Timing of volatile and magma ascent in the formation of the Bajo de la Alumbrera porphyry Cu-Au deposit

Abstract

Dacitic rocks at the Bajo de la Alumbrera porphyry Cu-Au deposit (NW Argentina) preserve textures that suggest volatiles streamed upwards through intrusions temporally related to ore formation and hydrothermal alteration. There is evidence that volatile phases accumulated in the carapace of some intrusions. Phase transformations measured from fluid inclusions in combquartz layered textures suggest pressure fluctuations, and imply that Cu-bearing volatiles were catastro phically released from the ascending magma into the adjacent wallrock. Other findings (including the results of low temperature thermochronology) also argue that magmatic volatiles were introduced independent of any new batches of magma. These volatiles must have been episodically sourced from deeper-seated magma bodies or blind intrusions beneath Bajo de la Alumbrera over millions of years.