Apatite is one of the accessory minerals in the UHP metamorphosed eclogites at Northern Dulan belt, North Qaidam. It appears in three kinds of occurrences: (1) included in garnet which often shows cracks along the apatite grains, (2) coexisting with omphacite, rutile and/or clinozoisite in matrix and often surrounded by garnet, and (3) coexisting with retrograded minerals. The three eclogite samples examined in the present study are enriched in garnet. Two of them contain up to 80 vol.% garnet and the other is a porphyry of medium-grained garnet. They commonly show cracks and features of retrograde metamorphism, such as fissure-filling of secondary minerals including calcite or greenschist facies minerals. There are two size-ranges of well-oriented sulfide minerals included in apatite. One is nanometer-sized sulfide needles (50 ¡Ñ 20 ~ 870 ¡Ñ 120 nm) and particles (55 ~ 370 nm). The other is micrometer-sized sulfide needles (~20 ¡Ñ 0.5 £gm) and rods (~2.5 ¡Ñ 0.5£gm). Fluid inclusions and the micrometer-sized sulfide minerals commonly occur in the apatite grains that are near the cracks. Both nanometer- and micrometer-sized sulfide minerals are elongated with their long axes being normal or parallel to the c axis of the apatite. We used SEM-EDS and TEM-EDS to analyze and found that the sulfide minerals are troilite, pyrrhotite, Cu-bearing pyrrhotite and chalcopyrite. There are two sets of preferred crystallographic orientations for the dominated troilite and host apatite. The rod troilite is elongated along its a axis and <001>troilite // <001>apatite, <48-3>troilite ∡ <13-3>apatite = ~ 0.6º, (2-10)troilite // (3-10)apatite, <100>troilite ∡ <100>apatite = ~ 10º. The needle troilite is also elongated along its a axis and <001>troilite ¡æ <001>apatite, <-110>troilite ∡ <-12-2>apatite = ~ 1.3º, (11-2)troilite // (0-1-1)apatite. The preferred crystallographic orientation relationships, in terms of the c axis of troilite being parallel or normal to the c axis of host apatite, are similar to those for oriented quartz precipitates and omphacite hosts in the previous studies. According to the observations that only few sulfide minerals included in other minerals, the occurrences of apatites, and the microtextures of sulfide minerals, we suggest that the origin of sulfide minerals may relate to metasomatism during plate subduction. Metal ions such as iron, copper, cobalt and nickel were carried by chlorine- and sulfur-enriched fluids, which might be trapped as primary fluid inclusions in the apatite. The sulfide minerals then formed at the sites of fluid inclusions with the aid of fluids and available ions.

The exploitation of metallic sulfide ores produces vast quantities of fine-grained wastes hosting potentially toxic elements (PTEs). There are concerns that, if improperly disposed of and managed, waste mineral particles can behave as vectors that disperse PTEs via aeolian and fluvial transport, subsequently contaminating soils and crops used to support human populations. The importance of these particles, as sources and influencers of PTE biogeochemistry in productive soils, has received limited research. Long-term (365 d) batch incubation experiments, field weathering experiments and phytoavailability trials, were performed to establish the rate, patterns and factors limiting PTE (Cd, As, Hg) release from grains of sphalerite (Zn(Fe,Cd)S), arsenopyrite (FeAsS) and cinnabar (HgS) into soil matrices (0.1 % mineral:soil m/m), and the bioavailability of the liberated PTEs to important food crops (Tricitum aestivum, wheat and Oryza sativa, rice). All three of the ores underwent chemical weathering in oxic agricultural soils of both temperate and sub-tropical provenance, during which nonessential PTEs (cadmium, mercury, arsenic) were released in bioavailable forms, at rates relevant to agricultural production. Sphalerite weathered at a rate of 0.6 to 1.2 % a-1 (Cd basis) in the experimental soils, releasing 0.5 to 1 μmol Cd g-1 ZnS a-1 into the soil matrix. Cinnabar weathering reached a maximum of 12.0 – 13.5 % (Hg basis) after 90 days exposure in oxic soils, whereas arsenopyrite weathering was rapid and extensive, reaching 56 to 66 % (S basis) after 180 days. The PTE concentrations accumulated in edible grains of wheat and rice grown in the sulfide-contaminated soils were higher than international food safety limits by factors of 8 (Cd in rice), 10 – 30 (Hg in wheat and rice) and 8 – 12 (As in wheat and rice). The primary geochemical factors controlling PTE release and bioavailability were solid-phase associations (i.e. PTEs complexed by clays, metal oxyhydroxides and organic matter) and the precipitation of secondary mineral phases. Weathering arsenopyrite grains were passivated from further oxidation by secondary iron-arsenate phases, which also co-precipitated arsenic liberated from the ore. Secondary phase formation was identified as the cause of decreasing extractable Hg (liberated from cinnabar) after mercury release from cinnabar peaked (≤ 90 days exposure). For sphalerite, the evidence indicates that secondary sulfide phases formed under flooded (sulfate-reducing) soil conditions (paddy rice), limited the bioavailability of cadmium previously liberated under oxic conditions. These key findings demonstrate a potential human health hazard relating to the dispersal of PTE-hosting sulfide ore particles produced by mining activities into soils supporting human populations via crop contamination. This work also highlights differences in ore geochemistry, showing the need for additional research on different ore minerals and their alteration products.

<p> The existence of significant and consistent fractionation of sulfur isotopes in natural coexisting sulfide mineral pairs is demonstrated. Such fractionations are shown to depend exclusively on the mineralogy of the assemblage and temperature of equilibration, consistent with a process of fractionation during equilibrium exchange of sulfur isotopes between the sulfide phases . The fractionation of sulfur isotopes between galena, sphalerite and pyrite has been determined experimentally over the temperature range 300-725°C . The fractionation for each mineral pair is shown to vary as T^(-2) so that three isotope geothermometer s have been calibrated. Experi mental results are applied to natural sulfide assemblages to determine the temperature of equilibration and extent to which isotopic equilibriurn has b een attained. </p> / Thesis / Doctor of Philosophy (PhD)