Trace Element Distribution in Chalcopyrite-Bearing Porphyry and Skarn Deposits

Abstract

In today's environment, large investments are being made to develop new, green energy sources. These technologies include wind turbines, solar panels, geothermal plants, and many others. Many of these new technologies are created or enhanced with the use of relatively uncommon elements. For example, rare earth element-bearing magnets are used in wind turbines and cadmium-tellurium films are used in the production of photovoltaic cells. Other applications of uncommon elements include catalytic converters, car batteries in hybrid vehicles, and high temperature superconductors. The distribution of uncommon elements is geographically uneven. For example, in 2009, 97% of the world's rare earth production came from China. This makes the rare earths critical elements from a U.S. perspective - elements that are subject to supply disruptions because they are dependent on only one or a few countries. Domestic sources of rare earth and other critical elements should be developed in order to lessen our dependency on foreign export. Much work has been done analyzing gold deposits for trace element association, but little work exists that examines trace elements in copper deposits. The focus of this project is to determine which, if any, trace elements occur within chalcopyrite-bearing deposits, if certain types of copper deposits are more enriched in trace elements than other types, which copper minerals host the trace elements, and if there is any geographical variation in trace elements within the same type of deposit. This project focuses on chalcopyrite-bearing porphyries and skarns due to the widespread nature of the mineral in these deposit types. The key trace elements of interest include tellurium, selenium, gallium, and indium, all of which have potential applications in solar panels. Over the course of this project an analytical error by a commercial laboratory was discovered. The initial geochemical data by ICP-OES determined by the commercial laboratory indicated an anomalously high concentration of tellurium. Iron-tellurium binary plots indicated a near-perfect positive correlation, implying interference. Follow-up analyses by the Denver-based USGS office utilizing an array of analytical techniques (ICP-MS, LA-ICP-MS) failed to reproduce the reported tellurium values. The data indicate that in the presence of high iron there is an iron interference with tellurium when analyzed using ICP-OES. Tellurium occurs as micron-sized inclusions associated with Ag-Bi-Au-Pb and typically hosted within chalcopyrite over other sulfides. Both porphyry and skarn deposits show erratic values, with no discernible preference in type of deposit. Selenium occurs presumably as a lattice substitution into the sulfur site of all sulfide phases but generally is preferentially enriched in chalcopyrite. Other sulfide phases as well as secondary copper phases do show selenium enrichment, but selenium in these phases typically is lower and more erratic than in chalcopyrite. The Yerington and Bingham porphyry deposits report the highest selenium values, with lower values from Grasberg, and none above the detection limit reported from Chuquicamata. Gallium was not identified in sulfide phases in any of the deposits, instead occurring as a lattice substitution into the iron site of magnetite. Indium shows a preferential enrichment in skarn deposits over porphyries. Indium values generally follow silver values, and both are more elevated and more consistent in chalcopyrite from skarns versus porphyry deposits. The Pumpkin Hollow deposit, Lyon County, Nevada, is a series of blind IOCG/skarn deposits genetically related to the emplacement of the Jurassic Yerington batholith. The primary host rocks in the region are the Triassic Mason Valley limestones and the Triassic Gardnerville Formation argillites, limestones, and shales. The sulfide mineralogy observed at Pumpkin Hollow consists of chalcopyrite, pyrite, pyrrhotite, and minor sphalerite, each with an associated trace element suite. The tellurium occurs as separate microscopic mineral inclusions within the chalcopyrite, the selenium as a substitution into all of the sulfides,; the gallium as a substitution into magnetite, and the indium as a substitution in the chalcopyrite.