GEOCHEMICAL AND SULFUR ISOTOPE COMPOSITIONS OF LATE PROTEROZOIC AND EARLY PALEOZOIC VOLCANOGENIC MASSIVE SULFIDE DEPOSITS, PRINCE OF WALES ISLAND AND VICINITY, SOUTHEASTERN ALASKA

Volcanogenic massive sulfide (VMS) deposits occur in two lithotectonic sequences on the southern half of Prince of Wales Island and vicinity. The older sequence, comprising greenschist- to amphibolite-facies volcanic and sedimentary rocks of the Late Proterozoic to Early Cambrian Wales Group, contains the Khayyam and nearby Stumble-On Cu-Zn-Ag deposits, the polymetallic (Zn-Pb-Cu-Ag) Ruby Tuesday deposit, and many small Cu±Zn±Ag deposits (e.g., Big Harbor, Copper City, Corbin, Keete Inlet). The younger Moira Group, a lower greenschist-facies volcanosedimentary sequence of Ordovician to Silurian age, contains small Zn±Cu±Ag deposits on the Barrier Islands and at Nichols Bay, and several Cu-Zn-Au-Ag deposits in the Niblack area, including Niblack, Dama, Trio, Moira, and the 2.5 Mt Lookout Mountain prospect. 

Geochemical analyses of 92 sulfide-rich rocks from 20 VMS deposits and occurrences reveal several patterns. Niblack-area deposits are distinguished by generally high Au (0.5-11 ppm) and Te (10-66 ppm); Khayyam and Stumble-On have mostly high Co (to 398 ppm) and relatively low Hg/Zn. In situ trace element analyses (n = 270) of pyrite, sphalerite, chalcopyrite, and galena from 16 deposits were acquired by laser-ablation ICP-MS on 50-mm spots. Pyrite from Barrier Islands deposits is distinct in typically containing high Mn (981-3552 ppm), As (3579-14412 ppm), Sb (128-283 ppm), and Tl (176-275 ppm); most Khayyam and Stumble-On pyrite has high Co (413-2057 ppm). Sphalerite from Niblack-area deposits contains uniformly low Fe (<0.7 wt %) and Mn (<60 ppm); Khayyam and Stumble-On sphalerite has relatively high Fe (2.9-7.5 wt %), and low Hg and In (<17 ppm each). Sulfur isotope d³⁴S values (n = 86) of sulfide separates and bulk sulfides range mainly from 6.2 to 17.4 ‰, recording variable contributions of seawater sulfate and footwall-derived sulfide sulfur.

Relatively Au- and Te-rich sulfide samples and low-Fe sphalerite in the Niblack area deposits reflect low-temperature VMS deposition under elevated fS₂ conditions; As-, Sb-, and Tl-rich pyrite from the Barrier Islands records similar low-T mineralization. Hg-poor sphalerite from Khayyam and Stumble-On reflects high-grade metamorphism.  Some trends in the data may be useful for discriminating VMS deposits in the Wales Group from those in the younger Moira Group.