Antimony quartz and antimony–gold quartz veins from northern Portugal

Antimony- and Pb–Sb-quartz veins from the Bragança district, Portugal, are mainly hosted by Silurian phyllites. Antimony–Au-quartz veins from the Dúrico–Beirã region are mainly hosted by a Cambrian schist–metagraywacke complex, as well as Ordovician phyllites and quartzites. The deposits were mostly exploited in the late 19th Century. Mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas. First and second generations of arsenopyrite precipitated at 390 and 300 °C, respectively. Berthierite and stibnite are the most abundant Sb-bearing minerals and precipitated between 225 and 128 °C, native antimony at < 200 °C. Drastic fluid cooling is the main cause of mineral precipitation. The Pb isotope compositions of stibnite suggest a homogeneous crustal source of lead, from the metasedimentary sequences, for Sb, Pb–Sb and Sb–Au deposits in both areas, which is consistent with the findings for comparable mineralizations elsewhere in Europe. Remobilization of Pb is related to Variscan metamorphism and deformation.     

P-T-X conditions of hydrothermal fluids and precipitation mechanism of stibnite-gold mineralization at the Wiluna lode-gold deposits,Western Australia: conventional and infrared microthermometric constraints

The Wiluna lode-gold deposits are located in the Archean Wiluna greenstone belt, in the northern sector of the Norseman-Wiluna belt in the Yilgarn Craton of Western Australia. They are hosted in subgreenschist facies meta-basalts, and controlled by the Wiluna strike-slip fault system and associated shear veins and breccias. The 13 individual lode-gold deposits have produced around 115 t Au from 1901 to 1946 and 1986 to today. Historically, they also produced 38.3 t As and 3.5 t Sb. Gold formed in two stages: stage 1 gold-pyrite-arsenopyrite is finely disseminated in the wallrock and breccia fragments, whereas stage 2 gold-stibnite is located in massive shear veins and breccia matrix, as fracture-fill and in banded-colloform textured veins. Stibnite-gold orebodies only occur in some of the deposits (e.g., Moonlight and northern part of the West Lode) and also display a restricted vertical extent, being preserved only in the uppermost 200 m of stibnite-bearing lodes.Petrographic, conventional, and infrared microthermometric and laser-Raman analysis on stibnite-bearing quartz veins and breccias reveal that the antimony- and gold-rich hydrothermal fluid was of mixed H₂O-NaCl-CO₂±CH₄ type. Microthermometric measurements reveal maximum homogenization temperatures of 340 °C (average 290±25 °C), and a wide range of salinities between 0.2 and 23 eq. wt% NaCl. Aqueous-carbonic fluid inclusions contain variable X_CO2+CH4 (0.03 to 0.82), with the carbonic phase containing a maximum X_CH4 of 0.21.Combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low-salinity, medium X_CO2+CH4, homogeneous parent fluid at about 290 °C and pressures between 700 and 1,700 bar. Fluid immiscibility was triggered by cyclic pressure release during fault-zone movement. The decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to lower temperatures, significantly reduced the degree of stibnite undersaturation, and caused stibnite to precipitate. The deposition of stibnite reduced the ore-fluid H₂S concentration, thereby destabilized gold bisulfide complexes in solution, and caused gold precipitation locally. This mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite-gold orebodies at Wiluna.Editorial handling: B. Lehmann     

Pentlandite sulfur core electron binding energies

A number of freshly abraded surfaces of pentlandite have been characterised by X-ray photoelectron spectroscopy to establish whether the initial intensity of the S 2p component near 161.4 eV, previously assigned to the 25% of S atoms in fourfold coordination by metal atoms in pentlandite, was always at least 25% of the total S 2p intensity. It was found that the intensity of this S 2p component could be lower than 20% for surfaces that were not significantly oxidised. To assess whether the proposed 0.75–0.8 eV 2p binding energy difference for the two sulfur environments in pentlandite was justified, ab initio calculations of the difference in core electron binding energies and of the densities of unfilled states have been carried out. The corresponding simulated S K near-edge X-ray absorption fine structure (NEXAFS) spectra have been compared with experimental spectra. The calculated S 2p and S 1s binding energy differences were 0.45 and 0.5 eV at most, in agreement with the experimental NEXAFS spectra. It was concluded that the S 2p component near 161.4 eV arises entirely from violarite present at the pentlandite surface rather than from 4-coordinate S in pentlandite itself. Ab initio calculations of the difference in S 2p binding energies for the 2- and 3-coordinate S in stibnite have also been carried out and found to be quite small, in agreement with previously reported experimental values. Nevertheless, for both pentlandite and stibnite, calculations have confirmed that an increase in coordination number is associated with an increase in sulfur core electron binding energies, even although that increase is barely measurable for the latter sulfide.     

贵州晴隆锑矿床成矿流体He-Ar同位素地球化学

晴隆锑矿床是华南中生代低温成矿域右江盆地Au-Sb-As-Hg矿集区内的大型锑矿床,成矿时代约150Ma。本文以该矿床矿石矿物辉锑矿中的流体包裹体为测试对象,研究了成矿流体的He和Ar同位素地球化学。研究表明,成矿流体的~3He/~4He为0.13~0.46Ra(Ra为空气的3He/4He值,1.4×10~(-6)),~(40)Ar/~(36)Ar为305~327,He、Ar同位素组成具有一定程度的正相关;成矿流体由两个端元组成:一是含地壳He的低温饱和空气的雨水,二是含地幔He的高温流体。含地幔He的高温流体可能来自右江盆地下部的侏罗纪壳幔混合成因花岗岩浆,这种岩浆的形成机制与华夏地块侏罗纪与钨锡成矿有关的花岗岩具有相似性。晴隆锑矿床以大气降水为主的成矿流体的加热、循环并浸取矿床围岩"大厂层"下伏地层中的成矿元素而成矿,受深部壳幔混合成因花岗岩浆释放出的含幔源He和岩浆S的热流所驱动。