Oxidised gold lode stains black: the historical Encardideira (Chico Rei) deposit, Ouro Preto, Brazil

Pub Date : 2023-02-01 DOI:10.1080/25726838.2023.2166282

A. Cabral, L. P. T. Nazareth, L. C. da Silva, L. E. D. Amorim, A. F. Amorim, I. L. F. Ferreira, M. H. S. Franco, M. C. Gomes, P. R. L. Ferreira, M. M. Marques, N. S. Martins, I. Mendes de Oliveira, S. D. Veiga, G. V. Terenzi, F. Rios

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Abstract

ABSTRACT The historical Encardideira gold mine, currently known as Chico Rei, represents a rare case of gold mine owned by a freed slave in Brazil’s colonial times. The term ‘encardideira’ alludes to the property of ingraining dirt on the surface of something. Here, we provide chemical and mineralogical evidence that the Encardideira mine likely had its name from the begriming character of its ore shoots. Gold-mineralised samples are enriched in graphite C, identified as tourmaline-enclosed euhedral graphite, and Mn-oxide minerals, mostly pyrolusite and cryptomelane-like material. Measurements for Au by atomic absorption spectrometry returned Au values above 1 ppm only after cupellation, suggesting the existence of graphite-bound gold in pervasively oxidised rocks. It is hypothesised that graphite-rich ore shoots, with superposition of Mn-oxide minerals, could account for enough gold and softness for manual removal by enslaved miners and their characteristic begriming.

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氧化的金矿染成黑色:历史悠久的恩卡迪拉(奇科雷)矿床，巴西欧鲁普雷图

摘要历史悠久的Encardideira金矿，目前被称为Chico Rei，代表了巴西殖民时代一个罕见的自由奴隶拥有的金矿案例。encardideira一词暗指在某物表面留下污垢的性质。在这里，我们提供了化学和矿物学证据，证明Encardideira矿的名字可能源于其矿石芽的起源特征。金矿化样品富含石墨C，被鉴定为电气石包裹的自形石墨，以及氧化锰矿物，主要是软锰矿和类隐玄岩材料。原子吸收光谱法对Au的测量结果显示Au值高于1 ppm，表明在普遍氧化的岩石中存在石墨结合的金。据推测，富含石墨的矿芽与氧化锰矿物叠加，可以为被奴役的矿工提供足够的黄金和柔软度，以供手工清除，并具有其特征。

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