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The Zhengguang gold deposit, a typical intermediate-sulfidation epithermal deposit, is located in the southeastern part of the Duobaoshan orefield, west of the Hegenshan-Heihe suture zone, in the eastern part of the Central Asian Orogenic Belt. The deposit comprises five ore zones with total Au reserves exceeding 35 tonnes, with potential additional resources at depth. All vein-type orebodies are hosted by Paleozoic volcanic rocks and comprise multiple vein sets 1–100 cm in thickness. Although gold generally occurs in native form, or as electrum in epithermal deposits like Zhengguang, both pyrite and sphalerite are known to accommodate modest concentrations of invisible gold. This study employs a combination of petrography and sulfide chemistry to determine the role of invisible gold in the Zhengguang ores and the mechanisms of gold incorporation into epithermal sulfides. Three sulfide stages are identified: an early quartz + pyrite (Py1a, Py1b) ± chalcopyrite (Ccp1) stage; a subsequent quartz + sphalerite (Sph2a, Sph2b) + pyrite (Py2a, Py2b, Py2c, Py2d) + chalcopyrite (Ccp2a, Ccp2b) ± galena ± calcite stage; and a late stage containing deformed quartz + pyrite (Py3a, Py3b) ± sphalerite. Petrography and sulfide chemistry allow three groups of pyrite (Au-poor, Au-rich, and a distinct Sb-rich group) to be distinguished, alongside three groups of chalcopyrite (Bi-rich, intermediate-Bi, and Bi-poor), and two groups of sphalerite (Au-poor, Au-rich). A potential porphyry system is indicated beneath the epithermal system by the appearance of Au-poor pyrite and Bi-poor chalcopyrite. After precipitation of early Au-poor sulfides, inflow of relatively low temperature epithermal fluids led to alteration and replacement of early porphyry-related sulfides, and to precipitation of Au-rich pyrite, Bi-rich and intermediate-Bi chalcopyrite, and sphalerite. Gold-rich pyrite contains up to 140 ppm Au, interpreted as both as lattice-scale substitution (Au1+) and as included particles of native gold (Au0). Epithermal chalcopyrite is an important silver carrier but, although Au is measurable, it is a not a good carrier for gold. A strong positive correlation between Au and Cu in pyrite from the first two stages indicate that gold and other metals were likely sourced from magma-derived hydrothermal fluids. The deposit was formed in the Early Paleozoic but some gold ores appear deformed and partially destroyed by a later metamorphic event during which a distinct Sb-rich pyrite crystallized. This study should catalyze exploration in the orefield as it provides further support for an as-yet undiscovered porphyry system close to the Zhengguang deposit.

The Olympic Dam Cu-U-Au-Ag deposit is dominantly composed of mineralised hematite-breccias and occurs entirely within the Roxby Downs Granite. Multivariate statistical analysis of a large whole-rock, 15 m-interval geochemical dataset (10,565 samples) was undertaken to identify geochemical signatures characteristic of iron-oxide copper gold (IOCG)-style mineralization and constrain the conspicuous lithogeochemical zonation observed at Olympic Dam. Statistical analyses include principal component analysis on centred logratio (clr)-transformed data coupled with hierarchical clustering. Certain groups of elements that can be interpreted in terms of an evolving hydrothermal system relative to host lithologies are derived from data analysis: granitophile (U-W-Sn-Mo); siderophile (Ni-Co); chalcophile (Ag-Bi) and related elements (As-Sb and Au-Te). The distributions of elements within each group are investigated through three vertical cross-sections and are compared with known lithological and Cu-(Fe)-sulphide zonation. Throughout the Olympic Dam Breccia Complex, the IOCG signature is defined by multi-element combinations of the commodity metals Cu, U, Au, and Ag, coupled with a range of trace elements. Overall, the IOCG signature overlaps well with Fe-metasomatism despite mismatch which is likely due to discrete styles of mineralisation found only on the margins of the deposit and also to the presence of mineralised domains within Fe-poor zones. The IOCG signature is composed of two geochemical associations, which exhibit distinct spatial distributions. The first group, Cu-U3O8-Se-S, shows concentric zonation whereas the second group, Au-W-Mo-Sb-As, forms a vertical ∼1800 m deep corridor in the southeastern lobe of the deposit. The specific Au-W-Mo-As-Sb signature could potentially be generic within IOCG systems across the Olympic Cu-Au province and if so, would provide a proxy model for near-mine exploration.

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.

Cu-Fe-sulphide mineral assemblages from the Olympic Dam (OD) Fe-oxide Cu-U-Au-Ag deposit, South Australia, are studied down to the nanoscale to explore the potential these minerals have for understanding genetic processes such as primary deposit zonation. Cu-Fe-sulphide pairs: ‘brown’ bornite associated with chalcopyrite (bornite-chalcopyrite zone); and symplectites of ‘purple’ bornite with species from the chalcocite group, Cu2 − xS (bornite-chalcocite zone), co-define an upwards and inwards deposit-scale zonation at OD. In the bornite-chalcocite zone, there is also an increase in the proportion of chalcocite relative to bornite within the symplectites towards upper levels. In this case, two-phase Cu2 − xS assemblages are also present, as anisotropic, hexagonal chalcocite (CcH) with lamellar exsolutions of digenite, distinguishable at the μm-scale. Using compositional data (electron microprobe) combined with Transmission Electron Microscopy (TEM) study of foils prepared in–situ via Focused Ion Beam (FIB)-SEM, we show that Cu-Fe-sulphides from different ore zones feature nanoscale intergrowths, lattice defects, superstructure domains (na) and antiphase boundary domains (APBs) that can be interpreted as due to exsolution, coarsening and phase transformation during cooling from high-T solid solutions in the system Cu-Fe-S and sub-systems according to published phase diagrams. ‘Brown’ bornite [(Cu + Fe)/S > 5] contains pervasive lamellae of chalcopyrite which extend down to the nanoscale; such specimens appear homogeneous at the μm-scale. ‘Purple bornite’ [(Cu + Fe)/S 300 °C (high-T phases, Cu-poor digenite), followed by cooling along distinct paths down to

The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ~ 1.595 Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (> 5 km) and proximal ( 27–34 ) is recognised, along with a more abundant, less-calcic plagioclase (~ An 12–20 ) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (~ Or 55 Ab 43 An 2 ) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite + sericite ± Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765 °C at 2.2 kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships.

There is an abundance of published trace element data for sphalerite, galena and chalcopyrite in natural systems, yet for a co-crystallized assemblage comprising these base metal sulphides, there is no detailed understanding of the preferred host of many trace elements. Laser-ablation inductively-coupled plasma mass spectrometry trace element maps and spot analyses were generated on 17 assemblages containing co-crystallized sphalerite and/or galena and/or chalcopyrite from 9 different ore deposits. These deposits are representative of different ore types, geologic environments and physiochemical conditions of ore formation, as well as superimposed syn-metamorphic remobilisation and recrystallization. The primary factors that control the preferred base metal sulphide host of Mn, Fe, Co, Cu, Zn, Ga, As, Se, Ag, Cd, In, Sb, Te, Tl and Bi are element oxidation state, ionic radius of the substituting element, element availability and the maximum trace element budget that a given sulphide mineral can accommodate. Temperature, pressure, redox conditions at time of crystallization and metal source, do not generally appear to influence the preferred base metal sulphide host of all the trace elements. Exceptions are Ga, In and Sn recrystallized at high metamorphic grades, when the preferred host of Ga and Sn usually becomes chalcopyrite. In more typical lower temperature ores, the preferred host of Ga is sphalerite. Indium concentrations also increase in chalcopyrite during recrystallization. At lower temperatures the partitioning behaviour of Sn remains poorly constrained and shows little predictable pattern among the data here. The results obtained may be used as a tool to assess co-crystallization. If trace element distributions in a given base metal sulphide assemblage match those reported here, and assuming those distributions have not been significantly altered post (re-) crystallization, then it may be suggestive of a co-crystallized assemblage. Such information provides a foundation for novel attempts to develop trace element-in-sulphide geothermometers.

The Zijinshan orefield is a Cu–Au–Mo–Ag porphyry–epithermal mineralization system of Cretaceous age in southeastern China, comprising the Zijinshan high-sulfidation (HS) Au–Cu deposit, Luoboling porphyry Cu–Mo deposits, Yueyang low-sulfidation Au–Ag–Cu deposit, and Wuziqilong and Longjiangting intermediate-sulfidation (IS) Cu deposit. A number of Sn-(W)-sulfides occur in the Zijinshan orefield, including kiddcreekite, hemusite, colusite, vinciennite, stannoidite, mawsonite and kesterite. The occurrence represents the first report of hemusite, colusite and vinciennite in China. The relative abundance and diversity of Sn-(W)-sulfide-bearing assemblages make the Zijinshan orefield possibly unique in the world, and carries implications for the evolution of the mineralizing environment. Detailed electron probe studies of the Sn-minerals reveal that kiddcreekite, hemusite, mawsonite, stannoidite and kesterite have compositions close to ideal formulae. In contrast, colusite displays grain-scale zoning expressed by W, Sn and As. Vinciennite displays compositional variation depending on mineralization style: an average composition of Cu 10.74 Fe 3.88 Sn(As 0.85 ,Sb 0.08 )S 15.4 in the Zijinshan HS deposit, Cu 10.71 Fe 3.9 SnAs 0.85 S 15.34 in the Wuziqilong Cu deposit and Cu 10.76 Fe 3.7 Sn 1.02 (As 0.5 ,Sb 0.45 )S 15.54 with higher Sb contents (3.32%) in the adjacent Longjiangting Cu deposit. In the Zijinshan HS Au–Cu deposit, the main Cu-mineral assemblage varies from bornite-rich to digenite- to covellite-rich with increasing depth. Correspondingly, tin mineralogy varies from kesterite at upper levels to stannoidite-dominant at mid-to-upper levels of the copper orebody, to vinciennite- and mawsonite-dominant at depth, implying an increase in sulfidation and oxidation state downwards. Moreover, kiddcreekite is always partially replaced by wolframite and (hypogene) covellite at depth, also indicating overprinting by high- f O 2 and - f S 2 fluids in the covellite stage of mineralization. Kiddcreekite, along with a primary chalcopyrite–pyrite mineral association, occurs widely in the Zijinshan orefield (Zijinshan, Yueyang, Luoboling, Wuziqilong, Longjiangting and Dayanli), and is likely indicative of the early IS stage of epithermal system. Analogous with published data, kiddcreekite most commonly occurs in porphyry-style mineralization, implying the presence of the mineral has potential as an indicator of porphyry systems. Vinciennite-bearing Cu–Sn–As mineral associations occur in a specific position in the Zijinshan HS Au–Cu, Wuziqilong and Longjiangting Cu deposits, indicative of a transitional zone between porphyry and HS epithermal-style mineralization. The occurrence of the identified Sn-(W)-sulfide-bearing assemblages has considerable potential implications for understanding the architecture of a complex porphyry–epithermal environment and for exploration for underlying porphyry copper deposits. There is potential for discovery of a subjacent porphyry Cu deposit within the Zijinshan orefield.

The supergiant Olympic Dam Cu-U-Au-Ag deposit in South Australia is a type example of the iron-oxide copper–gold (IOCG) deposit family. Hosted entirely within heterogeneous breccia in 1.59 Ga granite, the deposit contains a volumetrically substantial and mineralogically diverse component of carbonate minerals. Carbonate minerals are always associated with ore minerals (sulfides, uraninite), implying a genetic relationship and providing an opportunity to use gangue carbonates to better understand ore formation. This study provides the first detailed and comprehensive petrographic and chemical/isotopic study of Olympic Dam carbonates, with a particular emphasis on petrography and texture, and an attempt is made to relate carbonate formation to local and regional events that have affected Olympic Dam. Based on a set of 196 carbonate-bearing samples, carbonate minerals are observed in all lithologies present at Olympic Dam. Carbonates occur as cement in breccia and conglomerates, as breccia clasts, in veins crosscutting ore-rich breccia and other rock types, in pores and ooids, and in the form of laminated carbonate. Siderite and siderite-rhodochrosite-magnesite solid solution are by far the most common carbonate types, whereas calcite, dolomite-ankerite solid solution and REE-fluorocarbonates are locally abundant. Single carbonate grains typically show compositional zones (simple or oscillatory) and replacement textures (including mutual replacement of carbonates with other carbonates and with hematite) are common. In the absence of consistent, deposit-wide paragenetic relationships, the carbonates were placed in seven associations based on host rock, mineralogy and texture: (1) coarse-grained calcite veins in weakly brecciated granite and rhyolite, (2) carbonates in strongly brecciated granite, (3) carbonate veins in bedded clastic facies, (4) carbonates in mafic and ultramafic igneous rocks, (5) massive barite-fluorite-dominated veins with minor carbonate, (6) laminated siderite, and (7) carbonate matrix in conglomerate-breccia-sandstone above the unconformity. Some of these associations can be related to regional tectonic events based on local context and relationships with dated assemblages. δ13C (−6.5‰ to −2‰) values for the carbonates show a relatively limited range whereas δ18O is more variable (+9.4‰ to + 20.9‰). C-O isotopic compositions for the various carbonate associations tend to overlap, suggestive of mixed fluid sources, recycling of older carbonate and perhaps other fractionation processes. The C-O isotope data overlap the compositional fields of several major carbon–oxygen reservoirs (magmatic, sedimentary) and carbon sources in local granite, felsic volcanics, older BIF and sedimentary rocks are all possible at different stages of carbonate deposition.

Spatial associations between banded iron formation and iron-oxide Cu-Au (IOCG) style mineralization are well documented in the Gawler Craton (South Australia), but the possible genetic relationships between these two distinct types of mineralization are hitherto unclear. A texturally conspicuous generation of coarse-grained silician magnetite, intergrown with carbonates and quartz, is observed in drillholes intersecting the ‘outer shell’ of the Olympic Dam IOCG-type deposit. This magnetite is characterised by high U-content (~50 ppm), siliceous chemistry, and unusual zonal textures with respect to Si-Fe-nanoprecipitates. Direct dating of this magnetite by laser ablation inductively coupled plasma mass spectrometry yields reproducible 207Pb/206Pb dates (1761 ± 16 Ma) that are significantly older than the granite hosting the deposit (1593 Ma), or the mineralized breccias constituting the Cu-U-Au-Ag resource (~1592–1589 Ma). The older, Fe-rich crustal material can be correlated with the ~1.76–1.74 Ga (meta)sedimentary Wallaroo Group, host to Fe-rich horizons across the Gawler Craton, including locations ~15 km NW of Olympic Dam. A generation of granitic rocks, which intruded bedrock at ~1.75 Ga are present ~30 km NE of Olympic Dam, and likely exsolved hydrothermal fluids that enriched pre-existing magnetite-bearing protoliths in both U and REE. Such material was physically, and likely chemically, incorporated into the ‘outer shell’ at Olympic Dam some ~150 Ma later, during granite uplift along faults. The coincidence between Fe-rich horizons/BIF and ~1750 Ma granitoids may have provided IOCG systems with an additional source of both Fe and U that predates the ~1.59 Ga craton-scale metallogenic event. The uranium concentrations in some South Australian IOCG systems represent major global anomalies in the element. A combination of the fortuitous geological circumstances outlined here, may help explain the highly anomalous accumulation of uranium found at Olympic Dam.

The Basil Cu–Co deposit, Harts Range, central Australia, is hosted by the Riddock Amphibolite, a sequence that has been metamorphosed at upper-amphibolite- to granulite-facies conditions at 480–460 Ma (Larapinta Event), and subsequently reworked at amphibolite-facies conditions (450–300 Ma). As a result, many of the primary mineralization textures and other features that could characterise ore genesis have been obliterated. However, preserved textures and mineral relationships in the mineralized zone, allow some constraints to be placed on the genetic history of the deposit using mineralogical, petrographic and geochemical studies of host rocks and sulphides. Results of this study permit at least two genetic models to be ruled out. Firstly, whole rock geochemistry and garnet compositions suggest that the deposit is not a skarn system. Secondly, the lack of any significant Ni-signature, and the presence of abundant zircons in the host amphibolite (indicating that not all host rocks are mafic in composition and/or magmatic in character), make an orthomagmatic Ni–Cu–(PGE) system unlikely. Alternatively, Basil is assigned to a volcanic-hosted massive sulphide (VHMS)-style of mineralization, formed on the seafloor, within basaltic and sedimentary host rocks, typical of deposits occurring in such settings. The lack of a recognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone during metamorphism or detachment of the ore from alteration during later deformation. The occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn-metamorphic or earlier. Partitioning of trace elements between pyrite and co-existing pyrrhotite suggests that (re)crystallization occurred under equilibrium conditions. The composition of sphalerite coexisting with pyrite and pyrrhotite indicates crystallization at pressures of at least 10 kbar, consistent with peak metamorphism during the Early Ordovician Larapinta Event. Zr-in-titanite geothermometry indicates peak temperatures of 730–745 °C.

Scheelite (CaWO4), 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.

Enargite, Cu3AsS4, is a relatively common sulphide mineral and is considered diagnostic for deposits of intermediate- to high-sulphidation type. Analysis of enargite-bearing samples from deposits in the Zijinshan porphyry – high-sulphidation epithermal Cu-Au orefield, southeastern China, provides evidence for the diversity of trace elements that may be hosted within enargite and their range of concentrations. Enargite is shown to host Sb, Te, Sn, Zn and Ge at concentrations up to several thousand ppm. The mineral also incorporates measurable concentrations of Mo, Cd, Bi, Pb, Fe, Se, Ag, Au W, Ga and In. Element mapping (electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry) provides evidence for grain-scale heterogeneity in enargite in the form of oscillatory, grain-scale compositional zonation with respect to Sb, Sn, Te and several other trace elements. Element mapping also clearly shows an inverse correlation between the concentrations of As and Te, and between As and Sn. Incorporation of Sn and Te into the enargite structure is achieved by substitution of Sn4+ and Te4+ for As5+. Charge balance is maintained by incorporation of Fe2+, Zn2+ and other divalent cations (potentially including Cu2+) into the Cu+ site. The complex intra-grain zoning results from evolving fluids, multiple phases of growth – in turn leading to an overprinting of primary distribution patterns. Observed patterns are also influenced by equilibrium partitioning between enargite and co-existing minerals. Nevertheless, trace element signatures in enargite from different parts of the Zijinshan ore system show notable differences. Enargite from the high-sulphidation stage typically shows a marked enrichment in Te and Sn whereas enargite from intermediate-sulphidation stage is relatively depleted in Te and Sn, and comparatively enriched in Sb and Se. These differences represent a potential vector for exploration within porphyry – high-sulphidation epithermal systems. Furthermore, the presence of Te-rich enargite may be a prospective guide to high Au-grade mineralization. The notable concentrations of precious metals (Au, Ag) and critical elements (notably Te and Ge) within enargite make this mineral of particular interest from the perspective of potential recovery of these economically important elements. The observed grain-scale zoning and inherent variability within any given sample emphasize that spot analysis of trace elements alone without consideration of such heterogeneity may provide quantitative data of limited use and potentially, lead to misleading interpretations.

Martin Reich, Robert M. Hough, Artur Deditius, Satoshi Utsunomiya, Cristiana L. Ciobanu and Nigel J. Cook

Focussed ion beam–scanning electron microscopy (FIB–SEM) is a relatively new analytical tool that has been little applied to problems of ore genesis. The technique enables high-resolution (cross-section) imaging and can be used to prepare thinned foils for study by transmission electron microscopy (TEM). FIB–SEM methods applied to sulphides and related compounds represent an in-situ approach for sample characterisation and thus provides for crystal–chemical data that can be placed into the geological context of a given ore deposit. We present four study cases: these deal with minor element incorporation and release in ZnS; intergrowths and replacement among Cu–(Fe)-sulphides; fabrics in Au-bearing, As-free pyrite; and symplectites of Bi-sulphosalts within galena. The data is discussed in the context of polytypism and planar defects for minor element incorporation and release, superstructure ordering and formation of fine particles (100–2500 nm) or nanoparticles (

Laser-ablation ICP mass-spectroscopy has been used to investigate the geochemistry of sphalerite in a range of nine Zn–Pb deposits in South China. The deposits, which are of different ages and belong to different metallogenic provinces, have been assigned to the following genetic types: skarn (Hetaoping, Luziyuan), syngenetic massive sulphide (Dabaoshan, Laochang and Bainiuchang) and Mississippi-Valley-type (Huize, Mengxing, Niujiaotang) based on the features of the ore, even though their origin is heavily debated based on other criteria. The giant Jinding deposit is considered separately. Sphalerite from each genetic class of deposit shows a distinct chemical signature. Sphalerite from the skarn deposits is characterised by elevated, lattice-bound concentrations of Co and Mn. The distal character of these skarn systems is reflected by the low In content of sphalerite. The three syngenetic massive sulphide deposits feature sphalerite strongly enriched in In, Sn and Ga, whereas the deposits of MVT-type are typically enriched in Ge, Cd, Tl and As. These divergent characters are reflected in absolute element abundances as well as in element ratios. Time-resolved depth profiles for sphalerite from the Chinese deposits confirm the presence of elements such as Co, In, Ge, Ga, and Cd in solid solution, but the dataset expands the understanding of sphalerite mineral chemistry by also indicating that other elements, whose ability to enter the crystal structure of sphalerite has been previously debated (Ag, Sn, Tl, Sb), may also be in solid solution. Sphalerite is a refractory mineral and trace element analysis of sphalerite shows promise as a tracer of ore genesis even in overprinted ores. Systematic work on larger sample suites may help define the geochemical signature of different metallogenic epochs in regions as geologically complex as South China and help resolve the mechanism by which many of the debated ore deposits were formed.

We address the question of the predictability of skarn textures and their role in understanding the evolution of a skarn system. Recent models of skarn formation show that skarns are ideal for application of self-organisation theory, with self-patterning the rule in fluid-rock interaction systems rather than the exception. Zonation in skarn deposits, a consequence of infiltration-driven metasomatism, can also be treated in terms of self-organisation. Other less commonly described features, such as scalloping, fingering and mineral banding, can be understood by application of reactive infiltration and hydrodynamics at the skarn front. Devolatilisation may trigger formation of back-flow fluxes that overprint previously formed skarn. The range of textures formed from such events can be used to discriminate between prograde and retrograde stages. Refractory minerals, such as garnet, magnetite and pyrite, readily retain overprinting events. Skarns are also composed largely of minerals from solid solution series (garnet, pyroxene, pyroxenoids, etc.) and therefore skarn mineralogy helps to establish trends of zonation and evolution. The same minerals can act as ‘chemical oscillators’ and record metasomatic trends. The Ocna de Fier-Dognecea deposit was formed in a ∼10 km deep skarn system. Zonation and evolution trends therefore represent only the result of interaction between magmatically derived fluids emerging at the source and limestone. From the same reason, the transition from prograde to retrograde regime is not influenced by interaction with external fluids. Thirdly, the mineralisation comprises Fe, Cu and Zn-Pb ores, thus facilitating comparison with skarn deposits that commonly are formed in shallower magmatic-hydrothermal environment. Copper-iron ores (magnetite+Cu-Fe sulphides), hosted by magnesian (forsterite+diopside) skarn, occur in the deepest and central part of the orefield, at Simon Iuda. Their petrological character allows interpretation as the core of the skarn system formed from a unique source of fluids emerging from the subjacent granodiorite. It formed first as a consequence of the local setting, where a limestone indented in the granodiorite permitted strong reaction at ∼650 °C and focussed the up-streaming, buoyant fluids. The first sharp front of reaction is seen at the boundary between the Cu-Fe core and Fe ores hosted by calcic skarn (Di 70-90 -And 70-90 ), where Cu-Fe sulphides disappear, and forsterite gives way to garnet in the presence of diopside (Di 90 ). Following formation of forsterite, devolatilisation and transient plume collapse is interpreted from a range of piercing clusters and trails. We presume lateral flow to have been initiated at the source, as the emerging fluids are in excess to the fluids driven into reaction by the plume. Formation of the other orebodies, up to 5 km laterally downstream in both directions, is interpreted as skarn fingering at the limestone side. The metasomatic front is perpendicular to the flow along the channel of schists placed between the limestone base and the granodiorite. A metal zonation centred onto the source is defined, based on metal distribution: Cu-Fe/Fe/Zn-Pb. The second front of reaction, at the boundary between the Fe and Zn-Pb zone, has a sulphidation/oxidation character, with diopside giving way to a Fe-Mn-rich pyroxene, (HedJoh) >60 +pyroxmangite±bustamite; garnet is minor. Johannsenite-rich pyroxene (Di 20-40 Hed 20-40 Joh 40 ) is found in proximal skarn at the upper part of Simon Iuda, stable with Zn 0.95 Fe 0.05 S, at an inferred 570 °C. In distal skarn from Dognecea and Paulus, Mn-hedenbergite (Di Hed 70 Joh 20-30 ) formed at ∼400 °C is stable with Zn 0.84 Fe 0.16 S. Extensive compositional fields, eutectic decomposition and lamellar intergrowths characterise pyroxene in the Zn-Pb zone, formed at the magnetite-hematite buffer in the presence of pyrite. Distal skarn has a reducing character, in comparison with the proximal. A drop in both f S 2 and O 2 , with the zoned system moving closer to the pyrite-pyrrhotite buffer, is induced from the temperature gradient. Based on pyroxene mineralogy and calculated f S 2 , the metal zonation is confirmed as being formed upwards and outwards from the source. The Fe and Zn-Pb zones both have a patterned side coexisting with the unpatterned one. Patterning is seen at scales from macroscopic (rhythmic banding, nodular, spotted, orbicular, mossy, mottled textures) to microscopic scales (oscillatory zonation in garnet and silica-bearing magnetite). Following plume updraft, the path of decarbonation reaction controlled the motion of the skarn front until, towards the end of the prograde stage, a multiple steady state regime developed and produced rhythmic patterns on all scales. The activation of powerful patterning operators, represented by Liesegang banding alone, or coupled with competitive particle growth, show that the skarn front had the characteristics of an unstable coarsening front of reaction. A second retrograde event, carbofracturing, triggered by erratic decarbonation after cessation of infiltration, can be interpreted from overprinting textures in the Fe and Zn-Pb zone. A major drop in f O 2 is inferred from extensive, pseudomorphous replacement of hematite by magnetite. Textures show progressive destruction of prograde assemblages, i.e., piercing clusters, shock-induced, fluid-pressure assisted brecciation and deformation, followed by healing of the disrupted assemblages. Release of trace elements accompanies both retrograde events, with a Bi-Te-Au-Ag association common to both. The importance of shock-induced textures is emphasised in the context of Au enrichment, especially when the retrograde fluids cross the main buffers in f O 2 - f S 2 space. The presence of Bi-sulphosalt polysomes in the Fe zone indicates that patterning extends down to the nanoscale. The key role played by polysomatism in stabilising compositional trends that cannot otherwise be formed at equilibrium is a fertile ground yet to be adequately explored.