BULLETIN 46 Topical Study of Lead-Zinc Gossans by WILLIAM C. KELLY Field and laboratory studies of lead-zinc ore outcrops, mineralogy of gossan limonite, outcrop colors and porosity, with a discussion of nonsulfide boxwork structures 1 9 5 8 STATE BUREAU OF MINES AND MINERAL RESOURCES NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY CAMPUS STATION SOCORRO, NEW MEXICO NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY E. J. Workman, President STATE BUREAU OF MINES AND MINERAL RESOURCES Alvin J. Thompson, Director THE REGENTS MEMBERS Ex OFFICIO The Honorable Edwin L. Mechem .................. Governor of New Mexico Mrs. Georgia L. Lusk ......................... Superintendent of Public Instruction APPOINTED MEMBERS Robert W. Botts ................................................................ Albuquerque Holm 0. Bursum, Jr. .................................................................. Socorro Thomas M. Cramer ................................................................... Carlsbad John N. Mathews, Jr. ................................................................. Socorro Richard A. Matuszeski ...................................................... Albuquerque Contents Page ABSTRACT ....................................................................................................... 1 INTRODUCTION .......................................................................................... 3 Background, scope, and purpose ............................................................... 3 ACKNOWLEDGMENTS .............................................................................. 5 MINERALOGY OF LIMONITE IN LEAD-ZINC GOSSANS. ......... 6 General remarks ............................................................................................ 6 Equipment and procedure .......................................................................... 8 Results of thermal analyses of gossan limonites ...................................... 9 Predominance of goethite in lead-zinc gossans .................................... 14 Possible application of thermal analysis to copper gossans ................. 15 INTERPRETATION OF BOXWORK STRUCTURES ..................... 16 General remarks ......................................................................................... 16 Examples of nonsulfide boxworks ......................................................... 16 Copperside mine, Goodsprings, Nevada ............................................ 16 New Mayberry mine, Utah .................................................................... 17 Hidden Treasure mine, Ophir Hill area, Utah ................................... 17 Lark vein, Bingham district, Utah ........................................................ 18 Boxwork siderite ..................................................................................... 20 A case history in boxwork interpretation .............................................. 21 Chemistry of formation of supergene limonite and siderite box- works after limestone ............................................................................. 23 Problems involved .................................................................................. 23 Deposition of limonite or siderite ........................................................ 24 Leaching of the carbonate matrix ........................................................ 30 GOSSAN POROSITY .................................................................................. 32 General remarks ......................................................................................... 32 A case study of porosity at the Shingle Canyon mine, New Mexico 33 Sulfide estimates by ordinary porosity determinations ........................ 36 Application of microscopic counts to the measurement of gossan voids .......................................................................................................... 38 Limitations of the microscopic method ................................................. 41 Page OUTCROP COLORS AS GUIDES TO LEAD AND ZINC ORE 42 Previous studies ....................................................................................... 42 Present studies .......................................................................................... 43 Procedure ............................................................................................. 43 Colors associated with sphalerite ......................................................... 45 Colors associated with galena .............................................................. 45 Colors associated with pyrite ............................................................... 46 Colors of transported oxides ................................................................ 47 Concluding statement ............................................................................... 47 APPENDIX A ............................................................................................... 48 Features of oxidation in areas visited for sampling ................................... 48 Pewabic mine, Hanover, New Mexico ................................................ 48 Grant County mine, New Mexico ........................................................ 50 Bullfrog mine, Vanadium, New Mexico .............................................. 51 Anita mine, New Mexico ..................................................................... 52 Providencia mine, Mexico ................................................................... 53 Hidden Treasure mine, Utah ................................................................ 54 Bingham district, Utah ......................................................................... 56 Utah Copper Co. pit ............................................................................. 56 Lark vein .............................................................................................. 58 East Tennessee zinc district ................................................................. 59 Other sample sources ........................................................................... 60 APPENDIX B ............................................................................................... 63 Construction of the Eh-pH diagram for the system Fe-H2O-CO2-S at 25°C ........................................................................ 63 APPENDIX C ............................................................................................... 66 Method used for the determination of the porosity values (primary-ore samples) listed in Table 3 ............................................... 66 APPENDIX D ............................................................................................... 68 Calculation of volumetric percentages of sulfides in Shingle Can- yon ore samples from assay returns ..................................................... 68 APPENDIX E ............................................................................................... 69 X-ray diffraction data on gossan goethites ............................................... 69 REFERENCES ............................................................................................. 75 INDEX ......................................................................................................... 79 iv Illustrations TABLES Page 1. Gossan limonite samples tested by thermal analysis .................. 11 2. Reactions and equilibrium formulae for the system Fe-H2O-CO2-S in the oxide zone .................................................. 27 3. Sulfide estimates by gossan porosity determinations .................. 37 4. Outcrop colors ................................................................................. 44 5. Standard free energies of formation .............................................. 64 FIGURES 1. Standard differential thermal curves of the natural hydrous ferric oxides ......................................................................................... 7 2. Differential thermal curves (representative of 112 analyses) of gossan limonites ............................................................................... 10 3. Thermal curves of goethite mixtures .......................................... 13 4. Boxwork after limestone, Hidden Treasure mine, Utah ......... 19 5. Eh-pH diagram for the system Fe-H2O-0O2-S in the oxide zone 25 6. Variation of the siderite stability field with dissolved-CO2 concentration .................................................................................... 29 7. Oxidation subsidence, Shingle Canyon, New Mexico ............. 35 8. Microscopic estimates of sulfide volumes, Shingle Canyon, New Mexico ................................................................................................ 40 9. Zinc gossan of the Hanover mine, New Mexico ..................... 49 10. Perspective of the Hidden Treasure mine, Ophir Hill area, Utah .................................................................................................... 55 11. Pattern of oxidation in the Utah copper pit, Bingham, Utah .. 57 12. Pinnacle-saddle pattern of oxidation, Grasselli mine, east Tennessee .......................................................................................... 61 PLATES 1. Electron photomicrographs of goethite ..................... Following 78 2. Limonite boxworks ................................................................. " " 3. Limonite and smithsonite structures .................................... " " 4. Boxworks .................................................................................