Ore and Energy Resource Geology最新文献

{"title":"Gold nuggets: the inside story","authors":"C.R.M. Butt,&nbsp;R.M. Hough,&nbsp;M. Verrall","doi":"10.1016/j.oreoa.2020.100009","DOIUrl":"https://doi.org/10.1016/j.oreoa.2020.100009","url":null,"abstract":"<div><p>Gold nuggets have long captured the imagination of geologists, prospectors and the public alike, but their origin remains disputed. Supporting a supergene origin, most gold nuggets in Australia have been found at or near the soil surface. Many are intimately associated with, and even appear to enclose, soil materials and weathered rock. Even large nuggets (e.g., &gt;2 kg) have surface features and/or gross morphologies that suggest chemical reworking in the regolith. Conversely, other nuggets have been found at depths of ten metres or more in the regolith, and large masses of gold have been encountered at considerable depth, in completely unweathered, hypogene environments. Nuggets and particulate gold from many deposits in Australia, New Guinea, SE Asia and Brazil have been examined by optical and electron-optical techniques to determine characteristics that may indicate their genesis and stability in the regolith. The specimens have been collected at or close to the surface but all nuggets (mass range ~1 gm to &gt;8 kg) and many smaller grains appear to be hypogene. They have nearly homogeneous Ag contents, mostly in the range 3 to 20 wt%, although some have no detectable Ag. One specimen also contains up to 3 wt% Hg, but no other alloyed metals &gt; 0.1 wt% were detected. Enclosed minerals are rare – with only galena, Bi sulphide, galenobismutite and complex Ag-Hg tellurides in a few samples. The internal structure of the nuggets comprises nearly equigranular, randomly-oriented crystal domains. Many crystals display coherent twins and/or short incoherent twins that terminate within the crystal, all typical of thermal annealing at temperatures &gt;250°C. Some small nuggets from SE Asia, also with annealing fabrics are possibly the product of hydrothermal remobilization and re-precipitation. In comparisons, some specimens from New Guinea contain 10-&gt;30 wt% Ag and have internal structures such as zoning and ‘fern-like’ crystal habits. These are derived from epithermal deposits and have not been deformed or recrystallized since initial deposition. Even the largest nuggets have internal evidence of weathering. Many have secondary minerals such as Fe oxides, clays and calcite within them, but none of these is fully enclosed. Rather, they are all open to the outer margin of the nuggets, situated in interconnecting voids along crystal boundaries. These boundaries also exhibit Ag depletion, similar to the depletion rims on the external surface. EBSD analysis shows there is no variation in crystallographic orientation across, or into, the depletion zones. These characteristics show that nuggets are dissolving in the surface environment, not forming, with weathering reactions initiated on the external surface and, internally, along crystal boundaries.</p></div>","PeriodicalId":100993,"journal":{"name":"Ore and Energy Resource Geology","volume":"4 ","pages":"Article 100009"},"PeriodicalIF":0.0,"publicationDate":"2020-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.oreoa.2020.100009","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"92033973","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Use of QEMSCAN® to characterize oxidized REE ore from the Bear Lodge carbonatite, Wyoming, USA","authors":"Adrian D. Van Rythoven ,&nbsp;Katharina Pfaff ,&nbsp;James G. Clark","doi":"10.1016/j.oreoa.2020.100005","DOIUrl":"https://doi.org/10.1016/j.oreoa.2020.100005","url":null,"abstract":"<div><p>The Bear Lodge REE deposit is located in northeastern Wyoming. The Eocene carbonatite dyke and stockwork system intrudes trachytic-phonolitic rocks that contain multiple diatremes. The original magmatic characteristics of the REE-enriched carbonatite complex are strongly overprinted by carbo-hydrothermal and later supergene fluids. These fluids redistributed the REEs and created high variability in the ore mineral assemblage, as well as compositional variability within individual REE mineral species.</p><p>The REE ore minerals at Bear Lodge can be classified into four types: 1. fluorocarbonates (bastnaesite, parisite, synchysite), 2. phosphates (monazite, xenotime, florencite, rhabdophane, churchite), 3. cerianite, and 4. ancylite. These minerals vary greatly in abundance, grain size, and morphology. REE distribution is heterogeneous throughout the deposit.</p><p>Variations within a given REE mineral in terms of Ce depletion, Th content, degree of heavier REE enrichment, etc., create difficulties in the initial definition of discrete mineral species (i.e., by X-ray spectra) and their resultant species identification protocols for use in automated mineralogy (QEMSCAN^Ⓡ in this study). Prevalent submicron-scale supergene mineralization result in hybrid spectra from multiple phases. Iterative work in reconciling QEMSCAN^Ⓡ data interpretation with bulk assay, XRD, SEM, and optical petrography data allows for refinement of the protocols to quantify for both ore and gangue minerals. Use of automated mineralogy in the development of complex deposits requires rigorous review of these identification criteria in order to achieve results that can be applied with confidence to resolve mineral processing issues.</p></div>","PeriodicalId":100993,"journal":{"name":"Ore and Energy Resource Geology","volume":"2 ","pages":"Article 100005"},"PeriodicalIF":0.0,"publicationDate":"2020-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.oreoa.2020.100005","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"92050978","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Assessing the variability of expert estimates in the USGS Three-part Undiscovered Mineral Resource Assessment methodology: A call for increased skill diversity and scenario-based training","authors":"Rhys S. Davies ,&nbsp;Allan Trench ,&nbsp;David I. Groves ,&nbsp;Michael Dentith ,&nbsp;Marianne J. Davies ,&nbsp;John P. Sykes","doi":"10.1016/j.oreoa.2020.100006","DOIUrl":"10.1016/j.oreoa.2020.100006","url":null,"abstract":"<div><p>Future mineral exploration will necessarily be conducted in increasingly challenging and uncertain search spaces as near-surface, high-quality ore deposits are progressively depleted. Faced with this increase in task complexity, an important consideration from an exploration management perspective is the behavioural aspect of information interpretation and decision-making.</p><p>One such challenging search space is the Sandstone Greenstone Belt, Western Australia, covering an area of approximately 920 km², that is deemed prospective for the discovery of archean orogenic gold deposits, with mined (historic production), inferred and indicated resources (JORC 2004 and 2012) totalling 54 t Au. Gold endowment estimates made by geoscience experts, during an exploration project evaluation workshop, were compared with estimates from a group of non-geoscientists, made during a separate but identical workshop. Significant differences were identified between the estimates of the expert geoscientists and the non-geoscience expert group, with the latter proving more conservative. However, a portion of the geoscience experts (<em>N</em> = 11) group produced conservative estimates, comparable to a non-geoscience expert group (<em>N</em> = 10), with both suggesting the existence of additional gold deposits of similar size and quality to known resources (with group estimates for median total endowment of 99 t and 120 t Au, respectively). The remaining geoscience experts (<em>N</em> = 11) presented significantly more optimistic, albeit inconsistent, estimates for the gold endowment of the project area, predicting the existence of undiscovered deposits significantly larger than those already defined in the belt (with a group estimate for median total endowment of 350 t Au).</p><p>Although the true undiscovered gold endowment within the project area remains uncertain, several possible factors can explain the variations in estimates. These include the application of contrasting strategies, with participants opting to apply more empirical or conceptual methods, and to differences in background experience, resulting in distinct skillsets and varying ability to estimate uncertainty. To improve the quality of expert estimates, it is suggested that individual expertise and appropriate assessment strategies can be developed through scenario-based training courses, and that greater skill and experience diversity within exploration teams is desirable, leading to more balanced aggregate estimates. Further research is warranted to determine which, if any, of the proposed factors account for these disparities. This research could be used to adapt the composition of exploration teams and develop training programs to promote the development of expertise in predictive exploration targeting, in order to promote discovery of future mineral resources.</p></div>","PeriodicalId":100993,"journal":{"name":"Ore and Energy Resource Geology","volume":"2 ","pages":"Article 100006"},"PeriodicalIF":0.0,"publicationDate":"2020-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.oreoa.2020.100006","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78320935","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A revised stratigraphic model for the 2.7 Ga Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia","authors":"Martin J. Gole ,&nbsp;Erin Western ,&nbsp;Alexandre Diragitch","doi":"10.1016/j.oreoa.2019.100001","DOIUrl":"10.1016/j.oreoa.2019.100001","url":null,"abstract":"<div><p>The lower stratigraphy of Agnew-Wiluna greenstone belt is composed of two main elements; a mafic/komatiite domain and a felsic/komatiite/basalt domain. Previous stratigraphic models show the mafic domain overlying the felsic domain. Komatiites in the latter host the vast majority of the nickel sulphide endowment of the belt (&gt;20 significant deposits) whereas those in the mafic domain contain 3 and 4 relatively small deposits. Recently published geochemical data from well-preserved mafic domain rocks exposed in the Agnew area opens up the possibility to match these units with mafic rocks within the more structurally disrupted felsic domain. Analytical data from basalts at the Cliffs and Mount Keith Ni deposits and from the Wiluna Au mine sequence show that these can be matched to the basalt sequence stratigraphically below the Agnew Komatiite at Agnew and also show that basalts previously thought to occupy different stratigraphic positions (Centenary Bore and MacFarlanes Basalts) are laterally equivalent but structurally displaced. The revised stratigraphic model together with available age dates show that komatiites in both domains, Mount Keith and Cliffs/Agnew Komatiites, are laterally equivalent and part of the 2705 Ma Kalgoorlie-Kurnalpi komatiite LIP. This greatly enhances the Ni prospectivity of komatiites within the mafic domain which, previously being thought younger, were historically considered less prospective. The footwall to the komatiite is composed of basalt (Never Can Tell Basalt, in the mafic domain) and felsic sequences (Mount Keith Dacite in the felsic domain) that are laterally separated but occupy the same stratigraphic position and together with the komatiite correlate with the Kambalda Sequence in the south of the Kalgoorlie Terrane. The oldest crystallisation ages from the Mount Keith Dacite are 2719–2725 Ma but whether these rocks belong to the Kalgoorlie or Youanmi Terrane is currently unknown. The Kalgoorlie-aged sequence has an unconformable contact with underlying Youanmi-aged sequence (the latter including dates of 2724–2729, 2734, 2749 Ma) composed of basalt, komatiitic basalt, komatiite and minor felsic volcanic (in decreasing stratigraphic order; felsic volcanics, Songvang Basalt, Hickies Bore Basalt, Donegal Komatiite, Butchers Well Basalt). The Youanmi sequence is exposed throughout the AWB, is present in the Leonora area to the immediate south and extends eastward to other areas within the northern part of the Kalgoorlie-Kurnalpi Terranes.</p></div>","PeriodicalId":100993,"journal":{"name":"Ore and Energy Resource Geology","volume":"1 ","pages":"Article 100001"},"PeriodicalIF":0.0,"publicationDate":"2019-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.oreoa.2019.100001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"95682306","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}