MINERALOGY AND GEOCHEMISTRY OF MASSIVE SULFIDES FROM THE NORTH FIJI BASIN

During the SONNE cruise SO 134 new hydrothermal mound deposits were discovered in the SO 99 field. These mounds consist of chimney fragments of an older and younger massive sulfide generation, sometimes the younger generation chimneys top the mound deposit. Five different types of sulfide mineralisations can be distinguished. Precious metals like Ag and Au are fixed to a later-stage mesothermal sphalerite-silica mineral assemblage.

The Zn-rich paragenesis consists of sphalerite, amorphous quartz, pyrite, marcasite, and minor chalcopyrite. It could be distinguished into several sphalerite generations generally linked with amorphous SiO2. Zn-concentrations in this sulfide ore varies between 6.6 and 56 wt. %. Average Zn-contents are 32.5 wt. %, combined with high Cd concentrations between 0.1 and 0.96 wt. %. The samples are fragments of chimney walls with a typical bee-hive structure. Fluid inclusion studies indicate a “white smoker” epithermal mineralisation, with homogenisation temperatures around 230 °C. Multiple fluid remobilizations have formed sharp convex shaped “reaction fronts” in the relicts of the chimney walls. Along these thin boundaries inside the Zn-rich ores, Au and Ag are enriched with maximum concentrations of 5.4 ppm and 575 ppm.

The Cu-rich paragenesis consists of chalcopyrite, cubanite, bornite, covellite, and recrystallised Fe- sulfides (pyrite and marcasite). Cu-concentrations in this sulfide ore varies between 7 and 36 wt. %. Zoning and re-crystallisation structures are controlled by the mixing of hydrothermal fluids with cold seawater in the chimney walls. Chalcopyrite dominated smoker chimneys could be subdivided by there trace metal contents e.g. Mo, Se into two genetically different types. High Selenium contents (1240 ppm Se) suggest a high temperature of a katathermal stage (300-350 °C) for the ore building fluids. Similar Se-contents for high temperature “black smoker” mineralisations are known from the TAG field. Molybdenum is depleted in these high temperature chalcopyrite mineralisations. A strong Mo enrichment in sulfide ores is the result of mixing with low temperature fluids or suggests a relationship to a younger OIB-basalt type of a near triple junction point. Investigations of the Pb-isotope composition indicates basaltic source rocks of a mixed type of OIB and MORB.