Genetic Aspects of the Manto‐type Copper Deposits Based on Geochemical Studies of North Chilean Deposits

Abstract

Recent studies on mineralogy, geochronology, fl uid inclusion and stable isotope (Pb, Os, S, C, O, Sr) characteristics were reviewed to determine constraints for genetic models of the Chilean manto-type copper deposits. The Chilean manto-type deposits are divided into the two geologic categories of the northern areas (AricaIquique, Tocopilla -Taltal) and the central areas (Copiapó, La Serena, Santiago). The former is distributed in the coastal range composed of Jurassic andesite-dominated volcano-sedimentary piles and younger plutonic intrusions, and yields chalcocite (-digenite) and bornite as the principal hypogene copper sulfi des. The latter is hosted mostly in Lower Cretaceous volcano-sedimentary sequences, and has chalcopyrite-rich mineral associations. The fl uid inclusion data indicate that the primary copper mineralization was commonly generated in the temperature range 150 -360°C under low-pressure conditions near the boiling curve, mediated with relatively saline brines. Generally, homogeneous Pb and S isotope compositions for primary copper minerals imply direct magma source or leaching of igneous rocks. Pb and Os isotope data published for some deposits, however, suggest that ore-forming metals were derived mainly from the volcano-sedimentary host rocks. The noticeably negative isotope ratios of primary sulfi de sulfur and hydrothermal calcite carbon of some central area deposits indicate infl ux of sedimentary rock components, and the high 87 Sr/ 86 Sr initial ratios of hydrothermal calcite from the Tocopilla -Taltal area deposits imply contribution of the contemporaneous seawater or marine carbonates. These isotopic constraints imply a formation mechanism in which the Chilean manto-type copper deposits formed epigenetically in the process of hydrothermal interaction of non-magmatic surfacederived brine with the volcano-sedimentary host rocks, which is inferred to have been induced by a deepseated plutonic complex as the possible heat source.

Geological and Geophysical Studies of Sulfide Copper Mineralization in the Dochileh Area: An Example of Manto‐Type Deposit in the Sabzevar Zone, Iran

“…The δ 13 C values from the Dochileh area are comparable to Mantos Blancos and Mantos de la Luna deposits in Chile (Fig. ), which suggest a significant contribution of marine carbonates associated with igneous carbonates in the area (Kojima et al ., ).…”

“…The δ 13 C values of five calcite samples from the mineralized veins can be categorized into two groups; less negative values (−0.6 and −2.7‰) and more negative values ranging from −5.4 to −7.6‰. These data suggest that the more negative carbon in the mineralized veins may have been of magmatic origin (Richards & Spooner, ; Kojima et al ., ; Ramirez et al ., ) or sedimentary carbonate layers, which intercalated in the host rock (Kojima et al ., ). The δ 13 C value for magmatic carbon is typically around −4 to −8‰ (Ohmoto & Rye, ; Kyser, ; Burrwos & Spooner, ).…”

“…Overall, the host rocks, alteration, ore mineralogy, ore structure and texture, fluid inclusions, and stable isotope characteristics in the Dochileh district are similar to those copper manto types of the northern area deposits in Chile (Wilson & Zentilli, ; Kojima et al ., ). However, the manto‐type deposits in the northeast of Iran occur in volcanic and volcanosedimentary rocks of the Upper Eocene age, but the manto‐type deposits in northern Chile occur in the Jurassic period.…”

“…The δ 18 O and δ 13 C values obtained from the Dochileh area indicate that the source of hydrothermal water responsible for primary copper mineralization are surface-derived fluids such as meteoric water or seawater, including basinal brine, resulting in the digenesis of a deeper volcanosedimentary sequence (e.g., Kojima et al, 2009). This fluid has flowed downward through the marine carbonate or sedimentary carbonate layers within volcanic rocks, and the volcanic sequence leached copper values from ferromagnesian minerals such as pyroxene and amphibole.…”

Extremely negative and inhomogeneous sulfur isotope signatures in Cretaceous Chilean manto-type Cu–(Ag) deposits, Coastal Range of central Chile

“…Stratabound manto-type Cu-(Ag) deposits (Kojima et al, 2009;Maksaev and Zentilli, 2002;Sato, 1984;Sillitoe, 1989, Tristá Aguilera et al, 2006 are widely distributed in the Jurassic to Lower Cretaceous volcanosedimentary sequences of the Coastal Range of north and north-central Chile. These Chilean manto-type (CMT) deposits are relatively high grade, e.g.…”