Ulfers, Arne (2014) Mineralogy and geochemistry of seafloor massive sulfides from the Solwara 8 deposit, Papua New Guinea, with special emphasis on laser-ablation ICP-MS studies. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 89 pp.

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Abstract

Since the discovery of hydrothermal vent fields in the last century, several locations of active hydrothermal venting were discovered around the world. The metal-rich precipitates came into the focus of science, not only due to their role in understanding early genesis of volcanogenic massive sulfide deposits (VMS), but also as possible resource for future mining. Past studies disclosed that, especially precipitates from the eastern Manus back arc- basin, Papua New Guinea belong to the gold-richest sulfides yet discovered and most of the gold is linked to sphalerite in Zn-sulfide-rich samples. Surface samples were investigated for their mineral assemblage which lead to a classification in 4 groups: (1) Dense, chalcopyrite with minor secondary Cu-sulfides, (2) dense, secondary Cu-sulfides, (3) porous chalcopyrite and (4) porous sphalerite with minor chalcopyrite. Geochemical investigations of all groups revealed gold contents of 3.3 - 70.9 ppm Au, ø=25.9 (n=13). The highest gold-concentrations of 56.9 - 70.9 ppm Au, ø=64.3 (n=3) occurred in the porous-chalcopyrite group, while the gold-poorest samples were from the sphalerite-rich group 5.0-7.2 ppm Au, ø=6.0 (n=3). Amongst others, mineralogical investigations were carried out to count the grain-size and host-mineral distribution of gold-grains. Most grains were observed in chalcopyrite and the secondary Cu-sulfides bornite and digenite, whereas only a small fraction was found in fahlore and sphalerite. The size of the grains tends to be bigger in the secondary Cu-sulfides. Furthermore, the fineness of some gold grains was calculated after measurements with the electron microprobe. Additionally a complex coexistence of different bornite types and digenite was observed in samples from the dense, chalcopyrite-rich group 1 and the secondary Cu-rich group 2. In several areas a zonation of different colors in bornite was discovered. In general it is difficult to distinguish between bornite and digenite because solid solutions are common. Also segregation lamella with a width of only >1-10 μm are frequent throughout sections in group 2. Although electron microprobe analyzes for these narrow lamella was difficult, a differentiation could successfully be conducted and differences in the main components could be visualized by element specific mapping of the concerned areas. Selected samples from each group were analyzed with the LA-ICP-MS to determine trace element contents. The gold-content in sphalerite was 0.4 - 15.6 ppm Au, ø=5.5 (n=30). Since almost no gold grains were observed, it is likely that gold is distributed on a submicroscopic scale in sphalerite. Furthermore, measurements in bornite/digenite revealed values of up to 6.1 ppm Au, ø=0.8 (n=35) and chalcopyrite up to 4.3 ppm Au, ø=1.2 (n=9). This leads to the assumption that the high gold contents in the chalcopyrite- and bornite/digenite-rich groups is caused by solid gold grains of at least 1μm size and not by submicroscopic intergrowth like in the sphalerite-rich groups.

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