Abstract

The Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Perú, centered at latitude 15°11′ S, longitude 75°6′ W, incorporates Marcona, the preeminent central Andean iron oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one of the few major Andean IOCG deposits with economic copper grades (346.6 Mt @ 0.71% Cu). The emplacement of magnetite orebodies with uneconomic Cu grades (avg 0.12%) at Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine Middle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene Cu sulfide (~15 g/t Ag, 0.12 g/t Au) mineralization at Mina Justa was emplaced along reactivated listric-normal detachment faults during the mid-Cretaceous inversion of the contiguous, plate boundary-parallel, Aptian to Albian Cañete basin, accompanied by the earliest, largely granodioritic-dioritic, stocks of the Coastal batholith. Alteration and mineralization assemblages, supported by 40Ar/39Ar geochronology of biotite, phlogopite, actinolite, cummingtonite, and K-feldspars, reveal a history of magmatic and hydrothermal processes extending episodically for at least 80 m.y., from ca. 177 to 95 Ma, wherein metal-rich mineralization events were preceded and separated by episodes of barren alteration.

At Marcona, precursor, subocean-floor hydrothermal activity in the Aalenian (177 Ma) and Bajocian (171 Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperature Mg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocks and minor carbonate units underlying the nascent Río Grande Formation arc. Subsequent areally widespread, albite-marialite alteration (Na-Cl metasomatism) largely predated but overlapped with the emplacement of an en echelon swarm of massive magnetite orebodies, in turn overprinted by subordinate magnetite-sulfide assemblages. Magnetite and weak Cu and Zn sulfide mineralization coincided with a 156 to 162 Ma episode of andesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely by quartz-rich metaclastic rocks. From 162 to 159 Ma, iron oxide mineralization evolved from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite assemblages. These were overprinted at 156 to 159 Ma by chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, locally resulting in grades of 0.45 percent Cu and 0.5 percent Zn.

Hydrothermal activity was thereafter focused in the Mina Justa area, 3 to 4 km to the northeast of Marcona, where Middle Jurassic andesites experienced intense albite-actinolite alteration at ca. 157 Ma, i.e., contemporaneous with sulfide mineralization at Marcona, and magnetite-microcline alteration (K-Fe metasomatism) at ca. 142 Ma. Development of the Mina Justa Cu (-Ag) deposit proper, however, began much later, with, successively, actinolitization at ca. 109 Ma, the deposition of calcite and specular hematite, now entirely pseudo-morphed by magnetite, and the metasomatic emplacement of bodies of barren, massive magnetite and pyrite at 101 to 104 Ma. Finally, at 95 to 99 Ma, chalcopyrite-bornite-digenite-chalcocite mineralization, with abundant calcite and hematite, was emplaced as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays of breccia and stockwork, cored by preexisting magnetite-pyrite lenses. Supergene oxidation generated a chryso-colla-atacamite-covellite blanket, hosting ~40 percent of the Cu reserve, prior to the eruption of a 9.13 ± 0.25 Ma rhyodacitic ignimbrite flow.

Although areally contiguous, the major magnetite and copper-rich centers of the Marcona district record independent metallogenic episodes widely separated in age. Further, whereas the Cu-poor magnetite mineralization at Marcona was integral to the terminal eruptions of the Middle Jurassic arc, representing a shallow-marine analog of the Pliocene El Laco magnetite deposits of northern Chile, the Mina Justa Cu sulfide orebodies—like the other economic, mid-Cretaceous, Cu-rich IOCG deposits of the central Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, and Rául-Condestable—was the product of brines released during the inversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the Mesozoic Andes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration and mineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCG deposit clan.

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