Unconformity-type uranium systems: a comparative review and predictive modelling of critical genetic factors

A review of descriptive and genetic models is presented for unconformity-type uranium deposits with particular attention given to spatial representations of key process components of the mineralising system and their mappable expressions. This information formed the basis for the construction of mineral potential models for the world's premier unconformity-style uranium provinces, the Athabasca Basin in Saskatchewan, Canada (>650,000 t U3 O8), and the NW McArthur Basin in the Northern Territory, Australia (>450,000 t U3 O8). A novel set of ‘edge’ detection routines was used to identify high-contrast zones in gridded geophysical data in support of the mineral potential modelling. This approach to geophysical data processing and interpretation offers a virtually unbiased means of detecting potential basement structures under cover and at a range of scales. Fuzzy logic mineral potential mapping was demonstrated to be a useful tool for delineating areas that have high potential for hosting economic uranium concentrations, utilising all knowledge and incorporating all relevant spatial data available for the project area. The resulting models not only effectively ‘rediscover’ the known uranium mineralisation but also highlight several other areas containing all of the mappable components deemed critical for the accumulation of economic uranium deposits. The intelligence amplification approach to mineral potential modelling presented herein is an example of augmenting expert-driven conceptual targeting with the powerful logic and rationality of modern computing. The result is a targeting tool that captures the current status quo of geospatial and exploration information and conceptual knowledge pertaining to unconformity-type uranium systems. Importantly, the tool can be readily updated once new information or knowledge comes to hand. As with every targeting tool, these models should not be utilised in isolation, but as one of several inputs informing exploration decision-making. Nor should they be regarded as ‘treasure maps’, but rather as pointers towards areas of high potential that are worthy of further investigation

Leven Star deposit: An example of Middle to Late Devonian intrusion-related gold systems in the western Lachlan Orogen, Victoria

This study documents an example of atypical gold mineralisation in the central Victorian gold province of the western Lachlan Orogen, Australia. Unlike the vast majority of orogenic gold deposits in this region, the Leven Star deposit at Malmsbury is characterised by a disseminated-stockwork style of mineralisation, a close spatial and temporal association with post-tectonic felsic intrusions, complex alteration characteristics and a Au-As-Sb (±Bi-Te-Cu-Zn-Pb-Sn-W) ore assemblage. In contrast to orogenic-style, metamorphism-related gold mineralisation (ca 440 Ma), which pre-dated magmatism in the western Lachlan Orogen by tens of millions of years, ore formation in the Leven Star deposit was synchronous with, and is paragenetically younger than, Middle to Late Devonian (ca 370 Ma) magmatism. On the basis of these timing relationships, as well as whole-rock geochemistry, and structural, petrographic and fluid-inclusion data, it is suggested that the Leven Star deposit is not orogenic in character and instead should be classified as intrusion-related. © Geological Society of Australia.C

A new metallogenical association (Sn-Cd-In-Zn-Ag-Au) in the Deseado auroargentiferous province, Deseado Massif, Patagonia, Argentina

A new metallogenical association (Sn-Cd-In-Zn-Ag-Au) is reported for the low sulphidation epithermal Deseado Auroargentiferous Province, in Patagonia, Argentina. The anomalous presence of Sn, Cd and In, together with anomalies of Zn and Ag, and also high contents of Cu, Mn, Pb, W and Bi in a complex sulphur-rich mineralogy, represent a new metallogenical association. This assemblage could be related to a higher temperature mineralization or a different type into the epithermal range. The presence of this new metallogenical association increases the mining potential of the region, extending exploration targets in this metallogenical province and must be considered during future exploration duties in the region.Instituto de Recursos Minerale

Possible intrusion-related gold systems in the western Lachlan orogen, southeast Australia

Several gold deposits occurring in the western Lachlan orogen have geological, geochemical, and geochronological characteristics that distinguish them from typical vein-hosted orogenic gold deposits of the central Victorian gold province. The later are responsible for more than 90 percent, of primary (hard-rock) gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western Lach an orogen. Atypical gold occurrences at Malmsbury, Myrtle Creek, Mount Piper, and the Wonga deposit in the Stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions, disseminated to stockwork-style mineralization, alteration dominated by sericitization, sulfidation, silicification, carbonatization and tourmalinization, and associated complex An ± Mo-W-Bi-Te-Cu. The deposits have a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. Although production from this type of gold mineralization in the western Lachlan orogen has been small compared to orogenic gold deposits, the possible existence of intrusion-related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and Paleozoic metallogeny of eastern Australia. © 2005 Society of Economic Geologists, Inc.C

The role of carbonaceous "indicator" slates in the genesis of lode gold mineralization in the western Lachlan orogen, Victoria, southeastern Australia

"Indicator" slates have long been considered to represent a useful exploration guide for turbidite-hosted mesothermal lode gold mineralization in central Victoria. This assumption has been based on an apparent close spatial relationship between high gold grades and the proximity of thin, commonly pyritic and carbon-rich marker units, Detailed studies in a number of gold deposits throughout central Victoria, however, reveal that highest gold grades do not necessarily coincide with the presence of carbonaceous units. In many places where gold mineralization is associated with carbon-rich matter, the high C accumulations are the result of epigenetic remobilization during hydrothermal alteration and ore genesis. Petrographic, geochemical, and stable isotope IC. O, Si investigations into the origin and nature of the carbonaceous matter-mainly amorphous bitumens and pyrobitumen of organic origin, with biological fragments and rare graphite of both detrital and metamorphic origin-demonstrate that black shales within the Cambro-Ordovician succession ill central Victoria lacked the vital constituents to provide (1) a primary sink for precious metals, and (2) whereas the presence of carbonaceous matter was likely to affect the evolution of epigenetic hydrothermal fluids and, locally, may have facilitated gold enrichment, carbon-rich sedimentary rocks were not crucial for ore genesis on the deposit scale. Instead, the size of the hydrothermal cell, physicochemical conditions of the ascending fluids, and dynamic fault-valve behavior played far more significant roles in controlling gold precipitation. The importance of these processes has implications for exploration targeting sediment-hosted, lode- and disseminated-style gold mineralization in the western Lachlan orogen and in slate belt provinces elsewhere.C

The Fosterville (central Victoria, Australia) and globe-progress (Reefton, South Island, New Zealand) deposits : Examples of shear zone-related disseminated-style systems in low-grade metamorphic terrains

Orogenic gold deposits in central Victoria and Reefton formed during the evolution of a Paleozoic accretionary system along the Pacific margin of Gondwana. The majority of deposits in both camps are characterised by coarse-grained gold that is hosted in laminated to massive quartz veins. These ‘lode’ structures are developed in isoclinally folded turbidites that have been metamorphosed to lower greenschist facies. At Fosterville, diffuse zones of mineralization occur along a high-angle reverse fault zone consisting of en-échelon, strike-parallel segments with oblique slip and strike slip movement. The Globe-Progress deposit is associated with an arcuate shear that flattens with depth forming a listric ramp flat. In contrast to ‘classic’ lode-style deposits, mineralization at Fosterville and Globe-Progress is predominantly hosted by massive sandstone beds, in quartz/carbonate vein stockworks, and in clay-rich fault breccias of quartz vein and sulfidic wall rock fragments. The porous sandstones, which are intercalated with carbonaceous slates, have a bleached appearance, are invariably sericitized and carbonatized, and are 'impregnated' with a disseminated arsenopyrite-pyrite(±stibnite) assemblage. Gold occurs as sub-micron inclusions within the sulfides and rarely as free grains with a diameter of 1-10 microns. Highest-grade disseminated mineralization occurs within acicular arsenopyrite crystals. Fluid inclusions from Fosterville range in composition from high salinity and relatively high CO2, to low salinity and H2O-predominance, suggesting precipitation of at least a portion of the veins under epizonal conditions....

Nature of gold mineralisation in the Walhalla Goldfield, southeast Australia

The Walhalla-Woods Point Goldfield in southeast Australia is characterised by large gold deposits associated with a Late Devonian dyke swarm. The setting of this goldfield is unique because unlike the major gold deposits in Victoria, it occurs close to the eastern margin of the Western Lachlan Orogen, and highlights the disparities between the evolving phases of orogenic gold mineralisation in the Western Lachlan Orogen, and the contrasts between sediment hosted, dyke-associated and dyke-hosted gold mineralisation. This study integrates existing and new data from renewed mapping of the geology and geochemistry of three gold deposits near the township of Walhalla, in the historically important yet under-explored and under-researched Walhalla-Woods Point Goldfield. The ten highest yielding deposits within the goldfield are either hosted within, or adjacent to, intrusions of the Woods Point Dyke Swarm. This is due to the greater chemical reactivity of the calc-alkaline dykes, and the greater rheological contrast between the dykes and surrounding low-grade metasedimentary units, which allowed for the formation of dyke-hosted quartz breccia veins that are consistently favourable sites for gold mineralisation in the Walhalla Goldfield. This is in contrast to historical production, which concentrated on visible gold within the shear zone-hosted laminated quartz veins. Gold and As assay results have highlighted the increased levels of invisible gold disseminated along dyke margins in proximity to shear zones and quartz reefs. The high-yielding gold deposits hosted wholly by the dyke intrusions of the Woods Point Dyke Swarm are orogenic gold deposits, as they are not associated with elevated levels of Bi, W, As, Mb, Te and Sb, typical of intrusion-related gold deposits

Metallogeny of accretionary orogens - The connection between lithospheric processes and metal endowment

Accretionary orogens throughout space and time represent extremely fertile settings for the formation and preservation of a wide variety of mineral deposit types. These range from those within active magmatic arcs, either in continental margin or intra-oceanic settings, to those that develop in a variety of arc-flanking environments, such as fore-arcs and back-arcs during deformation and exhumation of the continental margin. Deposit types also include those that form in more distal, far back-arc and foreland basin settings. The metallogenic signature and endowment of individual accretionary orogens are, at a fundamental level, controlled by the nature, composition and age of the sub-continental lithosphere, and a complex interplay between formational processes and preservational forces in an evolving Earth. Some deposit types, such as orogenic gold and volcanic massive sulfide (VMS) deposits, have temporal patterns that mimic the major accretionary and crustal growth events in Earth history, whereas others, such as porphyry Cu-Au-Mo and epithermal Au-Ag deposits, have largely preservational patterns. The presence at c. 3.4 Ga of (rare) orogenic gold deposits, whose formation necessitates some form of subduction-accretion, provides strong evidence that accretionary processes operated then at the margins of continental nuclei, while the widespread distribution of orogenic gold and VMS deposits at c. 2.7-2.6 Ga reflects the global distribution of accretionary orogens by this time. (C) 2009 Elsevier B.V. All rights reserved.</p