Abstract 0 Road log begins at the Keystone Lodge in Keystone, Colorado. North of the hotel, Proterozoic granite and felsic gneiss are exposed on the upper elevations of the un-named mountain. The lower slopes of the mountain consist of Pleistocene till that overlies the Precambrian rocks. Drive west on U. S. Highway 6 to the intersection with Interstate 70 (exit 205). 0.6 North of the highway, the Williams Range thrust fault that placed Proterozoic rocks over Cretaceous shale and sandstone is exposed just north of the borrow pit (situated above the shale cliff). The trace of the fault south is covered by Quaternary rocks in the Snake River area, is then exposed along the lower slopes of Keystone Mountain ( Widmann et al., 2003 ). Shale and sandstone of Cretaceous age crop out adjacent the highway to Dillon and form the northern slope of Swan Mountain (south of the Snake River Arm of the reservoir) and the peninsula to the west ( Kellogg et al., 2008 ). 1.4 Highway crosses the Snake River. 2.4 Snake River. Pierre Shale (Cretaceous) forms the dark gray outcrops just north of the highway. 4.0 Dillon Reservoir. The west portal of the 21-mile long Harold Roberts Tunnel is located on the end of the peninsula west of here. The tunnel transfers water beneath the continental divide into the South Platte River which flows through Denver. Eastward, the Williams Range thrust is concealed beneath Quaternary units near the power transmission line on the bottom slopes of Tenderfoot Mountain (11,441 feet).

Abstract This field trip will visit a modern, large-scale underground block cave mining operation at the world-class Urad-Henderson porphyry molybdenum deposits on and beneath Red Mountain, in the historic Dailey-Jones Pass mining districts, Clear Creek County, Colorado. The underground tour summarizes the Henderson deposit geology and the current status of mining operations, and offers the opportunity to examine and collect rock specimens. The surface tour summarizes the regional and local geologic and structural setting of the deposits, and surface features that define and characterize the outer, peripheral parts of the intrusive-hydrothermal system. The mine is located in the northern Colorado Mineral Belt, in the Front Range of the Rocky Mountains, ∼75 km west of Denver. The deposits consist of molybdenite-bearing, quartz vein stockworks at the cupola apices of highly evolved, silica-rich, subalkaline, leucorhyolite/leucogranite porphyry stocks. The system formed over ∼3.0 m.y. between ca. 27 and 30 Ma by at least 23 intrusive events. Emplacement of the Red Mountain intrusive center and a second intrusive center at Woods Mountain may have been controlled by the NNE-trending Berthoud Pass–Vasquez Pass structural zone, a major Laramide-reactivated Precambrian shear/fault zone. A peripheral, 7.5 × 12.0 km, NNE-elongated, elliptical, pervasive chlorite alteration zone contains a well-developed system of radial quartz and base-precious metal veins.

This guidebook provides copies of the key previously published papers of Climax, Henderson and Leadville districts. This is a departure from the traditional SEG. Guidebook format in that it does not contain new papers of the deposits. The Climax and Henderson porphyry Mo and Leadville district carbonate-hosted manto deposits are within the Colorado Mineral Belt, a mineralized belt that contains the mostproductive porphyry molybdenum and some of the largest base- and precious-metal manto deposits in the world. Since the discovery of molybdenum-bearing stockwork veinlets at Climax in 1879, the Urad orebody in 1914, and Henderson in 1964, collectively these mines have produced over 2.9 billion pounds of Mo. In late 2009, Henderson produced its billionth pound of molybdenum and in 2012 Climax commissioned a new concentrator and is now producing ore from an open pit. Significant published reserves remain and for both Climax and Henderson that total 1 billion pounds of molybdenum metal. The Leadville district is noted for its long history of production having produced 3.1 million ounces of gold, 260 million ounces of silver and significant base metals since its discovery in 1860, research on carbonate-hosted Ag-Zn-Pb-(Au) deposits, and the founding of the Guggenheim mining fortune, including the formation of ASARCO, Inc.

This guidebook provides copies of the key previously published papers of Climax, Henderson and Leadville districts. This is a departure from the traditional SEG. Guidebook format in that it does not contain new papers of the deposits. The Climax and Henderson porphyry Mo and Leadville district carbonate-hosted manto deposits are within the Colorado Mineral Belt, a mineralized belt that contains the mostproductive porphyry molybdenum and some of the largest base- and precious-metal manto deposits in the world. Since the discovery of molybdenum-bearing stockwork veinlets at Climax in 1879, the Urad orebody in 1914, and Henderson in 1964, collectively these mines have produced over 2.9 billion pounds of Mo. In late 2009, Henderson produced its billionth pound of molybdenum and in 2012 Climax commissioned a new concentrator and is now producing ore from an open pit. Significant published reserves remain and for both Climax and Henderson that total 1 billion pounds of molybdenum metal. The Leadville district is noted for its long history of production having produced 3.1 million ounces of gold, 260 million ounces of silver and significant base metals since its discovery in 1860, research on carbonate-hosted Ag-Zn-Pb-(Au) deposits, and the founding of the Guggenheim mining fortune, including the formation of ASARCO, Inc.

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.