Alaska Science Center

U.S. Geological Survey
Professional Paper 1732-C

Geochemical and Sulfur-Isotopic Signatures of Volcanogenic Massive Sulfide Deposits on Prince of Wales Island and Vicinity, Southeastern Alaska

By John F. Slack, Wayne C. Shanks III, Susan M. Karl, Pamela A. Gemery, Peter E. Bittenbender, and W. Ian Ridley


Figure 1 is a full-color geologic map of southern Prince of Wales Island and vicinity in southeastern Alaska, with areas colored to show the various geologic units, including intrusive rocks (Mesozoic plutons, Paleozoic plutons, and Upper Proterozoic plutons) and stratified rocks (Silurian and younger strata, the Silurian through Ordovician Descon Formation, the Lower Silurian through Ordovician Moira Sound unit and correlatives, and the Cambrian through Upper Proterozoic Wales Group). Red dots and triangles show the locations of volcanogenic massive sulfide deposits and occurrences in the study area, mainly in the Wales Group and the Moira Sound unit.
Simplified geologic map of southern Prince of Wales Island and vicinity, showing locations of volcanogenic massive sulfide deposits and occurrences in study area (from figure 1).


Stratabound volcanogenic massive sulfide (VMS) deposits on Prince of Wales Island and vicinity, southeastern Alaska, occur in two volcanosedimentary sequences of Late Proterozoic through Cambrian and of Ordovician through Early Silurian age. This study presents geochemical data on sulfide-rich samples, in situ laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) of sulfide minerals, and sulfur-isotopic analyses of sulfides and sulfates (barite) for identifying and distinguishing between primary sea-floor signatures and later regional metamorphic overprints. These datasets are also used here in an attempt to discriminate the VMS deposits in the older Wales Group from those in the younger Moira Sound unit (new informal name). The Wales Group and its contained VMS deposits have been multiply deformed and metamorphosed from greenschist to amphibolite grade, whereas the Moira Sound unit and related VMS deposits are less deformed and generally less metamorphosed (lower to middle greenschist grade). Variations in the sulfide mineral assemblages and textures of the VMS deposits in both sequences reflect a combination of processes, including primary sea-floor mineralization and sub-sea-floor zone refining, followed by metamorphic recrystallization. Very coarse grained (>1 cm diam) sulfide minerals and abundant pyrrhotite are restricted to VMS deposits in a small area of the Wales Group, at Khayyam and Stumble-On, which record high-grade metamorphism of the sulfides.

Geochemical and sulfur-isotopic data distinguish the VMS deposits in the Wales Group from those in the Moira Sound unit. Although base- and precious-metal contents vary widely in sulfide-rich samples from both sequences, samples from the Moira Sound generally have proportionately higher Ag contents relative to base metals and Au. In situ LA-ICP-MS analysis of trace elements in the sulfide minerals suggests that primary sea-floor hydrothermal signatures are preserved in some samples (for example, Mn, As, Sb, and Tl in pyrite from the Moira Sound unit), whereas in other samples the signatures are varyingly annealed, owing to metamorphic overprinting. A limited LA–ICP–MS database for sphalerite indicates that low-Fe sphalerite is preferentially associated with the most Au rich deposits, the Niblack and Nutkwa.

Sulfur-isotopic values for sulfide minerals in the VMS deposits in the Wales Group range from 5.9 to 17.4 permil (avg 11.5±2.7 permil), about 5 to 6 permil higher than those in the Moira Sound unit, which range from -2.8 to 10.4 permil (avg 6.1±4.0 permil). This difference in δ34Ssulfide values reflects a dominantly seawater sulfate source of the sulfides and is linked to the δ34S values of contemporaneous seawater sulfate, which were slightly higher during the Late Proterozoic through Cambrian than during the Ordovician through Early Silurian.

Download this paper as a 37-page PDF file (pp1732c.pdf; 4.7 MB)

For questions about the content of this report, contact John Slack

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