CHLORINE ISOTOPE AND BROMINE EVIDENCE OF A JURASSIC SEDIMENTARY CHLORIDE SOURCE FOR ATACAMITE MINERALIZATION AT THE RADOMIRO TOMIC PORPHYRY COPPER DEPOSIT, NORTHERN CHILE

Supergene mineralization at the Radomiro Tomic porphyry copper deposit involves the copper-chloride mineral atacamite, distributed in a thick oxide zone as the main ore mineral. Aqueous fluids rich in chloride were introduced during supergene mineralization causing this atypical variation in copper-oxide mineralogy. The source of this chloride has been the subject of our research involving several geochemical techniques. Chlorine isotopes and bromine geochemistry have proved the most useful in determining potential chloride sources and evaluating their likely influence on copper mineralization. Typical geochemical characteristics of igneous rocks are low total bromine, (<2 ppm) and heavy chlorine isotopes (d37Cl=0.1 to 2.5 ‰). Un-mineralized, biotite-bearing whole rock samples from Radomiro Tomic conform to these typical values. In contrast, elevated bromine content, (22 to 180 ppm) and d37Cl values near 0.0 ‰ for the atacamite mineralization at Radomiro Tomic are clearly indicative of a non-magmatic origin for the mineralizing chloride. Moreover, atacamite mineralization at Chuquicamata and Mina Sur also have elevated bromine contents and d37Cl values near 0.0 ‰, indicating a similar source of introduced chloride.

The potential sources of non-magmatic chloride at Radomiro Tomic are: seawater, marine evaporites, or sediments deposited in a marine environment retaining a seawater signature. Seawater has a bromine concentration of 65 ppm and is used as the chlorine isotopic standard with a d37Cl value of 0.0 ‰. Marine evaporites have elevated bromine contents, >65 ppm and d37Cl values between 0.5 and -0.6 ‰. Regional Jurassic sediments have bromine concentrations from 10 to 25 ppm and d37Cl values of 0.5 to -2.5‰. Nearby Jurassic sediments appear to be the source of chloride in the atacamite mineralization at the Radomiro Tomic deposit, with the later influence of marine evaporites on exotic copper deposited in the overlying gravel. Numerous, small-scale deposits of atacamite occurring to the west of Radomiro Tomic also have geochemical signatures indicative of sedimentary derived chlorine, (20 - 100 ppm Br; d37Cl=-0.1 to -3.2 ‰).