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Mt Emmons |
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Colorado, USA |
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Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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the Mount Emmons porphyry molybdenum deposit is located in the central section of the Colorado Mineral Belt in Colorado, 200 km SW of Denver, Colorado, western USA (#Location: 38° 53' 18"N, 107° 3' 23"W).
Mineralisation at Mt Emmons is associated with the composite 17 Ma Miocene Mt Emmons Stock of overall rhyolitic composition. It occurs in the upper sections of the stock and within intruded hornfelsed sandstone and shale of the Mesaverde Formation and Mancos Shale respectively, both of Cretaceous age. All are overlain by post-ore Tertiary sandstones.
The Mt Emmons Stock has a plan diameter of approximately 480 m. It is made up of at least two plugs, the Keystone and the Red Lady, both of which are near vertical and cylindrical. The upper portion of the Keystone Plug is composed of a series of caps, comprising an upper border phase, an underlying aplitic and below that a porphyritic phase.
The orebody, as defined by the 0.2% Mo cut-off, is 'domal' in shape, capping the Mt Emmons Stock. It has a roughly circular plan outline, with a diameter of 660 m, is around 90 m thick, and buried at a depth of 300 m. Mineralisation is contained within fractures and is controlled by the geometry of the stock, with intense propylitisation and potassic alteration. Magnetite is abundant below the orebody. The principal sulphide is molybdenite, with traces of galena, hubnerite, cassiterite and sphalerite (USBM database; White, et al., 1981).
The main Mt Emmons deposit is about 400 m below surface and has a resource of 155 Mt @ 0.44% MoS2
Some 500 m to the NNW of the Mt Emmons Stock, the Redwell Breccia Pipe is developed. This pipe is approximately 300 x 450 m in plan, but broadens upwards and narrows downward. It extends from the surface to a depth of 600 m where it fades out in the upper reaches of a Miocene stock. The Miocene granite porphyry stock is capped by a series of layers, grading up through rhyolite-porphyry and rhyolite, with an adjacent skin of hornfels in the Cretaceous sediments. The breccia is multiphase, made up of several components, commencing with an early intrusion breccia related to a quartz porphyry, which now fringes the breccia pipe, but emanates from the stock below. This was followed by several explosion breccias with angular to rounded clasts, and matrices ranging in the different stages from fine clastic to ashy igneous material. The last breccia was post-ore as it includes ore clasts. Molybdenum mineralisation appears to be largely within the stock, with one zone concentrated in the roots of the breccia pipe as shown on plan Figure 99 (White, et al., 1981).
The most recent source geological information used to prepare this decription was dated: 1982.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Mt Emmons
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Thomas J A, Galey J T 1982 - Exploration and geology of the Mt Emmons Molybdenite deposits, Gunnison County, Colorado: in Econ. Geol. v77 pp 1085-1104
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