52 Economic Geology I: Magmatic PGE-Cr-Cu-Ni, VMS, Sed-hosted Pb-Zn-Cu, and Alaska Au Deposits

Monday, 28 October 2002
8:00 AM - 12:00 PM

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

Paper number: 52-8
Program: Technical Sessions
Time: 9:45 AM – 10:00 AM

Authors

Abstract

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.