GEO-CM: Trace metals in sphalerite from polymetallic, porphyry-lode mineral deposits of southwest Montana

Montana Technological University, Butte MT

Investigators

Abstract

Sphalerite is the primary ore mineral of zinc, and it oftens contains elevated concentrations of other metals as impurities, many of which are designated as “critical minerals” by the U.S. Geological Survey. Critical minerals include metals that are important to the country’s economy and security, but must currently be imported from other countries. Mines in Butte, Montana are rich in copper, silver, lead, zinc and manganese. The Philipsburg mining district is located 60 km northwest of Butte is smaller but shares many geologic features. Preliminary analyses of sphalerite samples from Butte and Philipsburg show that they are both highly enriched in the critical metals gallium, indium, germanium, and tungsten, as well as copper and silver. Recent research has shown that sphalerite that is richest in these metal impurities fluoresces bright red under a hand-held UV lamp. This phenomenon could provide a low-cost and simple way to sort sphalerite ores with different concentrations of trace metals. The first goal of this study is to analyze samples from Butte and Philipsburg to map out the distribution of metal impurities in sphalerite at the scale of each mining district (several square miles by up to 1 mile deep). The second objective is to make sphalerite grains in the laboratory that contain the same trace metals that are found in the natural specimens. The researchers will then compare the fluorescence behavior of the man-made and natural specimens. This project will train undergraduate and graduate students in the field of economic geology at Montana Technological University (MTU). MTU is a primarily undergraduate institution (PUI) in an EPSCoR state. Results from this project may reveal new ways that sphalerite and other ore minerals can be used to study mineral deposits. This research may also have applications to the use of luminescent sphalerite in materials science and engineering. Butte, Montana is the type example of a zoned, Cordilleran-style, porphyry-lode deposit, as well as being one of the nation’s top historic producers of Cu, Zn, Ag, and Mn. This study will take advantage of the Anaconda Research Lab (ARL) Collection, a set of over 50,000 hand samples collected from Butte and other mines in Montana that is archived at Montana Tech campus. Using the ARL Collection, as well as new samples from the active Continental porphyry Cu-Mo mine, the researchers will assemble a suite of > 200 polished ore specimens containing sphalerite and other sulfide minerals from known locations across the entire Butte district, to depths of up to 5000 ft. below the surface. The sphalerite samples will be analyzed by optical microscopy, photoluminescence spectroscopy (PLS), Raman spectroscopy, laser ablation ICP-MS, electron probe microanalysis (EPMA) and hyperspectral cathodoluminescence (HP-CL) to reveal spatial and temporal patterns in trace element distribution, from the micron scale to the km scale. Machine learning tools will be used to identify correlations between elements in the sphalerite database that will help illuminate the underlying mechanisms by which trace elements enter the ZnS structure. For example, the researchers hypothesize that the deep red fluorescence mentioned above is linked to coupled substitution of Cu(+1), Ga(+3), and W(+6) for Zn(+2) in the sphalerite lattice. They will synthesize sphalerite of known trace element compositions using hydrothermal methods and compare the HP-CL, Raman, and PLS signatures of the synthetic and natural specimens. Finally, they will establish collaboration with a synchrotron laboratory capable of using XANES to verify the oxidation state of tungsten and other trace elements in their metal-doped ZnS and natural sphalerite samples. This project will support cutting-edge research and graduate-student training in economic geology at Montana Technological University, formerly the Montana School of Mines, a primarily undergraduate institution (PUI) in an EPSCoR state. Results from this project may reveal new ways that sphalerite and other ore minerals can be used to study mineral deposits, and may have applications to the use of luminescent ZnS in materials science and engineering. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.