Bingham Canyon: The largest man-made pit

“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. 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. 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.“ (Source)

In September 2022, Rio Tinto approved a $55 million USD investment to start underground mining in 2023, initially focussing on an area known as the Lower Commercial Skarn (LCS) with a Resource of 7.5 million t @ 1.9% copper, 0.84 g/t gold, 11.26 g/t silver, and 0.015% molybdenum and a Probable Reserve of 1.7 million t @ 1.9% copper, 0.71 g/t gold, 10.07 g/t silver, and 0.044% molybdenum.

In September 2009, Rio Tinto announced this “massive molybdenum discovery beneath the floor of the open pit mine that could rival Colorado’s Climax and Henderson, two of the world’s biggest molybdenum deposits. The find is extraordinary because Bingham Canyon has been operating for more than a century and, while the main deposit still has legs, no one would have predicted that the world’s next big metal discovery would occur in such a mature environment. The other fascinating aspect of the news is that the discovery was actually made in 1970, but only became clear when Kennecott Utah Copper, a wholly-owned subsidiary of Rio Tinto, applied modern data processing and visualization techniques to historical drill logs… By 2004, molybdenum was becoming increasingly sought after for its ability to enhance the hardness, toughness, and corrosion resistance of metal alloys, particularly stainless steel. The price had increased 8-fold since the early 2000s to more than US$40 per pound and, as a result, Kennecott decided to revisit old drill logs that recorded molybdenum as part of a brownfields exploration program at Bingham Canyon.“

Today, the molybdenum price trades at $74 USD per kg ($34 USD per pound), down from $96 USD/kg in early March.

“The fresh rendering of the old drill data inspired the mine exploration team to launch a drill campaign below the floor of the pit, which is already more than kilometre deep and getting deeper everyday. Sixteen holes intersected high-grade molybdenum mineralization within a steeply dipping arcuate body that extends at least 1,000 metres below the pit floor. Based on the drilling so far, the deposit is estimated to contain in the order of 500-600 million tonnes grading 0.1-0.15% molybdenum. Hosted mainly by monzonite and occurring as quart-molybdenite veinlets and as molybdenite disseminations and fracture fillings, the mineralization remains open… Bingham Canyon, though primarily a copper deposit, has always been a significant gold and molybdenum producer… Bingahm Canyon is a classic copper porphyry with a fairly uniform distribution of sulphide mineralisation, mainly chalcopyrite. The existing pit will be worked out by 2019 but Kennecott is evaluating the feasibility of deepening the pit to add an extra 2.83 million tonnes of copper resources and extend the mine’s life from 2019 to 2036. The new discovery is considered a high-grade extension of the rich molybdenum zones with the main deposit. While the average grade of the molybdenum in the open pit reserve is 0.044% Mo, the grade within the new exploration target is up to 3.5 times richer. The mineralization appears to wrap around the barren core of the porphyry copper deposit in the northeast and southeast sectors of the deposit… Locally, the molybdenum mineralization reaches the surface on the eastern side of the pit bottom, where it has contributed to higher than average molybdenum head grades from 2004-2007. Batch flotation testing of composite samples indicated molybdenum recoveries of about 95% in rougher flotation… Companies never know what they might find when they pull out their dusty core logs and map sheets and start digitizing and visualizing the data in 3D. The Bingham Canyon molybdenum deposit may be the first major deposit to be found that way, but it is unlikely to be the last.“ (Source: “Discovery at the Bingham Canyon Mine“, 2009)

According to “Vectors to porphyry Cu-Au mineralization in carbonate wallrocks around the Bingham deposit“ (2021): “The Bingham Canyon porphyry Cu-Mo-Au occurrence (Utah, USA) is a highly productive mineral system with an extensive hydrothermal alteration halo. Such systems likely cause subtle, far-field alteration patterns which may be useful in vectoring toward the system center. This study presents initial results from our investigation into the effects of this hydrothermal system on two laterally extensive carbonate units, the Jordan and Commercial limestones… Bingham Canyon comprises a multi-phase sequence of Eocene-aged igneous rocks emplaced within a folded and thrusted Paleozoic carbonate and siliciclastic sequence, producing proximal Cu-Mo-Au porphyry-style mineralization and adjacent Cu-Au skarn deposits in carbonate host rocks, mainly in the Jordan and Commercial beds. Outboard from the porphyry-skarn zones (~0.5-2.5km) there are Pb-Zn-Ag vein and carbonate replacement orebodies, and even more distal (~7-9km) sediment-hosted Au deposits… Preliminary analysis reveals a decreasing trend in Fe, Mg, Ba, and several REEs from the Bingham Mine outboard to a distance of ~3-5km, while Mn and Zn display an increase to about 2km distance before dropping off to background levels ~4km outboard.“