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North Adelaide, Australia

O'Brien J.J.,Iowa State University | Spry P.G.,Iowa State University | Teale G.S.,Teale and Associates | Jackson S.E.,Geological Survey of Canada | Koenig A.E.,U.S. Geological Survey
Journal of Geochemical Exploration | Year: 2015

Gahnite occurs in and around metamorphosed massive sulfide (e.g., Broken Hill-type Pb-Zn-Ag (BHT), volcanogenic massive sulfide Cu-Zn-Pb-Au-Ag (VMS), sedimentary exhalative Pb-Zn (SEDEX)), and non-sulfide zinc (NSZ) deposits. In addition to occurring in situ, gahnite occurs as a resistate indicator mineral in unconsolidated sediments (e.g., glacial till) surrounding such deposits. The spatial association between gahnite and metamorphosed ore deposits has resulted in its use as an empirical exploration guide to ore. Major and trace element compositions of gahnite from BHT, NSZ, SEDEX, and VMS deposits are used here to develop geochemical fingerprints for each deposit type.A classification tree diagram, using a combination of six discrimination plots, is presented here to identify the provenance of detrital gahnite in greenfield and brownfield terranes, which can be used as an exploration guide to metamorphosed massive sulfide and non-sulfide zinc deposits. The composition of gahnite in BHT deposits is discriminated from gahnite in SEDEX and VMS deposits on the basis of plots of Mg versus V, and Co versus V. Gahnite in SEDEX deposits can be distinguished from that in VMS deposits using plots of Co versus V, Mn versus Ti, and Co versus Ti. In the Sterling Hill NSZ deposit, gahnite contains higher concentrations of Fe3+ and Cd, and lower amounts of Al, Mg, and Co than gahnite in BHT, SEDEX, and VMS deposits. Plots of Co versus Cd, and Al versus Mg distinguish gahnite in the Sterling Hill NSZ deposit from the other types of deposits. © 2015 Elsevier B.V.. Source

Heimann A.,East Carolina University | Spry P.G.,Iowa State University | Teale G.S.,Teale and Associates | Leyh W.R.,Eaglehawk Geological Consulting Pty. Ltd. | And 3 more authors.
Journal of Geochemical Exploration | Year: 2013

Sediment-hosted, Paleoproterozoic low-grade Zn-Pb-Ag and Cu-Au mineralization occurs at the Polygonum, Thunderdome, Hunters Dam, and Benagerie Ridge prospects under a thick sedimentary cover in the Mulyungarie Domain (MD) of the southern Curnamona Province (SCP), Australia. Host rocks and mineralization were metamorphosed to lower greenschist facies at Benagerie Ridge, upper greenschist facies at Hunters Dam, and lower to upper amphibolite facies at Polygonum and Thunderdome. The giant Paleoproterozoic Pb-Zn-Ag Broken Hill deposit, which was metamorphosed to the granulite facies, is hosted in the Broken Hill Domain (BHD). The general lithostratigraphy for each prospect is similar and can be subdivided into four main units. Unit 1, at the base of the stratigraphic section, hosts Cu-Au mineralization and is characterized by the presence of oxidized magnetic metapsammopelites. Unit 2 consists of sulfide-rich, pyritic carbonaceous metapelites, calcareous metashales/siltstones, Mn-bearing sideritic metacarbonates, metacalc-silicates, and carbonate and feldspar lenses and layers. Syn-sedimentary sulfide mineralization occurs as laminations of fine-grained pyrite and sphalerite and resembles that at the HYC deposit, Queensland. Differences in δ13C values (δ13CVPDB=-10.2 and -0.8‰) of calcite in unit 2 from the same area suggest variable amounts of original organic C. The δ13C values of carbonates in Broken Hill ore and those obtained here from the Esmeralda deposit near Broken Hill are extremely low (δ13C=-25 to -21‰) and likely resulted from volatilization and high temperature effects during high-grade metamorphism. Barren laminated carbonaceous metapelites in unit 3 constitute a marker unit throughout the SCP. Unit 4 at the top of the stratigraphic sequence consists of psammopelitic and laminated andalusite or chiastolite phyllites that contain garnet and gahnite, laminated garnetite, quartz garnetite, Mn-bearing banded iron formation, amphibolite, and stratabound hydrothermal Pb-Zn mineralization. This unit, which is present at the Polygonum and Thunderdome prospects, is stratigraphically equivalent to the upper Broken Hill Group, which hosts the Broken Hill deposit. Base metals occur in metacarbonates and metapelites and close to redox boundaries between units 1 and 2, and units 2 and 3. Geochemical indicators of stratabound Pb-Zn mineralization in the northern part of the SCP include whole-rock values of >4ppm Tl, >25ppm Cd, and >17ppm Se in units 2 and 4, (Mn+Fe+Mg)/(Mn+Fe+Mg+Ca) ratios>0.9 for carbonates, Mn/(Mn+Fe+Mg) ratios>0.6 for garnet in garnet-quartz rocks, and Mn/(Mn+Ca+Fe) ratios>0.3 for garnet in metacalc-silicate rocks. © 2013 Elsevier B.V. Source

O'Brien J.J.,Iowa State University | Spry P.G.,Iowa State University | Teale G.S.,Teale and Associates | Jackson S.E.,Geological Survey of Canada | Rogers D.,Perilya Ltd
Economic Geology | Year: 2015

Gahnite-bearing rocks are common throughout the Proterozoic Broken Hill domain, New South Wales, Australia, where they are spatially associated with Broken Hill-type Pb-Zn-Ag mineralization, including the supergiant Broken Hill deposit. In the past, such rocks have been utilized as exploration guides to ores of this type, but their presence has had mixed success in discovering new occurrences of sulfide mineralization. Major element chemistry of gahnite has previously been used to define a compositional range associated with metamorphosed massive sulfides deposits, including Broken Hill-type deposits, but it fails to distinguish sulfide-rich from sulfide-poor occurrences. Major and trace element data from LA-ICP-MS and electron microprobe analyses were obtained for gahnite from twelve Broken Hill-type deposits to determine whether or not gahnite chemistry may be used to distinguish prospective exploration targets from nonprospective occurrences. Major and trace element data were discriminated using a principal component analysis, and in a bivariate plot of Zn/Fe versus Ni + Cr + V to distinguish gahnite associated with the Broken Hill deposit from that associated with sulfide-poor lode pegmatite, and sillimanite gneiss. Bivariate plots of Zn/Fe versus trace element contents (e.g., Ga, Co, Mn, Co, Ni, V, Cd) suggest gahnite from the Broken Hill deposit has a relatively restricted compositional range that overlaps with some minor Broken Hill-type occurrences. Based on the ore grade (wt % Pb + Zn) of rocks hosting gahnite at each locality, gahnite in the highest grade mineralization from minor Broken Hill-type deposits possess compositions that plot within the field for gahnite from the Broken Hill deposit, which suggests that major and trace element chemistry (e.g., Zn/Fe = 2-4 vs. Co = 10-110 ppm, Ga = 110-400 ppm, Mn = 500-2,250 ppm; and Co = 25-100 ppm vs. Ga = 125-375 ppm) may be used as an exploration guide to high-grade ore. ©2015 Society of Economic Geologists, Inc. Source

O'Brien J.J.,Iowa State University | Spry P.G.,Iowa State University | Nettleton D.,Iowa State University | Xu R.,Iowa State University | Teale G.S.,Teale and Associates
Journal of Geochemical Exploration | Year: 2015

Various studies have focused on evaluating variability in the major-trace element chemistry of minerals as exploration guides to metallic mineral deposits or diamond-bearing kimberlites. The chemistry of gahnite has previously been proposed as an exploration guide to Broken Hill-type Pb-Zn-Ag mineralization in the Broken Hill domain, Australia, with the development of a series of discrimination plots to compare the composition of gahnite from the supergiant Broken Hill deposit with those in occurrences of minor Broken Hill-type mineralization. Here, the performance of Random Forests, a relatively new statistical technique, is used to classify mineral chemistry using a database (n=533) of gahnite compositions (i.e., Mg, Al, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, and Cd) from the Broken Hill deposit and 11 minor Broken Hill-type deposits in the Broken Hill domain. This statistical method has yet to be applied to geological problems involving mineral chemistry. Random Forests provide a framework for classification and decision making through a series of classification trees, which individually, resemble classification keys. Gahnite from the Broken Hill domain is classified here on the basis of the following schemes: 1. Random Forest 1 (RF1): gahnite in the Broken Hill deposit versus compositions of gahnite from other minor Broken Hill-type occurrences in the Broken Hill domain; 2. Random Forest 2 (RF2): gahnite in the Broken Hill deposit versus gahnite in minor Broken Hill-type deposits containing >0.25milliontonnes (Mt) of Pb-Zn-Ag mineralization versus gahnite in sulfide-free and sulfide-poor prospects containing<0.25Mt; and 3. Random Forest 3 (RF3): gahnite in sulfide-bearing quartz-gahnite lode rocks versus gahnite in sulfide-free samples. Misclassification rates, according to a ten-fold cross validation, of RF1, RF2, and RF3 are 1.6, 3.3, and 4.7% respectively. Results of this study suggest that Random Forests work well in classification problems involving mineral chemistry, and may prove useful in the exploration for Broken Hill-type and other types of metallic mineral deposits. © 2014 Elsevier B.V. All rights reserved. Source

Heimann A.,Iowa State University | Heimann A.,East Carolina University | Spry P.G.,Iowa State University | Teale G.S.,Teale and Associates | And 2 more authors.
Mineralogy and Petrology | Year: 2011

Garnet-rich rocks occur throughout the Proterozoic southern Curnamona Province, Australia, where they are, in places, spatially related to Broken Hill-type Pb-Zn-Ag deposits. Fine-scale bedding in these rocks, their conformable relationship with enclosing metasedimentary rocks, and their enrichment in Mn and Fe suggest that they are metamorphosed chemical precipitates. They formed on the floor of a 1.69 Ga continental rift basin from hydrothermal fluids mixed with seawater and detritus. Garnet in garnet-quartz and garnet-amphibole rocks is generally light rare earth element (LREE) depleted, and has flat heavy REE (HREE) enriched chondrite-normalized REE patterns, and negative Eu anomalies (Eu/Eu* < 1). Garnet in garnet-rich rocks from the giant Broken Hill deposit has similar REE patterns and either positive (Eu/Eu* > 1) or negative Eu anomalies. Manganese- and Mn-Ca-rich, Fe-poor garnets in garnetite, garnet-hedenbergite, and garnet-cummingtonite rocks at Broken Hill have Eu/Eu* > 1, whereas garnet in Mn-poor, Fe-rich quartz garnetite and quartz-garnet-gahnite rocks from Broken Hill, and quartz garnetite from other locations have Eu/Eu* < 1. The REE patterns of garnet and its host rock and interelement correlations among REEs and major element contents in garnet and its host rock indicate that the Eu anomaly in garnet reflects that of its host rock and is related to the major element composition of garnet and its host rock. The value of Eu/Eu* in garnet is related to its Mn, Fe, and Ca content and that of its host rock, and the distribution of REEs among garnet and accessory phases (e. g., feldspar). Positive Eu anomalies reflect high amounts of Eu that was preferentially incorporated into Mn- and Mn-Ca-rich oxides and carbonates in the protolith. In contrast, Eu/Eu* < 1 indicates the preferential discrimination against Eu by Fe-rich, Mn-poor precursor minerals. Precursors to Mn-rich garnets at Broken Hill formed by precipitation from cooler and more oxidized hydrothermal fluids compared to those that formed precursors to Mn-poor, Fe-rich garnet at Broken Hill and the other locations. Garnet from the Broken Hill deposit is enriched in Zn (> 400 ppm), Cr (> 140 ppm), and Eu (up to 6 ppm and positive Eu anomalies), and depleted in Co, Ti, and Y compared to garnet in garnet-rich rocks from other localities. These values, as well as MnO contents > 15 wt. % and Eu/Eu* > 1 are only found at the Broken Hill deposit and are good indicators of the presence of Broken Hill-type mineralization. © 2010 Springer-Verlag. Source

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