The San Cristobal silver district near Uyuni, Bolivia, was jointly studied in detail during the months of August to November 1966 by members of the Servicio Geologico de Bolivia and the U.S. Geological Survey, under the auspices of the Agency for International Development, U.S. Department of State. The rocks of the district are Cenozoic. The oldest rocks are the red beds of the Potoco Formation, which are unconformably overlain by agglomerate, conglomerate, sandstone, and tuff of the flat-lying Quehua Formation. Both formations are intruded by dacite porphyry and andesite porphyry stocks, and both are overlain by recent dacite porphyry lava flows. The intrusive and sedimentary rocks near the intrusive contacts are hydrothermally altered. Mineral deposits of the district ai*e associated with the intrusive rocks and are of four distinct types: 1. Sulfide veins containing lead, zinc, and silver associated with dacite porphyry intrusives (Colon and Tesorera mines). 2. Oxide veins containing iron and silver associated with andesite porphyry intrusives (Toldos mine). 3. Partial sulfide replacement of intrusive breccia containing silver, lead, and zinc (Animas mine). 4. Disseminated lead-zinc-silver minerals in altered dacite porphyi*y intrusives and in adjacent altered sedimentary rocks (Hedionda mine). Disseminated silver minerals may provide a basis for the development of open-pit silver mines. This possibility should be further investigated, beginning with geochemical surveys and hydrothermal alteration studies.
Introduction-______________________________________ Scope of report.________________________________ Previous investigations __________________________ Background and objectives of present investigation. Methods of operation.__________________________ Geochemical and geophysical field procedures. _____ Traverse-line preparation ____________________ Geochemical soil surveys___________________ Magnetic surveys.__________________________ Electrical surveys.__________________________ Resistivity. ____________________________ Self-potential__ _-_______---____-___--_ Electromagnetic ________________________ Induced polarization ____________________ Acknowledgments__ ___________________________ Geography _ ______________________________________ Topography ________________________________ Climate and vegetation__________________________ Proposed Pa Mong dam___._____________________ Regional geology___________________________________ Sedimentary and metamorphic rocks---______-._ Devonian or older rocks____________________ Devonian and lower Carboniferous rocks_____ Carboniferous and Lower Permian rocks_______ Triassic to Cretaceous rocks (Khorat Series) Quaternary alluvium and laterite--________-__ Igneous rocks__________________________________ Structure______________________________________ Metamorphism ________ _ ______________________ Mineral deposits._______________________________ Regional magnetic information. ______________________ Aeromagnetic surveys.__________________________ Magnetic interpretation-_ _ ______________________ Geochemical reconnaissance._________________________ Sampling and analysis_________________________ Geochemical background-_______________________ Geochemical anomalies._________________________ Mineral prospects_________________________________ Base-metal prospects..__________________________ Copper-Iead-zinc deposits.___________________ Phu Han-Phu Sang prospect. ____________ Phu Hin Lek Fai area_--_-_--__--_-----_ Phu Thong Daeng deposit. ________--__ Phu Poon-Phu Pa Phai prospect._________ Lead-zinc deposits. _________________________ Phu Khum 1 (Ban Khok Mon) prospect_ Phu Khum 2 (Ban Nong Ya Sai) prospect-Iron prospects..________________________________ Phu Lek (Chiengkarn) deposit______________ Phu Yang deposit_____________________ Phu Hia deposit.___________________________ Phu Ang deposit__________________________ 10. Summary of data used for calculating proven and probable ore, Phu Ang iron deposit. _ _ _ _ _ 11. Summary of drilling data, Ban Kham Duang gold prospect-__________________________ 12. Summary of analyses of Wang Saphung gypsum intersections-________________________ MINERAL INVESTIGATIONS IN NORTHEASTERN THAILAND
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