Geochemistry of Hanging-Wall Metasedimentary Rocks at the Greens Creek Massive Sulfide Deposit, Southeastern Alaska

Greens Creek is an unusual massive sulfide deposit in showing affinities to both seafloor magmatic processes and sedimentary processes. Geochronology suggests that the ores were formed in Triassic time by hydrothermal activity associated with subseafloor mafic and ultramafic plutonism. The presence of Co, Ni, and Cu in the ores--along with Zn, Pb, Ag, and Au--is consistent with a magmatic metal source. Argillite of Triassic age comprises the hanging wall of the deposit. Evidence that sedimentary processes were important includes low and variable δ³⁴S in ore sulfides (average: –13.9‰, range: –38.8 to 1.1‰) which implicates low-temperature bacterial sulfate reduction as the source of reduced sulfur. The rocks were overprinted by regional metamorphism in Cretaceous time.

Geochemical studies have been carried out on the hanging wall argillites to better constrain the role of sedimentary processes in ore formation. A few samples have S/C, degree-of-pyritization, and base- and precious metal concentrations like normal marine shales, but most are higher in these parameters. Dolomitic argillite has δ¹⁸O and δ¹³C values of 19±3‰ and –6±2‰, respectively, lower than normal marine carbonate but indistinguishable from the δ-values of hydrothermal dolomite within the orebody. Organic carbon, which is present in argillite in varying amounts, has a δ¹³C value of –25.9±0.9‰; the carbonate-organic carbon fractionation is uniform across a wide range of organic carbon contents. Dolomite δ¹³C is weakly correlated with organic carbon δ¹³C and with sulfur content.

We infer that (1) the argillite section experienced epigenetic additions of sulfur and metals, (2) the fluid that deposited sulfide-stage dolomite in the orebody also affected the overlying argillite section, (3) dolomite precipitated either from a single fluid over a range of temperatures or from a mixture of marine and hydrothermal fluids, and (4) H₂S was produced from marine sulfate by oxidation of sedimentary organic matter or organic-derived methane.