Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Geology and genesis of the George Fisher Zn-Pb-Ag deposit Mount Isa, Australia

Abstract: The George Fisher deposit (107 Mt @ 11.1% Zn, 5.4% Pb, and 93g/t Ag) is the northernmost significant Mount Isa-style deposit hosted by the -1653Ma Urquhart Shale in the Western Mount Isa Inlier. It is distinguished from the Mount Isa and Hilton Mines by a paucity of syn-late tectonic Cu. This has enabled recognition and examination of a previously ·unrecognized, syndiagenetic, hydrothermal Zn-Pb-Ag mineralization-system, despite superposition of intense deformation during the Diamantinan and Isan Orogenies. George Fisher contains eleven west-dipping, stacked, anastomosing, stratabound ore lenses contained within rhythmically laminated pyritic siltstones intercalated with banded mudstones, and separated by barren stylolitic mudstones. The ore-bearing sequence contains abundant carbonate banding including partly coalesced carbonate nodules in pyritic siltstones and planar white carbonate bands in siltstones and banded mudstones. The deposit is zoned from stratigraphically lower Zn-rich to higher Zn+Pb-rich ore bodies. Subeconomic Cu occurrences are coincident with Zn+Pb zones, but are predominantly restricted to upper portions of the sequence. This zonation was developed within two hydrothermal systems separated by 75-125Ma. Emplacement of Zn-Pb-Ag at George Fisher was the culmination of a complex, syndiagenetic, hydrothermal alteration system. Calcite is a major constituent of texturally distinct carbonate banding and its formation post-dates earliest dolomite-ferroan dolomiteankerite alteration. At George Fisher, these carbonates are distinguished by unique stable isotope geochemistry within the Urquhart Shale (δ¹⁸O = 17.1-18.5‰, δ¹³C = 4.1 to -1.7‰) and are enriched in (Fe,Mn)CO₃ relative to paragenetically equivalent carbonates located away from economic Zn-Pb-Ag. These carbonates formed during infiltration of warm, basin-derived fluids into cool sedimentary rocks, prior to stylolitization and at depths of a few hundred metres. Chemical zonation of early carbonates over 10-20km provides vectors for Zn-Pb-Ag-bearing fluid influx zones. Pyritization by thermochemical sulphate reduction of in situ organic material post-dated stylolitization. Infiltrating fluids lacked ore metals at the time. Bedding-parallel carbonate±quartz±celsian-hyalophane-K-feldspar vein development was localized in the immediate vicinity of, preceded and structurally prepared the sequence for, economic Zn-Pb-Ag mineralization. Sphalerite, galena, hydrophlogopite and bitumen were codeposited. Bitumen reflectance data and mesophase textures constrain maximum temperatures of mineralization and peak thermal conditions during burial to 200°C. Metals were transported in slightly oxidized, near-neutral fluids. Sulphur was either sourced from a separate reduced fluid or was transported as sulphate in the same fluid. Metal precipitation occurred via fluid mixing or sulphate reduction. Much sphalerite is preserved in primary depositional sites whilst galena occurs in syn-late tectonic veins and breccias but retains its primary spatial association with sphalerite at deposit scale. Nineteen galena samples, from a range of paragenetic settings, display homogeneous Pb isotope compositions. A Pb-model age of -1653 Ma is interpreted to reflect the timing of Zn-PbAg mineralization. Key characteristics of the syn-late tectonic Cu system include pyrrhotite, biotite-chloritedominant and ferroan dolomite-ferroan ankerite -dominant alteration. Phyllosilicates occur in stratabound lenses superimposed on Ba-K-feldspar alteration zones, whilst ferroan carbonate alteration is pervasive throughout the deposit hanging wall. These carbonates are texturally, chemically and isotopically (δ¹⁸O = 13.0-17.3‰, δ¹³C = 4.6 to -1.3 ‰) distinct from syndiagenetic carbonates. Copper-bearing fluids had temperatures of 250- 300°C based on phyllosilicate mineral stability relationships. The temporal-spatial-temperature zonation of the Zn-Pb-Ag and Cu systems at George Fisher is not preserved at Mount Isa and Hilton. Mineralogical and geochemical characteristics of carbonate and phyllosilicate alteration assemblages throughout Zn-Pbbearing strata at the latter deposits are products of syn-late-tectonic Cu mineralization, reflecting greater influx of Cu-bearing fluids at these sites. Poor preservation of syndiagenetic hydrothermal signatures at Mount Isa and Hilton has undoubtedly contributed significantly to the long-standing controversy surrounding the origin of Mount Isa-style Zn-Pb-Ag deposits.

The largest negative carbon isotope excursions in Neoproterozoic carbonates caused by recycled carbonatite volcanic ash

Abstract: The late Ediacaran Shuram Excursion (SE) records the most prominent negative δ13C excursions (δ13C = −12‰) during Earth’s history. It has been hypothesized to have resulted from oxidation of dissolved organic matter, diagenetic or authigenic precipitates. However, the origin of the SE remains enigmatic; current models face challenges regarding the significant amount of atmospheric oxygen needed to balance such extensive oxidation and sustained inputs of light carbon with extremely negative C isotope compositions. Here, we show that the Doushantuo Formation at the Jiulongwan section in South China, a key stratum recording the SE event, contains mineralogical and geochemical signatures related to igneous processes. Both the occurrence of ankerite, feldspar, moissanite and euhedral quartz in the SE samples and the relatively consistent Ce anomalies of carbonate and O isotopes of quartz indicate a contribution from an igneous source. In particular, the SE samples have trace element and C isotope compositions similar to those of recycled carbonatites formed by decarbonation and melting of sedimentary carbonate rocks. These observations suggest that the deep cycle of ancient carbonate rocks, which were subjected to decarbonation during subduction, melting and eruption related to the breakup of the Rodinia supercontinent, contributed to the SE. This igneous model for the SE may provide a connection between the deep and shallow carbon cycles of the Earth.

Geology of the Fishtie deposit, Central Province, Zambia: iron oxide and copper mineralization in Nguba Group metasedimentary rocks

Abstract: The Fishtie copper deposit, located in the Central Province of Zambia, contains approximately 55 Mt of 1.04 % Cu at a 0.5 % Cu cut-off in oxide, sulfide, and mixed oxide–sulfide ores. The deposit is hosted in Neoproterozoic diamictites and siltstones of the Grand Conglomerat Formation and overlying Kakontwe Limestone Formation of the lower Nguba Group. The Grand Conglomerat Formation at Fishtie directly overlies basement schists and quartzites. Mineralized zones are located adjacent to high-angle normal faults that appear to control thickness variations in the Grand Conglomerat Formation suggesting synsedimentary fault movement. Iron-rich rocks consisting of nearly monomineralic bands of magnetite and ankerite occur within the Grand Conglomerat Formation. The absence of magnetite-rich clasts in overlying diamictites and the presence of disseminated magnetite, ankerite, and apatite in adjacent diamictites suggest this iron-rich rock formed by replacement of siltstone beds. These magnetite-rich rocks thicken towards normal faults suggesting the faults formed conduits for oxidized hydrothermal solutions. The magnetite–ankerite–quartz rock was overprinted by later hydrothermal alteration and sulfide mineralization. Copper sulfide precipitation was associated with growth of both muscovite and chlorite, together with weak silicification. Sulfides are zoned relative to normal faults with bornite more common in proximity to faults and ore stage pyrite most common in an outer zone with chalcopyrite. Copper sulfides display generally heavy sulfur isotopic values, suggesting sulfide derivation from thermochemical reduction of Neoproterozoic seawater sulfate. Copper mineralized zones in the Grand Conglomerat at Fishtie are megascopically similar to those observed in the newly discovered Kamoa deposit in the southern Democratic Republic of Congo. Alteration and mineralization at Fishtie display lateral zoning relative to normal faults unlike the broad vertical zonation observed at the giant Kamoa deposit. The small size of the known mineralized zones at the Fishtie deposit relative to Kamoa is probably due to the absence of a thick siliciclastic palaeoaquifer beneath the Grand Conglomerat Formation as is present at Kamoa.