Geology and genesis of volcanic-hosted massive sulphide deposits in the Tasik Chini District, Central Peninsular Malaysia

The Tasik Chini volcanic-hosted massive sulphide district is located about 250 km southeast of Kuala Lumpur in Pahang State, Central Peninsular Malaysia. The district comprises two significant ore deposits, the Bukit Botol and Bukit Ketaya of precious metal-rich, polymetallic massive sulphides with barite and silica-iron-manganese oxide within the Permo-Triassic volcano-sedimentary sequence of rhyodacitic-rhyolite flows and related volcaniclastic units. LAICPMS U-Pb zircon dating of footwall and hangingwall sequences from the Bukit Botol deposit yielded an Early Permian (273 ¬¨¬± 8, 286 ¬¨¬± 4 and 292 ¬¨¬± 3 Ma) age. Similarly, the zircon U-Pb age results at Bukit Ketaya reveal a well-constrained age of Early Permian (286 ¬¨¬± 2 to 288 ¬¨¬± 4 Ma). These zircon U-Pb results demonstrate that the felsic volcanic units and associated mineralisation at both deposits are consistent with the broader Early Permian volcanism within the East Malaya Block. Triassic volcanic and plutonic rocks are also widespread in the Tasik Chini deposit and surrounding area. Their LA-ICPMS U-Pb zircon ages are constrained at 233 ¬¨¬± 4 to 242 ¬¨¬± 2 Ma. The XRF whole rock trace element compositions of the Early Permian host rhyodacite-rhyolite at the both Bukit Botol and Bukit Ketaya deposits show high to moderate HFSE (e.g., Nb, Y, Zr) contents and are characterised by transitional-calc-alkaline affinity of a subduction-related volcanic island arc type setting. Trace element patterns for these units normalised to primitive mantle show strong negative Nb and Ti anomalies relative to Th and La. The chondritenormalised REE patterns of these rocks are also enriched in the LREE showing near-flat trends for the HREE with negative Eu anomalies. All trace element and REE data are consistent with a tectonic environment of a volcanic arc setting. In comparison, the trace element data of the later Triassic volcanics and intrusions from the Tasik Chini area demonstrate a moderate to low HFSE composition, transitional to tholeiitic affinities, but has a similar magmatic arc signature. The differences in geochemical data between the Early Permian host rhyodacite-rhyolite and the later Triassic volcanics and intrusions are likely due to the Permo-Triassic tectonic progression from a volcanic arc environment to collisional setting within the East Malaya Block. The Sm-Nd isotopic studies also support the tectonic progression of arc-related magmatic events. Early Permian host rhyodacite-rhyolite is characterised by high ˜í¬µNd(T) values (-0.8 to +0.4) with calculated Nd model ages (TDM2) of 0.97 to 1.05 Ga, whereas Triassic volcanic and related intrusions (‚Äöv¢¬ß 250 Ma), show lower ˜í¬µNd(T) values (-3.6 to -1.0) for volcanic rocks and ˜í¬µNd(T) (-5.2 to -4.5) for intrusive rocks with the older TDM2 age (1.02 to 1.38 Ga). These differences show that the Early Permian host rhyodacite-rhyolite from Bukit Botol and Bukit Ketaya deposits are slightly evolved having less crustal influence, whereas the Triassic volcanic and plutonic rocks are significantly more evolved and relatively contaminated. The Bukit Botol and Bukit Ketaya deposits delineate a similar style of mineralisation and sulphide assemblages, but their alteration styles are different. In general, the stringer zone with minor massive sulphides/layers form directly below the mineralised zone at the footwall, whereas the barite, Fe-Mn and Fe-Si layers occur at the top of the mineralised zone or the upper part of the stratigraphic levels. The main sulphide phases include pyrite, chalcopyrite, sphalerite, rare galena and trace Sn-bearing minerals. Gold- and Ag-bearing minerals are present in the massive sulphide and barite layers at the Bukit Botol deposit but absent at the Bukit Ketaya deposit. At both the deposits, there are pure chemical sediments deposited during formation of the massive sulphide lenses, as a result of changing oxidation-reduction conditions and fluid compositions with increasing distance from the hydrothermal vent site in a local submarine environment. The Fe-Mn layer is discontinuously formed at Bukit Botol, whereas the Fe-Si layer has developed as a stratigraphic marker at Bukit Ketaya, both forming distinctive exhalite assemblages. The geometry of alteration assemblages at the Bukit Botol and Bukit Ketaya deposits show that they occur as semi-conformable or stratabound zones around the ore lenses. The Bukit Botol deposit is characterised by proximal quartz-sericite-pyrite and distal quartz-sericite alteration zones, whereas distal quartz-chlorite-sericite-pyrite-pyrophyllite¬¨¬±kaolinite and proximal quartzchlorite- pyrite¬¨¬±carbonate¬¨¬±pyrophyllite form the alteration assemblages of the Bukit Ketaya deposit. In addition, the molar elemental ratios of Na2O/Al2O3 versus K2O/Al2O3 and MgO/Al2O3 versus K2O/Al2O3 support that the abundance of muscovite (sericite) and chlorite controlled the intensity of alteration at the both deposits. Therefore, the difference in quartz-chlorite-sericite alteration assemblages between the Bukit Botol and Bukit Ketaya deposits, in combination with the presence of pyrophyllite and kaolinite as shown by SWIR and XRD results in Bukit Ketaya deposit suggest variable mixing of hydrothermal fluids with seawater and a possible minor magmatic contribution. Electron microprobe analysis of sphalerite from the Bukit Botol deposit reveals a range of 0 to 24.0 mole% FeS, whereas sphalerite from the Bukit Ketaya deposit shows a range of 0 to 3 mole% FeS. Although the sphalerite has a wide variation in composition, a discernible decreasing Fe trend is exhibited from the stringer zone towards massive sulphide. This compositional variation in sphalerites reflects variable temperature and activity of sulphur during ore formation. LA-ICPMS analytical data, coupled with textural characteristics, provide evidence for significant variations of trace elements in different pyrite types at Bukit Botol, having pyrite 1, pyrite 2 and pyrite 3 in paragenetic sequence. The suite of As, Se, Te, Cu, Zn and Pb trace elements show decreasing trends from pyrite 1 to pyrite 3, and a high Co but lower Ni contents in pyrite 1 and pyrite 3 compared to moderate Ni and low Co values in pyrite 2. Review of all data and trace element patterns from Bukit Botol suggests that the precipitations of Au, As and trace element in pyrites are likely to be related to reduction of sulphur from seawater under specific pressure and temperature conditions. However, a minor magmatic fluid contribution may also be inferred from the high Se (7‚Äöv†v¿650 ppm) and Co (0‚Äöv†v¿1192 ppm) concentrations in pyrites. Measured ˜í¬•34S values of sulphides from the Bukit Botol deposit range between -0.8 and +4.1‚ÄövÑ‚àû, and one sample displays a higher ˜í¬•34S value of +8.3‚ÄövÑ‚àû. Meanwhile, the ˜í¬•34S values for sulphides from the Bukit Ketaya deposit are characterised by a range of ˜í¬•34S between -2.9 to +3.6‚ÄövÑ‚àû. These data suggest that ore-forming fluids for these deposits are likely to have originated from a mixed sulphur source of reduced seawater sulphate with the possible addition of magmatic sulphur. The sulphur isotope values for barite from the Bukit Botol and Bukit Ketaya deposits are similar and range from 11‚ÄövÑ‚àû to 22‚ÄövÑ‚àû, with a mode of 13 to 19‚ÄövÑ‚àû. These ranges are close to the published composition of seawater sulphate during Permian time, and provide supporting evidence that these deposits formed during a submarine Permian volcanic event. Lead isotope values of sulphides from the Bukit Botol and Bukit Ketaya deposits are 18.04 to 18.20 for 206Pb/204Pb, 15.43 and 15.56 for 207Pb/204Pb and 37.96 to 38.35 for 208Pb/204Pb ratios, less radiogenic and similar to those of the host volcanic rocks (18.10 to 18.20 for 206Pb/204Pb, 15.53 and 15.59 for 207Pb/204Pb and 37.96 to 38.26 for 208Pb/204Pb). The Pb isotopes exhibit a primitive arc (i.e., island-arc setting) with a significance ocean island volcanic arc input and suggesting a mixed source from crust/juvenile arc and mantle. These similarities of lead isotopic composition of the sulphides and host volcanic rocks may indicate that lead for the both deposits were sourced mainly from the host sequence with some lead being derived from a basement lead reservoir during the Permian. Microthermometric analysis of fluid inclusions in quartz and barite from the Bukit Botol and Bukit Ketaya deposits yields homogenisation temperatures of 180-310oC with no fluid inclusion evidence of boiling. Salinities, densities, pressure and depth of ore forming fluids range from 1.0 to 14.3 wt% NaCl equivalent, 0.711 to 0.970 g/cm3, 12 to 93 bars, and ~ 1500 m depth of seawater. Laser Raman spectroscopic analysis show the presence of CO2 (100 mol%). Seawater is suggested as the main ore fluid in the formation of these deposits, but contribution from a magmatic source is indicated by higher salinities relative to seawater (3.2 wt % NaCl) and the presence of CO2. All present geological, geochemical, isotopic and fluid inclusions data indicate that the Bukit Botol and Bukit Ketaya deposits are seafloor volcanic-hosted massive sulphide deposit type, and correspond to many criteria of the bimodal felsic-hosted type. With respect to broad geological setting, this Early Permian volcanic-hosted massive sulphide deposits in the Tasik Chini district has similar features to the setting of the Miocene Kuroko deposits in Japan, which formed in an arc-related environment. The deposits are suggested to have formed during the Early Permian subduction-related arc/back arc volcanism, which is considered to be a part of the Palaeo Tethys Ocean evolution at the eastern Gondwana margin of the East Malaya Terrane. The deposits are considered to have formed during an early period of active volcanism and sedimentation, and the area was probably associated with a small rhyolite dome within a submarine felsic dominated volcanic centre. The exhalation of mineralising hy...