David L. Huston

Barite, BIFs and bugs: evidence for the evolution of the Earth’s early hydrosphere☆

Abstract The presence of relatively abundant bedded sulfate deposits before 3.2 Ga and after 1.8 Ga, the peak in iron formation abundance between 3.2 and 1.8 Ga, and the aqueous geochemistry of sulfur and iron together suggest that the redox state and the abundances of sulfur and iron in the hydrosphere varied widely during the Archean and Proterozoic. We propose a layered hydrosphere prior to 3.2 Ga in which sulfate produced by atmospheric photolytic reactions was enriched in an upper layer, whereas the underlying layer was reduced and sulfur-poor. Between 3.2 and 2.4 Ga, sulfate reduction removed sulfate from the upper layer, producing broadly uniform, reduced, sulfur-poor and iron-rich oceans. As a result of increasing atmospheric oxygenation around 2.4 Ga, the flux of sulfate into the hydrosphere by oxidative weathering was greatly enhanced, producing layered oceans, with sulfate-enriched, iron-poor surface waters and reduced, sulfur-poor and iron-rich bottom waters. The rate at which this process proceeded varied between basins depending on the size and local environment of the basin. By 1.8 Ga, the hydrosphere was relatively sulfate-rich and iron-poor throughout. Variations in sulfur and iron abundances suggest that the redox state of the oceans was buffered by iron before 2.4 Ga and by sulfur after 1.8 Ga.

A chemical model for the concentration of gold in volcanogenic massive sulphide deposits

Study and review of the gold distribution in selected volcanogenic massive sulphide deposits (Rosebery and Hellyer in Tasmania, Mt. Chalmers in Queensland and Millenbach in Quebec) indicates that gold has two distinct occurrences in these deposits: (1) a gold-zinc-lead-silver association which is commonly found in the upper part of zinc-rich massive sulphide lenses (e.g. Rosebery and Hellyer), and (2) a gold-copper association which is commonly present in the stringers and lower portions of copper-rich deposits (e.g. Mt. Chalmers and Millenbach). In each deposit one of these occurrences dominates almost to the exclusion of the other.

Design, calibration and geological application of the first operational Australian laser ablation sulphur isotope microprobe

This contribution describes the setup and operating procedures of the first operational laser ablation microprobe for stable (sulphur) isotope analysis in Australia as well as some brief geological applications. A significant feature on this laser ablation microprobe is automated gas purification and analysis; operator control is only required to locate and ablate sample targets. As with other laboratories, samples were ablated in an oxygen atmosphere, producing a SO2/O2 gas mixture. SO2 was separated from this mixture by either of two techniques. In the first technique, SO2 was condensed into a liquid N2 trap by cryogenic pumping, and O2 was pumped away. This resulted in the collection of 60–70% of the produced SO2. In the second technique, SO2 was condensed into a liquid N2 trap as the SO2/O2 mixture was slowly bled away. This technique collected 90–95% of the SO2, with a small fractionation of 0.16%. Laser ablation and SO2 collection via the second technique required a mineral dependent, additive corre...

Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons

Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.

Metallogeny and its link to orogenic style during the Nuna supercontinent cycle

Abstract The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nunas assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nunas interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting. Supplementary material: Supplementary material including (1) tables (S1–S3) of Euler poles and palaeopoles used, summary of Nuna orogens; (2) a figure (S1) of modelled plate velocities; (3) mp4 files (S1 & S2) of the model with age data; ore deposits and VGPs; and (4) a zip file (S1) of the Gplates model is available at http://www.geolsoc.org.uk/SUP18822.

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.

The effect of alteration and metamorphism on wall rocks to the Balcooma and Dry River South volcanic-hosted massive sulfide deposits, Queensland, Australia

Abstract Two types of visibly hydrothermally altered rocks are present in the Cambro-Ordovician Balcooma volcanic-hosted massive sulfide (VHMS) district of northeast Queensland: (1) quartz-chlorite ± biotite schist, and (2) pyritic quartz-muscovite schist. The quartz-chlorite ± biotite schist, which has gained Mg, Fe, Mn, Si, and S, but lost K, Na, Ca, Rb and Sr, occurs in a crosscutting pipe below Cu-rich massive sulfide lenses at the Balcooma deposit. Pyritic quartz-muscovite schist, which has gained Fe, Si, K, Rb, S, Cu, Zn and Pb, but lost Na, Ca and Sr, occurs in extensive zones below the Dry River South sulfide lens and Balcooma Zn-Pb-rich lenses as well as in stratiform zones west of Balcooma. With the possible exceptions of Nb and Y at Dry River South, Al, Ti, Zr, Nb and Y are all immobile in these alteration zones. An outer zone of weak Fe, S, Cu and Zn enrichment, and Na, Ca and Sr depletion surrounds the Balcooma deposit and extends up to distances of 200 m beyond visibly altered rocks. Evidence for syn-tectonic composition changes to these alteration zones exist as 0.1–10 m thick gradational zones between quartz-chlorite ± biotite schist and the outer weakly altered zone at Balcooma. These zones lack muscovite and contain highly poikilitic staurolite and biotite porphyroblasts. These zones gained Mg, Fe, Mn, Si, Ca, Cu and Sr, but lost K and S. These zones also gained Zr, which is considered to be immobile in most geological environments. Major and minor element concentrations in metasediments from the Balcooma district vary systematicaly with Zr/TiO 2 as a result of weathering and deposition during turbitidic sedimentation. Deviations from trends relating Na and Fe to Zr/TiO 2 can be used to define peripheral weak alteration zones not identifiable from geological mapping.

Microthermometry and geochemistry of fluid inclusions from the Tennant Creek gold-copper deposits: implications for ore deposition and exploration

Gold-copper-bismuth mineralization in the Tennant Creek goldfield of the Northern Territory occurs in pipe-like, ellipsoidal, or lensoidal lodes of magnetite ± hematite ironstones which are hosted in turbiditic sedimentary rocks of Proterozoic age. Fluid inclusion studies have revealed four major inclusion types in quartz associated with mineralized and barren ironstones at Ten nant Creek; (1) liquid-vapour inclusions with low liquid/vapour ratios (Type I), (2) liquid-vapour inclusions with high liquid/vapour ratios or high vapour/liquid ratios and characteristic dark bubbles (Type II), (3) liquid-vapour-halite inclusions (Type III), and (4) liquid-vapour inclusions with variable liquid/vapour ratios (Type V). Type I inclusions are present in the barren ironstones and the unmineralized portions of fertile ironstones, whereas Types II and III inclusions are recognized in fertile ironstones. Trails of Types II and III inclusions cut trails of Type I inclusions. Type I fluid inclusions have homogenization temperatures of 100° to 350 °C with a mode at 200° to 250 °C. Type II inclusions in mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44 deposits) have homogenization temperatures of 250 °C to 600 °C with a mode of 350 °C. Type I fluid inclusions have a salinity range of 10 to 30 NaCl equiv. wt %. Salinity measurements on fluid inclusions in the mineralized zones gave a range of 10 to 50 NaCl equiv. wt % with a mode of 35 NaCl equiv. wt %. Fluid inclusion studies indicate that the Tennant Creek ironstones were formed from a relatively low temperature and moderately saline fluid, where as gold and copper mineralization was deposited from later hydrothermal fluids of higher temperature and salin ity. Gas analysis indicates the presence of N2 and CO2, with very minor CH4 in Types II inclusions but no N2 or CH4 gases in Type I inclusions. Microprobe analysis of the fluid inclusion decrepitates indicates that the inclusions from Tennant Creek contain sodium and calcium as dominant cations and potassium in a subordinate amount. The high temperatures (≥ 350 °C), high salinities (≥ 35 NaCl equiv. wt. %) and cation composition of the Tennant Creek ore fluids suggest that the ore fluids were derived from upward migrating heated basinal brines, although contribution from a magmatic source cannot be ruled out. Close association of vapour-rich Type IIb and salt-rich Type III inclusions in the mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44) indicates heterogeneous trapping of ore fluids. This heterogeneous trapping is interpreted to be due to unmixing (exsolution) of a gas-rich (e.g. N2) fluid during the upward migration of the metal bearing brines and/or due to degassing caused by reaction of oxidized ore fluids and host ironstones. Fluid inclusion data have important implications regarding the deposition of gold in the ironstones, and may have application in discriminating fertile from barren ironstones.

Selenium and its importance to the study of ore genesis: the theoretical basis and its application to volcanic-hosted massive sulfide deposits using pixeprobe analysis

Distribution coefficients for the partitioning of selenium between aqueous fluids and sulfide minerals have been estimated from existing thermodynamic data. These data indicate that selenium levels in pyrite increase at lower temperature for a given fluid H2SeH2S ratio. Consideration of the aqueous speciation of selenium and sulfur indicates that above 200°C, H2SeH2S approximates ΣSeΣS in most hydrothermal fluids. Using pixeprobe data for pyrite from eastern Australian volcanic-hosted massive sulfide deposits, the ΣSeΣS ratio of zinc-rich ores is below 1.5 × 10−6, whereas the ΣSeΣS ratio of copper-rich ores is 7–300 × 10−6. Using ΣSeΣS ratios of 10−7 for sea water and 10−4 for fluids with magmatic-derived sulfur, zinc-rich ores contain minimal ( 7%) magmatic-derived sulfur.

Crustal evolution constraints on the metallogeny of the Yilgarn Craton

Improved understanding of the evolution of the Archean Yilgarn Craton, Western Australia, indicates a collage of crustal fragments formed through both autochthonous and allochthonous processes. Komatiite-associated nickel sulfide (KANS), volcanichosted massive sulfide (VHMS) Zn-Cu and orogenic Au deposits are associated with specific crustal domains. KANS deposits are concentrated in terranes with evidence for pre-existing crust. In contrast, VHMS deposits are associated with rift-like zones of isotopically primitive crust. Gold mineralization predominates in domains with evidence for komatiite magmatism, melts sourced from subduction-modified mantle, and lithospheric-scale orogeny. Development of the mineral systems likely reflects interaction of specific geotectonic processes with the pre-existing crustal and mantle reservoirs.