Geology and Geochemistry of Deep Molybdenum Mineralization at the Bingham Canyon Mine, Utah, USA
Gerry Austin, Geoff Ballantyne, 2010. "Geology and Geochemistry of Deep Molybdenum Mineralization at the Bingham Canyon Mine, Utah, USA", Tops and Bottoms of Porphyry Copper Deposits: The Bingham and Southwest Tintic Districts, Utah, Ken Krahulec, Kim Schroeder
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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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Tops and Bottoms of Porphyry Copper Deposits: The Bingham and Southwest Tintic Districts, Utah
A comprehensive look at the closely spaced blast hole samples, exploration drill data, and a resource model interpolated from diamond exploration drilling data show that the distinct molar tooth shape of the Bingham Canyon orebody has strong lithologic controls at depth. At and below the level of current mining Cu occurs in the Bingham stock as a ring approximately 4000 feet in diameter, centered on a barren core located at the eastern end of the quartz monzonite porphyry (QMP). A smaller ring of Cu (about 2500 feet in diameter) is centered on the southwest end of the QMP. The mineralized cylinders are broken where chemical and physical differences between the mineralized intrusive rocks and the barren quartzites on the south and west sides of the Bingham stock cause gaps in the regular Cu distribution, leading to the distinctive root zones of the Bingham porphyry system. These root zones are artifacts of favorable host rock and not products of multiple mineralization centers or structural control.
Blasthole and diamond drill hole assay data show broadly continuous grade through monzonite (MZ), QMP and latite porphyry (LP) dikes, though locally grade changes occur across LP contacts. A drop in grade within quartz latite porphyry (QLP) dikes is common. The overall consistency of grade across intrusive contacts including the LP suggests that the main pulse of mineralization, associated with the QMP, spanned the period of subsequent LP intrusion. Mineralization had notably waned by the time of QLP emplacement, leading to lower Cu grades in the QLP throughout the system.