C. J. Stanley

Chromite composition and PGE content of Bushveld chromitites: Part 1 – the Lower and Middle Groups

Abstract This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe2+) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe2+ for Mg and Al between spinel and liquid affecting the Mg-Fe2+ spinel-liquid Kd2. Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al2O3 as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the LG-5 (0·9 to 10) and all overlying chromitites (also documented by Scoon and Teigler). The Pt/Ru and Pd/Ru ratios are very variable, but the Ru/Ir, Ru/Rh and Ru/Os ratios of all chromitites are relatively constant, indicating that Pt and Pd respond to different concentration mechanisms to the other PGE. Rh, Ru, Ir and Os were likely concentrated by chromite itself, probably as grains of laurite and alloys incorporated in growing chromite crystals, but the bulk of the Pt, Pd along with lesser proportions of the other PGE were concentrated by sulphide liquid. Most chromitites now have very low contents of S, but mineragraphic and chemical data support the suggestion of Naldrett and Lehmann that vacancies in chromite forming above 900°C were filled by Fe2+ derived from the destruction of interstitial sulphide liquid. Data on En composition through the Bushveld CriticalZone, indicate that the LG-1 to LG-4 chromitites formed at a stage when influxes of magma into the chamber were rapid and primitive, and overrode the effect of fractional crystallization, whereas above this, fractionation mostly overrode influxes of new magma. Irvines model of mixing of resident magma with influxes of more primitive magma is invoked as the origin of the chromitite horizons. It is shown, using the equation for sulphur solubility and the programme MELTS, that influxes and mixing of fresh primitive magma from depth with that in the chamber (i.e. as envisaged for the LG-1 to LG-4) would not have caused sulphide immiscibility along with chromitite crystallisation, but that influxes and mixing of slower-ascending magma, that fractionated en route, could give rise to sulphide liquid segregating along with the chromitite (i.e. the scenario for the LG-5 and overlying chromitites). The modelling also shows that the more fractionated the magma in the chamber becomes, the more sulphide will form, accounting for the overall upward increase in Pt and Pd above the LG-5.

Sediment-Hosted Disseminated Gold Deposits in Southwest Guizhou, PRC: Their Geological Setting and Origin in Relation to Mineralogical, Fluid Inclusion, and Stable-Isotope Characteristics

The sediment-hosted disseminated gold deposits in Southwest Guizhou, Peoples Republic of China (PRC) are located in faults on the flanks of anticlines or domes in clastic sedimentary rocks of Late Permian to Middle Triassic age on the southwestern edge of the Yangtze paraplatform. Lamprophyres crop out in the vicinity of the gold deposits. Mineralization in the area coincides with belts of weak Bouguer gravity and magnetic anomalies. The Lannigou and Yata deposits, described in detail in the present study, together with the Baidi deposit, are situated in the southeastern domain where mineralization was emplaced in fine turbidites of basinal facies of Middle Triassic age. The structures guiding this mineralization are high-angle reverse faults on domes or anticlines. To the northwest, the Getang deposit is one of a group of deposits including Zimudang, Sanchahe, Dayakou, and Xiongwu, which were emplaced in silicified breccias in impure carbonates or marls of Permian to Lower Triassic age. They are controlled by low-angle and bedding-parallel faults on anticlines. The clastic sedimentary host rocks are rich in illite and organic matter. Mineralization takes the forms of pervasive silicification, veinlets of quartz and disseminated auriferous arsenic-bearing pyrite and arsenopyrite, veins of quartz and calcite, and veinlets of realgar, cinnabar, and stibnite. Gold is mainly associated with arsenic-rich pyrite. The main-stage gold mineralization in pyrite is accompanied by pervasive silicification of host rocks. The Permian Emeishan basalts, widely distributed in the northwestern area, contain high average gold contents and may have been the primary source of the gold in the sediment-hosted deposits in Southwest Guizhou. The distribution of arsenic, antimony, and mercury in the host rocks and country rocks shows a pattern similar to that of gold. Gold is found mainly in pyrite and partly in illite. High-resolution electron-probe microanalysis (EPMA) of samples from the Lannigou deposit revealed that gold is located in pyrite rims in zones of intermediate arsenic content (3-5 wt%). It is deduced that gold probably occurs as discrete submicron-sized particles rather than as a charged Au species in a coupled diadochic substitution with arsenic in the pyrite structure. The auriferous fluids at the Lannigou and Yata deposits are shown to be CO2-rich (Xco2 > 0.05) and of low salinity (<5 wt% equiv. NaCl), with relatively high homogenization temperatures (mainly 240° to 300°C) and were probably trapped under high confining pressures (1.5 to 2.3 kbar). They are not typical epithermal fluids. At Lannigou, the δ34SVCDT values of sulfides range from +8.4 to +12.5‰, the δ13CVPDB of carbon in calcite ranges from -0.1 to -3.6‰, and the δ18OVSMOW of quartz and calcite are mainly around +17.6 and around +25.8‰, respectively. At Getang, the isotopic compositions of hydrothermal minerals are in the range δ34SVCDT of -14.3 to +4.4‰ for sulfides, δ13CVPDB of -3.2 to -0.6‰ for calcite and σ18OVSMOW of +14.0 to +15.3‰ for calcite and quartz. These isotope analyses show that sulfur was probably derived mostly from the sedimentary country rocks, and therefore inherited from the marine reservoir in which they were deposited, although part of the sulfur in the Getang deposit could be from altered or weathered basalt. Most of the carbon in the hydrothermal fluids was probably derived from the dissolution of carbonates in sedimentary rocks, although decarbonation reactions caused by low-grade metamorphism at deeper levels could have contributed some of the CO2. The original hydrothermal fluids responsible for the gold mineralization are deduced to have formed by burial metamorphism at depths of 6-8 km with addition of meteoric water through deep fractures. Mineralization probably took place as a result of decompression when impermeable shales in the cover sequence were structurally breached during the Yanshanian tectonic cycle, allowing fluids to escape from confinement at near lithostatic pressures in permeable clastic horizons. Mixing between evolved formation water/burial metamorphic water and meteoric waters was an important process during the late stage of the hydrothermal evolution. The tectonic setting, structural control, hydrothermal alteration, and ore and gangue mineral assemblages of the deposits in Southwest Guizhou show many features in common with those of the Carlintype gold deposits in Nevada, United States, although the host rocks, relationship to igneous rocks, and the timing of mineralization are different.

Skaergaardite, PdCu, a new platinum-group intermetallic mineral from the Skaergaard intrusion, Greenland

Abstract Skaergaardite, PdCu, is a new mineral discovered in the Skaergaard intrusion, Kangerdlugssuaq area, East Greenland. It occurs in a tholeitiic gabbro associated with plagioclase, clinopyroxene, orthopyroxene, ilmenite, titanian magnetite, fayalite and accessory chlorite-group minerals, ferrosapo- nite, a member of the annite-phlogopite series, hornblende, actinolite, epidote, calcite, ankerite, apatite and baddeleyite. The mineral is found in composite microglobules composed of bomite, chalcocite, digenite, chalcopyrite, with rare cobalt pentlandite, cobaltoan pentlandite, sphalerite, keithconnite, vasilite, zvyagintsevite, (Cu5Pd5Au) and Pt-Fe-Cu-Pd alloys, unnamed PdCu3, (Pd,Cu,Sn), Au3Cu and PdAuCu. Skaergaardite occurs as droplets, equant grains with rounded outlines, subhedral to euhedral crystals and as irregular grains that vary in size from 2 to 75 pm, averaging 22 pm. It is steel grey with a bronze tint, has a black streak, a metallic lustre and is sectile. Neither cleavage nor fracture was observed. The mineral has a micro-indentation hardness of VHN25 = 257. It is isotropic, non-pleochroic and exhibits neither discernible internal reflections nor evidence of twinning. Skaergaardite varies from bright creamy white (associated with bomite and chalcopyrite) to bright white (associated with digenite and chalcocite). Reflectance values in air (and in oil) are: 58.65 (47.4) at 470 nm, 62.6 (51.1) at 546 nm, 64.1 (52.8) at 589 nm and 65.25 (53.95) at 650 nm. The average of 311 electron-microprobe analyses gives: Pd 58.94, Pt 1.12, Au 2.23, Cu 29.84, Fe 3.85, Zn 1.46, Sn 1.08, Te 0.28 and Pb 0.39, total 99.19 wt.%, corresponding to (Pd0.967Au0.020Pt0.010)∑0.997(Cu0.820Fe0.120Zn0.039Sn0.016Te0.004Pb0.003)∑1.002. The mineral is cubic, space group Pm3m, a = 3.0014(2) Å, V = 27.0378 Å3, Z = 1. Dcalc is 10.64 g/cm3. The six strongest lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.122(100)(110), 1.5000(20)(200), 1.2254(50)(211), 0.9491(20)(310), 0.8666(10)(222), 0.8021(70)(321). The mineral has the CsCl-type structure. It is believed to be isostructural with wairauite (CoFe), synthetic CuZn (β-brass) and is structurally related to hongshiite (PtCu). Skaergaardite developed from a disordered Pd-Cu-rich metal alloy melt that had exsolved from an earlier Cu-(Fe) sulphide melt. Ordering of Pd and Cu (beginning at T ≈ 600°C) results in development of the CsCl structure from a disordered face-centred cubic structure.

Mineralogy and geochemistry of trace elements in bauxites: the Devonian Schugorsk deposit, Russia

Abstract Processes of mineral alteration involving the mobilization and deposition of more than 30 chemical elements during bauxite formation and epigenesis have been studied on specimens from the Devonian Schugorsk bauxite deposit, Timan, Russia. Chemical analyses of the minerals were obtained by electron microprobe and element distribution in the minerals was studied by element mapping. Interpretation of these data also utilized high-resolution BSE and SE images. The main rock-forming minerals of the Vendian parent rock are calcite, dolomite, feldspar, aegirine, riebeckite, mica, chlorite and quartz; accessory minerals are pyrite, galena, apatite, ilmenite, monazite, xenotime, zircon, columbite, pyrochlore, chromite, bastnaesite and some others. Typically, the grainsize of the accessory minerals in both parent rock and bauxite is from 1 to 40 mm. However, even within these rather small grains, the processes of crystal growth and alteration during weathering can be determined from the zonal distribution of the elements. The most widespread processes observed are: (1) Decomposition of Ti-bearing minerals such as ilmenite, aegirine and riebeckite with the formation of ‘leucoxene’, which is the main concentrator of Nb, Cr, V and W. Crystal growth can be traced from the zonal distribution of Nb (up to 16 wt.%). Vein-like ‘leucoxene’ is also observed in association with organics. (2) Weathering of columbite and pyrochlore: the source of Nb in ‘leucoxene’ is now strongly weathered columbite, while the alteration of pyrochlore is expressed in the growth of plumbopyrochlore rims around Ca-rich cores. (3) Dissolution of sulphide minerals and apatite and the formation of crandallite group minerals: ‘crandallite’ crystals of up to 40 mm size show a very clear zonation. From the core to the rim of a crystal, the following sequence of elements is observed: Ca → Ba → Ce → Pb → Sr → Nd. Sulphur also shows a zoned but more complicated distribution, while the distribution of Fe is rather variable. A possible source of REE is bastnaesite from the parent rock. More than twelve crandallite type cells can be identified in a single ‘crandallite’ grain. (4) Alteration of stoichiometric zircon and xenotime with the formation of metamict solid solution of zircon and xenotime: altered zircon rims also bear large amounts of Sc (up to 3.5 wt.%), Fe, Ca and Al in the form of as yet unidentified inclusions of 1 →2 μm. Monazite seems to be the least altered mineral of the profile. In the parent rock, an unknown mineral of the composition (wt.%): ThO2−54.8; FeO − 14.6; Y2O5− 2.; CaO − 2.0; REE −1.8; SiO2− 12.2; P2O5 − 2.8; total 94.2 (average from ten analyses) was determined. In bauxite, another mineral was found, which has the composition (wt.%): ThO2 − 24.9; FeO − 20.5; Y2O5 − 6.7; CaO − 2.0; ZrO − 17.6; SiO2 − 8.8; P2O5 − 5.4; total 89.3 (F was not analysed; average from nine analyses). Presumably, the second mineral is the result of weathering of the first one. Although the Th content is very high, the mineral is almost free of Pb. However, intergrowths of galena and pyrite are observed around the partially decomposed crystals of the mineral. Another generation of galena is enriched in chalcophile elements such as Cu, Cd, Bi etc., and is related to epigenetic alteration of the profile, as are secondary apatite and muscovite.

Tin-bearing skarns of South China: Geological setting and mineralogy

Tin in skarns forms a significant part of the total Sn resource of South China, and erosion of these primary deposits has produced rich alluvial concentrations, creating the basis for a historically important tin-mining industry. The skarns are genetically related to S-type (Transformation Series) granites of Yanshanian age, emplaced in carbonate sequences, mainly of Devonian and Triassic age. These occupy the Hercynian-Indosinian structural depressions within the Caledonian Foldbelt, constituting the Nanling Ranges of South China. Most of the Sn skarns in this broadly-defined province contain calcic mineral assemblages, which include andradite-grossular, hedenbergite-diopside, actinolite-tremolite, idocrase, wollastonite, epidote, chlorite and magnetite, with or without various sulphides. In certain cases, Sn is located in minerals such as cassiterite, malayaite and nordenskioldine. In many other deposits, Sn substitutes in the structure of calc-silicates and oxides, in particular in andradite, titanite, actinolite, epidote, axinite and rutile. At Gejiu in Yunnan Province, andradite can carry up to 5.14 wt% SnO2. In this case, it is believed that Sn4+ together with Mg2+ substitutes for Fe3+ in the octahedral sites, a coupled substitution which can be represented by the equation: Sn4+ + Mg2+ = 2 Fe3+. The behavior of Sn during skarn formation is governed by the P-T-x conditions of the skarn-forming solutions. Eh and pH variations and changes in the chemical potentials of major and trace elements (e.g., ƒS2), play a crucial role in determining the prevailing equilibria. During the high-temperature and alkaline stage of prograde metasomatism, an association of andradite + diopside formed under oxidizing conditions when Sn entered the lattice of andradite; or, in environments deficient in Fe, Mg and Al, but rich in Sn, malayaite formed. The association hedenbergite + grossular was formed under reducing conditions, and in this case Sn was retained in the hydrothermal solutions, becoming available for reactions at lower temperature. The rare Sn mineral nordenskioldine was formed at a late stage, during prograde alteration where conditions were uncommonly B rich and depleted in F, CO2 and, especially, Si. Sn was not deposited as cassiterite until the system evolved to retrograde low-temperature and acid conditions. During this stage, cassiterite formed with magnetite, actinolite and fluorite at the expense of tin-bearing andradite (e.g., Shizhuyuan, Hunan Province), or precipitated from the ore-forming solution together with chlorite and sulphides (e.g., Debao, Guangxi Province) or with quartz and sulphides (e.g., Dachang, Guangxi Province). Cassiterite, together with calcite, even formed at the expense of nordenskioldine in place (e.g., Gejiu). Malayaite was also formed with quartz and calcite at this stage (e.g., Zhengjialong, Jiangxi Province).

Rare earth element anomalies in crandallite group minerals from the Schugorsk bauxite deposit, Timan, Russia

Crandallite group minerals from the Devonian bauxite deposit of Schugorsk, Timan, Russia, were studied by electron microprobe. Special attention was given to the distribution of the rare earth elements (REE). Of the A-site cations, essential are (oxides in wt.%) Ca (0.2 - 7.9), Sr (0.4 - 5.9), Ba (0.1 - 5.1), Pb (0.01 - 8.4), Y (to 2.4) and REE (to 21.6). Bi (A-site), Ga (B-site) and V (X-site) occur in trace amounts. Two main types of REE distribution in crandallite group minerals are distinguished. The first shows a clear negative Ce anomaly and partially depleted Pr; crandallite group minerals of this type formed under strong oxidising conditions (positive Eh and neutral to slightly alkaline pH values) and lost Ce due to its oxidation to Ce 4+ and accumulation in other minerals such as anatase. The second type displays a positive Sm anomaly and a negative Pr anomaly. Individual crandallite group crystals belonging to this type, I - 40 μm in size, show a clear compositional zoning: the core is enriched in Ca, while the rim is enriched in S, Sr, Pb and the REE. This crandallite formed under reducing conditions related to stripping of Fe from the weathering profile. The presence of Sm 2+ in the crandallite lattice is proposed, and the role of organic material in its reduction is discussed. A Pr anomaly was inherited from the parent rock. A comparison of REE distribution in crandallite group minerals from different weathering profiles suggests that these minerals can be used to distinguish conditions of weathering.

Tellurium-bearing minerals in zoned sulfide chimneys from Cu-Zn massive sulfide deposits of the Urals, Russia

Tellurium-bearing minerals are generally rare in chimney material from mafic and bimodal felsic volcanic hosted massive sulfide (VMS) deposits, but are abundant in chimneys of the Urals VMS deposits located within Silurian and Devonian bimodal mafic sequences. High physicochemical gradients during chimney growth result in a wide range of telluride and sulfoarsenide assemblages including a variety of Cu-Ag-Te-S and Ag-Pb-Bi-Te solid solution series and tellurium sulfosalts. A change in chimney types from Fe-Cu to Cu-Zn-Fe to Zn-Cu is accompanied by gradual replacement of abundant Fe-, Co, Bi-, and Pb- tellurides by Hg, Ag, Au-Ag telluride and galena-fahlore with native gold assemblages. Decreasing amounts of pyrite, both colloform and pseudomorphic after pyrrhotite, isocubanite ISS and chalcopyrite in the chimneys is coupled with increasing amounts of sphalerite, quatz, barite or talc contents. This trend represents a transition from low- to high sulphidation conditions, and it is observed across a range of the Urals deposits from bimodal mafic- to bimodal felsic-hosted types: Yaman-Kasy → Molodezhnoye → Uzelga → Valentorskoye → Oktyabrskoye → Alexandrinskoye → Tash-Tau → Jusa.

Platinum-group element mineralization in an As-rich magmatic sulphide system, Talnotry, southwest Scotland

Abstract Arsenic-rich magmatic sulphide mineralization is hosted by a diorite intrusion at Talnotry, southwest Scotland. A relatively abundant and diverse platinum-group mineral assemblage is present and is dominated by sperrylite, irarsite and electrum with subordinate merenskyite, michenerite and froodite. Early euhedral gersdorffite is enriched with respect to Rh, Ir and Pt and in some cases contains exsolved blebs of irarsite or euhedral grains of sperrylite. Sperrylite is also enclosed within silicates and sulphides indicating that it crystallized directly from an As-rich sulphide liquid. Pyrrhotite-chalcopyrite mineral assemblages are consistent with the fractional crystallization of monosulphide solid solution and are overlain by PGE-, Ni- and As-rich mineral assemblages indicative of crystallization from a NiAs liquid. Late-stage, cross-cutting, electrum-bearing chalcopyrite veins are consistent with the crystallization of Cu- and Au-rich intermediate solid solution. The chemistry, mineralogy and lithological relationships of the diorite suggest that it may be an appinite and as such is potentially analogous to the Au-rich lamprophyre dykes present within southwest Scotland.

Closed-vessel microwave digestion technique for lichens and leaves prior to determination of trace elements (Pb, Zn, Cu) and stable Pb isotope ratios

A reliable and robust procedure using closed-vessel microwave digestion of lichens and leaves for precise and accurate determination of trace elements (Pb, Zn and Cu) and stable Pb isotope ratios is presented. The method was developed using certified reference material CRM 482 Pseudovernia furfurea (Lichens), NIST 1515 (Apple Leaves) and NIST 1547 (Peach Leaves) and tested on lichens from a mining site in Russia. A mixture of 3 mL of HNO3, 3 mL of H2O2, 2 mL of H2O and 0.8 mL of HF ensured complete sample dissolution with 100 ± 5% recovery for Pb, Zn and Cu at a maximum temperature of 210°C and pressure of 350 psi. The amount of HF and microwave pressure significantly influenced Pb, Zn and Cu recovery. Comparison between EMMA-XRF and ICP-AES showed a good correlation between Pb, Zn and Cu concentrations. Using the newly developed digestion method, Pb isotopes in lichens from the mining site were determined with an internal precision better than 0.02%.

Characterisation of carbon nanotubes in the context of toxicity studies

Nanotechnology has the potential to revolutionise our futures, but has also prompted concerns about the possibility that nanomaterials may harm humans or the biosphere. The unique properties of nanoparticles, that give them novel size dependent functionalities, may also have the potential to cause harm. Discrepancies in existing human health and environmental studies have shown the importance of good quality, well-characterized reference nanomaterials for toxicological studies.Here we make a case for the importance of the detailed characterization of nanoparticles, using several methods, particularly to allow the recognition of impurities and the presence of chemically identical but structurally distinct phases. Methods to characterise fully, commercially available multi-wall carbon nanotubes at different scales, are presented.