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.

Lifting the cloak of invisibility: Gold in pyrite from the Olympic Dam deposit, South Australia

Ehrig, Kathy1,2, Ciobanu, Cristiana3, Verdugo-Ihl, Max3, Dmitrijeva, Marija3, Cook, Nigel2, Slattery, Ashley4

1BHP Olympic Dam, Adelaide, Australia, 2School of Civil, Environmental and Mining Engineering, University of Adelaide, Adelaide, Australia, 3School of Chemical Engineering and Advanced Materials, University of Adelaide, Adelaide, Australia, 4Adelaide Microscopy, University of Adelaide, Adelaide, Australia

’Invisible gold’ in pyrite refers to gold either present within the sulfide lattice or as discrete nanoparticle (<100 nm-diameter) inclusions (NPs), making it undetectable by conventional optical and scanning electron microscopy. Investigation of “invisible gold” in chalcopyrite-pyrite ores from the Olympic Dam Cu-U-Au-Ag deposit (one of the world’s largest Au deposits) confirms the presence of Au in some arsenic-bearing pyrites at concentrations measurable by laser ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS). Arsenic-bearing pyrite in the studied sample shows As-Co-Ni-oscillatory zoning patterns with variable complexity suggesting grain re-crystallization during replacement by chalcopyrite. LA-ICP-MS data obtained from 164 pyrite grains plot below the Au and As solubility limit empirically defined from studies of epithermal and Carlin-type deposits.

Several As-rich pyrite grains were analyzed using Scanning Transmission Electron Microscopy (STEM) and EDX-STEM analysis of foils obtained by Focused Ion Beam methods. Micron-scale, oscillatory zoning patterns observed on back-scattered electron (BSE) images and LA-ICP-MS element maps extend down to the nanoscale. Decoupling between trace elements is common, for example Ni depletion wherever As and Co are enhanced, with nucleation of discrete Co-As-bearing NPs (cobaltite/safflorite?)

Importantly, Au-bearing NPs are identified in all cases, in intimate association with other (sulpho)tellurides. In addition, abundant cassiterite and rare chalcopyrite NPs are also identified. Some of the largest Bi-Ag-telluride NPs contain electrum as tiny pore-attached NPs within the larger telluride. Nanometer-size electrum NPs were also identified in association with chalcopyrite. Silver-Au-telluride NPs form mono- or bi-component NPs. These NPs occur along trails displaying As-Co-enrichment, or formation of nm-wide lamellae of Bi-Pb-sulphotellurides marking pyrite twin boundaries. One wider lamella was identified from the layer stacking as a strongly disordered member of the aleksite series. Coarser tellurobismuthite (Bi2Te3), a few μm-wide, is associated with altaite (PbTe) at pyrite-chalcopyrite boundaries.

Pyrite displays kink- and screw-dislocations associated with trace element remobilization or NPs nucleation. These defects can be associated with either ‘marcasitization’ or loss of Fe (formation of pyrrhotite), within nanoscale domains affected by fluid percolation and pyrite recrystallisation. Twin planes in pyrite enriched in heavy elements (Bi-Pb-Te) represent zones of weakness and assist element exchange between host mineral and percolating fluids during coupled dissolution reprecipitation reactions (CDRR), analogous to those known for hematite from Olympic Dam.

Nanoscale textures in pyrite allow for interpretation of Au-NPs as Au released from solid solution in pyrite during CDRR associated with marginal chalcopyrite replacement. Nanoscale analysis lifts the cloak of invisibility for Au in pyrite at Olympic Dam. These results show that confirmation of whether gold occurs as NPs or in solid solution based solely on position above or below the solubility limit of Au in pyrite on a plot of Au vs. As is impossible without corroborative studies at the nanoscale.


Biography

Kathy completed a PhD from the University of California- Berkeley in 1991 and left San Francisco in 1992 to join the former WMC as a research geologist to work on the genesis of the Olympic Dam deposit. She currently leads the team who built the geometallurgical model of Olympic Dam.

Stability of gold nanoparticles in sulfur-bearing hydrothermal fluids: an experimental study

Liu, Weihua1, Chen, Miao1, Yang, Yi1, Mei, Yuan1, Etschmann, Barbara1, Brugger Joël1, Johannessen, Bernt 1

1CSIRO Mineral Resources, Clayton,, Australia, 2Monash University, Clayton,, Australia, 3Australian Synchrotron, Clayton,, Australia

Current theories of the gold deposit formation from hydrothermal fluids have been challenged by recent field and laboratory observations, suggesting that gold nanoparticles/colloids could be important in gold transport/deposition in ore fluids. In this study, the stability of gold nanoparticles (colloidal gold) in sulfur-bearing and citrate-bearing solutions was investigated at temperatures up to 225˚C using Synchrotron X-ray Near-edge Spectroscopy (XANES), and up to 350˚C with a visual check of colour change. The citrate-based colloidal gold solutions, with or without colloidal silica in the solution, are only stable up to 225˚C. In contrast, the gold colloids in Na2S solutions are not stable upon heating to 150˚C, but stable up to 300˚C when 0.5-1.5 wt% of colloidal silica is present in the solution. The colloidal gold particles started to aggregate and deposit from the solution with the aggregation and growth of silica particles at 350 ˚C. The concentrations of gold as colloids in the solutions are up to 0.5 mmol (~95 ppm), more than three orders of magnitude higher than gold solubility as aqueous complexes under the same condition calculated based on available thermodynamic data. These results provide the first evidence that high concentrations of gold nanoparticles are stable in sulfur-bearing fluids at elevated temperatures (~300˚C). This implies that the formation of gold nanoparticles is an effective way to concentrate gold in hydrothermal sulfur-bearing fluids to form high-grade gold ores.


Biography

Dr. Weihua Liu is a senior research scientist at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and was an Australian Research Council Future Fellow

Weihua’s research has been focused on the investigation of metal mobility and mineral/fluid reactions in hydrothermal systems, combining both experimental and theoretical techniques.

The unusual Imou porphyry Au-Cu deposit, Western Highlands PNG

Ireland, Timothy1; Federico Cernuschi2, Robert Sievwright1, Hannah Goswell1, Stefanova, Elitsa3, and John Dobe4.

1First Quantum Minerals Ltd, Canada, 2Eclectic Rock Ltd,  Punta del Este, Uruguay, 3Bulgarian Academy of Science – Geological Institute, Bulgaria, 4Footprint Resources Pty Ltd

Imou (4.944˚S; 142.806˚E) is a porphyry gold-copper deposit located western Papua New Guinea along structural strike between the deposits at Frieda River and Yandera. The area was highlighted during systematic geochemical exploration in the mid-1970s, when five porphyry centres were identified in a camp covering ~30 x 30 km. The Imou target was delineated in the 1990s as a stream sediment and soil Cu anomaly of >500 ppm coincident with Au anomalism >0.2 ppm and associated with a magnetic polyphase porphyritic intrusive complex. Two holes drilled in 1999 discovered the deposit, and a dozen recent holes permit this first attempt at geological description.

Magmatic rocks comprise equigranular and lesser porphyritic diorites that belong to the Nekiei intrusive suite, which was emplaced into a thrust duplex of obducted siltstone and oceanic crust. New zircon-U-Pb geochronology records at least 700 ky. but less than 2.1 my. of Late Miocene magmatic evolution. The intrusions are low to mid-K calc-alkaline series rocks composed primarily of calcic plagioclase and hornblende with rare biotite and quartz, and accessory titanomagnetite. Whole rock chemical proxies for porphyry fertility are comparable to causative suites in other deposits (Sr/Y up to 65 and V/Sc up to 16) but zircon in these rocks has relatively low concentrations of U, Th, LREE and Hf .The intrusions manifest in airborne magnetic surveys as discrete high amplitude magnetic ‘bullseye’ features, but no cogenetic large plutons are observed at surface, nor can be inferred from the magnetic response.

The outcropping manifestation of the porphyry deposit at Imou is in many ways typical: within a geochemical footprint of ~2 km2 there are domains of A-family quartz veins that sometimes contain Cu-Fe-sulphides and/or magnetite, the distribution of which broadly coincides with the occurrence of hydrothermal magnetite, feldspar, anhydrite and muscovite. These quartz vein domains reach 50 vol% quartz in the vicinity of the magnetite-bearing assemblage. There are structurally controlled domains of late D-veins and phyllic alteration, followed by post-mineral porphyritic dykes. However, there are numerous differences with economic PCD described elsewhere. Cu-Au grade is not associated with the abundance of A-family quartz veins. Further, there is no petrographic nor bulk chemical evidence for large volumes of potassic alteration. Observed feldspars are albite and sporadic hydrothermal biotite represents only remobilisation of local potassium. There is little disseminated sulphide associated with this alteration, instead, grade development is associated with chalcopyrite-pyrite-(magnetite) fracture paints, and sulphide-anhydrite veins that cross-cut the quartz vein stages. These paragenetically late brittle veins are widely distributed at low frequency, but are associated with best Cu-Au grades. These late hydrothermal features are not systematically associated with a particular wallrock alteration assemblage, although chlorite-montmorillonite is the dominant alteration assemblage outboard of localised sericite- and albite-bearing assemblages.

We advance several working hypotheses for why Imou differs from other porphyries, and especially why metals did not precipitate efficiently with the early high temperature alteration and quartz veining. We speculate that many barren or subeconomic porphyries may have inferior Cu-Au grade development for similar reasons.


Biography

I’m a jack-of-all-trades explorer with experience in porphyry-epithermal systems, carlin-type deposits, sed-hosted Cu and sed-hosted Zn. I’m currently the Principal Exploration Geologist – and informal resident geochemist – at FQM. I did my postgrad research at CODES, and have spent much of my career working in E Europe and the central African copperbelt.  

Evolution of magmatic fertility for porphyry Au & Cu deposits through the prism of zircon chrono-chemistry, Balkan Peninsula, SE Europe

Ireland, Timothy1; Bilyarska Teodora2, Bilyarski, Stoimen1, Protic, Nenad1, and Stefanova, Elitsa3

1First Quantum Minerals Ltd,, Vancouver, Canada, 2St Kliment Ohridski University of Sofia, Sofia, Bulgaria, 3Bulgarian Academy of Science – Geological Institute, Sofia, Bulgaria

The accreted arc terranes of the Balkan western Tethyan host a complex metallogeny that includes almost all Cu and Au mineralisation styles typical of subduction-related arcs and post-collisional settings. Broadly, magmatism in the belt began as a series of submarine arcs constructed on thinned continental basement, and evolved to subaerial post-collisional magmatism during and after arc-continent collisions in the Late Cretaceous and Eocene. Each collisional event is associated with a phase of metallogenesis that changes in terms of metal budget and deposit style as collision occurs and then collapses, e.g. in the Timok and Panagjurishte districts, three phases of magmatism, each lasting 5-10 my. record a transition in space and time from sub-volcanic diorites associated with porphyry Cu-Au deposits, to more voluminous effusive andesites associated with epithermal deposit styles, and then monzonitic intrusions with a Au-dominant metal budget. The rocks from all of these three phases are the products of hydrous, intermediate magmas that yield equivalent ‘fertile-looking’ results in terms of whole rock magma chemistry proxies such as Sr/Y that explorers may use for area selection in arc environments.

A similar metallogenic progression occurs among less well-documented magmatism that occurred subsequent to the Alpine orogeny. In this case the main, early metallogenic phase is represented by porphyry and epithermal Cu-Au deposits associated with trachyandesite and monzonitic volcano-plutonic complexes. As post-collisional extension comes to dominate the regional tectonic environment the metallogeny becomes dominated by Pb-Zn in a series of ‘ore fields’ that we infer to be genetically related to syn-extensional granitoids, and thereafter by epithermal gold deposits in terrestrial grabens.

In this study we compiled all the available published whole rock chemistry and zircon chronology and chemistry for magmatic rock units associated with each of these metallogenic stages, and collected new complementary data from rock samples and stream sediments. The result is a coherent record of the temporal evolution of magmas associated with each of these metallogenic stages. We observe that the detrital zircon populations are closely related to the results from rock samples, however, we contend that the detrital record is a superior medium as it avoids sampling bias and undersampling. These detrital results were interpreted as a holisitic record of the magmatism in each district. There are patterns among this zircon chrono-chemical evolution that correspond to consistent aspects of regional metallogeny. Short periods of zircon crystallisation (i.e. <3 my) in which zircons span a wide range of compositions suggesting both primitive and enriched or evolved source contributions tend to characterise the metallogenic events in which Au is the primary economic commodity. In contrast, more protracted zircon crystallisation history and slower evolution to fertility implied by proxies such as Eu/Eu* characterises the major porphyry Cu camps. This approach may be applicable in other terranes wherein the magmatic-metallogenic history is not well constrained.


Biography

I’m a jack-of-all-trades explorer with experience in porphyry-epithermal systems, carlin-type deposits, sed-hosted Cu and sed-hosted Zn. I’m currently the Principal Exploration Geologist – and informal resident geochemist – at FQM. I did my postgrad research at CODES, and have spent much of my career working in E Europe and the central African copperbelt.  

Controls on gold endowments of porphyry deposits

 Massimo, Chiaradia1

1Department of Earth Sciences, University of Geneva, Geneva, Switzerland

Porphyry deposits are natural suppliers of ~75% copper and ~20% gold to our society. Nonetheless, gold endowments of porphyry deposits are characterized by a wide range going from a few tons to >2500 tons of gold. Here, I propose a model to explain the reasons of the large variations in metal endowments of porphyry Cu-Au deposits.

Porphyry Cu-Au deposits define two distinct trends in Au versus Cu tonnage plots: Cu-rich (Au/Cu ~4*10-6) or Au-rich (Au/Cu ~80*10-6). Cu-rich porphyry deposits are related to Andean-type subduction and typical calc-alkaline magmatism in thick continental arcs. In contrast, Au-rich porphyry deposits are associated with high-K calc-alkaline to alkaline magmatism in late to post-subduction or post-collision and extensional settings, and also with calc-alkaline magmatism. The largest Au-rich porphyry deposits are associated with high-K calc-alkaline to alkaline magmatism. Geochronological data at individual porphyry deposits suggest that gold endowments for both trends grow larger the longer the mineralization process is. However, Au is precipitated at much higher rates in Au-rich (~4500 tons Au/Ma) than in Cu-rich porphyry deposits (~100 tons Au/Ma).

Monte Carlo modelling of petrologic processes suggests that the different rates of gold precipitation in Cu-rich and Au-rich porphyry deposits most likely result from a 5-12 times better efficiency of gold precipitation in Au-rich than in Cu-rich deposits. The reason of the different efficiencies of gold precipitation is the different depths of formation of Cu-rich and Au-rich porphyry deposits which favour (deep level) or not (shallow level) a decoupling of Au and Cu precipitated from the magmatic-hydrothermal fluids. Interestingly, Au-rich porphyry deposits formed at shallower levels are also associated with magmatic rocks that have evolved at average shallower levels than Cu-rich deposits, as suggested by systematically lower Sr/Y values of the former (Au-rich systems) with respect to the latter (Cu-rich systems). Monte Carlo modelling shows that the higher gold endowments of Au-rich porphyry deposits associated with alkaline magmas require higher gold contents in the parental magmas such as those that are typical of alkaline magmas but not of calc-alkaline ones. This suggests an additional petrogenetic control in the formation of the Au-richest porphyry deposits associated with variably alkaline magmas.

Whereas depth of porphyry formation and chemistry of magmas (alkaline versus typical calc-alkaline) seem to control the Au-rich versus Cu-rich nature of porphyry Cu-Au deposits, the correlation of the Cu and Au endowments with ore deposition duration suggests that the final Cu and Au endowments of these deposits are determined by the cumulative number of mineralizing steps that are ultimately controlled by magma volume and ore process duration. The difference is that variably alkaline systems and shallow crustal calc-alkaline systems are inherently associated with magmas, whose fluids are tectonically (i.e., shallow emplacement) and chemically (alkaline magmas) optimized for high gold precipitation efficiency. In contrast, typical calc-alkaline (high Sr/Y) magmas form in a geodynamic context that favours enormous magma accumulations, which are necessary to produce behemothian Cu-rich deposits, but are emplaced at depths at which the exsolved fluids are less efficient for gold precipitation.


Biography

Massimo Chiaradia obtained a MSc at Padova University (Italy) and a PhD at Fribourg University (Switzerland). After post-docs in Sydney (CSIRO), Geneva and Leeds he became Senior Lecturer at Geneva University. His main research focus is the petrogenesis of convergent margin magmas and their association with porphyry Cu-Au deposits.

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