Character and Origin of Climax-Type Molybdenum Deposits

Abstract

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 K2O > Na2O. 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.