Porphyry Gold Deposits of the Refugio District, Maricunga Belt, Northern Chile

摘要: The porphyry gold deposits of the Refugio district and similar in Maricunga belt contain lowest known copper to ratios (% Cu/ppm Au = ~0.03) any porphyry-type deposit. are associated with subvolcanic andesitic dacitic intrusions emplaced into coeval volcanic rocks. Both Verde Pancho zoned space from a deeper zone banded quartz veinlets chlorite-magnetite-albite and/or pyrite-albite-clay alteration shallow local quartz-alunite ledges. contains an additional, deepest, copperlike zone, (A-veinlets) potassic alteration. Relative Verde, is telescoped, all three zones present within 400-m-vertical interval. The at characterized by A-veinlets pervasive alteration, both restricted intrusive range hairline streaks magnetite ± biotite minor chalcopyrite, K feldspar envelopes sugary <1 cm width chalcopyrite no envelopes. Hypersaline liquid inclusions coexisting vapor-rich indicate temperatures above 600°C salinities as high 84 wt percent NaCl equiv. A pressure estimate 250 bars indicates depth 1,000 m, assuming lithostatic pressure. Potassic consists central magnetite-K feldspar-oligoclase that changes outward biotite-rich zone. Total sulfide content, predominately generally vol percent, whereas content 2 5 percent. Where predominate, grades typically 0.1 hypogene 0.5 1 ppm gold. Banded Pancho, where they occur mostly cut overlap. They less than thickness lack Dark gray bands, whose color due abundant fluid micrometer-sized grains magnetite, commonly symmetric pairs near vein walls. bands botryoidal continuous through grains, suggesting recrystallized silica gel. Rare liquid-rich <350°C <35 Estimated pressures <200 bars, depths 190 1,500 m under hydrostatic Gold occurs dark outside pyrite, chlorite, illite, feldspar. Banded occupy steeply dipping radial shallowly concentric fractures. Zones without early <0.05 copper. Most differences between can be attributed shallower formation—less km compared 1.5 4 typical for deposits. Shallower resulted lower concentrations, garnet veinlets, widespread albite-bearing most importantly which unique direct result episodic intrusion magma surface exposure high-temperature magmatic fluids pressures. Episodic rupturing brittle-ductile boundary surrounding centers led flashing fluids, loss sulfur vapor, low sulfide/gold ore.