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).

Size and composition (bulk metal ratios) of magmatic hydrothermal mineral deposits are affected by a number of chemical and physical processes including the nature of the source region and mode of emplacement. At shallow levels, rising plumes of vapour bubbles + melt, and the advection of water through interconnected vapour bubbles, allows access of the magmatic aqueous phase to the upper reaches of a magma chamber. These processes are operative at shallow levels where low water solubility and high molar volume for water make these processes more efficient. Partitioning experiments suggest that oxygen fugacity-dependent crystal/melt partitioning of ore metals leads to different efficiencies of removal of Cu, W, and Mo from silicate melts into ore-forming aqueous fluids. For example, the Mo/W ratio in magmatic hydrothermal deposits should increase as the oxygen fugacity of the magma increases. Further, Cu should behave as a crystal-compatible element in H 2 O-undersaturated, sulfide-saturated felsic magmas with f O 2 < NNO + 1 due to the strong partitioning of Cu from the melt into pyrrhotite. Cycling of oxidised, hydrated, sulfidised and Cl-enriched oceanic crust into mantle can give rise to magmas that contain S but are oxidised (≥NNO). The combination of high oxidation state, relatively hydrous but shallow conditions and a high C1/H 2 O ratio leads to saturation with respect to H 2 O early during crystallisation, and loss of a large proportion of magmatic Cu to the aqueous phase. Ores formed from these oxidised magmas also possess high Mo/W ratios due to the effect of oxygen fugacity on the sequestering of Mo vs W. In less oxidised magmas, Cu and Mo are partitioned into sulfides and Ti-bearing phases, respectively, resulting in lower efficiencies of removal of Cu and Mo from melts into aqueous fluids. Further, the partitioning of W into crystallising phases is reduced , producing a more efficient removal of W into ore-forming fluids. This ultimately leads to mineral deposits with higher W/(Mo + Cu) ratios relative to deposits associated with more oxidised systems. Silicic, high-F magmas with f O 2 ≈ NNO can be found in tensional environments (e.g. rocks associated with the Climax-type deposits of the Colorado Mineral Belt). High HF/H 2 O activity ratios in the source regions yield melts that evolve an aqueous phase late during crystallisation, leading to relatively low ratios of compatible/incompatible elements in the melt at H 2 O saturation.