Frank P. Bierlein

Geodynamic settings of mineral deposit systems

Mineral deposits represent extraordinary metal concentrations that form by magmatic, magmatic–hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and/or mechanical energy near plate boundaries. As they require the conjunction of specific environmental conditions to form, particular mineral deposit types tend to occupy specific geodynamic niches. The temporal distributions of mineral deposit types reflect both formational and preservational processes. In the Archaean and Palaeoproterozoic, these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle (SCLM), but were decoupled by the Neoproterozoic and Phanerozoic as a result of evolution to thinner, increasingly dense SCLM. The transition marks a change from mantle plume-influenced plate tectonics to modern-style plate tectonics, with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types. As mineral deposits represent an integral part of tectonic process, they are essential indicators of that process and geodynamic settings, and should be incorporated into any holistic tectonic terrane analysis. Their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data. Conversely, such reconstructions provide a first-order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover.

Orogenic disseminated gold in phanerozoic fold belts—examples from Victoria, Australia and elsewhere

Abstract Orogenic gold mineralisation in the Lachlan Orogen of southeastern Australia and in other Phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins. However, an improved understanding of the tectonic, structural and geochronological framework in which these occurrences form, and recent advances towards the extent of wallrock alteration around these veins, cast doubt on the validity of this deeply entrenched ‘lode-gold paradigm’. Ore-grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits (Lachlan Orogen, Australia; Buller Terrane and Otago Schist, New Zealand; Meguma Terrane, Canada; Tien Shan Mountains in Uzbekistan and Kyrgyzstan; Yana-Kolyma Province, Baikal, Verhoyansk and Allakh-Yun fold belts in north–central Mongolia and Far East Russia). Disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks. These occurrences also share a number of characteristics with some Carlin-type gold deposits in the Great Basin of western North America (e.g., Getchell) regarding structural relationships, fluid composition, mineral paragenesis and a common association of carbonaceous matter with mineralisation. Features of alteration, physico-chemical characteristics of the ore-bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation. The two styles are considered to represent end-members of a crustal continuum of orogenic gold emplacement in Phanerozoic fold belts, with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation. These conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems. Efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids, and accelerated by the presence of carbonaceous matter at the site of deposition.

Timing relationships between felsic magmatism and mineralisation in the central Victorian gold province, southeast Australia*

New SHRIMP U–Pb data from major felsic intrusives and dykes confirm that initial gold emplacement in the central Victorian gold province was metamorphic‐related, pre‐dating the emplacement of Devonian intrusions in the western Lachlan Fold Belt by as much as 80 million years. Importantly, together with previously published geochronological constraints, these data strongly suggest that no genetic link exists between the main phase of ‘metamorphism‐related’ orogenic gold mineralisation and magmatism in the central Victorian gold province. A subsequent ‘intrusive‐related’ phase of gold mineralisation, often accompanied by elevated copper, tungsten, molybdenum or antimony, formed synchronously with, or post‐dated, the emplacement of Devonian plutons and dykes, and thus is spatially (if not genetically) related to melt generation at depth. These timing relationships differ from those found in many other orogenic gold districts in which ore emplacement is closely associated, spatially, with felsic magmatism. The introduction of metamorphically derived fluids well before magmatism is seen to reflect variations in the timing of peak metamorphic conditions at different crustal levels. The extensive development of melt in this environment is thought to occur in response to the transfer of thermal energy from the mantle into the overlying, thickened crust in a Cordilleran‐type setting, which is dominated by episodic subduction‐accretion processes. The new SHRIMP U–Pb data also demonstrate that many of the major granites throughout the central Victorian gold province have a composite intrusive history and are the product of multiple tectonomagmatic events.

Tectonic and economic implications of trace element, 40Ar/39Ar and Sm–Nd data from mafic dykes associated with orogenic gold mineralisation in central Victoria, Australia

Abstract Mafic to intermediate dykes are spatially and temporally closely associated with major post-tectonic granitic complexes in the western Lachlan Orogen of SE Australia. These dykes, which range petrographically from basaltic to andesitic, are concentrated within several, north- to northwest-trending zones and were emplaced during two broad intervals of extensive magmatic activity during the Silurian–Devonian period. Geochemical and Sm–Nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction-related tectonic setting. Interaction between mantle-wedge material, sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region. Field evidence and 40 Ar/ 39 Ar data show that in the eastern part of the Stawell Zone and in the northwest portion of the Bendigo Zone, mafic dyke were intruded between 410 and 400 Ma (Late Silurian/Early Devonian). Further emplacement in the Bendigo Zone and the eastern part of the Melbourne Zone took place at between 375 and 365 Ma (Middle to Late Devonian). Episodic mantle-derived magmatism was possibly related to step-wise rollback, slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses. A close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central Victorian gold province. Mafic to intermediate dykes both crosscut, and are host to, mineralisation in a number of goldfields. Although there is little evidence for a direct genetic association, the two processes are linked by the common utilisation of translithospheric structures, which facilitated the rapid ascent into shallow crustal levels of both mantle-derived magma and crustal-scale ore-forming fluid systems. Previous studies have suggested that transfer of heat into the crust via ascending mafic mantle magmas can provide a thermal engine which triggers and sustains extensive crustal melting, thus explaining the commonly observed close association of mafic to intermediate and felsic intrusive suites. This study supports the viability of this mechanism and in addition, indicates that a causal link exists between the formation of mantle magmas in collisional zones and the generation of orogenic gold deposits.

Metallogenic relationships to tectonic evolution – the Lachlan Orogen, Australia

Abstract Placing ore formation within the overall tectonic framework of an evolving orogenic system provides important constraints for the development of plate tectonic models. Distinct metallogenic associations across the Palaeozoic Lachlan Orogen in SE Australia are interpreted to be the manifestation of interactions between several microplates and three accretionary complexes in an oceanic back-arc setting. In the Ordovician, significant orogenic gold deposits formed within a developing accretionary wedge along the Pacific margin of Gondwana. At the same time, major porphyry Cu–Au systems formed in an oceanic island arc outboard of an evolved magmatic arc that, in turn, gave rise to granite-related Sn–W deposits in the Early Silurian. During the ongoing evolution of the orogen in the Late Silurian to Early Devonian, sediment-hosted Cu–Au and Pb–Zn deposits formed in short-lived intra-arc basins, whereas a developing fore-arc system provided the conditions for the formation of several volcanogenic massive sulphide deposits. Inversion of these basins and accretion to the Australian continental margin triggered another pulse of orogenic gold mineralisation during the final consolidation of the orogenic belt in the Middle to Late Devonian.

Geodynamics and metallogeny of the Lachlan Orogen

The Palaeozoic Lachlan Orogen of eastern Australia is an accretionary orogen made up of structurally thickened oceanic successions, including turbidites from deep‐sea fans, andesitic volcanics from remnant island arcs, forearc sediments and slices of oceanic crust. Accretion of these units has been inferred to occur by collapse of a marginal basin during double divergent subduction within a complex southwest Pacific‐style oceanic setting. The small basin closed behind, and in the backarc position to, a major, long‐lived subduction system off Gondwanaland in the Palaeozoic. Stepwise deformation and metamorphism from Late Ordovician through Early Carboniferous times formed three distinct geological subprovinces that have interacted with each other through time and space. Metallogeny relates directly to these three provinces and their position within this tectonic framework. In the Western Subprovince, Ordovician turbidites host major lode Au deposits (e.g. Bendigo and Ballarat, central Victoria) because of structural thickening and metamorphism within a coupled thick turbidite wedge ‐ oceanic crustal layer above an inferred flat, west‐dipping subduction zone. In the Central Subprovince a Late Ordovician to Early Silurian high‐T metamorphic belt hosts Sn–W ± Mo skarn/greisen deposits as part of a more evolved, differentiated magmatic arc associated with northwest‐trending, east‐dipping subduction beneath the Tabberabbera Zone. In the Eastern Subprovince, porphyry Cu–Au deposits (e.g. Goonumbla and Lake Cowal, central‐northern New South Wales) formed within an Ordovician oceanic island arc, and volcanic‐hosted massive‐sulfide deposits (e.g. Captains Flat and Woodlawn, eastern New South Wales) formed in younger Silurian forearc sediments. Both deposit types are related to the outboard, long‐lived, west‐dipping subduction zone off Gondwanaland.

Palaeozoic Lachlan orogen, Australia; accretion and construction of continental crust in a marginal ocean setting: isotopic evidence from Cambrian metavolcanic rocks

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.

Stable isotope (C, O, S) systematics in alteration haloes associatedwith orogenic gold mineralization in the Victorian gold province,SE Australia

This study investigates whether hydrothermal alteration of metasedimentary rocks associated with orogenic gold mineralization in the Victorian gold province produces systematic changes in oxygen, carbon and sulphur isotope compositions that might provide explorationists with vectors towards ore. The presence of systematic isotopic trends across wallrock alteration haloes in gold deposits in the Stawell, Percydale, Ballarat, Bendigo and Fosterville goldfields clearly supports the interpreted hydrothermal origin of these haloes. General increases in carbonate δ13C towards the ore zones in all deposits record the influx of hydrothermal CO2. Whole-rock silicate and quartz δ18O, carbonate δ18O, and sulphide δ34S shifts equally mirror petrological and geochemical changes that resulted from prolonged and extensive interaction between externally derived hydrothermal fluids and the surrounding siliciclastic wallrocks. With the possible exception of sulphur, however, the isotopic enrichment/depletion trends are subtle and also vary significantly from deposit to deposit. Results of this study and comparisons with published data suggest that the oxygen and carbon isotopic ratios of silicates and carbonates in whole-rock samples within the alteration haloes are buffered by the wallrocks. Hydrothermal sulphides in Victorian orogenic gold deposits are characterized by δ34S values generally in the range −5 to +5‰. However, there is relatively little influx of hydrothermal sulphur beyond c. 5 to 10 m from the vein margins and disseminated pyrite well removed from the lode systems is characterized by either greatly negative or positive δ34S values typically greater than±10‰.

New constraints on the polychronous nature of the giant Muruntau gold deposit from wall-rock alteration and ore paragenetic studies

Muruntau in north-central Uzbekistan is one of the biggest gold deposits in the world with >5200 t of contained Au. Detailed observations from thin-section petrography of ore samples from the super-giant deposit and quantitative XRD analyses of variably hydrothermally altered host-rocks are reported here. These new data are used to argue that the unique size and nature of the deposit is the result of a combination of distinct and potentially temporally separated ore-forming events that probably spanned at least 60 Ma. Consequently, describing Muruntau as an entirely orogenic, thermal-aureole, or intrusion-related type of deposit might not be valid, and caution should be used when using deposit-scale features of this enormous occurrence in the formulation of genetic or exploration models. †Present address: Paladin Energy Pty Ltd, 502 Hay Street, Perth WA 6008, Australia.

Advanced methodologies for the analysis of databases of mineral deposits and major faults

The effectiveness of some novel software tools used for clustering and classifying multivariate data is tested and used to evaluate mineral exploration criteria by examining a mineral deposit and major fault database. The database containing 364 diverse mineral deposits is divided into natural groups utilising a vector quantisation data-mining approach based on a self-organising map (SOM), and phenetic and cladistic analysis packages. The last two approaches are loosely based on biological principles of numerical taxonomy and evolutionary relationships, respectively. Based on the assumption of a common process of formation, the analyses are used to define the group (or class) of Archean orogenic-gold deposits, as distinct from gold deposit types such as turbidite-hosted orogenic gold, Carlin-type, and porphyry Cu – Au that should be excluded from this group. The main findings from this study are: (i) large, global-scale databases, representing the full range of commodity types, geographic locations, ages and variation in deposit characteristics, are required in order to classify new deposit examples using these techniques; (ii) traditional classifications are broadly correct but inadequately define deposit types; and (iii) SOM, and phenetic and cladistic analysis packages can aid in the identification of the characteristics (i.e. commodity, rock type, alteration, vein morphology, fluid composition) of the deposits that are mainly responsible for defining individual deposit groups. Applied to metallogenic terranes that host many different styles of mineralisation and deposit groups (such as Archean cratons), this approach can aid in identifying which deposits belong to a single coherent group. Analysis of the major fault database in this pilot study emphasises the need to obtain a significantly larger number of entries (total of 138 entries used, whereas ≫200 entries are required). It also highlights the impact of incomplete attribute data and the categorical nature of many of the datafields that describe faults. Nevertheless, preliminary results of several statistical analyses (Boolean, SOM, phenetic, cladistic) of the major fault database confirm the importance of empirically derived criteria for mineralisation, such as proximity to crustal-scale faults and anticlinal hinge zones, dilational jogs and fault roughness, strong rheological contrasts at lithological boundaries and metamorphic grade. Presence and concurrence of these parameters determine the extent of metallogenic endowment of a given fault system and segments within it.