Copper(-Iron) Mineralization and Superposition of Alteration Events in the Punta Del Cobre Belt, Northern Chile

Abstract

Two superimposed alteration events control the regional distribution of five main alteration types in the Punta del Cobre belt, south of Copiap6, Chile. An early event of alkali metasomatism caused extensive albitization and was overprinted locally by potassic alteration. Albite-quartz-chlorite or K-feldspar-quartz-chloritelbiotite assemblages characterize the metasomatized rocks. The alteration resulted in marked changes in chemical composition, with extremes of 10 wt percent Na20 or 11 wt percent K20. The second alteration event was associated with emplacement of a middle Cretaceous batholith, which is exposed in the western part of the area. The intrusion produced contact metamorphism which overprinted, with variable intensity, the areas affected by the alkali metasomatism. The contact effects are expressed as north-northeast-trending, largely overlapping zones which, from west to east, are characterized by: Ca-amphibole± biotite ± sericite, biotite± chlorite ±sericite ± epidote, and epidote-chlorite ± quartz± calcite. The Ca-amphibole is mainly actinolite, actinolitic hornblende, and magnesio-hornblende. The three alteration zones show only minor chemical modifications compared to the inferred average compositions of unaltered precursors. Chalcopyrite, pyrite, magnetite, and hematite are the principal ore minerals. Alkali metasomatism, in particular potassic alteration, is related spatially to mineralization both east and west of the Copiap6 River. In contrast, calcsilicate assemblages, unrelated genetically to the Cu mineralization, occur only west of the river, close to the batholith contact. In the Punta del Cobre and Ladrillos districts, the mineralization appears to be controlled by north-northwest- to northwest-trending faults, which were active since the middle Cretaceous. A middle Cretaceous age for the mineralization is supported by a 40 Ar/39 Ar inverse isochron age of 114.9 ·±0.5 Ma for hydrothermal biotite. The alteration pattern and ore formation temperatures (up to 400°-500°C) support an association of the Cu mineralization with deep-seated magmatic intrusion(s). Fluid inclusion data for postore calcite indicate involvement of saline fluids. A magmatic source of the sulfur is indicated by isotope ratios determined for chalcopyrite and pyrite (∂345 between -0.7 and + 1.1 %o). The deposits in the Punta del Cobre belt are somewhat similar and possibly transitional to both the Chilean magnetite (-apatite) deposits and porphyry copper deposits.