Broken Hill Domain Research Articles

Gahnite, garnet and magnetite compositions of metamorphosed sediment-hosted Zn-Pb-(Cu[sbnd]Ag) deposits of the Mesoproterozoic Nova Brasilândia Group: Vectors for SEDEX deposits with Broken Hill-type affinities in the western Amazonian Craton, Brazil

Stratiform and stratabound Zn-Pb-Ag sulfide deposits hosted in siliciclastic sedimentary rocks constitute some of the world's largest accumulations of base metals. This research paper is the first to describe the DM-1 Zn-(PbAg) and PQ Zn-(CuAu) sulfide deposits, located about 20 km apart in the Mesoproterozoic Nova Brasilândia belt at the southwest margin of the Amazonian Craton. These deposits occur in a NW-SE trending sequence of metamorphosed siliciclastic rocks. The Zn-rich mineralization in both deposits occurs as massive, semi-massive and banded sulfide orebodies concordant with the layering of the country rocks. The DM-1 deposit (1.2 Mt at 12 wt% Zn) has up to 30 m thick zones of sulfide ore consisting predominantly of sphalerite, galena, pyrrhotite and pyrite while the PQ deposit (5 Mt at 5.6 wt% Zn) contains up to 20 m thick zones of sulfide ore consisting mainly of pyrite, pyrrhotite, and sphalerite along with minor chalcopyrite. The host rocks of both deposits consist of schist, gneiss and quartzite with variable modal proportions of garnet, sillimanite, gahnite, magnetite and biotite. These host rocks are up to 50 m thick and commonly show gradational contacts with country rocks. Country rocks, Zn-rich orebodies and their host rocks were overprinted by a post-ore upper amphibolite facies regional metamorphic event.Gahnite compositions of the PQ and DM-1 deposits are characterized by high contents of Zn (68 to 81 % of the gahnite end member) and moderate contents of Fe+2 and Mg. Their major elements compositions plot within the field for metamorphosed massive sulfide deposits hosted by hydrothermally altered Fe-Al-rich metasedimentary and metavolcanic rocks. Moreover, they display compositional trends like those reported for Pb-Zn-Ag deposits in the Broken Hill Domain, Australia. Gahnite from the DM-1 and PQ deposits shows unusually low Cd concentrations (<0.04 ppm), which are 1 to 2 orders of magnitude lower than values reported for gahnite in other Zn-rich sulfide deposits. Very high (from 400 to 900 ppm) Cr concentrations were obtained in gahnite from the PQ deposit. Garnet from the DM-1 and PQ deposits varies from Mn-rich (30 to 62 % of the spessartine end member) in samples from the ore zone or closely associated sulfide-rich host rocks to Mn-poor in sulfide-poor host rocks (<5.5 % of the spessartine end member). Magnetite crystals from ore samples from both deposits have distinctively higher MnO contents than those from host rocks, which follows the Mn-rich and Mn-poor garnets distribution. Our results support the concept that the DM-1 and PQ deposits share common features with BHT deposits as well as some SEDEX deposits affected by high-grade metamorphism, suggesting that they may be classified as clastic-dominated SEDEX deposits with BHT affinities. In addition, gahnite in heavy mineral concentrates is the best pathfinder for regional exploration, and the identification of Mn-rich garnet and/or Mn-rich magnetite in heavy mineral concentrates or preliminary drilling may be used as a proximal pathfinder for Zn-rich sulfide ore.

Major and Trace Element Chemistry of Gahnite as an Exploration Guide to Broken Hill-Type Pb-Zn-Ag Mineralization in the Broken Hill Domain, New South Wales, Australia

Research Article| June 01, 2015 Major and Trace Element Chemistry of Gahnite as an Exploration Guide to Broken Hill-Type Pb-Zn-Ag Mineralization in the Broken Hill Domain, New South Wales, Australia Joshua J. O’Brien; Joshua J. O’Brien † 1Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212 †Corresponding author: e-mail, jjobrien@iastate.edu Search for other works by this author on: GSW Google Scholar Paul G. Spry; Paul G. Spry 1Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212 Search for other works by this author on: GSW Google Scholar Graham S. Teale; Graham S. Teale 2Teale and Associates, P.O. Box 740, North Adelaide, South Australia 5006, Australia Search for other works by this author on: GSW Google Scholar Simon E. Jackson; Simon E. Jackson 3Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar Dean Rogers Dean Rogers 4Perilya Limited, P.O. Box 5001, Broken Hill, New South Wales 2880, Australia Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (4): 1027–1057. https://doi.org/10.2113/econgeo.110.4.1027 Article history received: 02 Jun 2014 accepted: 02 Nov 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joshua J. O’Brien, Paul G. Spry, Graham S. Teale, Simon E. Jackson, Dean Rogers; Major and Trace Element Chemistry of Gahnite as an Exploration Guide to Broken Hill-Type Pb-Zn-Ag Mineralization in the Broken Hill Domain, New South Wales, Australia. Economic Geology 2015;; 110 (4): 1027–1057. doi: https://doi.org/10.2113/econgeo.110.4.1027 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Using Random Forests to distinguish gahnite compositions as an exploration guide to Broken Hill-type Pb–Zn–Ag deposits in the Broken Hill domain, Australia

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.

Geochemistry and genesis of low-grade metasediment-hosted Zn–Pb–Ag mineralization, southern Proterozoic Curnamona Province, Australia

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.

Correlation of Olary and Broken Hill Domains, Curnamona Province:Possible Relationship to Mount Isa andOther North Australian Pb-Zn-Ag-Bearing Successions

The 300-Mt Broken Hill Pb-Zn-Ag orebody is in the Broken Hill Domain, which, together with the contiguous Olary Domain, constitutes the southern part of the Curnamona province of eastern Australia. The Curnamona province is dominated by medium- to high-grade metamorphic rocks of the Paleoproterozoic Willyama Supergroup and is the focus of exploration for base metals, gold, and uranium. A robust chronology of the ~1720 to 1640 Ma Willyama Supergroup provides the basis for temporal correlations on a regional scale between the Olary and Broken Hill Domains, and possibly on an interbasin scale between the southern Curnamona province and the Carpentaria zinc belt in northern Australia (Mount Isa, McArthur River). SHRIMP U-Pb zircon analyses from tuffaceous metasedimentary rocks provide ages for two new chronostratigraphic marker horizons in the Olary Domain. Firstly, the 1693 ± 3 Ma Plumbago Formation, and an equivalent metasiltstone in the Broken Hill Domain, confirm earlier suggestions that the base metal-enriched Bimba Formation is stratigraphically equivalent to the Ettlewood Calc-Silicate Member in the lower part of the ~1685 to 1695 Ma Broken Hill Group. Secondly, tuffaceous sedimentary rocks in the Mount Howden Subgroup of the Olary Domain have a maximum age of 1651 ± 7 Ma, thus suggesting correlation with a similar lithostratigraphic succession in the middle Paragon Group at Broken Hill. The lithostratigraphy, depositional ages, and inferred intervals of missing rock in the Curnamona province are broadly similar to the basin evolution of the Carpentaria zinc belt of northern Australia, suggesting possible chronostratigraphic correlations between Broken Hill and the Mount Isa-McArthur regions. The Broken Hill Group (~1695–1685 Ma) is considered to be time-equivalent to the Prize Supersequence in northern Australia, with possible implications for their respective depositional and metallogenic histories. Depositional ages from the middle Paragon Group (1655 ± 4 Ma, 1657 ± 4 Ma) at Broken Hill are closely comparable to the ca. 1655 Ma ages of tuff units associated with the Mount Isa and Hilton Pb-Zn-Ag orebodies (Gun Supersequence of northern Australia). The upper Paragon Group is considered to be the same age (ca. 1640 Ma) as that part of the Isa superbasin that hosts the McArthur River (HYC) deposit of the McArthur basin, the Walford deposit, and other Pb-Zn-Ag–mineralized sequences of the Lawn Hill platform.

Discussion on detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

C. H. H. Connor, W. V. Pries, R. W. Page, B. P. J. Stevens, I. R. Plimer and P. M. Ashley write: Gibson & Nutman (2004) postulate that the Willyama Supergroup in the southern Curnamona Province contains a 1690–1670 Ma metamorphic core complex, and imply that this is relevant to the genesis of the Broken Hill Pb–Zn–Ag deposit. We contend that this is a model-driven, speculative interpretation predicated upon unsupported assertions that conflict with substantial stratigraphic and geochronological data. Not only does the paper fail to demonstrate an early high-grade event and formation of a metamorphic core complex at 1690–1670 Ma, but it is also factually incorrect in several critical geological and geochronological aspects. The detachment concepts of Gibson & Nutman may appear persuasive, but only by omitting reference to a huge body of published and recent mapping and laboratory research, by companies, universities, and geological surveys, some of which are cited here. Surprisingly, Gibson & Nutman ignore much recent work (e.g. Page et al . 2004) to which one of them has contributed. The essence of the model is that a low-angle, extensional ‘detachment’ formed along the boundary between the Broken Hill and Sundown Groups in the Broken Hill Domain, and between the Bimba and Plumbago Formations in the Olary Domain (Gibson & Nutman, fig.1). The ‘detachment’ was alleged to have occurred as part of a sillimanite-grade D1 deformation between 1690 and 1670 Ma, and to have provided a channel for hydrothermal fluids that could have been responsible for regional scale Na (–Fe) alteration and formation of Pb–Zn mineral deposits, including the Broken Hill orebody. As pointed out below, most of the evidence is misinterpreted, misleading and/or ambiguous, and the geochronological case advanced by Gibson & Nutman for such an early (1690–1670 Ma) high-grade event has no foundation. …

Neotectonic disruption of silicified palaeovalley systems in an intraplate, cratonic landscape: Regolith and landscape evolution of the Mulculca range‐front, Broken Hill Domain, New South Wales

The landscape expression of a wide range of ancient and contemporary regolith materials in the vicinity of the Mulculca Fault demonstrates two important points: (i) tectonism can be a significant factor in the evolution of landscapes in some parts of the Australian craton; and (ii) ancient and young regolith‐landform features can coexist within a tectonically active landscape. Tectonic activity along the Mulculca Fault has created a range‐front near the Broken Hill Domain ‐ Murray Basin margins. This tectonism defeated a now silicified palaeodrainage system that flowed from the area now occupied by the Barrier Ranges towards the present area of the Murray Basin. Stream defeat led to the development of lacustrine‐overbank conditions within the area of the fault‐angle depression, which was later breached by stream incision across the range‐front. The area of lacustrine‐overbank deposition is now dominated by alluvial (channel and overbank) deposits with minor colluvial and aeolian deposition. Silicification of palaeovalley sediments and adjacent saprolite has been occurring over a broad range of times during landscape development, including several stages of palaeovalley evolution and as minor red‐brown hardpan development in the contemporary landscape. Ferruginised regolith has been developing at many different times during the evolution of the landscape, including: (i) prior to the defeat of the palaeodrainage system; (ii) during the sedimentary infilling of the fault‐angle depression; and (iii) within the contemporary landscape. The variable preservation and, therefore, landscape expression of a wide range of regolith materials that formed over a long period of landscape evolution has persisted even though there has been tectonic activity along the Mulculca Fault. For example, topographically inverted, and therefore ancient, silicified alluvial deposits occur alongside contemporary colluvial and alluvial deposits along the tectonically active range‐front. This is in contrast to simple models invoking long‐term tectonic stability to account for the expression of ancient regolith and landforms in Australian cratonic landscapes.

Compilation of U–Pb zircon data from the Willyama Supergroup, Broken Hill region, Australia: Evidence for three tectonostratigraphic successions and four magmatic events?

The Willyama Supergroup of the Broken Hill region in southern Australia consists of supracrustal sedimentary and magmatic rocks, formed between 1810 and 1600 Ma. A statistical analysis of nearly 2000 SHRIMP U–Pb zircon spot ages, compiled from published and unpublished sources, provides evidence for three distinct tectonostratigraphic successions and four magmatic events during this interval. Succession 1 includes Redan Geophysical Zone gneisses and the lower part of the Thackaringa Group (Cues Formation). These rocks were deposited after 1810 Ma and host granite sills of the first magmatic event (1710–1700 Ma). Succession 2 includes the upper Thackaringa Group (Himalaya Formation), the Broken Hill Group and the Sundown Group and was deposited between 1710 and 1660 Ma. These rocks all contain detrital zircons from the first magmatic event (1710–1700 Ma) and in some cases from the second magmatic event (1690–1680 Ma). The second magmatic event (1690–1680 Ma) was bimodal, resulted from crustal extension, and was coeval with deposition of the Broken Hill Group and deepening of the basin. With this event a mafic sill swarm focused in the Broken Hill Domain. Mafic sills lack any trace of inheritance, unlike the granitoids that commonly contain inherited zircons typical of the supracrustal sediments. Succession 3, the Paragon Group and equivalents were deposited after 1660 Ma, but before a regional metamorphic event at 1600 Ma. Metamorphism was closely followed by inversion of the succession into a fold‐and‐thrust belt, accompanied by a fourth late to post‐orogenic magmatic event (ca 1580 Ma) characterised by granite intrusion and regional acid volcanism (the local equivalents of the Gawler Range Volcanics in South Australia).

Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary Domain, South Australia

Iron-formations occur as massive to compositionally layered, Fe oxide-rich, concordant bodies in the Palaeoproterozoic Willyama Supergroup of the Olary Domain, South Australia. They have constitutional similarities to those occurring in the neighbouring Broken Hill Domain. The most abundant iron-formations are in the Quartzofeldspathic Suite and comprise magnetite-quartz assemblages (± hematite, barite, actinolite, apatite). Hematite, magnetite, albite, quartz, Ca(Na) amphibole(s), CaNaFe clinopyroxene and andraditic garnet are major constituents of rare calc-silicate iron-formations in the Bimba and Calcsilicate Suites, whereas magnetite, quartz, almandine-spessartine, manganoan fayalite, manganoan grunerite and apatite form manganiferous iron-formations in the Pelite Suite. The pronounced differences in mineralogy of the three iron-formation types are the result of regional metamorphism of diverse hydrothermal precipitates with variable elastic components, together with the local effects of high-temperature metasomatic alteration. Metasomatic fluids were produced as a result of devolatilisation of the evaporite-bearing volcanosedimentary sequence, during and following amphibolite grade metamorphism and deformation, which led to localised and regional-scale hydrothermal alteration. In places, there was extensive metasomatic reconstitution (veining, brecciation, replacement) of iron-formations and associated rocks, caused by high-temperature (350°–650°C), oxidising, saline fluids. The resulting epigenetic ironstones are dominated by magnetite-hematite-quartz with minor sulfides and display enrichment in Fe, Ti, Cu, Au, Sc, U, V, Y, Zn and HREE relative to parental iron-formations.