K. A. Baublys

Stable isotope geochemistry of authigenic clay minerals from Late Permian coal measures, Queensland, Australia: implications for the evolution of the Bowen Basin

Oxygen and hydrogen isotope analyses were carried out on authigenic clay minerals from Late Permian coal measures of the Bowen Basin (Australia). In the northern Bowen Basin, the oxygen isotope compositions of the mixed-layer illite/smectite show significant irregular variations with respect to depth, which parallel the changes in the extent of the illitisation reaction and are interpreted as reflecting changes in water/rock ratio in turn related to permeability. The δO and δD values of illite-smectite and kaolinite in the northern Bowen Basin and the calculated fluid isotopic composition (δO = -3‰ to +1‰; δD = -70‰ to -90‰) in equilibrium with these clays are considerably lower than those typically reported for deeply buried sedimentary basins. These stable isotope data, together with relatively high inferred palaeotemperatures (up to 235°C) and abnormally high geothermal gradients are consistent with a hydrothermal origin for clay mineral formation in the northern Bowen Basin. The hydrothermal system is interpreted to be a result of the Late Triassic extensional tectonic regime, which developed in large parts of eastern Australia and affected the northern part of the Bowen Basin. In the southern Bowen Basin, by contrast, clays are more enriched in O and deuterium, which is explained by lower temperatures (in the shallow coal measures) and a significant enrichment in the fluid isotopic composition (δO = -3.6‰ to +5.6‰, δD = -66‰ to -35‰) under low water/rock ratio conditions, especially in deeper pelitic rocks

Earliest Seafloor Hydrothermal Systems on Earth: Comparison with Modern Analogues

Recent developments in multiple sulfur isotope analysis of sulfide and sulfate minerals provide a new tool for investigation of ore-forming processes and sources of sulfur in Archean hydrothermal systems, with important implications for the Archean sulfur cycle, the origin and impact of various microbial metabolisms and the chemistry of surface waters. In the current study we show that most of the sulfides and sulfates in the 3.49 Ga Dresser Formation and 3.24 Ga Panorama Zn–Cu field of Western Australia have non zero Δ33S values that indicate variable proportions of seawater sulfate and elemental sulfur of UV-photolysis origin were incorporated into the deposits. Our results show that the multiple sulfur isotope systematics of the Dresser Formation sulfides and sulfates mainly reflect mixing between mass independently fractionated sulfur reservoirs with positive and negative Δ33S. Pyrite occurring with barite is depleted in 34S relative to the host barite that has been interpreted as evidence for microbial sulfate reduction. We note, however, that the reported quadruple sulfur isotope systematics of pyrite-barite pairs are equally permissive of a thermochemical origin for this pyrite, which is consistent with inferred formation temperatures for the chert-barite units in excess of 100°C. The variably positive Δ33S anomalies of the Panorama VHMS deposits, disequilibrium relations among sulfides and sulfates and general trend of increasing sulfide Δ33S with stratigraphic height in individual ore systems most likely reflects temperature evolution and fluid mixing through the life of the hydrothermal system. The absence of sulfides with significant negative Δ33S anomalies suggests that volcanic sulfur, not seawater sulfate, was the dominant sulfur source for the Panorama mineral system. The data presented here require Paleoarchean seawater to be at least locally sulfate bearing.

Carbon dioxide-rich coals of the Oaky Creek area, central Bowen Basin: a natural analogue for carbon sequestration in coal systems

High-CO2-containing coal seams in the Oaky Creek area of the Bowen Basin, eastern Australia provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. We conducted mineralogical, stable and radiogenic isotope and major element analyses of mudstones and sandstones adjacent to the coal seams and stable isotope and compositional studies of coal seam gas desorbed from the coals to establish the impact of the high CO2 levels and the mechanisms that keep the CO2 naturally sequestered. Siderite is the earliest carbonate phase present and occurs with kaolinite in mudstones and sandstones. It is interpreted to have formed under low-temperature, reducing conditions where methanogenesis has produced residual 13C-enriched CO2. Enhanced kaolinite concentrations adjacent to a low-CO2-containing coal seam reflect interaction with acidic fluids produced during the coalification of organic matter. Stable isotope data for carbonates and Rb–Sr isochron ages for illitic clays indicate that illitic clay–carbonate assemblages adjacent to both coal seams formed as a result of meteoric hydrothermal activity in the Upper Triassic with more intensive mineralogical reactions evident in the high-CO2 coals. The present-day CO2 in the high-CO2 coals at Oaky Creek was emplaced in the Upper Triassic based on dating of illitic clay minerals from the high-CO2 well and is magmatic or deep crustal in origin. Methane in the coals is of mixed origin, with secondary biogenic CH4 formed by microbial reduction of CO2 predominant in the high-CO2 coals. This suggests that methanogenesis may provide an additional sequestration mechanism for CO2 in coal seams.

Understanding mine site water and salt dynamics to support integrated water quality and quantity management

Abstract Water reuse is becoming an integral component of the water management strategy on mine sites. This practise is being driven by corporate sustainability goals, community and societal pressures to demonstrate improved water stewardship, as well as climate and regulatory pressures. However, water reuse often results in water quality compromise which can then result in decreased recovery through problems in processing circuits, product quality, and an increased likelihood of discharge of water that cannot meet environmental regulatory requirements. On most mine sites, there is usually a disjuncture between water quantity management and water quality management with the latter being managed solely as an environmental problem. It is becoming increasingly clear that water quality and quantity must be managed as an integrated system. In order to integrate water quality and quantity management to achieve multiple objectives the dynamics of water and constituents must be understood. Using examples from a study being conducted at a coal mine in the Bowen Basin, this paper will outline the dynamics of water and salts on the site.

Source and Timing of Coal Seam Gas Generation in Bowen Basin Coals

Coal seam gases collected from Bowen Basin cores have moderately negative methane carbon isotope compositions (−51 ± 9 per mil) which overlap the published range for Australian coal seam methane of −60 ± 11 per mil. No systematic relationship between coal rank and methane δ13C value is apparent. A thermogenic origin for methane has been assigned when its carbon isotope composition is heavier than −60 per mil, although biogenic methane generated in closed systems may have similar δ13C values from −60 to −40 per mil depending on the methanogenic pathway and the carbon isotope composition of the source. Subordinate inputs from biogenic methane could account for some of the isotopic variability of the desorbed methane; however, a good correlation between desorbed methane volumes and bitumen/pyrobitumen content suggests that much of the methane sorbed in the coal was produced by secondary cracking of bitumen.

Nature and source of carbonate mineralization in Bowen Basin coals, eastern Australia

The Permo-Trias sic Bowen Basin is the northernmost structural element of the ca. 2000 km long Bowen—Gunnedah—Sydney Basin System in eastern Australia (Figure 1). The Bowen Basin formed through back-arc extension and foreland subsidence prior to the break-up of Gondwana, resulting in a series of distinct domains reflecting basement morphology and the effects of tectonism (Fielding et al., 1990; Murray, 1990; de Caritat and Braun, 1992). It contains up to 10 000 m of terrestrial and shallow marine siliciclastic sediments (Totterdell et al., 1995), and is economically significant, containing abundant coal and widespread accumulations of coal seam and natural gas.