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The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded 13CVPDB values of –11 to +1 and 18OVSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. 34SVCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D1 deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz 相似文献
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柴达木盆地东部表土花粉分析 总被引:21,自引:1,他引:20
柴达木盆地东部荒漠区 8个样地、 2 3个样点的表土花粉分析较好地揭示了荒漠区花粉组合的特点及其与植被、环境的关系。研究发现 ,荒漠区表土花粉以藜科 (Chenopodiaceae)和蒿属 (Artemisia)为主 ,并以前者占优势地位 ,麻黄属 (Ephedra)亦较常见 ,禾本科(Gramineae)、菊科 (Compositae)植物在群落中也占有一定的比例。花粉百分比含量DCA排序与相关分析发现 ,荒漠区不同植物群落下表土花粉组合特征各不相同 ,而且与相应的植物群落及其所代表的环境特征相一致。荒漠区表土花粉蒿 /藜 (Artemisia/Chenopodiaceae)比对干旱程度具有指示作用 ,比值越小 ,越干旱。 相似文献
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在线渗析-离子色谱法测定土壤中有效氟 总被引:1,自引:1,他引:0
土壤样品经超声波超声静置,提取上清液,经渗析进入色谱柱,采用Metrohm A Supp 4-250阴离子交换柱,以1.8 mmol/L碳酸钠-1.7 mmol/L碳酸氢钠混合溶液作为流动相,用在线渗析技术与离子色谱法联用测定土壤中的有效氟。与常规方法相比,建立的方法操作简单,避免了繁琐的样品前处理。加标回收率为90.5%~95.7%,精密度(RSD,n=6)为1.60%。该方法具有高效、快速、灵敏等特点,能够用于环境土壤样品中有效氟的检测。 相似文献
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土壤环境质量评价是了解土壤环境质量状况、采取保护措施的重要前提,本文用土壤环境质量标准(GB15618-1995),对河北平原表层土壤重金属环境质量进行了评价,结果表明:河北平原表层土壤环境质量优良,一、二级土壤分别占全区93.02%和6.62%。同时又根据重金属元素的环境地球化学行为,采用富集因子,有效地区分了重金属污染的自然影响和人为影响,结果显示,河北平原表层土壤人为污染的面积较大,但以轻微污染为主。 相似文献