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OUTCROPS 213 kaolin. At slight depth argentiferous galena formed the principal ore; this gave way in depth to argentiferous enargite, chalcocite, bornite and chalcopyrite, which in turn were underlain by lean primary pyritic sulphides, v^ At Cripple Creek, Colorado, the telluride gold ores occur with a very scanty gangue, and the outcrops of the veins are not conspicuous. "As elements of geological structure, the lode fissures at Cripple Creek are exceedingly inconspicuous. They are marked neither by bold outcrops of quartz nor by superficial bands of ferruginous gossan. They seldom fault perceptibly the structures that they traverse, and are not sufficiently different froin the enclosing rocks as regards resistance to erosion, to have influenced perceptibly the topographic development of the district. It is this obscurity that retarded the discovery of the ore-deposits, and that to-day renders it impossible to follow the veins over the surface without first stripping off the soil and loose rock, or sinking test pits.''^ The surface ores here contain dull native gold associated with tellurites in ferruginous clays, kaolin and alunite. Hydrother- mal metamorphism was an inconspicuous process in this district. ; In the Black Hills, South Dakota,^ the large Homestake ore-bodies outcropped as iron stained chloritic slates, quartz, and porphyry, which in places carried as high as $16 in gold. These altered gold-bearing rocks were not- sufficiently resistant to ero- sion to produce prominent outcrops. In the open cuts it is not possible for the unpractised eye to distinguish the slates that are sufficiently mineralized to constitute ore from those that are practically barren. In general, the rocks within the ore zone appear to be somewhat more completely silicified, and to carry more iron stain than the country rocks, and also to have been subject in greater degree to fracturing and folding. In the sur- face ores the quartz occurs in three phases: as thin silicious layers intercalated in the slates, as thin seams that frequently follow the lamination and bedding, but occasionally cut across ^ Waldemar Lindgren and S. F. Emmons, P. P. 54, U. S. G. S., p. 153. ^ ^J.B. Irving, P. P. 26, U. S. G. S., p. 57. 214 EXAMINATION OF PROSPECTS them, and also as veinlets independent of the structural features of the containing rock, which last are the most important. The gold in these deposits occurs in finely disseminated particles, and appears to be of later origin than the quartz and rock gangue. ^ In the Mogollones District, New Mexico,^ the country rocks are a series of flows, f ragmental beds and tuffs, composed of soda- rhyolite, andesite and basalt. The rocks in the mineralized areas exhibit propylitic alteration; silicification, resulting in a green- ish-gray hornstone, is prominent along the veins. Sericitic alter- ation is absent. The veins are partly filled fissures and partly the result of replacement of the walls along fractured zones. The topography is rugged, and the veins being harder than the altered wall rocks, form bold outcrops. The veins carry native silver, argentite, chalcocite, pyrite, chalcopyrite, and bornite associated with specularite in a gangue of quartz, calcite and fluorite. In certain of the veins in which copper is present in small quantity the ore minerals are finely disseminated through the gangue. In another type of vein copper forms a large proportion of the total value of the ore and sulphides are abundant. Oxidation has reached a slight depth only in these veins and secondary enrich- ment does not appear^ to have effected any important rearrange- ment of the values. The water level in this district is deep, and the slight depth of secondary alteration and oxidation must be explained by the geologic youth of the region, erosion having proceeded more rapidly than oxidation. The degree to which the rocks have been shattered and the presence of abundant druse- lined cavities in the veins, together with the type of rock altera- tion, indicate a slight depth at the time of ore deposition, and that no great depth of rock has been removed by erosion since the veins were formed. The oxidized ores found at the surface contained chiefly malachite, cerargyrite, and gold, associated with limonite. 1 L. C. Graton, P. P. 68, U. S. G. S., p. 195. INDEX Adirondack Mts., deposits of, 87 Aereal geology, 40 Aguilera, J. G., 80 Alaska-Treadwell mine, 7, 47, 77 Alteration, alunitic, 150 contact, 146 decomposition, 153 depth of, 167 dynamo-regional, 145 to greisen, 150. hydrometamorphic, 153 hydrothermal, 146 influence of channels on, 160 kaolinitic, 155 leaching, 154 metasomatic, 82, 84 migration, 158 oxidation, 156 primary, 145, 204 propyUtic, 148 secondary, 153 sericitic, 149 silicification, 150 by surface agencies, 153 of wall rocks, 145 weathering, 153 Alunitic alteration, 150 Anticlines, 31 ore-deposits at tops of, 134 Antiguas, examination of, 4 Apatite veins, 83 Apex, 7 Appalachians, deposits of, 71 Aspen, Colo., 126, 141 Associations, of metals, 160 of minerals, 76 Bain, H. F., 193 Bald Mountain, S. D., 59, 106 Ballerat, Australia, 141 Basin, 31 Bassick mine, Colo., 54 Bayley, W. S., 30 Beck, R., 32, 33, 53, 55, 63, 79, 80, 109, 110, 112, 128, 129, 185 Bed vein, 45 Bedded ore-deposits, 110 Bendigo, Australia, 124, 135 Bertha mines, Va., 206 Bibliography, search of, 6 Bingham, Utah, 58, 108, 126, 184, • 210 Bisbee, Ariz., 79, 150, 181, 209 Black ffiUs, S. D., 32, 103, 106, 129, 132, 213 Blanket vein, 45 Boehmer, Max, 160 Booms, mining, 3 Boutwell, J. M., 58, 80, 105, 108, 126, 184, 210 Branching veins, 54, 61, 81 Breccia lodes, 52 Broken Hill, N. S. W., 189 Bullfrog, Nev., 198 Bunker Hill mine, Ida., 104 Burro Mts., N. M., 108 Butte, Mont., 118, 140 Carbon, precipitation by, 138 Casts in resistant minerals, 200 Cassiterite veins, 83 Catalina Mts., deposits of, 7 Cerro de Potosi, S. A.,, 59 215 216 INDEX Chalcanthite, 9, 15 Chalcocite, enrichments, ISO disseminated, 182 Chalcopyritic enrichments, 184 Chimneys, 53 Church, J. A., 133, 134 Clark, F. W., 195 Classification of ore-deposits, 86 Cleavage, 26 CUfton, Ariz., 78, 108, 179, 181 Clinton ore measures, 111 Coeur d'Alenes, Ida., 46, 80, 104, 117, 124, 210 Compensating faults, 35 Comstock lode, Nev., 212 Compound veins, 46, 47 Concentrations, surface, 171 Concretions, oolitic, 111 Conglomerate beds, 109 Conjugate faults, 35 joints, 56 veins, 56, 57 Contact deposits, 87, 88, 157 metamorphism, 146, 152 ores, 79, 124 veins, 47 Contouring, 40 Copper, enrichment of, 180 oxidized ores of, 169 primary ores of, 78, 79 secondary ores of, 176 Copper King mine, Ariz., 181 Cordilleras, deposits of, 71, 72 Cornwall, England, 137, 138 Cost of mining, 22 Cripple Creek, Colo., 48, 49, 59, 60, 77, 119, 129, 213 Crosscutting, 9, 42, 49, 57 Crosscut tunnels, 9 Cnistification, 39, 74, 94, 96, 97, ,136 Crystallization, interrupted, 77, 200 Curie, J. H., 5, 24, 141 Darton, N. H., 29 Decomposition of rocks, 153 Deep-seated veins, 83 Deformation, 37 De la Beche, 138 De la Mar mine, Utah, 171 Development, 40 Devil's Tower, Wyo., 29 DiUer, J. S., 78 Displacement, 33, 34, 37 Disseminated enrichments, 182 mineralizations, 107 Dolcoath mine, England, 137 Dolomitization, 152 Dome, 31 Drag, 36 Dressing a mine, 10 Ducktown, Tenn., 78, 91, 181 Dunnington, F. P., 195 Efflorescent salts, 15, 159, 203 Emmons, S. F., 35, 80, 171, 189, 215 Emmons, W. H., 75, 79, 91, 92, 122, 164, 165, 181, 191 Encampment, Wyo., 30, 56 Enrichment, chalcocitic, 180 chalcopyritic, 184 of copper, 180 depths of, 163, 165 disseminated, 182 of gold, 189 of gold and silver with copper, 185 irregularity of, 162 of lead, 191 of lesser metals, 195 of metals, 158 relation of, to topography, 161 relation of, to water level, 161 of silver, 186 of zinc, 194 zones of, 158, 163 INDEX 217 Epigenetic deposits, 26 Examinations, of antiguas, 4 formal, 1 preliminary, 1 preparing a property for, 9 of prospects, 3 for rescue of capital, 2 Exploration, 71, 139 amount of, compared with results attained, 9 and development, 40 for faulted ore-body, 36 of prospects, 6 Extrusive rocks, 69 Fahlbands, 90 Falun, Sweden, 79 Faribault, E. R., 135 Faults, 33 blocks, 40 compensating, 35 depth of formation, 35, 36 development from folds, 31 expression in topography, 39, 199 lodes, 51 normal, 34 reverse, 34 step, 35 torsion, 34 Filled fissures, 39, 74, 93, 95, 96, 97, 136 Fining of open spaces, 93 Finlay, J. R., 22 Fissihty, 27 Fissures, gouge filled, 39 Fissuring, time of, 68 Flexures, 32, 33 Folds, 31 Formal examinations, 1 Foster, Le Neve, 137 Fractures, 32 Freiberg, Germany, 63 Gale, H. S., 30 Gallup, N. M., 31 Garrey, G. H., 64, 66, 187, 188, 192 Gash veins, 44 Gemmel, R. C, 13 Geological maps, 3, 4, 8, 37, 40 sections, 37, 40 Geology, aereal, 40 structural, 26 Georgetown, Colo., 64, 80, 130, 187, 192 Gilbert, G. K., 70 Globe, Ariz., 199 Gneissic structure, 27, 67 Gold, enrichment of, 185 distribution of by oxidation, 171 primary ores of, 77, 78 Goldfield, Nev., 43, 151 Gordon, C. H., 90 Gossan, 202 Gouge filled fissures, 39 Granby mine, B. C, 8 Granite mine^ Mont., 79, 164, 191 Grant Co., N. M., 133 Grant, U. S., 193 Grass Valley, Cal., 78, 123 Graton, L. C, 90, 105, 109, 214 Guanajuato, Mex., 80 Gympie, AustraUa, 141 Hand picking, 2, 21 Hanging wall, relative mineraliza- tion of, 85 strength of, 8 Harder, E. C, 207 Harz, Germany, 55 Heave, 33 Helena-Frisco mine, Ida., 117 Hermosa, N. M., 133 Herron, J., 56 High assays, 19 Highland Boy mine, Utah, 105, 184 218 INDEX Himmelsfahrt, Germany, 128 Homestake mine, S. D., 213 Horn Solver, Utah, 171 Howell, E. E., 31 Himdt, R., 31 Hydrometamorphism, 153] Hydrothermal metamorphism, 146 alimitic, 150 distinction from weathering, 148 to greisen, 150 propyUtic, 148 sericitic, 149 silicification, 150 Idaho Springs, Colo., 66 Inclusions, fluid, 74 Indicator seam at Ballerat, 142 Intercalated veins, 67 Intergrowth, structural, 74 Intrusive rocks, relation of minerali- zation to, 69, 70 Irving, J. D., 32, 35, 79, 105, 129, 132, 188, 213 Jerome, Ariz., 208 Joints, 27 compression, 27 conjugate, 56 mineralization of, 51, 56 tension, 28, 31 Jones sampler, 13 KaoUn, precipitation by, 168 Kaolinization, 154, 155 Kemp, J. F., 34, 44, 87, 90, 110, 111 Keweenaw Point, Mich., 110 Kingston, N. M., 133 Krumb, H., 184 Laccoliths, 70 Lake aty, Colo., 79, 188 Lake Valley, N. M., 133 Laney, F. B., 91, 181 Large vs. small properties, 11 Leaching, 154, 159 Leaching (process), 9 Lead, enrichment of, 191 oxidized ores of, 169 primary ores of, 80 Leadville, Colo., 35, 80, 160, 195 Ledges, 44 Lincohi, F. replacement, 99 Metcalf, Ariz., 108, 177, 209, 211 Miami, Ariz., 184 Michigan, copper deposits of, 110 Microscopic slides, 73, 206 Migration of metals, 158 of gangue minerals, 195 upward, 160 Mill runs, 17 Mine dressed for examination, 10 Minerals, accessory, 76 both primary and secondary, 75 primary, 74, 75 secondary, 76 Mining, cost of, 22 Mining examinations, 1 formal, 1 for rescue of capital, 2 preliminary, 1 of prospects, 3 Mogollones, N. M., 214 Monocline, 31 Monte Cristo, Wash., 51, 191 Montpelier, Ida., 30 Monzonite, association of ores with, 72 Morenci, Ariz., 89, 108, 174, 177, 184, 209 Nacozari, Mex., 8, 207 Negaunee, Mich., 30 Neihart, Mont., 65, 67, 140, 189 Nieggen, Germany, 31 Nogal, N. M., 109 Normal faults, 34 Nova Scotia, gold deposits of, 135 Offset, 33 Old Jordan mine, Utah, 210 Oolitic concretions. 111 Open spaces, 95 Ordonez, E., 80 Ore deposition, loci of, 69, 70 Ore-deposits, associated with cer- tain rocks, 71 behavior of in depth, 81 deep-seated, 81 distribution in districts, 71 epigenetic, 26 formed at shallow depths, 81 lenticular, 92 pegmatitic, 90 regionally metamorphosed, 91 replacement, 99 syngenetic, 26 Ores, contact, 79, 88 low grade, 11 oxidized, 169, 170 primary, 69 criteria of, 73, 115 decrease in value of, in depth, 123 depth to which persistent, 72, 81, 124, 139 refractory, 8 residual, 73 secondary, 173 criteria of, 174 minerals of, 176 Ore reserves, calculation of, 19, 24 Ore-shoots, distinction between pri- mary and secondary, 115 primary, 24, 114 at tops of anticlines, 136 deep-seated, 24, 124, 143 due to impounding, 130 due to intersections, 127 due to open spaces, 127 factors determining, 114 influence of wall rocks on, 136 irregularity of, 114, 118 lenticular, 120, 144 overlapping, 120 220 INDEX Ore-shoots, primary, shapes of, 118 structure, influence of, on, 125 terms used to describe, 116 residual, 171 secondary, 24, 173, 176 effect of porosity on, 179 effect of primary mineral- ization on, 179 effect of structure on, 178 effect of wall rocks on, 178 effect of water level on, 178 lenticular, 92 shapes of, 190 Ouary, Colo., 90, 13o, 132 Outcrops, 196 casts in, 200 composition of, 201 conditions of indicating enrich- ments, 202 of contact deposits, 205 down hill creep of, 198 of kaoUnized rocks, 204 meandering of, 197 porosity of, 200 relation of length to depth, 196 size of compared with size of vein, 197 of superficial deposits, 200, 205 Overlapping veins, 57 Oxidation, 156 minerals resistant to, 157 irregularity of, 162 relation to topography and water level, 161 Pachuca, Mex., 80, 195 Paige, Sidney, 96, 175 Park aty, Utah, 80, 210 Pecos, N. M., 90 Pegmatitic deposits, 90, 124 PhiUipsburg, Mont., 164 Pilares mine, Sonora, 8, 207 Pinos Altos, N. Sections, geological, 37, 40 Segregation of values, indications of, 76 Segregations, magmatic, 86 Selective precipitation, 88, 139 Sericitization, 82, 149 Shafts, sampUng, 13 Shasta County, Cal., 78, 105 Shear zones, 52 Sheeted zones, 49 Sheeting, 28, 34 Sierra Mojada, Mex., 102 Sierra Nevada, deposits of, 71 SiUcificationl50 Silver, distribution of, by oxidation, 172 enrichment of, 185, 186 primary ores of, 79, 80 Silver Lake, Colo., 54 Silver Peak, Nev., 86, 87 SUverton, Colo., 50, 94, 120, 185, 212 Size of ore-deposits, 11 Smith, C. H., 112 Snowstorm mine, Ida., Solution, enlargement of cavities by, 94 Solution breccias, 207, 208 Specific gravity, determination 20 Spencer, A. C, 30, 47, 56, 77 Spurr, J. E., 33, 80, 87, 126, 130, 141, 187, 188, 191, 192, 212 Step faults, 35 Stocks, 53 Stockworks, 53 Stoping width, 20 Stratification, 26 Striations, 36 Stringer lodes, 50 222 INDEX Structural geology, 26 Structure, gneissic, 27 schistose, 27 Surficial deposits, 205 Syncline, 31 Syngenetic deposits, 26 Systems of related veins, 57 Tecoripa, Mex., 143 Telluride, Colo., 118 Terms of sale, 5, 6 Throw, 33 Title, 7 Tomboy mine, Colo., 56 Tombstone, Ariz., 134 Tonopah, Nev., 36, 41, 80, 84, 130, 212 Topography, expression of faults in, 39, 199 of mineralization in, 198 Trail, 36 Tularosa, N. M., 113 Unionville, Nev., 65 United Verde mine, Ariz., 208 Values, minerals indicating a segre- gation of, 76 Van Hise, C. R., 26, 27, 30, 62, 95 Veins along dikes, 48 bed, 45 blanket, 45 branching, 54, 61, 81 compound, 46, 47 conjugate, 56, 57 contact, 47 deep-seated, 83 Veins, distinction between inter- calated and fissure, 67 fissure, 43 follow faults of small displace- ment, 62 formed near the surface, 82 gash, 44 in schist, 68 intercalated, 67 linked, 55 movement along, 63 overlapping, 57 persistence of, in depth, 61 pinching and swelling of, 127 pockety, 73 relation between depth and num- ber of, 62 replacement, 98, 136 structure, influence of country rock on, 63 systems of, 57 Virginia aty, Nev., 212 Watson, T. L., 206 Weathering, 153 minerals resistant to, 88 Weed, W. H., 67, 78, 79, 93, 107, 136, 140, 156, 189, 190, 191 Witwatersrand, South Africa, 109 Yung, M. B., 79 Zinc, enrichment of, 194 oxidized ores of, 170 primary ores of, 81 Zones developed by surface agen- cies, 163 1 \ THia BOOK 18 DTTB OK THE IJLST DATE I YB 15518 y -.^ ■9^M ni". 