Search

Proximal and distal garnets from skarns at Jiaduobule, Tibet, are used to demonstrate how variation in the fluid composition and parameters such as salinity, pH, fO.sub.2, and [Formula omitted] will impact on rare earth element (REE) distribution in garnets, and also to constrain skarn evolution across the orefield from proximal (Fe mineralization) to distal (Cu mineralization). These garnets display a diversity from proximal to distal skarn which is expressed in mineral assemblages, textures, major to trace element contents, and particularly, chondrite-normalized REE fractionation trends. The empirical variation among REE fractionation trends, determined from laser ablation inductively coupled-plasma mass spectrometry data, can be numerically modelled in terms of variable fluid compositions and physicochemical parameters, among which the key determining factors are salinity, pH, [Formula omitted] and Ca content buffered from the rock-fluid reaction with carbonate rocks. Modelling REE trends in skarn garnet is shown to be valuable for constraining conditions during garnet formation and a useful tool for monitoring the evolution of complex skarn deposits., Author(s): Jing Xu [sup.1] [sup.2] [sup.3], Cristiana L. Ciobanu [sup.3], Nigel J. Cook [sup.3], Youye Zheng [sup.1], Xiaofeng Li [sup.2] [sup.4], Benjamin P. Wade [sup.5], Max R. Verdugo-Ihl [sup.3], Wenyuan [...]

The Mount Weld rare earth element (REE) deposit, Western Australia, is one of the largest of its type on Earth. Current mining exploits the high-grade weathered goethite-bearing resource that lies above, and which represents the weathering product of a subjacent carbonatite. The mineralogy, petrography, deportment of lanthanides among the different components, and variation in mineral speciation, textures, and chemistry are examined. Microanalysis, involving scanning electron microscope (SEM) imaging, electron probe microanalysis (EPMA) and laser ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS), was conducted on sized fractions of three crushed and ground laterite ore samples from current and planned production, and a representative sample from the underlying carbonatite. High-magnification imaging of particles in laterite samples show that individual REE-bearing phases are fine-grained and extend in size well below the micron-scale. Nanoscale inclusions of REE-phosphates are observed in apatite, Fe-(Mn)-(hydr)oxides, and quartz, among others. These have the appearance, particularly in fluorapatite, of pervasive, ultrafine dusty domains. Apart from the discrete REE minerals and abundant nano- to micron-scale inclusions in gangue, all ore components analysed by LA-ICP-MS contain trace to minor levels of REEs within their structures. This includes apatite, where low levels of REE are confirmed in preserved igneous apatite, but also Fe- and Mn-(hydr)oxides in which concentrations of hundreds, even thousands of ppm are measured. This is significant given that Fe-(Mn)-(hydr)oxides are the most abundant component of the laterite and points to extensive mobility and redistribution of REEs, and especially HREE, during progressive lateritisation. Late-formed minerals, notably tiny grains of cerianite, reflect a shift to oxidising conditions. REE-fluorocarbonates are the main host for REEs in carbonatite and are systematically replaced by hydrated, Ca-bearing REE-phosphates (largely rhabdophane). The latter displays varied compositions but is characteristically enriched in HREE relative to monazite in the same sample. Fine-grained, compositionally heterogeneous rhabdophane is accompanied by minor amounts of other paragenetically late, hydrated phosphates with enhanced MREE/HREE relative to LREE (although still LREE-dominant). Minor, relict xenotime and zircon are significant HREE carriers. Ilmenite and pyrochlore group members contain REE but contribute only negligibly to the overall REE budget. Although the proportions of individual mineral species differ, the chemistry of key ore components are similar in different laterite samples from the current resource. Mineral signatures are, however, subtly different in the lower grade southeastern part of the deposit, including higher concentrations of HREE relative to LREE in monazite, rhabdophane, florencite and Fe-(Mn)-(hydr)oxides. [ABSTRACT FROM AUTHOR]

Trace element signatures in apatite are used to study hydrothermal processes due to the ability of this mineral to chemically record and preserve the impact of individual hydrothermal events. Interpretation of rare earth element (REE)-signatures in hydrothermal apatite can be complex due to not only evolving fO2, fS2 and fluid composition, but also to variety of different REE-complexes (Cl-, F-, P-, SO4, CO3, oxide, OH- etc.) in hydrothermal fluid, and the significant differences in solubility and stability that these complexes exhibit. This contribution applies numerical modeling to evolving REE-signatures in apatite within the Olympic Dam iron-oxide-copper-gold deposit, South Australia with the aim of constraining fluid evolution. The REE-signatures of three unique types of apatite from hydrothermal assemblages that crystallized under partially constrained conditions have been numerically modeled, and the partitioning coefficients between apatite and fluid calculated in each case. Results of these calculations replicate the measured data well and show a transition from early light rare earth element (LREE)- to later middle rare earth element (MREE)-enriched apatite, which can be achieved by an evolution in the proportions of different REE-complexes. Modeling also efficiently explains the switch from REE-signatures with negative to positive Eu-anomalies. REE transport in hydrothermal fluids at Olympic Dam is attributed to REE--chloride complexes, thus explaining both the LREE-enriched character of the deposit and the relatively LREE-depleted nature of later generations of apatite. REE deposition may, however, have been induced by a weakening of REE--Cl activity and subsequent REE complexation with fluoride species. The conspicuous positive Eu-anomalies displayed by later apatite with are attributed to crystallization from high pH fluids characterized by the presence of Eu3+ species. [ABSTRACT FROM AUTHOR]

The Middleback Ranges is a major iron ore belt in the southeastern region of the Gawler Craton, South Australia, interpreted to be of BIF origin. Iron ore deposits are hosted within ∼2550 Ma metasedimentary rocks of the Middleback Group and occur as a series of N-S trending hills, forming a ∼60 km-long magnetic anomaly. A petrographic-geochemical study of iron-oxides from BIFs and iron ores was undertaken on samples from thirteen locations spanning the strike of the belt. Iron-oxides are texturally diverse due to multiple processes accompanying and postdating ore formation. Primary magnetite features preserved in the southern segment of the belt display distinct overprinting features (e.g., increased porosity, reworked grain boundaries) and multiple generations of growth associated with deposition of trace minerals, including native gold. Northwards along strike, this overprint is expressed by the pseudomorphic replacement of magnetite by hematite (martite) and is locally associated with brecciation, the presence of rare earth element (REE)-minerals, and replacement by iron-hydroxides. Whereas evidence of microplaty hematite accompanying martitization and predating iron-hydroxides is observed throughout the belt, distinct iron-oxide generations postdating the iron-hydroxides are inferred based on compositional zoning with respect to Si and intimate relationships between iron-oxides and -hydroxides. Chondrite-normalized fractionation trends obtained from the various iron-oxides and from different BIF types generally display LREE-enrichment and distinct positive Eu- and Y-anomalies. An increasing ΣREY- and LREE-trend accompanies martitization. The presence of specific element groups, e.g., granitophile elements (U, W, Sn, Mo), or transition metals (Cr, Mn, Co, Ni, Ti, V, Nb) within the lattice of iron-oxides suggests their formation in evolving environments associated with the emplacement of felsic and mafic lithologies, respectively. The impact of local setting on iron-oxide formation is highlighted by complex trace element signatures of iron-oxides; the northern segment of the belt is relatively enriched in As and Sb, while the southern segment is enriched in granitophile elements and REY. Further complexity is shown in the local variation of Mn and Zn in iron-oxides in the southern segment of the belt, where the Iron Magnet deposit shows the strongest correlation between the two elements. Various depositional settings for BIFs are inferred based upon Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides and various water types. These settings include environments where iron-minerals precipitate from mixtures of 1) anoxic seawater and high-temperature hydrothermal fluids, 2) anoxic seawater and low-temperature hydrothermal fluids, and 3) oxygen-richer seawater and low-temperature hydrothermal fluids. A genetic model for ore formation is proposed based upon textural and compositional variations observed in iron-oxides throughout the belt and includes formation resulting from supergene fluids rich in elements leached from local granites penetrating BIF, interaction with granite-derived hydrothermal fluids, and heat generated during emplacement of younger dikes. Recognition of petrographic features linked to changes in composition demonstrates the utility of iron-oxides to trace iron ore formation in a temporal and spatial context, with implications for ore genesis and models of mineral exploration. [ABSTRACT FROM AUTHOR]

Atomic-scale high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and electron diffractions are used to address the complexity of lattice-scale intergrowths of REE-fluorocarbonates from an occurrence adjacent to the Olympic Dam deposit, South Australia. The aims are to define the species present within the intergrowths and also assess the value of the HAADF STEM technique in resolving stacking sequences within mixed-layer compounds. Results provide insights into the definition of species and crystal-structural modularity. Lattice-scale intergrowths account for the compositional range between bastnäsite and parasite, as measured by electron probe microanalysis (at the µm-scale throughout the entire area of the intergrowths). These comprise rhythmic intervals of parisite and bastnäsite, or stacking sequences with gradational changes in the slab stacking between B, BBS and BS types (B--bastnäsite, S--synchysite). An additional occurrence of an unnamed B2S phase [CaCe3(CO3)4F3], up to 11 unit cells in width, is identified among sequences of parisite and bastnäsite within the studied lamellar intergrowths. Both B2S and associated parisite show hexagonal lattices, interpreted as 2H polytypes with c = 28 and 38 Å, respectively. 2H parisite is a new, short hexagonal polytype that can be added to the 14 previously reported polytypes (both hexagonal and rhombohedral) for this mineral. The correlation between satellite reflections and the number of layers along the stacking direction (c*) can be written empirically as: Nsat = [(m × 2) + (n × 4)] − 1 for all BmSn compounds with S ≠ 0. The present study shows intergrowths characterised by short-range stacking disorder and coherent changes in stacking along perpendicular directions. Knowing that the same compositional range can be expressed as long-period stacking compounds in the group, the present intergrowths are interpreted as being related to disequilibrium crystallisation followed by replacement. HAADF STEM imaging is found to be efficient for depiction of stacking sequences and their changes in mixed-layer compounds, particularly those in which heavy atoms, such as rare-earth elements, are essential components. [ABSTRACT FROM AUTHOR]

Olympic Dam is a world-class breccia-hosted iron-oxide copper-gold-uranium ore deposit located in the Gawler Craton, South Australia. It contains elevated concentrations of rare earth elements (REE) which occur as the REE minerals bastnäsite, synchysite, florencite, monazite, and xenotime. This is the first study to focus on the mineralogy and composition of the most abundant REE mineral at Olympic Dam, bastnäsite, and subordinate synchysite. The sample suite extends across the deposit and represents different sulfide mineralization styles (chalcopyrite-bornite and bornite-chalcocite) and breccias of various types, ranging from those dominated by clasts of granite, dykes, and hematite. The REE-fluorocarbonates (bastnäsite and synchysite) typically occur as fine-grained (<50 µm) disseminations in Cu-Fe-sulfides and gangue minerals, and also within breccia matrix. They are also locally concentrated within macroscopic REE-mineral-rich pockets at various locations across the deposit. Such coarse-grained samples formed the primary target of this study. Three general textural groups of bastnäsite are recognized: matrix (further divided into disseminated, fine-grained, and stubby types), irregular (sulfide-associated), and clast replacement. Textures are largely driven by the specific location and prevailing mineral assemblage, with morphology and grain size often controlled by the associated minerals (hematite, sulfides). Major element concentration data reveal limited compositional variation among the REE-fluorocarbonates; all are Ce-dominant. Subtle compositional differences among REE-fluorocarbonates define a spectrum from relatively La-enriched to (Ce + Nd)-enriched phases. Granite-derived hydrothermal fluids were the likely source of F in the REE-fluorocarbonates, as well as some of the CO2, which may also have been contributed by associated mafic-ultramafic magmatism. However, transport of REE by Cl-ligands is the most likely scenario. Stubby bastnäsite and synchysite may have formed earlier, coincident with hydrothermal alteration of granite releasing Ca from feldspars. Other categories of bastnäsite, notably those co-existing with sulfides, and reaching the top of the IOCG mineralization at Olympic Dam (chalcocite + bornite zone) are relatively younger. Such an interpretation is concordant with subtle changes in the REE patterns for the different categories. The common association of bastnäsite and fluorite throughout the deposit is typical of the hematite breccias and can be deposited from neutral, slightly acidic fluids (sericite stability) at T * 300 °C. [ABSTRACT FROM AUTHOR]

Apatite is a common magmatic accessory in the intrusive rocks hosting the giant ∼1590Ma OlympicDam(OD) iron-oxide copper gold (IOCG) ore system, SouthAustralia. Moreover, hydrothermal apatite is a locally abundant mineral throughout the altered and mineralized rocks within and enclosing the deposit. Based on compositional data for zoned apatite, we evaluate whether changes in the morphology and the rare earth element and Y (REY) chemistry of apatite can be used to constrain the fluid evolution from early to late hydrothermal stages at OD. The ∼1.6 Ga Roxby Downs granite (RDG), host to the OD deposit, contains apatite as a magmatic accessory, locally in the high concentrations associated with mafic enclaves. Magmatic apatite commonly contains REY-poor cores and REY-enriched margins. The cores display a light rare earth element (LREE)-enriched chondrite-normalized fractionation pattern with a strong negative Eu anomaly. In contrast, later hydrothermal apatite, confined to samples where magmatic apatite has been obliterated due to advanced hematite-sericite alteration, displays a conspicuous, convex, middle rare earth element (MREE)-enriched pattern with a weak negative Eu anomaly. Such grains contain abundant inclusions of florencite and sericite. Within high-grade bornite ores from the deposit, apatite displays an extremely highly MREE-enriched chondrite-normalized fractionation trend with a positive Eu anomaly. Concentrations of U and Th in apatite mimic the behaviour of ΣREY and are richest in magmatic apatite hosted by RDG and the hydrothermal rims surrounding them. The shift from characteristic LREE-enriched magmatic and early hydrothermal apatite to later hydrothermal apatite displaying markedMREE-enriched trends (with lower U, Th, Pb and ΣREY concentrations) reflects the magmatic to hydrothermal transition. Additionally, the strong positive Eu anomaly in theMREE-enriched trends of apatite in high-grade bornite ores are attributable to alkaline fluid conditions. [ABSTRACT FROM AUTHOR]

Trace element concentrations, particularly rare earth elements and yttrium (REY) in feldspars and accessory minerals, have been determined in a suite of albitized igneous, metasedimentary and metasomatite rocks from the Moonta-Wallaroo district, Olympic Cu–Au Province, South Australia. Results show that changes in REY-fractionation trends and concentrations in feldspars and common accessories are associated with key textures in albite-bearing associations from different lithologies. In granitic rocks, pseudomorphic replacement of pre-existing feldspars is typified by porous albite with cleavage-oriented intergrowths of sericite and pore-attached hematite. These observations are comparable with albitization features of granitic terranes elsewhere. A mineral association (albite-sericite ± chlorite), similar to that from granitoids, is observed as pervasive spots in limestone, inferring prograde skarnoid reactions at low fluid/rock ratio in an impure carbonate. In metasedimentary and metasomatite rocks with comparable Na 2 O content (~ 5–6 wt.%), fine-grained granoblastic albite suggests growth under high fluid/rock ratios irrespective of lithology. In such cases, albite with the highest REY content (ΣREY ~ 200 ppm) accounts for the entire REY budget, e.g., in albite–biotite-schist with the lowest abundance of accessory minerals. Nanoscale investigation confirms this albite to be a REY carrier (elements incorporated within the crystal lattice); no pore-attached inclusions are observed. In contrast, albite with the lowest REY-concentration (~ 14 ppm) is encountered in the metasomatite. In such rocks, recording the highest ΣREY (~ 1000 ppm) in whole-rock, partitioning of REY is favoured among the abundant accessories (titanite, apatite) and calc-silicates (actinolite, clinozoisite) rather than albite. Comparable low-REY albite is also found in granitoid-derived albitite (Na 2 O ~ 5 wt.%), in which abundant accessories and discrete REY-minerals formed during albitization account for the high ΣREY content (~ 700 ppm) in whole rock. The role of coupled dissolution–reprecipitation reactions (CDRR) is critical for REY (re)distribution within albitized igneous rocks, where REY-release from early magmatic accessories and/or feldspars assists REY-enrichment into late albite. The presence of abundant nanopore-attached inclusions in plagioclase demonstrates the nanoscale nature of CDRR-driven albitization in granitoids, consistent with published experimental work on altered granites. Such porosity offers sites for REY entrapment seen within discrete REY-minerals in new-formed K-feldspar. Similarly, release and uptake of REY, concurrent with albitization, is seen in formation of coarser REY-minerals (xenotime, bastnäsite, synchysite) during CDRR-driven replacement of accessory Fe–Ti-oxides by symplectites of chlorite and hematite. Based on the differences identified between the albitization pathways in igneous and metasedimentary rocks, we discuss how albitization proceeds via a series of complex fluid–mineral reactions, each involving the redistribution, accumulation and retention of REY. These reactions are critical for defining the endowment and deportment of REY in rocks that have undergone sodic alteration. Contrary to previous models, albitization appears controlled by pH rather than redox conditions. Despite regional differences in local geological environment and alteration style across the Olympic Cu–Au Province, albitization, the initiation of hydrothermal alteration, is a pre-requisite stage for REY-enrichment in Iron-Oxide–Copper–Gold (IOCG) systems. REY distribution patterns in feldspars may thus have value in mineral exploration as criteria enabling alteration associated with mineralization to be distinguished from the regional background. Strong albitization without superposition of later potassic alteration may not, however, be automatically linked [ABSTRACT FROM AUTHOR]

The Hillside Cu–(Au) deposit, Yorke Peninsula, South Australia, is a recently-discovered ore system within the 1.6Ga World-class Olympic iron oxide–copper–gold (IOCG) Province. The deposit is characterized by a skarn-style alteration zone. Analyses of feldspar, calcite, skarn minerals (garnet, pyroxene, clinozoisite and actinolite) and accessories (titanite, apatite and allanite), and grain-scale element mapping by laser-ablation inductively-coupled plasma mass spectrometry are used to assess the distributions of rare earth element (REE), incompatible and ore-forming elements in host rocks, prograde and retrograde skarn. Garnet is a major repository of HREE, especially in prograde skarn, whereas LREE-enriched clinozoisite is the principal REE-host in retrograde skarn. REE distribution patterns define a pronounced partitioning of elements among the dominant coexisting minerals. Compositional variation between assemblages, and also within individual grains, defines an evolution from early feldspar–pyroxene skarn through main-stage calcic skarn to the ore-stage. A switch from a prograde, HREE-dominant signature to a LREE-enriched signature is observed in both retrograde and distal skarn. Zr-in-titanite geothermometry supports transition from magmatic to hydrothermal, skarn-forming processes at temperatures of ~660°C; the initiation of ore-stage is about 100°C lower. Understanding REE distributions in all minerals within a complex, multistage ore system assists the development of vectoring tools that use trace element chemistry in exploration for similar IOCG deposits beneath regolith cover across the Olympic Province. Titanite and apatite show particular promise because of their characteristically distinct REE patterns in magmatic and hydrothermal stages, trace element responses to redox changes, and their widespread abundance throughout different lithologies in the area. [ABSTRACT FROM AUTHOR]

Abstract: ‘Green energy futures’ are driving unprecedented demand for Rare Earth Elements (REE), underpinning significant exploration activity worldwide. Understanding how economic REE concentrations form is critical for development of exploration models. REE mineralisation in the Browns Ranges, Gordon Downs Region, Western Australia, comprises xenotime-dominant mineralisation hosted within Archaean to Palaeoproterozoic metasedimentary units (Browns Range Metamorphics). Mineralogical, petrographic and mineral–chemical investigation, including trace element analysis by Laser-Ablation Inductively-Coupled Plasma Mass Spectroscopy, gives insights into the mineralogical distribution and partitioning of REE, and also provides evidence for the genetic evolution of the Browns Range REE mineralisation via a succession of hydrothermal processes. Two main REE-bearing minerals are identified: xenotime [(Y,REE)PO4], which is HREE selective; and subordinate florencite [(REEAl3(PO4)2(OH)6] which is LREE selective. Two morphological generations of xenotime are recognised; compositions are however consistent. Xenotime contains Dy (up to 6.5wt.%), Er (up to 4.35wt.%), Gd (up to 7.56wt.%), Yb (up to 4.65wt.%) and Y (up to 43.3wt.%). Laser Ablation ICP–MS element mapping revealed a subtle compositional zoning in some xenotime grains. LREE appear concentrated in the grain cores or closest to the initial point of growth whereas HREE, particularly Tm, Yb and Lu, are highest at the outer margins of the grains. The HREE enrichment at the outer margins is mimicked by As, Sc, V, Sr, U, Th and radiogenic Pb. Florencite is commonly zoned and contains Ce (up to 11.54wt.%), Nd (up to 10.05wt.%) and La (up to 5.40wt.%) and is also notably enriched in Sr (up to 11.63wt.%) and Ca. Zircon (which is not a significant contributor of REEs overall due to its low abundance in the rocks) is also enriched in REE (up to 13wt.% ΣREE) and is the principal host of Sc (up to 0.8wt.%). Early, coarse euhedral xenotime has undergone fracturing, partial breakdown and replacement by florencite. Second generation xenotime occurs as abundant small blades commonly associated with acicular hematite. Mineralization is attributed to percolation of a volatile-rich, acidic fluid, possibly granite-derived, through porous arkose units. Late hematite may suggest mixing with meteoric water and subsequent oxidation. Field observations suggest that faults acted as fluid conduits and that brecciation, possibly associated with release of volatiles from the fluid, occurred along these faults. The data provide valuable constraints on chemical compositional trends in xenotime and coexisting minerals. Given the current surge in exploration for REE, this information will assist in the development of exploration models for comparable terranes. [Copyright &y& Elsevier]

Minerals form in all types of chemical and physical environments. Such glimpses of the ore-forming process can be recorded by the incorporation and release of trace elements from host minerals, the degree of order/disorder in mixed-layer compounds or complex sulphides, or the distribution of nanometer-scale inclusions relative to nanopores or reaction boundaries. We would single out the work of Yin et al. [[22]], who have demonstrated the important role of mineral nanoparticles at fluid-mineral interfaces during trace-element uptake in hydrothermal systems, which is a topic also discussed by several papers in this special issue. 5 Cook N.J., Ciobanu C.L., George L., Zhu Z.-Y., Wade B., Ehrig K. Trace Element Analysis of Minerals in Magmatic-Hydrothermal Ores by Laser Ablation Inductively-Coupled Plasma Mass Spectrometry: Approaches and Opportunities. [Extracted from the article]

Scheelite (CaWO4) and powellite (CaMoO4) are isostructural minerals considered as a non-ideal solid solution series. Micron- to nanoscale investigation of a specimen of skarnoid from Zhibula, Gangdese Belt, Tibet, China, was carried out to assess the identity of the phases within a broad scheelite-powellite (Sch-Pow) compositional range, and to place additional constraints on redox changes during ore formation. An electron probe microanalysis shows that Mo-rich domains within complex oscillatory-zoned single crystals, and as thin sliver-like domains, have a compositional range from 20 mol.% to 80 mol.% Pow. These occur within a matrix of unzoned, close-to-end-member scheelite aggregates (87 mol.%–95 mol.% Sch). Laser-ablation inductively coupled plasma mass spectrometry spot analysis and element mapping reveal systematic partitioning behaviour of trace elements in skarn minerals (grossular50, diopside80, anorthite, and retrograde clinozoisite) and scheelite-powellite aggregates. The Mo-rich domains feature higher concentrations of As, Nb, and light rare earth elements LREE, whereas W-rich domains are comparatively enriched in Y and Sr. Transmission electron microscopy (TEM) was carried out on focused-ion-beam-prepared foils extracted in situ from domains with oscillatory zoning occurring as slivers of 20 mol.%–40 mol.% Pow and 48 mol.%–80 mol.% Pow composition within an unzoned low-Mo matrix (20 mol.% Pow). Electron diffractions, high-angular annular dark field (HAADF) scanning-TEM (STEM) imaging, and energy-dispersive spectroscopy STEM mapping show chemical oscillatory zoning with interfaces that have continuity in crystal orientation throughout each defined structure, zoned grain or sliver. Non-linear thermodynamics likely govern the patterning and presence of compositionally and texturally distinct domains, in agreement with a non-ideal solid solution. We show that the sharpest compositional contrasts are also recognisable by variation in growth direction. Atomic-scale resolution imaging and STEM simulation confirm the presence of scheelite-powellite within the analysed range (20 mol.%–80 mol.% Pow). Xenotime-(Y) inclusions occur as nm-wide needles with epitaxial orientation to the host scheelite-powellite matrix throughout both types of patterns, but no discrete Mo- or W-bearing inclusions are observed. The observed geochemical and petrographic features can be reconciled with a redox model involving prograde deposition of a scheelite+molybdenite assemblage (reduced), followed by interaction with low-T fluids, leading to molybdenite dissolution and reprecipitation of Mo as powellite-rich domains (retrograde stage, oxidised). The observation of nanoscale inclusions of xenotime-(Y) within scheelite carries implications for the meaningful interpretation of petrogenesis based on rare earth element (REE) concentrations and fractionation patterns. This research demonstrates that HAADF-STEM is a versatile technique to address issues of solid solution and compositional heterogeneity. [ABSTRACT FROM AUTHOR]

Many analytical techniques for trace element analysis are available to the geochemist and geometallurgist to understand and, ideally, quantify the distribution of trace and minor components in a mineral deposit. Bulk trace element data are useful, but do not provide information regarding specific host minerals—or lack thereof, in cases of surface adherence or fracture fill—for each element. The CAMECA nanoscale secondary ion mass spectrometer (nanoSIMS) 50 and 50L instruments feature ultra-low minimum detection limits (to parts-per-billion) and sub-micron spatial resolution, a combination not found in any other analytical platform. Using ore and copper concentrate samples from the Olympic Dam mining-processing operation, South Australia, we demonstrate the application of nanoSIMS to understand the mineralogical distribution of potential by-product and detrimental elements. Results show previously undetected mineral host assemblages and elemental associations, providing geochemists with insight into mineral formation and elemental remobilization—and metallurgists with critical information necessary for optimizing ore processing techniques. Gold and Te may be seen associated with brannerite, and Ag prefers chalcocite over bornite. Rare earth elements may be found in trace quantities in fluorapatite and fluorite, which may report to final concentrates as entrained liberated or gangue-sulfide composite particles. Selenium, As, and Te reside in sulfides, commonly in association with Pb, Bi, Ag, and Au. Radionuclide daughters of the 238U decay chain may be located using nanoSIMS, providing critical information on these trace components that is unavailable using other microanalytical techniques. These radionuclides are observed in many minerals but seem particularly enriched in uranium minerals, some phosphates and sulfates, and within high surface area minerals. The nanoSIMS has proven a valuable tool in determining the spatial distribution of trace elements and isotopes in fine-grained copper ore, providing researchers with crucial evidence needed to answer questions of ore formation, ore alteration, and ore processing. [ABSTRACT FROM AUTHOR]

The Wirrda Well and Acropolis prospects are located ~ 25 km SSE and SW, respectively, from the giant Olympic Dam Cu-U-Au-Ag deposit within the Olympic Cu-Au Province of South Australia. Mineralisation in the two prospects displays a temporal and spatial zonation characteristic of other IOCG-type deposits and prospects within the province, including Olympic Dam, in which an early high-temperature, magnetite-dominant mineralisation is transitional to a hematite-dominant mineralisation style, accompanied by subordinate and often localised carbonate alteration. Both prospects contain conspicuous hydrothermal apatite which is characteristic in terms of host assemblage and geochemical signature. Chondrite-normalised rare earth element (REE) fractionation patterns for apatite depict the evolution of hydrothermal fluids during ore formation. An early generation of apatite is abundant within magnetite-dominant mineralisation in both prospects, and displays a characteristic light-REE enriched fractionation trend. Overprinting of this initial high-temperature assemblage results in the loss of REE and Y (REY), as well as Cl, along fractures within this apatite, as well as the formation of new generations of apatite that are also depleted in these elements. The transition from early reduced, to later oxidised assemblages within both prospects is accompanied by an evolution of the REY signature of apatite in which LREE-enriched patters with negative Eu-anomalies are replaced by convex middle-REE (MREE)-enriched patterns, positive Eu-anomalies and the development of a conspicuous negative Y-anomaly. Comparable trends are recognised elsewhere within the district and are interpreted as the hallmark of productive mineralised IOCG ore systems. The evolution of chondrite-normalised REY patterns can be explained in terms of changes in fluid parameters, speciation of REY in ore-forming fluids, and the capacity of REY to precipitate and partition into apatite in ways that contrast with those expected by simple consideration of crystal structure. Results are concordant with modelling of REY-speciation, which show that, under hydrothermal conditions typical of IOCG mineralisation, a decrease in salinity, pH and temperature is associated with hematite-sericite alteration sufficient to produce MREE-enriched apatite. These data offer encouragement for the use of apatite geochemistry in mineral exploration within the Olympic Cu-Au Province, and potentially in analogous terranes elsewhere, given the clear association between MREE-enriched apatite and often well-mineralised, hematite-dominant domains within these large IOCG systems. [ABSTRACT FROM AUTHOR]

[Display omitted] Several Pb–Zn deposits in the Nyainqêntanglha metallogenic belt have unique features whereby either the causative pluton is not exposed, or there is evidence for complex multistage and multi-type ore-forming processes. These scenarios are typified by the Lawu and Yaguila Pb–Zn skarn deposits, respectively. In this contribution, we selected skarn garnets from these two deposits as the research objects and utilized LA–ICP–MS to measure the U–Pb isotope and trace element compositions. Our new garnet U–Pb data, combined with published age constraints on Pb–Zn mineralization in this belt, suggest a magmatic-hydrothermal origin for corresponding mineralization, with the causative magma likely derived from partial melting of ancient continental material due to rollback and subsequent breakoff of the Neo-Tethys oceanic slab during India–Asia continental collision. • The Lawu Pb–Zn skarn mineralization event is constrained at 54.6 ± 2.9 Ma. • Garnet U–Pb ages for the Yaguila Pb–Zn skarn are older: 65.0 ± 4.7 ─ 68.5 ± 3.4 Ma. • Garnet U–Pb geochronology support a magmatic-hydrothermal origin for Pb–Zn skarns. • Reliable garnet U–Pb dating underpins genetic models for Pb–Zn skarns. Lawu and Yaguila are two Pb–Zn skarn deposits in the eastern Nyainqêntanglha metallogenic belt, central Lhasa subterrane, Tibet. The genesis of Pb–Zn deposits in this belt and the crustal processes leading to their formation remain ill-constrained, mostly because of the lack of precise mineralization ages. We integrate textural information, geochemistry, and in-situ garnet U–Pb geochronology to constrain the timing and genesis of Pb–Zn mineralization. Garnets from the two deposits display oscillatory compositional zoning (And 14 Gr 86 to And 100) and contain variable U contents (0.07–5.3 ppm). Aluminum-rich garnet displays a chondrite-normalized rare earth element (REE) fractionation pattern in which LREEs are depleted relative to flattish HREE segments. In contrast, Fe-rich garnet shows LREE-enriched, relatively HREE-poor patterns with positive Eu-anomalies. Uranium concentration is correlated with total REE and Fe components in garnets, implying that U incorporation is largely controlled by coupled substitution mechanisms. The same garnets also contain measurable contents of other metals (up to hundreds of ppm), such as Sn, W, and In. The distribution and fractionation of major and trace elements in zoned garnets record periodic fluid pluses with different compositions during hydraulic fracturing. The new garnet U–Pb data show that the Yaguila deposit formed between 68.5 ± 3.4 and 65.0 ± 4.7 Ma, and the Lawu deposit formed at 54.6 ± 2.9 Ma. Within the geochronological framework of igneous rocks and Pb–Zn mineralization in the Nyainqêntanglha metallogenic belt, the new garnet ages suggest a magmatic-hydrothermal origin for related mineralization, with the causative magma likely derived from partial melting of ancient continental material due to rollback and subsequent breakoff of the Neo-Tethys oceanic slab during India–Asia continental collision. This study highlights the opportunities offered by garnet U–Pb dating for elucidating the formation age and ore genesis of base metal skarn systems. [ABSTRACT FROM AUTHOR]

The study provides an opportunity to examine the geochemistry of scheelite from porphyry-skarn systems. For the first time, three distinct groups of scheelite are recognized in the same W-Mo porphyry-skarn deposit. Each shows different geochemical features in terms of (87Sr/86Sr) i , Mo, La, LREEs, MREEs, and HREEs. After emplacement of porphyry rocks, the initial W-Mo rich fluids derived from unconsolidated magma mixed with formation waters. During this process (M1), the initial fluids inherited the REE patterns with slight enrichment from magma and P-group scheelite was precipitated. This type of scheelite has a higher (87Sr/86Sr) i of 0.70893–0.71082, strong enrichment in Mo, and depletion in Ba, Ta, and U compared to the porphyry rocks. Fluids then migrated from the porphyry, infiltrating biotite or quartz hornfels and precipitating H-group scheelite. During this process (M2), fluids interacted with hornfels and mixed with formation waters, which have higher Sr-isotope compositions. At the contact zone between porphyry rocks and Cambrian limestone, skarn ores were formed. During this process (M3), mixing took place between magma-derived fluids and formation waters and interacted with limestones, allowing precipitation of S-group scheelite. This type of scheelite precipitated at an oxidized stage after precipitation of early skarn minerals which scavenged REE (especially HREE) from fluid. • Three groups of scheelite are recognized in the Gaojiabang W-Mo deposit. • Fluid mixing and fluid-rock interaction play important roles during ore formation. • REE fractionation in scheelite is influenced by evolving fluids and redox states. • Redox state of fluids is a key factor controlling Mo behavior in scheelite. • Scheelite geochemistry fingerprints genetically different types of metal deposits. Scheelite (CaWO 4), a hydrothermal mineral commonly displaying enrichment in Mo (up to 16%) and/or rare earth elements (REE), is the main economic mineral in the Gaojiabang porphyry-skarn type W-Mo deposit, East China. Based on microscopic observations and in-situ LA-ICP-MS and LA-MC-ICP-MS analysis, three groups of scheelite, each with different geochemical characteristics, can be recognized. This evidence provides a good basis for considering the behavior of some trace elements in scheelite and how they may constrain ore genesis in porphyry-skarn systems. P-group scheelite occurs inside the porphyry rocks in the form of vein-hosted or disseminated scheelite. These have the lowest (87Sr/86Sr) i values (0.7089–0.7108), lowest Mo concentration (mean 213 ppm; n = 51), LREE/HREE ratios of 16.33–84.91, and highest ΣREE concentration (191–405 ppm) with downwards-sloping REE fractionation trends. S-group scheelite occurs in skarns, coexists with skarn minerals (garnet, diopside, epidote, etc.) and has the highest (87Sr/86Sr) i values (0.7103–0.7113), highest Mo concentration (mean 1,323 ppm; n = 29), highest LREE/HREE ratio (62.6–164), and lowest ΣREE concentrations (68.5–112 ppm), also with strongly downwards-sloping REE fractionation trends. H-group scheelite occurs within veins hosted by hornfels and displays moderate (87Sr/86Sr) i (0.70104–0.70122), Mo (mean 806 ppm; n = 17), LREE/HREE ratio (16.32–42.55), and ΣREE concentration (98.3–167.6 ppm). During hydrothermal precipitation of scheelite, changing redox state plays a major role in controlling Mo behavior. Both the precipitation of early skarn minerals and changing redox states of ore-forming fluids likely result in an increase in Mo, and corresponding decrease of HREE, in scheelite (particularly in S-group scheelite). The Sr-isotope study indicates that crustal materials provided the main source for W-Mo-bearing ore-forming fluids. Furthermore, both fluid mixing and fluid-rock interaction played an important role in the evolution of ore-forming fluids: Formation water or groundwater was likely involved in ore formation. A geochemical model is described combining data for the different types of scheelite, and the behavior of Mo, REEs and Sr-isotopes, to constrain the evolution of ore-forming fluids and constrain ore genesis in the porphyry-skarn system at Gaojiabang. This study contributes to the fingerprinting of ore deposits using scheelite geochemistry. [ABSTRACT FROM AUTHOR]