Evolution of the Giant Marcona-Mina Justa Iron Oxide-Copper-Gold District, South-Central Peru

摘要: The Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Peru, centered at latitude 15°11′ S, longitude 75°6′ W, incorporates Marcona, the preeminent central Andean oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one few major IOCG deposits with economic copper grades (346.6 Mt @ 0.71% Cu). emplacement magnetite orebodies uneconomic Cu (avg 0.12%) Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine Middle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene sulfide (~15 g/t Ag, 0.12 Au) mineralization Justa emplaced along reactivated listric-normal detachment during mid-Cretaceous inversion contiguous, plate boundary-parallel, Aptian Albian Canete basin, accompanied earliest, largely granodioritic-dioritic, stocks Coastal batholith. Alteration assemblages, supported 40Ar/39Ar geochronology biotite, phlogopite, actinolite, cummingtonite, K-feldspars, reveal a history magmatic hydrothermal processes extending episodically for least 80 m.y., from ca. 177 95 Ma, wherein metal-rich events were preceded separated episodes barren alteration. At precursor, subocean-floor activity in Aalenian (177 Ma) Bajocian (171 generated, respectively, cummingtonite phlogopite-magnetite assemblages through high-temperature Mg-Fe metasomatism previously metamorphosed Lower Paleozoic Formation siliciclastic rocks minor carbonate units underlying nascent Rio Grande Subsequent areally widespread, albite-marialite alteration (Na-Cl metasomatism) predated but overlapped en echelon swarm massive orebodies, turn overprinted subordinate magnetite-sulfide assemblages. Magnetite weak Zn coincided 156 162 Ma episode andesitic eruption dacitic intrusion which terminated growth arc, hosted quartz-rich metaclastic rocks. From 159 evolved magnetite-biotite-calcic amphibole ± phlogopite fluorapatite magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite These chalcopyrite-pyrite-calcite pyrrhotite sphalerite galena locally resulting 0.45 percent 0.5 Zn. Hydrothermal thereafter focused area, 3 4 km northeast where andesites experienced intense albite-actinolite 157 i.e., contemporaneous magnetite-microcline (K-Fe 142 Ma. Development (-Ag) proper, however, began much later, with, successively, actinolitization 109 deposition calcite specular hematite, now entirely pseudo-morphed magnetite, metasomatic bodies barren, pyrite 101 104 Finally, 99 chalcopyrite-bornite-digenite-chalcocite mineralization, abundant as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays breccia stockwork, cored preexisting magnetite-pyrite lenses. Supergene oxidation generated chryso-colla-atacamite-covellite blanket, hosting ~40 reserve, prior 9.13 0.25 rhyodacitic ignimbrite flow. Although copper-rich centers district record independent metallogenic widely age. Further, whereas Cu-poor integral terminal eruptions representing analog Pliocene El Laco northern Chile, orebodies—like other economic, mid-Cretaceous, Cu-rich Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, Raul-Condestable—was product brines released back-arc volcanosedimentary basins. latter environment recurred comparable orogenic settings elsewhere, extended histories incorporating numerous events, may therefore represent salient feature clan.