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Distal signatures of, and vectors to, hydrothermal ores in carbonates in the Candelaria-Punta del Cobre district, Chile
Piurkowsky, Stephen
Piurkowsky, Stephen
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2021
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2022-09-10
Abstract
The Candelaria-Punta del Cobre district is a NE trending Cu-Au-(Zn-Ag) mineralized zone within the Chilean Iron Belt. Magnetite and copper-gold mineralization in the district is hosted in andesitic volcanic rocks of the Punta del Cobre Formation and in calcareous rocks of the Chañarcillo Group. Deposits may be flat lying (e.g., Candelaria) or in structurally controlled breccias (Santos, Alcaparrosa). Lower grade and tonnage mineralization also occurs in skarns hosted in calcareous sedimentary rocks of the Chañarcillo Group located stratigraphically above the volcanic rocks. The Chañarcillo group is well exposed in the eastern and southern peripheries of the district. Developed in these sediments are carbonate veins that are believed to be the distal expressions of spent mineralizing fluid(s) from the hydrothermal system. These veins and their associated wall rocks provide an excellent opportunity for studying the distal signatures of mineralization in carbonates. In this study, 87 surface samples and 44 drill core samples were collected to study the geological, geochemical, isotopic, and ultraviolet luminescence features of carbonate veins and wall rock. Samples were taken from traverses along faults through the orebodies (e.g., Traverses #6 and #7; mainly N-trending) and traverses not along any faults (e.g., Traverses #1, #2, #3, and #5; mainly E-trending).Veins proximal to mineralization are large (1-2 m width), extensive along strike (~100s of m), and in addition to calcite may contain other minerals like quartz, specularite and Cu sulfides (now malachite). Distal veins are composed solely of calcite and are small (<1 cm width and <0.5 m length). Proximal veins are hosted in altered rocks (e.g., skarn) whereas distal veins are hosted in carbonate or calcareous wall rocks. Wall rocks ~1-2 km from mineralization may have small (~mm scale) disseminated pyrite; pyrite disappears beyond this distance.
Multi-element and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data of calcite from both veins and wall rocks show maximum concentrations of Mn (>10,000 ppm) at ~1-3 km from mineralization, with samples along faults showing anomalies at a further distance than samples not along faults. Manganese concentrations return to background (~1,275 ppm) at distances of ~5-6 km. Other elements like Zn and Pb are detected in vein and wall rock multi-element geochemistry, but not as trace elements in calcite. These elements have closer (~1-2 km) maximum concentrations (>600 ppm for Zn, >100 ppm for Pb in veins), with Cu having the most proximal (within 100m) maximum concentrations (>1,000 ppm in veins).
The δ18O of H2O in equilibrium with vein calcite decreases from near magmatic signatures of ~13-2.5‰ 0-0.5 km close to orebodies to ~2.5 to -10‰ 4.5-8 km away from orebodies. δ13C values of CO2 in equilibrium with vein calcite also decrease away from orebodies, from ~1.5 to -2‰ at ~0.5 km distance, to -8 to -4‰ at ~4.5 km. Depletion in δ18O of wall rock calcite below ~20‰ due to hydrothermal alteration can be detected up to ~3.5 km from mineralization. Ratios of 87Sr/86Sr for vein and wall rock calcite were investigated along one traverse (n=14). The ratios from proximal veins have relatively primitive igneous signatures (~0.704). At more distal locations, wall rocks have signatures typical of Phanerozoic open ocean carbonates (~0.707). Veins from intermediate locations have mixed signatures (~0.704-0.705) and signatures from wall rock become progressively more mixed closer to mineralization.
Shortwave ultraviolet fluorescence of calcite in veins and wall rock was dominantly red in color (~94%), with a few samples having blue-white or green fluorescence. Red UVF is primarily due to the presence of Mn2+, along with other minor activators and sensitizers. A Manganese Fluorescence Index (MFI) is designed as a ratio of Mn times the sum of fluorescence activators and sensitizers to quenchers in calcite (MFI = Mn * (Sm + Eu + Tb + Dy + Pb +Ce) / (Fe + Co +Ni)) and was calculated based on LA-ICP-MS trace element data. MFI discriminates between red UVF and other colors. MFI also has a spatial trend with maximum values at ~3-4 km along faults and ~1 km from mineralization not along faults. Samples from orebodies have the lowest MFI score.
The above trends are interpreted to be related to the outflowing of mineralizing fluids, the decrease of fluid/rock ratio, and other parameters including temperature, pH, and redox state. Isotopic data is consistent with mineralization by magmatic-hydrothermal fluids and late interaction with meteoric waters. The results of this study indicate that distal carbonate wall rocks and veins contain important geological, geochemical, luminescence, and isotopic characteristics that may be useful for exploration. Geochemistry and mineral chemistry provide vectors (e.g., Mn in calcite) while C, O, and Sr isotopes provide for both vectoring and discrimination between barren carbonate veins and veins related to mineralization. UVF is less conclusive, but preliminary results indicate that red UVF is a useful indicator of Mn-rich calcite veins. The related MFI may provide a useful vector.
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