The Mantoverde district: example of the distal chlorite-sericite-carbonate facies of zoned IOCG systems
|Location||International Geological Congress,oslo 2008|
|Author||Rieger, Ana Alejandra۱; Marschik, Robert۱; Diaz, Manuel۲; Chiaradia, Massimo۳; Spangenberg, Jorge E.۴|
|Holding Date||04 October 2008|
The Mantoverde IOCG district is located in the Chilean Coastal Cordillera about 100 km north of Copiapó. The orebodies are mainly breccias with a Cu-Au-bearing specularite and/or magnetite-rich matrix. They are hosted in a tectonic block of Jurassic andesitic rocks, which is intruded by granitoids at depth.
This andesitic block is separated by branches of the Atacama Fault Zone from Lower Cretaceous granitoids exposed to the E and W. Four main ore zones, Kuroki, Mantoverde Norte, Mantoverde Sur, and Franco, occur along the Mantoverde Fault (MVF), a major brittle NW-trending, 40-50°E dipping structure, which divides the block. Other major orebodies are isolated andesite-specularite breccia pipes (Manto Ruso, Manto Monstruo) or hosted by subvolcanic intrusions in some distance from the MVF (Celso project). Barren massive magnetite replacements bodies occur in deeper parts of the deposits along the MVF. Mining still focusses on the near surface supergene, leachable Cu-Au ores at Mantoverde Norte, and Mantoverde Sur. However, the hypogene Cu mineralization is of interest for mining activities in the near future. It is dominated by chalcopyrite and subordinate bornite. Composite samples of these hypogene ores show that there is a good positive correlation of Cu and Au with a Cu/(Au •10000) of around 3.5 - 4. Magnetite from Mantoverde Sur and Norte has low TiO2 contents, averaging 0.16 wt.%, and low average V contents, which are close to 0.5 wt.%. The paragenetical sequence of the hypogene mineralization in the Mantoverde district shows close similarities with the Candelaria-Punta del Cobre IOCG deposits, near of Copiapó, with early specularite followed by magnetite-mushketovite, and finally hematite-martite. Sulfides occur interstitial to specularite. Enrichments of LREE have been detected locally.
Although, the Mantoverde district differs markedly from Candelaria-Punta del Cobre at a first glance, the abundant specular hematite and spatially associated Kfeldspar-chlorite-sericite-calcite alteration, which is characteristic for most of the mineralized zones, resembles the upper part of the deposits in the Punta del Cobre district. The Mantoverde district therefore shows features of an upper, more distal facies of zoned IOCG systems. This is also manifested in the range of sulfur isotope ratios of sulfides as a function of the sulfides spatial distribution. The sulfur isotope ratios of sulfides from the proximal portions of the Mantoverde system cluster around 0 ‰ consistent with a magmatic sulfur source, whereas sulfides from the distal portions are enriched in 34S suggesting a non-magmatic sulfur source.
The sulfides from Mantoverde have low Pb contents and their lead isotope signature shows a significant radiogenic lead component. The Pb isotope ratios overlap with those of sulfides from the Candelaria-Punta del Cobre district and of Early Cretaceous plutons of the Coastal Cordillera, which is suggestive for a magmatic Pb component.