Nodular sillimanite rocks as field indicators to metamorphosed massive sulfide deposits

Abstract

Nodular sillimanite rocks (NSR), some of which are spatially associated with massive sulfide deposits, consist of intergrowths of quartz, sillimanite (mainly fibrolite), and muscovite (up to 30 cm in length) that occur predominantly in Archean-Proterozoic metapelites, ortho- and para-gneisses, and metavolcanic rocks metamorphosed to the amphibolite/granulite facies. Such rocks are particularly abundant in central Colorado where they either enclose or occur stratigraphically above or below at least 10 metamorphosed Proterozoic Cu-Zn-(Pb-Au-Ag) deposits. A plot of NSR compositions in terms of SiO2/Al2O3 vs Na2O/K2O and a K2O-MgO-Na2O ternary diagram suggest that the protoliths of NSR spatially associated with sulfide occurrences in Colorado were predominantly shale, arenite, lithic arenite or greywacke. North American shale composite (NASC)-normalized rare earth element (REE) patterns of NSR associated with these deposits show compositions close to NASC values with flat patterns and slightly negative Eu anomalies, which also supports the concept of a sedimentary protolith. Such a protolith contrasts with NSR spatially associated with massive sulfide deposits elsewhere in the world that more commonly have volcanoclastic/igneous precursors. Mass-balance calculations of NSR from the Cotopaxi deposit (Colorado) show major and trace element mobility with mostly mass gains in K2O, MgO, Ba, Cu, Pb, and Zn, and depletions in CaO, and Na2O relative to their protolith (lithic arenite), an unaltered biotite-muscovite-feldspar gneiss. Although Al2O3 and SiO2 show both gains and losses, the mass gains are considerably larger for Al2O3. The gains in base metals (Cu, Pb and Zn), the enrichment of Cu and Zn (rather than Pb) that mimics the ore concentrations, the presence of gahnite (ZnAl2O4) in sillimanite nodules in some metallic deposits in Colorado, and the existence of chalcopyrite and pyrrhotite in NSR suggest the sedimentary protoliths were hydrothermally altered and that NSR, where spatially associated with sulfide deposits, represent zones of metamorphosed stratabound alteration zones. Nodular sillimanite rocks are easy to recognize in the field and constitute an exploration guide for massive sulfide deposits that were subjected to regional metamorphism at the amphibolite to granulite facies.