Petrology and mineralogy of the Old Workings Area, El Salvador, Chile

The \u27Old Workings Area,\u27 so called for its early mining by the Incas, is located 4 1/2 miles northeast of the El Salvador Porphyry copper deposit, the third largest copper producer in Chile. The area mapped in this study is shown to consist of rhyolite pebble conglomerate, two volcanic flows, andesite and rhyolite porphyry, and two intrusions. Quartz porphyry and tourmaline-bearing breccia. Petrographic study of surface and drill core rock samples confirms the field nomenclature used for the five rock types. All of the rock types are altered, but the quartz porphyry and andesite are the most intensely altered. Both hydrothermal and weathering actions have contributed to the alteration. Ore microscopic investigations of selected polished surfaces showed that pyrite, chalcopyrite, and molybdenite are the primary sulphides in the \u27Old Workings Area.\u27 The secondary copper minerals, chalcocite and covellite, replace the primary sulphide minerals. At the surface the copper sulphides are oxidized to brochanthite, chrysocolla, and antlerite. Hematite, hausmannite, and braunite locally are present as fracture coatings --Abstract, page i