University of Nevada, Reno Trace Element Distribution in Chalcopyrite
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University of Nevada, Reno Trace Element Distribution in Chalcopyrite-Bearing Porphyry and Skarn Deposits A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Reid I. Yano Dr. Tommy B. Thompson/ Thesis Advisor August, 2012 Copyright by Reid I. Yano 2012 All Rights Reserved THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by REID I. YANO entitled Trace Element Distribution In Chalcopyrite-Bearing Porphyry And Skarn Deposits be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Tommy Thompson, Ph.D., Advisor Jonathan Price, Ph.D., Committee Member Thom Seal, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Dean, Graduate School August, 2012 i 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 ii 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. iii Acknowledgements This thesis would not have been possible without the tireless support and guidance of Dr. Jonathan Price. I am also deeply indebted to Dr. Tommy Thompson for his guidance through the course of my graduate career. Furthermore, this project would not have been completed if not for the support and assistance of Poul Emsbo, Alan Koenig, and the other USGS staff; Hank Ohlin and Greg French of Nevada Copper for allowing me unrestricted access to samples and geochemical data and providing housing during my field work in Yerington; Terri Garside (formerly) of the NBMG, and the UNR College of Sciences for funding my thesis. I would also like to thank my classmates both within the CREG program and the geology department as a whole. I also want to thank my parents for instilling in me a love for the outdoors at an early age, and my brother and sister for always being there. Finally, this paper is dedicated to Brigid Doran, without whom I would not have made it to or through grad school. Thank you for believing in me. iv Table of Contents Introduction . 1 Focus of Project . 3 Trace Elements . 8 Tellurium . 9 Selenium . 10 Gallium . 11 Indium . 12 Analytical Discrepancies . 14 Distribution of Trace Elements . 17 Bingham Canyon, Utah, USA . 18 Chuquicamata, Antofagasta Province, Chile . 20 Yerington, Nevada, USA . 22 Grasberg, Irian Jaya, Indonesia . 24 Cananea District, Sonora, Mexico . 27 Republic, Arizona, USA . 31 Outokumpu, Finland . 33 Case Study – Pumpkin Hollow . 35 Regional Geology . 36 Deposit Geology . 37 Sulfide Mineralogy . 39 Trace Element Package . 44 Comparison . 46 Conclusions . 50 v Reference . 55 Appendix . 57 vi List of Tables Table 1: Global production of critical elements in 2009 . 6 Table 2: Summary of trace elements from spot analyses . 48 Table 3: Summary of trace elements from bulk rock analyses . 48 vii List of Figures Figure 1: Crustal abundance versus price . 5 Figure 2: Periodic table with critical elements . 13 Figure 3: Comparison plots of tellurium versus iron . 16 Figure 4: Comparison of element composition by analytical technique . 16 Figure 5: Pumpkin Hollow deposit map . 38 Figure 6: Chalcopyrite photomicrographs . 41 Figure 7: Pyrite photomicrographs . 43 Figure 8: Magnetite photomicrograph and SEM image . 44 Figure 9: SEM images of inclusions . 45 Figure 10: Selenium frequency plots by phase . 52 Figure 11: Bulk rock binary plots of bismuth versus tellurium . 52 1 Introduction In today’s political environment, the mining industry has become an international business. As new technologies are developed and produced, more and more resources are necessary to maintain a steady supply. One of the fastest-growing industries is in the field of green energy, due in no small part to the recent push throughout much of the world to reduce greenhouse gas emissions. Many of these new green energy sources (such as solar, wind, and geothermal) require a large amount of resources that historically had little use or demand. Due to the lack of historical use, relatively little is known about occurrence or distribution of such elements as opposed to the base or precious metals. This project came about as a result of the increasing demand for such elements, and the need for a better understanding of how these particular elements are distributed within copper systems. The University of Nevada, Reno (UNR) is fortunate to possess the Mackay-Stanford Ore Deposits Collection, donated to UNR by Stanford University after the closure of Stanford’s economic geology program. This collection provides an excellent opportunity to study a wide number of deposits from across the world, including many classic deposits that were studied by Stanford professors and students over the last 150 years. Utilizing this collection, as well as other ore samples collected by myself and Mackay School of Earth Sciences and Engineering colleagues, splits were taken from larger specimens (preserving a specimen in the collection for future examination and analysis) and analyzed by colleagues at the USGS Denver office to determine, primarily using their newly developed laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique (Wilson et al., 2002), what trace elements are present and in which mineral they reside. A field study, involving sampling of core from the Pumpkin Hollow IOCG/skarn deposit, provides an excellent comparison to skarn and porphyry-copper 2 samples selected from the Mackay-Stanford collection. Finally, comparisons of trace elements are being made to determine which deposit types preferentially host which trace elements, and if there are variations within deposit types. 3 Focus of Project This project is part of a joint effort between the USGS, the Nevada Bureau of Mines and Geology, and various other academic and government agencies. The overall goal of this project is to identify trace element distributions in various ore deposits in order to lower the United States’ dependence on foreign import of critical elements. The elements of interest are those which are rare in the earth’s crust, are vital in the development of both green energy sources and military applications, and are at risk of restricted supply.