GEOLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE MARIGOLD MINE, BATTLE MOUNTAIN-EUREKA TREND, NEVADA by Matthew T. Fithian ! A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date ____________________ Signed: _______________________ Matthew T. Fithian Signed: _______________________ Dr. Elizabeth A. Holley Thesis Advisor Signed: _______________________ Dr. Nigel M. Kelly Thesis Co-Advisor Golden, Colorado Date ____________________ Signed: _______________________ Dr. Paul Santi Department Head Department of Geology and Geological Engineering ! ii! ABSTRACT The Marigold mine is located on the northern end of Nevada’s Battle Mountain-Eureka trend, approximately 55 km east-southeast of Winnemucca, Nevada in the Battle Mountain mining district. Marigold defines a N-S trending cluster of economic gold anomalies approximately 7 km long. Marigold has been historically described as a porphyry-related distal disseminated deposit based on the presence of porphyritic intrusions, proximity to known porphyry systems (e.g. Phoenix, Converse, Elder Creek), inferred high Ag:Au ratio, and limited understanding of sulfide mineralogy related to gold mineralization. The aim of this research was to examine the genesis of the gold mineralizing system at Marigold by determining the age of felsic porphyritic intrusions throughout the Marigold mine and the genetic relationship between these intrusions and gold mineralization. Geochronologic data were supplemented by geochemical sampling to understand the effect of the intrusions on the host rock, the effect of alteration on the intrusions, and the geochemical signature of gold ores. In addition to geochronologic and geochemical data, a secondary goal of the project was to determine the ore mineralogy below the redox boundary. The gold deposits at Marigold formed in Ordovician to Permian meta-sedimentary and sedimentary sequences that were intruded by at least 14 plagioclase-biotite-hornblende phyric quartz monzonite porphyry dikes, three of which crosscut the sedimentary host rock packages in the Basalt, Mackay, and Target II open pits. U-Pb isotopic ages from CA-TIMS analysis of zircon grains show that the intrusions were emplaced in the Late Cretaceous (~97.6-92.2 Ma). Geochemical analysis of quartz monzonite intrusions indicates that increased alteration of dike margins is associated with elevated concentrations of gold and gold pathfinder elements such as arsenic, antimony, and thallium, suggesting the introduction of gold post-dates emplacement of Late Cretaceous intrusions. Petrographic analyses of sulfide-bearing rock suggest that two mineralization events occurred at Marigold: a base metal mineralization event (stannite, gersdorffite, argentiferous tennantite, chalcopyrite, pyrite, sphalerite, chromite) and a gold mineralization event (native gold, pyrite, stibnite). No textural evidence was found to suggest the relative timing between gold and base metal mineralization, although porphyry-related base metal mineralizing systems formed during the late Cretaceous elsewhere in the Battle Mountain mining district. Gold occurs natively ! iii! within quartz veinlets and within As-bearing pyrite in unoxidized rock. Electrum, which is common in distal disseminated Ag-Au deposits, was not documented at Marigold. Apatite fission track ages indicate a heating event occurred in the Eocene and are interpreted to represent the age of gold mineralization. The Ag:Au ratio of gold ores is 0.64, which is typical of Carlin-type deposits but significantly lower than many distal disseminated deposits, which have considerably more silver. The deposits at Marigold have no clear genetic affiliation with a porphyry system and display many characteristics similar to Carlin-type deposits. As a result, Marigold should be considered a Carlin-type deposit predominantly hosted by metasedimentary rocks of the Roberts Mountain allochthon. ! iv! TABLE OF CONTENTS ABSTRACT...... ............................................................................................................................. iii LIST OF FIGURES....................................................................................................................... vii LIST OF TABLES ......................................................................................................................... ix ACKNOWLEDGEMENTS ............................................................................................................ x CHAPTER 1 INTRODUCTION.................................................................................................1 1.1 Thesis organization .............................................................................................. 4 1.2 Statement of Problem ........................................................................................... 5 1.3 Overview of Carlin-type deposits ........................................................................ 6 1.4 Overview of Distal Disseminated Ag-Au deposits .............................................. 8 1.5 Deposit Classification .......................................................................................... 8 1.6 Geologic Setting ................................................................................................. 10 1.7 Approach and methods....................................................................................... 13 1.7.1 Data collection ....................................................................................... 14 1.7.2 Fieldwork ............................................................................................... 15 1.7.3 Whole rock geochemistry ...................................................................... 15 1.7.4 Mineralogical and textural analysis ....................................................... 16 1.7.5 U-Pb CA-TIMS geochronology ............................................................. 18 1.8 References .......................................................................................................... 20 CHAPTER 2 GEOLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE MARIGOLD MINE, BATTLE MOUNTAIN-EUREKA TREND, NEVADA 26 2.1 Regional Geology .............................................................................................. 27 2.2 Deposit Geology ................................................................................................ 29 2.2.1 Marigold Stratigraphy............................................................................. 29 2.2.2 Marigold Structure ................................................................................. 36 2.2.3 Marigold Ore Bodies .............................................................................. 37 2.3 Alteration styles and distribution ....................................................................... 39 2.4 Mineralization .................................................................................................... 44 2.4.1 Base Metal and Silver Mineralization .................................................... 45 2.4.2 Gold Mineralization ............................................................................... 50 ! v! 2.4.3 Paragenesis ............................................................................................. 56 2.4.4 Geochemical Comparison of Mineralization and Alteration Styles ...... 58 2.4.5 Comparison of Alteration and Mineralization Styles Among Host Rock Types ............................................................................................ 62 2.5 U-Pb Geochronology ......................................................................................... 68 2.6 Discussion .......................................................................................................... 70 2.6.1 Magmatism in the Battle Mountain Mining District........................... ... 72 2.6.2 Alteration ............................................................................................... 73 2.6.3 Base metal mineralization ...................................................................... 75 2.6.4 Gold mineralization................................................................................ 76 2.6.5 Controls on gold mineralization ............................................................. 78 2.6.6 Comparison to distal disseminated deposits .......................................... 78 2.6.7 Comparison to Carlin-type deposits ....................................................... 80 2.6.8 Genetic model for the Au deposits at Marigold ..................................... 82 2.7 Conclusions ........................................................................................................ 83 2.8 References Cited ................................................................................................ 84 CHAPTER 3 DEPOSIT AND REGIONAL SCALE EXPLORATION IMPLICATIONS AND SUGGESTIONS FOR FUTURE WORK ................................................ 93 3.1 Regional-scale suggestions for future work .............................................. 97 3.2 Concluding statement ................................................................................ 98 3.3 References Cited ....................................................................................... 98 APPENDIX A CROSS SECTIONS ......................................................................................... 100 APPENDIX