Abstract

A porphyry gold deposit has been documented for the first time at Marte in the Maricunga belt of the Andean Cordillera, northern Chile. Exploration conducted from 1981 through 1987 resulted in definition of 66 metric tons of contained gold. The deposit entered production in late 1989 as an open-pit, heap leach operation.The gold deposit is part of a linear, calc-alkaline volcanoplutonic arc constructed during the mid- to late Miocene as eastward-directed subduction was shallowing. The consequent compression of continental basement underlying the deposit and its environs created a series of high-angle, reverse faults. Stock emplacement and gold mineralization took place within a coeval andesitic stratovolcano at 13 to 14 Ma. The hornblende-biotite diorite stock is subdivided into three phases: two mineralized porphyries and a weakly porphyritic, late mineralization microdiorite. Much of the microdiorite constitutes the matrix of an intrusion breccia, which is transected locally by a closely related hydrothermal breccia.Gold mineralization is coincident with a stockwork of quartz veinlets surrounded by chloritesericite-clay alteration of intermediate argillic type. Pyrite and iron oxides are both present in dominantly disseminated form, with each making up as much as 10 vol percent of the rock. Hematite exceeds magnetite in abundance and was generated in large part by hypogene martitization. Minor chalcopyrite and even lesser quantities of molybdenite, bornite, tennantite, and enargite also occur. Gypsum after anhydrite commonly exceeds 5 vol percent and calcite and tourmaline exist as traces. Isolated remnants of hydrothermal biotite and alkali feldspar attest to the former presence of K silicate alteration, which is believed responsible for introduction of much of the veinlet quartz, magnetite, and gold. Chlorite-dominated alteration affected the late-stage microdiorite and its associated breccias. An erosional remnant of the andesitic volcanic roof to the stock is largely replaced by a chalcedony- and alunite-rich advanced argillic assemblage, which is part of a formerly more extensive tabular alteration zone. Abundant pyrite, gypsum, native sulfur, and barite and trace amounts of enargite and subeconomic gold are present in this advanced argillic cap.The relative impermeability of the cap caused development of an inverted supergene profile in which sulfide-bearing ore is underlain by jarositic leached ore. The steep, elongate orebody is made up of both ore types, which do not differ in gold content. However, the hypogene copper content, about 550 ppm, was more than halved by supergene oxidation. The gold ore is poor in silver (Ag/Au = 0.44), but highly anomalous in molybdenum as well as copper and weakly anomalous with respect to lead, arsenic, and mercury. The advanced argillic cap is marked by pronounced Au, Bi, Hg, Tl, Pb, Mo, and As anomalies, which contrast with a broad zinc halo to the orebody.Preliminary fluid inclusion studies suggest that the auriferous stockwork is a product of boiling fluids which ranged in temperature from 155 degrees to 375 degrees C and in salinity from 2 to 20 wt percent NaCl equiv. A reconstruction of the volcanic edifice combined with pressure estimates based on the fluid inclusion data implies that mineralization took place about 600 to 700 m below the paleosurface. The geologic, alteration, and mineralization characteristics of the Marte gold deposit are closely similar to those of gold-rich porphyry copper deposits, especially those in the Philippines. The only significant mineralogic difference is a deficiency of copper at Marte. The fluid inclusion population at Marte is also more reminiscent of porphyry rather than epithermal environments, although high-salinity inclusions common in porphyry copper deposits were not encountered in the few samples studied.

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