Distinct Au and Ag precipitation mechanism in the Xiayingfang Au–Ag deposit, North China Craton

Abstract

The Yan-Liao Au polymetallic ore belt, located in the northern margin of the North China Craton, hosts a number of sizeable hydrothermal Au–Ag deposits with abundant Pb-, Sb-, Bi-minerals. These minerals are closely linked to the Au and Ag precipitation, but are rarely studied. The Xiayingfang deposit (22.5 Mt @ 1.9 g/t Au and 9.9 g/t Ag) is a typical Au–Ag deposit in this ore belt, and has characteristic Pb–Sb–Bi–Au–Ag-mineral assemblages. We conducted an integrated study on the mineral assemblages and geochemical components and sulfur isotopes to constrain the Au–Ag precipitation mechanism at Xiayingfang.The Xiayingfang mineralization can be divided into three stages: (I) quartz-pyrite, (II) quartz-polymetallic sulfide, and (III) quartz-sphalerite-galena-electrum (main Au–Ag ore stage), which comprises the sphalerite-galena-native bismuth-pyrite-electrum (IIIA) and galena-freibergite-bournonite-pyrrhotite-electrum (IIIB) substages. The δ34SV-CDT of the paragenetic pyrite and galena pairs reveals a general temperature drop from 300 to 330 °C (Stage I), through 250–290 °C (Stage II), to 170–250 °C (Stage III). The Stage I and II fluid temperature is higher than the melting point of Bi-melt (271 °C) and Au–Bi eutectic (241 °C), which probably caused the “liquid bismuth collector model”. Bi-minerals are widely distributed in later Stage II, and are represented by the coexisting native bismuth and maldonite with dissolution pits on the surface (occurring as droplet-shape or eutectic texture in Stage II galena), which indicates the temperature evolved to lower than 241 °C. Such inference is also supported by fluid inclusion data (216–286 °C for Stage II). The pH generally shifts from the felspar-illite buffer (∼7.0) in Stages I and II to the illite-calcite buffer (ca. 5.2–5.6) in Stage IIIA, which could lead to Au solubility decrease (but minor impact on Ag solubility) in the fluid, favoring electrum precipitation (19.0–31.2 wt% Ag). Moreover, oxygen fugacity drop is revealed by pyrite stable from the Stage IIIA (–41 to –46) to pyrrhotite stable in Stage IIIB (≤–46), which may have markedly decreased both Au and Ag solubility in the fluid. Meanwhile, freibergite coexisting with bournonite and galena in Stage IIIB is highly heterogeneous under EPMA element map, suggesting that it was mainly derived from retrograde and solid-state reactions. These reactions could effectively remobilize Ag from galena and Pb–Sb–Ag-sulfosalts via decomposition and exsolution during fluid cooling. These processes may have contributed to the second-phase precipitation of electrum with a steep rise of Ag (57.5–77.8 wt%) in Stage IIIB. The different mechanisms of Au and Ag precipitation at Xiayingfang are also consistent with the different Au-/Ag- mineral assemblages, i.e., electrum-pyrite-chalcopyrite-enargite assemblages in Stages I and II, and electrum-maldonite-freibergite in Stage III.