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Geology, geochemistry and metallogenesis of selected sediment-hosted gold deposits in the central gold belt, Peninsular Malaysia

posted on 2023-05-26, 13:38 authored by Charles MakoundiCharles Makoundi
The Tersang, Selinsing, and Penjom gold deposits are located at the east of the Bentong-Raub Suture Zone in the State of Pahang, Central Malaysia. The Tersang gold deposit is hosted in sandstone, breccia and felsic rhyolite of probably Triassic in age. LA ICP-MS U-Pb zircon dating of the host sandstone at Tersang indicates a possible age that ranges from ca.319 Ma to ca.219 Ma. However, the age of the sandstone could be as young as Upper Triassic. The alkali rhyolite, which intruded the host sandstone sequence, yielded a Late Triassic age of 218.8 ± 1.7 Ma. Structural and textural investigation has shown two sets of NNW-SSE and ESE-WNW oriented mineralised veins at Tersang and ore minerals include pyrite, arsenopyrite, sphalerite, galena, geocronite [Pb14 (Sb, As)6S23)], covellite, and gold. Normalised quantitative analysis of gold grains in veins indicates two generations: high fineness gold (959 to 965) and low fineness gold (769 to 792). The argillic alteration is dominant in the Tersang deposit and characterised by an assemblage of sericite, illite, and montmorillonite. Detailed paragenetic studies at Tersang revealed four pyrite types including euhedral to subhedral “spongy” pyrite with internal fracturing (pyrite 1), euhedral clean pyrite (pyrite 2) overgrown on pyrite 1, amorphous pyrite with high As and high Au (pyrite 3), crack-fill or vein pyrite with high As and low Au (pyrite 4). LA ICP-MS analysis of different pyrite types indicates that they are enriched in Au, As, Zn, Ni, Co, Sb, Tl, Se and depleted in Cu, Ag, Te, V, Bi, and Ag. Pyrite 1 and Pyrite 3 are particularly enriched in Au, As, Co, Ni, Se, and Tl. Pyrite 1 and pyrite 2 show zoning of Co, Ni, As, and Se and they have low Ag/Au, and Ni/Co ratios indicating that these pyrites are likely of metamorphic origin. Sulphur isotope composition of pyrite, arsenopyrite and galena has a range of δ34S values from -6 to 2.5 ‰ at Tersang. Additionally, these sulphide minerals are associated with primary hematite indicating that the sulphur may have been derived from the oxidation of a magmatic hydrothermal fluid at Tersang. The Tersang ores contain lead isotope ratios of 18.59 for Pb206 /Pb204 and 15.75 for Pb207 /Pb204 suggesting that lead was probably derived from the lower crust source rocks. At Tersang, fluid-inclusion studies yielded a homogenisation temperature range from 210 to 348 °C with salinities between 2.41 and 8.95 wt % NaCl equiv. indicating magmatic input due to relatively high salinity. Laser Raman Spectrometry analysis indicates the presence of CO2 (98.1 to 100 mol %) in fluids with a minor amount of CH4 (0 to 1.9 mol %). In comparison, the Selinsing gold deposit is hosted by phyllite, siltstone, cataclasite, mylonite and argillite units of probably Triassic in age. LA ICP-MS U-Pb zircon dating of the host siltstone indicated a maximum Carboniferous age between 324.1 ± 3.5 and 346 ± 6 Ma. Structural and paragenetic studies of quartz veins revealed two vein generations: NS-oriented early veins and NNW-SSE-trending pinch and swell structures formed during D2 extensional regime and later brecciated veins, which resulted from D3 compressional regime. At Selinsing, ore minerals consist of pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, pyrrhotite and gold. Metal distribution in drill holes shows that economic mineralisation is mostly found in cataclasite and breccia units suggesting a structural control on gold mineralisation. The host sequences are altered to quartz-ankerite alteration in the high-grade zones in the catalasite units. However, sericite and illite alteration is present in phyllite and siltstone units. Detailed textural and chemical studies revealed seven pyrite types, which include framboidal pyrite (pyrite 1), rounded recrystallised pyrite (pyrite 2), euhedral clean pyrite (pyrite 3), euhedral to subhedral with inclusions (pyrite 4), overgrown pyrite (pyrite 5), euhedral to subhedral overgrown pyrite (pyrite 6), and euhedral overgrown on pyrite 6 (pyrite 7). LA ICP-MS analysis of the pyrite types indicates that they are enriched in Au, Pb, As, Zn, Ni, Co, and Sb and depleted in Mo, Te, V, Bi, and Ag relative to the Tersang pyrites. Pyrite 1 and pyrite 2 have almost the same trace element assemblages. Compared to pyrite 1, pyrite 2 shows slight enrichment in Bi, Te, and Se and depletion in Co and Mo. However, Pyrite 3 and pyrite 6 are particularly enriched in Au, As, Cu and Ni, whereas pyrite 7 is enriched in Pb and Mo. Sulphur isotope composition of ores from the Selinsing deposit ranges from 1.5 to 9.2 ‰. In addition, Pb isotope values of pyrite range from 18.94 to 19.10 for Pb206 /Pb204 and from 15.72 to 15.73 for Pb207 /Pb204 suggesting a high uranium source rock of the lead. Fluid-inclusion in quartz at Selinsing yielded a homogenisation temperature range from 194 to 348 °C and salinities between 1.23 and 9.98 wt % NaCl equiv. Similar to Tersang, Laser Raman Spectrometric analysis revealed CO2 -rich (95 to 100 mol %) inclusions with minor amount of N2 (0 to 5 mol %). A limited study was undertaken at the Penjom gold deposit. This deposit is hosted in Permian tuffaceous siltstone, carbonaceous shale, and tuffaceous conglomerate sequence that is intruded by rhyodacitic and trachyandesitic igneous rocks. LA ICP-MS U-Pb zircon dating of the host tuffaceous siltstone and tuffaceous breccia indicated ages ranging from 264.9 ± 2.6 to 259.8 ± 4.9 Ma (Permian). Gold mineralisation occurs in carbonate-rich zones hosted within dilated quartz veins, which contain disseminated pyrite and arsenopyrite. These veins are found in NS and NE trending shears which dip 60-75° to the east. Sulphur isotope composition at the Penjom deposit ranges from -8.4 to -3.9 ‰. The Pb isotope composition from galena ranges from 18.54 to 18.57 for Pb206 /Pb204 and 15.70 for Pb207 /Pb204 . This Pb isotopic signature is less radiogenic indicating a lower crust source rock. Fluid inclusion studies at Penjom yielded a homogenisation temperature range from 260 to 300 °C and salinities between 6.30 and 8.68 wt % NaCl equiv. The geological, structural, geochemical and isotopic data together with mineral paragenesis, pyrite chemistry and ore fluid characteristics of the Tersang, Selinsing and Penjom gold deposits indicate that they are comparable to Bendigo and Meguma type deposits. The geochemical and isotopic evidence also suggests that the ore fluids have been predominantly derived from a mixture of magmatic and metamorphic sources with a minor meteoric fluid. The ore fluids ascended and deposited in structurally favorable traps, such as shear zones, saddle reefs, and fold hinges during metamorphism and deformation. The source of Au is still conjectural; however, LA ICP-MS pyrite geochemistry indicates multiple stages of Au enrichment and remobilisation from diagenetic conditions. The enrichment of VAMSNAZ (V, As, Mo, Se, Ni, Ag and Zn) metals implies a possibility of diagenetic origin of the gold. Pyrite chemistry, alteration, Pb isotopes and structural setting at Tersang, Selinsing, and Penjom provide exploration guides. Elements such as As, Sb, Pb, Ag, Co, Ni, Tl, Te, Se, Zn, Mo, U and V occurring together with Au can be used as pathfinders and vectoring tools. Occurrences of quartzankerite and quartz-illite-chlorite alteration assemblages are also indicative of potentially Au enriched geological terrains. Additionally, Pb isotope ratios for Pb206 /Pb204 that varies from 15.72 to 19.10 are useful geochemical criteria to search for gold.





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