SULFUR ISOTOPIC COMPOSITION OF HYDROTHERMAL PRECIPITATES FROM THE LAUBACK-ARC - IMPLICATIONS FOR MAGMATIC CONTRIBUTIONS TO SEA-FLOOR HYDROTHERMAL SYSTEMS

The sulfur isotopic composition of sulfides and barite from hydrotherm al deposits at the Valu Fa Ridge back-are spreading center in the sout hern Lau Basin has been investigated. Sulfide samples from the White C hurch area at the northern Valu Fa Ridge have delta(34)S values averag ing +3.8 parts per thousand (n = 10) for bulk sphalerite-chalcopyrite mineralization and +4.8 parts per thousand for pyrite (It = IO). Barit e associated with the massive sulfides exhibits an average of +20.7 pa rts per thousand, (rr = 10). Massive sulfides from the active Vai Lili hydrothermal field at the central Valu Fa Ridge have much higher delt a(34)S ratios averaging +8.0 parts per thousand for bulk sphalerite-ch alcopyrite mineralization (n = 5), +9.3 parts per thousand for pyrite samples (n = 5), and +8.0 parts per thousand, and +10.9 parts per thou sand for a chalcopyrite and a sphalerite separate, respectively. The i sotopic composition of barite from the Vai Lili field is similar to th at of barite from the White Church area and averages +21.0 parts per t housand (n = 8). Sulfide and barite samples from the Hine Hina area at the southern Valu Fa Ridge have delta(34)S values that are considerab ly lighter than those observed for samples from the other areas and av erage -4.9 parts per thousand, for pyrite (n = 9), -4.0 and -5.7 parts per thousand for two samples of sphalerite-chalcopyrite intergrowth, and -3.4 parts per thousand, for a single chalcopyrite separate. The t otal spread in the isotopic composition of sulfides from Vai Lili and Hine Hina is more than 20 parts per thousand over a distance of less t han 30 km. The delta(34)S values of sulfides at Hine Hina are the lowe st values so far reported for volcanic-hosted polymetallic massive sul fides from the modem seafloor. Barite from the Hine Hina field also ha s unusually light sulfur with delta(34)S values of +16.1 to +16.7 part s per thousand, (n = 5). Isotopic compositions of the sulfides at Hine Hina indicate a dramatic decrease in delta(34)S from ordinary magmati c values and. in the absence of biogenic sulfur and/or boiling. imply a unique S-34-depleted source of probable magmatic origin. Sulfide-bar ite mineralization in the Hine Hina area is associated with a distinct ive alteration assemblage consisting of cristobalite, pyrophyllite, ka olinite, opal-CT, talc. pyrite, native sulfur, and alunite. Similar st yles of alteration are typically known from high-sulfidation epitherma l systems on land. Alunite-bearing, advanced argillic alteration in th e Hine Hina field confirms the role of acidic. volatile-rich fluids, a nd a delta(34)S value of +10.4 parts per thousand for the sulfur in th e alunite is consistent with established kinetic isotope effects which accompany the disproportionation of magmatic SO2 into H2S and H2SO4. The Hine Hina field occurs near the propagating tip of the Valu Fa bac k-are spreading center (i.e., dominated by dike injections and seafloo r eruptions) and therefore may have experienced the largest contributi on of magmatic volatiles of the three fields. The sulfur isotopic rati os of the hydrothermal precipitates and the presence of a distinctive epithermal-like argillic alteration in the Hine Hina field suggest a d irect contribution of magmatic vapor to the hydrothermal system and su pport the concept that magmatic volatiles may be an important componen t of some volcanogenic massive sulfide-forming hydrothermal systems.