Origin and fluid geochemistry of unconformity-related gold at the Black Ridge gold deposit, Clermont, Queensland
Origin and fluid geochemistry of unconformity-related gold at the Black Ridge gold deposit, Clermont, Queensland
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昆士兰州克莱蒙 Black Ridge 金矿床中与不整合面相关的金矿的成因和流体地球化学
DOI：
10.2113/gsecongeo.89.7.1469
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发表时间：
1994
期刊：
影响因子：
--
通讯作者：
G. Dong
中科院分区：
文献类型：
--
作者：
Taihe Zhou;S. Dobos;G. Dong
Black Ridge 矿床中的金主要集中在元古代（？）Anakie 变质岩与二叠纪 Blair Athol 煤系砾岩之间的不整合面。包括第三纪玄武岩在内的所有上层岩石单元的金浓度要低得多。该矿床通常被视为化石砂矿。基于详细的地表和地下测绘以及 57 个钻孔的测井，对 Black Ridge 矿床的矿物学、流体包裹体、稳定同位素地球化学和热力学进行了系统研究，为该矿床提供了新的地质、矿物学和地球化学数据，这些数据与砂矿模型不一致，有利于热液流体混合模型。在包括第三纪玄武岩在内的所有岩石单元中都观察到了菱铁矿蚀变。岩心测井和现场观测揭示了金与菱铁矿的密切空间关联。根据化学分析和显微镜观察，金品位与菱铁矿蚀变强度之间的相关性进一步证实了这种空间关系。金与菱铁矿和白铁矿的直接共生共存也经常在显微镜、扫描电镜和电子显微镜下观察到。金的品位与菱铁矿中分带的成分变化相关。此外，主要矿带和单个矿体在结构上受到控制。菱铁矿中碳和氧的稳定同位素研究表明，有两种不同的流体来源参与了蚀变。一种（流体I）是不整合面上方砾岩中的下降流体，另一种（流体II）是从不整合面下方的变质岩中上升的。碳和氧同位素数据都表明沿不整合面的流体混合过程。黑岭金的沉积与菱铁矿直接相关，可能是由流体混合引起的。矿物学和流体包裹体研究以及热力学分析表明，流体 II 相对还原，温度约为 250 摄氏度，pH > 或 = 6.6，而混合流体 (III) 被轻微氧化（在硫化物-氧化物边界），温度约为 120 至 150 摄氏度，pH < 或 = 5。原始流体 I 可能是冷的，相对氧化且呈弱酸性，尽管定量参数仍未知。主要由流体II引入，菱铁矿沉淀主要受宿主环境中铁的含量和氧化态控制。热力学模型表明，在不整合面上方的流体混合应该导致黑岭流体中金的溶解度下降四个数量级以上。计算进一步表明，在与菱铁矿平衡的系统中，金可能在非常低的流体浓度下沉淀。热液蚀变和矿化可能是由二叠纪沉积后的火成岩活动或与构造运动相关的变质作用产生的。无论哪种情况，昆士兰中部和澳大利亚东部的二叠纪后热液金矿化潜力可能都很大。
Gold in the Black Ridge deposit is mainly concentrated along the unconformity between the Proterozoic(?) Anakie Metamorphics and the conglomerates of the Permian Blair Athol coal measures. Much lower gold concentrations occur through all upper rock units including the Tertiary basalts. The deposit is conventionally regarded as a fossil placer. Systematic studies of the mineralogy, fluid inclusion, stable isotope geochemistry, and thermodynamics of the Black Ridge deposit based on detailed surface and underground mapping and on logging of 57 drill holes have provided new geologic, mineralogical, and geochemical data for the deposit which are inconsistent with the placer model and favor a hydrothermal fluid mixing model.Siderite alteration has been observed in all rock units including the Tertiary basalt. A close spatial association of gold with siderite is revealed by core logging and field observations. This spatial relationship is further confirmed by the correlation between gold grade and intensity of siderite alteration according to chemical analysis and microscope observation. The direct paragenetic coexistence of gold with siderite and marcasite is also commonly observed under the microscope, SEM, and electron microprobe. The grade of gold is correlated with compositional variation of zoning in siderite. Moreover, the main ore zones and the individual orebodies are structurally controlled.Stable isotope studies of carbon and oxygen in siderite suggest that there were two different fluid sources involved in the alteration. One (fluid I) was a descending fluid in the conglomerates above the unconformity, and the other (fluid II) rose from the metamorphic rocks below the unconformity. Both the carbon and oxygen isotope data indicate fluid-mixing processes along the unconformity. The deposition of Black Ridge gold, which is directly associated with siderite, may have been caused by fluid mixing. Mineralogic and fluid inclusion studies and thermodynamic analyses suggest that fluid II was relatively reduced and roughly 250 degrees C, with a pH > or = 6.6, whereas the mixed fluid (III) was slightly oxidized (at the sulfide-oxide boundary), and about 120 degrees to 150 degrees C, with a pH < or = 5. The original fluid I was probably cool, relatively oxidized, and slightly acidic, although the quantitative parameters remain unknown.It is proposed that carbon may have been largely introduced by fluid II, and siderite precipitation controlled mainly by the content and oxidation state of iron in the host environments. Thermodynamic modeling indicates that fluid mixing just above the unconformity should have led to the gold solubility dropping more than four orders of magnitude in the fluids at Black Ridge. The calculation further suggests that in a system which is in equilibrium with siderite, gold may precipitate at very low fluid concentrations.Hydrothermal alteration and mineralization may have been generated by either igneous activity or tectonic movement-related metamorphism after Permian sedimentation. In either case, the potential for post-Permian hydrothermal gold mineralization may be significant in central Queensland and eastern Australia generally.