Origin of the Paleoproterozoic “Giant Quartz Reef” System in the Bundelkhand Craton, India: Constraints from Fluid Inclusion Microthermometry, Raman Spectroscopy, and Geochemical Modelling

AbstractThe Bundelkhand “giant quartz reef” (BGQR) system comprises 20 major quartz reefs which run for tens of km in strike length of average width of 40 m and occurs in spatial intervals of 12–19 km in the Bundelkhand craton, North Central India. The BGQR system is distinct from quartz vein systems originating from crustal scale shearing observed in ancient as well as modern convergent tectonic settings. Fluid inclusions studied in BGQR system are intriguingly diverse although dominated by aqueous fluid which exhibit a broad range of salinity from ~0 to 28.9 wt% NaCl equivalent and temperature of homogenization range of 58 to 385°C. Primary and pseudosecondary aqueous inclusions in assemblages in grain interiors and growth zones vary randomly in their Th—salinity characteristics that preclude identification of discrete fluid events. Aqueous fluid in the BGQR system evolved through mixing of two distinct sources of fluids—a meteoric fluid and a moderate temperature—moderate salinity fluid that was possibly derived from the Bundelkhand granodiorite based on an important clue provided by hydrous mineral bearing fluid inclusions detected by Raman microspectrometry. The results of modeling with PHREEQC indicate that mixing of fluids could be a suitable mechanism in formation of these giant reefs. The available 1-dimensional diffusive transport model for deposition of silica helps in putting constraints on the time span of deposition of silica in the context of the BGQR system. The BGQR system is a possible result of shallow-crustal sources of fluid and silica and could be visualized as a “Paleoproterozoic geothermal system” in a granitic terrane

Compositional zoning of fluid inclusions in the Archaean Junction gold deposit, Western Australia: a process of fluid � wall-rock interaction?

The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron-rich, tholeiitic dolerite still. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid-inclusion populations with a wide range of compositions in the H2O-CO2-CH4-NaCl± CaCl2 system. Pre-shearing, pre-gold, molybdenite-bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H2O-CO2-CH4 ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥ 150°C fluid. The syn-gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz-calcite ± albite ± chlorite ± pyrrhotite veining. Fluid-inclusion analyses indicate that gold deposition occurred during the unmixing of a 400°C, moderately saline, H2O-CO2 ± CH4 fluid at pressures between 70 MPa and 440 MPa. Post-gold quartz-calcite-biotite-pyrrhotite veins occupy normal fault sets that slightly offset the Junction shear zone. Fluid inclusions in these veins are predominantly vapour rich, with CO2>>CH4. Homogenisation temperatures indicate that the post-gold quartz veins precipitated from a 310 ± 30° fluid. Finally, late secondary fluid inclusions show that a <200°C, highly saline, H2O-CaCl2-NaCl-bearing fluid percolated along microfractures late in the deposite's history, but did not form any notable vein type. Roman spectroscopy supports the microthermometric data and reveals that CH4-bearing fluid inclusions occur in syn-gold quartz grains found almost exclusively at the vein margin, whereas CO2-bearing fluid inclusions occur in quartz grains that are found toward the centre of the veins. The zonation of CO2:CH4 ratios, with respect to the location of fluid inclusions within the syn-gold quartz veins, suggest that the CH4 did not travel as part of the auriferous fluid. Fluid unmixing and post-entrapment alteration of the syn-gold fluid inclusions are known to have occured, but cannot adequately account for the relatively ordered zonation of CO2:CH4 ratios. Instead, the late introduction of a CH4-rich fluid into the Junction shear zone appears more likely. Alternatively, the process of CO2 reduction to CH4 is a viable and plausible explanation that fits the available data. The CH4-bearing fluid inclusions occur almost exclusively at the margin of the syn-gold quartz veins within the zone of high-grade gold mineralisation because this is where all the criteria needed to reduce CO2 to CH4 were satisfied in the Junction deposit

New constraints on fluid sources in orogenic gold deposits, Victoria, Australia

Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern Australia. Victorian gold deposits vary from large turbidite-hosted 'orogenic' lode and disseminated-stockwork gold-only deposits, formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO 2-bearing fluids. The majority of these fluid inclusions also have similar 40Ar/ 36Ar values of less than 1500 and 36Ar concentrations of 2. 6-58 ppb (by mass) that are equal to or much greater than air-saturation levels (1. 3-2. 7 ppb). Limited amounts of nitrogen-rich fluids are present at a local scale and have the highest measured 40Ar/ 36Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity aqueous-carbonic fluids with low 40Ar/ 36Ar values, in both 'orogenic' and 'intrusion-related' gold deposits, is attributed to fluid production from common basement volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in the Stawell-Magdala deposit of western Victoria (including those associated with N 2) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10 -6 in the Melbourne Zone. Br/I ratios in this zone range from 0. 5 to 3. 0 that are characteristic of fluid interaction with organic-rich sediments. The maximum I/Cl and characteristic Br/I ratios provide evidence for organic Br and I released during metamorphism of the shales. Therefore, the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation, but are consistent with deeper metamorphic fluid sources from basement volcano-sedimentary rocks. The overlying sediments are probably involved in gold mineralisation via fluid-rock interaction

Unravelling the Consequences of SO₂-Basalt Reactions for Geochemical Fractionation and Mineral Formation

Palm is supported by a Commonwealth Supported Place for the Masters of Earth Science (Advanced) degree and the John and Kerry Lovering Scholarship (RSES, ANU). Palm, King, Renggli, Troitzsch and Mernagh are supported by Australian Research Council grants to King (DP150104604 and FT130101524)

Analytical techniques for probing small-scale layers that preserve information on gas-solid interactions

This work was supported by Australian Research Council funding to King (DP150104604 and FT130101524) that provided for the contributions from King, Renggli, Troitzsch, and Palm; Australian Synchrotron funding to King, Troitzsch, Mernagh, Yue and Palm; plus an Australian Commonwealth Supported Place and the Kerry and John Lovering scholarship to Pal