Abstract Laverton is an important region in the Y ilgarn C raton for gold mineralization. Previous studies suggested that the premineralization structure was dominated by a fault restraining step‐over structure with the W allaby fault and S unrise S hear Z one as the key fault splays hosting the world‐class W allaby and S unrise D am gold deposits, respectively. Two major gold mineralization phases occurred during two of a series of tectonic events with different far‐field stress orientations. 3D coupled deformation and fluid flow modelling was used to investigate the effects of varying far‐field stress orientations on reactivation of the fault structure in the region. The results show that structural reactivation is sensitive to regional shortening directions. Two shortening directions are identified to be favourable for reactivation of the Wallaby fault and Sunrise Shear Zone: (i) NW – SE shortening and (ii) ENE – WSW to E – W shortening. The reactivated segments exhibited localization of shear strain and dilation as well as fluid focusing at locations corresponding to the W allaby and S unrise D am deposits. This is in contrast to the models with shortening direction between NNW – SSE and NE – SW , which showed little fault reactivation at both locations. These results support previous independent interpretations of the controls on gold mineralization at L averton. The two shortening directions favourable for structural reactivation inferred from the models are consistent with the kinematics of the two main gold mineralization events in the region. Our results suggest that strain localization, dilation and fluid focusing during structural reactivation could be some of the key factors governing gold mineralization at L averton.
Gold precipitation at Bendigo postdates cleavage development and was initiated during folding, increased during a reverse faulting stage in bedding-parallel quartz veins, and culminated in quartz reefs associated with a strike-slip faulting event. Some gold was remobilized along carbonate-rich cataclasites associated with later brittle faults. Understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis, from source to sink. In this study, a petrographic approach, closely tied to macroscopic structural observations, along with stable isotope and fluid inclusion analyses, places constraints on the relative timing of gold mineralization with respect to deformation. Early sulfide assemblages in bedding- and cleavage-parallel veins are dominated by pyrite-pyrrhotite-siderite, and are free of visible gold. Later assemblages in reactivated bedding-parallel veins and other fold-related veins are characterized by the presence of arsenopyrite-ankerite-gold. Fault-related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold, but lack pyrrhotite. Small amounts of late-stage antimony-bearing minerals occur in many vein types and postdate the precipitation of gold. Decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins. Stable oxygen isotope data show that quartz from all vein types ( δ 18 O = 15.9–19.0) homogenized with the host rocks, whereas carbon ( δ 13 C =–14.0 to 4.1) and oxygen ( δ 18 O = 4.5–24.0) in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature. This corresponds with early siderite, pyrrhotite, and anhedral pyrite, and with later ankerite and ferroan dolomite associated with arsenopyrite, galena, and euhedral pyrite that are crosscut by calcite veins. Fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide, with smaller proportions of nitrogen and methane, particularly in the later strike-slip faults. The sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks. Although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the Bendigo orogenic gold deposits, we believe there is little direct or deposit-scale evidence for a relationship between original metal content in the host-rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites. It is suggested that deep-seated faults act as conduits for fluid flow and the source for the gold at Bendigo; the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites.
3D models and computer-based numerical simulations have been used in the exploration industry for some time to visualise the geometry and mechanisms resulting in the formation of orebodies. However, due in part to computational limitations, few numerical simulations have been run on complex (real) geometries in order to predict the location of new ore systems. Presented here are the results of an exploration program developed by the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) and MPI (now Leviathan Resources) in the orogenic-gold system of western Victoria that utilised 3D modelling and numerical finite-element simulations to successfully target several new orebodies and predict their geometries and extent. Existing drillcore databases were utilised to constrain the geometries of known deposits and associated mafic domes, the effects of known post-mineralisation faulting was systematically removed and syndeformation fluid flow was then modelled within the system. The results of these simulations were compared with the known geometry of the mineralised systems about these deposits in order to test the simulation parameters and accuracy. 3D models were also developed of poorly constrained target domes in regions with no outcrop utilising potential-field datasets and limited drilling data. Simulations were then run on these model geometries using the tested parameters in order to predict the likelihood of mineralisation in these systems, its geometry and (most importantly) its location. These targets were then drilled resulting in the discovery of previously unknown gold deposits associated with the Kewell Dome northwest of Stawell.
We present coupled mechanical, fluid‐flow and chemical numerical models that place constraints on some of the processes that formed gold deposits in the Bendigo‐Ballarat Zone of Victoria, Australia, and specifically the Bendigo goldfield. Although many goldfields in this region are located close to major intrazone faults, these major faults are rarely mineralised, and such goldfields are commonly located in the hangingwall of the faults, 5–10 km from the fault plane. Individual gold deposits are hosted within quartz veins associated with reverse faults and chevron folds. Regional‐scale models are aimed at determining if the intrazone faults were more or less permeable than the surrounding host rocks, if these faults acted as conduits to supply fluids to goldfields, if the presence of a blind fault below the Bendigo goldfield was necessary to deliver fluids to the area, and if the upper units of the Castlemaine Supergroup (Darriwilian and Yapeenian) acted as a low permeability cap to facilitate gold and quartz precipitation in the lower units (Castlemainian to Lancefieldian).The models indicate that the combination of permeable intrazone faults, and relatively low permeability units at the top of the Castlemaine Supergroup, allows the greatest fluid flux to occur in the area of the Bendigo goldfield. The presence of a blind fault beneath the Bendigo goldfield is not critical to the supply of fluids to this area. Small‐scale models of a chevron fold with a permeable fault simulate fluid pumping and suggest that fluids transported along the fault mixed with those in the host rocks. Fluids are expelled from, and drawn back into, the fault in a cyclical manner and are controlled by deformation‐induced changes in volume. Intermittent high fluid pressures in the host rocks relative to the fault cause the rocks to yield in tension, simulating hydrofracturing and the formation of quartz veins associated with gold‐bearing reverse faults. Models coupling deformation, fluid flow and chemical reactions demonstrate that gold is precipitated within the fault region when CH4 and H2S are sourced at depth and CO2 is transported along the fault. In the models presented, gold precipitation is strongly controlled by H2S, although the effects of fluids interacting with graphite have not been modelled. Rates of gold mineralisation of 0.09–0.023 g/t over 1 million years suggest that the permeability of the fault was at least two orders of magnitude higher than the permeability modelled here.
The sediment-hosted McArthur River Zn-Pb-Ag deposit is located in the southern McArthur Basin of northern Australia. 3D numerical models are used to explore the role of thermal convection in the mineral system that formed the McArthur River deposit. The model geometry is a simplified representation of the area, comprising gently dipping hydro-stratigraphic units intersected by a steeply dipping fault. An aquifer represents the source of mineralising fluid, while the fault provides a pathway for this fluid to reach the site of mineralisation, either on the seafloor (syngenetic mineralisation) or within sedimentary rocks adjacent to the fault (diagenetic/epigenetic mineralisation). Two fault permeability scenarios are investigated, representing an open fault with high permeability, and a closed fault with low permeability in the top 300 m of the model and high permeability below. In both scenarios, thermal convection occurs within the fault and aquifer due to their high permeability, with upwellings of hot fluid spaced at ~ 14 to 23 km along the fault. The results show that convection results in sufficient fluid exchange between the aquifer and fault to account for known mineralisation at McArthur River (~20 Mt Zn). In the open fault scenario, the convective upwellings provide sufficient flow of hot fluid onto the seafloor to form a 20 Mt syngenetic deposit in ~ 0.4 to 0.9 Myr. Syngenetic mineralisation would be accompanied by minor diagenetic mineralisation within the sediments adjacent to convective upwellings in the fault. In the closed fault scenario, some of the hot upwelling fluid flows into sediments adjacent to the fault at ~ 300 m depth, sufficient to create a 20 Mt diagenetic/epigenetic deposit in ~ 2.3 Myr. The rate and temperature of fluid flowing onto the seafloor or into the host rocks increases with fault permeability and heat flux, with higher heat flux and/or permeability being required to generate a diagenetic/epigenetic deposit than a syngenetic deposit within a geologically reasonable timeframe. Geometric complexities (e.g. fault intersections, bends or offsets) and areas of anomalously high heat flow or permeability are likely to focus convective upwelling, and are therefore suggested as targets for mineral exploration.
The Valhalla complex, a Cordilleran metamorphic core complex, is a domal culmination made up of gently dipping interlayered sheets of igneous and supracrustal rocks that were deformed and metamorphosed in the Middle Jurassic and Late Cretaceous, and exhumed by extensional faults in the Eocene. Mapping, fabric, and metamorphic studies of predominantly metasedimentary rocks in Valhalla and Passmore domes in the northern part of the complex, together with published geochronological data, reveal a significant Late Cretaceous tectonic history. This includes extensive magmatism, the culmination of upper amphibolite facies metamorphism (approx. 800°C and 8 GPa), migmatization, development of a dominant penetrative transposition foliation, and localization of strain on ductile thrust faults termed the Gwillim Creek shear zones. The Valhalla assemblage, a package of metasedimentary rocks in Valhalla and Passmore domes, comprises a heterogeneous sequence of pelitic schist, marble, calc-silicate gneiss, psammitic gneiss, metaconglomerate, quartzite, amphibolite gneiss, and ultramafic rocks. Based on the presence of distinct laterally continuous marker units and similar lithologic ordering, we propose that the Valhalla assemblage is correlative with part of the Palaeozoic North American stratigraphic succession. If this is correct, then the Valhalla assemblage represents an inverted sequence of strata that has been thinned by as much as 60%; thinning may have occurred during Late Cretaceous transposition foliation development. The Gwillim Creek shear zones, originally mapped in a restricted locality in Gwillim Creek, were found to merge into one broad, ductile shear zone beneath Valhalla dome and extend throughout the entire Valhalla complex. The general style and timing of Late Cretaceous deformation in the Valhalla complex is characteristic of that found throughout the Shuswap complex in a belt of rocks that were at mid-crustal levels during the Cretaceous. This zone is thought to have accommodated Cretaceous - Early Tertiary shortening in the eastern Cordillera, and is the ductile equivalent of the higher level Rocky Mountain thrust belt to the east.
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