Hydrothermal fluids during mineralization and alteration in the Hualilan Mining District, province of San Juan

Abstract

The detailed study of the fluid inclusions in the dacitic intrusions of Hualilan mining district has been carried out to evaluate the evolution of the hydrothermal processes during the stages of mineralization and alteration that have affected these rocks. Hualilan is located 120 km to the north-northwest of San Juan city. The study of representative samples from the northern sector (Falla Sánchez) and from the southern sector (Magnata) shows two periods of circulation of hydrothermal fluids with well defined characteristics. In the first one, which can be considered the responsible of the primary sulphide mineralization, the temperature has overcome the 300ºC, with a boiling episode responsible for the deposition of ore minerals, and an average salinity of 12 % in weight of NaCl eq., with an important participation of CaCl₂. The second period had a temperature below 300ºC, also with a boiling episode, and a salinity average of 4.5 % in weight of NaCl eq., without participation of calcium. This second episode can be considered the responsible of the intense hypogenic oxidation that has suffered the primary mineralization, ending up to its disappearance in wide sectors, and producing also the masking of the calc-silicate minerals, the alteration of the dacitics dikes, the superficial formation of jasperoid bodies related to normal faults, and the formation of minerals typical of an acid-sulfate alteration, superpose to the previous illite, caolinite and adularia alteration. The second episode has been more intense in the northern sector (Falla Sánchez) than in the southern one.