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Origin of placer laurite from Borneo: Se and As contents, and S isotopic compositions

¹ University of Ottawa, Ottawa, Canada
² 99 Fifth Avenue, Suite 122, Ottawa, Canada
³ Geological Survey of Finland, Espoo, Finland
⁴ US Geological Survey, Spokane, USA

View larger version (71K): [in a new window]   FIG. 1. (a) Map showing the laurite sample locality (open star), towns and villages (open squares) and selected features relevant to the placer PGM occurrences in southeastern Kalimantan. They include diamond-Au-PGM placers (dotted areas), PGM-Au placers (solid colour), podiform chromitite deposits (solid circles), ultramafic rocks (hatched pattern), and the Manunggul Formation (inclined stripe). The detailed geology of the dotted rectangular area close to the laurite location is shown in Fig. 1b. The Late Cretaceous sedimentary and mafic volcanic rocks of the Manunggul Formation appear to be the source of the diamond-bearing placers. Modified from Zientek et al. (1992). (b) Geological map of the Riam Pinang area, South Kalimantan. The location of the laurite samples is shown relative to the native gold-platinum-iron alloy-bearing placers downstream from Riam Pinang (area with PGM observed in pan concentrate samples). Modified from Zientek et al. (1992).

View larger version (91K): [in a new window]   FIG. 2. Morphology and BSE images of selected laurite grains. X marks the areas chosen for the analyses. Scale bars are all 100 µm. The brighter areas correspond to higher Os and the darker areas to higher Ru contents. (a) Morphology of grain P-1-1.; (b) BSE image of the grain, P-1-1; (c) spherical shape of P-2-6; (d) BSE image of the grain, P-2-6, with several inclusions of amphiboles; (e) angular shape with abundant fractures of P-2-9; and (f) BSE image of the grain P-2-9.

View larger version (73K): [in a new window]   FIG. 3. Laurite grains used for the S isotope study listed in Table 3: (a) Gr-3; (b) Gr-4; (c) Gr-5.

View larger version (122K): [in a new window]   FIG. 4. Laurite grain, P-1-2, containing a sulphide inclusion: (a) Subhedral shape of the grain; (b) photomicrograph of the grain; (c) BSE electron image of the grain; and (d) BSE image of the sulphide inclusion. The compositions of the areas marked by X are listed in Table 2. Abbreviations: Bn = bornite, Cp = chalcopyrite, Hz = heazlewoodite, Pn = pentlandite.

View larger version (22K): [in a new window]   FIG. 5. Compositions of laurite plotted on the Ru-Ir-Os ternary. The compositions of the laurite studied (open circles for Pontyn samples, solid circles for Tambanio samples) are compared with those from the Bushveld layered igneous complex (South Africa) and Bird River Complex (Canada). The compositional fields of laurite from ophiolitic ultramafic complexes are shown on the right side of the ternary. They include Oman, Othrys (Greece), Skyros (Greece) and Vourinos (Greece), Ray-lz (Polar Ural, Russia), and Rhodope (Bulgaria). Data sources: Bird River (Cabri and Laflamme, 1988; Ohnenstetter et al., 1986), Bushveld (Schwellnus et al., 1976; Kingston and El-Dosuky, 1982), Oman (Ahmed and Arai, 2003); Othrys (Garuti et al., 1999b), Ray-Iz (Garuti et al., 1999a), Vourinos (Garuti and Zaccarini, 1997), Skyros (Tarkian et al., 1992), and Rhodope (Tarkian et al., 1991).

View larger version (21K): [in a new window]   FIG. 6. Compositional variations of laurite grains, Ir vs. As and Ir vs. Os. Lines of calculated correlation and correlation coefficients, r, are shown. Open circles = Pontyn samples, solid circles = Tambanio samples. (a) Iridium vs. As, (b) Iridium vs. Os, (c) Arsenic vs. Se, (d) Osmium vs. As.

View larger version (26K): [in a new window]   FIG. 7. Ir and Os contents of laurite compared to those from the Merensky Reef of the Bushveld Complex (solid triangles), Bird River complex (X), Ray-Iz (open circles), Ojen in Ronda Complex (open diamonds), and Oman (solid squares). The data from Ojen are from Torres-Ruiz et al. (1996). The other data sources are listed in Fig. 5.

View larger version (21K): [in a new window]   FIG. 8. Ir and As contents of laurite compared to those from other locations. Data sources are listed in Fig. 5.

View larger version (15K): [in a new window]   FIG. 9. S/Se ratios of the samples studied (laurite and Cu-Ni-Fe sulphides) compared to the values for the primitive mantle, sulphides from sub-arc mantle, mid-oceanic ridge basalts, and sedimentary rocks. Data sources: primitive mantle (McDonough and Sun, 1995), sulphides from sub-arc mantle (Hattori et al., 2002), mid-oceanic ridge basalts (Hamlyn and Keays, 1986; Peach et al., 1990), sedimentary rocks (Stanton, 1972; Leutwein, 1978) and seawater (Measures and Burton, 1980).