The environment of formation of Cu+Ag-bearing calcite veins, Sacramento Mountains, California Item Type text; Thesis-Reproduction (electronic) Authors Schuiling, William Thys 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 25/09/2021 08:48:53 Link to Item http://hdl.handle.net/10150/566689 THE ENVIRONMENT OF FORMATION OF Cu+Ag-BEARING GALGITE VEINS, SACRAMENTO MOUNTAINS, CALIFORNIA by William Thys Sehuiling 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 19 7 8 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements 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 acknowledgment 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 head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: R. E. BEANE f D^te Associate Professor of Geosciences ACKNOWLEDGMENTS I would like to thank Dr. R. E. Beane for his aid in choosing a study area, for providing background information based on his knowledge of the region, for suggestions and assistance during the research phase of the investigation, and for his critical editing of the manuscript. Conversations with Stephen Reynolds and Larry Bradfish, of the Depart­ ment of Geosciences, University of Arizona, clarified geological ques­ tions and provided information on other occurrences of mineralization similar to that described in this thesis. I would also like to thank Drs. D. Norton and S. R. Titley for reading and improving the final draft of this thesis. Robert Bodnar, Roger Nielsen, and Robert Schafer assisted with the fluid inclusion studies and electron microprobe analyses. I am also grateful to Mr. Tom.Teska for his advice on use of the electron microprobe. Funding for this work was provided in part by National Science Foundation grant EAR77—13642. iii TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS . ............. o ..... v ABSTRACT oo.e e e e e e ooo e e « e «« • e . a . o Vli INTRODUCTION ............................. .. ............... .. 1 GEOLOGIC SETTING .............. .......... ... 4 VEIN MINERALIZATION AND ALTERATION ....... ............. 7 Mineral Assemblages . .................................. i ...... 7 Paragenetic Relationships . ........ ......... 8 Alteration ................................................ 13 FLUID INCLUSION STUDIES . ........... ........................ .. 15 CHEMICAL ENVIRONMENT OF FORMATION ...... ................ ... 23 Concentration of Silica in Solution ............ 23 Composition and Stability of Chrysdcolla ........... .. 24 Limits on the Fugacities of Carbon Dioxide and Sulfur Trioxide ..................... 35 Mineral Stabilities as a Function of fOg and fS^ ..... 38 Solubility of Chrysocolla at 200°C ....... 44 Solubility of Iron .......... .............. 46 Stabilities of the Alteration Phase ....................... 49 SUMMARY AND CONCLUSIONS . .... ........ .. 53 APPENDIX . , . ................... 56 BIBLIOGRAPHY ............ .................. 58 iv LIST OF ILLUSTRATIONS Figure Page 1. Location Map of Eastern California and Western Arizona Showing the Sacramento Mountains near Needles, California . ................... .. ................. .. 2 2. Geologic Map of the Northern Sacramento Mountains . 5 3. Paragenetic Relations among Minerals in the Three Common Vein Assemblages ...... ........ .. 9 4. Covellite and Argentite Textures as Shown by Ag (Top) and S Electron Backscatter Photography ............ 11 5. AKF Triangle Illustrating Compositions of Chlorite and Illite from the Sacramento Mountains ....... 14 6. Histogram of Fluid Inclusion Homogenization Tempera­ tures in Calcite, Barite, and Quartz ......... 18 7. Phase Diagram for the System NaCl-H^O ........ .. 22 8. Solubility Curves for Quartz, and Amorphous Silica as a Function of Temperature ............ .. 25 9. Compositional Triangle Showing Distribution of Chrysocollas from the Literature and from the Sacramento Mountains ................. 27 10. Correlation Plot Showing Third Law Entropies of MgSiOg, MnSiOg, and CaSiO3 versus the Summation of the Component Oxides for Each ........... 31 11. Temperature-stability Plot of Chrysocollas as a Function of Aging and Concentration of Silica .... 34 12. Stability Relations among Calcium, Barium, and Copper Oxides, Silicates, Carbonates, and Sulfates at 200°C as a Function of fSO3 and fC02 ................ .. ............... 36 13. Stability Relations among Calcium, Barium, and Copper ■ Oxides, Silicates, Carbonates, and Sulfates at 150°C as a Function of fSOg and fC02 ........ 36 v vi LIST OF ILLUSTRATIONS— Continued Figure Page 14. Stability Relations among Iron Oxides and Sulfides at 200°C as a Function of fSg and f O g ............ 39 15. Stability Relations among Copper and Silver Sulfides, Oxides, and Silicates at 200°C. as a Function of fS2 and f02 . ................ 39 16. Stability Relations among Iron, Copper, and Silver Sulfides, Oxides, Silicates, and Calcite-barite Costability at 200°C as a Function of fS2 and f02 . 40 17. Stability Relations among Iron, Copper, and Silver Sulfides, Oxides, Silicates, and Calcite-barite Costability at 150°C as a Function of fS2 and f02 . 40 18. Solubility of Iron in Calcite as a Function of Temperature ................... .. ........... .. 42 19. Concentration of Iron in Solution at Hematite Satura­ tion as a Function of Temperature, pH, and f02 . 47 20. Stability Relations among Phases, in the System ^O-MgO-AlgOg-FegOg-SiOg-HgO, Conserving. Alumina, at 200oC, aH20 = 1, and Amorphous Silica and Hematite S a t u r a t i o n .............. ............ .. 51 21. Stability Relations among Phases in the System KgO-MgO-AlgOg-FegOg-SiOg-HnO, Conserving Silica,. at 200°C, aH20 = 1 , Log = 0 , and Hematite Saturation ...... .a?+ . ........ 52 ABSTRACT Brown calcite-barite-hematite-chrysocolla veins with minor co- vellite and argentite cut granitic and gneissic rocks. Tertiary(?) ande­ sites , and a deformed Tertiary(?) conglomerate in the Sacramento Mountains of California. Paragenetic relations among the vein minerals and fluid inclusion studies indicate that most of the mineralization was deposited from hydrothermal solutions having salinities in the range 5-14 weight percent NaCl equivalent. Temperatures decreased from more than 300°C to less than 150°C, depositing early, high-temperature calcite-barite-covellite-argentite, as well as later, lower temperature calcite-barite-hematite-chrysocolla. Mineral stability considerations suggest a strong decrease in fSg and moderate increase in fC^ occurred within the system during cooling. Goethite included in high-temperature calcite is incompatible with the accompanying barite-covellite- argentite, and it is likely that this phase formed by oxidation of fer­ rous iron exsolved from the calcite during cooling. Available evidence indicates that chrysocolla formed at temperatures as high as 200°C from neutral or slightly basic solutions which were supersaturated with re­ spect to quartz, and which were also responsible for chloritic and il- litic alteration in the Tertiary(?) arkosic conglomerate. vii INTRODUCTION Mineral assemblages are commonly used as a means of interpreting and defining physical and chemical conditions under which mineralization has taken place. In.the Sacramento Mountains approximately 24 km north­ west of Needles in southeastern California (Figure 1), veins cutting Cenozoic rocks contain iron oxides, chrysocolla, and covellite, an as­ semblage which is commonly interpreted as having formed as a product of supergene oxidation of copper-iron sulfide mineralization. Also present at this locality, however, and apparently oogenetic with the first set of phases, are barite, acanthite, and dark calcite,.a group of minerals considered to be typical of the epithermal environment. This entire, apparently contradictory assemblage has been suggested to be of primary hydrothermal origin (Beane, 1968a) based on the presence of barite para- genetically later than, much of the iron oxide, covellite, and chryso­ colla mineralization. It is the purpose of this study to define the chemical environment under which the mineralization was deposited in order to. resolve this interpretations! conflict. Representative samples of vein mineralization, were collected at eight localities containing this mineralization along the north and east flanks of the Sacramento Mountains, and were analyzed for purposes of mineral identification, composition,, and paragenetic relations among vein-forming minerals. Fluid inclusions in several of the phases were studied to determine temperatures of formation of the minerals