ABSTRACT

   The Mantoverde IOCG district is located in the Chilean Coastal Cordillera about 100 km north of the city of Copiapó, Chile. With an SX-EW fine copper production of 60 000 tonnes/year from supergene ores, AngloAmerican's Mantoverde operation is one of the most important ventures in the world mining this deposit class. The hypogene ores have been explored recently. They are mainly Cu-Au-bearing specularite and/or magnetite-rich breccias and stockworks. The orebodies are hosted in a tectonic block of Jurassic andesitic rocks between two branches of the north-south trending Atacama Fault Zone. The Mantoverde Fault (MVF), a major brittle northnorthwest-trending, 40 to 50°E dipping structure, divides this block into an upper southwestern and a tectonically down-thrown northeastern part. Four main ore zones, Kuroki, Mantoverde Norte, Mantoverde Sur and Franko, occur along the MVF. Other major orebodies, Manto Ruso and Celso, are located in structures subparallel and at some distance to the MVF. Chalcopyrite is the main hypogene Cu mineral. The mineralisation can be subdivided into three hydrothermal stages: a) an early high temperature Iron Oxide Stage forming the bulk of specularite-magnetite present in the district, b) a Sulphide Stage responsible for the main Cu-Au mineralisation, and c) a Late Stage mainly with calcite ± specularite veining, specularite veining, and pervasive carbonatisation. The host rocks at Mantoverde suffered variably proportions of K-feldspar, chlorite, sericite, quartz, and carbonate alteration, characteristic for the distal facies of zoned IOCG-systems. The distrital distribution in iron oxide phases reveals an up- and outward zonation from magnetite-rich ores representing the more proximal portions, towards specularite-dominated ores in the distal portions of the hydrothermal system. A zonation is also recognised in the sulphur isotope signatures of chalcopyrite as a function of its spatial distribution.  δ³⁴S_VCDT values centered around 0 ‰, suggesting a magmatic-derived sulphur component, are characteristic for chalcopyrite in orebodies with a close spatial relationship with the MVF in the southern and deeper central part of the Mantoverde district, representing the more internal parts of this IOCG hydrothermal system. Higher δ³⁴S_VCDT values, roughly around +6 ‰, are typical for northern part of the district or shallower levels of the hydrothermal system and suggestive for sulphur input from a nonmagmatic source. The new data is consistent with previously suggested genetic models in which a metal- and sulphur-bearing magmatic-hydrothermal brine ascends, cools, and mixes with a nonmagmatic more dilute surficial fluid, which also contributes with sulphur (and metals?).

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