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Henderson, Urad |
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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 Urad-Henderson porphyry molybdenum mine, like the Mt Emmons and Climax deposits lies within the Colorado Mineral Belt of central Colorado, USA, ~72 km west of Denver (#Location: 39° 46' 5"N, 105° 51' 1"W).
The 0.2% MoS2 grade contour at Henderson defines the orebody with plan dimensions of the order of 1000 x 700 m and is 180 to 300 m thick. The deposit is associated with the upper Oligocene Red Mountain Intrusive Suite, emplaced episodically over a 5 m.y. period from 28 to 23 Ma. This suite intrudes the Middle Proterozoic (1350 to 1480 Ma) Silver Plume Granite, similar to that at Climax (White, et al., 1981).
The Red Mountain Intrusive Suite comprises eight intrusive units, including from oldest to youngest: i) the Henderson Granite; ii) the Primos Porphyry (made up of a Border, Crowded, Interior and Seriate Phase); iii) Urad Porphyry; iv) Red Mountain Porphyry (made up of the Peak Breccia, Igneo-fragmental rock, Concentric dykes, Red Mountain Porphyry and Rubble Rock Breccia); v) Rhyolite Porphyry of radial dyke swarms; vi) Square Quartz Porphyry; vii) East Knob Unit (comprising a coarse and a fine phase); and viii) the Crowded Quartz Porphyry. Of these the Urad Porphyry, Primos Porphyry and the Henderson Granite are the most closely associated with the Henderson Orebody, with all three serving as hosts to ore. The Urad Porphyry is pre-ore in age and hosts most of the deposit, while the Primos intrusion lies directly below the orebody and is interpreted to have been the principal source of mineralisation. The Henderson Granite contains small, scattered zones of ore-grade mineralisation near its cupola (White, et al., 1981).
The Primos intrusion is a four phase mass, emplaced within the older Urad Porphyry body, comprising from oldest to youngest of the Border, Crowded, Interior and Seriate Phases. Each phase, with the exception of the latest forms, a concentric shell around the next younger and deeper phase. The Primos Border Phase comprises phenocryst-poor aplite with chilled margins and discontinuous intrusion breccias along the contact with the Urad Porphyry. The contact with the inner Crowded Phase is gradational, while the Interior Phase is a cumulophyric porphyry. The lithologies of the Red Mountain Intrusive Suite are similar in composition to those described at Climax above. The Primos intrusion phases has 40 to 50% alkali feldspar, 20 to 25% plagioclase and 30 to 35% quartz, with 0.5 to 5 mm phenocrysts of all three set in a fine (<0.1 to 0.5 mm) matrix (White, et al., 1981).
The smaller and topographically higher Urad Orebody is related to the Square Quartz Porphyry (White, et al., 1981).
More than 90% of the molybdenum mineralisation at Henderson, as at Climax and Mt Emmons, is present in thin (<3 mm thick) moderately to steeply dipping quartz-molybdenite veinlets that form a stockwork. Molybdenite is most commonly concentrated along the veinlet walls, but also occurs in discontinuous layers within quartz veins, or as disseminations throughout quartz veins. Recurrent fracturing and movement during mineralisation is indicated by vein relationships. Minor molybdenite is also present as fracture coatings and as disseminated crystals in aplites, pegmatites, breccias and porphyries. While most stockwork veinlets dip steeply, less abundant, gently dipping veins, locally up to 1 m thick contain clots and rosettes of molybdenite up to cm's across. The paragenesis at Urad from oldest to youngest is barren quartz to stockwork quartz-molybdenite to quartz-molybdenite in gently dipping veins to quartz-pyrite to fluorite-rhodochrosite (White, et al., 1981).
Up to 1100 m of altered rock overlie the Henderson Orebody. Five zones of major rock alteration are recognised, destroying the primary rock forming minerals. These comprise the potassium feldspar; quartz-sericite-pyrite; upper argillic; lower argillic; and propylitic zones. Five minor alteration zones of less areal extent overlap and crosscut the major alteration zones, but still bear a spatial relationship to the ore. These include silicification as either quartz veining in the Vein-silica Zone or nearly total silica metasomatism in the Pervasive Silica Zone; Magnetite-topaz Greisen; and the Garnet Zone (White, et al., 1981).
The Urad-Henderson operation commenced in 1976 as an underground block caving mine. In 1994 some 4.54 Mt of ore were mined at a head grade of 0.3% Mo, containing 11 794 t of Mo.
Published production and reserve figures for the Urad-Henderson orebody include:
260 Mt @ 0.49% Mo (Reserve 1979, start-up 1976, USBM).
160 Mt @ 0.23% Mo (Proved + Probable Reserve, 1994, AME, 1995).
Remaining recoverable reserves at December 31, 2011 (Freeport-McMoRan, 2012):
Proved reserves - 118 Mt @ 0.174% Mo;
Probable reserves - 3 Mt @ 0.171% Mo;
Proved + probable reserves - 121 Mt @ 0.174% Mo.
Remaining recoverable reserves at December 31, 2018 (Freeport-McMoRan, 2018):
Proved + probable reserves - 71 Mt @ 0.17% Mo.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1997.
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.
Henderson
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Carten R B, Geraghty E P, Walker B M, Shannon J R 1988 - Cyclic development of igneous features and their relationship to high-temperature hydrothermal features in the Henderson Porphyry Molybdenum deposit, Colorado: in Econ. Geol. v83 pp 266-296
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Seedorff E, Einaudi M T, 2004 - Henderson Porphyry Molybdenum System, Colorado: II. Decoupling of Introduction and Deposition of Metals during Geochemical Evolution of Hydrothermal Fluids: in Econ. Geol. v99 pp 39-72
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Seedorff E, Einaudi M T, 2004 - Henderson Porphyry Molybdenum System, Colorado: I. Sequence and Abundance of Hydrothermal Mineral Assemblages, Flow Paths of Evolving : in Econ. Geol. v99 pp 3-37
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