Exploration and geology of the Mt. Emmons molybdenite deposits, Gunnison County, Colorado

Abstract

During the period 1970 to 1976, three stockwork molybdenite deposits were discovered beneath Mt. Emmons, a prominent peak of the Elk Mountains in the central portion of the Colorado mineral belt. Two of the deposits are below Redwell Basin on the northwestern side of the mountain and the third, and richest, deposit is under Red Lady Basin on the southeast side of Mt. Emmons.The uppermost molybdenite deposit in Redwell Basin is at a depth of about 2,400 ft, where it is associated with a small cupola of rhyolite porphyry. The cupola is expressed at the surface by a composite breccia pipe which appears to have been fed through a zone of crackled hornfels lying between the base of the pipe and the apex of the cupola. Rhyolite porphyry in the cupola grades downward into granite porphyry of the Redwell stock. The lower molybdenite deposit is about 1,000 ft below the west-dipping contact of the stock.The molybdenite deposit under Red Lady Basin is a contact-related stockwork draped over the granite porphyry Red Lady stock, whose apex is about 1,400 ft beneath the surface. This deposit, referred to as the Mt. Emmons deposit, contains about 155 million short tons of altered hornfels and porphyry with an average grade of 0.44 percent MoS 2 Both the Redwell and Red Lady stocks are compositionally granites; both have the same range of porphyritic textures and have K-Ar ages that group between 16 and 18 m.y. Because of the above similarities, the stocks are thought to have risen out of a common igneous source.Intruded rocks include carbonaceous shale and sandstone beds of Cretaceous age which have been metamorphosed along the contacts of the stocks to a pyrite-pyrrhotite-bearing hornfels. The hornfels zone surrounding the two stocks includes an innermost 1,000-ft-thick facies of brown hornfels which grades outward to a 500-ft-thick facies of black hornfels.Pyrrhotite, which is a characteristic mineral of the contact metamorphic halos surrounding the two stocks, is depleted in the phyllic zones above the Mt. Emmons and upper Redwell deposits.Hydrothermal effects observed in the rocks above the Redwell cupola include, from the periphery inward, mineral assemblages characteristic of propylitic, phyllic, and possibly potassic alteration. The Red Lady stock, 3,000 ft to the southeast, also represents a center of hydrothermal activity which includes an outer zone of propylitically altered rocks that grade inward through phyllically altered rocks (pervasive quartz-sericite-pyrite) to a small zone of K-feldspathized rock in the apex of the stock. The K-feldspathized zone is veined and pervasively flooded by quartz, magnetite, and biotite.Widely spaced veins and veinlets in the prophylitic zones surrounding the Redwell and Red Lady stocks contain various proportions of quartz, pyrite, pyrrhotite, epidote, chlorite, calcite, and base metals. These veinlets are transitional into, or are later than, the more closely spaced veinlets of the phyllic zones which are typically filled with quartz, pyrite, sericite, and fluorite. As the distance to the Redwell cupola and Red Lady stock decreases, phyllic zone veinlets become depleted in pyrite and increasingly enriched in molybdenite, fluorite, orthoclase, and huebnerite.Several of the faults mapped on Mt. Emmons are mineralized with base metals, and a few contain rhyolite or intrusive breccia, but none are filled with molybdenite or porphyritic phases of the underlying stocks. These observations imply that the faults originated after emplacement of the stocks and before or contemporaneous with the period of base metal mineralization.An intrusive breccia dike in Red Lady Basin and an intrusive breccia phase of the pipe in Redwell Basin contain at the surface sparse fragments of molybdenite-mineralized rhyolite porphyry carried 1,500 to 2,700 ft upward from underlying deposits. The intrusive breccias are clearly later than the periods of emplacement of the granite porphyry stocks and associated molybdenite mineralization and provide the only direct evidence at the surface of porphyry-molybdenite mineralization at Mt. Emmons.