Geology of the Manto Verde Copper Deposit, Northern Chile: A Specularite-Rich, Hydrothermal-Tectonic Breccia Related to the Atacama Fault Zone
TomÁS Vila, Zamora Richard, Nicholas Lindsay, 1998. "Geology of the Manto Verde Copper Deposit, Northern Chile: A Specularite-Rich, Hydrothermal-Tectonic Breccia Related to the Atacama Fault Zone", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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The Manto Verde copper deposit is associated with specularite-rich breccias emplaced during extensional tectonism along brittle faults of the Atacama fault zone (AFZ). The AFZ is a complex sinistral strike-slip/dip-slip system that originated during oblique subduction in a Jurassic to Early Cretaceous magmatic arc. Activity of the AFZ is documented from the Middle Jurassic, with intermittent movements occurring at least into the late Miocene. South of Manto Verde, the AFZ is a major control on the emplacement of apatite-bearing magnetite deposits in the Cretaceous Iron Belt (CIB).
The Manto Verde deposit is hosted by cataclased andesitic volcanics and coeval (?) diorite porphyry stocks of Early Cretaceous age affected by low-grade regional metamorphism. The deposit consists of three breccia units paralleling the Manto Verde fault (MVF) for at least 1500 m. The MVF is a 12 km long, north-northwest — striking- 40°-50°E — dipping structure that joins two major branches of the AFZ, and shows evidence of premineralization strike-slip motion followed by postmineralization, east-side-down, dip-slip movement. The hanging - wall Manto Atacama breccia is a ±100 m wide, specularite-rich, matrix-supported hydrothermal breccia that follows the MVF along strike and down dip, although it becomes narrower with depth. A Transition Zone is defined between the Manto Atacama breccia and the andesitic country rocks as brecciation intensity and associated copper mineralization decrease progressively away from the MVF. In contrast, the footwall Manto Verde breccia is represented by a ±20m wide cataclasite displaying a relatively sharp mineralization boundary.
These units are all deeply oxidized to 200 m, so that the present mineralogy consists of a suite of copper carbonates, sulfates, and silicates with minor copper chlorides, which occur as fracture fillings, patches, and disseminations. Only a thin, poorly developed supergene enrichment zone exists. Hypogene sulfide mineralization is poorly explored and corresponds to chalcopyrite ± pyrite disseminated in a specularite matrix.
District-wide chloritization and K-silicate alteration, characterized by microcline replacement and veining of the andesitic country rock, is recognized. Superimposed sericitic alteration is localized by the MVF and neighboring fracture zones. This late alteration event accompanied the main deposition of specularite and associated copper(-gold) mineralization dated by the KlAr method at 121 ± 3 and 117± 3 Ma.
Preliminary studies indicate a predominance of three-phase fluid inclusions as well as the coexistence of vapor-rich and liquid-rich, two-phase inclusions evidencing boiling. Homogenization temperatures for two and three-phase inclusions in translucent quartz veinlets and fragments accompanying early iron - copper - (gold) mineralization range between 180° and 320° C, although they are mainly between 180° and 250° C. Salinities for the three-phase inclusions vary from 30 to 47 wt percent NaCl equivalent, although a decrease in fluid salinity (14-21 wt % NaCl equiv.J is detected in two-phase inclusions from late-mineralization, chalcopyrite-bear-ing calcite veins.
The geologic features of the Manto Verde deposit suggest a common magmatic-hydrothermal origin with the apatite-bearing magnetite deposits of the CIB. Manto Verde probably represents the copper-rich, specularite-dominated, and possibly younger end member of a continuum of deposits that extends to copper-poor, magnetite-dominated deposits at the other extreme.