Economic Geology, Volume 6

acid alteration amount amphibole amphibole magnetite analyses andesite anticlinal arsenopyrite assays associated average bands barite beds bornite calcite cent chalcocite chalcopyrite chemical chert cherty iron carbonate chlorite clay coal contain copper country rock crystals Cuyuna deposits depth described diabase dikes district dolomite drilling economic geology fault feet ferruginous chert field fissures fractures galena geologists gold grains granite graphite hematite igneous rocks intrusion iron formation itabirite limestone limonite lode magma magnetite masses metallic metamorphic minerals mining occur oil and gas oil rock ore-bodies origin outcrop oxide plates pore space porphyry present probably production pyrite pyrrhotite quartz quartzite region replacement salt samples sand sandstone schist secondary sedimentary sediments shale siderite silica silver slate solution specimens strata structure sulphides surface tellurium thickness tion U. S. G. S. Bull U. S. Geol veins zinc zone