Fluid inclusions in quartz from a gold-mineralized granodioritic intrusion at Carlés, Asturias, Spain

Abstract

The granodioritic stock of Carlés, located near Ovideo (Asturias, Spain) is intruded in Devonian limestones which are transformed by contact metamorphism. Small inclusions of native gold are associated with arsenopyrite in the granodiorite and metamorphic aureole, either in quartz veins or in fractured rocks. Heating and freezing measurements carried out on the fluid inclusions in the quartz veins reveal the presence of four types of fluid inclusions: aqueous two-phase inclusions (type A); carbonic two-phase inclusions (type B); aqueous three-phase inclusions containing a halite cube (type C); and CO 2-rich three-phase inclusions (type D). Aqueous inclusions (types A and C) show two generations, the homogenization of the whole occurring in a wide temperature interval (100–380°C). The carbonic and hydrocarbonic inclusions (types B and D, respectively) show each as a single generation, with the homogenization temperature of the CO 2 component occurring — either in the liquid or in the vapor phase — from 21° to 26°C, and the total homogenization temperature for type D varies between 280° and 380°C. Estimated pressures and temperatures from the stated inclusions would have maximum values of 2.4 kbar and 550°C. Two very important aspects of the fluids associated with the mineralization are: (1) the presence of CO 2 as a factor which could help the Au mobilization; and (2) the occurrence of important differences in density and salinity for apparently contemporaneous fluid inclusions, which could be related to boiling phenomena — by pressure drap — facilitating the Au precipitation.