Cristiana L. Ciobanu

Modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids

The effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution–mineral–melt system. The calculations for the Au-Bi-Na-Cl-S-H-O system, performed at temperatures between 300 and 450 °C, demonstrate that Au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid, indicating the possible formation of economic gold deposits from undersaturated aqueous fluids, in which mineralization would not be expected in the absence of a bismuth melt. The model applies to any deposit where a Bi melt is stable and coexists with a hydrothermal fluid; examples of such deposits are known from skarn, intrusion-related, orogenic, and volcanogenic massive sulfide (VMS) gold systems. In sulfur-poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the Au grade in the ore (e.g., Escanaba Trough VMS deposit). These results also illustrate important principles generally applicable to understanding magmatic-hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetallic melts.

Bismuth tellurides and sulphosalts from the Larga hydrothermal system, Metaliferi Mts., Romania: Paragenesis and genetic significance

Abstract Intermediate levels of the Larga-Faţa Báii field, Metaliferi Mts., Romania, are host to epithermal vein mineralization, small skarn-like bodies and ‘geode’-like replacement ores, that together form a complex hydrothermal system rooted in the subjacent andesite stock. Silver-Pb-Bi sulphosalts, some with Sb and/or Se, are intimately associated with hessite and a range of Bi-tellurides with Bi/(Te+S+Se) ≤ 1 (tetradymite, tellurobismuthite, tsumoite, Pb-free rucklidgeite) within all these ore types. Lillianite homologues, both ordered 4L and 7L derivatives (lillianite, gustavite), and disordered varieties are the most abundant sulphosalts, with subordinate bismuthinite derivatives (aikinite, krupkaite, bismuthinite). An exceptionally Ag-rich lillianite homologue, intimately intergrown with tetradymite and hessite, is identified as treasureite on the basis of chemical composition. Porphyry-style mineralization from 600 m beneath the studied level contains rucklidgeite as the single stable Bi-telluride. The trace mineralogy of the system is suggestive of a hydrothermal system strongly zoned with respect to fTe₂ and, to a lesser extent, also fS₂. Mineralogical and textural evidence suggests that initial crystallization took place at temperatures in excess of 400ºC. The association of gold with Bi-minerals is linked to the role of Bimelt as a scavenger for gold, and deposition within ‘droplets’ simultaneous with the formation of arsenopyrite from löllingite + pyrrhotite. Both Bi-tellurosulphides and sulphosalts are commonly non-stoichiometric, a fact that can be correlated with order-disorder in both series. When widely distributed, as in the Larga system, trace amounts of Bi-minerals (sulphosalts, tellurides/ tellurosulphides) have considerable, untapped potential as tracers of the physical-chemical character of an evolving hydrothermal system.

Gold-telluride nanoparticles revealed in arsenic-free pyrite

Abstract Pyrite, the most abundant sulfide on Earth and a common component of gold deposits, can be a significant host for refractory gold. This is the first documentation of pore-attached, composite Autelluride nanoparticles in “arsenic-free” pyrite. Trace elements mapping in pyrite from an intrusionhosted Au deposit with orogenic overprint (Dongping, China) shows trails of tellurides overlapping Co-Ni-zonation. Intragranular microfracturing, anomalous anisotropy, and high porosity are all features consistent with devolatilization attributable to the orogenic event. The pyrite-hosted nanoparticles are likely the “frozen,” solid expression of Te-rich, Au-Ag-Pb-bearing vapors discharged at this stage. Nanoparticle formation, as presented here, provides the “smallest-scale” tool to fingerprint Au-trapping during crustal metamorphism

Multivariate Analysis of an LA-ICP-MS Trace Element Dataset for Pyrite

Application of multivariate statistics to trace element datasets is reviewed using 164 multi-element LA-ICP-MS spot analyses of pyrite from the Moonlight epithermal gold prospect, Queensland, Australia. Multivariate analysis of variance (MANOVA) is used to demonstrate that classification of pyrite on morphological and other non-numeric factors is geochemically valid. Parallel coordinate plots and correlation cluster analysis using Spearman’s coefficients are used to discover unexpected elemental relationships without making assumptions a priori. Finally, principal component analysis and factor analysis are used to demonstrate the presence of sub-classes of pyrite. Corroborated with geological data, statistical analysis provides evidence for successive generations of hydrothermal fluids, each introducing specific metals, and for partial or complete replacement of different minerals. The data permit reinterpretation of Moonlight as a telescoped system where epithermal-Au (± base metals) is superposed onto early porphyry-Mo mineralization.

Trace and minor elements in galena: a reconnaissance LA-ICP-MS study

Abstract Minor and trace elements can substitute into the crystal lattice of galena at various concentrations. In situ LA-ICP-MS analysis and trace element mapping of a range of galena specimens from different deposit types are used to obtain minor/trace element data, aimed at achieving insight into factors that control minor/trace element partitioning. The previously recognized coupled substitution Ag++(Bi,Sb)3+↔ 2Pb2+ is confirmed. However, the poorer correlation between Ag and (Bi+Sb) when the latter elements are present at high concentrations (~>2000 ppm), suggests that site vacancies may come into play: [2(Bi,Sb)3++o ↔ 3Pb2+]. Galena is the primary host of Tl in all mapped mineral assemblages. Along with Cu, Tl is likely incorporated into galena via the coupled substitution: (Ag,Cu,Tl)++(Bi,Sb)3+↔ 2Pb2+. Tin can reach significant concentrations in galena (>500 ppm). Cd and minor Hg can be incorporated into galena; the simple isovalent substitution (Cd,Hg)2+↔ Pb2+ is inferred. This paper shows for the first time, oscillatory and sector compositional zoning of minor/trace elements (Ag, Sb, Bi, Se, Te, Tl) in galena from two epithermal ores. Zoning is attributed to slow crystal growth into open spaces within the vein at relatively low temperatures. The present data show that galena can host a broader range of elements than previously recognized. For many measured elements, the data sets generated display predictable partitioning patterns between galena and coexisting minerals, which may be dependent on temperature or other factors. Trace element concentrations in galena and their grain-scale distributions may also have potential in the identification of spatial and/or temporal trends within individual metallogenic belts, and as markers of ore formation processes in deposits that have undergone superimposed metamorphism and deformation. Galena trace element geochemistry may also display potential to be used as a trace/minor element vector approach in mineral exploration, notably for recognition of proximal-to-distal trends within a given ore system.

Determination of the oxidation state of Cu in substituted Cu-In-Fe-bearing sphalerite via μ-XANES spectroscopy

Abstract In situ spatially resolved X-ray absorption near edge structure (m-XANES) spectra are obtained for natural Cu-In-bearing sphalerite. Copper K-edge spectral data show that, in Cu-In-bearing sphalerite, in which an excellent correlation between the Cu and In contents is noted, Cu is present in the Cu+ state. This offers indirect proof for the coupled substitution 2 Zn2+ ↔ Cu+ + In3+, which allows indium to enter the sphalerite structure. The study clearly demonstrates the utility of synchrotron radiation to accurately determine oxidation state in small volumes of mineral in which the concentration of the element of interest is low or very low. The study also demonstrates that good quality μ-XANES spectra can be collected on TEM foils prepared in situ at a chosen position on the surface of a polished sample using the focused ion beam-scanning electron microscope method.

Intergrowths of bismuth sulphosalts from the Ocna de Fier Fe-skarn deposit, Banat, Southwest Romania

The Ocna de Fier skarn deposit is typified by texturally and compositionally specific intergrowths of Bi sulphosalts. Bismuthinite derivatives, makovickyite, lillianite homologues and galenobismutite have been documented by microanalysis. Among the bismuthinite derivatives, two new homogeneous phases in the series, “Phase 70” CuPbBi 7 S 12 and “Phase 88.6” Cu 0.33 Pb 0.33 Bi 7.67 S 12 , are reported; both had earlier been predicted as possible stable superstructures. Intergrowth textures occur within assemblages from the entire metasomatic evolution of the deposit and reflect relative stabilities of individual phases in the context of local equilibrium gradients. Primary features are exemplified by skeletal intergrowths in the gladite-krupkaite compositional range. These have been preserved during cooling and are distinct from other intergrowths resulting from the breakdown of earlier-formed minerals. The latter are typified by vermiform intergrowths in the range gladite-“Phase 88.6” and by lamellar and symplectitic lillianite-galena intergrowths containing remnants of initial heyrovskýite. Microanalytical data shows high levels of substitution in the sulphosalt structures, notably Cu in makovickyite and lillianite homologues. Bulk analyses of fine symplectites, together with some of the heyrovskýite-type remnants clearly show that small chemical differences existed within the initial crystals prior to decomposition. Despite the complex intergrown character, textural analysis and microanalytical data suggest that these assemblages could be related to crystal-growth processes evolving in a looping manner within the skarn-forming environment, parallel to oscillatory zoning patterns in garnet. Consequently, some of these intergrowths, e.g. gladite-krupkaite, characterised by order-disorder phenomena within a specific compositional range in the assemblage, could be subject for analysis of stacking sequences in terms of oscillatory growth processes.

Bismuth tellurides as gold scavengers

Bi-Te melts have potential as Au scavengers in different types of gold deposits. Using phase equilibria, we define ‘melt-(precipitation) windows’ within which the Bi-Te scavenging mechanism can operate. Predicted assemblages correlate well with those in natural samples and can be applied in other cases to explain the distribution of telluride minerals in Au deposits.

Trace and minor elements in sphalerite from metamorphosed sulphide deposits

Sphalerite is a common sulphide and is the dominant ore mineral in Zn-Pb sulphide deposits. Precise determination of minor and trace element concentrations in sulphides, including sphalerite, by Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) is a potentially valuable petrogenetic tool. In this study, LA-ICP-MS is used to analyse 19 sphalerite samples from metamorphosed, sphalerite-bearing volcanic-associated and sedimentary exhalative massive sulphide deposits in Norway and Australia. The distributions of Mn, Fe, Co, Cu, Ga, Se, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb and Bi are addressed with emphasis on how concentrations of these elements vary with metamorphic grade of the deposit and the extent of sulphide recrystallization. Results show that the concentrations of a group of trace elements which are believed to be present in sphalerite as micro- to nano-scale inclusions (Pb, Bi, and to some degree Cu and Ag) diminish with increasing metamorphic grade. This is interpreted as due to release of these elements during sphalerite recrystallization and subsequent remobilization to form discrete minerals elsewhere. The concentrations of lattice-bound elements (Mn, Fe, Cd, In and Hg) show no correlation with metamorphic grade. Primary metal sources, physico-chemical conditions during initial deposition, and element partitioning between sphalerite and co-existing sulphides are dominant in defining the concentrations of these elements and they appear to be readily re-incorporated into recrystallized sphalerite, offering potential insights into ore genesis. Given that sphalerite accommodates a variety of trace elements that can be precisely determined by contemporary microanalytical techniques, the mineral has considerable potential as a geothermometer, providing that element partitioning between sphalerite and coexisting minerals (galena, chalcopyrite etc.) can be quantified in samples for which the crystallization temperature can be independently constrained.

Paragenesis of Cu-Fe ores from Ocna de Fier-Dognecea (Romania), typifying fluid plume mineralization in a proximal skarn setting

Abstract Trace mineral assemblages in the bornite- and chalcopyrite-rich Cu-Fe zone of the Ocna de Fier- Dognecea skarn deposits, Banat, SW Romania provide additional constraints on the genesis of this classic zoned skarn system. Observed assemblages substantiate a model, in which the Cu-Fe zone forms the proximal fluid-plume root of the system. Observed trace mineral assemblages in the magnesian forsterite-bearing skarns crystallized at ~650°C in a volatile-rich environment, evidenced by widespread phlogopite, ludwigite, valleriite and apatite. The entire assemblage thus belongs to the initial stage of skarn formation. Prolonged cooling led to sequential exsolution of trace mineral phases from bornite and chalcopyrite during the retrograde stage, although still at temperatures in excess of 500°C. Bornite is typified by the abundance of exsolved phases along cleavage planes and along crystal margins, notably chalcopyrite and pyrrhotite, but also cobalt pentlandite, carrollite, wittichenite, galena, mawsonite, silver and electrum. Chalcopyrite hosts cobalt pentlandite, carrollite, wittichenite, galena and a sequence of Se- and Te-bearing minerals (kawazulite, bohdanowiczite, hessite, volynskite), along, although not restricted to, grain margins. The assemblage bornite-chalcopyrite-magnetite, with the trace phases, cobalt pentlandite, carrollite, wittichenite and various Se- and Te-bearing minerals represents a characteristic assemblage common to a disparate range of deposits formed at temperatures in excess of 500°C in the presence of volatiles and typified by relatively low fS₂ fluids.