Lead and oxygen isotope evidence for the origin of the Monte Rosa gold lode deposits (Western Alps, Italy); a comparison with Archean lode deposits

Abstract

In the Monte Rosa area of the northwestern Italian Alps swarms of epigenetic, quartz sulfide gold-bearing veins occur within a terrane affected by Tertiary regional metamorphism of greenschist to amphibolite grade. They are preferentially associated with the felsic rocks of the upper Penninic nappes. Field evidence and available geochronologic data imply emplacement ages postdating the metamorphic peak (38 + or - 2 m.y.). The spatial distribution of the veins is related to late metamorphic large-scale tectonic disturbances, which displace, truncate, or fold the isograds.Lead isotope analyses of galenas and other sulfides were compared with those of feldspars, clinozoisites, and whole rocks from various unaltered host rocks. The data suggest derivation of the ore lead from Caledonian metapelites, with a possible contribution from Hercynian granitoids, but they rule out significant contributions from the neighboring ophiolitic greenstones and associated stratiform massive sulfide deposits. The separation of the ore leads into two distinct data fields is probably the result of ore deposition from at least two hydrothermal systems, which were separated in space and possibly also in time. The hypothesis of two distinct mineralization ages is also supported by geochronologic data.Oxygen isotope data indicate that the ore fluids had isotopic compositions consistent with a metamorphic origin, with the notable exception of the Val Toppa mine, where lighter, possibly surface-derived water was involved. Oxygen isotope fractionations between mineral pairs in metasediments and granites indicate metamorphic peak temperatures of 530 degrees to 605 degrees C, in agreement with those inferred from petrologic evidence. These temperatures and preliminary fluid inclusion data were used to predict oxygen isotope compositions of gangue quartz. The comparison of predicted with measured delta 18 O values shows consistency with the assumption that the fluids were generated by metamorphic devolatilization within the metapelites and granitoids, in agreement with the lead isotope data.The results suggest that the gold-bearing fluids and the lead were supplied by rocks derived from the continental crust, in opposition to the data from many greenstone-hosted Archean gold lode deposits, which indicate a mantle source. This implies that metamorphic-hosted quartz-gold lodes do not necessarily derive metals and fluids from source beds within a greenstone sequence. Therefore, the assumption that the Archean gold was derived from greenstones should be checked by isotope tracing studies. The common lead method appears to be particularly promising in distinguishing greenstone metamorphic from granitic sources in Precambrian terranes, in view of the contrasting U/Pb ratios usually stated in these lithologies.