NW Queensland Research Articles

Paragenetic and geological setting of the Starra iron oxide copper–gold deposits, Mount Isa Inlier, Queensland, Australia: constraints on IOCG deposit models

The five Starra iron oxide copper–gold (IOCG) deposits, which are located in the Cloncurry district of the Mount Isa Inlier, NW Queensland, are hosted in a N-trending zone of deformed iron-rich metasedimentary rocks that are underlain by two mineralogically distinct mafic–intermediate meta-igneous rocks. Copper–Au mineralisation is subdivided into two stages—an early main stage that transitioned from a magnetite-dominated Cu substage comprising chalcopyrite, pyrite, biotite and chlorite, to a hematite–muscovite-bearing Cu–Au substage with chalcopyrite, bornite and rare chalcocite, as well as Au–Ag-tellurides and native Au. This second stage was followed by a subordinate but nonetheless important carbonate–quartz–barite vein stage with minor but locally high-grade Cu–Au mineralisation characterised by abundant chalcocite and bornite, minor native bismuth and wittichenite (Cu3BiS3), together with minor Au–Ag telluride minerals. Bornite in these carbonate–quartz–barite veins contains considerable amounts of Au (>10 ppm), which appears to be deported either as lattice substitution in the bornite crystal structure or as homogeneously distributed Au0 nanoparticles. An earlier, coarse-grained mafic intrusion was affected by early Na–Ca alteration, whereas a second mafic intrusion lacks evidence of Na–Ca alteration but was exposed to K–Fe alteration and subsequent mineralisation at Starra 222. Mushketovite (magnetite after specular hematite) is only observed in ironstone lodes proximal to the mafic–intermediate rocks. The close spatial and mineralogical associations of the mafic–intermediate bodies and the ironstone lodes point to a stronger link between magmatic activity and IOCG mineralisation than has previously been recognised at Starra. Remobilisation of gold by late-stage hydrothermal fluids aided by the relatively low fluid temperatures during the main pulse of Cu–Au mineralisation refined gold grades at Starra, making it a relatively gold-rich IOCG deposit. We show that the Cu–Au deposits of the Starra trend feature mineralogical and geochemical elements common to both Cloncurry IOCG deposits (e.g. abundant magnetite) and IOCG deposits of the Tennant Creek district (e.g. Au–Cu–Bi mineralisation; hematite–bornite–chalcocite assemblages), making them a unique and important member of this complex family of deposits.

Comments on “A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes” by Cave, B., Lilly, R., Simpson, A., McGee, L. [ore geol. Rev., 154, 105311

Comments on “A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes” by Cave, B., Lilly, R., Simpson, A., McGee, L. [ore geol. Rev., 154, 105311

Genesis of the Mount Colin Cu-Au deposit, NW Queensland: Evidence from geology, multi-mineral U–Pb geochronology, S-, O- and C-isotope constraints

The Mount Colin Cu-Au deposit is located in the Mary Kathleen Domain of the Mount Isa Inlier, NW Queensland. The deposit is hosted in the Corella Formation, which transitions into the Burstall Granite at a depth of ~400 m. The host rocks of the deposit are overprinted by Na-Ca alteration and a Ca-Mg-Fe skarn assemblage. Na-Ca alteration yields a titanite 207Pb/206Pb weighted mean age of 1717 ± 8 Ma (MSWD = 0.32) and is composed of albite, actinolite, zircon, titanite and scapolite with retrograde calcite, prehnite and sericite. The Ca-Mg-Fe skarn yields a titanite 207Pb/206Pb weighted mean age of 1712 ± 7 Ma (MSWD = 1.2) and is composed of hastingsite, K-feldspar, clinopyroxene, albite, titanite, calcite, biotite, and zircon with retrograde quartz and chlorite. Based on temporal relationships, Na-Ca alteration and the Ca-Mg-Fe skarn is interpreted to have formed from a hydrothermal magmatic fluid during late-stage magmatism of the (1740–1710 Ma) Wonga-Burstall plutons. A second generation of titanite (denoted titanite II) is present within Na-Ca alteration and the Ca-Mg-Fe skarn assemblage. Titanite II yields a 207Pb/206Pb weighted mean age of 1576 ± 7 Ma (MSWD = 0.37), and is interpreted to have formed from an external metamorphic hydrothermal fluid during widespread deformation associated with the (1610–1500 Ma) Isan Orogeny. The Mount Colin Cu-Au deposit consists of a predominantly coarse-grained, mineralogically zoned assemblage that infills along the NW-striking Mount Colin Fault. The centre of the deposit comprises a largely unmineralized core composed of calcite and/or quartz ± apatite ± microcline. Cu-Au mineralization is positioned along the outer margins of the deposit and consists of biotite, actinolite, chalcopyrite and pyrrhotite with trace abundances of magnetite, pyrite, galena, melonite and arsenopyrite. Apatite from the core of the deposit yields a U–Pb age of 1523 ± 16 Ma (MSWD = 1.2). The δ34SCDT values of chalcopyrite and pyrrhotite range from 0.1–2.1 ‰ and 0.9–1.5 ‰ respectively. The δ13CVPDB values of calcite range from −5.54 ‰ to −4.54 ‰ with corresponding δ18OVSMOW values ranging from 8.16–11.26 ‰. Based on the temporal and isotopic constraints presented in this study, a hydrothermal magmatic origin for Cu-Au mineralization is favoured. The timing of Cu-Au mineralization is coeval with a regional-scale metamorphic/hydrothermal event throughout the Mary Kathleen Domain and widespread magmatism throughout the Eastern Fold Belt of the Mount Isa Inlier.

A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes

The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa. Although previous workers have noted the existence of a dolerite dyke at the Hilton Zn-Pb-Ag deposit (Valenta, 1994b; Valenta, 1994a), a dolerite dyke at the George Fisher Zn-Pb-Ag deposit has not been previously documented. Progressive underground expansion at the George Fisher mine has resulted in the discovery of a dolerite dyke in the southernmost portion of the deposit. The temporal and petrological relationship between the dolerite dykes and the adjacent Zn-Pb-Ag mineralisation at the George Fisher and Hilton Zn-Pb-Ag deposits has not been considered by previous studies, and has significant implications for the currently accepted syn-diagenetic metallogenic model. In assay data, the dolerite dykes correspond with a large spike in TiO2 and V values. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 ± 21 Ma (MSWD = 0.93) and 1619 ± 22 Ma (MSWD = 0.53) for the dolerite dykes at the George Fisher and Hilton deposits, respectively. This age is consistent with dyke intrusion during the earliest phases of deformation during the (1620–1500 Ma) Isan Orogeny. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining, which have paragenetic equivalents in the adjacent Zn-Pb-Ag orebodies. The hydrothermal stages include: (1) Quartz ± albite ± K-feldspar alteration/veining; (2) Dolomite alteration/veining; (3) Biotite-chlorite-sulphide alteration; (4) Paragenetically late calcite, dolomite and/or sulphide veining. Monazite from a quartz ± albite ± K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 ± 16 Ma (MSWD = 1.1), constraining the maximum age of alteration within the dolerite dykes. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein. Calcite from the pre-mineralisation alteration assemblage produces a Lu-Hf age of 1501 ± 32 Ma (MSWD = 1.04), which is interpreted to constrain the maximum age of stratabound Zn-Pb-Ag mineralisation. Calcite from the late cross-cutting vein produced a Lu-Hf age of 1289 ± 26 Ma (MSWD = 1.00), which is interpreted to constrain the age of post-mineralisation faulting throughout the deposit. The evidence presented in this study indicates that the dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralisation.

Hyperspectral cathodoluminescence, trace element, and U-Pb geochronological characterization of apatite from the Ernest Henry iron oxide copper-gold (IOCG) deposit, Cloncurry district, Queensland

Hyperspectral cathodoluminescence (CL), geochemical, and geochronological characterization of a series of apatite-bearing samples from within and around the Ernest Henry IOCG deposit, NW Queensland, Australia, have revealed complex mineral parageneses and a spectrum of U-Pb ages that point to the effects of multiple geological processes. No two samples are identical, either in geochemistry or texture, despite their relative proximity to one another (all samples within 5 km from Ernest Henry). Hyperspectral CL maps reveal diverse internal textures and emissions ranging from near infrared (NIR) to near ultraviolet (UV) with a complex series of spectra in all samples, requiring the fitting more than 40 individual peaks (both sharp and broad) to capture the observed variability. Imaging analyses via LA-ICPMS show that apatite from the Ernest Henry district is enriched above background in a variety of trace elements, including Na, Mg, Al, Si, V, Mn, As, Sr, Y, the rare Earth elements (REEs), Pb, Th, and U. Samples outside the ore zone display chondrite-normalized REE profiles that are consistent with either a magmatic or hydrothermal origin, whereas ore zone apatite exhibits profiles that are decidedly hydrothermal in nature. Moreover, specific zones within ore zone apatite grains are very As-rich (up to 7 wt% As2O5), and the effect of such high As on the hyperspectral CL signature of these zones is a pronounced dampening of CL emission, regardless of REE concentrations. Uranium-Pb dating of the same samples (via LA-ICPMS) has yielded a diverse array of overlapping Mesoproterozoic ages ranging from 1,580 ± 34 Ma to 1,533 ± 61 Ma. These results correlate to published ages that constrain hydrothermal alteration in the Ernest Henry area, both before and during Cu-Au mineralization. Collectively, these data highlight the complexity of apatite studies at Ernest Henry, the broader Cloncurry district, and probably analogous terranes elsewhere. A combination of micro-scale methods such as those used in this study are shown to be essential for accurately deciphering geological information contained within petrogenetic indicator minerals.

Reversal of Ages of the Kalkadoon/Leichhardt Complex and the Magna Lynn Metabasalt, NW Queensland

Reversal of Ages of the Kalkadoon/Leichhardt Complex and the Magna Lynn Metabasalt, NW Queensland

The impact of hydrothermal mineral replacement reactions on the formation and alteration of carbonate-hosted polymetallic sulfide deposits: A case study of the Artemis prospect, Queensland, Australia

Hydrothermal sulfidation of carbonate rocks and the reverse replacement of sulfides by carbonates are important processes responsible for the formation and subsequent alteration of ore deposits. These processes have been recognized in the case study of the Artemis Zn-Cu-Au prospect, which is located in the Cloncurry district within the Eastern Succession of the Mount Isa Inlier, NW Queensland, Australia. The high-grade polymetallic mineralization displays a complex association of massive sulfides and carbonates hosted in a vertical marble lens adjacent to garnet-bearing schist, staurolite-muscovite schist, and quartzo-feldspathic psammite. Petrographic analyses of drill core specimen by optical and electron microscopy, micro-X-ray florescence elemental maps, electron probe microanalyses, cathodoluminescence, and bulk geochemistry suggested three major stages of mineralization: (i) pre-ore stage marble (calcite-0); (ii) ore stage involving sulfidation of the marble and precipitation of calcite-1 and spherical sulfide inclusions in the first episode and the formation of calcite-2 and massive sulfide in the second episode; and (iii) post-ore stage involving replacement of sulfides by calcite-3 and garnet.Mineral replacement reactions are dominant processes in rocks of ore stage and post-ore stage. The ore stage involved the dissolution of coarse and pristine calcite-0 (Ca0.95Mn0.3Fe0.2CO3) and the precipitation of sulfides and calcite-1 (Ca0.96Mn0.3Fe0.1CO3) and calcite-2 (Ca0.96Mn0.3Fe0.1CO3). The sulfide assemblage consists of pyrrhotite, sphalerite, chalcopyrite, galena, arsenopyrite, cobaltite, costibite, Bi, cubanite ± Ag-sulfides and Au, in decreasing order of abundance. In episode 1, calcite-1 contains spherical sulfide inclusions while in episode 2, calcite-2 envelopes calcite-1. The replacement of Co-rich arsenopyrite (Fe0.75Co0.26As1.13S0.83) by relatively Co-poor arsenopyrite (Fe0.86Co0.15As0.99S0.99) via a coupled dissolution-precipitation reaction occurred in episode 2. Based on arsenopyrite geothermometry, the temperatures for the first and second episode are 650 ± 50 and 450 ± 70 °C, respectively. Micro-XRF elemental maps of the garnet-bearing schist, adjacent to the sulfidized marble, revealed the sulfidation front where pre-ore garnet and amphibole were replaced by pyrrhotite, chalcopyrite, Au, biotite, and muscovite. In the post-ore stage, calcite-2 was partially replaced by siderite and calcite-3 (Ca0.94Mn0.4Fe0.2Mg0.1CO3), which shows sectoral and oscillatory zoning. Galena was replaced by calcite-3 as documented by embayed lobate grain boundaries between these minerals, and pyrrhotite Fe(1-x)S, (x = 0.091–0.119) was pervasively replaced by garnet (Mn1.2Fe1.2Ca0.6)Al2Si3O12. The microscale investigations of the mineralization processes in the Artemis orebody document the impact of hydrothermal mineral replacement reactions on metal sequestration. Mineral replacement reactions facilitate the formation of ore-deposits and subsequent alteration processes, which may lead to the loss of economic value of the deposit.

A new spatially continuous basement heat flow map for NW Queensland

A recently released open file study of the depth-to-basement and basement heat flow is presented, which covers the Queensland portion of the South Nicholson Basin and includes basins underlying the Lawn Hill Platform and Georgina Basin. The present-day basement heat flow model is derived from an analysis of basement composition, structure and history, with the crustal radiogenic and mantle heat flow assessed separately. Resulting from an integrated, iterative interpretation and analysis of a wide range of publicly available spatially continuous geophysical and geological datasets, the heat flow model reproduces faithfully sharp and high-amplitude variations of the published heat flow at small distances. Variations are replicated through the integration of interpreted basement composition and a geologically driven determination of heat production within the radiogenic crustal layer. The values of mantle heat flow based on lithosphere thickness derived from seismic tomography models are consistent with published stable mantle heat flow under terranes of similar age. The long-wavelength regional variations can be attributed to the change in the thickness of the lithosphere. Regionally, the highest values of heat flow are found where radiogenic crust is the thickest and the composition is interpreted to comprise radiogenic intrusives.

U-Pb Geochronology of Apatite and Calcite at The Ernest Henry Deposit, NW Queensland; Implications for Hydrothermal Evolution and Ore Genesis

The Ernest Henry deposit represents the largest known Iron Oxide Copper Gold (IOCG) deposit in the Eastern Succession of the Mount Isa inlier. The orebody consists of a structurally controlled pipe-like breccia hosted in complexly altered Proterozoic volcanics with mineralization occurring post-peak metamorphism during a regional transpressional deformational event (D3). Ore formation was controlled by the mixing of magmatic, metamorphic and basal fluids, resulting in the precipitation of chalcopyrite, pyrite, calcite, quartz, magnetite and accessory gold. Coarse-grained apatite is present as an accessory mineral in areas of high sulphide mineralization and in shear zones adjacent to the orebody. The paragenesis and relative timing of the alteration and mineralisation stages have been well constrained by previous workers. However, advances in U-Pb geochronology via the in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) method means that for the first time apatite and calcite from the ore body and adjacent structures may be suitable for age dating. This project will use an established apatite dating technique and aims to develop a calcite dating technique to provide dates for individual paragenetic stages. These time constraints will improve the current understanding hydrothermal evolution and ore genesis at Ernest Henry. As calcite is a common accessory mineral in ore deposits, this technique could be widely implemented to date mineralization events and may allow hydrothermal events at different deposits to be linked.

Geology, Apatite Geochronology, and Geochemistry of the Ernest Henry Inter-lens: Implications for a Re-Examined Deposit Model

The Ernest Henry Iron-Oxide-Copper-Gold deposit is the largest known Cu-Au deposit in the Eastern Succession of the Proterozoic Mount Isa Inlier, NW Queensland. Cu-Au mineralization is hosted in a K-feldspar altered breccia, bounded by two major pre-mineralization shear zones. Previous research suggests that Cu-Au mineralization and the ore-bearing breccia formed simultaneously through an eruption style explosive/implosive event, facilitated by the mixing of fluids at ~1530 Ma. However, the preservation of a highly deformed, weakly mineralized, pre-mineralization feature (termed the Inter-lens) within the orebody indicates that this model must be re-examined. The paragenesis of the Inter-lens is broadly consistent with previous studies on the deposit, and consists of albitization; an apatite-calcite-quartz-garnet assemblage; biotite-magnetite ± garnet alteration; K-feldspar ± hornblende alteration; Cu-Au mineralization and post-mineralization alteration and veining. Apatite from the paragenetically early apatite-calcite-quartz-garnet assemblage produce U–Pb ages of 1584 ± 22 Ma and 1587 ± 22 Ma, suggesting that the formation of apatite, and the maximum age of the Inter-lens is synchronous with D2 deformation of the Isan Orogeny and regional peak-metamorphic conditions. Apatite rare earth element-depletion trends display: (1) a depletion in rare earth elements evenly, corresponding with an enrichment in arsenic and (2) a selective light rare earth element depletion. Exposure to an acidic NaCl and/or CaCl2-rich sedimentary-derived fluid is responsible for the selective light rare earth element-depletion trend, while the exposure to a neutral to alkaline S, Na-, and/or Ca-rich magmatic fluid resulted in the depletion of rare earth elements in apatite evenly, while producing an enrichment in arsenic. We suggest the deposit experienced at least two hydrothermal events, with the first event related to peak-metamorphism (~1585 Ma) and a subsequent event related to the emplacement of the nearby (~1530 Ma) Williams–Naraku Batholiths. Brecciation resulted from competency contrasts between ductile metasedimentary rocks of the Inter-lens and surrounding shear zones against the brittle metavolcanic rocks that comprise the ore-bearing breccia, providing permeable pathways for the subsequent ore-bearing fluids.

The Lawn Hill annulus: An Ordovician meteorite impact into water‐saturated dolomite

AbstractThe Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. 40Ar‐39Ar dating of impact‐related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low‐strength, porous, and water‐saturated, with all three properties affecting the crater morphology. The water‐saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid‐Ordovician (470 ± 6 Ma).

Raman spectroscopy enables the classification of molybdenite phases in the presence of carbonaceous material from Merlin Mo-Re deposit, Australia

A Raman band shift has been observed for the first time between the two key primary generations of molybdenite at an ore deposit. Samples were studied from Merlin, the world’s highest-grade molybdenum-rhenium (Mo-Re) deposit, which is primarily hosted by metamorphosed graphitic sediments belonging to the Mt. Isa Inlier, NW Queensland, Australia. Two molybdenite textural varieties occur here; the first (molybdenite 1) contains abundant sub-nano scale inclusions of carbonaceous material, whereas the second (molybdenite 2) is relatively inclusion free. Re is found to not vary between molybdenite 1 and 2 in any systematic way. Raman spectroscopy is one of the best analytical approaches that can be used to distinguish between two molybdenite varieties and detect the presence of graphitic and other carbonaceous material at Merlin.

Geochemical exploration for supergene copper oxide deposits, Mount Isa Inlier, NW Queensland, Australia

Extensive, variable, and generally thick regolith is a major challenge to mineral exploration in regolith-dominated terranes. The landscape of the Mount Isa Inlier, NW Queensland, Australia, is the product of several episodes of complex weathering, erosion and sedimentation. Weathering profiles were studied from more than 100 drill holes along three traverses crossing the Lady Annie, Mount Kelly and Anthill sulfide deposits to assess the nature, lateral extent and thickness of the supergene Cu oxide mineralization. A typical regolith profile consists, from the base up, of saprock, ferruginous saprolite (Cu-oxide zone), kaolinitic saprolite and silcrete bands (leached zone), and a pedolith of mottled, lateritic duricrust and soil zones. Oxidation of sulfides (primarily pyrite) in host rocks has created an acidic pH gradient, mobilizing Fe, Mn, Cu, Mo, Zn, Co, Ni, As, Bi, In, Sb, Be, P, U and Cd. The acidic pH is buffered by calcareous lithologies of the host rocks, particularly in the vicinity of the primary sulfides at Lady Annie and Mount Kelly resulting in formation of thick, mushroom-shaped Cu oxide halo, in which Cu may reach >15wt.%. The Cu values decrease to <500ppm in the bleached kaolinitic saprolite and silcrete. Conversely, siliceous bedrock lithologies at Anthill act as a weak buffer to the acidic fluids; thus, Fe, Cu and other mobile elements are laterally dispersed from the sulfide ore body and precipitated at different lithological boundaries where they are buffered by carbonates. Therefore, thick kaolinitic saprolite and silcrete are formed over sulfide mineralization, particularly along faults. Ferruginous duricrusts marginal to fault-controlled sulfide mineralization are anomalous in Cu, Zn, Cd, Sb and Be, and thus can be used as useful medium in geochemical exploration. Gold, Ag, Pb and As are residually concentrated in pedolith and kaolinitic saprolite over the primary sulfide mineralization. Thus, these elements can be used effectively as pathfinder elements, particularly in truncated landscape, where pedolith is eroded and the exposed kaolinitic saprolite is depleted in Cu. Ferruginous soil (850–300, 300–80 and <80μm size fractions), lag (>1.6 and >7.6mm) and termite mounds have been applied successfully as sampling media for Cu oxide exploration in Anthill. Soil and lag over sulfide mineralization are rich in Fe, As, Pb, Ag, Bi, In, V, U and Mo. The best response over sulfide mineralization is recorded in the >7.6mm ferruginous lag. However, broadest signature of well-defined Au, Bi and In anomalies are better detected using termite mounds than soil and lag. Successful use of soil in exploration for Cu oxide deposits in Mount Isa requires a genetic discrimination between residual and transported origin.

Anisotropy of Magnetic Susceptibility (AMS) and Paleomagnetism applied to the differentiation of structural and metallogenic controls on Iron Oxide Copper-Gold (IOCG) mineralization: a case study from Monakoff, NW Queensland

The temporal and metallogenic relationships between BIFs and IOCGs in the Mt Isa Block are controversial and difficult to assess quantitatively. In this study we examine new magnetic data, anisotropy of magnetic susceptibility (AMS) and remanent magnetisation to define potential structural controls on mineralisation, and try to determine temporal relationships between a barren hematite-magnetite BIF and a carbonate-hosted IOCG, Monakoff. The results showed that the BIF was magnetically isotropic, but that it had a high remanent component (Q=13). The remanent magnetisation has Dec= 238.4°, and Inc= -38.6°, which is offset from the present field and previous data from Cloncurry, so it likely formed during deposition, or early in the deformation history e.g., D1, D2. Conversely, the ore has very low remanent magnetisation (Q<0.2) but significant AMS, oriented Azimuth=225°, Plunge=75°, which is consistent with NE-SW (D3) shortening. 3-D magnetic modelling, constrained by magnetic property data and geophysical enhancements, showed that the BIF horizon formed a tight synform. However, modelling of the ore body showed it to be sub vertical. Based on the recognition that the ore body formed during D3 shortening, we infer that the ore formed in a zone of dilation sub-perpendicular to the shortening direction, rather than in a layer sub-parallel jog. The results suggest that the intersection of NW-oriented, sub-vertical faults with stratiform BIF horizons, adjacent to mafic volcanics are prospective for Monakoff-style IOCGs.

Ostracods (Crustacea) with soft part preservation from Miocene cave deposits of the Riversleigh World Heritage Area, NW Queensland, Australia

In an extensive ostracod assemblage recovered from the Bitesantennary Site, an Early Miocene (23–16 Ma) cave deposit within the Riversleigh World Heritage Area in north-west Queensland, Australia, 26 specimens were found to have soft part preservation. This is the first known example of soft part preservation in ostracods that were living in a palaeo-subterranean habitat. These exceptionally preserved specimens represent three new species, described herein: Heterocypris collaris sp. nov., Candonocypris fimibolus sp. nov. and Newnhamia mckenziana sp. nov. The appendages of a few of the specimens showed a highly detailed mode of preservation, typical of mineralization by Ca-phosphate (apatite). This preservation can be ascribed to an elevated supply of phosphate from guano derived from bats living in the cave. Two specimens feature an extraordinary mineral surface precipitation, partly embedding surface structures, such as fines setules on setae. After the decay or mechanical loss of the latter, only sharp imprints are left. Another specimen is characterized by a differential preservation of the ovaries and hepatopancreases, visible from outside the carapace as differences in coloration, a first in ostracod fossil preservation.

Phosphate amendment of metalliferous waste rocks, Century Pb–Zn mine, Australia: Laboratory and field trials

The aim of the study was to determine whether the application of phosphate compounds (phosphorite rock, phosphate fertilizer) to polyminerallic waste rocks can inhibit sulfide oxidation and metal mobility (Cu, Pb, Zn, Cd, Ni, Mn, Mg). Waste rocks comprised sulfidic carbonaceous shales and were sourced from the Century Pb–Zn mine, NW Queensland, Australia. The acid producing, Pb–Zn rich rocks consisted of major quartz, muscovite/illite, dolomite, siderite and kaolinite as well as smaller amounts of sulfide minerals (e.g. galena, sphalerite, pyrite). Laboratory leach experiments were conducted on finely granulated phosphate-treated waste rocks (>2 to <30 mm) over 13 weeks, whereas phosphate amendment of coarsely granulated waste rocks (sand to boulder size) was investigated using heap leach piles at the mine site over an 11 months period. Results of the laboratory experiments demonstrate that the treatment of finely granulated waste rocks with phosphorite rock produced leachates with near-neutral pH values due to calcite dissolution. This in turn did not allow the leaching of apatite, formation of secondary phosphate phases and phosphate stabilization to occur. Metal mobility in these amended wastes was restricted by the dissolution of calcite and the resultant near-neutral pH conditions. By contrast, the application of the water-soluble phosphate fertilizer MKP (KH2PO4) to polyminerallic sulfidic waste rocks during the short-term laboratory experiments led to the formation of phosphate coatings and precipitates and inhibited acid and metal release (Cd, Mn, Ni, Pb, Zn). At least in the short term, the application of phosphate fertilizers proved to be an effective method. However, results of the long-term field trials demonstrate that coarsely granulated waste rocks were not coated by secondary phosphate phases and that amendment by phosphorite rock or superphosphate fertilizer did not improve leachate quality compared to the unamended waste. Thus, phosphate stabilization appears ineffective in suppressing oxidation of sulfides in coarsely granulated mine wastes.

Mixed messages in iron oxide–copper–gold systems of the Cloncurry district, Australia: insights from PIXE analysis of halogens and copper in fluid inclusions

Proterozoic rocks of the Cloncurry district in NW Queensland, Australia, are host to giant (tens to hundreds of square kilometers) hydrothermal systems that include (1) barren regional sodic–calcic alteration, (2) granite-hosted hydrothermal complexes with magmatic–hydrothermal transition features, and (3) iron oxide–copper–gold (IOCG) deposits. Fluid inclusion microthermometry and proton-induced X-ray emission (PIXE) show that IOCG deposits and the granite-hosted hydrothermal complexes contain abundant high temperature, ultrasaline, complex multisolid (type 1) inclusions that are less common in the regional sodic–calcic alteration. The latter is characterized by lower salinity three-phase halite-bearing (type 2) and two-phase (type 3) aqueous inclusions. Copper contents of the type 1 inclusions (>300 ppm) is higher than in type 2 and 3 inclusions ( 1,000 ppm) are found both in the granite-hosted systems and in inclusions with Br/Cl ratios that are consistent with a magmatic source. The Br/Cl ratios of the inclusions with lower Cu contents are consistent with an evaporite-related origin. Wide ranges in salinity and homogenization temperatures for fluid inclusions in IOCG deposits and evidence for multiple fluid sources, as suggested by halogen ratios, indicate fluid mixing as an important process in IOCG genesis. The data support both leaching of Cu by voluminous nonmagmatic fluids from crustal rocks, as well as the direct exsolution of Cu-rich fluids from magmas. However, larger IOCG deposits may form from magmatic-derived fluids based on their higher Cu content.

Biogeochemistry of Pb–Zn gossans, northwest Queensland, Australia: Implications for mineral exploration and mine site rehabilitation

Abstract The concentration and distribution of metals were studied in metallophytes, growing on and in the vicinity of Pb–Zn gossans, NW Queensland. The study investigated the accumulation of metals in plant species and assessed their potential use as indicators in geobotanical and biogeochemical prospecting and as metal excluders in mine site rehabilitation. Plant species colonising the gossans tolerate high concentrations of metals. Total mean metal concentrations of soils ranged from minima of 14 ppm Cu, 28 ppm Pb and 34 ppm Zn in background areas to maxima of 660 ppm Cu, 12000 ppm Pb and 2100 ppm Zn over mineralised soils. Over the gossans, the grass species Eriachne mucronata forma, Enneapogon lindleyanus and Paraneurachne muelleri replace the characteristic grass Triodia molesta where the soils have high Pb and Zn concentrations. Of the 16 plant species identified, 3 of them, Hybanthus aurantiacus, Clerodendrum tomentosum and Bulbostylis barbata, were confined to the gossan sites. B. barbata appears to be of particular use in geobotanical prospecting as it indicates base metal mineralisation in the region. The biogeochemical analyses indicate significant enrichment of Cd, Pb and Zn in the tissue of all plant species, with the abundance of Cd, Pb and Zn in dried vegetation from the gossans being up to one order of magnitude above background. In particular, the uptake of Cd, Pb and Zn by Sida sp., T. molesta, Cleome viscosa and Eriachne mucronata forma increases linearly with DTPA-extractable soil metal concentrations. The biogeochemistry of these plants provides the best anomaly definition of the exposed gossans. Furthermore, the analysis of roots demonstrate that the plant species T. molesta, Eriachne mucronata forma and P. muelleri allow the transport of Cd, Pb and Zn from the roots to the above-ground biomass. Hence, the species best suited for biogeochemical prospecting for base metals in the region and semi-arid inland northern Australia are Eriachne mucronata and the genus Triodia. The analyses also reveal differences in the ability of each species to accumulate metals. Among all gossan plants, Eremophila latrobei displays distinctly low Pb concentrations and low correlations with soil DTPA extractable Pb. This plant is the most efficient in excluding Pb from its biomass, making it most suitable for the revegetation of Pb contaminated soils. This study demonstrates that biogeochemical examinations of gossans can reveal indicator and excluder plants, which are of potential use in mineral exploration as well as mined land reclamation.

Distribution of Gold in Hypogene Ore at the Ernest Henry Iron Oxide Copper-Gold Deposit, Cloncurry District, NW Queensland

Petrographic studies, assay records, and laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) analyses of major sulfides suggest nearly all of the gold at the Ernest Henry Cu-Au deposit occurs in the form of native gold or electrum (95–65 wt.% Au); sylvanite and dissolved gold in cobaltite, chalcopyrite, and pyrite make only minor contributions. LA-ICP-MS analyses of the gold contents of chalcopyrite gave from 8 to 139 ppb (mean = 56 ppb), more than an order of magnitude lower than those experimentally determined in chalcopyrite in equilibrium with native gold at the estimated mineralization temperatures (350°–450°C). Gold content is more variable in pyrite (1–400 ppb, mean = 77 ppb) and highest in narrow growth zones of a subordinate type of complexly zoned pyrite wherein it is associated with elevated As, Ag, Sn, Sb, Te, and Bi. Deposition of gold-electrum was closely associated with, but preceded some of the chalcopyrite deposition, as indicated by microtextures and lower bulk Au: Cu ratios in late-stage chalcopyrite-rich veins. Gold grains have maximum dimensions from <1 to 50 micrometers, and are heterogeneously distributed with most occurring in microscale clusters. Fifty-five out of 76 grains recorded in polished sections are in contact with both chalcopyrite and pyrite, and many of the rest are in sites where such a relationship could exist in the third dimension. The dominant textural sites are intergrowths with chalcopyrite in microveinlets cutting pyrite, and on faces of pyrite grains in contact with chalcopyrite. Scanning electron microscopy, electron microprobe analysis, and proton induced X-ray emission studies show that pyrite associated with gold includes complexly zoned, weakly zoned, arsenian, and arsenic-poor types. The textures are compatible with the possibility that most gold was deposited electrochemically as native metal on pyrite surfaces. The textural distribution of gold has important metallurgical implications given that the ore processing circuits at Ernest Henry and most other sulfide copper mines are designed to exclude pyrite (and arsenic) from concentrates.

Combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization: Application to copper deposits of the Mount Isa Inlier, NW Queensland, Australia

The clustering of mineral occurrences and their spatial associations with particular geological features are critical aspects of mineral distributions for exploration and understanding ore genesis. Variations in the degree of clustering of mineral occurrences or geological features can be measured by fractal dimensions, obtained from a shifting box counting method. Spatial associations between mineral occurrences and geological features can be quantified by the weights of evidence (WofE) method using the contrast value, which increases with the strength of the spatial relationship. A new method is proposed to evaluate mineral occurrence distributions by combining the power of fractal analysis of clustering with the WofE approach. The method compares the correlation between the variation in degree of clustering of mineral occurrences and a geological feature in a study area, with the contrast value of the same feature. The possible outcomes can be simplified into four scenarios, depending on whether the correlation in variation of clustering and the contrast are high or low, respectively. Each outcome has specific exploration implications. If either a high correlation in variation of clustering or a high contrast value is obtained, the geological feature can be used for exploration targeting. The integrated fractal and WofE approach is applied to copper occurrences in the Proterozoic Mount Isa Inlier, NW Queensland, Australia, which hosts large numbers of copper deposits (1,869 occurrences), including the world class Mount Isa copper deposit. Variation in clustering of copper occurrences has a positive correlation with variation in clustering of fault bends ( R = 0.823), fault intersections ( R = 0.862) and mafic rocks ( R = 0.885). WofE results indicate that the copper occurrences are spatially associated with fault intersections and bends and with mafic rocks. Analyses were carried out separately for the two major lithostratigraphic sequences in the Inlier, the Eastern and Western Successions. The Western Succession copper occurrences are apparently more clustered than those of the Eastern Succession, which may reflect a lower degree of exploration and/or geological factors. The association of copper occurrences with mafic rocks compared with fault bends and intersections is greater in the Eastern Succession, which may reflect genetic factors. Correlations in the variation of clustering of mineral occurrences and geological features have a linear relationship with the contrast values, and the spatial association between all geological features and copper occurrences constitute high correlation/high contrast cases. The linear relationship suggests that the geological features that control the clustering of the copper occurrences could be the same features that control their localization.