River(ina) of gold – historical activity and structural controls

Stuart, Dr Cait1, Ricketts, Mel; Gilmore1, Phil1

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia

The Riverina region of New South Wales is well known for agriculture, but the region also has a long history of gold mining, dating back to the late 19th century. The Riverina has seen relatively little modern mineral exploration. The five-year (2014 to 2019) East Riverina Mapping Project undertaken by the Geological Survey of NSW has highlighted the prospectivity of the region for structurally-controlled, low-sulfide (orogenic) gold, as well as intrusion-related gold and porphyry copper–gold mineralisation.

Structurally-controlled, low-sulfide gold mineralisation within the eastern part of the Riverina region, which extends from West Wyalong to Albury, is hosted by Ordovician to Devonian units and largely located adjacent to the Gilmore Fault Zone. The area has a current endowment (past production and identified resources) of 33.45 t (1.4 M ounces) of gold in 23 identified goldfields situated along the Gilmore Fault Zone and other major, parallel structures.

For each goldfield, a review of historical data, host lithologies, endowment, structural setting, style and timing of mineralisation was undertaken. Each goldfield was considered in a regional structural context. In particular, the structural setting and timing of mineralisation were related to movement along the Gilmore Fault Zone and associated structures.

Structurally-controlled, low-sulfide gold mineralisation was found to occur in three main settings:

  1. In narrow vein arrays in orientations that fit within a Riedel shear model

Veins are typically found in more competent rock types adjacent to second or third order splay faults of the Gilmore Fault Zone or major parallel structures. Mafic dykes are commonly associated with shear zones and may have caused desulfidation of gold-bearing fluids, resulting in mineralisation. Examples of this setting include Adelong, West Wyalong and Sebastopol.

2. Along the contacts between intrusions and metasedimentary country rock, or in pressure shadows of intrusions

Competency contrast between intrusions and metasedimentary rocks is thought to have focussed fluids along these contacts or around competent bodies. Pressure changes during fluid migration likely played a role in gold mineralisation. Examples include Yalgogrin, Weethalle and Grong Grong.

3. Gold found in fold hinges and fold limbs adjacent to the Gilmore Fault Zone

Barmedman is the only recognised example of this style in the area.

Geochronological data for gold mineralisation in the project area are rare and further work is required. However, field relationships indicate the majority of structurally-controlled, low-sulfide gold mineralisation occurred during the Devonian, in the Bindian Event or the Tabberabberan Contraction, with minor mineralisation also potentially occurring during the Ordovician Benambran and Carboniferous Kanimblan contraction events.


Biography

Cait Stuart was a Graduate Geoscientist with the Geological Survey of NSW and is now a Geologist with the Northern Territory Geological Survey, where she undertakes mapping and mineral systems projects on NT geology. She has experience in structural and metamorphic geology and has BEnvSc and PhD degrees.v

The Late Archean Au epoch: By-product of Earth degassing

Walshe, John L.1, Bath, Adam, B.1

1CSIRO Mineral Resources, PO Box 1130, Bentley, Western Australia 6102, Australia

It is possible to think of the mineral systems that created the Earth’s major mineral deposits and provinces as volato-thermally driven chemical engines with roots deep in the mantle. It may be argued that the chemical potential of systems was linked to this degassing history and reflects the interplay of deep-Earth anhydrous fluids with the Earth’s hydrous outer layers.  Metal transport and deposition capacity was closely linked to propagation of redox and related physico-chemical gradients through systems. Such arguments imply quite specific links between the formation of the Earth’s resources across time and space, secular changes in architecture and geochemistry of the planet over some 4.5 billion years of evolution and Earth phenomena such as mass extinction events, global anoxia and atmospheric evolution. Conceivably a better understanding of large-scale linkages may lead to better techniques for differentiating productive terrains and epochs of Earth history as well as the links between metallogenesis and other Earth processes.

The Late Archean Au deposits, at ~ 2.7 to 2.63 Ga, can be interpreted as one manifestation of planetary degassing of highly reduced and oxidized volatiles. The Late Archaean gold deposits are known from five continents, are hosted within supra-crustal volcano-sedimentary sequences and are spatially associated with trans-crustal structures, 100s of kilometres in length, as exemplified by the deposits of the eastern Yilgarn Craton, Western Australia. All the productive gold camps of the eastern Yilgarn gold province show evidence of deposit to district scale mineral zoning and systematic patterns in 13Ccarbonate and δ34Ssulfide that can be related to chemical gradients sustained by fluxes of reduced (H2, CH4) and oxidized volatiles (SO2) of mantle origin, triggered by tectonic events and related magmatism. Gold transport and deposition was favoured by chemically zoned volato-thermal plumes with an inner core of reduced, alkaline & quartz under-saturated fluids and outer zones of oxidized fluids. Gradients in activity of H2O were sustained by pulsed injection of anhydrous mantle-volatiles into pre-existing crustal hydrothermal systems. Local electrical discharges occurred over distances of ~10 to 100 m, coupled to rock fracturing in the anhydrous parts of systems. Gold deposition was controlled by water activity gradients, coupled with pH and redox gradients.

Sustaining these gradients was key to sustaining gold transport and formation of high-grade resources. Collapse led to dispersion of the gold ± low grade mineralization.

It is hypothesized that the ultimate driver of the Late Archean Au epoch was an electron flux from the core-mantle boundary that released H from the mantle (e + OH_ = O2- + H), drove redox reactions and sustained the volatile flux of the late Archean.


Biography

Dr John Walshe is Chief Research Scientist, CSIRO Mineral Resources. Since joining CSIRO in 1995, Dr Walshe has contributed to developing mineral systems concepts and applications to mineral exploration. Prior to joining CSIRO, Dr Walshe lectured at the Australian National University. He is a graduate of the University of Tasmania.

Apatite chemistry indicates that oxidized auriferous fluids along the world-class Boulder Lefroy – Golden Mile fault system were significantly different to fluids from global porphyry systems

Bath, Dr Adam, B.1, Walshe, John, L., Ireland, Tim., Cobeñas, Gisela., Sykora, Stephanie., Cernuschi Federico., Woodall, Katie., MacRae, Colin., Williams, Morgan., Schmitt, Leanne.

1CSIRO Mineral Resources, PO Box 1130, Bentley, Western Australia 6102, Australia, 2First Quantum Minerals Ltd, 1/24 Outram St, West Perth WA 6005, 3Lundin Mining, El Bosque Norte 500, piso 11, Of. 1102,  Las Condes, CP 7550092, Santiago, Chile

The Boulder Lefroy – Golden Mile (BL-GM) fault system contains >70 Moz of gold and accounts for more than a quarter of the known gold mineralization in the Archean Yilgarn Craton, Western Australia. Alteration along the BL-GM fault system shows a wide range of redox conditions (hematite/anhydrite- to pyrrhotite-stable), and this range in many cases can be linked to mineralization. Oxidized fluids have been linked to adakitic magmas or more generally, Au fluids linked with sub-arc mantle wedges. However, invariably in Archean Au systems there is a lack of evidence of causative intrusions at the time of oxidized alteration and mineralization, leading numerous authors to reject the possibility that magmas were a major source of auriferous fluids. Despite this, oxidized fluids derived from deeper concealed intrusions cannot be ruled out, particularly given the length and breadth of anhydrite alteration can be spatially significantly large (km-scale). Here we present mineral and isotopic datasets on oxidized fluids from the BL-GM fault system with the aim to view these datasets in comparison to those from porphyry systems. Porphyry Cu-Au systems are associated with oxidized magmatic fluids generated in arc environments, and here we test if apatite chemistry can be used to make any links between oxidized fluids in Archean Au systems and global porphyry systems. Close to 1000 apatite analyses were obtained on the electron microprobe. The apatite crystal formula is Ca5(PO4)3X, where X represents the channel volatile site that runs parallel to the crystallographic c-axis, occupied by OH, F, and Cl. Results from our dataset show that apatite from gold-bearing oxidized alteration assemblages in Archean Au system have variable amounts of F (2.5 to 4.1 wt. %), whereas Cl values remain low (<500ppm) over the wide range of F values. In contrast, apatite from porphyry Cu-Au deposits with anhydrite have a wide range of F values (2.2 to 4.8 wt. %) and show a significant linear increase in Cl (<300ppm to 1.7 wt. %) with decreasing F. The stark difference in the X-site substitution appears the main variance between the chemistry of apatite from the two environments. In hydrothermal Archean Au and porphyry Cu-Au systems, higher amounts of F tend to correspond to lower temperatures (based on biotite-apatite thermometer). Also, previous experimental studies show that relatively low Cl and F concentrations in acidic (H20-HCI) fluids result in high ratios of XClAp/XOHAp and XFAp/XOHAp in apatite, whereas much higher abundances of CI and F are required in basic fluids to achieve the same result. Data from previous studies also show that the dilution of aH2O with higher aCO2 results in apatites with higher ratios of XCIAp/XOHAp and XFAp/XOHAp from fluids with comparatively lower levels of Cl or F. Our data shows no evidence of these processes. The most plausible explanation for the comparatively low XCIAp/XOHAp apatites from Archean Au systems is that the fluids were comparatively low saline compared to those from oxidized alteration assemblages in porphyry Cu-Au systems. Furthermore, results of this study show that apatite from oxidized and reduced alteration assemblages of Archean Au systems across the Yilgarn Craton plot along the same trend on the Cl versus F diagram. One notable difference between the two populations is that those forming part of reduced alteration assemblages have lower F values (1 to 3.3 wt. %) and appear to be associated with hotter fluids.


Biography

Mineral system scientist with experience of mapping, characterising and exploring for Cu and Au mineral systems in a range of terranes. Experience developing and implementing new and novel techniques towards mapping hypogene alteration and exploring in Archean gold systems in various terranes of the Yilgarn Craton at the camp-scale.

Source of gold in Neoarchean orogenic-type deposits in the North Atlantic Craton, Greenland: Insights for a proto-source of gold in sub-seafloor hydrothermal arsenopyrite in the Mesoarchean

Saintilan, Dr Nicholas J.1,7*, Selby, D.1,2, Hughes, J. W.1,3, Schlatter, D. M.4, Kolb, J.5, Boyce, A.6

1Department of Earth Sciences, University of Durham, Durham DH1 3LE, United Kingdom, 2State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Resources, China University of Geosciences, Wuhan, China. 3Bluejay Mining Plc, 2nd Floor, 7-9 Swallow Street, London, W1B 4DE, United Kingdom, 4Helvetica Exploration Services GmbH, Carl-Spitteler-Strasse 100, 8053 Zürich, Switzerland, 5Institute of Applied Geosciences, Department of Geochemistry and Economic Geology, Karlsruhe Institute of Technology, Adenauerring 20b, 76131 Karlsruhe, Germany, 6Isotope Geoscience Unit, SUERC, Rankine Avenue, East Kilbride, Glasgow G75 0QF, United Kingdom, 7Present address: Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zürich, Clausiusstraße 25, 8092 Zürich, Switzerland

Given that gold (Au) mostly remained in the incipient Earth mantle until ca. 3.9–3.8 Ga, a “proto-source” of gold may have been present in the dominantly mafic crust precursor born through first-stage melting of the early Earth mantle. In south-westernmost Greenland, a fragment of the North Atlantic Craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks, and harzburgite cumulates that were emplaced at ca. <3.19–3.01 Ga (e.g., Tartoq greenstone belt). Here, combining detailed sulphide petrography with rhenium-osmium-sulphur (Re-Os-S) isotope geochemistry of individual mineral separates of arsenopyrite from gold-sulphide mineralised shear zones, we pinpoint the precipitation of ca. 3.18–3.13 Ga (Re-Os model ages) hydrothermal arsenopyrite associated and coeval with arc-related magmatism of the Tartoq Group. We consider sub-seafloor hydrothermal alteration of the oceanic crust and magmatic activity to have supplied arsenic (As), Re, and Au, to result in the precipitation of the ca. 3.18–3.13 Ga arsenopyrite with primary invisible gold. Additionally, in major shear zones in a rigid juvenile continental crust, retrograde greenschist-facies metamorphism overprinted the ca. >3.0 Ga prograde amphibolite-facies metamorphic assemblages and caused local dissolution of arsenopyrite. During this retrograde tectono-metamorphic stage, in gold-rich shear zones, the Re-Os geochronometer in arsenopyrite was reset to a Neoarchean age while invisible gold was liberated and deposited as free gold with 2.66 Ga pyrite (Re-Os isochron ages). The initial Os isotope ratios of Neoarchean arsenopyrite (187Os/188Osi = 0.13 ± 0.02) and gold-bearing pyrite (0.12 ± 0.02) overlap with the estimated 187Os/188Os ratio of the Mesoarchean mantle (0.11 ± 0.01) and preclude contribution of radiogenic crustal Os from evolved lithologies in the accretionary arc complex, but instead, favour a local contribution in Os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids. Thus, the ca. 2.66 Ga lode gold mineralisation identified in the North Atlantic Craton may illustrate a gold endowment in shear zones in Earth’s stabilizing continental crust at the time of the 2.75–2.55 Ga Global Gold Event, through metamorphic upgrading of bulk gold which had originally been extracted from the Mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced Mesoarchean ocean.

Reference:

Saintilan et al. (2020) Source of gold in Neoarchean orogenic-type deposits in the North Atlantic Craton, Greenland: Insights for a proto-source of gold in sub-seafloor hydrothermal arsenopyrite in the Mesoarchean. Precambrian Research 343, 105717.


Biography

Nicolas Saintilan has set up and manages a Re-Os geochronology laboratory at ETH Zürich in Switzerland. He is an Ambizione Fellow of the Swiss SNF and holds a PhD from the University of Geneva with postdocs with Rob Creaser and Dave Selby in Canada and the UK successively.

Do rocks deposited during time periods with high gold in sedimentary pyrite host more gold mineralization?

Gregory, Daniel1, Lui, Timothy1, Wu, Selina1, Large, Ross2

1University of Toronto, Toronto, Canada, 2ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Hobart, Australia.

Over the past 10 years it has become increasingly common for people to view host rocks, often sedimentary rocks, to be a source of metals for orogenic gold systems. For sediment hosted deposits it has been proposed that syngenetic and/or diagenetic pyrite may be an important source of the gold. This model states that the gold is released during metamorphism, after which it migrates with metamorphic fluids and is deposited in trap sites. During the same time period it was shown that pyrite trace element content, including gold, varies significantly through geologic time. Thus, it stands to reason that an initial step to finding new gold districts may be to identify basins / periods of time when gold is elevated in sedimentary pyrite. Databases of trace element content of sedimentary pyrite show that gold was elevated at approximately: 3 Ga, 2.5-2.7 Ga, 1.9 Ga, 0.9 Ga, 550 Ma, 450 Ma, and 300 Ma (Precambrian ages +/- 100 Ma; Phanerozoic +/-50 Ma to encompass the length of time that sedimentary pyrite is generally elevated in gold). These are also the ages of the host rocks for many important gold districts (for example, the Superior and Yilgarn craton orogenic gold deposits, the Witwatersrand deposits, the Sukoi Log deposit, the Bendigo district, and the Carlin district). In this study we went to an area where gold deposits occur but is not one of these more famous districts where gold is well established to be hosted by sediments the same age as those with elevated gold in pyrite: Queensland Australia. Approximately 8658 known gold deposits or occurrences are present in Queensland. Of these 3023 (35%) are host by the stratigraphy of the ages given above, a further 898 (10%) are within 1 km and 1579 (18%) are within 5 km. Furthermore, the geologic units of prospective age encompass less than 5% of the land area of Queensland. This suggests that indeed gold deposits are more likely to form in areas that are likely to have elevated gold in sedimentary pyrite and these stratigraphic packages should be prioritized when searching for new gold districts.


Biography

Daniel Gregory is an Assistant Professor at the University of Toronto. He completed his PhD focussing on pyrite chemistry at CODES, University of Tasmania in 2014 before spending 3 years as a post doc at the NASA Astrobiological Institute at the University of California Riverside.

A view of orogenic gold deposits as nonlinear systems: Nonlinear analysis of data

Ord, Alison1, Hobbs, Bruce2

1The University of Western Australia, Perth, Australia, 2CSIRO, Perth Australia

Despite many studies of orogenic gold systems, the underlying processes involved in their formation and in defining their location remain enigmatic. This arises because such processes are multiscale and nonlinear so that patterns of alteration and mineralisation are apparently irregular and unpredictable. The goal of a nonlinear dynamical analysis of spatial data is to extract the dynamics of the underlying nonlinear and multiscale physical and chemical processes that produced these data. We review nonlinear analysis methodology and explore hyperspectral and gold assay data for a drill-hole in an orogenic gold system. The analysis is non-parametric and purely data driven. We use recurrence, cross- and joint-recurrence plots to extract the invariant measures of the system including the embedding dimension, the first positive Lyapunov exponent and the entropy and construct the attractor for the mineral distributions. The resulting dynamical model is tested using nonlinear prediction algorithms. Cross recurrence analysis shows strong spatial correlations of gold with carbonates and weaker correlations with phengitic micas and chlorite. Joint recurrence analysis reveals that all parts of the system are part of the same dynamical attractor and hence parts of the same physical-chemical system. We speculate on the coupled processes, compatible with the nonlinear analysis, responsible for the mineralising system. The overall aim is to constrain the structure and organisation of the coupled non-equilibrium dynamics that define this system. We propose that autocatalytic reactions associated with quartz and carbonate deposition control pH variations responsible for gold deposition and present a new view of mineralising systems.


Biography

Alison is a structural geologist interested in the mechanics of hydrothermal systems, computer modelling of coupled deforming systems with heat and fluid transport and the thermodynamics of chaotic systems. She aims to apply the tools developed for nonlinear dynamical systems to quantify and fingerprint various classes of mineralising systems

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.