University of Tasmania
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Geochemistry of Phanerozoic carbonaceous black shales, sandstones and cherts in Malaysia : insights into gold source rock potential

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posted on 2023-05-27, 11:51 authored by Makoundi, C
The trace element geochemistry of marine black shales has been successfully used to interpret sediment provenance and deep ocean conditions during sedimentation. More recently, it has been proposed that certain pyritic black shales may act as sources of gold and arsenic for orogenic gold deposits. Four aspects of the geochemistry of the Phanerozoic black shales from the central gold belt/East Malaya terrane and the Sibumasu terrane in Malaysia are investigated in this thesis: 1) the whole rock major and trace element geochemistry of shales has been used to determine provenance and redox depositional conditions; 2) relationships of whole rock gold concentration to other elements in the shales has been used to assess the mechanism of gold transport and concentration; 3) textures and Laser Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS) geochemistry of pyrite in the shales is employed to determine origin of the pyrite; sedimentary, metamorphic or hydrothermal; and 4) LA-ICP-MS gold content of the sedimentary pyrite is determined to assess the source rock potential of the shales to form orogenic deposits. The oldest, organic-poor, grey shales of the Late Cambrian-Early Ordovician Machinchang Formation have a felsic provenance (Th/Sc>1; La/Th>2.8) and were deposited under oxic conditions as determined by the absence of framboidal pyrites. The Machinchang Formation shales display a good relationship among alumina (Al2O3) and Au, V, Cr, Ni, Co, Sn, and Sb implying that these trace elements were deposited attached to clay particles into the sedimentary basin.The shales have Au contents that range from 43 to 49 ppb (mean 46 ppb). LA ICP-MS gold content is present from 0.05 to 3.6 ppm (0.6 ppm) within euhedral pyrite. The texture and composition (Ag/Au<1; As/Ni>10) suggests a hydrothermal origin for pyrite in Malaysia. A lack of visible sedimentary pyrite in the Machinchang Formation grey shales in Malaysia leaves a gap in the understanding of the Late Cambrian-Early Ordovician ocean chemistry in Malaysia. Consequently, trace metals in existing euhedral pyrites may have an epigenetic origin related to circulation of hydrothermal fluids and deposition of trace elements into pyrite after sedimentation. The hydrothermal activity is also evidenced by the occurrence of quartz veins containing euhedral pyrite grains. The Early Ordovician-Silurian Setul Formation sandstones are very siliceous, alumina poor but organic carbon-rich. The negative europium anomaly and the ratio Th/Sc>1 suggest a felsic provenance. The sandstone gold content varies from 5 to 75 ppb (mean 26 ppb) and shows a strong positive relationship with alumina indicating gold content is clay-related. The geochemistry indicates a strong control of organic processes during deposition of trace metals due to a positive correlation between total organic carbon and V, U, Cr, Se, As, Zn and Ni. The Pennsylvanian-Early Permian Singa Formation black shales from the Sibumasu terrane show a ratio of Th/Sc>1 indicative of felsic-intermediate provenance. Framboidal pyrites are bigger than 10 ˜í¬¿m across with a consistent ratio of V/Cr<2 suggesting that they were deposited in an oxic marine environment. The black shales display a positive relationship between Al2O3 and the trace elements Ni, Cu, Zn, As, Sb and the organic carbon content is low (<0.5 wt %). The gold content in the shales ranges from 40 to 62 ppb (51 ppb) and correlates weakly with V supporting a possible control of organic processes during deposition of gold in the sediments. Gold content in sedimentary pyrite ranges from 0.05 to 1.6 ppm (mean 0.3 ppm) and varies positively with As, Co, Ag, Sb, Te and Bi. The pyrites are enriched in Mn, Co, Ni, Cu, Zn, As, Cd, Sb, Te, Au, Pb, and Bi but depleted in Ag, Mo, and strongly depleted in Tl compared to Worldwide Diagenetic Pyrite (WDP, Gregory et al., 2015). The Late Devonian BRSZ Unit 1 black shales have a Th/Sc ratio that ranges from 0.6 to 1, suggesting an intermediate provenance likely related to the East Malaya terrane granitoids as parent rocks. The shales contain framboidal pyrite commonly less than 10 ˜í¬¿m implying deposition under anoxic to euxinic conditions. The gold concentrations in shale ranges from 4 to 39 ppb (mean 23 ppb). Whole rock trace elements such as V, Cr, Zn, As, Se, Mo, Sn, Sb, and U show a positive correlation with both alumina and organic carbon implying a sedimentary (clay) and organic matter control for these trace elements during their deposition into basin. Sedimentary pyrites of the BRSZ Unit 1 are enriched in Mn, Zn, Te, Au, and Pb and depleted in Co, As, Mo, Cd, Sb, Tl, and Bi relative to WDP. Gold content in sedimentary pyrite varies from 0.06 to 5 ppm (mean 1 ppm). The Selinsing gold deposit is hosted by the Carboniferous-Permian Raub Group. Framboidal pyrites, proximal to the orebody, are enriched in Mn, Zn, and Au depleted in Ni, Cu, As, Se, Mo, Ag, Cd, Sb, Te, Bi and strongly depleted in Tl compared to WDP. The gold content in diagenetic pyrite varies from 0.03 to 0.84 ppm (mean 0.3 ppm). In the Middle Triassic Karak Formation, gold content ranges from 0.4 to 0.6 ppm (mean 0.5 ppm) in diagenetic pyrite whereas it ranges between 0.06 and 0.8 ppm (mean 0.2 ppm) in metamorphic pyrite. The good correlation of V with Au, Zn, Cd, and Sb in diagenetic pyrite suggests that these trace elements were all concentrated by organic processes. The Middle Triassic Semantan Formation black shales have ratios of Th/Sc<1 indicating a mafic provenance. Framboidal pyrites are mostly less than 10 ˜í¬¿m indicating deposition under anoxic to euxinic conditions. Both alumina and organic carbon contents vary positively with V, Cr, Mo, Sb, and U indicating these trace elements were introduced adsorbed onto clays or alumino-silicates and organic matter into the Semantan basin. Gold content in sedimentary pyrite varies between 0 and 4.6 ppm (mean 0.8 ppm) and changes positively with Se, Zn, and Cd. Compared to WDP, the Semantan pyrites are strongly enriched in Zn, moderately enriched in Cu, As, Se, Ag, Cd, Te, Au, and Bi but depleted in Mo, Sb, and Tl. The Middle Triassic Gua Musang Formation black shales have a ratio of Th/Sc<1 implying an intermediate-mafic provenance. The shales contain framboidal pyrites which diameter is higher than 20 ˜í¬¿m indicative of oxic-dysoxic conditions. Gold content in black shales varies from 3 to 51 ppb (mean 28 ppb). Al2O3 content shows a weak positive correlation with Au, Ni, Cr, and Sn suggesting that Au, Ni, Cr and Sn may have been introduced into sediments attached to clay minerals or contained in the aluminosilicate mineral structure. The gold content of framboidal pyrite varies from 0.2 to 1.8 ppm (mean 0.8 ppm) whereas in metamorphic pyrite, it is between 0 to 0.1 ppm (mean 0.02 ppm). Compared to WDP, the Gua Musang framboidal pyrites are enriched in Au, Te, and midly in Ag, and depleted in Co, As, Mo, Sb, Tl, Pb, and Bi. The Late Triassic BRSZ Unit 2 cherts that crop out in the vicinity of the BRSZ have elevated contents of SiO2, S, Cu, Mo, and U and depleted in other major elements including Cr, Zn, Ga, and Th compared to PAAS. Framboidal pyrites are enriched in Au, Zn, Cd, Sb and depleted in Ni, Cu, Te and strongly depleted in Bi, Tl, Pb, Mo, Co relative to WDP. Through time, there are two peaks of Au, Se, Cd, Tl contents in sedimentary pyrite that have been revealed by the study in black shales from Malaysia. A peak in the period from 207 to 237 Ma in sediments of East Malaya terrane (e.g. Gua Musang and Semantan formations) when the seawater conditions were dysoxic to euxinic across the Central Gold Belt. Additionally, a peak of Au, Se, Cd, Tl concentrations at 368 Ma has been found in sedimentary rocks of intermediate provenance (e.g. the BRSZ Unit 1) when the redox depositional conditions were dominantly anoxic to euxinic. The highest gold contents which vary from 0.01 to 5 ppm were measured from sediments that deposited under anoxic-euxinic depositional conditions. Overall, both in the Central Gold Belt and the Northwest Domain in Langkawi Islands, gold content in sedimentary pyrite correlates strongly with V, U, and Se suggesting a strong control of organic processes on Au introduction into sedimentary pyrite. There is considerable variation in the Pb isotopic composition of the sedimentary pyrite across the region. Machinchang and Semantan formation pyrites tend to be less radiogenic than the BRSZ Unit 1 pyrite. Partial variation in each sample is related to the in situ decay of U and Th but the remainder relates to initial isotopic differences during crystallization of pyrite. The Singa Formation contains pyrites with two different compositions: one is similar to the pyrite found in the Machinchang and Semantan formations but slightly lower in 207Pb/204Pb, and the other is much more radiogenic than the BRSZ Unit 1 pyrite. The Pb isotope model age of sedimentary rock-hosted gold-bearing pyrite do not coincide with the age of tuffs which is 233 Ma in the southern part of the Central Gold Belt indicating no link between gold mineralisation in the Semantan Formation and volcanism. Gold-bearing ore pyrites from the Selinsing and Tersang gold deposits have isotopic composition indicating Pb model age around 250 ¬¨¬± 50 Ma. This age may be interpreted as the age of gold mineralisation in the district. The S isotope data from sedimentary pyrites ranges from -32.1 ‚ÄövÑ‚àû to 6.2 ‚ÄövÑ‚àû (mean -12.4 ‚ÄövÑ‚àû) and show a broadly parallel trend to the seawater sulphate curve suggesting that sulphur originated from reduction of seawater sulphate. This study has clearly demonstrated that diagenetic pyrites from black shales located far from gold deposits are enriched in gold. The gold content of all sedimentary pyrites ranges from 0.01 to 5 ppm (mean 1 ppm). The mean value of 1 ppm (1000 ppb) is far above the value of 250 ppb documented by Large et al. (2011) in diagenetic pyrite indicative of carb...


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