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Book Chapter

Geology and Geochemistry of Deep Molybdenum Mineralization at the Bingham Canyon Mine, Utah, USA

By
Gerry Austin
Gerry Austin
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Geoff Ballantyne
Geoff Ballantyne
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Published:
January 01, 2010

Abstract

The molybdenum grade of the ore mined from the Bingham Canyon porphyry Cu-Mo-Au deposit has historically been about 0.05% Mo. Molybdenum has been an important byproduct since 1936 when the first molybdenite recovery plants were constructed. Recent drilling has provided a clearer picture of the distribution of molybdenum at depth. An inverted-cup shaped zone, or “shell”, of >0.09% Mo mineralization is present. The molybdenum shell overlapped part of the >0.35% Cu shell within the portion of the deposit that has already been mined. The molybdenum shell has a lower center of mass and a smaller diameter than the copper shell, and below the bottom of the current pit, the Mo and Cu shells show little spatial overlap.

Most of the deep molybdenum mineralization occurs in quartz-molybdenite veins, with a lesser amount occurring as molybdenite-only veins. The quartz-molybdenite veins typically contain rare pyrite and/or chalcopyrite grains, and below detection levels of gold and silver. The veins lack obvious alteration halos. Whereas quartz veins containing significant chalcopyrite and bornite are more abundant in earlier intrusions than in later intrusive phases, the molybdenite-rich veins cut all intrusive phases; they therefore appear to be younger than most or all of the copper mineralization, a finding consistent with previously published Re-Os ages. Narrow, transparent, quartz-molybdenite veins, 1 to 4 mm in width, with high concentrations of molybdenite, cut more common, wider, milky-white veins.

Because the pit has mined the top of the inverted-cup shaped shell, the remaining molybdenum-rich zone takes the form of a thin walled annular cylinder. Only part of that cylinder has been drilled out to date. Based on blast hole sampling, the >0.09% Mo zone is a maximum of 200 meters in width. At its outer edge molybdenum grade decreases abruptly from 0.2% to less than 0.05% Mo over a distance of about 35 meters.

Molybdenite within the Bingham deposit contains elevated levels of rhenium. If the rhenium is assumed to occur entirely within molybdenite, rhenium contents for molybdenite, based on drill core geochemistry, are 308 ppm in an upper Cu-Mo-Au domain (Mo >0.05%, Cu >0.35%), 117 ppm in a deep, molybdenum-rich and copper-poor domain (Mo >0.05%, Cu <0.35%), and 135 ppm in the barren core (Mo <0.05%, Cu <0.35%).

Although several lines of evidence indicate that most of the molybdenum mineralization is younger than most or all of the copper mineralization, the copper and molybdenum ore fluids appear to have ascended the same zone of permeability. The conditions under which molybdenite precipitated are not yet well understood but the remarkably sharp outer boundary of the higher-grade molybdenum mineralization suggests that the molybdenum may have been transported by a plume of buoyant fluid that was ascending rapidly through significantly cooler fluids. That plume appears to have had a diameter of approximately 1.8 km, and been generally coincident with the boundaries of the composite Bingham Stock.

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Contents

Society of Economic Geologists Guidebook Series

Tops and Bottoms of Porphyry Copper Deposits: The Bingham and Southwest Tintic Districts, Utah

Ken Krahulec
Ken Krahulec
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Kim Schroeder
Kim Schroeder
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Society of Economic Geologists
Volume
41
ISBN electronic:
9781934969946
Publication date:
January 01, 2010

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