Abstract

The Caspiche porphyry gold-copper deposit, part of the Maricunga gold-silver-copper belt of northern Chile, was discovered in 2007 beneath postmineral cover by the third company to explore the property over a 21-year period. This company, Exeter Resource Corporation, has announced a proven and probable mineral reserve of 1,091 million tonnes (Mt) averaging 0.55 g/t Au, all but 124 Mt of which also contain 0.23% Cu, for a total of 19.3 Moz of contained gold and 2.1 Mt of copper.

The deposit was formed in the latest Oligocene (~25 Ma) during the first of two volcanic and corresponding metallogenic epochs that define the Maricunga belt. The gold-copper mineralization is centered on a composite diorite to quartz diorite porphyry stock, within which five outward-younging phases are routinely distinguished. The centrally located, early diorite porphyry (phase 1) hosts the highest-grade ore, averaging ~1 g/t Au and 0.4% Cu. The subsequent porphyry phases are quartz dioritic in composition and characterized by progressively lower gold and copper tenors. Stock emplacement was both pre- and postdated by the generation of large-volume, andesite-dominated breccias, with tuffaceous matrices, which are believed to be shallow portions of diatremes. The deposit is characterized by a central gold-copper zone and partially overlapped but noneconomic molybdenum halo. The gold-copper mineralization in the lower half of the deposit accompanies quartz ± magnetite-veined, potassic-altered rocks, whereas the shallower mineralization occurs within quartz-kaolinite–dominated, advanced argillic alteration. Upper parts of the advanced argillic zone are characterized by siliceous ledges, some auriferous, composed of vuggy residual quartz and/or silicified rock. The chalcopyrite-pyrite mineralization in the potassic zone was partially transformed to high sulfidation-state sulfides and sulfosalts during the advanced argillic overprint, although the underlying chalcopyrite-bornite assemblage was mainly too deep to be affected. The deposit terminates downward in a sulfide-deficient, potassic-calcic zone defined by K-feldspar, actinolite, and magnetite, which formed at the expense of biotite. A relatively minor, shallowly inclined zone of intermediate sulfidation epithermal gold-zinc mineralization, comprising narrow veinlets and disseminations, abuts the late-mineral diatreme contact. Supergene sulfide oxidation throughout the deposit is relatively shallow, and chalcocite enrichment extremely minor.

The Caspiche deposit is thought to have been emplaced at relatively shallow paleodepths, within the southern, flat-bottomed part of the premineral diatreme vent. Earliest porphyry system development, probably north of the present deposit, appears to have been aborted by diatreme formation. Much of the gold and copper in the Caspiche deposit was introduced during the potassic alteration stage, with the highly telescoped, advanced argillic overprint being responsible for only minor redistribution of the two metals, and the addition of arsenic. The late-mineral diatreme was emplaced west of the Caspiche deposit, and caused destruction of only its uppermost peripheral parts. The late-mineral diatreme was both pre- and postdated by advanced argillic alteration. Finally, the intermediate sulfidation epithermal gold-zinc zone was localized by the enhanced permeability provided by intense fracturing along the underside of the upward-flared, late-mineral diatreme contact.

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