Tassos A. Grammatikopoulos

RECOVERY OF METAL VALUES FROM COPPER—ARSENIC MINERALS AND OTHER RELATED RESOURCES

Copper is often associated with arsenic in mixed sulphide minerals such as enargite (Cu3AsS4) and tennantite (Cu12As4S13). Enargite, in particular, is the principal mineral in many deep epithermal copper–gold deposits. Most mining companies avoid exploiting such resources, because the arsenic can become a serious environmental liability or may considerably reduce the resource value due to hefty treatment charges. The few enargite deposits that have been exploited so far are usually rich in gold and silver. The first challenge in the exploitation of copper–arsenic sulphides is the effective separation of arsenic phases from other valuable minerals. In the last decade, though, it was shown that this is possible by pulp-potential adjustment (oxidative conditions) combined with pH adjustments (basic conditions) prior to flotation. In this way, two types of concentrate can be produced: one rich in arsenic and another low in arsenic but rich in other valuable metals. Arsenic-rich concentrates have traditionally been processed pyrometallurgically by reduction roasting to gaseous arsenic sulphide, which is then converted to arsenic trioxide. New pyrometallurgical technologies for the treatment of copper–arsenic sulphides include sulphidization roasting, sulphidization roasting and halogenation, and carbothermic reduction to copper arsenide. The hydrometallurgical treatment of copper–arsenic-antimony resources has been done by atmospheric leaching in alkaline sodium-sulphide solutions. Ultrafine grinding and ferric oxidation at atmospheric pressure, total pressure oxidation at temperatures above 220°C, and bacterial leaching have recently been tried on copper–arsenic sulphides, some with considerable success.

Listwaenite evolution in the ophiolite mélange of Iti Mountain (continental Central Greece)

Rare listwaenite occurrences with the assemblage quartz + calcite + magnetite + goethite + hematite ± talc ± ankerite have been found within the ophiolite melange of the Iti Mountain. They also contain relic Cr-spinel, and have retained mesh and hourglass textures indicating an origin from serpentinite. Geochemical investigation, using isocon analysis and assuming Ti as immobile element, reveals conservation of mass. During the listwaenite-forming metasomatic event there was influx of Si, Ca, Ba, Zr, light rare-earth elements (LREE), Ir and Pt, while Mg, Mn, heavy rare-earth elements (HREE) and Pd were removed from the system. Al, Cr, Ni and Fe tot behaved as immobile elements. The release of Mg and formation of quartz may have occurred from breakdown of the serpentine minerals at low pressure, involving very high X CO2 in the fluid phase. Two distinct fluids are thought to have been involved in the alteration process. Fractionation of REE is explained by their mobility mainly as REE-carbonate complexes, which are favoured by a CO 2 -rich and mildly alkaline fluid. Ca influx in the listwaenite is attributed to this fluid, which is likely related to the serpentinization of the peridotites. Transportation of SiO 2 , as well as of platinum-group elements (PGE), was favoured by another low-pH, highly-oxidized, saline fluid, at low temperatures. Mobility of REE probably persisted, but likely as chloride complexes. The latter fluid may have been also responsible for oxidation of Cr-spinel to goethite and of magnetite to hematite. The presence of goethite and calcite in the listwaenite account for the enrichment of LREE, given that these minerals can selectively absorb the LREE into their structures. The HREE, which were released during the breakdown of serpentine and olivine, were subsequently removed from the system. At very low temperatures the two fluids were miscible and the interaction of the acid, oxidized fluid with the calcite probably led to precipitation of Pt and Ir in the listwaenite. Direct dissolution of olivine relics has likely occurred at a late stage and further depleted the listwaenite in Mg.

Platinum-group mineral characterisation in concentrates from low-grade PGE chromitites from the Vourinos ophiolite complex, northern Greece

Abstract The Vourinos ophiolite complex, located in northern Greece, hosts various chromite deposits characterised by very low platinum-group element (PGE) grades. Total PGE (excluding Os) concentrations in the fourteen chromitite samples collected for use in this study varied from 200 to 300 ppb. Previous reports on the platinum-group minerals (PGM) from the Vourinos chromitites, obtained data by in-situ investigation on polished sections. Consequently, we used the technique of hydroseparation to study the PGM from the concentrates in the Vourinos chromitites. More specifically, we investigated two separate composite samples from Voidolakkos and Xerolivado chromitites. The Voidolakkos concentrate sample contains 74 PGM that include: laurite (Ru, Os)S2; irarsite (Ir, Ru, Rh, Pt)AsS; erlichmanite (Os, Ru)S2; ruthenium pentlandite; iridium (Ir(Os)); osmium (Os(Ir, Ru, Pt)); secondary phases composed of Ru, Os and Cu; alloys of Ir–Fe; Rh– and Ru–Ni–Fe alloys; and Os–Ir–Fe alloys. The investigation of Xerolivado concentrate sample yield 45 grains of PGM, including laurite, irarsite, erlichmanite, minor other PGE sulphides, Os–Ir–Ru alloys, iridium and secondary phases of Ru–Os alloys. PGM occur as both single and polyphase particles in both samples. The bulk of mineralisation in Voidolakkos is dominated by a finer variety (<10 μm) of PGM than the Xerolivado sample (mainly <20 μm). The former occurrence hosts considerably more altered PGM grains, less laurite and a larger variety of PGM than the latter, whereas Os–Ir alloys are present in almost equal amounts in both samples. The hydroseparation process has recovered significantly more, as well as novel, PGM grains than were known from previous in-situ mineralogical examination of single chromitite samples. Although, most of the PGM occur as free particles and in-situ textural information is lost, single grain textural evidence is observed. The mineralogical and grain size differences between the two samples may reflect styles of mineralization and indicate significant remobilization of PGE. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Voidolakkos and Xerolivado chromitites. In summary, this study provides significant new information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.