Abstract

Spence is a supergene-enriched copper porphyry deposit located between Antofagasta and Calama in the Atacama Desert of northern Chile. Porphyry intrusion and hypogene mineralization took place during the Palaeocene. Following supergene enrichment that produced an atacamite–brochantite assemblage in the oxide zone, the deposit was covered by 50 to 100 m of piedmont gravels of Miocene age. RioChilex discovered the deposit in 1996 by reconnaissance drilling. This paper describes elemental and isotopic data for groundwaters collected within and peripheral to the deposit and relates these compositions to geochemical anomalies in gravel soils over the deposit. There are two distinct types of groundwater with distinctive isotopic and elemental composition: saline water (average Cl= 11 600 mg/l) extending downflow from the axis of the deposit; and non-saline water (average Cl=1300 mg/l) upflow from the axis. Oxygen and hydrogen isotope ratios imply that the former is a deep formation water and the other is meteoric water derived from precipitation in mountains to the east. The data suggest that the saline formation water flows upward from a permeable fault zone coincident with the long axis of the deposit. This fault may have originally guided the intrusion of the porphyries and the copper-bearing hydrothermal fluids. The saline water has much greater contents of As and Se (up to 129 ppb and 800 ppb, respectively) than the meteoric water (both less than detection limits of 10 and 50 ppb, respectively), but both types have high amounts of Cu within the deposit area. Dispersion of Cu away from the deposit is restricted by adsorption of Cu2+ on negatively charged hydroxide colloids, whereas As and Se freely disperse dissolved as anions. Reactivation of the axial fault created a permeable fracture zone in the gravels above the deposit. During earthquake activity there was pumping of saline formation water to the surface up this fracture zone, which created soil anomalies. The anomalies are characterized by NaCl, As, Se and Cu. One kilometre to the east of the deposit there is another fracture zone in the gravels, overlying unmineralized basement. Soils above this zone have anomalies for NaCl, As and Se, indicative of flooding by formation water, but lacking Cu.

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