The integrated approach of field work, microscopy, whole-rock and mineral-scale geochemistry, and in situ U-Th-Pb zircon geochronology has proven to be useful for recognizing the type, timing, and sequence of complex Na and K fluid alteration related to the development of Kiruna-type magnetite-apatite deposits and the tectonic evolution of the granites that host these deposits. The Lyon Mountain Granite in the northeastern Adirondack Mountains of New York State has undergone multiple episodes of hydrothermal fluid alteration and Fe mineralization. Perthite granite containing ubiquitous 1060–1050 Ma zircon grains was overprinted by potassic alteration, which in turn was overprinted by pervasive Na alteration. During the Na alteration, preexisting orebodies, consisting of magnetite, clinopyroxene, and apatite, were overprinted and remobilized to form new deposits that contain magnetite, apatite, quartz, and zircon.
The U-Th-Pb zircon geochronology data suggest that the Lyon Mountain Granite intruded the Adirondack Highlands during the Ottawan orogeny between ca. 1060 and 1050 Ma. However, subsequent alteration obscured much of the prehistory of the LMG. Amphibolite layers within the Lyon Mountain Granite and granitic dikes and pegmatites that crosscut the foliation in the Lyon Mountain Granite have been dated between 1045 and 1016 Ma. These ages coincide with previous published zircon age data from second-generation orebodies associated with Na alteration.
Low-Ti Kiruna-type magnetite-apatite, economic-grade mineral deposits are often included with the class of deposits commonly referred to as Fe oxide (Fe-Cu-Au; iron oxide-copper-gold [IOCG]) deposits, the characteristics of which are variable (e.g., Hitzman et al., 1992; Barton and Johnson, 1996; Corriveau et al., 2007). Some of these deposits contain significant amounts of Cu, Au, and U such as those at Olympic Dam, Australia, where the IOCG name is appropriately used (e.g., Reynolds, 2000), whereas in the Norrbotten region of northern Sweden (e.g., Kiirunavaara, Luossavaara, Svappavaara, Malmberget), the ores are composed mainly of magnetite and apatite (e.g., Hitzman et al., 1992; Harlov et al., 2002) and lack economic concentrations of Cu or Au. Genetic models for the origin of IOCG deposits include two main types; those that are directly related to magmatism, and those related to brines and evaporites (Barton and Johnson, 2004). The deposits that are clearly magmatic are characterized by a direct relation to a particular intrusion (e.g., intermediate to felsic calc-alkaline or alkaline), typically occur in an arc setting, have widespread K alteration, and are enriched in Au and Cu with moderate amounts of magnetite and few rare earth elements (REE) (e.g., Olympic Dam). Brine-related deposits are characterized by Na, K, or acid alteration (sericite and chlorite production), abundant magnetite and apatite, enrichment in REE, and are correlated with the presence of evaporites (e.g., Kiruna). In addition, brine-related deposits lack a direct connection to any particular intrusive event; however, they still require magmatism to drive hydrothermal fluid circulation in an extensional tectonic setting (Barton and Johnson, 2004). Specifically, the Kiruna-type ores have been debated as being volcanic-sedimentary in origin (Parák, 1975), or magmatic (Frietsch, 1978). In general, many deposits contain evidence for multiple generations of fluid alteration, including Na, K, Ca, Si, and Fe mineralization, and trace element enrichment (REE and high field strength elements, HFSE) and the remobilization of early-formed ores (e.g., Hitzman et al., 1992).
The magnetite-apatite deposits within the Lyon Mountain Granite (LMG) in the northeastern Adirondack Highlands of New York State are similar to the deposits in northern Sweden and lack Cu and Au in economic concentrations, but have high concentrations of REE in some apatite associated with mineralization (Lindberg and Ingram, 1964; Roeder et al., 1987; Valley et al., 2010). Extensive outcrops and relatively easy access make the LMG and its associated ore deposits an ideal natural laboratory for studying the relationships and hydrothermal alteration of the host rocks and the magnetite-apatite Kiruna-type mineralization. Understanding the timing and geochemical evolution of the hydrothermal alteration and related Fe mineralization, as well as the origin and geologic history of the LMG, has important implications for the origins of magnetite-apatite deposits globally, the conditions through which they form, and further exploration in other locations.
The Adirondack Mountains comprise a 27,000 km2 area of exposed Proterozoic Grenville Province rocks in northern New York State. The region is divided into the Adirondack Lowlands in the northwest and the Adirondack Highlands to the east and south (Fig. 1). The Adirondack Highlands are separated from the Adirondack Lowlands by the Carthage-Colton Shear Zone, which is marked by highly ductile fabrics and small granitic intrusions. Tonalite and granodiorite composition magmas were emplaced into the southern and eastern Adirondacks in the interval between 1350 and 1250 Ma. These rocks are interpreted to be juvenile, arc-related magmas emplaced along an Andean-style active margin prior to the onset of the Shawinigan orogeny (e.g., 1210–1170 Ma) (McLelland and Chiarenzelli, 1990; Rivers, 1997). Widespread metamorphism and deformation affected all of the Adirondacks during the Shawinigan orogeny as a result of the Adirondack–Green Mountain block colliding with the southeast margin of Laurentia (Wasteneys et al., 1999). Immediately following cessation of the contractional phase of the Shawinigan orogeny, extensive anorthosite, mangerite, charnockite, and granitic (AMCG) magmas were emplaced into the Adirondack Highlands between 1170 and 1120 Ma (e.g., McLelland et al., 2004; Hamilton et al., 2004). This time period is coincident with amphibolite-grade metamorphism in the Adirondack Lowlands. A subsequent period of deformation and metamorphism in the Adirondack Highlands was during the Ottawan phase of the Grenville orogeny. The timing of Ottawan metamorphism and deformation in the highlands was between 1100 and 1040 Ma, based on U-Pb zircon dating of deformed and undeformed intrusions (McLelland et al., 2001a). The penetratively deformed Hawkeye granite, dated as 1100 Ma, was used as an upper age limit for Ottawan deformation, while emplacement of the LMG during extension along the Carthage-Colton Shear Zone and elsewhere has been proposed to mark the end of the Ottawan orogeny (Mezger et al., 1991; McLelland et al., 2001a; Selleck et al., 2005).
The LMG crops out extensively in the northeastern Adirondack Highlands (Fig. 1) and is the host to numerous Kiruna-type magnetite-apatite deposits (e.g., Whitney and Olmsted, 1988; McLelland et al., 2002). The LMG underwent widespread K and Na fluid alteration, accompanied by the addition of F, Cl, and P, and mobilization of HFSE, including Zr, Y, U, and REE during at least one stage of Fe mineralization (Lindberg and Ingram, 1964; Roeder et al., 1987; Foose and McLelland, 1995). Sample localities for this project include Lyon Mountain, Dannemora, Hawkeye, Ausable Forks (fayalite granite), Palmer Hill, Arnold Hill, Clintonville, Dry Bridge, Rutgers, Mineville, and Skiff Mountain. Some of these localities were previously reported (Dannemora, fayalite granite, and Dry Bridge in McLelland et al., 2001; Lyon Mountain, Dannemora, and Skiff Mountain in McLelland et al., 2002). Outcrops of mineralogically similar, and possibly LMG related, rocks occur throughout the western Adirondack Highlands, some of which contain magnetite ore deposits (e.g., Star Lake). These deposits, however, remain to be explored in detail and their similarities (or lack of) to the ore deposits studied here are unclear. Quartz-sillimanite vein emplacement as a result of fluid alteration is present in some outcrops of the LMG in the western Adirondacks, but is rare in the northeastern Adirondacks (Selleck et al., 2004).
Sampling of the LMG host rock and ores, and other nearby units, was undertaken to quantify the extent of fluid alteration within the LMG and throughout the northeastern Adirondack Highlands. Detailed sampling and mapping were done in and around (i.e., from the host rock to the ore) a number of orebodies in the LMG to characterize fluid alteration and Fe mineralization associated with a given deposit. Similarly, sampling and mapping across contacts between the LMG and neighboring units were undertaken to better understand the origins of the LMG.
Cathodoluminescence (CL) microscopy was done with a PATCO ELM-3 cathodoluminoscope at Memorial University of Newfoundland. Operating conditions were 12 kV and 0.7mA, a vacuum of 50–70 mTorr, and an unfocused beam. Images were acquired using a KAPPA DX-30C Peltier cooled charge-coupled device (CCD) camera and KAPPA Image software. All CL images were acquired using consistent red-green-blue (RGB) settings so that CL colors could be directly compared between samples.
Whole-rock powders were prepared by using a tile saw to remove weathered material, a “chipmunk” jaw crusher (to produce pea-sized material), a disk mill (set to produce material with a grain size between 500 and 63 μm), and an alumina ceramic shatter box to produce pulverized powders in house, and subsequently analyzed for major and trace elements by Activation Laboratories Ltd. (Ancaster, Ontario, Canada), using inductively coupled plasma mass spectrometry for the Actlabs WRA +trace 4Lithoresearch analysis package. Reference materials AGV-1 and BCR-1 were also analyzed as unknowns and are included in Table 1 (Govindaraju, 1994).
Electron Probe Microanalysis
Electron microprobe analyses were done using a Cameca SX-51 electron probe microanalyzer (EPMA) with Probe for Windows software at the University of Wisconsin-Madison. Feldspars were analyzed for Si, Al, Fe, Ca, Na, K, and Ba. The operating conditions were an accelerating voltage of 15 kV, a Faraday cup current of 10 nA, and a defocused beam of 10 μm. Clinopyroxene and olivine were analyzed for Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and Cr, and magnetite was analyzed for Si, Ti, Al, Fe, Mg, Mn, Ca, Cr, V, and Zn. The operating conditions for both minerals were an accelerating voltage of 15 kV, a Faraday cup current of 20 nA, and a focused beam. We used 20 s count times on peak and background for all the elements that were measured. Natural mineral standards were used and the matrix correction algorithm of Armstrong/Love Scott phi-rho-z (Armstrong, 1988) was used for all analyzes.
Zircon Processing and Imaging
Zircon separation was done using standard crushing (to produce material with a grain size between 500 and 63 μm), heavy liquids (bromoform and methylene iodide), and magnetic methods (i.e., Frantz isodynamic magnetic separator). Zircon crystals were hand-picked from the nonmagnetic fraction and mounted in epoxy resin and polished to reveal the crystal centers. The epoxy mounts were gold coated and imaged with a Hitachi S-4300 scanning electron microscope (SEM) using CL to identify internal structures and zoning prior to ion probe microanalysis. Backscattered electron (BSE) imaging was also used to identify fractures and inclusions within the grains and to identify analytical spot locations precisely after the analyses were done at Memorial University of Newfoundland using a FEI Quanta 400 environmental SEM. A 20 kV accelerating voltage and 50 nA current were used for BSE imaging.
Secondary Ion Mass Spectrometry
The U-Th-Pb and Pb-Pb zircon data were acquired to determine the ages of the samples in this study using the Cameca IMS 1270 ion microprobe at the Swedish Museum of Natural History (Nordsim) following the methods described in Whitehouse et al. (1999) and modified in Whitehouse and Kamber (2005). The calibration of U/Pb is based on the 1065 Ma zircon standard 91500 with U and Pb concentrations of 80 and 15 ppm, respectively (Wiedenbeck et al., 1995). Initial data reductions were done using Excel macros developed at the Swedish Museum of Natural History. Age determinations and errors were calculated using Isoplot version 3.34 (Ludwig, 2003). The U-Pb data are plotted as 2σ error ellipses (all age errors quoted in the text are 2σ unless specifically stated otherwise). Common Pb corrections were applied to all ion probe data; an average composition for modern-day terrestrial common Pb is assumed (Stacey and Kramers, 1975).
Field Relations, Structure, and Deformation of the LMG
Several authors have discussed the extent of deformation within the LMG, and the name of the Lyon Mountain suite of rocks has varied between Lyon Mountain Granite (e.g., McLelland et al., 2001a; Selleck et al., 2005), Lyon Mountain granite gneiss (e.g., Postel, 1952), and Lyon Mountain gneiss (e.g., Whitney and Olmsted, 1993). We have adopted the term Lyon Mountain Granite as the unit is not always a gneiss but it is always a granite and most igneous rock types in the Adirondack Highlands retain their igneous name regardless of their deformational or metamorphic history (e.g., Marcy anorthosite, Hawkeye granite).
Contacts between the LMG and other lithologies in the Adirondack Highlands are typically poorly exposed, and the nature of these contacts (tectonic versus intrusive) is not obvious. However, dikes of probable LMG intrude the ca. 1100 Ma Hawkeye granite near the town of Hawkeye, New York, and a gabbronorite body (ca. 1150 Ma AMCG suite [anorthosite-mangerite-charnockite-granite]) on the south side of Skiff Mountain. In the northern part of the study area, the LMG overlies domes or core complexes of Hawkeye granite and anorthosite (Fig. 1). Much of the Fe mineralization in the northern area occurs just above the contact with one these domes of Hawkeye granite. The dominant mineral lineations and trends of the orebodies in the LMG plunge to the north-northeast, parallel to the axis of this regional folding (Postel, 1952). In the cores of some of the synforms (e.g., Dannemora), an assemblage of highly deformed and migmatized metasediments is present above the LMG. In the central part of the study area, the LMG has been folded into a north-plunging upright syncline between domes of anorthosite (Whitney and Olmsted, 1993) (Fig. 1). Mineralization in the central area is associated with faults or the hinge regions of folds (Postel, 1952). In the southern part of the study area, the LMG is typically in contact with calc-silicate marbles, other metasedimentary rocks, and gabbros (Isachsen and Fisher, 1971) (Fig. 1). Iron mineralization in the southeastern LMG frequently occurs near these contacts (e.g., Skiff Mountain, Mineville). Late northeast-trending faults of uncertain age crosscut all lithologies in the southern and central study area (Isachsen and Fisher, 1971).
There is little evidence of widespread penetrative deformation in the LMG, but localized deformation is present in the form of stretched or sheared clinopyroxene layers (Fig. 2A), and quartz veins (Fig. 2B). These layers and veins are typically ambiguous as to the direction of shearing, showing only stretching without rotation. Migmatization and mylonitization were observed in and around the zone of mineralization of some orebodies (e.g., Lyon Mountain) (Figs. 2C, 2D). At these localities, melting has taken place in situ, as revealed by mafic selvages around the leucosome.
Rock Descriptions and Petrography
Detailed rock and petrographic descriptions of all the LMG rock types and associated rocks for this study are presented in the Appendix. A brief summary is presented here.
Lyon Mountain Granite
The LMG consists of three primary end-member rock types based on the dominant feldspar present: perthite granite, microcline granite, and albite granite. The microcline granite and the albite granite are most likely the result of fluid alteration and do not constitute true magmatic compositions. Mineral assemblages related to Na fluid alteration clearly overprint the perthite and microcline granite. The paragenetic sequence for the LMG and subsequent alteration events are presented in Figure 3A.
The perthite granite (e.g., Clintonville, sample 99–1a; Rutgers Mine, sample 06–4d; Lyon Mountain, sample LM-06–2a) is the dominant rock type of the LMG suite of rocks. The perthite granite is considered unaltered, or the least altered, due to its occurrence away from orebodies and hydrothermal alteration zones. Its general resemblance to perthitic granites from other igneous and metamorphic terranes in the Adirondacks and elsewhere and the coherent or semicoherent nature and regular distribution of lamellae throughout the feldspar grains suggest that it is a primary igneous or metaigneous rock type (e.g., Parsons et al., 2005).
The primary mineral assemblage in the perthite granite is perthitic feldspar, quartz, euhedral magnetite, and minor zircon, hematite-ilmenite intergrowths, titanite, and apatite. Perthite grains exhibit coarse, anastomosing albite lamellae hosted by microcline (Figs. 4A, 4B). Many samples of the perthite granite contain secondary clinopyroxene and coarse masses of magnetite. In addition, the unit commonly contains minerals related to subsequent Na- and K-rich fluid alteration, especially albite replacement of perthite grains. In CL, microcline regions in perthite grains luminesce blue (typical of microcline; e.g., Finch and Klein, 1999), and albite lamellae are brownish-gray-green to red. Red CL in feldspars is attributed to the presence of Fe3+ or trivalent REE (e.g., Sm or Eu) (Finch and Klein, 1999).
The microcline granite is the simplest mineralogically and typically more homogeneous than either the perthite or albite granite. Foliations in the microcline granite are much less distinct and difficult to observe than in the perthite granite owing to the lack of mafic and planar minerals or subsequent K alteration and recrystallization. The mineral assemblage consists predominately of microcline, quartz, magnetite or hematite, and minor titanite, apatite, and zircon. The microcline granite is pristine in some localities (e.g., Dannemora, sample 99–6c) or overprinted by minerals related to Na-rich fluid alteration (e.g., Palmer Hill, sample 99–4b). In addition, CL imaging shows multiple generations of K-feldspar growth (Figs. 4C, 4D).
The albite granite, like its potassium counterpart the microcline granite, is probably the product of pervasive Na-rich fluid alteration and overprints both the perthite granite and the microcline granite. Earlier rock types are completely converted to albite granite (e.g., albitized), with only small remnants of the preexisting lithology remaining, or veins and patches of albite present where replacement is incomplete (Figs. 4E, 4F). The albite granite appears little deformed except in localized shear zones, where the unit has acquired a moderate foliation. The mineral assemblage of the albite granite is highly variable, consisting of granoblastic albite, quartz, zircon, apatite, magnetite ± clinopyroxene ± amphibole ± titanite ± garnet ± fluorite. The albite granite in some localities (e.g., Arnold Hill, sample AH-06–1a) has been overprinted by a second phase of K alteration as well as by Ca, or Si-rich fluids in the form of untwinned K-feldspar, calcite, and jasper.
In addition to the three main rock types in the LMG, there is a small body of hydrothermally altered fayalite granite (Ausable Forks, sample 99–3a) that crops out in the town of Ausable Forks and probably represents a later intrusion or reduced magma of the LMG series (Fig. 1) (Postel, 1952; Whitney and Olmsted, 1993). The primary mineral assemblage consists of perthitic feldspar, quartz, clinopyroxene, minor zircon, and fayalite. It is not clear if the fayalite is primary or the result of hydrothermal alteration. Secondary mineral growth as a result of Na fluid alteration consists of albite, quartz, fluorite, apatite, and magnetite. Perthitic feldspar and clinopyroxene grains in the fayalite granite are distinct from those of the perthite granite. In feldspar grains, thin, straight, uniform lamellae are crosscut by hydrothermal alteration–related albite flame lamellae containing minute inclusions of Fe oxides (Figs. 4G, 4H). Clinopyroxene grains contain exsolution lamellae of Fe-rich pyroxene that are not present in other LMG rock types (see following mineral chemistry results).
Ore deposits in the LMG can be divided into two main types: clinopyroxene-magnetite (skarn-like deposits) and those associated with Na-rich fluid alteration and the growth of magnetite-apatite and quartz. Figure 3B shows the proposed sequence of mineral paragenesis for Fe mineralization. The clinopyroxene-magnetite deposits are typified by the mineral assemblage clinopyroxene, magnetite ± apatite ± amphibole ± K-feldspar. Enclaves and pods of clinopyroxene-magnetite ore veins can be observed within the perthitic granite (e.g., Rutgers Mine, sample 06–4d; Fig. 5A). The clinopyroxene-magnetite deposits are crosscut by granitic dikes and pegmatites, may contain miarolitic cavities (Fig. 5B), and in many localities have been overprinted and remobilized by subsequent Na-rich fluid alteration (Fig. 5C) to form a second generation of deposits (Fig. 5D). Deposits associated with Na alteration comprise magnetite, apatite, quartz, and zircon, and can contain albite ± fluorite ± clinopyroxene ± amphibole ± biotite ± titanite ± chlorite ± calcite. Inclusions from earlier assemblages (e.g., perthite granite, clinopyroxene-magnetite ore) were present at some localities. In most samples zircon was not identified in thin section, and only identified later during heavy mineral separation.
Regardless of the ore type, certain physical features are shared between the different ores. The contacts between the ore and the host granite are either sharp (Figs. 5D, 5E) or in some localities gradational (Fig. 5F). Often, thin layers of magnetite (or magnetite and clinopyroxene) gradually increase in number and thickness until the main layer of the ore is reached (Fig. 5F). It is common above or below the mineralized zone to find the granite nearly devoid of disseminated magnetite, and for that matter any other ferromagnesian minerals that are present elsewhere in the rock further away from the ore-host contact. These barren zones range from a few meters (e.g., Lyon Mountain, Skiff Mountain) to ∼100 m (e.g., Palmer Hill).
Related Rock Types
Related minor units within the LMG include amphibolite layers, and crosscutting felsic dikes. These units, as well as the nearby Hawkeye granite and AMCG suite–related charnockite, were collected to better understand the evolution of the LMG, including the fluid alteration history and Fe mineralization.
Amphibolite layers (e.g., Skiff Mountain, 00–11; Ausable Forks, F-06–1a) are relatively common within the LMG, but constitute little of the total volume of the granite. Though not the focus of this study, they are considered here to help better characterize the LMG and its evolution and origin. The layers typically have a “salt and pepper” appearance and comprise varying amounts of biotite, amphibole, plagioclase, and apatite, with minor orthoclase, magnetite or hematite-ilmenite intergrowths, titanite, and zircon, and in the sample collected at Skiff Mountain, alteration-related albite.
Granitic dikes and pegmatites are common throughout the LMG and the Adirondack Highlands. These dikes crosscut layering in the LMG and the regional fabric of the highlands. Both potassium feldspar–rich dikes and plagioclase-rich dikes were observed. The potassium feldspar–rich dikes (e.g., Dannemora, sample 99–6b; Skiff Mountain, sample 00–03) typically contain coarse microcline, quartz, and minor magnetite, zircon, and apatite ± muscovite ± amphibole ± biotite ± calcite. A plagioclase-rich dike collected at Lyon Mountain (sample LM-06–1a; also reported in McLelland et al., 2001b) consists of coarse quartz, oligoclase, magnetite, microcline, apatite, titanite, calcite, zircon, and minor sulfides. This dike was overprinted by minerals related to Na alteration (i.e., albite).
The Hawkeye granite (sample H-06–1c) mineral assemblage contains highly recrystallized quartz, perthitic feldspar, myrmekite plagioclase, and zircon ± amphibole ± orthopyroxene ± garnet ± biotite ± ilmenite ± apatite. Perthite grains in the Hawkeye granite typically contain short, fat lamellae that are often absent in the rims of the grains. The mineral assemblage of the charnockite comprises plagioclase, perthitic feldspar (with short or rounded lamellae), clinopyroxene (with exsolution lamellae), orthopyroxene, garnet, apatite, and zircon and minor quartz.
Whole-Rock Major and Trace Elements
Whole-rock geochemical analyses were done on the three main rock types of the LMG and the associated units mentioned above (including the Hawkeye granite and the AMCG suite charnockite). These data are shown in Table 1 (locality stated here, and marked on the geologic map in Fig. 1). The whole-rock geochemistry presented here is in general agreement with earlier findings from the different LMG rock types (e.g., Whitney and Olmsted, 1988; McLelland et al., 2002). The LMG shows a wide range of compositions in both major and trace elements as a result of the original composition of the rock types and the subsequent hydrothermal alteration and Fe mineralization, and it is not clear how much the major and trace element chemistry accurately reflect the original rock compositions. The SiO2 content in LMG samples varies between 65.78 and 73.53 wt%, and total alkalies (Na2O + K2O) vary from 6.03 to 10.57 wt%. It is unclear what portion of the alkali content is related to alteration and what is related to the original composition of the protolith. Samples that have been subjected to complete alkali replacement contain as much as 10 wt% K2O or Na2O (Table 1). There is a one to one correlation between Na and K when normalized to aluminum (Fig. 6A). LMG rock types range from peralkaline to peraluminous and both the microcline granite (K alteration) and the albite granite (Na alteration), and have a higher aluminum saturation index relative to the perthite granite (least altered) (Fig. 6B). Most samples from the LMG as well as the AMCG suite charnockite have high Fe relative to Mg (Figs. 6C, 6D); the most Na-altered samples (e.g., Arnold Hill, sample AH-06–1a), Mineville, sample BH-LMG-2) contain the highest levels of Fe [to 10 wt% Fe2O3 (total, tot)]. The perthite granite typically averages 5 wt% Fe2O3 (tot). Samples immediately adjacent to the orebody often show a depletion in Fe with Fe2O3 (tot) values as low as 1.69 wt% (e.g., Lyon Mountain, sample LM-06–1c). Samples with lower Si content typically have higher Ti and Al values but with no clear correlation to a particular type of alteration (Figs. 6E, 6F). Elevated Ca is observed in some samples regardless of rock type or alteration (e.g., Arnold Hill, RC-06–2; Dry Bridge, 99–2b) (Table 1; Fig. 7A). Samples that have undergone K alteration (Dannemora, 99–6c) are enriched in the large ion lithophile elements (LILE) Rb (170% increase), Cs (333% increase), and Ba (540% increase) relative to the perthitic granite (Clintonville, 99–1a). Albite granite and Na-altered samples are depleted in these same elements relative to the perthite granite (Figs. 7B–7E).
Plots of elements generally considered to be immobile (e.g., Ti, Al, Zr, Y, and Nb) are presented in Figure 8. The Ti and Al contents are variable in the LMG with a slight decrease of TiO2 with increasing Zr and a slight increase of Al with increasing Ti (Figs. 8A, 8B). Most samples that have undergone Na alteration are enriched in most HFSE (Figs. 8C–8F; Table 1). Thorium does not increase with Na alteration. A specific comparison between the least altered perthite granite from Clintonville (99–1a) and the Na-altered albite granite at Skiff Mountain (00–02) shows that Zr increased by 522%, Y by 400%, Nb by 167%, and Ta by 197% in the albite granite.
Chondrite-normalized whole-rock REE concentrations for the LMG and associated rocks are plotted in Figure 9. All LMG samples have similar negative slopes with minor variations in La in the least altered samples and a more pronounced negative Eu anomaly in the Na-altered samples (Fig. 9; Table 1). Samples that have undergone extreme Na alteration have the highest whole-rock REE concentrations and the widest range of REE concentrations relative to the other samples. Lanthanum has increased by 205%, Nd by 191% and Yb by 357% in the albite granite at Skiff Mountain compared to the perthite granite at Clintonville. The least altered samples and those dominated by potassic alteration are broadly similar in concentrations and variability (Fig. 9A; Table 1). Overall, all rock types in the LMG have elevated REE relative to the Hawkeye granite and AMCG charnockite (Fig. 9A). The AMCG suite charnockite, however, has a pronounced positive Eu anomaly. The concentrations of REE in the ore deposits of the LMG are highly variable depending on the mineralogy of the ore (Fig. 9B; Table 2). Those ore samples containing abundant (i.e., 25–30 modal percent) apatite are highly enriched in REE relative to those containing only pyroxene or those ore deposits associated with quartz and feldspar pegmatites (Fig. 9B).
Electron probe analyses of minerals from the different LMG rock types were analyzed to better understand the fluid geochemistry of alteration and the differences between primary and alteration-related minerals.
Representative samples of various feldspars from each of the three main LMG rock types were analyzed and are shown in Table 3 and plotted in Figure 10. Plagioclase chemistry from all lithologies is highly sodic, containing 94%–99% albite (Ab) component. In the perthite granite, there is a distinction between alteration-related albite and the albite lamellae. Electron microprobe transects across single perthitic feldspar grains reveal that rims and patches of alteration-related albite are consistently near end-member compositions (e.g., Ab99), while albite lamellae contain as much as 5% anorthite (An) (Figs. 10A, 11A, and 11B; Table 3). Alteration-related flame lamellae in the fayalite granite have similar end-member compositions (e.g., Ab99) (Figs. 10B and 11B; Table 3). The perthite grains in the fayalite granite also contain small, thin plagioclase lamellae related to granite crystallization and cooling, but these were too small for EPMA analysis (<5 μm) (Figs. 4G, 4H). Plagioclase compositions in the albite granite have chemical composition of Ab94–95 or contain near pure albite (e.g., Ab99) (Fig. 10C; Table 3).
The microcline regions in the perthite granite range from (orthoclase) Or92 to Or97 (Figs. 10A, 11A and 11B; Table 3), while perthite grains in the fayalite granite are typically Or90 to Or94 (Figs. 10B and 11B; Table 3). Potassium feldspar in the microcline granite was the most variable in composition of all the samples analyzed. Orthoclase values typically range from Or86 to Or96. This range of Or values may be due to remnant Na from the original perthitic feldspar or chemical variations in multiple generations of potassium feldspar growth (see microcline granite description). Barium was below the minimum detection limit of the EPMA analyses for the conditions used in potassium feldspar of the perthitic granite and fayalite granite, but was as high as 1 wt% BaO in the microcline granite.
Clinopyroxene was not present in our samples of the microcline granite, but has been reported by other workers (Postel, 1952). The composition of all pyroxene grains (regardless of LMG rock type) is aegirine-augite with varying amounts of Ca, Na, Fe, and Mg (Table 4). Clinopyroxene from the perthitic granite contains a greater diopside component than either the albite or fayalite granites, while clinopyroxene from the albite granite has increasing Na and Fe and decreasing Ca and Mg. Clinopyroxenes from the fayalite granite have a higher ferrohedenbergite component (Fe rich and Mg poor) than other LMG clinopyroxene grains and contain exsolution lamellae of nearly pure ferrosilite or inverted pigeonite (Figs. 1C, 1D). Olivine grains in the fayalite granite are near end-member fayalite composition.
The chemistry of magnetite was compared between the perthite granite, a granite sample that has undergone both K and Na alteration (Palmer Hill), and various ore samples (Table 5). There are few differences in major elements between ore and granitic magnetite compositions. Magnetite is relatively low in Ti and other transition metals. One exception is the sample from the granite just a few meters above the orebody at the Palmer Hill mine that contained anomalously high Ti (to 11 wt%). This sample contains both microcline and albite, but no perthitic feldspar. The magnetite in this sample underwent extensive oxidation and replacement by hematite (martite in some grains).
Three samples that represent the three main rock types within the LMG were chosen for zircon U-Th-Pb geochronology. Results from two additional samples from the LMG at Palmer Hill and Arnold Hill were published in Valley et al. (2009). In addition to the LMG samples, two amphibolite layers and three crosscutting dikes (discussed above) were dated to constrain the timing of fluid alteration and Fe mineralization relative to the regional tectonics of Adirondack Highlands. These dates, along with previously published U-Th-Pb dates, are summarized in Table 6. Individual U-Th-Pb analyses are presented in Tables 7, 8, and 9. All errors are given as 2 sigma and the stated mean square weighted deviation (MSWD) is of concordance and equivalence.
Lyon Mountain Granite
A representative sample of the LMG perthite facies was collected along the Ausable River near Clintonville, New York (sample 99–1A), for geochronology (Fig. 1). Zircon grains from that sample are brown to pink or clear, well faceted, and can contain irregularly shaped cores that are highly fractured. Zircon grains are typically 200–400 μm in length with an aspect ratio that approximates 2:1. Some zircon grains exhibit growth zoning and relict cores (in BSE), while others (especially those lacking relict cores) are relatively homogeneous (e.g., Figs. 12A, 12B). Zircon grains without inherited cores and rims on zircon grains with cores from the perthitic granite at Clintonville yield a concordant age of 1047.2 ± 9.2 Ma (2σ) (n = 7) (Fig. 13A).
A sample of microcline granite was collected ∼1.6 km west of the town of Dannemora, New York (sample 99–6c) (Fig. 1). The zircon grains are typically 200–500 μm in length, with an aspect ratio that approximates 2:1, well faceted, and brown to pink or clear in color, and in some cases have obvious cores (Figs. 12C, 12D) of AMCG suite age (e.g., ca. 1150 Ma) that are dark brown, and are often cracked relative to the outer regions in the zircon crystals. New zircon growth embays or cuts into these relict cores. Most zircon cores and outer regions show growth zoning in BSE images. Single age zircon grains, and rims on grains with cores, from the microcline granite give a concordant age of 1050.5 ± 5.2 Ma (2σ) (n = 11) (Fig. 13B). This age agrees with the data of McLelland et al. (2001a), who obtained a U-Pb zircon age of 1055 ± 7 Ma (2σ) by sensitive high-resolution ion microprobe (SHRIMP) from a sample collected at the same outcrop.
Samples of the albite granite were collected at Dry Bridge ∼3.2 km northeast of the town of Ausable Forks, New York (sample 99–2b) (Fig. 1). Zircon grains are typically smaller, and much less abundant, than those of the microcline granite or the perthitic granite and have aspect ratios of 2:1 and range from 100 to 200 μm in length (Figs. 12E, 12F). Some of the zircon grains are similar to those of the perthite and microcline granites, with dark irregular cores of ca. 1150 Ma and rims or single age zircon grains ca. 1060 Ma (filled error ellipses; Fig. 13C); however, these analyzes did not produce a statistically reliable concordia age. McLelland et al. (2001a) obtained a similar date of 1055 ± 7 Ma. The albite granite at Dry Bridge, however, contains a significant population of zircon that displays patchy zoning, or no zoning in BSE and CL, and is consistently younger than the typical 1060–1050 Ma zircon population found throughout the other LMG rock types. These zircon grains with patchy zoning yield a concordant age of 1033 ± 9 Ma (2σ) (n = 4) (Fig. 13C). We interpret this age to reflect hydrothermal zircon growth during Na alteration at the Dry Bridge locality. This interpretation is supported by U-Pb age data from nearby magnetite orebodies associated with Na alteration that have been dated between 1040 and 1015 Ma (Valley et al., 2009). These ages have not been detected in the microcline granite despite several attempts to find younger zircon.
Crosscutting Granitic Dikes
Relatively undeformed granitic dikes that crosscut the fabric (when present) of the LMG are common. Representative samples were collected and dated to provide a minimum age for fabric development within the LMG and to better constrain the ages of various types of alteration.
A crosscutting plagioclase-rich granitoid dike was collected at the Chateaugay Mine in the town of Lyon Mountain, New York (Fig. 1) (sample LM-06–1a). The dike crosscuts the granitic fabric as well as the main orebody. The dike has undergone secondary Na alteration, which in turn was overprinted by a minor potassic alteration. Zircon grains from this sample are clear and elongate or equant. Equant grains are as long as 300 μm on a side, and elongate grains are typically 400–600 μm with some grains as long as 900 μm. These zircon grains contain growth zoning or display patchy zonation in BSE and CL images (Figs. 14A, 14B). No obvious inherited cores were observed. These zircons yield a concordant age of 1040.9 ± 6.8 Ma (2σ) (n = 11) (Fig. 13D). The age of this dike provides a maximum age for Na alteration at the Chateaugay Mine in Lyon Mountain and minimum age for at least one stage of ore formation (e.g., clinopyroxene-magnetite ore).
A granite pegmatite dike was collected at the same locality as the dated microcline granite sample mentioned above, just west of the town of Dannemora, New York (sample 99–6b). Here the pegmatitic dike crosscuts the fabric in the microcline granite. Zircon grains are clear, elongate, and 300–600 μm in length, typically exhibit patchy zonation, and can contain obvious inherited cores in BSE images (Figs. 14C, 14D). Zircon rims and single age zircon grains from this sample are concordant at 1016.5 ± 6.9 Ma (2σ) (n = 8) (Fig. 13E).
A granite pegmatite dike was collected from a road cut at the base of Skiff Mountain, New York (sample 00–03), and crosscuts the microcline granite. Both the dike and the microcline granite have undergone minor sodic alteration. Zircon crystals from this sample are elongate (300–500 μm long), clear with patchy zonation in BSE images, and typically contain large inherited cores of both AMCG (ca. 1150 Ma) and LMG (ca. 1060 Ma) age (Figs. 14E, 14F). The zircon rims from zircon grains with relict cores and grains without cores have a concordant age of 1030.4 ± 1.8 Ma (2σ) (n = 5) (Fig. 13F).
At Skiff Mountain, two parallel amphibolite layers are present 2 m below the upper ore seam on the west side of Skiff Mountain (sample 00–11) (Fig. 1; Appendix Fig. A5A). These layers are continuous across the extent of the exposed outcrop. Zircon grains are small (100–200 μm), clear, and irregularly shaped (Fig. 14G), and yield a concordant age of 1046 ± 11 Ma (2σ) (n = 5) (Fig. 13G). Rare inherited cores (ca. 1130 Ma) are sometimes present, as well as a subset of zircon that are younger and equivalent in age to zircon from the orebody at Skiff Mountain (ca. 1000 Ma) (Table 9).
Samples from an amphibolite layer were collected ∼100 m east of the cemetery in the town of Ausable Forks, New York (sample F-06–1a). Its relation to the amphibolite layers within the LMG is unclear as this unit separates Na-altered LMG from outcrops of the fayalite granite mentioned above. This unit is probably related to the metagabbro and amphibolite bodies mapped locally by Postel (1952) and Whitney and Olmsted (1993). Zircon grains are clear, small (100–200 μm), and rounded or irregularly shaped with little internal zonation or obvious inherited cores in BSE images (Fig. 14H). This sample has a concordant age of 1018.8 ± 7.6 Ma (2σ) (n = 12) (Fig. 13H).
Understanding the origins and the subsequent timing and sequence of the Na and K hydrothermal fluid alteration and Fe mineralization of the LMG has important implications for the ore genesis of Kiruna-type deposits globally. In the following discussion we try to relate petrographic and field observations, whole-rock and mineral geochemistry, and U-Th-Pb zircon dates to form a cohesive emplacement, fluid alteration, and mineralization history of the LMG and the associated magnetite-apatite deposits.
Sequence of Alteration
A model for the sequence of alteration of the LMG and related rocks is presented in Figure 15. Combining U-Th-Pb geochronology with field data, detailed petrography, and geochemistry, it is possible to assign absolute ages or at least maximum and minimum ages to the various phases of alteration, mineralization, and tectonic conditions that have affected the LMG.
Age data presented here and in earlier work (e.g., McLelland et al., 2001a; Valley et al., 2009) suggest that the LMG cannot be younger than ca. 1060–1050 Ma, and likely intruded rocks of the AMCG suite at that time (see LMG evolution discussion). The earliest mineral assemblage (e.g., perthite granite) consists of perthitic feldspar, quartz, magnetite, hematite-ilmenite intergrowths, titanite, apatite, and zircon (Fig. 3). Clinopyroxene and coarse magnetite are present in some samples of the perthite granite and form subsequent to the primary mineral assemblage. The secondary growth of clinopyroxene and magnetite within the LMG and the textural similarities to the clinopyroxene-magnetite ± apatite orebodies suggest that they may be related. Secondary growth of titanite occurs after magnetite in both the granite and clinopyroxene-magnetite orebodies.
In some localities, the perthite granite is unaltered or has undergone minor alteration. In other localities the perthite granite served as the protolith for the microcline and albite granites. The perthite granite is overprinted by widespread and pervasive potassic alteration associated mainly with the growth of microcline and quartz. Complete replacement of the primary perthitic feldspar results in the formation of the microcline granite. The similarity of zircon ages in the microcline granite and perthite granite suggests that K alteration did little to modify preexisting zircon grains or promote new zircon growth, or that K alteration took place within error of the zircon ages of the perthite granite. Whole-rock geochemistry indicates that the LILE were mobile during K alteration (Table 1; Fig. 7). Potassic alteration must have occurred prior to ca. 1040 Ma, as the plagioclase-rich dike at Lyon Mountain crosscuts the microcline granite, and 1040 Ma hydrothermal zircon growth associated with sodic alteration Fe mineralization at Palmer Hill also overprints the preexisting microcline granite (Valley et al., 2009).
Sodic alteration is associated with the growth of albite, quartz, apatite, titanite, hydrothermal zircon ± amphibole ± biotite ± fluorite ± garnet. Complete replacement of perthitic feldspar or the microcline granite results in the formation of the albite granite. In rocks where partial replacement has occurred, varying proportions of earlier assemblages related to the perthite granite, microcline granite, and clinopyroxene-magnetite orebodies are present. In addition, some amphibolite layers and granitic dikes have been affected by Na alteration. These overprinting relationships, when combined with zircon age data from the same samples, provide maximum ages for Na-rich fluid alteration at those localities (Fig. 15). The development of albite flame lamellae, similar to that observed in the fayalite granite, typically occurs during deformation at upper greenschist or lower amphibolite facies conditions (Pryer and Robin, 1995; 1996), suggesting that Na alteration of the fayalite granite occurred under similar conditions and must be younger than the ca. 1047 Ma age of the fayalite granite.
Whole-rock geochemical data (and CL imaging of thin sections) suggests that the extreme variation in K and Na and an increase in Al in the hydrothermally altered rocks can be explained by alkali exchange in feldspar or the addition of feldspar during fluid alteration (Figs. 6A, 6B). Many immobile elements (including Zr, Nb, Y, and Ta) and REE are enriched in Na-altered rocks relative to the other LMG rock types, with apatite as the main repository for REE. The presence of fluorite and fluoroapatite (as well as fluoroamphibole and fluorotitanite; P. Whitney, 2006, personal commun.) and NaCl-rich fluid inclusions (McLelland et al., 2002) in the Na-altered rocks suggests that Na fluids were rich in F and Cl and most likely responsible for the transport of HFSE and REE (e.g., Bau and Dulski, 1995; Salvi et al., 2001).
Sodic alteration coincides with the probable remobilization of disseminated magnetite and the breakdown of clinopyroxene to form amphibole and biotite through hydration reactions. The remobilization of the clinopyroxene-magnetite ore deposits, and further concentration of disseminated magnetite from the granite, results in the formation of second-generation ores composed primarily of magnetite-apatite-quartz and zircon. Zirconium and REE are released from the clinopyroxene and apatite, respectively, resulting in the growth of hydrothermal zircon rich in radiogenic Hf (Valley et al., 2010). These zircon dates from the ore directly date the Na alteration and second-stage Fe mineralization (Valley et al., 2009) (Fig. 15). Hydrothermal zircon growth occurs mainly in the magnetite-apatite orebodies and possibly in the rocks of the LMG that have undergone extensive interaction with sodic fluids, such as at Dry Bridge. Zircons from Dry Bridge have an age population that is significantly younger (ca. 1033 Ma) than the zircon ages in the microcline and perthite granites (1060–1050 Ma). This 1033 Ma zircon population is similar to zircon ages from the orebodies that range from ca. 1040 Ma at Palmer Hill to ca. 1000 Ma at Skiff Mountain (Valley et al., 2009). Based on these hydrothermal zircon ages, alteration by Na-rich fluids in the LMG must be episodic over a period of at least 40 m.y. In addition, the age of hydrothermal zircon is similar to the zircon ages of crosscutting dikes and pegmatites (ca. 1040–1016 Ma) found throughout the LMG (and elsewhere in the Adirondack Highlands; see Selleck et al., 2004), suggesting that fluid circulation and secondary ore formation coincide with dike emplacement and continued extension of the Adirondack Highlands during that time. The extent of Na alteration is difficult to assess due to the lack of exposure. At Lyon Mountain, the Na alteration in the hanging wall is ∼10 m wide. Other outcrops of albite granite are hundreds of meters long.
A second, more limited period of potassic fluid alteration (and locally including Ca alteration) postdates all previously mentioned mineral assemblages and fluid events. This late alteration was observed at Lyon Mountain, Arnold Hill, and Dannemora (see feldspar CL petrography discussion). This phase of K alteration is limited to the growth of twinned and untwinned potassium feldspar on rims and grain boundaries or preexisting perthite, microcline, and albite grains. This final stage of K alteration overprints the plagioclase-rich granite dike at Lyon Mountain (ca. 1040 Ma), the granite pegmatite at Dannemora (ca. 1016 Ma), and Na alteration and associated Fe mineralization at Arnold Hill (ca. 1016 Ma). Calcium alteration is present in the form of calcite and at Arnold Hill, and is associated with the growth of jasper, chlorite, and the nearly complete oxidation of magnetite to hematite.
Implications for Evolution of the LMG
In addition to the timing and sequence of fluid alteration and Fe mineralization in the LMG, the data presented here provide some information on the origins and evolution of the LMG and thus provide further constraints on the ore genesis of magnetite-apatite deposits. Debate over the origins of the LMG centers around the meaning of the 1060–1050 Ma zircon rims and single-age grains found in the LMG, i.e., (1) whether the LMG was emplaced (or erupted) ca. 1150 Ma and is coeval with the AMCG suite rocks in the Adirondacks (Whitney and Olmsted, 1988) and was subsequently metamorphosed and/or hydrothermally altered between 1060 and 1050 Ma; or (2) if the LMG was emplaced between 1060 and 1050 Ma during the middle to late Ottawan orogeny (McLelland et al., 2001a; Selleck et al., 2005). If the former, the implication is that the LMG subsequently underwent granulite facies metamorphism during the Ottawan orogeny and that the 1060–1050 Ma zircon ages are metamorphic or a combination of metamorphic and hydrothermal recrystallization of the earlier formed zircon. If the LMG was emplaced between 1060 and 1050 Ma, it is interpreted as a syntectonic to late tectonic granite.
The Hawkeye granite (1100 Ma), which is geochemically similar to the LMG and clearly has undergone granulite facies metamorphism and Ottawan deformation, contains a pervasive quartz ribbon lineation that is lacking in the LMG. McLelland et al. (2001a) cited this as evidence that the LMG did not undergo the main phase of Ottawan deformation and metamorphism and was emplaced later than the Hawkeye granite (e.g., 1060–1050 Ma). The Hawkeye granite contains garnet and orthopyroxene that can be indicative of granulite facies metamorphism in certain rock types (e.g., Frost and Frost, 2008, and references therein) and the LMG lacks these minerals. Both the Hawkeye granite and the LMG contain perthitic feldspar with coarse plagioclase lamellae; however, the rims of perthite grains in the Hawkeye granite show evidence of recrystallization and elimination of the lamellae while perthite grains in the LMG do not (Figs. 4A, 4B, A1, and A6).
Zircon grains from the LMG are elongate and often contain faceted and well-developed terminations and growth zoning in BSE or CL imaging (McLelland et al., 2001a) (Fig. 12). The majority of zircon ages from the Hawkeye granite are 1090 Ma or older; there is a small population of 1060–1050 Ma zircon ages (McLelland et al., 2001a; Valley, 2008, personal observ.) despite being geochemically similar to the LMG. The differences in deformation, mineralogy, and zircon ages suggest that the LMG and Hawkeye granite had different metamorphic and deformational histories.
This is not to say the LMG did not experience any metamorphism or deformation. Clearly the LMG underwent prolonged periods at elevated temperatures. The presence of coarse, wavy albite lamellae and triclinic microcline in perthite grains requires very slow cooling at temperatures below the transition from low sanidine to intermediate microcline (∼500 °C) (e.g., Yund and Davidson, 1978; Brown and Parsons, 1984). In addition, the LMG has undergone regional-scale upright folding or core complex development (Isachsen and Fisher, 1971; Whitney and Olmsted, 1988), locally exhibits a pronounced tectonic fabric (e.g., recumbent folding), and can contain boudinaged or stretched clinopyroxene and migmatite layers, or quartz veins (Fig. 2).
The third possibility for the origin of 1060–1050 Ma zircon ages in the LMG is that those zircon grains, or rims of grains, are hydrothermal in origin and this fluid event erased much of the tectonic fabric and metamorphic mineral assemblage. This idea is based on the unusual internal structures observed in BSE and CL imaging and the presence of embayments from the outer zircon regions intruding, with no obvious crystallographic control, into the inner regions of some zircon crystals (Fig. 12D). If this hypothesis is true, it would suggest that the LMG protolith is ca. 1150 Ma, that Ottawan deformation has been obliterated by the hydrothermal alteration, or went unrecorded, and that the entire LMG underwent this hydrothermal event. This seems unlikely given the following. (1) This 1060–1050 Ma hydrothermal zircon event is not recorded in any of the orebodies, as zircon ages in the ore deposits are 20–60 m.y. younger than those of the granite (Valley et al., 2009). (2) A pervasive 1060–1050 Ma fluid event would be concurrent with dry granulite facies metamorphism during the Ottawan orogeny (e.g., Valley et al., 1990; Rivers, 1997; McLelland et al., 2001a. (3) If the entire LMG has undergone fluid alteration, why did this fluid event not affect the Hawkeye granite? The data obtained in this study suggest that the perthite granite (e.g., Clintonville, sample 99–1a; Rutgers Mine, sample 06–4d; Lyon Mountain, sample LM-06–2a) is not the product solely of hydrothermal fluid alteration, given its ubiquitous presence away from the orebodies and lack of zones of obvious fluid alteration. Thus the perthite granite should retain any Ottawan deformational and metamorphic effects if it was emplaced at 1150 Ma. (4) If the LMG is 1150 Ma, this forces us to explain previously mentioned differences between the Hawkeye granite and the LMG regarding metamorphism, deformation, and mineralogy. A more likely reason for the observed zircon textures is that they are inherited zircon grains that were assimilated by 1060–1050 Ma LMG magmas.
The most likely hypothesis suggested by the data presented here is that the LMG was originally a syntectonic to late tectonic granite, as proposed by McLelland et al. (2001a) and Selleck et al. (2005). Intrusion of the LMG occurred between ca. 1060 and ca. 1047 Ma, concurrent with the Ottawan orogeny. Intrusion of the LMG probably marked the onset of extension (if melts are the result of decompression) or crustal anatexis from an overthickened crust in the Adirondack Highlands (e.g., metamorphic dehydration melting, the input of mantle heat from delamination of the lower crust or frictional heating during Ottawan thrusting). Synextensional intrusion of the LMG has been documented along the Carthage-Colton Shear Zone in the western Adirondack Mountains (Selleck et al., 2005). Similarly, extension in the Maurice region of Quebec was active from ca. 1090 to ca. 1040 Ma and contemporaneous with emplacement of younger granitoids that have a chemistry similar to that of the LMG (Corrigan and van Breeman, 1997). The LMG must have undergone slow cooling for the development of coarse lamellae and triclinic microcline to develop in perthite grains and the formation of hematite-ilmenite intergrowths (e.g., Haggerty, 1991; Brownlee et al., 2010). By ca. 1047 Ma the orogen had cooled enough to allow for the preservation of feldspars and clinopyroxene (with Fe-rich exsolution lamellae) in the fayalite granite that are more typical of intrusive felsic igneous rocks that have not undergone long periods at elevated temperatures or extensive deformation.
We suggest that the LMG intruded along extensional shear zones that developed near or along the anorthosite massif that acted as a ridged impermeable barrier (Figs. 1 and 16) prior to hydrothermal alteration and Fe mineralization. Here initial clinopyroxene-magnetite ore deposits formed as a result of the late stages of LMG magmatism from immiscible Fe-rich liquids. From 1040 Ma to 1000 Ma there was a period of orogenic extension, as evidenced by the emplacement of dikes that crosscut the regional fabric of the LMG, and massive remobilization and modification of preexisting orebodies that took place by K-, Na-, Cl-, Ca-, and F-rich fluids. In the northeastern Adirondacks, extension was dominantly to the northeast, as evidenced by northeast-plunging mineral lineations. Orebodies are often coincident with these lineations. Northeast-directed extension is supported by the development of the Tawachiche shear zone, which records oblique northeast extension in the Maurice region of Quebec and occurs along strike to exposures of the LMG in the northeastern Adirondack Highlands (Corrigan and van Breeman, 1997).
Postmagmatic hydrothermal fluid circulation and mineralization were driven by rapid uplift of the hot interior of the orogen and juxtaposition against cooler upper level rocks. High heat flow is suggested by the presence of the dikes. Given that the ages of hydrothermal zircon from the Lyon Mountain ores are 20–60 m.y. younger than zircon ages in the host granite, fluids responsible for hydrothermal zircon crystallization and secondary ore formation cannot be directly related to the LMG emplacement. Currently there are no major intrusive events younger than ca. 1040 Ma identified in the Adirondack Highlands. This suggests that the fluids responsible for ore remobilization and extensive Na alteration were externally derived. Halogen-bearing fluids or brines could have penetrated the LMG along extensional shear zones and interacted with the clinopyroxene-magnetite ores and scavenged disseminated magnetite from the LMG to form new orebodies associated with albitization (McLelland et al., 2002; Barton and Johnson, 2004; Valley et al., 2010). Many of the LMG orebodies are located immediately above the contact of the LMG with other units (e.g., Hawkeye granite, calc-silicate marbles, gabbros); this suggests that these contacts were fluid pathways. These contacts are typically covered with vegetation and direct observation of alteration in the adjacent rock types is not easily observed, but in many localities, ore deposits occur only tens of meters away. Near Skiff Mountain the LMG is in contact with a mixed calc-silicate marble and gabbro unit. As the contact with the LMG is approached, clinopyroxene in the marble is converted to biotite and amphibole, calcite grain size is reduced, and quartz and albite appear in the marble. Near Mineville, rare quartz-sillimanite veins and nodules occur with magnetite at the contact of the LMG and a metasedimentary unit, suggesting chemical exchange between the metasedimentary rocks and the LMG. Field relations suggest that these veins and nodules are contemporaneous with Na alteration.
Prograde metamorphism and penetrative deformation in the northeastern Adirondack Highlands must have ended by ca. 1045 Ma, given the presence of dikes that crosscut the fabric in the LMG (e.g., Lyon Mountain dike, 1040.9 ± 6.8 Ma), the preservation of typical igneous minerals in the fayalite granite (dated as 1047.5 ± 2.2 Ma), and the lack of a penetrative fabric in much of the LMG. In addition, titanite from the eastern and central Adirondacks was dated as between 1033 and 1021 Ma (Mezger et al., 1991). These ages are interpreted as cooling ages (∼500 °C), which further suggests that prograde metamorphism had ended by that time.
Implications for Magnetite-Apatite Deposits and Related Alteration
The formation of clinopyroxene-magnetite orebodies occurred during the latest stages of pluton emplacement (see ore deposit descriptions in the “Rock Descriptions and Petrography” section; Fig. 5). The formation of clinopyroxene-magnetite ore deposits during dry granulite facies metamorphism suggests that these initial deposits could be the result of Fe-rich immiscible liquids or the result of incorporation of mafic material from the AMCG suite. Alternatively these ores could be hydrothermal in origin if the LMG magmas contained fluids derived from the dehydration melting of mica-rich metasedimentary rocks (e.g., Thompson, 1982; Patiño-Douce and Johnston, 1991) or produced residual magmatic fluids (McLelland et al., 2002; Selleck et al., 2004). The U-Th-Pb age data presented here and in Valley et al. (2009) show that hydrothermal systems related to Kiruna-type magnetite-apatite deposits are episodic and may reuse the same plumbing system repeatedly over tens of millions of years. The Palmer Hill and Arnold Hill orebodies are only 5 km apart, but their hydrothermal zircon ages are ∼25 m.y. apart (ca. 1040 Ma and ca. 1016 Ma, respectively; Valley et al., 2009). Despite the age difference between the ore deposits at Palmer Hill and Arnold Hill, the host granite at both localities is nearly identical in age (ca. 1060 Ma). Secondary Fe mineralization and remobilization of preexisting clinopyroxene-magnetite orebodies is associated with Na alteration and hydrothermal zircon growth. The similarities in ages between crosscutting dikes, pegmatites, and Na alteration suggest that Na alteration and secondary Fe mineralization was the result of fluid circulation during extension of the LMG and the Adirondack Highlands.
The source of Fe in the ore deposits is most likely the result of sequestration and concentration of disseminated magnetite from the adjacent granite. The previously mentioned depleted zones in the granite (see discussion of field relations in “Rock Descriptions and Petrography” section) can account for the Fe needed to form the some of the ore deposits. Mass-balance calculations by Hagner and Collins (1967) indicate that the 100-m-wide magnetite-depleted zones on either side of the Palmer Hill ore could produce more than enough Fe to form the ore deposit. However, using these same calculations for the upper ore seam at Skiff Mountain (3-m-wide depleted zone, and 0.6-m-wide ore seam) we conclude that a maximum of two-thirds of the magnetite in the orebody can be accounted for by removal of disseminated magnetite from the host granite (see Appendix Table A1 for these calculations). We suggest that the rest of the magnetite came from remobilization of the early-formed clinopyroxene-magnetite ores.
The various types and styles of Na- and K-rich fluid alteration in other magnetite-apatite Kiruna-type deposits are typically associated with depth (i.e., changes in temperature and pressure) (Hitzman et al., 1992). The sequential nature of fluid alteration (Na overprints K) observed in the LMG further suggests that the crustal level of the LMG changed between 1050 and 1000 Ma. Fluid alteration in the LMG was episodic and lasted for at 40 m.y., as revealed by the ages of hydrothermal zircon ages in the orebodies. The data presented here suggest that magnetite-apatite mineralization is both directly related (e.g., primary clinopyroxene-magnetite deposits) and indirectly related (e.g., secondary magnetite-apatite-quartz deposits) to magmatism and that the mineralization can continue long after magmatic crystallization.
A multidisciplinary approach using field relations, whole-rock geochemistry, microscopy, mineral chemistry, and U-Th-Pb zircon geochronology has proven useful in unraveling the origins of the LMG, its tectonic history, and the sequence of fluid alteration and subsequent Fe mineralization. The data presented here show that the LMG most likely intruded the Adirondack Highlands between 1060 and 1050 Ma and was hydrothermally altered tens of millions of years later. The data presented here suggests that these deposits in the LMG are related to both magmatism and to externally derived fluids (i.e., brines). Early clinopyroxene-magnetite mineralization is presumably related to granite emplacement and was subsequently altered and remobilized, producing new deposits by Na-rich (and rarely K rich) fluids during continued extension of the Adirondack Highlands. The presence of U-Th-Pb zircon ages 20–60 m.y. younger than the host granites of the ores and the highly variable mineralogy of the ore deposits (e.g., clinopyroxene-magnetite and quartz-magnetite-apatite) suggest that multiple processes are responsible for ore formation and modification of preexisting ores. However, the presence of ore deposits almost exclusively within the LMG implies a fertility requirement of the host granite (or the presence of Fe-rich liquids or incorporation of mafic material by LMG magmas to form clinopyroxene-magnetite ores) and that magnetite-apatite mineralization may be directly or indirectly related to the magmatism that produced the LMG.
APPENDIX. ROCK DESCRIPTIONS AND PETROGRAPHY
Lyon Mountain Granite
The Lyon Mountain Granite (LMG) consists of three primary end-member rock types based on the dominant feldspar present, i.e., perthite granite, microcline granite, and albite granite. The three rock types are fine to medium grained, in some cases with a sugary granoblastic texture, and are often difficult to differentiate in the field. The LMG has a foliation or layering that can be locally pronounced, but lacks continuity especially along strike (Whitney and Olmsted, 1988).
The perthite granite is the most prevalent rock type of the LMG. In outcrop, the unit appears pinkish-white to salmon colored and contains thin coarse-grained quartz-potassium feldspar-rich pegmatite layers. The unit ranges from gneissic to unfoliated or massive with blocky layering.
The primary mineral assemblage in the perthite granite is perthitic feldspar, quartz, euhedral magnetite, hematite-ilmenite intergrowths, and minor zircon and apatite. Perthitic feldspar grains in this unit contain coarse, sinuous, anastomosing, or discontinuous lozenge-shaped albite lamellae in microcline with well-developed twinning (Figs. A1A, A1B). Quartz grains are equant and in some cases contain zircon. Zircon also occurs along feldspar grain boundaries.
Clinopyroxene is present in most samples, is intimately associated with coarse-grained magnetite, and contains inclusions of perthite or partially encloses perthite grains (Figs. A1C, A1D). It is unclear if this is the result of recrystallization during deformation, hydrothermal alteration, or the growth of clinopyroxene during skarn development in the latest stages of crystallization of the LMG (Valley et al., 2010).
In some samples the perthite granite has undergone varying amounts of alteration by sodic fluids. Sodic alteration is associated with the crystallization of albite, quartz, magnetite, zircon, and apatite along grain boundaries, and replacement of perthitic feldspars by albite. In addition, sodic alteration is accompanied by the growth of titanite, amphibole (optically determined as arfvedsonite and hornblende), garnet, and biotite. Titanite rims or encloses grains of coarse magnetite (Fig. A1E) and amphibole and biotite replace pyroxene, but are later than titanite. Rarely ilmenite or rutile appears as rims on magnetite in the absence of titanite.
In cathodoluminescence (CL), microcline regions in perthite grains typically luminesce blue (typical of microcline; e.g., Finch and Klein, 1999) and albite lamellae are brownish-gray-green to red. Red CL in feldspars is generally attributed to the presence of Fe3+ or trivalent rare earth elements (REE; e.g., Sm or Eu) (Finch and Klein, 1999). Our preliminary CL spectroscopic measurements (J. Götze, J.M. Hanchar, and P. Valley, 2009, personal observ.) of the CL suggests that the red CL emission is due to Fe3+ and not trivalent REE, based on the relatively broad full width at half maximum CL emission peak typical of d-electron elements and the position of the red CL peak centered at ∼755 nm (Wenzel and Ramseyer, 1992). The red CL tends to be patchy and discontinuous. Apatite in the perthite granite typically occurs along grain boundaries in the least altered samples and in some cases as inclusions in magnetite (Fig. A1C). Both occurrences luminesce purple-orange, which in apatite is indicative of trivalent REE and not Mn2+ CL activation, and is the more common color of the CL emission in plutonic rocks (Roeder et al., 1987).
Reflected light microscopy reveals that some oxides are a mixture of magnetite grains and minor hematite-ilmenite intergrowths (Fig. A1F) or magnetite rimmed by ilmenite. The visible characteristics of these oxide intergrowths are nearly identical to the perthitic feldspar.
Microcline Granite—Potassic Alteration
The microcline granite is most likely the result of hydrothermal Na- and K-rich fluid alteration of preexisting granitoid rocks, and in many cases there are no remnants of the perthite granite. The unit appears pink to orange and rarely reddish-orange in outcrop. Foliations in the microcline granite are much less distinct and difficult to observe than in the perthite granite owing to the lack of mafic and planar minerals or subsequent K alteration and recrystallization.
The microcline granite is the simplest mineralogically and typically more homogeneous than the perthite or albite granite. The mineral assemblage consists predominately of microcline, quartz, magnetite, and minor titanite, apatite, and zircon. The microcline texture is granoblastic and mafic silicates are rare. Quartz occurs interstitially or as rounded grains within microcline grains and locally shows evidence of recrystallization and shape-preferred orientation. Euhedral magnetite grains, quartz, zircon, and trace perthitic feldspar are found as inclusions in microcline or secondary quartz. Apatite typically occurs at grain boundaries in contact with quartz. Rare pyroxene is altered to biotite or chlorite. Coarse euhedral magnetite is present without pyroxene, and titanite rims are ubiquitous on both magnetite and pyroxene grains.
The microcline granite can appear pristine (i.e., unaltered) when distal to the ore deposits (Figs. A2A, A2B), but is typically overprinted near the ore zones by minerals related to Na alteration (Figs. A2C, A2D) and Fe mineralization. Minerals associated with Na alteration observed in the microcline granite include albite, quartz, apatite, zircon, and magnetite. In addition, some samples show multiple generations of potassium feldspar growth (Figs. A2A, A2B). A second period of K alteration overprints all earlier assemblages, as revealed by thin rims or veins of potassium feldspar crosscutting the earlier formed microcline grains and, in samples where Na alteration has occurred, on albite grains.
In CL microscopy, the microcline luminesces blue, a characteristic CL emission color of potassium feldspar (e.g., Finch and Klein, 1999.). In some samples, three distinct shades of blues are visible (Fig. A2B). These differences in luminescence could represent different generations of potassium feldspar growth. Blue-gray luminescent grains can form larger grains than those that are darker blue, have less developed twinning, and crosscut darker blue grains (Figs. A2A, A2B). A third, brighter blue, potassium feldspar appears along grains boundaries of both dark blue and blue-gray grains. In the microcline granite, apatite luminesces orange-purple.
Albite Granite—Sodic Alteration
The albite granite, like its potassium counterpart the microcline granite, is the product of pervasive fluid alteration. The albite granite is typically white, gray, or pink, but can appear red due to hematite staining.
The mineral assemblage of the albite granite is the most variable of LMG lithologies studied. The dominant mineral assemblage in the albite granite consists of granoblastic albite, quartz, zircon, apatite, magnetite ± clinopyroxene ± amphibole ± titanite ± garnet ± fluorite (Fig. A3). Grains of rounded quartz, euhedral magnetite, titanite, zircon, apatite, and remnant perthite and microcline grains are found as inclusion within albite. Clinopyroxene and magnetite are intimately associated. Rims or grains of titanite typically form around or in close proximity to pyroxene and magnetite. In some samples garnet is associated with pyroxene instead of magnetite. Pleochroic blue-green amphibole (i.e., arfvedsonite) and hornblende grew at the expense of preexisting clinopyroxene, and both amphibole and pyroxene were subsequently altered to biotite or biotite-quartz intergrowths. Apatite is typically associated with albite, quartz, and coarse magnetite. Zircon occurs predominately within grains of quartz and albite and along grain boundaries.
The albite granite in some localities has recorded subsequent alteration events by K-, Ca-, or Si-rich fluids. Thin rims of potassium feldspar on albite were observed in some samples. Calcium alteration is recorded typically by the growth of calcite and is associated with brittle deformation and a lower temperature mineral assemblage (e.g., chlorite, hydrothermal quartz) and hematization.
Replacement of preexisting feldspars by albite ranges from near completion in some samples to only grain margins in other. In samples where Na replacement is complete, well-twinned granoblastic albite is predominant (Figs. A3A, A3B) and only small remnants of earlier formed feldspars are present (Figs. A3C, A3D). In samples that have undergone partial or limited Na alteration, flame or patch perthite is often present (Fig. A3E, A3F). Alteration of feldspars in the perthite granite shows replacement of the microcline portions of grains, leaving earlier exsolution-related albite lamellae protruding into the replacement albite (Fig. A1A, A1B) and as rims of albite around the perthite.
Cathodoluminescence imaging of the albite granite samples shows that alteration-related albite is most often associated with red luminescence (Figs. A1–A4), although dark gray, gray-green, and purple-gray luminescences are also present. Red areas can grade into gray within the same grain or area of alteration. In samples with nearly complete albitization, red luminescence tends to be more subdued. Our preliminary CL spectroscopy measurements (J. Götze, J.M. Hanchar, and P. Valley, 2009, personal observ.) of the red CL suggest that it is due to Fe3+ and not trivalent REE, as noted herein for the microcline. However, electron probe microanalysis (EPMA) data show there is not a significant difference in the iron content between any of the feldspars analyzed (see following EPMA discussion). Apatite grains luminesce purple to orange, indicative of high REE content and trivalent REE CL activation (Roeder et al., 1987).
Related Rock Types
A body of fayalite granite in Ausable Forks, New York, is separated from outcrops of albite granite on its western edge by a layer of coarse-grained amphibolite, and its eastern, northern, and southern, margins are covered. This unit appears dark green-brown to rust-colored, is massive, and is medium grained.
The fayalite granite is mineralogically distinct from other lithologies in the LMG. The primary igneous assemblage consists of perthitic feldspar, quartz, clinopyroxene, and zircon (Fig. A4). It is not clear if the fayalite is primary or the result of hydrothermal alteration. Secondary mineral growth as a result of Na-rich fluid alteration consists of albite, quartz, fluorite, apatite, and magnetite. Perthitic feldspar and clinopyroxene grains in the fayalite granite are clearly distinct from those of the perthite granite (Fig. A4). In the feldspar grains, thin, straight, uniform lamellae are crosscut by hydrothermal alteration–related flame lamellae containing minute inclusions of Fe oxides and other unidentified phases (Figs. A4C, A4D). Twinning in the potassium regions of perthitic feldspar is not well developed. Clinopyroxene grains contain exsolution lamellae of Fe-rich pyroxene. Fayalite grains are often altered extensively to iddingsite and are associated with clinopyroxene. The clinopyroxene is not altered to amphibole or biotite in this sample. Zircon is present along grain boundaries and inclusions in quartz and feldspar. Apatite, fluorite, and quartz typically occur together.
In CL microscopy, Na alteration–related flame lamellae are bright red and potassium feldspar grains luminesce gray-green. Apatite grains luminesce purple-orange, similar to the other samples investigated, indicative of high REE content and trivalent REE CL activation (Roeder et al., 1987).
Ore Deposits, Iron Mineralization
Iron mineralization in the LMG is characterized by magnetite as the main Fe-bearing oxide phase; however, a few deposits contain appreciable quantities of martite (incomplete pseudomorphic replacement of magnetite by hematite) and hematite. The ore deposits can be divided into two main types: clinopyroxene-magnetite and those associated with Na-rich fluid alteration and the growth of albite and quartz. A smaller subset of magnetite deposits consists of either quartz and/or potassium feldspar pegmatites and magnetite with minor apatite and zircon, or more rarely some magnetite deposits are associated with K alteration and the microcline granite (Postel, 1952). Alteration and mineralization can be highly variable within a single deposit and multiple overprinting fluid events recorded in these rocks are common.
Clinopyroxene-magnetite deposits typically occur in layers ranging from <1 m to 4 m wide. Clinopyroxene-magnetite migmatite or granite contaminated by the clinopyroxene-magnetite assemblage is also present. Enclaves of clinopyroxene-magnetite ore observed within the perthitic granite suggest that the clinopyroxene-magnetite ore was contemporaneous with the emplacement of the LMG. Clinopyroxene-magnetite deposits are typified by the mineral assemblage clinopyroxene, magnetite ± apatite ± amphibole. Coarse magnetite is typically secondary to pyroxene, but small euhedral crystals of magnetite appear as inclusions within clinopyroxene. Secondary amphibole commonly rims clinopyroxene and encloses magnetite. Apatite luminesces intense orange-purple, which is indicative of the high REE content (Lindberg and Ingram, 1964; Roeder et al., 1987). In clinopyroxene-magnetite migmatites, coarse microcline or orthoclase rims pods of clinopyroxene-magnetite or forms alternating layers with clinopyroxene and magnetite (Fig. 5B). Inclusions of perthite or other early feldspars are present within magnetite or pyroxene grains in clinopyroxene-magnetite contaminated granites. Apatite occurs interstitially along grain boundaries of magnetite and pyroxene and often makes up a significant portion of the rock. Apatite luminesces purple-orange in CL. Zircon was not observed in thin sections from clinopyroxene-magnetite ore samples or in heavy mineral separates (e.g., Rutgers Mine).
The most prevalent type of ore deposit is associated with Na alteration and albitization. Field and petrographic observations clearly show that Na alteration overprinted and was subsequent to clinopyroxene-magnetite ore formation and that Na alteration can occur at the same locality as clinopyroxene-magnetite deposits (e.g., Lyon Mountain, Palmer Hill, Arnold Hill). Deposits associated with Na alteration are typified by coarse or massive magnetite, apatite, quartz, and zircon, but also contain albite ± fluorite ± clinopyroxene ± amphibole ± titanite ± garnet ± chlorite ± calcite. Apatite occurs interstitially between magnetite grains and exhibits intense orange-purple luminescence. In most samples zircon was not identified in thin section, and was only identified during heavy mineral separation. However, some samples (e.g., Mineville) contain extremely large zircon (and apatite) (e.g., millimeter scale). These large zircons are highly metamict from the high U content and subsequent radiation damage.
The positions of the orebodies are structurally controlled. Orebodies occur in the hinges, or limbs, of folds and immediately above the contact of the LMG with various other units and are tabular, or rod-like (Postel, 1952). One or more of the following characteristics are locally present at a given ore deposit: migmatization, mylonitization, boudinage of the ore and host rocks, breccias, crosscutting dikes and pegmatites, and miarolitic cavities.
Amphibolite layers of uncertain origin are common within the LMG, but constitute little of the total volume. Although they are not the focus of this study, they are considered here to better characterize the LMG and its evolution and origin. Layers range from a few centimeters to 3 or 4 m in width and can appear as a single layer or occur in groups spaced a few centimeters to many meters apart (Fig. A5). The layers can be followed the length of the outcrop with little variation. The layers typically have a salt and pepper or banded appearance, and are conformable with the local granitic fabric when present. Early workers described the amphibolite layers as anastomosing and pinching out and that some samples contain pyroxene or relict pyroxene (though none was observed in this study; Gallagher, 1937; Postel, 1952). The samples collected in this study comprise varying amounts of biotite, amphibole, plagioclase apatite, minor orthoclase, magnetite or hematite-ilmenite intergrowths, titanite, and zircon. Plagioclase is generally granoblastic. Biotite in thin section is fresh in appearance and ranges from brown to green pleochroic in plane light microscopy. Plagioclase luminescence ranges from blue-gray to red. Apatite typically luminesces purple-orange.
Two amphibolite layers (∼10 and 20 cm thick) just below the ore horizon at Skiff Mountain have relatively sharp contacts with thin layers (1–2 cm) of albite separating the amphibolite from the albite granite (Fig. A5A). An amphibolite layer at the top of Skiff Mountain has a lower contact with the LMG that appears to be gradational, with thin layers of amphibolite gradually increasing in thickness and abundance until becoming a massive layer 3 or 4 m across (Fig. A5B). The upper contact with the LMG is sharp but wavy, with evidence for partial melting and shearing at the interface of the granite and the amphibolite and partial melting within the amphibolite as a result of the layer being pulled apart.
In addition to the amphibolite layers within the LMG, an amphibolite layer ∼10 m across separates the LMG from the fayalite granite at Ausbale Forks. The relationship between this sample and the amphibolite layers that are clearly internal to the LMG is uncertain, but they are considered here together based on the similarity of mineralogy and texture. The amount of amphibole and biotite varies between 20% to ∼100% across the outcrop with some areas of the outcrop consisting entirely of biotite or amphibole. Apatite is ubiquitous, typically occurs along grain boundaries, and contains inclusions of plagioclase (e.g., Ausable Forks). The oxide and Ti-bearing phases are different in the amphibolite layer at Ausable Forks and the one collected at Skiff Mountain. The amphibolite body at Ausable Forks contains magnetite with rims of ilmenite, whereas the amphibolite layer at Skiff Mountain contains hematite-ilmenite intergrowths and minor magnetite with ubiquitous titanite throughout the thin section.
Granitic dikes are common throughout the LMG and the Adirondack Highlands. These dikes crosscut layering in the LMG. Two main types of dikes are present in the LMG: potassium feldspar–rich dikes that appear orange to pink in the field, and plagioclase-rich dikes that appear white or light gray. Some dikes contain coarse masses of magnetite and minor pyrite.
The potassium feldspar–rich dikes typically contain coarse microcline, quartz, and minor magnetite, zircon, and apatite ± muscovite ± amphibole ± biotite ± calcite. The dike samples collected at Dannemora and Skiff Mountain are pegmatitic. Limited fluid-enhanced Na alteration has added albite and calcite to the mineral assemblage of these samples. Blue or blue-gray luminescent microcline is typical. Two populations of apatite are present; yellow luminescent grains (typical of igneous apatite; Roeder et al., 1987) that occur with quartz and magnetite, and those with purple-orange luminescence typical of REE-enriched apatite (Roeder et al., 1987) that occur with quartz and microcline. Zircon grains are present at grain boundaries and rarely along fractures in magnetite.
The plagioclase-rich dike collected at Lyon Mountain consists of coarse quartz, oligoclase (McLelland et al., 2001b), magnetite, microcline, apatite, titanite, calcite, zircon, and minor sulfides. Secondary Na and K alteration has masked the original igneous composition and mineral assemblage (Table 1). Primary oligoclase (blue-gray CL) is replaced first by albite (with red CL) and then by potassium feldspar (blue CL) along grain boundaries. Apatite occurs near quartz, in areas of fine-grained alteration, and is associated with clusters of titanite grains, which grew later than apatite. Apatite typically luminesces yellow with a subset of purple-orange luminescent grains. Zircon grains are present along grain boundaries and in association with quartz.
The Hawkeye granite is dated as 1100 Ma, is highly deformed and metamorphosed as a result of the Ottawan orogeny, and intrudes rocks of the 1150 Ma AMCG (anorthosite, mangerite, charnockite, and granite) suite (e.g., Buddington, 1939; McLelland et al., 2001a). The Hawkeye granite is white to light gray or pink, and contains a distinctive quartz ribbon lineation; the lineations are as long as 10 cm.
The Hawkeye granite mineral assemblage contains highly recrystallized quartz, perthitic feldspar, myrmekite plagioclase, and zircon ± amphibole ± orthopyroxene ± garnet ± biotite ± ilmenite ± apatite. Perthite grains in the Hawkeye granite typically contain short, fat lamellae that are often absent in the rims of the grains (Fig. A6). Zircon occurs at grain boundaries and as inclusion in perthite grains. The mafic mineral content is variable in the Hawkeye granite; some samples contain only minor magnetite while others would be better classified as charnockite. Potassium feldspar luminesces blue with brown to gray-brown plagioclase lamellae, and apatite, when present, luminesces orange-purple.
AMCG Suite Charnockite
Charnockite is common throughout the Adirondack Highlands as part of the ca. 1150 Ma AMCG suite. A charnockite sample near Skiff Mountain was collected for this study as a comparison between the LMG and granitoid rocks of the AMCG suite that did not undergo alteration and mineralization. The unit is typically orange to orange-gray and has a tiger-striped appearance as a result of the presence of elongated mafic minerals and gneissic layering.
The mineral assemblage consists of plagioclase, perthitic feldspar, clinopyroxene (with exsolution lamellae), orthopyroxene, garnet, apatite, zircon, and minor quartz. Perthite grains contain short, fat, or rounded lamellae. Feldspar grains and quartz are typically granoblastic and can form composite strings between clinopyroxene and garnet-rich layers. Clinopyroxene and garnet are rimmed by amphibole and magnetite, and all exhibit a shape-preferred orientation. In addition, magnetite occurs as small euhedral grains within orthopyroxene and garnet. Elongated apatite grains are present throughout the thin section. Plagioclase luminesces brown-green and perthitic feldspar luminesces blue with brown-green lamellae. Apatite exhibits a purple-orange luminescence.
This research was supported by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant and startup funds to Hanchar provided by the Memorial University of Newfoundland. The Nordsim ion microprobe facility is financed and operated under an agreement between the research councils of Denmark, Norway, Sweden, the Geological Survey of Finland, and the Swedish Museum of Natural History; this is Nordsim publication 271. We thank Marian Lupulescu, Phil Whitney, Pat Bickford, Bruce Selleck, William Peck, and James McLelland for helpful discussions.