Invisible gold in arsenian pyrite from the high-grade Hishikari gold deposit, Japan: Significance of variation and distribution of Au/As ratios in pyrite
Invisible gold in arsenian pyrite from the high-grade Hishikari gold deposit, Japan: Significance of variation and distribution of Au/As ratios in pyrite
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DOI：
10.1016/j.oregeorev.2018.02.029
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发表时间：
2018-04
影响因子：
3.3
通讯作者：
Y. Morishita;N. Shimada;K. Shimada
中科院分区：
文献类型：
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作者：
Y. Morishita;N. Shimada;K. Shimada
利用二次离子质谱仪(SIMS)和电子探针微区分析(EPMA)技术，对Hishikari矿床黄铁矿中Au和As的含量进行了广泛的测定，以探讨Au和As之间的微观关系。低硫化浅成热液Hishikari矿床自1985年以来(截至2016年3月)已生产超过224 t的黄金，以其高品位矿石而闻名世界。通过制备注入Au的黄铁矿标准样品并使用适当调整的SIMS分析方法，可以准确地对黄铁矿中的Au进行SIMS定量分析。由于Hishikari矿床的黄铁矿颗粒具有不均匀的As浓度，因此选择了3 μm的分析规模。本文给出了分布广泛的高品位含金黄铁矿3 μm范围内Au和As含量的首次综合数据集。用SIMS和EPMA对Hishikari金矿床中的砷黄铁矿进行了SIMS和EPMA分析，结果表明，3 m范围内0~7 %的As含量与Au含量呈正相关。Hishikari脉的形成分为两个阶段，又分为早期和晚期两个亚阶段。黄铁矿颗粒的边缘在核形成后沉淀，但核和边缘形成的时间尺度远远短于整个成矿序列的时间尺度。黄铁矿中Au/As比值的特定值具有特定的含金脉及其深度的特征。因此，根据Au/As比值可以推断未知矿脉的性质。虽然不同矿脉的Au/As值不同，但Hishikari矿床各矿带的Au/As值有一个总的趋势。三晋矿带Au/As值高，富含高品位脉体的石漫托群白垩系增生沉积岩及其以上。本子矿带Au/As值适中，该矿带由多条高品位矿脉组成，赋存于基底沉积岩及其以上。另一方面，山田矿带产于更新世安山期火山碎屑岩中，矿脉品位较低，不整合地覆盖于基底沉积岩之上。与其他矿带相比，山田黄铁矿中Au/As的比值相对较低。已有文献提出，黄铁矿中的As−可能有助于As氧化将Au+还原为Au+As Au纳米颗粒(NPs)；然而，在本研究中，砷黄铁矿中的Au和As浓度落在或低于Au的溶解度极限。此外，黄铁矿中As和S的浓度呈负相关，这与还原环境下天然砷黄铁矿固溶体Fe(S1−xAsx)2固溶体中As取代S作为阴离子As−是一致的。目前的研究表明，Hishikari Au以固溶体形式存在于Hishikari砷黄铁矿中，尽管Au和As的浓度降至Au的溶解极限或以下，但仍有部分以NPs的形式存在的可能性。
Au and As concentrations in pyrite grains from the Hishikari deposit were extensively determined by secondary ion mass spectrometry (SIMS) and electron probe microanalysis (EPMA) to investigate the microscale relationship between Au and As. The low-sulfidation epithermal Hishikari deposit, which has produced more than 224 t of gold since 1985 (as of March 2016), is world-famous for its high-grade ore. Accurate quantitative SIMS analyses of Au in pyrite were possible by preparing an Au-implanted pyrite standard sample and by using a properly tuned SIMS analytical method. An analysis size of 3 μm was chosen since the pyrite grains from the Hishikari deposit have heterogeneous As concentrations. This paper presents the first comprehensive data set of Au and As concentrations in a 3 μm area of pyrite obtained from the widely distributed high-grade Au-bearing Hishikari veins.Microanalyses of arsenian pyrite from the Hishikari gold deposit by SIMS and EPMA revealed that Au concentrations of 0.1–2800 ppm are positively correlated with As concentrations of 0 to 7 wt% in a 3 μm area. The formation of Hishikari veins is divided into two stages that are further subdivided into the early and late sub-stages. The rim of a pyrite grain precipitates after formation of the core; however, the time scale of core and rim formation is far shorter than that of the entire mineralization sequence. The specific value of the Au/As ratio in pyrite proves to be characteristic of the specific Au-bearing vein and its specific depth. Therefore, it could be possible to infer the nature of unknown veins by the Au/As ratio obtained. Although the Au/As ratio varies from vein to vein, a general tendency is observed for each ore zone of the Hishikari deposit. The Au/As ratio is high in the Sanjin ore zone, which consists of numerous high-grade veins in or above the Cretaceous accretionary sedimentary rocks of the Shimanto Group. The Au/As ratio is moderate in the Honko ore zone, which consists of several high-grade veins and also occurs in or above the basement sedimentary rocks. On the other hand, the Yamada ore zone, which consists of relatively low-grade veins, occurs in Pleistocene andesitic pyroclastic rocks that overlie the basement sedimentary rocks unconformably. The Au/As ratio in pyrite of the Yamada ore zone is relatively low compared with that of other ore zones.Several previous papers have proposed that the As−in pyrite may contribute to the reduction of Au+to Au0as Au nanoparticles (NPs) by As oxidation; however, Au and As concentrations of arsenian pyrites fall on or below the Au solubility limit in this study. Moreover, the As and S concentrations in pyrite are inversely correlated, which is consistent with the substitution of As for S as anionic As−in the Fe(S1−xAsx)2solid solution of natural arsenian pyrite under reducing environments. The present study suggests that the Hishikari Au exists as a solid solution in the Hishikari arsenian pyrite, although there remains a possibility that it exists partially in the form of NPs even though the Au and As concentrations of arsenian pyrites fall on or below the Au solubility limit.