Fluid evolution in the Cu-Au deposit related to the Carlés granodiorite (Asturias)
Arcos, David; Soler, Albert; Delgado, Jοrdi
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 975 - 985
published: Oct 30, 1996
manuscript accepted: May 13, 1996
manuscript received: Sep 22, 1995
ArtNo. ESP147050805008, Price: 29.00 €
Abstract The Carles copper-gold deposit, located in the Cantabrian Zone (the outermost zone of the Hercynian Iberian Massif), is associated with a small intrusive body of granodioritic composition of late Hercynian age (283 ± 6 Ma). Four different types of alteration characterize the deposit: a skarn developed in marbles; an amphibolearsenopyrite replacement of the marbles and of the earlier metasomatic skarn; a K-feldspar alteration mainly developed in the granodiorite; and a greisen-type alteration also developed in the granodiorite. The exoskarn and an endoskarn alteration were the first alterations formed in a high-temperature episode (720-500°C), while the other three types of alteration and a veining stage hosted in the exoskarn, were formed along with the ore minerals in a lower temperature stage (500-300°C). Ore mineralogy in the veins of the skarn and K-feldspar alteration consists of copper sulfides and significant amounts of gold; in the greisen and amphibole-arsenopyrite alterations the dominant sulfides are arsenopyrite, lollingite, and pyrite, and gold has been not detected. Fluid inclusions in garnets from the exoskarn and in quartz from veins of the skarn and the K-feldspar alteration, are highly saline (up to 48 wt% Nad equiv.), with small quantities of CO2- Fluid inclusions in quartz from greisen and amphibole-arsenopyrite alterations show low salinity (< 6 wt% NaCl equiv.), significant amounts of CO2 (XCo2 - 0.15-0.03), and small amounts of CPU, N2, and H2S were also detected by micro-Raman analysis. The similarity of δ18O values in quartz from all alteration types, and the fluid inclusions relationships suggest that an unmixing process took place during the low-temperature episode, related to a fracturing event. The distributions of ore minerals and fluid-inclusion types suggest that gold and copper were fractionated preferentially into the high-salinity fluid. A decrease in temperature probably caused the precipitation of gold and copper sulfides in the skarn and K-feldspar alteration.