Fresh rendering of old drill data inspires deeper drill campaign and leads to new discovery at Rio Tinto’s Bingham Canyon mine
It’s hard to imagine a better poster child for the practice of marrying historical data with modern processing technology than the recent molybdenum discovery at the Bingham Canyon copper mine in Utah.
In late September, Rio Tinto announced the discovery of 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.
“We have put tremendous effort put into converting our geologic and drillhole data into a digital format,” says Gerry Austin, Superintendent of Geology for the Bingham Canyon mine. “Most of the effort was completed in the 1990s but it was not until the early 2000s that computer speed and advancements in 3D software allowed geologists to view deposits in 3D space.”
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.
“It was the ability to spin and view the deposit in any direction that highlighted the two 1970 drillholes that had intercepted the top part of the mineralized zone,” says Austin.
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.
“We expect to see more as we explore deeper and laterally,” says Austin.
Bingham Canyon, though primarily a copper deposit, has always been a significant gold and molybdenum producer. In 2008 the company announced plans to build a US$270 million facility to improve molybdenum recoveries. The proposed project is expected to increase Bingham Canyon’s molybdenum production by 15% over the 2008 rate of 10,600 tonnes (23.4 million pounds).
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.
“My initial thought is that the moly is geologically controlled within favourable horizons,” says Austin, though he says his team needs more time to study the new exploration target to determine how it fits with Bingham Canyon’s current geological model.
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.
Austin says further exploration of the giant new deposit and potential similar deposits in the area are on hold while management determines its exploration priorities. The price of molybdenum has dropped to less than US$15 per pound since the exploration commenced in 2004 and the whole Rio Tinto group has slashed exploration for both greenfield and brownfield projects to US$115 million this year from US$235 million in 2008. Expenditures will remain flat in 2010 as Rio Tinto awaits a global recovery.
And there are, as yet, no plans to develop the new deposit.
“While it is likely that underground development could access both the deep molybdenum body and deep porphyry copper and skarn mineralization, the feasibility of developing such a mine has not been established,” Rio Tinto states in a fact sheet on its web site.
Still, it seems a sure bet that Rio Tinto will develop the giant resource as molybdenum prices improve in step with the global recovery and consequent increased demand for stainless steel. Molybdenum price forecasts from producers are calling for US$11 per lb average price for 2009, down from US$29 in 2008, but up from the first-half average of US$9, according to purchasing.com, a free on-line magazine that provides the latest market forecasts, supply chain tactics, and technologies of goods purchasers.
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.