Andrew Tomkins

• PhD
• Professor at Monash University (Australia)

Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs Thermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which libera...

Gold has historically been a key strategic commodity, and the study of gold deposit formation has long been investigated. Debate on the genesis of so-called “orogenic gold deposits” (>75% of gold recovered through history; [Phillips, 2013][1]), continues largely because it is difficult to

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 observation...

Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO4-rich compositions in upper amphibolite facies...

Ureilite meteorites are arguably our only large suite of samples from the mantle of a dwarf planet and typically contain greater abundances of diamond than any known rock. Some also contain lonsdaleite, which may be harder than diamond. Here, we use electron microscopy to map the relative distribution of coexisting lonsdaleite, diamond, and graphit...

Gold nuggets occur predominantly in quartz veins, and the current paradigm posits that gold precipitates from dilute (<1 mg kg⁻¹ gold), hot, water ± carbon dioxide-rich fluids owing to changes in temperature, pressure and/or fluid chemistry. However, the widespread occurrence of large gold nuggets is at odds with the dilute nature of these fluids a...

Hyperenrichment of Au in orogenic ores occurs overwhelmingly within quartz veins, but the formation pathway of quartz veins in orogenic systems remains enigmatic. We conducted hydrothermal experiments simulating coprecipitation of Au and amorphous silica and subsequent recrystallization to test whether this is a viable mechanism to generate Au nugg...

The addition of cathodoluminescence (CL) and soft x-ray emission spectrometers (SXES) to microprobes gives access to spectroscopies that provide sample information not available from wavelength dispersive or energy dispersive spectrometers. We have developed software and hardware to allow collection of the backscatter electron signal, WDS, EDS, SXE...

Ureilites are primitive achondrite meteorites that result from fractional melt extraction from deep within the ureilite parent body [1]. These meteorites contain up to 7% carbon and exhibit a range of carbon phases, including amorphous carbon, graphite, diamond, and diamond-like structures [2-4]. The hexagonal form of diamond, lonsdaleite, has also...

Lonsdaleite is a hexagonal allotrope of carbon found in nature in meteorites and at meteorite impact sites. It has been predicted to have an indentation hardness greater than cubic diamond by first principles calculations. However, this has not been demonstrated experimentally. Here, nanoindentation was used to measure the hardness of two different...

A long-standing debate on the genesis of magmatic-hydrothermal Cu-Au deposits is whether the elevated oxygen fugacity (fO2) and metal endowment of their parental magmas are inherited from the mantle source or acquired during fractionation within the crust. This debate is built mainly on the composition of magmas emplaced into the upper crust that u...

Evidence for a hexagonal form of diamond, known as lonsdaleite, was discovered in the Canyon Diablo meteor impact site in 1967. Since then, there have been many observations of lonsdaleite in natural and laboratory samples. However, recent reports have argued that lonsdaleite does not exist as a discrete material and that prior observations can be...

The Wulong gold deposit (> 80 t Au) is located at the northeastern margin of the North China Craton (NCC). Gold in the most economically important quartz veins (Stages 2 and 3) is associated with a varied assemblage of Bi and Te minerals. Stage 2 is characterized by, in a temporal order, native gold-native bismuth-maldonite-hedleyite, bismuthinite-...

Meteorites that fall to Earth quickly become contaminated with terrestrial microorganisms. These meteorites are out of chemical equilibrium in the environments where they fall, and equilibration promotes formation of low-temperature alteration minerals that can entomb contaminant microorganisms and thus preserve them as microfossils. Given the well...

Ureilite meteorites contain regions of localized olivine reduction to Fe metal widely accepted to have formed by redox reactions involving oxidation of graphite, a process known as secondary smelting. However, the possibility that other reductants might be responsible for this process has largely been ignored. Here, 17 ureilite samples are investig...

In many orogenic gold deposits, gold is located in quartz veins. Understanding vein development at the microstructural scale may therefore provide insights into processes influencing the distribution of gold, its morphology, and its relationship to faulting. We present evidence that deformation processes during aseismic periods produce characterist...

Combined phase equilibrium and thermal modelling has been used to investigate the evolution of asteroid 4 Vesta. Orthopyroxene compositions of 200 natural diogenite meteorites are used as a basis for constructing a staged mantle melting model for Vesta, which is then used to develop a staged thermal evolution model. Our pMELTS models find that remo...

The origin of sediment-hosted copper-cobalt deposits (SCDs) within metamorphic terranes remains contentious, particularly in regard to the timing of mineralization relative to basin evolution. Here, we link the timing of Cu-Co mineralization in the Zhongtiao Mountains district, central China, to basin closure during development of the Trans-North C...

Reaction-induced porosity is a key factor enabling protracted fluid-rock interactions in the Earth's crust, promoting large-scale mineralogical changes during diagenesis, metamorphism, and ore formation. Here, we show experimentally that the presence of trace amounts of dissolved cerium increases the porosity of hematite (Fe 2 O 3) formed via fluid...

Metal transport in ore forming fluids is highly dependent on pH, ligands species (S, Cl, F) and redox conditions. The scapolite group is a family of tetragonal aluminosilicate consisting of meionite (Me; O 24 (SO 4 ,CO 3), respectively rich in [CO 3 ] 2-, Cl-, and [SO 4 ] 2-. During fluid/rock interaction occurring during mineralising process of sc...

Scapolite is abundant in medium to high grade metamorphic rocks with evaporitic calc-silicate bulk compositions, and because it can be Cl⁻ and SO4²⁻ rich, is able to provide a unique view into the evolution of deep brines during orogenic cycles. In the Zhongtiao Mountains, part of the Trans-North China Orogen (TNCO), scapolite occurs in two distinc...

Metamorphic reactions within the Earth’s crust produce fluids of variable composition that play a major role in the evolution of continents. Metamorphic fluids facilitate reactions that alter crustal rheology, reduce melting temperature, cycle elements between geological reservoirs and form ore deposits. These fluids are relatively inaccessible, ot...

We provide the first detailed analysis of the carbonaceous chondrite Dhofar (Dho) 1988. This meteorite find was recovered in 2011 from the Zufar desert region of Oman and initially classified as a C2 ungrouped chondrite. Dho 1988 is a monomict breccia composed of millimetre-sized clasts, between which large (∼50-250µm) intermixed sulphide-Ca-carbon...

The Fosterville gold (Au) deposit is hosted in the Bendigo zone within the western Lachlan orogen, southeast Australia, and contains three distinct mineralization styles: (1) refractory Au in fine-grained arsenopyrite and arsenian pyrite disseminated throughout metasedimentary rocks near brittle faults, (2) visible Au hosted in fault-controlled qua...

The past Martian atmosphere is often compared to the Archean Earth’s as both were dominated by CO2-rich and O2-poor chemistries. Archean Earth rocks preserve mass-independently fractionated sulfur isotopes (S-MIF; non-zero Δ³³S and Δ³⁶S), originating from photochemistry in an anoxic atmosphere. Thus, Martian crustal rocks might also be expected to...

The world's meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroi...

IOCG deposits are economically important providing amongst other resources, around 12% of global copper production and 47% of Australian copper production. A number of different genetic models have been proposed for the formation of IOCG deposits including ore systems for which fluids and metals are sourced from igneous bodies (Hauck, 1990; Groves...

Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire know...

The meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) toge...

The advent of microfluidics has revolutionized the way we understand how microorganisms propagate through microporous spaces. Here, we apply this understanding to the study of how endolithic environmental microorganisms colonize the interiors of sterile rock. The substrates used for our study are stony meteorites from the Nullarbor Plain, Australia...

Orogenic Au deposits have contributed the majority of Au recovered globally throughout history. However, the mechanism that concentrates Au to extremely high bonanza grades in small domains within these deposits remains enigmatic. The volume of fluid required to provide extreme Au endowments in localized occurrences is not reflected in field observ...

We describe how the world-class Proterozoic Century zinc deposit in northwest Queensland has been sculptured by an Ordovician meteorite impact, the Lawn Hill Impact Structure. The deposit is located at the SW edge of the crater, is dismembered by crater-related faults and is overlain by breccias (suevite) related to fall-back and resurge processes...

At the Fosterville gold deposit, high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz-carbonate veins. They are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence. Here we distinguish the influence of faults versus folds at Fosterville, by assessin...

Scapolite is an accessory mineral in high grade metamorphic terranes, and abundant in medium-grade evaporitic calc-silicate rocks. It varies in composition through a ternary solid solution with end-members of marialite (Na4Al3Si9O24Cl), meionite (Ca4Al6Si6O24CO3) and silvialite (Ca,Na)4 Al6Si6O24(SO4,CO3), which are rich in chlorine (Cl), carbon di...

The atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre‐existing mineralogy, texture, and organic material. The degree of heating depends predominantly on the gravity and atmospheric density of the planet on which they fall. For particles falling on Earth, the alteration can be significant, leading to the destructio...

NASA's strategy in exploring Mars has been to follow the water, because water is essential for life, and it has been found that there are many locations where there was once liquid water on the surface. Now perhaps, to narrow down the search for life on a barren basalt‐dominated surface, there needs to be a refocusing to a strategy of “follow the n...

The genesis of diogenites and olivine-bearing diogenites has been debated for decades, with current models favouring formation via either mineral settling in a homogeneous magma ocean, or as late stage intrusions into the crust of asteroid 4 Vesta. Using pMELTS, both equilibrium and fractional crystallisation modelling was conducted on a large rang...

We report the first Ag isotope data for the Paleozoic orogenic Au deposits in the Victorian Goldfields, southeast Australia, a world-class province with a historic production of 2,400 tonnes of Au. Despite their relatively uniform geology—similar host-rock types, age, mineralization style—deposits in Victoria show a wide range in 107Ag/109Ag ratios...

Arsenic (As) and gold (Au) are closely associated in many gold deposits, both being hosted in Fe-sulfide minerals (pyrite, marcasite, and arsenopyrite), partly because As geochemistry controls Au accumulation. Yet, the partitioning behavior of As between pyrite, arsenopyrite, and hydrothermal fluids remains poorly understood. Here, we introduce sol...

The Maryborough meteorite is a new H5 ordinary chondrite discovered about 2 km south of Maryborough, Victoria, in May 2015. It is a single stone measuring approximately 39 × 14 × 14 cm and with a mass of 17 kg. Plentiful indistinct chondrules are up to 1 mm across in a strongly recrystallised plagioclase-bearing matrix. Olivine and orthopyroxene in...

The fluorine(F)-rich nature in iron oxide-copper-gold (IOCG) deposits has received much attention since it was recognized that the giant Olympic Dam (OD) deposit contains >2 wt.% fluorite. Yet, the significance of the F-U-REE-Fe association remains poorly understood, with four existing hypotheses regarding the role of F: (i) fluoride increases the...

Changes in the oxygen fugacity (fO2) of the Earth’s mantle have been proposed to control the spatial and temporal distribution of arc-related ore deposits, and possibly reflect the evolution of the atmosphere over billions of years. Thermodynamic calculations and natural evidence indicate that fluids released from subducting slabs can oxidise the m...

The world's major source of iron ore is hosted in Precambrian banded iron formations. These chemical (meta-) sedimentary rocks are composed of alternating laminae of iron oxide minerals and chert. Despite the economic significance of high-grade iron ore deposits, controversy persists after decades of research on how banded iron formations became up...

Precambrian banded iron formations record the composition of Earth’s atmosphere and hydrosphere during the global rise of oxygen. It has been suggested that the banded texture of these rocks points to fluctuations in ocean chemistry although this remains a subject of debate. Here we show, by petrographic and electron microscopy of Palaeoproterozoic...

I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology...

In recent years, there has been an increased interest in Ag isotope variations in natural systems. Considering gold is mono-isotopic, but is closely associated with silver, a particular field of interest is the Ag isotopic composition of gold and its relation to ore-forming processes and source regions. In this study, a suite of native gold samples...

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 h...

Unambiguous identification of biosignatures on Mars requires access to well-characterized, long-lasting geochemical standards at the planet’s surface that can be modified by theoretical martian life. Ordinary chondrites, which are ancient meteorites that commonly fall to the surface of Mars and Earth, have well-characterized, narrow ranges in trace...

Nitrogen Defects in Diamond Examined by an Electron Microprobe - Volume 23 Issue S1 - N. C. Wilson, C. M. MacRae, A. Torpy, A. Tomkins

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 an...

It is generally thought that orogenic gold deposit formation is associated, in many cases, with metamorphic fluids. These fluids are mainly liberated by chlorite breakdown in the upper greenschist facies, and there is some debate as to whether the pyrite-to-pyrrhotite desulfidation reaction precedes or coincides with this fluid release. Since pyrit...

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 bee...

Magnetite breakdown during subduction of serpentinised ultramafic rocks may produce oxidised fluids that oxidise the deep Earth and/or the sub-arc mantle, either via direct transport of ferric iron, or via redox reactions between ferric iron and other elements, such as sulfur. However, so far, there is no consensus on the oxidation state of fluids...

The oxygen fugacity (fO 2) of the Earth's mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle fO 2 such as the introduction of oxidised material by subduction are still unresolved...

One hurdle to finding evidence for life beyond Earth is being able to identify its chemical, textural and isotopic fingerprints. This is a challenge in environments that can mask biomarker discovery and/or where geochemical and mineralogical limits are still being established. An ideal solution would be to use standards with known chemical and isot...

The Martian meteorites comprise mantle-derived mafic to ultramafic rocks that formed in shallow intrusions and/or lava flows. This study reports the first in situ platinum-group element data on chromite and ulvöspinel from a series of dunitic chassignites and olivine-phyric shergottites, determined using laser-ablation ICP-MS. As recent studies hav...

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sectio...

The formation of banded iron formations (BIF) remains controversial despite their potential to provide key information on Precambrian atmospheres and hydrospheres. It is widely agreed that BIF are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases. Many models address the chem...

The oxygen fugacity (fO 2) of the sub-arc mantle has controlled the availability of ore-forming elements to magmatic arcs and the speciation of volcanic gases emitted into the atmosphere since the onset of plate tectonics. Fluids generated from dehydration of subducted lithologies transfer oxidation potential inherited from sea water – crust intera...

The importance of fluorine for understanding the genesis of the Olympic Dam (OD) supergiant Fe-Cu-Au-U-REE deposit has been recently postulated by a number of studies that noted the abundance of this element at the OD deposit. The extracted ore contains ~2.5 wt% of fluorite (CaF2), adding up to ~106 Mt fluorine. McPhie et al. (2011) linked the high...

Both gold-rich sulphides and ultra-high grade native gold oreshoots are common but poorly understood phenomenon in orogenic-type mineral systems, partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations. The world-class Obuasi gold deposit, Ghana, has gold-r...

The Fe and Ni isotopic composition of ferromagnesian silicates in chondrules from unequilibrated ordinary chondrites (UOCs) have been used to estimate the initial abundance of the short-lived radionuclide, 60Fe, in the early Solar System. However, these estimates vary widely, and there are systematic discrepancies in initial 60Fe/56Fe ratios inferr...

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...

Keywords: sulfur sulfate sulfur cycle subduction arc magmatism porphyry copper Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little in...

An essential ingredient in the genesis of magmatic-hydrothermal ore deposits in arc settings, such as porphyry Cu-Au and epithermal Au, is thought to be a relatively high oxygen fugacity (fO 2) of the associated magma. fO 2 in magmatic systems controls the speciation of sulfur: at fO 2 > FMQ+1.5 sulfur is present mainly in its oxidized form, and is...

Scavenging of gold during metamorphic devolatilization is a widely accepted model for fluid and metal sourcing in orogenic gold deposits. In order to further constrain this process and quantify the capacity of autogenous fluids to release metals from pelites, we investigated the behaviors of S, Au, As, Cu, Pb, and Zn during pelite metamorphic devol...

Oxybarometric calculation show that lavas at convergent plate margins are, on average, more oxidised than mantle-derived magma from other tectonic settings. One way of oxidising the source of arc lavas in the mantle wedge is the addition of oxidised species, such as S 6+ , C 4+ and possibly Fe 3+ in slab-derived fluids. If so, this requires an effi...

Structural measurements and geochemical analyses, including bulk and in situ pyrite geochemistry, sulfur isotopes, and whole-rock geochemistry, are presented for the No. 3 orebody of the Jiaojia gold deposit (JJ3), located in the Jiaodong district of northeast China. The JJ3 orebody is distinct from the main orebody of the Jiaojia deposit (JJ1) bec...

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...

The Ok Tedi porphyry Cu-Au deposit in Papua New Guinea is a world-class mineral deposit. In its premining stage, it contained 5.5 Mt Cu and 18.1 Moz Au; skarn accounts for ~80% of the current 1.9 Mt Cu and 7.7 Moz Au in total resources. Two composite felsic stocks intruded a tectonostratigraphic sequence of clastic and carbonate rocks at ~1.16 ± 0....