Abstract

Copper mineralization at Yandera is associated with low-K tholeiite-calc-alkaline porphyry intrusives into the middle Miocene Bismark Intrusive Complex. Three episodes of porphyry emplacement are recognized, the youngest generating intrusive breccia pipes.A broad area of secondary biotite alteration with peripheral epidote-chlorite is superimposed upon the igneous terrain, postdating all igneous events within it. Fracture-controlled calcic and phyllic/argillic alteration zones symmetrically disposed about an elongate central core of sulfide-barren quartz veining overprint the secondary biotite and epidote-chlorite.Ore-grade mineralization of > or =0.3 percent copper equivalent flanks both sides of the two main centers of quartz veining within the central core and is also localized into breccia pipes.Inclusions of "hydrosaline melt," homogenizing- between 270 degrees and 470 degrees C, occur in vein quartz through the central core and, to a much lesser extent, the calcic zone immediately haloing it. Moderately saline liquid-dominant gas-liquid inclusions homogenizing between 270 degrees and 380 degrees C and vapor-dominant gas-liquid inclusions occur in vein quartz of the quartz core, calcic, and phyllic/argillic alteration zones. The moderately saline fluid probably boiled indicating a hydrostatic depth of 1,200 + or - 300 meters for the hydrothermal system.Electron microprobe analyses of secondary biotite indicate a systematic variation in molecular Mg/Fe ratio across the system, the most phlogopitic coinciding with the central core. This composition zoning correlates with color zoning of the biotite.

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