Lower Mesozoic volcanic rocks exposed in an 8- to 15-km-wide belt between Westwood and Taylorsville, northern California, are the northernmost exposures of a volcanic sequence that originally spanned the eastern flank of the present-day Sierra Nevada batholith. The northwest-striking, southwest-dipping, west-facing, homoclinal volcanic section is only weakly metamorphosed and deformed. The more than 11-km-thick volcanic sequence records at least four episodes of subaerial, alkalic to calc-alkalic, andesitic to dacitic volcanism punctuated by intervals of clastic deposition. Rock types include monolithic intrusive breccia, porphyritic flow rock, tuff breccia, and hypabyssal intrusive rock. The interbedded clastic rocks consist of fluvial to shallow-water bedded volcanic arenites, conglomerates, laharic breccias, debris flows, siltstones, and shales. Major- and trace-element abundances for the volcanic rocks, including rare earth elements (REE), are similar to those in modern continental arc sequences. Ce/Y:Y, La:Ba, La:Th, K 2 O:Na 2 O, and Ba:K 2 O resemble those in volcanic rocks from high-K provinces. On the basis of K 2 O:SiO 2 relations, most of the volcanic rocks are classified as latites or toscanites. The morphology, stratigraphy, and major- and trace-element chemistry of the lower Mesozoic rocks exposed in northeastern California resemble those of other mildly alkalic, lower Mesozoic volcanic sequences along the eastern flank of the Sierra Nevada, California, in western Arizona, and western Nevada. It is proposed that this belt of high-K volcanic rock (the high-K magmatic province) represents postorogenic magmatic activity associated with a phase of long-lived extensional tectonics following the Permian-Triassic Sonoma orogeny.

The alphabet soup of granite types and its association with certain ore deposit types is called into question. We now realize that granites do not retain a menu of ingredients inherited at the site of their generation, and that initial magma chemistry is only one of several influences on the potential for later ore formation in granite systems. Rather, processes leading to fluid release and crystal-melt and fluid-melt fractionation of critical elements, mainly metal species and volatile complexing agents, are central to the formation of ore. These processes are certainly influenced by tectonic setting, source mineralogy and chemistry, assimilation, contamination, and geothermal gradient, but are not strictly controlled by them. The delicate relations between a granite magma, its crystallizing phases, volatile content and species, and oxygen fugacity, plus the timing and mechanism of fluid release and the efficiency of metal extraction, ultimately control the formation of an ore deposit. As experimental data become more available, models for granite-related ore deposits must incorporate and emphasize the important role of crystal and volatile fractionation in the system. In other words, we need to model the process as well as the source.

Constraints may be placed on the functional dependence of the partitioning of ore and other trace, minor, and major elements in melt (liquid)-vapor-crystal systems by relating empirical Nernst-type partition coefficients to true mass action expressions and equilibrium constants. This process is herein termed stoichiometric analysis. The resulting expressions can be used to determine the functional dependence of partition coefficients on the bulk composition of the phases in question. In this chapter, stoichiometric constraints are used to place limits on the partitioning of europium between melt and vapor, to determine the functional dependence of the HCl concentration in the magmatic aqueous phase on the basis of the composition of the melt, and to suggest controls on the iron concentration in the magmatic aqueous phase. Based on this analysis and data available in the literature, it can be shown that the Nernst partition coefficient for europium is probably a function of the square of the chloride concentration in the magmatic aqueous phase; at the very least, it is limited to a polynomial dependence on the chloride concentration raised to the second and third power. The HCl concentration in the aqueous phase is shown to increase with the peraluminosity and chlorine concentration in the melt phase, and the aqueous iron concentration is shown to increase with increasing activity of HCl and decreasing f O 2 for a given activity of magnetite component in a magmatic system.

The distribution of F and Cl between aqueous fluids and melts of haplogranite and topaz rhyolite composition was determined experimentally at 0.5 to 5 kbar and 775° to 1,000°C. The distribution coefficients, D i , for F and Cl were calculated as the parts per million by weight (ppmw) of i in the fluid/parts per million by weight (ppmw) of i in the melt. D Cl ranges from 0.8 to 85; however, under typical geologic conditions, Cl partitions more strongly into an aqueous fluid relative to F-bearing granitic melts. Cl partitions increasingly in favor of the fluid as F in the fluid and magma decrease and as the X H 2 O fl (molar H 2 O/H 2 O + CO 2 in fluid), temperature, pressure, and Cl in the fluid and melt increase. Cl partitions in favor of haplogranite melts that contain >7 wt % F and ⩽1,200 ppm Cl at 1,000°C and 2 kbar. D F ranges from 0.2 to >1.0; however, F typically is concentrated in granitic melts relative to aqueous fluids. F concentrates more strongly into topaz rhyolite melt as pressure, F in the fluid, and melt, temperature, and the X H 2 O fl decrease. However, F partitions in favor of aqueous fluids relative to topaz rhyolite melts at 800°C and 2 kbar, if the melt contains ⩾7 wt % F. Computations indicate that extreme enrichments in F (>4 wt %) and Cl (>5,000 ppm) may occur in magmas and in associated magmatic hydrothermal fluids during the end stages of crystallization of topaz rhyolite magmas and magmas associated with Climax-type molybdenum deposits if the initial H 2 O, CaO, and ferromagnesian contents in the magma are low, and if the pressure at which water saturation occurs is high.

The processes leading to the generation of pegmatite fabrics and rare-element ore deposits are evaluated in terms of pressure, temperature, and compositions of fluids through the combination of recent field, fluid inclusion, and experimental data. The results indicate that primary consolidation of miarolitic and massive rare-element (Li-Be-Ta) pegmatites occurs in the range of 700° to 450°C at 400 to 250 MPa. The model for pegmatite genesis proposed here involves disequilibrium crystallization of quartz and feldspars in a hydrous melt-crystal system that approaches chemical equilibrium but rarely attains it. Fluxing anionic components of boron, phosphorus, and fluorine are not essential to this model, but augment the zoning process by promoting expansion of the liquidus fields of quartz and K-silicates (feldspar and mica) and by driving residual melt toward alkaline, Na-rich compositions enriched in lithophile trace elements. Aqueous vapor, if present, may have a nominal affect on primary zonation and fabric development within pegmatites. Various lines of evidence suggest that rare-element pegmatites may not become vapor saturated until they approach solidus conditions, at which point internal retrograde recrystallization and wall-rock alteration occur. Recent experiments have succeeded in replicating most aspects of pegmatite geology, including fractionation trends, mineral fabrics, and spatial zonation, at vapor-undersaturated conditions.

Late-stage volatile-rich topaz granites occur widely but sparsely throughout the southwest England Sn-Cu-polymetallic mineralized S-type biotite granite batholith. New observations from the St. Austell area have clarified field relations, and demonstrate the importance of an aureole of tourmalinization affecting both granitic and sedimentary host rocks. Topaz granite contacts are often marked by pegmatitic zones showing undirectional solidification textures and carrying vugs of quartz-tourmaline; minor intrusive sheets have symmetrical haloes of tourmalinization within adjacent host rocks. The topaz granites are mineralogically complex, containing primary topaz, zinnwaldite, or lepidolite, amblygonite (and other phosphates), and various Nb-Ta rich accessory phases, as well as albite, orthoclase, and quartz. Fluorite is secondary. They are chemically distinct from the biotite granites, showing markedly higher concentrations of Li 2 O (as much as 0.5 percent), F (as much as 1.5 percent), P 2 O 5 (0.5 percent), Nb (as much as 65 ppm), Ta (as much as 30 ppm), Ga (as much as 50 ppm), and Rb (as much as 2,000 ppm), in particular. Late differentiates of the biotite granites include tourmaline granites, but this differentiation trend principally involves an increase in B with little change in Li, F, or P. It is considered that the topaz granites are unlikely to be derived by fractional crystallization of the southwest England biotite granite magma (they are intruded by rhyolite porphyry dikes, which belong to the biotite granite suite), and an origin involving limited partial melting of subbatholithic fusion residues during an episode of potassic basic magmatism is preferred. Comparison with other volatile-rich granitic rocks indicates that certain lithium pegmatites (e.g., Tanco), other topaz granites (Seward Peninsula, Erzgebirge, etc.) and volcanic glasses (e.g., Macusani) share important characteristics with the southwest England topaz granites. It is suggested that these rock types may represent a fundamentally similar volatile-rich granite magma type whose formation, while debatable, may be controlled by limited partial melting of lower crustal fusion residues that had previously generated more “normal” granite magmas.

In this chapter we describe the petrogenesis of aplitic segregations in the fluorine-rich Proterozoic Butler Hill and Graniteville granites of the St. Francois Mountains volcano-plutonic terrane, southeastern Missouri. Both plutons contain an early coarse-grained type of granite that grades into or is crosscut by fine-grained aplitic segregations. The aplitic segregations are generally enriched in fluorine and alkalis, have more pronounced negative Eu anomalies in their rare earth element (REE) patterns and higher concentrations of heavy REEs compared to their coarse-grained high-silica counterparts. In the Butler Hill granite, the distinction in fluorine concentrations between the two rock types was obliterated in part by subsolidus hydrothermal alteration, which is indicated by complete chloritization of biotite, sericitization of feldspar, and recrystallization or reequilibration of muscovite with water-rich fluids. These chemical relations suggest that the aplitic segregations in both the Butler Hill and Graniteville plutons are the products of crystal-liquid fractionation in which fluorine played a significant role. Initially, the slightly peraluminous to metaluminous compositions of the fluorine-containing magmas were near the composition of a minimum melt in the Q-Ab-Or system. However, early crystallization of quartz and feldspars resulted in enrichment of the remaining melt in fluorine, causing the pseudoternary minimum to move away from quartz. As a consequence of the enlargement of the quartz field, quartz became the sole crystallizing phase, yielding the silica-enriched composition of the coarse-grained granites. The aplitic rocks crystallized from the relatively alkalic residual melt, which separated from the crystalline assemblage as the melt’s viscosity decreased due to an increase in the fluorine and water content. These results are in accord with published experimental data that show that the effect of fluorine is to decrease the silica content of residual liquids, contrary to normally observed fractionation trends in igneous rocks.

The quartz phenocrysts of rhyolitic rock that have erupted over the last 40 m.y. in the Thomas Range, west-central Utah, preserve glass inclusions that may contain trapped liquidus phases. These glass inclusions, analyzed by electron microprobe, show the melts of the Joy Tuff (crystal tuff member), Dell Tuff, and Topaz Mountain Rhyolite to be peraluminous potassic rhyolites, the latter enriched in fluorine (as much as ~1.9 percent). The older units (Joy and Dell Tuffs), tuffaceous calc-alkaline rhyolites, contain trapped Mn-rich aluminous clinopyroxene, zircon, fluorapatite, feldspar(?), titanite(?), and quartz. The anorogenic Topaz Mountain rhyolite preserves evidence of a ternary feldspar and magnetite (as well as quartz) on the liquidus. The potassic and peraluminous nature of the melts is consistent with anatexis of a crustal sequence. The distribution of fluorine, unlike that of chlorine, is heterogeneous in the Topaz Mountain Rhyolite. In this F-rich unit, primary β -quartz gave way to primary α -quartz. Both Ca and Fe were added to the bulk rocks following their devitrification.

Peraluminous rhyolites that are chemically somewhat similar to topaz rhyolites and anorogenic granites occur in an orogenic setting near Sierra Blanca in the Tertiary Trans-Pecos magmatic province. The Sierra Blanca rhyolites are even more enriched in most incompatible trace elements than are topaz rhyolites. Some of the extreme enrichments may in part be the result of chemical modification by crystallization from an F-rich vapor phase. The rhyolites were intruded as laccoliths at 36 Ma, during the main phase of Trans-Pecos igneous activity, which is characterized by ash-flow eruptions from numerous calderas and widespread mafic, intermediate, and silicic intrusions. A dominant east-northeast orientation of dikes and veins throughout the region indicates mild compression that was residual from Laramide deformation. This compressive tectonic setting, coupled with concurrent volcanism in Mexico and the east-northeast change in magma chemistry from calc-alkalic in western Mexico through alkali-calcic to alkalic in Texas, suggests that the rhyolites were emplaced in a continental arc. Extension did not begin in Trans-Pecos Texas until after 32 Ma; 31- to 17-Ma dikes are dominantly oriented north-northwest, perpendicular to the direction of extension during early Basin and Range deformation. Thus, the tectonic setting of the Sierra Blanca rhyolites contrasts with that of typical topaz rhyolites, most of which were emplaced during periods of crustal extension. The Sierra Blanca rhyolites are chemical and mineralogic oddities for the region, where most rhyolites are peralkaline or metaluminous. The rhyolites are depleted in the same elements as topaz rhyolites (Mg, Ca, Ti, Sr, Ba) but are more highly evolved than topaz rhyolites. Extreme trace-element enrichments (Li, F, Zn, Rb, Y, Zr, Nb, Sn, Ta, Pb, HREE, Th, U) are accommodated in Li-rich white mica, Zn-rich biotite, Rb-rich feldspars, and numerous trace minerals, including cassiterite, changbaiite, columbite, thorite, xenotime, yttrium- and REE–rich fluorides, and zircon. The rhyolites are large-tonnage, low-grade resources of several rare metals. Also enriched in Be (as much as 180 ppm), the rhyolites are the sources of Be and F in beryllium deposits in fluoritized limestones along the contacts with the laccoliths. Interaction with the limestones probably locally elevated the Ca, Mg, and Sr contents of the rhyolites. Vapor-phase crystallization has modified the original magmatic chemistry of the rocks. Evidence of vapor-phase crystallization includes the presence of minerals typical of pegmatites: cryolite (from 0 to 3 volume percent), alkali feldspars with nearly end-member compositions, polylithionite-zinnwaldite mica, and rutilated quartz, plus fluid inclusions defining quartz overgrowths on magmatic grains. Extreme HREE enrichments (Yb to 72 ppm; chondrite-normalized REE patterns with positive slopes) may also be the result of vapor-phase crystallization.

Late Cretaceous to Early Tertiary granitic plutons associated with W skarn or Sn greisen-skarn occur interspersed in a belt 70 × 200-km-long just northeast of Fairbanks, Alaska. All plutons intrude the late Precambrian–early Paleozoic Yukon-Tanana terrane and are similar in major-element compositions (dominantly granodiorite to monzogranite), initial Sr isotopic ratios (0.710 to 0.719), and Pb isotopic signatures ( 206 Pb/ 204 Pb = 19.17 to 19.37). Biotite compositions and opaque mineral abundances indicate both types of plutons crystallized along a buffered path intermediate between nickel–nickel oxide and quartz-magnetite-fayalite. Both suites contain multiple igneous units, with younger, usually equigranular, units spatially related to mineralized zones. Isotopic, trace-element, and mineralogical data suggest an “I-type,” “ilmenite-series” classification for both pluton suites. Because the W and Sn plutons appear to represent magmas with similar origins and source materials, differences in observed metallogeny are thought to be related to differences in environment of crystallization and vapor loss. Such differences include: age (102 to 87 Ma for W plutons, 73 to 50 Ma for Sn plutons), crystallization pressure (1 to 2 kbar for W plutons, <0.5 kbar for Sn plutons), vapor loss history (late for the W plutons and early + late for the Sn plutons), and fluorine trends (decreasing F with increasing differentiation for the W plutons and increasing F for the Sn plutons). Differences in confining pressure (depth) and vapor loss history are associated with differences in age: the younger (Sn) plutons are shallower, and the older (W) plutons are deeper. Trace-element patterns (e.g., Rb, B, Be, W, Sn, Li) are similar for least differentiated units of both pluton types, increasing modestly with increasing differentiation for the W plutons and increasing strongly for the Sn plutons. Data are most compatible with 80 to 95 percent fractionation (crystal-liquid) followed by vapor loss for the W plutons and 80 to 90 percnt fractionation (crystal-liquid) for the Sn plutons, with early vapor loss followed by (liquid-liquid?) “ultrafractionation.” Ultrafractionation and subsequent ore element enrichment occurs in the Sn plutons by early vapor loss and subsequent F enrichment in the residual magma. The data suggest that metallogeny differences for W vs. Sn plutons in our study area are not a function of differences in initial metal contents of the magmas but are more likely due to differences in magmatic evolution.

The amount and character of the tin resources in the Seward Peninsula tin granite belt in western Alaska are directly related to the depth of erosion of the plutonic system. Plutons that have little or no outcrop (little erosion), such as Kougarok or Lost River, are the site of lode tin deposits, whereas placer tin deposits are associated with somewhat eroded plutons (Cape Mountain, Ear Mountain) with a modest outcrop area. The largest pluton in the tin belt, the Oonatut Complex, is a deeply eroded pluton with little tin in either lode or placer deposits. Textural units in the granite plutons also vary with depth in the plutons. Late-stage, fractionated (Differentiation Index = sum of normative quartz + albite + orthoclase = 91.5 to 96.6) equigranular biotite granite is found at high levels in the plutons, and earlier, less fractionated (Differentiation Index = 71.8 to 92.5), seriate and porphyritic biotite granite is more abundant at greater depth. Fluids associated with the porphyritic and seriate granites were H 2 O-NaCl–rich and produced metal-poor, idocrase-scapolite-diopside–rich skarns in the carbonate host rocks. Fluids evolved during the late-stage crystallization of the equigranular granite were enriched in incompatible components and produced tin-bearing greisen in granite and hedenbergitic pyroxene-garnet-tourmaline-axinite-cassiterite skarns in marble. Mineral assemblages and chemistry reflect magma fractionation and record a two-stage evolution for the granite and associated skarns in the tin belt. Biotite compositions show progressive Fe enrichment (Fe/(Fe + Mg) = 0.69 to 0.99) with increasing fractionation of the enclosing granite. Biotite-bearing pegmatite and aplite dikes are associated with the early-crystallizing seriate and porphyritic units of some plutons. Biotite compositions in these dikes are very similar to those in the seriate and porphyritic granites, indicating the H 2 O-rich fluids associated with the dikes were magmatic in origin. Tourmaline is found as an interstitial phase in the equigranular granites; it is Fe-rich, similar to the coexisting biotite, suggesting a magmatic origin for this tourmaline. Later, secondary tourmaline is Ca-Mg–rich, and is associated with white mica alteration of biotite in granite and with sulfide deposition in skarns.

The Devonian batholiths of western Tasmania represent a diverse assemblage of highly fractionated intrusions (70 to 77 percent SiO 2 ) that are the products of different source materials. The Housetop batholith exhibits compositional affinities to a fluorine-rich I-type magma. The Meredith batholith also has characteristics indicative of I-type source materials. The Heemskirk batholith is composite, and consists of a volatile (F, B, H 2 O)–rich S-type granite underlying an I-type granite. The Three Hummock Island, Interview River, Sandy Cape, and Conical Rocks plutons probably have an S-type source and are grouped together as the Sandy Cape Suite. Rapakivi texture is common in the Housetop, Meredith, and Heemkirk batholiths. Quartz-tourmaline nodules are found in the Conical Rocks pluton and the S-type portion of the Heemskirk batholith. The Conical Rocks and Interview River plutons yield high initial Sr isotopic ratios of 0.74242 and 0.76009, respectively. The Housetop and Meredith batholiths yield the lowest initial Sr isotopic ratios of 0.71041 and 0.71445, respectively. The S-type portion of the Heemskirk batholith has an initial Sr isotopic ratio of 0.76387. The 40 Ar/ 39 Ar release spectrum and Rb/Sr mineral isochron analyses corroborate previously reported Devonian to Carboniferous age estimates for these batholiths. A relatively low-temperature thermal event (<200°C) caused argon loss from the K-feldspars at about 105 Ma. This heating event is probably related to the continental breakup of Australia from Antarctica. Major-element compositions of the western Tasmanian granites are very similar. The highly fractionated Sandy Cape Suite leucogranites exhibit high Ga/Al ratios typical of A-type granites, but not their extreme Zr, Y, or Ce enrichments. A distinctive feature of the Sandy Cape Suite is the increase in P 2 O 5 concentration with fractionation. The increase in P 2 O 5 with fractionation is apparently due to extremely low Ca activity, which precludes the formation of apatite, thus allowing P 2 O 5 to behave incompatibly in the melt. All of the granitoids have LREE–enriched chondrite-normalized rare earth element patterns. REE fractionation within the individual granitoids can be summarized by two trends: those with LREE >> HREE depletion (Housetop, Meredith, and Heemskirk batholiths), and those with LREE = HREE depletion (Sandy Cape Suite). The first trend is caused by the initial undersaturation of accessory mineral assemblage that resulted from high concentrations of volatiles and/or alkali complexes. The second trend is caused by early saturation of accessory phases and/or refractory accessory phases.

The East Kemptville greisen-hosted tin deposit (58 million tonnes of 0.165 percent Sn), southwestern Nova Scotia, Canada, occurs beneath undulations in the contact between granitic rocks of the Davis Lake complex (DLC) and Meguma Group metawacke. Cassiterite-topaz ore precipitated from a F- and Sn-rich fluid derived from the East Kemptville leucomonzogranite. Controls on tin mineralization include the unusual primary Sn and F abundances of the DLC magma, the vertical chemical zonation of the magma prior to crystallization, generation of an aqueous phase, and associated Sn-Cl complexing before the separation of F from the magma and the flat-lying granite-metawacke contact. The Carboniferous DLC is composed of biotite-bearing monzogranites, leucomonzogranite, and high-F, low-B topaz greisen. The chemical and isotopic signatures of the monzogranites reflect the unusual source of this highly evolved pluton. Magmatic evolution was focused toward the granite-metawacke contact and culminated, at the current erosional level, in the East Kemptville deposit. Chemical variation is attributed to the vertical zonation of the magma prior to the fractionation of biotite, K-feldspar, ilmenite, and REE-bearing phosphate minerals. The remarkably high F and P contents resulted in unusual Al and P distributions and concentration of metals beneath the contact as a result of increased magmatic depolymerization and diffusion. Pegmatitic segregations record the evolution of a F- and Sn-rich aqueous fluid. This aqueous phase scavenged Cl, alkali elements, P, Sn, and other metals from the magma, concentrating them beneath the contact. In contrast to porphyry-style deposits, this fluid was not expelled from the granite. When crystallization was complete, more than 10,000 tonnes of F partitioned into this aqueous fluid, forming a hydrofluoric fluid that was neutralized by reaction with the leucomonzogranite. Quartz-topaz rock is either a direct precipitate from the F-rich fluid or extensively replaced leucomonzogranite. Quartz-mica greisen and incomplete greisen formed as the F/OH ratio in the fluid decreased. Veins and fractures emanating from massive greisen zones have alteration envelopes, indicating these structures were conduits for the F-rich fluid. Their orthogonal orientation suggests these veins are related to cooling, not hydraulic fracturing. Whole-rock Rb-Sr isotopic data from leucomonzogranite and quartz-topaz rocks yield dates and initial ratios statistically identical to the biotite monzogranite. High mean square of weighted deviations (MSWDs) resulting from the former data reflect contamination of the late-magmatic fluids by Sr derived from the metawacke and later thermal overprinting. Deformation, probably related to regional tectonothermal events, affected all rocks of the DLC, but postdates ore formation.