Geology of the Little Whiteman Carbonate-Hosted Replacement Zn-Pb-Ag-(Cu) Prospect, Western Fortymile District, Alaska
Published:January 01, 2010
- PDF LinkChapter PDF
Chris R. Siron, Murray W. Hitzman, Robert Mcleod, 2010. "Geology of the Little Whiteman Carbonate-Hosted Replacement Zn-Pb-Ag-(Cu) Prospect, Western Fortymile District, Alaska", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
Download citation file:
The Little Whiteman prospect is located in the western part of the historic Fortymile mining district of the Yukon-Tanana Uplands of east-central, Alaska. The prospect consists of steeply dipping Zn-Pb-Ag-(Cu) massive and semimassive sulfide chimneys and mantos that replace marbles of the greenschist-grade Nasina assemblage of the Yukon-Tanana terrane. A prominent northeast-trending, sinistral strike-slip fault and accessory structures occur within a complex structural zone referred to as the Kechumstuk fault. Normal dip-slip displacement on the southeast-dipping Kechumstuk fault juxtaposed unreactive metavolcaniclastic footwall rocks adjacent to reactive hanging-wall carbonate rocks. Transtension along the Kechumstuk fault has resulted in left-lateral dilation at the northern part of the Little Whiteman prospect. Hydrothermal fluids were channelized along the Kechumstuk fault and vertically restricted by an overlying quartz diorite sill. Hydrothermal alteration ranges from dolomitization of the marble near fault contacts to a distal siliceous zone often containing abundant manganese-oxide stockwork veinlets. Acidic and partly oxidized hydrothermal fluids caused strong local alteration of porphyry dikes, resulting in a muscovite, quartz, pyrite, and kaolinite mineral assemblage. Sulfide bodies extend for >700 m along strike and to depths >300 m. Replacement-style sulfide deposition is localized near and along contacts of steeply dipping structures and felsic porphyry dikes. The sulfide-rich replacement bodies display a sulfide mineral paragenesis of early iron- and subordinate arsenic-bearing sulfide minerals, followed by zoned sphalerite with iron-rich margins containing abundant chalcopyrite inclusions. Continued sulfide mineral precipitation formed a galena and sulfosalt mineral assemblage that became increasingly silver rich through time. Most silver resides in tetrahedrite, which forms inclusions in galena or in late-stage carbonate-sulfide veinlets. Mineralization at the Little Whiteman prospect is interpreted to be the result of hydrothermal fluids driven by Late Cretaceous volcanism. The spatial relationship between the sulfide bodies and felsic porphyry dikes suggest they are related and, perhaps time equivalent to the adjacent Middle Fork caldera that has an age of 69 Ma.
The little Whiteman Zn-Pb-Ag-(Cu) prospect is located in the Eagle A-4 1:63,360-scale quadrangle, approximately 450 km east of Fairbanks, Alaska (Fig. 1). The prospect lies along a 90-km northeast-trending base metal belt in the western part of the historic Fortymile mining district, the oldest placer gold camp in Alaska. Since 1886, more than 15.2 metric tons (t; 535,000 oz) of placer gold has been produced (Szumigala, 2000). However, no large-scale source for the Fortymile district’s placer gold has been identified. Other prospective mineral occurrences in the Fortymile district include intrusion-hosted gold deposits, porphyry copper-molybdenum deposits, ultramafic-hosted platinum deposits, volcanogenic massive sulfide deposits, and Zn-rich skarn and replacement deposits (Burleigh et al., 1994; Szumigala, 2000).
During the 1970s, large-scale reconnaissance mineral exploration programs were conducted in the Fortymile region by Apex Exploration, Arctic Resources Inc., ASARCO, Central Alaska Gold Co., Cities Services, Duval, Exxon, IMC, Kennecott, Phelps Dodge, Placid Oil, RAA, Strauss Resources, U.S. Steel, and Watts, Griffis, and McOuat (WGM) Inc. (WGM, 1977). Many companies tailored their exploration efforts to porphyry copper-gold and molybdenum deposits, whereas others explored the asbestos potential in the northern and eastern parts of the district (WGM, 1977). Base metal occurrences were discovered throughout the western part of the Fortymile district (Fig. 2), including the Little Whiteman prospect, but most received little to no attention.
The Little Whiteman prospect was originally identified in 1975 by a vegetation anomaly, or “kill zone,” caused by the development of acidic soils from supergene sulfide weathering. Watts, Griffis, and McOuat (WGM) Inc. conducted reconnaissance mapping and detailed soil, rock, heavy-mineral concentrate, and stream-sediment geochemical surveys at the Little Whiteman Creek and identified an area of Zn-Pb-Ag-(Cu) anomalies, including one anomaly that was associated with a mineralized gossan. Anglesite and malachite were identified within a mineralized breccia in a small marble outcrop within the kill zone. The Little Whiteman and neighboring Fish prospect were “rediscovered” in the mid-1990s and were believed to have potential for sedimentary exhalative-style, shale-hosted Zn-Pb mineralization, typical in the Selwyn basin of central Yukon Territory.
Massive and semimassive sulfide bodies were discovered at Little Whiteman in 2006 by Full Metal Minerals through diamond core drilling targeting a soil geochemistry and gravity anomaly within the area of the kill zone. Follow-up drilling resumed in 2007, encountering both sulfide and oxide zones. The 2008 drilling program at Little Whiteman focused on extending the strike length of the mineralized zone. At the end of 2008, Full Metal Minerals had completed 60 diamond core drill holes on the property for a total of 13,343 m. This exploration identified two subparallel sulfide bodies. The largest and deeper of the two zones extends for more than 700 m in strike length and to depths >300 m (Siron, 2008). The Little Whiteman prospect is currently in an advanced exploration stage.
Recent work by Full Metal Minerals has discounted the SEDEX model and has classified the Little Whiteman prospect as an intrusion-related, high-temperature, Zn-Pb-Ag-(Cu) carbonate-hosted replacement deposit (Megaw, 1998). This paper focuses on the geology of the Little Whiteman prospect. The development of a geologic foundation to constrain the formation of mineral deposition at the Little Whiteman prospect was conducted through structural analysis, and petrographic and scanning electron microscopy (SEM) studies of alteration and sulfide mineral assemblages, as well as geophysical and geochemical studies. The objective of the study is to provide a robust geologic framework for continued exploration for other epigenetic, high-temperature, intrusion-related, carbonate-hosted, base metal deposits in the Fortymile region of Alaska.
Regional Geologic Setting
The western part of the Fortymile district contains greenschist-to amphibolite-facies Paleozoic metasedimentary, metaplutonic, and metavolcanic rocks that form part of the composite Yukon-Tanana terrane, as originally defined by Coney et al. (1980). The Yukon-Tanana terrane is bounded to the north by the right-lateral Tintina fault and to the south by the right-lateral Denali fault. The composite Yukon-Tanana terrane extends from the Yukon-Tanana Upland of interior Alaska, through the central Yukon Territory and southeastern Alaska, and into northern British Columbia (Fig. 1).
The composite Yukon-Tanana terrane contains a series of fault-bounded assemblages of metamorphosed sedimentary, volcanic, and plutonic rocks that formed along a tectonically disrupted continental margin (Dusel-Bacon et al., 2004). The rocks of the composite Yukon-Tanana terrane have been subdivided into assemblages on the basis of composition, protolith origin, and the structural and metamorphic histories of their components (Foster et al., 1985; Hansen et al., 1991; Mortensen, 1992; Dusel-Bacon and Cooper, 1999; Dusel-Bacon et al., 2002).
Nelson et al. (2006) proposed retaining the term Yukon-Tanana terrane for only the Paleozoic arc and back-arc assemblages that were rifted from the continental margin. These rifted, outboard assemblages (e.g., the Fortymile and Nasina assemblages and Chicken Metamorphic Complex) make up the majority of the Fortymile district. Nelson et al. (2006) referred to the rest of Coney et al.’s (1980) composite Yukon-Tanana terrane as parautochthonous North America. Because of the long-standing usage of the Yukon-Tanana terrane term for both components in Alaska, Dusel-Bacon and Williams (2009) proposed the unambiguous terms “allochthonous Yukon-Tanana terrane” for the rifted component and “parautochthonous Yukon-Tanana assemblage” for the component that remained landward.
The Fortymile River assemblage
The Fortymile River assemblage, which is synonymous with the Taylor Mountain terrane and Taylor Mountain assemblage of Dusel-Bacon et al. (1995) and Hansen and Dusel-Bacon (1998), respectively, is composed of amphibolite-facies metasedimentary, metavolcanic, and meta-igneous rocks containing garnet amphibolite, biotite-hornblende gneiss, marble, quartzite, and pelitic schist. Most amphibolites have volcanic arc trace element geochemical signatures, with a minor population having normal or enriched midocean ridge basalt (MORB) or ocean-island basalt (OIB) trace element compositions (Dusel-Bacon and Cooper, 1999; Day et al., 2002; Dusel-Bacon et al., 2006).
Zircons from metaplutonic intermediate-composition gneisses, augen gneiss, and meta-rhyolite tuff within the Fortymile River assemblage yield early Mississippian (Tournasian) U-Pb crystallization ages of 360 to 343 Ma (Day et al., 2002; Dusel-Bacon et al., 2006). A marble in the assemblage contains Meramecian to Sakmarian (mid-Mississippian to Early Permian) age conodonts (Dusel-Bacon and Harris, 2003). A minimum protolith age for this assemblage is provided by a zircon U-Pb crystallization age of 263 ± 1.5 Ma from an undeformed granitic dike that intrudes the assemblage along the Fortymile River (Dusel-Bacon et al., 2006).
The Nasina assemblage
The Little Whiteman prospect is hosted in the Nasina assemblage, which is composed of greenschist-facies metamorphic rocks including variably carbonaceous quartzite, phyllites, schists, marble, greenstones, and meta-tuffaceous rocks (Dusel-Bacon et al., 2006, 2009b). A possible Mississippian depositional age for at least part of the assemblage is suggested by poorly preserved conodonts in marble (Dusel-Bacon and Harris, 2003).
The Nasina assemblage rocks are described as being depositionally interlayered with rocks of the Fortymile River assemblage east of the Alaska-Yukon Territory border (Dusel-Bacon et al., 2006). The interlayered nature of the Nasina and Fortymile River assemblage rocks suggests a Mississippian depositional age and shared depositional setting for both assemblages. Meta-igneous rocks from both assemblages display volcanic arc geochemical signatures. The age and structural relationships suggest that the Fortymile and Nasina assemblages developed as adjacent volcanic-sedimentary packages related to a convergent continental margin ( Dusel-Bacon et al., 2006).
Chicken Metamorphic Complex
The Chicken Metamorphic Complex consists of greenschist-facies volcanic and sedimentary rocks including meta -gabbro, metadiabase, marble, slate, quartz-mica phyllite, and quartzite (Werdon et al., 2001; Dusel-Bacon et al., 2009b). Conodont fossils suggest a late Paleozoic depositional age (Dusel-Bacon and Harris, 2003). This package of rocks was proposed by Werdon et al. (2001) to have originated as a volcanic arc adjacent to a sedimentary basin. Dusel-Bacon et al. (2006) proposed a shared protolith origin for the Chicken Metamorphic Complex and the Fortymile River assemblage based on volcanic arc geochemical signatures of meta-igneous rocks.
The rocks of the allochthonous Yukon-Tanana terrane record a complex igneous and metamorphic history. Creaser et al. (1999) proposed, on the basis of geochemical and Nd data for eclogites associated with allochthonous Yukon-Tanana terrane rocks in the Yukon Territory, a Mississippian high-pressure metamorphic event associated with coeval subduction-zone magmatism, followed by another high-pressure metamorphic event in the Permian, associated with subduction of the allochthonous rifted fragment. It is unclear whether both of these metamorphic episodes affected the rocks of the western part of the Fortymile district; the earliest event of metamorphism and deformation predates the intrusion of 212 Ma plutons (Dusel-Bacon et al., 2002). This event resulted in regional greenschist- to amphibolite-facies metamorphism and folding of the sedimentary rock sequence and appears to have ended in the late Paleozoic with the tectonic emplacement of the amphibolite-facies Fortymile River assemblage (and Chicken Metamorphic Complex?) over the greenschist-facies Nasina assemblage.
An arc-related magmatic event, present only in the allochthonous Yukon-Tanana terrane, began during the Late Triassic and continued into the Early Jurassic. Plutons from this time are observed in both the Nasina and Fortymile assemblages. Hornblende 40Ar/39Ar cooling ages in these foliated Mesozoic plutons suggest that a regional metamorphic event overlapped with the waning stages of magmatic activity (Dusel-Bacon et al., 2002). Regional wrench tectonism between 135 and 110 Ma was accompanied by widespread, mostly volcanic arc-associated magmatism in both the allochthonous Yukon-Tanana terrane and the parautochthonous Yukon-Tanana assemblage from the Middle to Late Cretaceous (Dusel-Bacon et al., 2002, and references therein). This event resulted in the mid-Cretaceous rhyolitic volcanism in the northern Tanacross and southwestern Eagle quadrangles (Bacon et al., 1990). Bimodal, within-plate magmatism, as a result of extensional tectonism, followed at approximately 60 to 50 Ma (Foster et al., 1994; Newberry et al., 1995).
The current structural framework of the region consists of prominent northeast-trending structural zones together with northwest-trending conjugate faults (Day et al., 2007). These northeast-trending structures exhibit both left-lateral strike-slip and normal displacements. On a regional scale these faults cause clockwise rotation of blocks as the result of right-lateral Tintina and Denali fault movement (Dusel-Bacon et al., 2002).
Geology of the Little Whiteman Prospect
Exposure of the bedrock geology in the Little Whiteman area is limited. The stratigraphy of the area is known primarily through drilling, which reached a maximum depth of 575 m. None of the metasedimentary rocks at the Little Whiteman prospect contain fossils, so the exact age of the sequence remains unknown. The metasedimentary rocks are believed to be part of the Nasina assemblage (Dusel-Bacon et al., 2006, 2009b). Relic bedding textures preserved in the metasedimentary rocks suggest that the section is right-side up. Contacts between different lithologic units are mainly structural in nature (Fig. 3). Foliation in a quartzite subcrop to the south of the prospect strikes 070o and dips to the southeast between 25° and 40° (Fig. 4).
Quartz-rich metavolcaniclastic rocks
Quartz-rich metavolcaniclastic rocks occur west of the Kechumstuk fault zone at Little Whiteman, forming the footwall to the fault zone (Fig. 3). They are the lowermost stratigraphic unit encountered to date in the drilled area and the thickness of this unit is not known. They are tentatively correlated with the Nasina assemblage.
These rocks consist of quartz and plagioclase grains in a chlorite and white mica matrix. Petrographic analysis indicates two populations of quartz: (1) subrounded, medium-grained ribbon quartz; and (2) subrounded, medium-to coarse-grained, polygonal quartz showing undulatory extinction and 120° triple junctions. Coarse-grained, subhedral to euhedral, polysynthetic twinned plagioclase feldspar grains showing very weakly sericitized margins are also prominent. Plagioclase grains typically show feathery embayments of recrystallized quartz and very fine grained plagioclase. Foliated chlorite and subordinate white mica compose the main metamorphic fabric within the rock. Carbonate minerals fill space between quartz and plagioclase grains and form crosscutting veins. Minor disseminated fine-grained euhedral to subhedral pyrite and anhedral chalcopyrite are evident. The presence of subrounded, detrital metamorphic quartz grains and euhedral igneous plagioclase grains suggests derivation from a mixed metamorphic-igneous terrain. The abundant chlorite could have been derived from intermediate-composition volcanic ash.
Quartzite breccia within the Kechumstuk fault zone
A ubiquitously faulted interval composed of quartzite breccia clasts within a matrix of graphitic gouge marks the Kechumstuk fault zone and separates the quartz-rich meta-volcaniclastic footwall rocks from an overlying thick marble unit. The contacts above and below this unit are structural in nature and the quartzite breccia is discontinuous in the drill area. The quartzite breccia is composed predominately of quartz which occurs as: (1) subangular, medium-size detrital fragments of fine-grained ribbon quartz; (2) subangular clasts of medium- to coarse-grained polygonal quartz grains showing undulatory extinction and 120° triple junctions; and (3) coarse-grained subangular to round individual quartz grains that show very subtle undulatory extinction. Muscovite primarily forms a foliation cleavage between polygonal quartz clasts. Carbonate minerals, primarily calcite, occur along grain boundaries. The matrix supporting the quartzite breccia clasts is composed of noncalcareous graphitic fault gouge containing abundant coarse-grained euhedral pyrite. The graphitic gouge material is interpreted to have originally been an organic-rich mudstone that was interbedded with quartz-rich beds.
Marble accounts for approximately 80 m of section at Little Whiteman. Unaltered marble is white to grayish-blue and consists of medium- to coarse-grained calcite with very sparse (less than 1%) detrital quartz grains showing undulatory extinction. Graphitic stylolites are common throughout the marble section. The marble forms the primary host for sulfide mineralization at Little Whiteman (Fig. 3).
Metasedimentary clastic rocks
The structurally highest interval at Little Whiteman consists of well-bedded carbonaceous argillite, graywacke, thinly bedded sandy marble, and quartzite. Most of the metasedimentary rocks contain minor to significant primary calcite. Barren carbonate and carbonate-sulfide veins are also locally prominent. The metasedimentary clastic rocks are a minimum of several hundred meters thick and define the uppermost unit currently recognized at Little Whiteman. The steeply dipping, thin marble beds of this section of rocks often act as host for strata-bound (manto) replacement sulfide mineralization.
Quartz diorite sill
A quartz diorite body lies within the drill area at Little Whiteman. It is generally found between the marble and the metasedimentary clastic rocks. The contact between the quartz diorite body and the surrounding metasedimentary rocks is often conformable, suggesting it formed a sill. The quartz diorite contains medium-grained biotite and hornblende crystals, both of which are commonly altered to chlorite. Zoned potassium feldspar and plagioclase crystals are variably altered to white mica. Calcic-pyroxene accounts for 1 to 2 percent of the rock by volume. Fine- to medium-grained titanite and fine-grained, partially oxidized magnetite (depending on depth and proximity to fractures) occurs sporadically in the groundmass. Secondary epidote is found as selvages along veins and variably occupying the quartz diorite groundmass. A weak alignment of chlorite forms the metamorphic fabric in this rock. A zircon U-Pb crystallization age of 210 ± 3 Ma was determined for this intrusive rock (Dusel-Bacon et al., 2009b).
Dacite and trachy-andesite porphyry dikes
Dacite porphyry dikes (Fig. 5) occur throughout the mineralized interval at Little Whiteman. These dikes postdate the quartz diorite and cut all hanging-wall lithologic units at angles between 60° and 75°. Dacite dikes generally range in thickness between centimeter scale and upward to several meters and are often laterally continuous across the property. Most of the dacite dikes are pervasively sericitically altered. Prior to alteration, these dikes contained coarse-grained potassium feldspar phenocrysts, quartz, and biotite. Alteration resulted in replacement of the coarse-grained potassium feldspar by white mica; X-ray diffraction indicates that the white mica is muscovite. Fine- to coarse-grained euhedral pyrite is commonly intergrown with quartz and muscovite.
Trachy-andesite porphyry dikes (Fig. 5) are less frequently intercepted in drill core. These dikes are commonly observed cutting the metasedimentary clastic rocks and are found laterally for several hundreds of meters from the Kechumstuk fault zone. The trachy-andesite dikes are porphyritic and contain fine- to medium-grained, euhedral to subhedral biotite, clinopyroxene, and hornblende. The groundmass is composed of aphanitic to very fine grained plagioclase that has undergone weak white mica alteration. The margins of biotite and hornblende crystals typically show weak replacement by chlorite. Minor calcite replacement of calcic-pyroxene is common.
Dacite and trachy-andesite porphyry dikes at Little Whiteman contain depleted Nb and Ta values (Fig. 6), which is characteristic of calc-alkalic melts formed along a convergent margin. The distinct concave shape of the normalized rare earth element plots suggests that amphibole fractionation occurred during the magmatic evolution of these rocks. The trace element signatures are similar among all the porphyry dikes sampled, suggesting that both the dacite and trachy-andesite porphyry dikes were derived during a single magmatic event (Fig. 6).
The most prominent structure observed at the Little Whiteman prospect is the northeast-trending Kechumstuk fault zone (Figs. 7, 8), which forms a topographic low through the prospect area. The fault zone has been intersected in most drill holes at Little Whiteman. This structural zone can also be identified from both airborne and ground geophysical surveys (Burns, 2008; Full Metal Minerals, pers. commun., 2008). The Kechumstuk fault zone separates barren metavolcaniclastic footwall rocks to the west from mineralized marble in the hanging wall to the east. Numerous minor structures splay off the main Kechumstuk fault zone.
Drilling indicates that the Kechumstuk fault zone is structurally complex. An airborne resistivity survey suggests that the fault motion is left-lateral strike-slip. The Kechumstuk fault zone strikes approximately 050° and dips steeply to the southeast at 60° to 75°. Drilling indicates that the Kechumstuk fault zone has a steeper dip in the southeastern part of the drill area.
Regional tectonic studies suggest that northeast-trending structures in the Fortymile district, such as the Kechumstuk fault zone, developed during the mid-Cretaceous (Day et al., 2009). Extensional (or oblique-slip) motion may have been initiated during the Late Cretaceous to Early Tertiary. Apatite fission track studies suggest that dip-slip components of the northeast-trending structures in the Fortymile district have a vertical separation of as much as 3 km (Dusel-Bacon and Murphy, 2001). The present-day topographic relief in the Fortymile region is thought to be due to Neogene dip-slip movement on these structures (Newberry and Burns, 1999).
Lithified graphitic fault breccias occur within the Kechumstuk fault zone. These annealed polylithic, matrix-supported breccias contain subrounded to angular fragments of unaltered marble, dolomitized marble, altered and strongly pyritized dacite porphyry, and rarely Zn and Pb sulfide clasts. The matrix is composed of graphite with minor disseminated pyrite. Wall-rock fragments in the fault breccias indicate that the Kechumstuk fault experienced movement after dacite porphyry dike emplacement and sulfide mineralization.
Synthetic strike-slip faults, common in wrench fault systems, exhibit an en echelon array to the Kechumstuk fault zone. In the southwestern part of the drilled area, Riedel shears achieve a smaller angle to the main structure as a result of inflection of the Kechumstuk fault zone. Altered dacite porphyry dikes have near-identical orientation as the synthetic structural trend (Fig. 9) and may have been emplaced along these structures. Northwest-trending antithetic strike-slip faults occur to the south of the Little Whiteman prospect.
Airborne and ground geophysical data (Burns, 2008; Full Metal Minerals, pers. commun., 2008) suggest that the Kechumstuk fault zone forms a left-lateral jog resulting in a rhomboidal pull-apart releasing bend in the northern Little Whiteman area. Drilling indicates that the Kechumstuk fault zone is terminated to the northeast within the quartz diorite.
The Kechumstuk fault zone is interpreted to be part of a wrench fault system. The stresses developed along this structure are modeled using a fault kinematic diagram (Fig. 10). Determination of the compressive stress vector or greatest horizontal principal stress (σ1) along the fault zone can be calculated from the bisecting angle between synthetic and antithetic (R') structures. The direction of greatest horizontal principal stress along the Kechumstuk fault zone at Little Whiteman is approximately 005°. The tensional stress vector or least horizontal principal stress (σ3) occurs orthogonal to the greatest horizontal principal stress and is oriented at 095° along the fault zone (Fig. 10).
Alteration and Mineralization
Marbles adjacent to the Kechumstuk fault zone and subsidiary structures are commonly altered to dolomite. Faulting and fracturing of the marble created permeability exploited by hydrothermal fluids to locally dolomitize the calcite-rich host rocks. Thus, structures are invariably enveloped in a halo of medium- to coarse-grained dolomitized carbonate rock. Use of carbonate stain (Hitzman, 1999) and SEM analysis indicates that the dominant hydrothermal carbonate species is nonferroan, magnesian dolomite.
The fringes of the sulfide replacement bodies contain silicified marble cut by a stockwork of Mn oxide veins. At Little Whiteman, silicification may have occurred during early hydrothermal alteration. Silicification appears to have resulted in permeability reduction of the host rocks, making the carbonates less favorable hosts for later metal-bearing hydrothermal fluid migration. The transition to distal manganese is observed in many carbonate-hosted replacement deposits from northern Mexico (Megaw et al., 1988). At Little Whiteman, manganese occurs in oxide minerals either disseminated in the rock or occupying veinlets. Abundant crosscutting manganese oxide veinlets produce a brecciated texture referred to as crackle breccia. Manganese-rich zones at Little Whiteman generally contain negligible precious metal concentrations. However, supergene zinc enrichment does occur within this alteration halo. The distribution of manganese-oxide and the growth of abundant manganese dendrites higher in the mineralizing system probably occurred as a result of late supergene processes.
The quartz diorite sill often shows sericitic alteration at contacts with mineralized marble and adjacent to crosscutting fracture-fill carbonate-sulfide veins and dacite and trachy-andesite porphyry dikes. Sericitic alteration halos within the quartz diorite surrounding such zones rarely exceed 1 m in thickness. Hematite staining of the surrounding quartz diorite wall rock is ubiquitous along contacts with the crosscutting porphyry dikes and, to a lesser extent, leached halos enveloping carbonate-sulfide veins. The hematite staining appears to be due to the conversion of primary magnetite to hematite within the groundmass of the quartz diorite. Hematization probably occurred during the emplacement of the porphyry dikes but could also be the result of younger fluid migration.
Dacite and trachy-andesite porphyry dikes cutting the marble are not replaced by sulfide minerals. However, some dikes contain disseminated euhedral pyrite and minor anhedral masses of sphalerite and chalcopyrite. Where these dikes are in contact with mineralized marble, they display strong sericitic alteration characterized by replacement of potassium feldspar phenocrysts and biotite by muscovite. Fine-grained, euhedral to subhedral quartz, coarse-grained muscovite, and pyrite are typically part of the alteration mineral assemblage. Most dikes contain euhedral apatite, which occur disseminated throughout the groundmass. Some highly altered dikes are cut by quartz-carbonate and carbonate (ankerite)-sulfide veins. Clay typically occurs in late-stage calcite veins cutting the porphyry dikes and could have formed, both during the mineralization-related hydrothermal alteration and supergene alteration of the sulfide bodies.
Massive sulfide bodies
The polymetallic Zn-Pb-Ag-(Cu) sulfide bodies at the Little Whiteman prospect occur as northeast-trending, steeply dipping, chimney-shaped, and likely sheeted, sulfide-rich replacement bodies in marble of the Nasina assemblage (Fig. 11A). Sulfide replacement occurs within the hanging-wall marble of the Kechumstuk fault zone, adjacent to subsidiary fault splays, and along the contacts of steeply dipping porphyry dikes. The largest sulfide replacement bodies occur within approximately 25 m of the Kechumstuk fault zone and are best developed in hydrothermally dolomitized carbonate rocks. Smaller sulfide bodies occur along synthetic faults generally distal to the main structure, as strata-bound manto bodies in the marble layers within the metasedimentary clastic rocks and as fracture-fill carbonate-sulfide veins, commonly intercepted high in the Little Whiteman section.
The replacement sulfide bodies at Little Whiteman typically contain 60 to 90 percent sulfide minerals with the remainder comprised of predominately quartz and carbonate gangue. Drill hole LWM-32 intersected several sulfide bodies that occupy a 20-m-wide zone averaging 12.5 percent Zn, 8.1 percent Pb, 0.18 percent Cu, and 158.7 g/t Ag (McLeod, pers. commun., 2008). Thicknesses of the replacement sulfide bodies are extremely variable. Most drill core intercepts are roughly 1- to 2-m-true thickness but can reach widths >3 m of continuous sulfide replacement. Most sulfide replacement bodies are stacked, with several multimeter-wide zones (Fig. 11A), typically separated by highly altered porphyry dikes. Massive and semimassive sulfide replacement bodies have irregular contacts with the marble host rocks. Sharp, often irregular, and anastomosing replacement textures suggest that the sulfide bodies formed predominantly by the replacement of carbonate wall rock, rather than by open-space filling. Many of the sulfide replacement bodies at Little Whiteman locally display a breccia texture consisting of subrounded to angular clasts of dolomitized marble encased in a sulfide matrix.
The available drill data suggest that replacement sulfide bodies pinch and swell vertically and along strike. Steeply dipping, strata-bound, manto-shaped, sulfide replacement bodies also occur in thin marble beds of the metasedimentary clastic rocks. However, in drilling to date, these replacement bodies have been highly oxidized to gossan and rarely contain primary hypogene minerals.
Polymetallic fracture-fill carbonate-sulfide veins (Fig. 11B) often occupy steeply dipping structures. Most commonly, they are intercepted high in the section and are typically <1 m in thickness. Fracture-fill carbonate-sulfide veins are laterally continuous along strike of prominent northeast-trending synthetic structures in the northeast part of the drill area. These veins are typically small and relatively low grade.
The principal sulfide mineral assemblage at Little Whiteman consists of sphalerite, chalcopyrite, galena, argentiferous tetrahedrite, bournonite, pyrite, and minor arsenopyrite (Figs. 12, 13). The paragenetic sequence of hypogene ore mineral deposition began with early pyrite and subordinate arsenopyrite, followed by coarse-grained, dominantly iron poor, reddish-brown sphalerite rimmed by iron-rich, black sphalerite. Disseminated chalcopyrite is concentrated within the iron-rich sphalerite rims. Minor pyrite and lesser galena are also found as inclusions in the iron-rich sphalerite.
The period of major sphalerite precipitation was followed by deposition of coarse-grained galena containing rounded blebs of Ag-bearing tetrahedrite, with minor chalcopyrite, pyrite, and sphalerite. Galena often mantles earlier sphalerite that sometimes displays dissolution along the contacts with the galena. Chalcopyrite, Ag-rich tetrahedrite, and bournonite occur in late-stage, very thin, carbonate-sulfide veins cutting earlier mineral assemblages. Chalcopyrite appears to replace earlier massive sphalerite along the margins of some late-stage carbonate-sulfide veins.
The SEM investigations indicate that the galena is not argentiferous. Most silver at Little Whiteman occurs as Ag-bearing sulfosalt minerals, dominantly tetrahedrite inclusions in galena, or as tetrahedrite in late-stage carbonate-sulfide veins. Silver was introduced with mid-stage galena and continued to be precipitated through the late-stage carbonate-sulfide veining event. Where galena is the dominant sulfide mineral, silver grades are typically high. Dusel-Bacon et al. (2009b) tentatively identified pyrargyrite in Little Whiteman drill core, based on its distinctive optical properties, and high Ag and Sb contents in whole-rock geochemical analyses. The final mineralizing event was the precipitation of coarse-grained pyrite that rims and cuts earlier mineral assemblages.
Euhedral crystals of dolomite, ankerite, calcite, and quartz typify the gangue mineral assemblage throughout the period of mineralization. Dolomite appears to have precipitated during the early stages of hydrothermal alteration, prior to the sulfide replacement. Ankerite and quartz are the dominant gangue minerals that precipitated during the sulfide deposition. SEM analyses indicate that intergrown ankerite, nonferroan dolomite, and minor rhodochrosite comprise the synsulfide carbonate gangue mineralogy. Late-stage calcite veins, sometimes containing barite and kaolinite, cut all earlier assemblages.
Early-stage metaliferous fluids were initially enriched in Zn and Cu and became depleted in these elements during the formation of later and more distal mineralization. Consequently, the contoured metal values are expected to vector toward the upflow direction of the metaliferous hydrothermal fluids. Zinc and copper contours represent normalized Zn and Cu sulfide values as projected in plan view (Fig. 14). Care was taken to exclude zones that displayed significant supergene oxidation, as supergene effects often result in Zn/Pb ratios that are elevated several orders of magnitude relative to hypogene values. For example, the strata-bound sulfide gossan in the upper metasedimentary rocks typically have Zn/Pb ratios that exceed 100/1.
The Zn/Pb ratio calculated at Little Whiteman would suggest a slight enrichment in Zn sulfide compared to Pb sulfide. The average Zn/Pb ratio is calculated at 1.2 for sulfide intercepts analyzed from nine drill holes. In general, most carbonate replacement deposits show element zoning from Cu-Zn-(Au)-rich zones near intrusions to Ag-Pb-Mn−rich in the distal fringes of the system (Megaw, 1998). However, Little Whiteman contains little to no gold.
Supergene alteration at Little Whiteman extends to variable depths, generally not exceeding the depth of the quartz diorite sill. However, deeper oxidation along structures is common. Massive sulfides are particularly susceptible to oxidation. Hence, the sulfide bodies close to the surface and adjacent to the Kechumstuk fault have been strongly oxidized. Gossans derived from oxidized sulfide bodies often have a strong Fe oxide to whitish-tan (leached) color and contain minor siliceous boxwork textures. “Zinc Zap” (Hitzman et al., 2003) was utilized to identify secondary zinc enrichment.
The gossans contain significant secondary zinc minerals, including hemimorphite, smithsonite, and hydrozincite. These zinc-bearing minerals replace sphalerite, occupy fractures, and most commonly occur as disseminations within marble and along fractures within the quartz diorite sill. Cerussite after galena is commonly developed in situ within oxidized sulfide bodies. Copper carbonate minerals, particularly malachite, occur on fractured surfaces as coatings and rim copper-rich carbonate-sulfide veins in oxidized zones.
Sulfate minerals commonly occur in oxidized parts of the section. Minor barite and abundant gypsum occupy fractures and occur as late-stage veins. Barite occurs in late crosscutting calcite veins and no primary barite has been identified in the sulfide bodies. The barium was probably derived from feldspar breakdown during the alteration of the porphyry dikes.
Meteoric karst, dissolution sanding, and brecciation are evident within the carbonate beds of the metasedimentary clastic rocks and within the thick marble unit. Matrix-supported dissolution breccias typically contain angular to subrounded metasedimentary rock fragments. The matrix of these breccias is composed of very fine grained, insoluble material, which commonly displays sedimentary structures. Mineralized clasts have not been observed in these meteoric breccias; fragments of dolomitized and/or silicified marble rarely occur. Dissolution breccias are best developed adjacent to more impermeable lithologic units, such as crosscutting dikes.
Rare, open-space and ice-filled cavities have been intercepted in the thick marble unit. Cavities can reach diameters of 2.5 m in width. These cavities often contain clay-rich material that is similar to fault gouge, suggesting that some of these cavities may be related to fault movement. The dissolution cavities present at Little Whiteman appear to have developed postmineral deposition.
Most carbonate replacement deposit systems are genetically associated with intrusions that are typically felsic in composition (Megaw, 1998). Dacite and trachy-andesite porphyry dikes occur ubiquitously throughout the mineralized interval at Little Whiteman and are likely the hypabyssal polyphase expression of an unexposed pluton at depth. Currently, the only age constraint for the porphyry dikes is the crosscutting relationship with the Late Triassic quartz diorite sill.
Other possibly related base metal occurrences in the western part of the Fortymile district, such as the Oscar, Fish, and West Little Whiteman prospects (J. Ridley, pers. commun., 2008; Dusel-Bacon et al. 2009b), are spatially associated with Late Cretaceous volcanic and intrusive rocks (Dusel-Bacon et al., 2009a). The West Little Whiteman prospect is intruded by leucocratic hypabyssal dikes of unknown affinity (J. Ridley, pers. commun., 2008). Zircon U-Pb geochronology indicates a 70.5 ± 1.1 Ma age for a rhyolite porphyry at the Fish prospect (Dusel-Bacon et al., 2009b) and 69 ± 1 Ma ages for granites adjacent to Oscar and West Little Whiteman (Dusel-Bacon et al., 2009a). The Oscar prospect lies on the southern border with the Middle Fork caldera dated at 69.1 ± 0.2 Ma (Bacon and Lanphere, 1996). A considerably younger age for calcic Zn-Pb skarns and replacement bodies of the eastern interior are reported in Newberry et al. (1997). The Oscar prospect, for example, is described to occur near 55 Ma Wrich greisens (Burleigh et al., 1994). Regardless, the spatial relationship of volcanism and caldera formation in the proximity of Zn-rich skarn and carbonate replacement deposit formation in the southwestern Eagle quadrangle appears to be associated with Late Cretaceous to Early Tertiary volcanism.
A 90-km northeasterly trend of base metal occurrences through the Fortymile district (R. McLeod, pers. commun., 2009) could represent a series of intrusion-related carbonate replacement deposits and Zn-rich skarns. The potential for a newly recognized carbonate replacement deposit district in the Fortymile region is promising and additional high-temperature, carbonate-hosted replacement deposits should be present along structural corridors in the Fortymile district of Alaska.
Exploration for base metal carbonate replacement deposits in the Fortymile district should focus on several important features. Major northeast-trending structural zones, particularly areas that appear to have dilational jogs and releasing bends or fault intersections with northwest-trending conjugate faults, may be the most attractive targets. Fold hinges in marble, similar to that at the West Little Whiteman prospect, are also prospective structural targets for carbonate replacement deposit occurrences. Identification of major structural zones through the use of geophysical methods or GIS linear analysis is recommended. Exploration should focus on the location of carbonate rocks, as these are the most reactive host rocks susceptible to replacement. Negligible marble is present in the parautochthonous Yukon-Tanana assemblage, thus it is less prospective than the allochthonous Yukon-Tanana terrane, which makes up the majority of the Fortymile mining district (Dusel-Bacon, writ. commun., 2010). Areas adjacent to caldera complexes and felsic- to intermediate-composition hypabyssal porphyry dikes appear to be most favorable because there may be a genetic link and spatial association between eruptive volcanic centers and carbonate replacement deposit and skarn mineralization (Megaw et al., 1988). Exploration should also seek to locate kill zones indicative of acid leaching of sulfide bodies at or near the surface. It is critical, however, to ensure that vegetation anomalies are not the result of permafrost or snow patches, as these often resembles kill zones caused by oxidation of sulfides. Discovery of hydrothermal carbonates will also be useful as well as the identification of zones containing supergene zinc-oxide minerals.
The work presented in this manuscript is part of an M.Sc. study at the Colorado School of Mines by Chris R. Siron. Nigel Kelly provided helpful suggestions and edits to this manuscript. Warren Day is acknowledged for comments on the geologic framework of the Fortymile district and Eric Nelson helped with the structural interpretations. John Skok is thanked for analytical support. This publication benefited from discussions with Rainer Newberry and Laurel Burns. We thank Cynthia Dusel-Bacon and David Szumigala for their reviews which significantly improved this manuscript. Chris R. Siron thanks Full Metal Minerals, the Society of Economic Geologists, and the Colorado School of Mines for financial support.
Figures & Tables
The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.