Complexity in the Au–Ag–Hg system: New information from a PGE (‘osmiridium’) concentrate at Waratah Bay, Victoria, Australia

IF 2.8 3区 地球科学 Q2 MINERALOGY
William D. Birch, Chi Ma
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Abstract

Au–Hg–Ag phases have been described from a variety of metallogenic orebodies and the placer deposits derived from them. In many documented placer deposits, the phases typically occur intergrown as ‘secondary’ rims to primary Au–Ag grains. The origin of these rims has been ascribed to supergene redistribution reactions during deposition or to the effects of amalgamation (i.e. use of mercury) during mining for gold. Difficulties in determining compositions and crystal structures on such a small scale have made full characterisation of these phases problematic. This paper describes a new occurrence of these phases, found by accident during investigation of a historical concentrate of ‘osmiridium’ containing a number of gold grains from beach sands at Waratah Bay, in southern Victoria, Australia. The phases occur as rims to gold grains and are intergrown on a scale of tens of micrometres or less. Application of electron microprobe analysis (EPMA) and limited electron back-scattered diffraction (EBSD) was required to characterise them. These techniques revealed the presence of the approved mineral weishanite (Au–Hg–Ag) and a phase with compositional range Au2Hg–Au3Hg surrounding primary Au–Ag (electrum) containing trace amounts of Hg. EBSD analysis showed weishanite is hexagonal P63/mmc and Au2Hg to be hexagonal P63/mcm. Comparison with published data from other localities (Philippines, British Columbia and New Zealand) suggests weishanite has a wide compositional field. Textures shown by these phases are difficult to interpret, as they might form by either supergene processes or by reaction with anthropogenic mercury used during mining. However, in the absence of any historical evidence for the use of mercury for gold mining at Waratah Bay, we consider the formation of the Au–Hg phases is most probably due to supergene alteration of primary Au–Ag alloy containing small amounts of Hg. In addition to revealing some of the reaction sequences in the development of these secondary Au–Hg–Ag rims, this paper illustrates methods by which these phases can be more fully characterised and thereby better correlated with the Au–Hg synthetic system.

金-银-汞系统的复杂性:来自澳大利亚维多利亚州瓦拉塔湾的一块 PGE(锇)精矿的新信息
从各种成矿矿体及其衍生的砂矿床中描述了金、氢、银相。在许多有记录的砂矿床中,这些相通常作为“次生”边缘与原生金银颗粒共生。这些边缘的起源被认为是沉积过程中的表生再分配反应,或者是金矿开采过程中汞化(即汞的使用)的影响。在如此小的尺度上测定成分和晶体结构的困难,使这些相的全面表征变得困难。本文描述了这些相的一个新出现,在对澳大利亚维多利亚南部Waratah湾海滩砂中含有大量金粒的“锇”的历史浓缩物进行调查时偶然发现。这些相以金颗粒的边缘形式出现,并在几十微米或更小的尺度上相互生长。需要应用电子探针分析(EPMA)和有限电子背散射衍射(EBSD)对其进行表征。这些技术结果表明,在含有微量Hg的原生Au-Ag (electroum)周围存在一组成分范围为Au2Hg - au3hg的矿物(Au-Hg-Ag)和一组成分范围为Au2Hg - au3hg的相。EBSD分析表明,魏山石为六方P63/mmc, Au2Hg为六方P63/mcm。与其他地区(菲律宾、不列颠哥伦比亚省和新西兰)已发表的资料比较表明,伟山石具有广泛的组成领域。这些相所显示的结构很难解释,因为它们可能是由表生过程形成的,也可能是与采矿过程中使用的人为汞反应形成的。然而,由于没有任何历史证据表明在Waratah Bay使用汞进行金矿开采,我们认为Au-Hg相的形成很可能是由于含有少量Hg的原生Au-Ag合金的表生蚀变。除了揭示这些次生Au-Hg - ag边缘发育过程中的一些反应顺序外,本文阐述了可以更充分地表征这些相的方法,从而更好地与Au-Hg合成体系相关联。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Mineralogical Magazine
Mineralogical Magazine 地学-矿物学
CiteScore
4.00
自引率
25.90%
发文量
104
审稿时长
6-12 weeks
期刊介绍: Mineralogical Magazine is an international journal of mineral sciences which covers the fields of mineralogy, crystallography, geochemistry, petrology, environmental geology and economic geology. The journal has been published continuously since the founding of the Mineralogical Society of Great Britain and Ireland in 1876 and is a leading journal in its field.
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