Genesis of the Volcan Copiapo Sulphur Deposits, Maricunga District, Chile

Abstract

The Miocene age Copiapo Volcanic Complex is located in the heart of the Maricunga gold mining district in the southernmost part (Latitude 27oS) of the Andean Central Volcanic Zone. The largest cone in this complex, Volcan Copiapo or Azufre, has many voluminous sulphur deposits which contain porous sulphur impregnated bodies, hydrothermal breccia vents, silica sinter, and silica caps. Ore deposits in this district are formed by epithermal acid-sulphate (high-sulphur) alteration processes (e.g. Marte, Lobo).

Petrographic, X-ray diffraction, chemical, and microprobe studies indicate that the sulphur deposits contain the alteration mineral assemblage: native sulphur +/- alunite +/- quartz +/- gypsum +/- tridymite +/- minimilite +/- cinnabar. Clay minerals are generally rare. This assemblage resembles the surficial expression of an epithermal high-sulphur ore deposit.

Sulphur isotope, thin section, and hand sample studies indicate that: 1.) native sulphur in Copiapo sulphur deposits formed from 34S-depleted H2S(g) which was generated by the disproportionation of magmatic SO2(g); 2.) these sulphur deposits contain supergene sulphate minerals from the partial oxidation of 34S-depleted H2S(g) which is formed by the disproportionation of magmatic SO2(g); 3.) hypogene alunite in gold-bearing quartz veins at Marte formed from 34S-enriched sulphuric acid which is formed by the disproportionation of magmatic SO2(g).

A conventional K-Ar date on alunite from the sulphur deposits within the Copiapo Volcanic Complex gave an age of 13.3 +/- 0.5 M.Y. This date is coeval with the gold mineralization at Marte and Lobo in the periphery of the complex and much lower elevation. A 40Ar/39Ar date on an andesite flow, which partially covers and thus preserves the soft sulphur deposits, gives an age of 6.6 +/- 0.3 M.Y. This constitutes the youngest known volcanism in the Copiapo Volcanic Complex.

This work seems to confirm the hypothesis that native sulphur accumulations in the Volcan Copiapo Complex represent the surficial crown of high-sulphur epithermal precious metal deposits at depth.

Keywords: Acid-Sulphate Alteration, Epithermal, Gold, Native Sulphur, Alunite, Sulphur Isotopes, Geochronology, Volcanic. Pages: 139 Supervisors: Marcos Zentilli