Silver enrichment and trace element deportment in hydrothermal replacement reactions: Perspective from the Nageng Ag-polymetallic deposit, East Kunlun Orogen, NW China

Abstract

Mineral replacement reactions (dissolution-reprecipitation and solid-state diffusion) have been reported in various types of hydrothermal deposits. However, the metal transfer and partitioning between the parent and daughter minerals and the role of different replacement reactions in silver enrichment remains poorly understood. Here we present the petrographic textures, major and trace elements, and sulfur isotopes of different generations of pyrite and marcasite associated with replacement reactions from the Nageng epithermal Ag-polymetallic deposit, East Kunlun Orogen (EKO, NW China). The mineralization includes the pyrite-quartz (I), quartz-siderite-sulfides (II), quartz-fluorite-rhodochrosite-sulfides-sulfosalts (III), and quartz-calcite (IV) stage. This deposit is characterized by a metal zonation from deep-level Cu-Pb-Zn mineralization to shallow-level Ag-Pb-Zn mineralization, which correspond to main ore stages II and III, respectively. Stage II ores contain the colloform pyrite (Py1) and porous pyrite aggregate (Py2), both of which have higher Co, Ni, and W contents than stage III pyrite (Py3 and Py4) and marcasite (Mc1 and Mc2). In contrast, stage III pyrite/marcasite are enriched in Ag and other ore metals (e.g., Pb, Zn, Sb, Mn, and Sn). The sharp contact between Py3 and Py4/Mc1, together with the porosity in Py4/Mc1, suggests that Py3 and Py4/Mc1 represent the parent and daughter phase of dissolution and reprecipitation. The crystallographically-oriented and metallic inclusion-rich Mc1 has higher Ag concentrations (median 246 ppm) than Py3 (median 180 ppm), and commonly occurs in breccia-type ores related to hydraulic fracturing. This suggests that (i) the dissolution-reprecipitation reaction may have remobilized and concentrated silver and associated ore metals, and (ii) the fluid oxidation related to hydraulic fracturing may contribute to dissolution-reprecipitation reaction. However, the randomly-oriented Mc2 is porous and has homogeneous compositions. This, together with the prevailing occurrence with rhodochrosite in vein-type ores, indicates that Mc2 was transformed from Mc1 via Mn-rich fluid-driven solid-state diffusion under relatively mild physicochemical changes. Mc2 has the most homogeneous and highest Ag concentrations (avg. 1142 ppm) and other ore meals among all the studied Fe disulfides, suggesting that the marcasite has higher trace element compatibility than pyrite, and that the solid-state diffusion reactions promoted Ag enrichment more efficiently than dissolution-reprecipitation reactions. The mostly positive δ34S values (−1.09 to 8.50‰) of the Nageng pyrite and marcasite are consistent with a magmatic origin. The δ34S variations among different pyrite and marcasite generations are attributed to the fluctuations between episodic replenishment of magmatic fluids and subsequent oxidation in respond to hydraulic fracturing and mixing with meteoric water. Our findings reveal higher Ag compatibility of marcasite over pyrite, and that replacement reactions (especially solid-state diffusion) greatly promote ore metal enrichment.