Textural control on gold distribution in As-free pyrite from the Dongping, Huangtuliang and Hougou gold deposits, North China Craton (Hebei Province, China)

Abstract

Pyrite from the Dongping, Huangtuliang and Hougou gold deposits, Hebei Province, China, was investigated using a combination of ore microscopy, including back-scattered imaging, and in situ laser-ablation inductively-coupled plasma mass spectroscopy (LA-ICP-MS). A range of elements, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni and As, was analyzed with the aim to constrain the textural controls and influence of superimposed deformation on the distribution of invisible gold in As-free pyrite. The dataset shows excellent correspondence between measured high gold values and three key textural criteria: (i) areas of clustered telluride inclusions (especially in Dongping); (ii) microshearing and fracturing/brecciation (Dongping and Huangtuliang); and (iii) pyrite recrystallization (Huangtuliang). Invisible gold is only at trace levels in the Hougou and gold-bearing telluride minerals are not observed. This is considered to result from larger-scale remobilization of gold during advanced brecciation and recrystallization of pyrite. Pyrite grains containing clustered inclusions have by far the highest gold concentrations (up to 1 wt.%) with consistent values of thousands of ppm Au. Interpretation of textures and LA-ICP-MS data infers that in these areas the distribution of telluride inclusions extends pervasively from micron- to nanoscale, i.e., as fields of nanoparticles. Local employment of coupled dissolution–reprecipitation reaction between pyrite and fluids driving the K-metasomatic alteration (dated at ~ 150 Ma) is invoked to explain such patterns. Syn-deformational grain-scale mobilization of gold and other elements took place leading to a portion of the gold being recrystallized within fractures and microshears. The deformational event leads to gold enrichment in the Huangtuliang pyrite, where brittle fracturing brecciation has been followed by a resorption and recrystallization. In the latter case, ‘foam’-textured pyrites, again without As, can contain up to hundreds of ppm gold. The interpreted sequence of pyrite post-depositional events is in accordance with the protracted magmatic evolution of the area, starting with Variscan alkaline intrusions (~ 390 Ma) and reactivated during the Yanshanian orogeny (~ 150 Ma). The study shows: (i) that As-free pyrite can readily contain significant amounts of invisible gold, both as nanoparticles and locked in the sulfide lattice; (ii) the critical importance of understanding ore textures in ‘mapping’ gold distributions and the capability of LA-ICP-MS methods in such studies; (iii) that gold is highly susceptible to small-scale mobilization under a range of conditions, creating unique distribution patterns within each grain dependent upon the local expression of overprinting (defined in turn by grain size, rheology, oriented stress/strain etc.); and (iv) the role played Te and other “low melting point chalcophile elements” (e.g., Bi in the case of Huangtuliang) in governing gold distribution patterns and leading to high concentrations.