Gold mineralization in the Black Cloud #3 carbonate replacement orebody, Leadville Mining District, Lake County, Colorado Item Type text; Thesis-Reproduction (electronic) Authors Gray, Matthew Dean, 1933- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 30/09/2021 01:36:16 Link to Item http://hdl.handle.net/10150/558083 GOLD MINERALIZATION IN THE BLACK CLOUD #3 CARBONATE REPLACEMENT OREBODY, LEADVTLLE MINING DISTRICT, LAKE COUNTY, COLORADO by Matthew Dean Gray Copyright(g>Matthew Dean Gray 1988 A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 1988 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder. APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: Professor of Geosciences ACKNOWLEDGMENTS The writer wishes to express his sincere appreciation to a number of individuals and organizations whose help and support made this thesis project possible and enjoyable. In particular, I would like to thank my advisory committee at the University of Arizona, Dr. Spencer Titley, Dr. John Guilbert, and Dr. Joaquin Ruiz. Special thanks to Dr. Titley for his eminently practical guidance. I would also like to greatly thank Doug Smith and ASARCO Inc. for their generous help, both financial and logistical, and I am indebted to Doug Smith as well for his sharing of ideas on Leadville geology. For graciously assisting in the field work, as well as making it a pleasure, I thank the staff at the Black Cloud mine. Additional thanks go to Tom Teska for his invaluable help with the electron microprobe studies, to Mike Gustin for many useful discussions, and to St. Joe American Corp. for their generous financial assistance. Finally, special thanks to Nancy A. Gray for without her support the writer could not have completed this project. iii TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS.................................................................. ................... ... v LIST OF T A B L E S ..................................................................................................................... vii A B S T R A C T ......................................................................................................................... viii INTRODUCTION................................................................................................................. 1 Purpose of Study................... ..................................................................................... 1 Methods of Study............................ ............................................................................. 1 Geography............................... 2 Previous Work ....................................... 2 • DISTRICT GEOLOGY ............................... 4 Stratigraphy............................... 4 Intrusive A c t iv it y ................... 9 Structure......................................................................................................................... 11 Mineralization...................................................... 11 BLACK CLOUD #3 OREBODY............................... 13 Introduction................................................................................................................. 13 Mineralization and Ore Petrography ................................... 13 Relative Timing of the Mineralization S t a g e s ................... 14 Main Stage Mineralization ...................................................................................... 15 Vein Stage Mineralization.................................................................. 30 Bismuth-Tellurium M ineralization............................... ... ............................ 37 Geochemistry........................... 50 Interpretation of Geochemical Results .................................... 53 SUMMARY OF GOLD OCCURRENCE .............................................................. 55 DISCUSSION ..................................................................................................................... 57 Comparisons with Previous Work ....................................................................... 57 Relavance to other Leadville O rebodies................................... 58 Exploration Significance ...................................................................................... 60 CONCLUSIONS............................ 63 REFERENCES ...................................................... 64 iv i I LIST OF ILLUSTRATIONS Figure Page 1. Simplified Geologic Map of the Black Cloud Mine A r e a ............................... 5 2. Stratigraphic Section for the Leadville D is tr ic t................... 7 3. Photograph of Massive-Granular Ore .............................................. 17 4. Photograph of Mixed Sulfide O r e ............................................................................... 18 5. Photograph of Siliceous O r e .................................................................. 19 6. Photomicrograph of Chalcopyrite Inclusions Concentrated along Marmatite Grain Boundary . ............................... ...........................................................22 7. Photomicrograph of Chalcopyrite Stringer and Inclusions in Marmatite . 23 8. Photomicrograph of Marmatite Replacing Galena and Pyrite................................ 25 9. Photomicrograph of Native Gold Grain in P y r ite ........................... 27 10. Photomicrograph of Native Gold Grain in P y r ite ....................... 28 11. Paragenetic Diagram for Main Stage Mineralization . .................................31 12. Paragenetic Diagram for Type 1 V einlets................................................................... 32 13. Paragenetic Diagram for Type 2 Veinlets ................................... 33 14. Photomicrograph of Electrum Grain in P y r i t e .................. 35 15. Photomicrograph of Electrum Grain in M a rm a tite.................................. 36 16. Photomicrograph of Electrum Grain at Marmatite/Pyrrhotite Boundary . 38 17. Photomicrograph of Intergrowths of Hessite and Chalcopyrite........................... 41 18. Photomicrograph of Quartz/Hessite Micro veinlets in Pyrite . ............................ 42 19. SEM Images of Telluride M ineralization...................................................................43 20. SEM Images of Schapbachite Replacing P y r it e ....................................................... 46 21. SEM Images of Electrum and Tellurides in Pyrite....................... 48 v vi LIST OF ILLUSTRATIONS - Continued 22. SEM Images of Au-Ag Tellurides ...................................................................... 51 Plate 1. Stope M a p ......................................................................................................... in pocket LIST OF TABLES Table Page 1. Igneous Rocks of the Leadville District.............................................................. 10 vii ABSTRACT The Black Cloud #3 orebody is a massive sulfide carbonate replacement lead- zinc-silver-gold orebody. Although gold occurs in significant amounts its style of occurrence was unknown. A detailed petrographic, geochemical, and electron microprobe study of mineralization from the #3 orebody was undertaken in order to characterize the gold occurrence. Three stages of gold mineralization are recognizable in the #3 orebody. The most important is an early stage where gold was deposited with pyrite early in the formation of the massive sulfide orebody. Gold was again introduced to the orebody in a later stage as electrum grains contained within crosscutting sulfide veinlets. Finally, minor gold deposition associated with bismuth- tellurium mineralization overprinted the earlier mineralization. viii INTRODUCTION Purpose of Study The Black Cloud mine in Lake County, Colorado, produces lead, zinc, silver, and gold from what has been described as a mesothermal carbonate replacement deposit, the #3 orebody (Arehart, 1978). Although gold occurs in economically significant amounts, the systematics of its occurrence and distribution are not known in detail. The purpose of this study is to provide a detailed picture of gold occurrence within the orebody, detailing the distribution of gold and its geological/mineralogical controls. Methods of Study In order to characterize gold occurrence within the #3 orebody, an active gold producing stope was chosen for detailed investigation. The slope chosen for study, slope 12-68-121 (10th level of the #3 orebody) has been a significant gold producer and at the time of study production gold values were averaging approximately 0.1 oz. per ton. During the fall of 1986 the stope was mapped at a