Abstract

“Invisible” gold in hydrothermal ores is frequently scattered in the most abundant minerals of the Fe–As–S system. It is assumed that “invisible” gold does not incorporate into the mineral structure (nanoscale inclusions of the metal or its compounds) or is chemically bound (isomorphous substitution). The aim of this study is to determine the concentration range of “invisible” gold, species of its occurrence in arsenopyrite, and conditions facilitating the formation of gold-bearing arsenopyrite using synthetic crystals and natural samples from the Vorontsovka Carlin-type deposit in the Northern Urals. Arsenopyrite crystals have been synthesized using the ampoule method in a eutectic melt of alkali metal chlorides and Al at a stationary thermal gradient and 400–500°C at the cold ampoule end. The chemical composition of arsenopyrite has been measured by electron probe microanalysis. The chemical composition of synthesized arsenopyrite is, at %: 32.6–34.4 Fe, 30–36.5 As, and 29.4–36.0 As. The gold concentration ranges from below the detection limit (<45 ppm) to 3 wt %. The obtained chemical data for synthetic crystals are compared with theoretical trends calculated for various gold species. It has been established that the slope of the trends of the average arsenopyrite compositions is very close to that of the theoretical line of isomorphous substitution Au ↔ Fe. It allows the assumption that the isomorphous solid solution in which Au occupies the Fe site formed during experiment. In general, all our data on synthetic and natural arsenopyrite show a strong negative correlation between Au and Fe, which supports the formation of the solid solution with Au at the cation site. In addition, a weak positive correlation between Au and As is observed: the higher As concentration is characteristic of As-rich (As/S > 1 at %) arsenopyrite and is close to stoichiometry, whereas in the S-rich variety, the Au content is as low as 0.25 wt %. This dependence is not only within individual grains, but also at the deposit in general: later As-rich arsenopyrite formed at lower temperature and sulfur fugacity (T = 250–370°C, log f S₂ = –12 to –17) is enriched in Au compared to early arsenopyrite (T = 270–400°C, log f S₂ = –7 to –9). Comparison of our data with the literature shows that an increasing Au content with increasing As concentration and decreasing Fe content in arsenopyrite is a common feature of Carlin-type deposits. We believe that in contrast to the negative correlation between Au and Fe, the correlation between Au and As is not obvious and may be caused by external factors, different composition of hydrothermal fluids, and sulfur fugacity.

中文翻译：

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合成和天然毒砂晶体中的“隐形”金，沃伦佐夫卡矿床，北部乌拉尔

摘要

热液矿石中的“无形”金经常散布在Fe-As-S系统中最丰富的矿物中。假定“不可见的”金不掺入矿物结构（金属或其化合物的纳米级夹杂物）或化学键合（同晶取代）。这项研究的目的是使用北部北部Vorontsovka Carlin型矿床的合成晶体和天然样品，确定“无形”金的浓度范围，其在毒砂中的存在种类以及促进形成含金毒砂的条件。乌拉尔。使用安瓿法在碱金属氯化物和Al的共熔熔体中以固定的热梯度和在400-500°C的冷安瓿端合成了毒砂晶体。毒砂的化学成分已通过电子探针显微分析测定。合成毒砂的化学成分为％：32.6–34.4 Fe，30–36.5 As和29.4–36.0 As。金的浓度范围从低于检测极限（<45 ppm）到3 wt％。将获得的合成晶体化学数据与针对各种金物种计算出的理论趋势进行比较。已经确定，平均毒砂组成的趋势的斜率非常接近于同构取代金Au铁的理论线的斜率。可以假设在实验过程中Au占据了Fe位的同构固溶体。总的来说，我们所有有关合成和天然毒砂的数据都显示出金和铁之间的强烈负相关，它支持在阳离子位点与金形成固溶体。此外，观察到Au和As之间的弱正相关性：较高的As浓度是富砷（As / S> 1 at％）毒砂的特征，并且接近化学计量，而在富S品种中，Au含量低至0.25重量％。这种依赖性不仅存在于单个晶粒中，而且通常也存在于沉积物中：后来在较低的温度和硫逸度下形成了富砷的毒砂（与早期毒砂（T = 270–400°C，log  f S ₂ = –7至–9）相比，T = 250–370°C，log  f S ₂ = –12至–17）富含Au 。我们的数据与文献的比较表明，毒砂中金的含量随砷浓度的增加而铁含量的降低是卡林型矿床的共同特征。我们认为，与金和铁之间的负相关性相反，金和砷之间的相关性并不明显，并且可能是由外部因素，热液的不同成分和硫逸度引起的。