Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry
Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry
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DOI：
10.1016/0169-1368(95)00026-7
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发表时间：
1996-05
影响因子：
3.3
通讯作者：
K. Zaw;J. Gemmell;R. Large;T. Mernagh;C. Ryan
中科院分区：
文献类型：
--
作者：
K. Zaw;J. Gemmell;R. Large;T. Mernagh;C. Ryan
Hellyer矿床是塔斯马尼亚西部寒武纪里德火山带中典型的大吨位、高品位、丘式火山岩型块状硫化物(VHMS)矿床。在矿床正下方的下盘，有一条变化很大的管道，其中包含一个发育良好和保存完好的管弦带。Hellyer的矿脉共生表明，矿化前阶段1的矿脉全部由石英组成，分布在整个蚀变管道中。同矿化2期脉是脉带中最丰富的脉，由3个亚期组成：2期脉为碎屑状石英、黄铁矿和碳酸盐脉体，少量黄铜矿、闪锌矿和方铅矿；2期脉为富含贱金属硫化物、少量石英、碳酸盐和重晶石脉体；2期脉为粗晶重晶石脉，含不同量的黄铁矿、闪锌矿、方铅矿和碳酸盐。3-6期脉为矿化后脉，与泥盆纪塔伯拉贝兰造山带有关。Hellyer Stringer系统中流体包裹体的结构、岩相学和显微测温研究表明，I型原生液蒸汽包裹体沿碎屑状石英晶体的生长面或在带状闪锌矿的色带内赋存。包裹体大小为10~15μm，均一温度分别为1 70~2 2 0℃、1 65~32 2℃、1 90~2 5 6℃。这些数据表明了一段热史的兴衰。然而，平均盐度保持在8-11个氯化钠当量之间。WT%在所有2期静脉中。在富贱金属的2B期脉中发现了含黄铜矿的原生流体包裹体。微量测温法没有记录到二氧化碳存在的证据(例如，笼状化合物的形成)。然而，激光拉曼光谱(LRS)分析表明，在2B阶段的矿脉中存在CO2(1摩尔%)，而在2A和2C阶段的矿脉中没有检测到CO2。流体包裹体的半定量SEM/WDS微探针分析表明，相对于海水，Hellyer成矿流体富钾、富钙，贫镁。石英中流体包裹体的PIXE微区分析表明，2B期成矿流体的基本金属含量明显高于2A期矿脉。矿化后阶段4脉基本金属含量较低，但变化较大。在这项研究中，没有流体包裹体沸腾的证据。阳离子组成、相对于海水更高的盐度以及二氧化碳的存在，表明循环海水本身不是矿液的唯一来源。这一解释与以前在Hellyer Stringer系统中的同位素研究是一致的。尽管目前的流体包裹体数据不能证明来自岩浆室的大量矿石成分的直接输入，但不能排除来自岩浆来源的矿液的这种贡献。来自岩浆源的可能输入可能发生在富贱金属的2B阶段，其特征是沉积温度升高，贱金属和CO2含量较高。
The Hellyer deposit is a classic, large tonnage, high-grade, mound style volcanic-hosted massive sulphide (VHMS) deposit in the Cambrian Mt Read Volcanic belt of western Tasmania. In the footwall directly underlying the deposit, there is an extensively altered pipe which contains a well developed and preserved stringer zone. The vein paragenesis at Hellyer indicates that premineralization Stage 1 veins consist entirely of quartz, and occur throughout the alteration pipe. The synmineralization Stage 2 veins are the most abundant veins in the stringer zone and consist of three sub-stages: Stage 2A veins of crustiform quartz, pyrite, andc arbonate with minor amounts of chalcopyrite, sphalerite and galena, Stage 2B veins with abundant base metal sulphides, minor quartz, carbonate and barite gangue and Stage 2C veins of coarsely crystalline barite with variable amounts of pyrite, sphalerite, galena and carbonate. Stages 3–6 veins are postmineralization veins and are related to the Devonian Tabberabberan Orogeny. Textural, petrographic and microthermometric investigations of fluid inclusions in the Hellyer stringer system indicate that Type I, primary, liquid-vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite. These inclusions are 10–15 μm in size, and yielded homogenisation temperatures of 170–220°C in early 2A veins, 165–322°C in main-stage 2B veins and 190–256°C in late-stage 2C veins. These data suggest a waxing and waning thermal history. However, the average salinity remained between 8–11 NaCl equiv. wt% in all Stage 2 veins. Chalcopyrite-bearing primary fluid inclusions have been recognised in the base metal-rich Stage 2B veins. No evidence for presence of CO2(e.g. formation of clathrates) was recorded by microthermometry. However, Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2(< 1 mole%) in the Stage 2B veins, and no detectable CO2in 2A and 2C vein stages. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz indicates that the Stage 2B ore fluids have a significantly higher base metal concentration compared to the Stage 2A veins. The postmineralization Stage 4 veins have a variable but lower base metal content. In this study, there was no fluid inclusion evidence of boiling. Cation composition, higher salinities relative to seawater and the presence of CO2, suggest that recycled seawater alone cannot be the sole source of the ore fluids. This interpretation is in agreement with previous isotopic studies in the Hellyer stringer system. Although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data, such a contribution of ore fluids from a magmatic source cannot be ruled out. The possible input from the magmatic source may have occurred during the base metal-rich Stage 2B vein formation characterised by the intensifying temperature of deposition, higher base metals and CO2contents.