Geology and Geochemistry of Highly Metamorphosed Footwall Alteration Zones in the Hongtoushan Volcanogenic Massive Sulfide Deposit, Liaoning Province, China

The Hongtoushan Archean Cu–Zn volcanogenic massive sulfide (VMS) deposit, which was metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650°C, occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. Stratiform cordierite–anthophyllite gneiss (CAG) that occurs hundreds of meters below the ore horizon in the Hongtoushan district corresponds to the metamorphosed semi-conformable alteration zone of the VMS hydrothermal system, whereas the CAG that contains abundant deformed sulfide-bearing quartz veins immediately below the main ore layer represents the metamorphosed discordant alteration zone. Whole-rock geochemistry indicates that stratiform CAG was derived ultimately from five lithologies (basalt, basaltic andesite, andesite, dacite, and rhyolite), while discordant CAG derived from a single lithology (rhyolite). Amphibolite and biotite-rich gneiss are identified as a metamorphosed least-altered precursor for these CAGs. Mass change calculation indicates that, compared to the least-altered rocks, stratiform CAG is enriched in Fe and Mg, and depleted in Na, K, Ca, Cu, Pb and Zn, while discordant CAG is enriched in Fe, Mg, Si, Na, Pb, Cu and Zn, and depleted in K. HREE and HFSE (Zr, Ti, Nb and Ta) behaved inertly during submarine alteration, whereas Rb, Sr, Ba and LREE (especially Eu) were leached off. Both stratiform and discordant CAGs are depleted in 18O, with values up to 7‰ lower than their corresponding least-altered precursors. Addition of Fe, Mg, and depletion of Ca, K, Sr, and 18O, indicate that hydrothermal alteration for both types of CAGs was characterized by chloritization prior to metamorphism. Stratiform CAG could be used to evaluate the mineralization potential of VMS in metamorphic terranes, while discordant CAG containing sulfide-bearing quartz veins could be a good indication for overlying stratiform massive sulfide ores as well as an exploration target itself.