Abstract

The gold deposits of the upper Anzasca Valley are part of the Monte Rosa gold district, northwestern Alps, Italy. The mineralization occurs within polymetamorphic Paleozoic metasedimentary and metagranitic rocks of the Pennidic Monte Rosa unit. The orebodies consist of more or less concordant veins where they are hosted by metasedimentary rocks, whereas in the metagranites they occupy fractures and shear zones discordant with the foliation. Field evidence suggests, however, a common late Alpine age for the emplacement of both types of orebodies.The mineralogy of the orebodies is dominated by quartz. Gold is invariably associated with sulfides (pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, and sulfosalts).The characteristics of fluid inclusions in quartz from gold-bearing veins are remarkably consistent: the inclusions are moderately saline brines containing, at room temperature, about 20 to 60 percent vol of a low-density, rather pure CO 2 phase; they were probably homogeneous at the time of trapping. Small amounts of CH 4 occur in some inclusions. Total homogenization occurs either in the liquid or in the gas phase, or by near-critical phenomena, at temperatures mostly on the order of 300 degrees to 350 degrees C. By combining these temperatures with geologic pressure estimates and with temperatures inferred from arsenopyrite compositions, trapping conditions of about 400 degrees to 450 degrees C at pressures of 1 to 1.5 kbars are derived. A less common, presumably later, type of CO 2 -poor inclusion is interpreted as a late influx of cooler and less saline fluids.Quartz from barren veins also contains CO 2 -bearing inclusions; however, these show higher T (sub m clathrate ) , lower T (sub h (sub CO 2 ) ) , and lower total homogenization temperatures than mineralized veins.Results of reconnaissance studies on fluid inclusions from other gold deposits (Val Sesia, Val Toppa) in the Monte Rosa district are somewhat different from those of the upper Anzasca Valley: the CO 2 phase is of higher density, and total homogenization temperatures are lower.Combined with field evidence and available Pb and O isotope data, the fluid inclusion characteristics are consistent with a model of ore genesis in which the gold deposits of the upper Anzasca Valley were formed in the late stages of the Alpine orogeny by presumably metamorphic fluids, probably produced by devolatilization of deep unexposed portions of Monte Rosa crustal rocks. The strict association of gold with sulfides suggests that gold possibly was transported as thio complexes. Possible precipitation mechanisms include cooling, dilution, minor wall-rock reaction, and decrease of sulfur activity in the fluid.Similar processes probably were active in the formation of other neighboring gold deposits, but there is evidence that gold deposition, on a regional scale, was related to at least two distinct hydrothermal systems separate in space and also in time.

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