Abstract Porphyry molybdenum deposits are spatially, temporally, and genetically associated with porphyritic intrusions of quartz monzonite to high silica, alkali-rich granite composition. Most molybdenum is in quartz-molybdenite veinlets that are part of an intrusion-centered stockwork of veinlets. Associated minerals are pyrite and fluorite. Recoverable to geochemically anomalous tungsten, tin, copper, lead, and zinc commonly occur marginally and/or peripherally to the molybdenum ore. Stockwork deposits associated with intrusions of high silica, alkali-rich rhyolite, and granite porphyry are herein recognized as a distinct class, referred to as “Climax type.” These deposits generally are dome shaped, with each deposit centered on an intrusive cupola, such that the molybdenite zone mimics the shape of, and commonly straddles, the igneous contact. The Climax (Ceresco, Upper, and Lower orebodies), Red Mountain (Urad and Henderson orebodies), and Mount Emmons-Redwell deposits are composite Climax-type systems that formed by multiple pulses of intrusion and mineralization. Host rocks of Climax-type intrusions typically are warped, attenuated, domed, and fractured. Steeply dipping radial and concentric dikes, veins, faults, and joints indicate vertical orientation of the maximum principal stress during forceful emplacement of magmatic cupolas. Sparse inclusions of host rocks near contacts indicate magmatic stoping. Discontinuous stockwork veinlets resulted from forces generated by hydrothermal fluids that evolved from the magmas. Gently outward-dipping concentric veins and faults probably formed during cooling and contraction of intrusive cupolas. Climax-type rocks are silica rich, aluminous, calcium poor, and alkali rich, with K 2 O > Na 2 O. Essential minerals are quartz, potassic feldspar, and albite. Accessory minerals include fluorine-bearing biotite, fluorite, fluorine-rich topaz, spessartine, zircon, ilmenorutile, rutile, columbite, brannerite, uraninite, thorite, monazite, fluocerite, apatite, xenotime, aeschynite, and euxenite. Numerous textural features in Climax-type intrusions suggest that ore-forming fluids ex-solved directly from crystallizing magmas. Rhythmic quartz layers in porphyry indicate high water pressure and episodic build-up and release of volatiles. Replacement of albite pheno-crysts by nearly pure orthoclase in a groundmass containing albite suggests the presence of a separate hydrothermal fluid before formation of the groundmass. Zones of micrographic textures indicate areas of hydrothermal fluid accumulation prior to overpressure relief and release of the fluid. Aplitic groundmass textures suggest pressure quenching. Veins near igneous contacts commonly have both igneous and hydrothermal characteristics. Fluid inclusions from the Henderson mine that contain as much as 65 wt percent NaCl suggest that molybdenite mineralization formed at temperatures above 500°C, probably between 500° and 650°C. Consideration of phase equilibria in fluid inclusions indicates overpressures 150 to 250 bars greater than lithostatic pressure during mineralization. These overpressures probably caused the stockwork fracturing. Hydrothermal alteration is best recorded at Red Mountain. Five major pervasive rock alteration zones include the potassium feldspar zone, quartz-sericite-pyrite zone, upper and lower argillic zones, and the propylitic zone. Five additional zones of less areal extent include the vein silica zone, pervasive silica zone, magnetite-topaz zone, greisen zone, and garnet zone. Strontium and lead isotope data and trace element concentrations in Climax-type systems suggest that Climax-type igneous rocks represent extreme differentiates of parent magmas which formed by fractional partial melting of mafic- to intermediate-composition materials in the lithospheric mantle and lower crust, and that upper crustal rocks were not significantly involved in the generation or evolution of Climax-type magmas and ore leads. Age determinations, structural observations, and plate tectonic reasoning suggest that the Climax-type magmas of Colorado formed during a relatively atectonic transition, after subduction-related calc-alkaline igneous activity but before extension-related normal faulting and without or before known local basaltic volcanism. The high concentrations of silica, alkalis, and Rb in Climax-type rocks, coupled with the low concentrations of Ca, Sr, and Ba, suggest fractional crystallization of plagioclase and potassic feldspar. This, together with gravitational crystal-liquid separation, probably was the dominant differentiation mechanism in the deep crustal environment. The high inferred concentrations of water, F, Mo, W, Sr, U, Th, and Nb in Climax-type magmas suggest upward enrichment of these constituents by convection-driven thermogravitational diffusion (Hildreth, 1979), a process which probably became dominant as magma columns traversed steepening thermal gradients in progressively shallower environments. At depths between 2,000 and 10,000 ft, particularly volatile and molybdenum-enriched magma cupolas forcefully expelled ore-forming fluids. This caused stockwork fracturing of host rocks and pressure quenching of aplitic cupolas. Fracture filling by quartz, molybdenite, pyrite, fluorite, topaz, and/or huebnerite formed the orebodies.

Abstract There are two principal categories of porphyry molybdenum deposits: the quartz monzonite type and the Climax type. The quartz monzonite-type deposits are broadly distributed throughout the western cordillera of North America, and one or more are present and known in Peru, Yugoslavia, Sulawesi, and China. The Climax-type deposits are fewer in number, are restricted to the Colorado mineral belt, and are related to highly evolved rhyolitic rocks. Deposits classed as sub-Climax type are located in New Mexico, Utah, Nevada, Texas, and Greenland. The Climax-type deposits are the product of more complex igneous hydrothermal systems than are deposits of other types; multiple intrusive and mineralizing events are common features, and average ore grades are significantly higher. The Climax mine was the first producer of molybdenite from a porphyry system, and this deposit has the longest history of operation and model development. The story of events leading to the present interpretation of Climax geology is the focus of this paper. The first bona fide geologic model for Climax was proposed in 1926, when B. S. Butler of the U.S. Geological Survey noted that ore developed on one side of a zone of pervasive silicification should have a counterpart on the other side. A hole drilled in 1929 to test this thesis was successful, and the result was the truncated cone model of Butler and Vanderwilt (1933). This cone (the upper orebody) was interpreted as a mineralization chimney formed by solutions issuing from a postulated intrusion below the chimney. Extreme alteration and paradoxical relationships between dike rocks and mineralization led to misidentification of rock types (Tertiary porphyries versus Precambrian metamorphic rocks and granites) and misinterpretation of events; even the discovery of a second orebody (lower orebody) in 1936 did not change the concept of a single event from a single source. Work by Wallace and others in the 1950s and 1960s added a third orebody (Ceresco orebody) and demonstrated the importance of time and of multiple intrusive and mineralizing events fueled by a master reservoir. The development of new highly sensitive, accurate, and rapid analytical methods enabled T. G. Moore and H. T. Schassberger of AMAX Exploration to demonstrate during the early to mid-1960s the highly evolved nature of Climax-type intrusions. This, plus the recent developments in tectonic theory and the concept of fractionation by thermogravitational diffusion (Hildreth, 1979), provided the elements necessary for the formulation of the “atectonic diapir” model by White et al. (1981).

Abstract Porphyry Cu and porphyry Mo deposits are large to giant deposits ranging up to >20 and 1.6 Gt of ore, respectively, that supply about 60 and 95% of the world’s copper and molybdenum, as well as significant amounts of gold and silver. These deposits form from hydrothermal systems that affect 10s to >100 km 3 of the upper crust and result in enormous mass redistribution and potential concentration of many elements. Several critical elements, including Re, Se, and Te, which lack primary ores, are concentrated locally in some porphyry Cu deposits, and despite their low average concentrations in Cu-Mo-Au ores (100s of ppb to a few ppm), about 80% of the Re and nearly all of the Se and Te produced by mining is from porphyry Cu deposits. Rhenium is concentrated in molybdenite, whose Re content varies from about 100 to 3,000 ppm in porphyry Cu deposits, ≤150 ppm in arc-related porphyry Mo deposits, and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-type) porphyry Mo deposits. Because of the relatively small size of porphyry Mo deposits compared to porphyry Cu deposits and the generally low Re contents of molybdenites in them, rhenium is not recovered from porphyry Mo deposits. The potential causes of the variation in Re content of molybdenites in porphyry deposits are numerous and complex, and this variation is likely the result of a combination of processes that may change between and within deposits. These processes range from variations in source and composition of parental magmas to physiochemical changes in the shallow hydrothermal environment. Because of the immense size of known and potential porphyry Cu resources, especially continental margin arc deposits, these deposits likely will provide most of the global supply of Re, Te, and Se for the foreseeable future. Although Pd and lesser Pt are recovered from some deposits, platinum group metals are not strongly enriched in porphyry Cu deposits and PGM resources contained in known porphyry deposits are small. Because there are much larger known PGM resources in deposits in which PGMs are the primary commodities, it is unlikely that porphyry deposits will become a major source of PGMs. Other critical commodities, such as In and Nb, may eventually be recovered from porphyry Cu and Mo deposits, but available data do not clearly define significant resources of these commodities in porphyry deposits. Although alkali-feldspar rhyolite-granite porphyry Mo deposits and their cogenetic intrusions are locally enriched in many rare metals (such as Li, Nb, Rb, Sn, Ta, and REEs) and minor amounts of REEs and Sn have been recovered from the Climax mine, these elements are generally found in uneconomic concentrations. As global demand increases for critical elements that are essential for the modern world, porphyry deposits will play an increasingly important role as suppliers of some of these metals. The affinity of these metals and the larger size and greater number of porphyry Cu deposits suggest that they will remain more significant than porphyry Mo deposits in supplying many of these critical metals.

Several types of mineralization appear to be related to the emplacement of fluorine-rich silicic lava flows and domes. An important example is the beryllium deposit at Spor Mountain, west-central Utah, where bertrandite, fluorite, amorphous silica, and Mn-Fe oxides replace dolomite fragments in tuffaceous surge deposits just beneath a topaz-bearing rhyolitic lava flow. The Be-mineralized zone is also highly enriched in F, Sn, W, Nb (and presumably Ta), Zn, Pb, and several other metals (but not in Mo). The uniform lateral character of the mineralization, the restriction of Be mineralization to the upper-most few meters of tuff, and the lack of mineralization in fluorite-bearing breccia pipes (tuffaceous vent breccias, in some cases) in underlying dolomite suggest (Bikun, 1980) that the beryllium mineralization resulted from the devitrification of the overlying lavas (a “steam iron” model). Mass-balance calculations based on comparisons of the chemical compositions of glassy and devitrified rhyolite are consistent with this unconventional interpretation. A second important example is provided by “Mexican-type” fumarolic tin deposits, characterized by cassiterite in carapace breccias of rhyolitic domes. Low-temperature dissolution and reworking of early fumarolic cassiterite may produce the colloform “wood tin” common in this deposit type. Deposits of this type occur in Nevada and New Mexico, as well as in many areas of northern Mexico. Fluorine-rich intrusive domes (better known as plutons) may also host metal mineralization, generally of the porphyry type. Examples include the well-known Climax-type porphyry molybdenum deposits of Colorado, New Mexico, and Utah, and the porphyry tungsten deposit at Mount Pleasant, New Brunswick, Canada. These subvolcanic deposits, in common with those associated with extrusive silicic lava flows and domes, are believed to have been derived by the crystallization-devolatization of highly fractionated magma.

High-silica, topaz-bearing rhyolites of Cenozoic age are widely distributed across the western United States and Mexico. Topaz rhyolites are characteristically enriched in fluorine (>0.2 wt%) and contain topaz crystallize during post-magmatic vapor-phase alteration. In the United States, their ages span much of the Cenozoic Era (50 to 0.06 Ma). Their emplacement followed or was contemporaneous with calc-alkaline and basaltic magmatism in the Basin and Range province, along the Rio Grande rift, and in Montana, and coincided with episodes of extensional tectonism in these regions. Nearly all topaz rhyolites extruded as small, endogenous lava domes with or without lava flows; no topaz-bearing ash-flow tuffs have yet been identified with certainty in the western United States. Most domes are underlain by a precursory blanket of non-welded tephra. A few are small, shallowly emplaced intrusive plugs. Volumes of rock (<1 to 100 km 3 ) in individual complexes composed of 1 to many separate extrusions suggest that the lavas were erupted from small to medium sized magma bodies. In addition to topaz, these rhyolites also contain garnet, bixbyite, pseudobrookite, hematite, and fluorite in cavities or in their devitrified groundmasses. All of these phases may form during vapor-phase crystallization. Magmatic phenocrysts include sanidine (ca. Or 50 ), quartz, sodic plagioclase (usually oligoclase), and F- and Fe-rich biotite in order of usual abundance. Fe-rich hornblende or clinopyroxene occur in a few lavas. Common magmatic accessory minerals include magnetite, ilmenite, zircon, apatite, allanite, and fluorite. Titanite and REE-rich phosphates have been identified in a few lavas. The rhyolites crystallized over a wide temperature interval (850 to 600°C, with most at the lower end of this range) and at variable oxygen fugacities. Titanite-bearing lavas crystallized above the NNO buffer under oxidizing conditions. Most others appear to have crystallized near the QFM oxygen buffer. For individual complexes, temperatures correlate negatively with F-content. All topaz rhyolites are high-SiO 2 rhyolites with elevated F, Na, K, Fe/Mg and low Ti, Mg, Ca, and P. Samples with F concentrations of about 1% have notably lower Si and higher Al and Na than other topaz rhyolite glasses. Most glasses from topaz rhyolites are metaluminous, but many appear to be slightly peraluminous. Fluorine concentrations in glasses range from slightly less than 0.2 to more than 1.0 wt%, and F/Cl ratios are high (3 to 10) compared to F-rich peralkaline glasses (<3). Topaz rhyolites are characteristically enriched in incompatible lithophile elements (Rb, U, Th, Ta, Nb, Y, Be, Li, and Cs). Elements compatible in feldspars (Sr, Eu, Ba), ferromagnesian minerals (Ti, Co, Ni, Cr), and zircon (Zr, Hf) are depleted. The REE patterns of most topaz rhyolites are almost flat (La/Yb N = 1 to 3) and have pronounced negative Eu anomalies (Eu/Eu* = 0.01 to 0.02). Both of these parameters decrease with differentiation as indicated by increasing F, U, Cs, and other incompatible elements. Titanite-bearing rhyolites have prominent middle REE depletions. Initial Sr-isotope ratios range from 0.705 to over 0.710. Geochemical trends at individual complexes are interpreted as arising from fractional crystallization of an initially more “mafic” rhyolite with about 0.2% fluorine. Extensive fractionation of sanidine, quartz, plagioclase, biotite, and Fe-Ti oxides (in proportions consistent with their modes) produced much of the trace element patterns. Zircon, apatite, and a REE-rich phase (allanite, monazite, or titanite) were minor but important fractionating phases. No liquid-state fractionation is required to explain the geochemical trends. The high F content and F/Cl ratios of topaz rhyolite melts may have modified phase relations so as to produce Na and Al enrichments for highly evolved magmas. Topaz rhyolites are intimately related to economic deposits of lithophile elements (i.e. Be, U, F, Li, and Sn). The volcanic rocks were probably ore- and, in some cases, fluid-sources for these mineral deposits. In their age, tectonic setting, mineralogy, chemistry, and style of emplacement, topaz rhyolites bear resemblance to the rhyolitic stocks associated with Climax-type Mo deposits, and some may be surface manifestations of such deposits. In their chemical composition and mineralogy, topaz rhyolites are distinct from both peralkaline rhyolites and calc-alkaline rhyolites with which they may be spatially and temporally associated. Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the relative effects of F and Cl, on melt structure and phase relationships in their parental magmas. The F/Cl ratios of the melt or its source may determine the alumina saturation of the magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, and Eu, and higher F, Rb, U, and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites. Topaz rhyolites are similar to other aluminous rhyolites erupted in bimodal associations with basalt in the western United States. They may be the equivalent of the topaz-bearing ongonites of central Asia. Topaz rhyolites from the western United States are not the eruptive equivalents of S-type granites; we liken them to the highly evolved, non-peralkaline, and F-rich anorogenic granites. Topaz rhyolites appear to have evolved from partial melts of a residual felsic granulite source in the lower or middle crust of the Precambrian continent. According to the proposed model, the passage of contemporaneous mafic magmas through the crust produced necessarily small volumes of partial melts as a result of the decomposition of small amounts of F-rich biotite that persisted in a high-grade metamorphic protolith. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas. Subsequent fractionation led to their extreme trace element characteristics.