Gj Davidson

Early Mesoproterozoic intrusive breccias in Yukon, Canada: the role of hydrothermal systems in reconstructions of North America and Australia

In northern Yukon, Canada, numerous breccia zones of early Mesoproterozoic age (ca. 1.6 Ga) are targets for mineral exploration. Collectively termed Wernecke Breccia, they are characterized by disseminated specular hematite and local enrichment of Cu, Co, U and Au. The breccias are hosted mainly by the Paleoproterozoic Wernecke Supergroup, a 13-km thick basinal to platformal succession of carbonate and fine-grained clastic rocks. Brecciation occurred after the Wernecke Supergroup was fully lithified, deformed, and locally metamorphosed. The breccia zones were generated by forceful explosions of volatile-rich fluids within the crust. The source of the fluids is uncertain, but may be related to igneous intrusions at depth. Rapid expansion of the fluids shattered large volumes of country rock, mainly sedimentary rocks of the Wernecke Supergroup, and dioritic to syenitic rocks of the Bonnet Plume River intrusions. In the central parts of the breccia zones, fragments underwent considerable motion, and in some cases became rounded from abrasion. Venting of brecciated rock and fluid is considered likely, but surface deposits are nowhere preserved. At one locality, large blocks of country rock foundered into open space near the top of a breccia zone, forming a fallback megabreccia. Faulting may have been active concurrently with brecciation. Breccia fragments are cemented together by hematite, quartz, carbonate, chlorite, feldspar, mica, and other minerals. In most cases, clasts and wallrocks were hydrothermally altered, leading to metasomatic growth of secondary minerals including flecks of hematite or rhombs of dolomite. Widely disseminated earthy hematite and local potassic alteration in the breccia clasts resulted in color changes from original drab hues of gray and brown to striking pink and red. Clasts with embayments rimmed with secondary minerals such as specular hematite are evidence for the dissolution of clasts or their diagenetic cements by hydrothermal fluids. The main phase of brecciation and metasomatism occurred at ca. 1.6 Ga, as indicated by a 15955 Ma U-Pb date on titanite. Subsequent minor hydrothermal events related to emplacement of the Hart River intrusions and Bear River dykes occurred at 1382.87.4 Ma (U-Pb rutile) and ca. 1270 Ma (U-Pb baddeleyite), respectively. Mineralized breccias at and near the Olympic Dam deposit in South Australia mineralogically and texturally resemble, and have nearly the same age as, the Wernecke Breccias. These similarities suggest that both breccia provinces developed from related systems of hydrothermal activity, and provide additional evidence for models linking the cratons of North America and Australia in Proterozoic time.

Microbial involvement in the formation of Cambrian sea-floor silica-iron oxide deposits, Australia

The Cambrian-Ordovician Mount Windsor volcanic belt in northern Australia is host to stratiform lenses of massive ferruginous chert that are spatially associated with volcanogenic massive sulfide occurrences, in particular the Thalanga zinc-lead-copper-silver deposit. The rocks are composed principally of Fe2O3 and SiO2, with very low concentrations of alkalic elements, and lithogenous elements such as Al, Zr, and Ti; they are interpreted as nearly pure chemical sediments. Textural evidence is documented of the integral role of filamentous bacteria (and/or fungi) in depositing iron from hydrothermal fluids, and of the inorganic precipitation of silica-iron-oxyhydroxide gels that subsequently matured to subcrystalline and crystalline silica forms. At least three distinct iron-accumulating microbial forms are distinguished: networks of septate filaments, nonseptate filament networks, and extremely coarse branching filaments that do not reconnect. Values for δ34S in disseminated pyrite are up to 50‰ lighter than those of contemporaneous Cambrian seawater, suggesting postdepositional colonization of some ironstones by sulfur-reducing bacteria. The site not only preserves the textural interplay of biological and inorganic depositional processes in exhalites, but also extends the oldest known instance of microbial mediation in vent-proximal hydrothermal iron precipitation to at least 500 Ma.

Stable isotope evidence for thermochemical sulfate reduction in the Dugald river (Australia) strata-bound shale-hosted zinclead deposit

Abstract Sulfur isotopes vary along strike and down-dip in the mid-Proterozoic Dugald River strata-bound shale-hosted zinclead ore deposit, Australia. Although the metal grade has been substantially increased at its southern end by tectonism, geological relationships indicate that mineralisation occurred during early diagenesis, probably in an organic-rich, shallow-water to evaporitic setting (Muir, 1983). Cu, Cu/(Cu + Pb + Zn) and Pb/(Pb + Zn) decrease in the mineralisation northwards independent of the main structural thickening, and so are likely to represent a pre-deformational primary geochemical dispersion. Average δ34S values in pyrite and sphalerite range from ∼ −1‰ in the south, coincident with the highest copper content of the ores, to ∼ +8‰ in the north, accompanying deposit thinning. In contrast, adjacent footwall and hangingwall iron sulfide does not exhibit isotopic zonation, and has a δ34S peak between +3 and +4‰ (n = 33), with some values as light as −14.5‰, whereas overlying dolomites contain δ 34 S py = + 5.5 to + 17.5‰ ; each population is attributed to varying degrees of closed-system biogenic sulfate reduction. Two populations of carbonate carbon isotopes are present. The first (δ 13 C = −11 to −5‰) mainly characterises graphitic Footwall Limestone of the Lady Clayre Dolomite, and is interpreted to reflect the metamorphic equilibration of carbonate and organic carbon isotopes. The second (δ 13 C = −25 to −15‰) characterises the sulphide lens and its immediate sedimentary host, and is most consistent with formation by the oxidation of organic carbon. Thermochemical sulfate reduction (TSR) by organic matter is preferred to account for the δ34S zonation, because of: (1) mass-balance calculations; (2) the δ13C values of ore and near-ore carbonate; (3) the high temperatures likely for ore formation (150–250°C), which would have prevented biogenic sulfur reduction but promoted TSR; and (4) the trend toward δ34Sseawater. The proposed mineralisation model involves an H2S − and metal-bearing fluid with δ 34 S ≈ −3 to 0‰ . This fluid ascended into the Dugald basin in the south, permeating laterally northward through carbonaceous sediments, below an evaporitic to shallow marine carbonate platform. During this migration, the hydrothermal fluid reacted with organic matter and sulfate to form 34S-rich sulfide (δ 34 S ≈ + 8 to + 10‰) by TSR. Reaction was catalysed by deep sourced H2S, biogenic H2S and H2S that evolved by hydrothermal-cracking of organic matter. The sources of sulfate for the reaction were: (1) evaporites locally within the sequence; and (2) dissolved sulfate diffusing from surface waters into the diagenetic zone.

Isotopic composition of gypsum in the Macquarie Island ophiolite: Implications for the sulfur cycle and the subsurface biosphere in oceanic crust

The O, S, and Sr isotope compositions were determined for 17 samples of gypsum that replaced anhydrite in the sheeted-dike complex of the Macquarie Island ophiolite. Elevated d34S (26.2%-29.0%) and d18O values (12.5%-14.4%) of gypsum compared to those of seawater sulfate are the result of microbial sulfate reduction. Low organic carbon contents and little sulfate reduction in sediments, plus a large basaltic Sr component in the gypsum (87Sr/86Sr = 0.70446-0.70524), indicate that the sulfate source was not pore waters in the overlying sediment. Low d34S values of sulfide in basalt lavas are consistent with microbial reduction of seawater sulfate within the volcanic rocks. Tectonic activity at the slowspreading ridge allowed evolved formation waters to enter hot sheeted-dike complex basement, resulting in heating and precipitation of anhydrite. Results show that microbes can leave geochemical tracers of their activity in oceanic basement and that anhydrite can be preserved in oceanic crust and may be of significance for the oceanic sulfur budget.

Metal leaching and inorganic sulfate reduction in volcanic-hosted massive sulfide mineral systems: Evidence from the paleo-Archean Panorama district, Western Australia

Comprehensive studies of the well-preserved, paleo-Arehean (3.6-3.2 Ga) Panorama volcanic-hosted massive sulfide district of Western Australla provide compelling evidence that metals were leached from the hase of the volcanic pile and redeposited at its top in volcanic-hosted massive sulfide deposits. This leaching provided more than enough metal to form known deposits, implying that direct magmatic input of metal is not required. Sulfur depletion from the base of the volcanic pile was associated with an increase in Fe2O3/FeO and hematitic alteration. These data, combined with sulfur isotope data, indicate that seawater sulfate reduction was facilitated by the olddation of rock FeO to hematite at high temperature in the H2S stability field. This is the first time that seawater sulfate reduction has been demonstrated regionally in an ancient volcanic-hosted massive sulfide mineral system. The data presented here require pah-Archean seawater to be sufate bearing.

Two sulphur isotope provinces deduced from ores in the Mount Isa Eastern Succession, Australia

The sulphur isotopic characteristics of ore deposits in the Australian Mount Isa Eastern Succession are not well known, unlike those of the Western Succession. In this study new detailed analyses are provided for recently discovered Eastern Succession mineralisation, such as the Starra and Osborne BIF-hosted Cu-Au ores, the Dugald River sediment-hosted Pb-Zn prospect, and four vein-hosted Cloncurry-style Cu±Au deposits (Hampden, Mt Elliot/Swan, Mt Cobalt, and the Answer Mine). All of the deposits of the Eastern Succession have δ34Ssulphide between −8 and +9%., regardless of their genesis. Empirically a moderate (δ34S range averaging close to 0%. characterises Starra-style Cu-Au and Pegmont Pb-Zn BIF ores, whereas shear and vein-style Cu mineralisation populations are tighter and do not average close to 0%. This is a particularly surprising result for Dugald River, where a larger isotopic variation more typical of stratiform sediment-hosted Pb-Zn ores in the region might have been expected. By comparison, Western Succession stratiform Pb-Zn and vein-style Cu deposits span a huge range of-15 to 51%. Large sulphur isotope ranges typify sulphate evaporite or organic sulphur-rich sedimentary successions. The lack of such variation in the Eastern Succession in turn suggests that primary evaporite sequences there were halite-dominated but sulphate-poor, and/or contained only limited volumes of organic-sulphur-rich sediment. Eastern Succession sequences were therefore less likely hosts for giant stratiform Pb-Zn deposits, because of their paucity of sulphur, although local sulphur sources permitted small deposits such as Dugald River to develop. Sedimentary conditions were more favourable for the development of sulphur-poor synsedimentary hydrothermal systems such as Starra, Osborne, and Pegmont, although sulphur isotope evidence is equivocal about the origin of these. Epigenetic deposits close to the Williams Batholith (Mt Dore, Hampden) owe their clustering around 0%. to their granitic fluid source.

Gold metallogeny and the copper-gold association of the Australian Proterozoic

Australian Proterozoic gold-producing deposits, emplaced mainly at 1.55–2.00 Ga, are divided into the following categories: (1) iron oxide-dominated, brecciahosted, Cu-U±Au replacement deposits spatially associated with felsic intrusions (273t Au); (2) stratabound Au±Cu-bearing iron formations (152.4t Au); (3) unconformity-style U ±Cu/PGM/Au deposits (53t Au); (4) Iron oxide-dominated Au±Cu mineralisation hosted within elements of ductile deformation (146.7t Au); (5) Broken Hill and volcanic-hosted massive sulphides (150t Au); (6) iron-sulphide-dominated veins and replacement zones spatially related to felsic intrusions (150.7t Au), and (7) iron-sulphide-dominated veins and replacement zones spatially related to elements of regional deformation (159.9t Au). Categories (1) to (4) are mainly confined to Proterozoic rocks, constituting an association in which Au and Cu are commonly present together, with variable amounts of U, Bi, Co, W, Se, Te and REE. Most examples in categories 1–4 fall into either of two groups: Cu-Aumagnetite ±hematite types formed at relatively high temperature (300–450 °C), and Cu-U±Au-hematite types formed at 150–300 °C. We postulate that these ores formed from a common high salinity (15–35 wt. % NaCl equiv.), low total sulphur (aΣS = 10−3 to 10−2), high fO2 fluid-type, in which metal transport was dominated by chloride-complexing. The most effective method of metal deposition was fluid mixing, achieving a synchronous decrease in fO2 and temperature. This unusual oxidised fluid association was favoured in high heat-flow extensional settings containing oxidised and/or oxidised-evaporitic sedimentary sequences. The intrusion of oxidised fractionated granites, which are commonly temporally associated with metal emplacement, acted in some places to heat and focus basinal fluids, and in others was the ultimate source of metals.

Variation in copper‐gold styles through time in the Proterozoic Cloncurry goldfield, Mt Isa Inlier: A reconnaissance view

The Cloncurry copper‐gold field, Mt Isa Inlier, contains a variety of copper‐gold mineralisation styles, in which Cu/Au ratios vary radically from Cu‐ to Au‐only. Several of these mineralisation styles are not recognised elsewhere in the Mt Isa Inlier and are rare in Proterozoic metallogeny, They developed concurrently with intense, high‐temperature, saline metasomatism exposed in the Cloncurry metasomatic zone 13 km to the southwest and may represent mineralisation styles that occur lateral to and above such albite‐actinolite alteration. Mineralisation varied through time in response to changing structural style. Initially small quartz vein‐hosted Au‐only deposits (Gilded Rose‐style) formed during regional D1 thrusting. These are known only from one east‐west oriented antiform in the Soldiers Cap Group, and do not display the high‐temperature saline fluid‐related alteration of subsequent systems. Quartz vein/replacement‐hosted Cu‐Au ± Zn‐As‐Co‐Bi‐Mn deposits (Eloise‐style) were formed in ductile D2/D3‐sh...

Alkali alteration styles and mechanisms, and their implications for a ‘brine factory’ source of base metals in the rift‐related McArthur group, Australia

McArthur Group alkali metasomatism occurred in several phases, none of which fit a classic model of alkali‐mineral zonation towards an evaporitic basin centre as previously proposed. They comprise: (i) trough‐margin alteration along the Emu Fault Zone, formed by gravity‐driven meteoric‐evaporitic brines sourced from adjacent shelves, and focused up to 3.5 km out into the adjacent deeper water clastic succession of the Barney Creek Formation; these formed zoned albite‐microcline assemblages, variably overprinted by B‐bearing illite; (ii) ore‐related alteration at the HYC Zn‐Pb‐Ag deposit, which overprinted trough‐margin alteration in places, adding disseminated ankerite, adularia and base‐metal sulfides to some feldspathised beds; (iii) early diagenetic alkali metasomatism that was associated with descending/advecting brines sourced from overlying evaporite sequences, and transmitted through porous clastic sediments (e.g. parts of the Lynott Formation, Mara Dolomite and Tatoolla Sandstone); and (iv) extens...

The Glyde Sub‐basin: A volcaniclastic‐bearing pull‐apart basin coeval with the McArthur River base‐metal deposit, Northern Territory

Sedimentological, geochemical and tectonic studies have been carried out on the Glyde Sub‐basin, a fault‐bounded depocentre adjacent to the margin of the Batten Trough, 80 km south of the HYC Pb‐Zn‐Ag ore deposit, in the mid‐Proterozoic McArthur Basin. Although it is unmineralized, the basin is, in some aspects, morphologically similar to the HYC Sub‐basin and provides an insight into processes which occurred coevally along strike from a giant shale‐hosted base‐metal deposit. The geometry of the sub‐basin supports an origin in a releasing bend of the Emu Fault during oblique right‐lateral extension of the Emu Fault Zone, resulting in the deposition of a very thick sequence of below wave‐base Barney Creek Formation carbonaceous siltstone. Prior to sub‐basin development the area was covered by hypersaline carbonate tidal flats of the Coxco Dolomite Member of the Teena Dolomite. Internal syn‐sedimentary normal faulting fractured the sub‐basin into seven major blocks, establishing a basic geometry of northern...