Tin-bearing veins in the Llallagua, Huanuni, and Morococala districts and tin-silver veins of the Potosi and Oruro districts are closely associated with volcanic rocks and quartz porphyry intrusions of middle? Tertiary age, and occupy steeply dipping fractures whose trends probably reflect a combination of regional northeast-southwest compression and varied local stresses. The deposits were formed at relatively shallow depth at temperatures which were initially as high as 400-500 degrees C (cassiterite) and which decreased through the mesothermal range (stannite, chalcopyrite, tetrahedrite, andorite) to about 100-150 degrees C. The ore-bearing solutions were neutral to weakly alkaline during the period of rock alteration and early vein growth, but apparently became acid in late stages. The tin ores exhibit a greater degree of telescoping than the tin-silver ores.

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