Andrew George Tomkins

Wetting facilitates late-stage segregation of precious metal-enriched sulfosalt melt in magmatic sulfide systems

The wetting behavior of As-rich sulfosalt melts against monosulfide solid solution (MSS) is investigated using experiments to elucidate late-stage fractionation processes in magmatic sulfide systems, which may control the distribution of platinum group elements. A range of As-rich melt compositions is found to wet MSS, including those that contain significant proportions (1%–45%) of precious metals (Pt, Pd, and Au). However, extremely Au rich or Pt rich sulfosalt melts (>∼40% Au; >∼50% Pt + 5% Au) do not wet MSS. These results imply that if magma contamination and/or fractionation processes were able to produce a late-stage As-rich melt (sulfosalt melts crystallize cooler than MSS) that exceeds ∼0.2% of the rock volume, then an interconnected melt drainage network would be able to form along MSS crystal triple junctions. The dense sulfosalt melt could thereby drain downward, progressively sequestering incompatible Bi, Sb, Te, Pt, Pd, and Au to form sulfosalt melt accumulations that continue to fractionate to form the platinum group minerals. This late-stage fractionation model is consistent with the observed mineral distribution in numerous magmatic sulfide deposits.

Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere

It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.

Sulfur isotope evolution in sulfide ores from Western Alps: assessing the influence of subduction-related metamorphism

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with δ34S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

New insights into the size and timing of the Lawn Hill impact structure: relationship to the Century Zn – Pb deposit

The Lawn Hill circular structure in northwest Queensland contains unambiguous evidence of an extraterrestrial impact, including planar deformation features in quartz, impact diamonds, widespread shatter cone formation and impact melt breccia in the Mesoproterozoic basement. The question of its relevance to ore genesis is investigated because the world-class Century Zn – Pb deposit is situated at the conjunction of the 100+ km Termite Range Fault and the previously defined margin of the impact structure. The impact structure is considered to be a 19.5 km wide feature, this constrained in part by the outer margin of an annulus of brecciated and highly contorted limestone. New evidence is presented indicating impact into this Cambrian limestone, including: (i) ‘dykes’ of brecciated Cambrian limestone extending hundreds of metres into the Mesoproterozoic basement; (ii) highly contorted bedding in the limestone annulus compared with essentially undeformed limestone away from the impact site; as well as (iii) a 1 Mt megaclast of Mesoproterozoic Century-like ore suspended in the limestone. Through aerial photograph analysis, large-scale convoluted flow structures within the limestone are identified, and these are interpreted to indicate that parts of the Cambrian sequence may have been soft or only semi-consolidated at the time of impact. This highly contorted limestone bedding is suggested to represent slump-filling of an annular trough in response to impact-induced partial liquefaction of a sediment veneer. The age of impact is therefore considered to be concurrent with limestone formation during the Ordian to early Templetonian, at 520 – 510 Ma. Formation of the Century deposit is found to be unrelated to impact-generated hydrothermal activity, although some minor hydrothermal remobilisation of metals occurred. However, there was macro-scale remobilisation of gigantic ore fragments driven by impact-induced lateral and vertical injection of limestone into the Proterozoic sediments. The limestone-filled annular trough surrounds a 7.8 km diameter central uplift, consistent with formation of a complex crater morphology.

Release of uranium from highly radiogenic zircon through metamictization: The source of orogenic uranium ores

Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone– and shear zone–hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element–rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ?100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons.

Microbial populations of stony meteorites: substrate controls on first colonizers

Finding fresh, sterilized rocks provides ecologists with a clean slate to test ideas about first colonization and the evolution of soils de novo. Lava has been used previously in first colonizer studies due to the sterilizing heat required for its formation. However, fresh lava typically falls upon older volcanic successions of similar chemistry and modal mineral abundance. Given enough time, this results in the development of similar microbial communities in the newly erupted lava due to a lack of contrast between the new and old substrates. Meteorites, which are sterile when they fall to Earth, provide such contrast because their reduced and mafic chemistry commonly differs to the surfaces on which they land; thus allowing investigation of how community membership and structure respond to this new substrate over time. We conducted 16S rRNA gene analysis on meteorites and soil from the Nullarbor Plain, Australia. We found that the meteorites have low species richness and evenness compared to soil sampled from directly beneath each meteorite. Despite the meteorites being found kilometers apart, the community structure of each meteorite bore more similarity to those of other meteorites (of similar composition) than to the community structure of the soil on which it resided. Meteorites were dominated by sequences that affiliated with the Actinobacteria with the major Operational Taxonomic Unit (OTU) classified as Rubrobacter radiotolerans. Proteobacteria and Bacteroidetes were the next most abundant phyla. The soils were also dominated by Actinobacteria but to a lesser extent than the meteorites. We also found OTUs affiliated with iron/sulfur cycling organisms Geobacter spp. and Desulfovibrio spp. This is an important finding as meteorites contain abundant metal and sulfur for use as energy sources. These ecological findings demonstrate that the structure of the microbial community in these meteorites is controlled by the substrate, and will not reach homeostasis with the Nullarbor community, even after ca. 35,000 years. Our findings show that meteorites provide a unique, sterile substrate with which to test ideas relating to first-colonizers. Although meteorites are colonized by microorganisms, the microbial population is unlikely to match the community of the surrounding soil on which they fall.

Fluorine and chlorine behaviour during progressive dehydration melting: Consequences for granite geochemistry and metallogeny

Dehydration melting of biotite is the main control on crustal differentiation in the mid to lower continental crust because this reaction produces the most voluminous and most mobile granitic melts. Biotite breaks down over a broad temperature interval, so the partitioning behaviour of elements between biotite and melt is likely to vary. It has been hypothesized that fluorine may stabilize biotite to higher melting temperatures because biotite is typically F-rich in ultra-high temperature (UHT) metamorphic rocks. If true, F would be an important influence on crustal differentiation because not only would it broaden the temperature range of melting but also elevated F concentration decreases melt viscosity. Furthermore, ligand partitioning between biotite and melt may be an important influence on the metallogeny of magmas. This study used electron microprobe analysis of biotite in rocks from the Ballachulish and Rogaland metamorphic aureoles to investigate the concentration of F and Cl in biotite heated to 600–1,000°C. Results show a broad increase in biotite F content (up to 5.04% F) with temperature until 850–920°C, beyond which F content decreases (<2.5% F). Chlorine concentrations in biotite are consistently lower (<1% Cl), and show a progressive decrease after the onset of partial melting. It was found that Mg content, and the processes that control Mg distribution, are most strongly correlated with F and Cl concentration in biotite. Calculations based on these results indicate that F-enriched biotite could be a significant source of F for continental crust-derived melts. Generation of a hot, F-rich melt at UHT conditions could be important for transporting lower crustal metals to the upper crust.

Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia

Abstract This paper describes the new mineral nataliyamalikite, the orthorhombic form of thallium iodide (TlI), from high-temperature fumaroles from the Avacha volcano, Kamchatka Peninsula, Russia. We also present some chemical analyses showing extreme enrichment of Tl in the volcanic gases at the Avacha volcano, and a review of thallium geochemistry that highlights the fascinating processes that led to the formation of nataliyamalikite. Nataliyamalikite occurs as pseudo-cubic nanocrystals (≤0.5 µm) within vacuoles in an As-(Te)-rich amorphous sulfur matrix and rarely as irregularly shaped aggregates up to ~50 µm in diameter within the amorphous sulfur matrix. Associated minerals include an unidentified Tl-As-S mineral, barite, and rare inclusions of a Re-Cu-bearing phase. The mean empirical composition based on four EDS analyses is Tl1.00(I0.95Br0.03Cl0.02), corresponding to the ideal formula TlI. Nataliyamalikite crystallizes in the orthorhombic system, space group Cmcm, which is consistent with the low-temperature (<175 °C) synthetic TlI polymorph. EBSD data reveal that some grains retain the cubic symmetry (Pm3m) of the high-temperature polymorph, although most analyzed grains display the orthorhombic symmetry. Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron, determined the following cell dimensions: a = 4.5670(9), b = 12.803(3), c = 5.202(1) Å, V = 304.2(1) Å3, and Z = 4. The six strongest calculated X-ray reflections and their relative intensities are: 3.31 (100), 2.674 (73), 3.20 (43), 2.601 (28), 2.019 (21), and 2.284 Å (19). The combination of EBSD analysis (providing an efficient test of the crystallinity and crystal symmetry of a population of micrometer-sized grains) and synchrotron single-crystal X-ray micro-diffraction (beam size ~7.5 µm) on micro-aggregates extracted using FIB-SEM opens the way to the characterization of challenging specimen—in this case, the sulfur matrix is highly beam sensitive, and the nataliyamalikite grains could not be isolated using optical microscopy. The high-temperature (>600 °C) sulfidic (~1.2 wt% S) vapors at Avacha are extremely enriched in thallium; with 34 ppm, they contain an order of magnitude more Tl than the richest volcanic gases analyzed to date and ~100× more Tl than most metal-rich fumarolitic fluids associated with volcanic arcs. The formation of nataliyamalikite illustrates the complex processes that control thallium geochemistry in magmatic arc systems. Thallium minerals have now been reported in andesitic (Avacha), basaltic (Tolbachik, Kamchatka), as well as rhyolitic (Vulcano, Eolian Islands, Italy) volcanoes. Ultimately, these thallium minerals result from the transfer of thallium from subducted sediments to volcanic gases in arc volcanoes. We suggest that the extremely thallium-enriched vapors from which nataliyamalikite formed result from complex and transient interactions between Tl-rich sulfosalt melts and magmatic vapors, a process that may be important in controlling metal distribution in boiling epithermal systems.