Sw Bull

Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits: Part 2. HYC deposit, McArthur River, Northern Territory

The giant stratiform Zn-Pb-Ag HYC deposit displays a broad Zn, Pb and Tl halo which extends laterally along the favourable pyritic black shale facies of the Barney Creek Formation for at least 15 krn west of the deposit. A ferroan dolomite/ankerite halo overlaps with the Zn-Pb-Tl halo extending up to 250 m into the immediate stratigraphic hangingwaD, and 50 to 100 m into the footwall sediments close to the deposit. A manganese carbonate halo is offset from the ferroan dolomite/ankerite halo, being concentrated in the immediate footwall of the deposit and extending laterally along the W-Fold Shale Member. Manganiferous carbonate forms the most pronoW1ced and laterally extensive halo at RYC extending well beyond the Zn-Pb-TI and ankerite halos. The geometry and extent of the halos described above are based on sampling and analyses from two drill holes repolted in this study and a further six drill holes reported previously by I.B. Lambert and K.M. Scott [1. Geochem. Explor. 2, 307-330, 1973]. The SEDEX alteration index previously proposed as a vector for the Lady Loretta deposit [R.R. Large and P.I. McGoldrick, 1998, I. Geochem. Explor. 63, 37-56] has been shown to have applications in the McArthur Basin for defining the favourable stratigraphic unit hosting stratiform Zn-Pb-Ag mineralisation. A modified alteration index (AI Mark 3) has also been developed which eliminates the effect of the shale/dolomite ratio on the index and thus highlights the control of carbonate chemistry and its relationship to stratiform mineralisation. The application of a group of indices including Zn, Pb, Tl, SEDEX alteration index, AI Mark 3 and manganese content of dolomite (MnOd) is proposed for defining the most favourable stratigraphic units for stratiform Zn-Pb-Ag mineralisation within carbonate-bearing sedimentary basins.

Sedimentology of the Palaeoproterozoic Barney Creek formation in DDH BMR McArthur 2, southern McArthur basin, northern territory

The 122 m‐long diamond drillcore DDH BMR McArthur 2 was collared in Reward Dolomite approximately 23 km southwest of McArthur River Mine. It intersected 25 m of Reward Dolomite and was terminated in the underlying Barney Creek Formation, comprising 97 m of grey to black dolomitic siltstone and shale. Facies and geochemical characteristics suggest that the lowermost 15 m of the intersection represent the upper part of the W‐Fold Shale Member, and the overlying 82 m has a close affinity to the HYC Pyritic Shale Member. Sedimentary‐facies‐based analysis of the core indicates that the entire succession accumulated in a quiet, reduced, sub‐wave‐base environment. However, the restriction of key facies to discrete stratigraphic intervals indicates subtle fluctuations in environmental conditions, chiefly water depth, during deposition. A schematic relative water depth curve has been constructed that identifies two periods of increasing water depth (one from the W‐Fold Shale Member into the lower part of the HYC P...

Salt as a fluid driver, and basement as a metal source, for stratiform sediment-hosted copper deposits

The source of copper for stratiform sediment-hosted copper deposits is considered to be redbed clastics situated stratigraphically below the deposits. However, for one of the principal copper provinces in the world, the Zambian Copperbelt, there is insufficient thickness of redbeds to constitute a viable source. Numerical modeling demonstrates that high-salinity sedimentary brines, generated beneath a halite seal, will develop convective hydrothermal plumes that penetrate through the redbeds, deep into the crystalline basement, despite its low permeability and regardless of the availability of cross-stratal conduits. This greatly expands the volume of the potential metal source for this style of ore deposit.

Processes of ore formation in the stratiform sediment-hosted ZnPbdeposits of Northern Australia: testing the Century model

Abstract Numerical simulations of chemical processes of ore formation are used to test hypotheses concerning the genesis of northern Australian Proterozoic stratiform sediment-hosted Zn Pb deposits. Exhalation of metal-bearing brines into anoxic marine waters is shown to be effective for massive sulfide ore formation. The ‘Century’ model, late diagenetic massive sulfide deposition involving metal-laden brines interacting with hydrocarbon reservoirs, is also demonstrated to have potential as an ore-forming process. It requires that H2S(g) is available in the gas reservoir and that acidification is prevented by dissolution of wall rock carbonate. Seawater mixing is capable of producing laminated sulfide ore. Brine-sour gas interaction will most likely produce discordant Zn Pb ore due to carbonate dissolution.

Tupilakosaur-like vertebrae in Bothriceps australis, an Australian brachyopid stereospondyl

ABSTRACT A new locality near the Permo-Triassic boundary in Tasmania, Australia, has yielded a number of specimens of the brachyopid stereospondyl, Bothriceps australis, whose provenance was unknown previously. An articulated diplospondylous vertebral column of a morphology typical only of the Tupilakosauridae was found at the same locality. Although the Brachyopidae and Tupilakosauridae have skulls of similar shape, most authors have regarded them as not closely related. Phylogenetic analysis confirms that the Brachyopidae and Tupilakosauridae are both members of the Stereospondyli but are not closely related, despite the remarkable similarity of the Bothriceps and tupilakosaur vertebral columns.

Setting the stage for the genesis of the giant Bendigo ore system

Abstract The Ordovician sediments that host the giant Bendigo saddle reef gold deposits consist of a 3 km-thick sequence of turbiditic sandstones and interbedded siltstones and mudstones. Sedimentological studies suggest the succession formed within a major deep marine channel–levee complex similar to those described from contemporary continental margin to basin plain settings outboard of major river systems (e.g. the Amazon, Mississippi and Congo). Black shales, which are commonly the immediate host rocks to the epigenetic gold reefs, are interpreted to be over-bank deposits or abandoned channel fills, developed adjacent to active channels which were sandstone-dominated and had an incised axial thalweg marked by the coarsest-grained sediments present. Organic carbon content of the black shales at Bendigo varies from 0.2 to 2 wt%, compared with the grey shales, siltstones and sandstones, which vary from 0.05 to 0.2 wt%. Trace elements fall into two main groups: (a) elements that have a linear relationship with aluminium, and are controlled by the detrital clay content (Sn, Ba, Rb, Li, Cs, Mn, Cr and Tl); and (b) elements that show relationships with both aluminium and organic carbon (V, U, Ni, Zn, Cu, Bi, Pb, Se, Ag and Au) and are controlled by both the clay and organic matter content in the carbonaceous shales. The elements in the second group are enriched in the black shale facies. The background gold content of the black shales in the drill holes distal from mineralization averages 8.9 ppb, compared with the sandstones with 1.5 ppb. Most of the gold in the shales is present in diagenetic pyrite and marcasite, which laser ablation inductively coupled mass spectrometer (LA-ICPMS) analyses indicate varies from 5 to 3850 ppb and averages 370 ppb Au. The geochemical data suggest that this syngenetic gold was most likely sourced by erosion of the hinterland, and transported attached to detrital clay particles or as colloidal gold, by a high-volume feeder river system. High Rb/K ratios in the shales support a highly weathered source typical of a giant river system. By analogy with modern systems, following transport into deep marine channel–levee complexes via continental margin canyons, gold and other redox sensitive trace elements were ultimately trapped by reduction, adsorption and complexation with organic matter in the sub-oxic to anoxic over-bank deposits. Oxidation of much of the organic matter during diagenesis released the gold and certain trace elements (Ni, Co, Se, Ag, Cu, Bi, Pb), which became incorporated into diagenetic pyrite. Enrichment of gold in diagenetic pyrite of the black shale facies of the Ordovician turbidites at Bendigo was the first stage in a two-stage process that produced the world-class quartz–gold saddle reef deposits. Supplementary material: Whole rock analyses for sedimentary rocks in drill holes NBD005 and NBD186, Kangaroo Flat Mine, Bendigo, are available at http://www.geolsoc.org.uk/SUP18732

New evidence of large Permo-Triassic dicynodonts (Synapsida) from Australia

Dicynodonts are herbivorous synapsids found in Permian and Triassic deposits in all of the Pangean continents. The fossil evidence of this group from Australia is limited to a quadrate, part of a tusk (Thulborn, 1983a, 1990), and a partial femur (Northwood, 1997), all recorded from the earliest Triassic Arcadia Formation in central Queensland. Thulborn (1983a, 1983b, 1990) attributed the quadrate from the Arcadia Formation to the Kannemeyeriidae, a taxon commonly found in Middle Triassic rocks worldwide. This identification was disputed by King (1983) because of the earliest Triassic age. Northwood (1997) attributed the femur to the earliest Triassic genus Lystrosaurus.Dicynodont footprints have been identified in Early Triassic strata of the Sydney Basin (Retallack, 1996). Fragments of a skull from the Lower Cretaceous of Queensland have also been attributed to dicynodonts (Thulborn and Turner, 2003). In this paper we report on the first dicynodont material recovered from Tasmania, and describe its sedimentary context and its paleogeographic significance in comparison to other Permian and Triassic dicynodonts.