RB-SR ISOTOPIC AGE AND GEOCHEMISTRY OF TH E MANTOS BLANCOS COPPER-DEPOSIT - TEMPORAL RELATION OF THE JURASSIC MAGMATISM

The Mantos Blancos volcanic sequence, host of the stratabound Cu (-Ag) deposit of the same name, has been studied for its Rb-Sr isotope geoc hemistry, to provide insights into the age and genesis of this importa nt mineral deposit. Whole rock Rb-Sr ages of rock units characterized for their alteration pattern, allow the following generalizations: a- errorcrons from the NK unit (rocks with normal K2O contents and with l ittle sodic alteration), yield ages of ca. 175 Ma, interpreted as the age of the host Mantos Blancos volcanic sequence; b- the ages of about 150 Ma obtained for the strongly altered ANa unit(sodic alteration), are interpreted to represent the age of hydrothermal alteration respon sible for sodium metasomatism and its associated copper mineralization ; c- intrusives emplaced into La Negra Formation and Mantos Blancos vo lcanic sequence may have been responsible for the thermal event causin g the hydrothermal alteration process related to the ore mineralizatio n. The initial Sr-87/Sr-86 of both NK and ANa units, and in calcite, w hich crystallized late in the paragenetic sequence of the ores, sugges ts that the hydrothermal fluids could not have been purely magmatic, n or of seawater origin, but could represent meteoric waters that have p artially equilibrated with the host formations, or mixtures of differe nt fluids.