Fluid evolution and gold precipitation in the Muping gold deposit (Jiaodong, China): Insights from in-situ trace elements and sulfur isotope of sulfides

Abstract

The Early Cretaceous Muping gold deposit is hosted in the Late Jurassic monzogranite. Crosscutting relations of ore veins implied a presence of multiple fluid pulses, and the mineralization process comprises four mineralization stages. Three generations/types of pyrite (Py1, Py2, and Py3) have been discriminated. Coarse-grained and euhedral Py1 pertains to pyrite-quartz veins. Coarse-fine grained and subhedral Py2 comes from quartz-pyrite veins. Fine-grained and anhedral Py3 is from quartz-polymetallic sulfides veins. Py3 further can be divided into three sub-types (Py3a, Py3b, and Py3c), in which Py3a did not have a core-rim texture, Py3b is the core and Py3c is the rim of pyrite with core-rim texture.Py1 and Py2 contain relatively higher Co (738 and 29 ppm on median) and Ni (30 and 7.6 ppm on median), but lower As (754 and 15 ppm on median) and Au (median < 0.1 ppm) content. However, Py3 has extremely low Co and Ni (medians below detection), but elevated As (from 16,290 to 206 ppm on median) and Au (from 36 to 0.4 ppm on median) content. Co and Ni contents of pyrite gradually decreased but As content increased from Py1 to Py3, implied that the temperature of ore-forming fluids progressively decreases. Extremely higher As content and high δ34S of Py3c could be associated with an incursion of As-rich and heavy δ34S meteoric water which has flowed through meta-sedimentary rocks. Sulfur isotopes revealed a gradual decrease from Py1 (+11.6‰) to Py3b (+7.5‰). As for the pyrite with core-rim texture, the sulfur isotope progressively increased from the core (Py3b, +6.0‰) and the rim (Py3c, +11.8‰). The high δ34S could be derived from the metasomatic and fertile lithospheric mantle, and the difference between Py1 and Py2 attributed to suffering from degassing processes with different condition. Rayleigh fractionation process caused a “reservoir effect” can explain the inside variations of Py2, while the increase of fO2 caused by phase separation is the main factor leading to δ34S decrease for Py3. It is the fluid mixture that resulted in the inter-crystalline sulfur isotope variation of pyrite with core-rim texture. Gold and As contents for all types of pyrite plot well below the solubility limit for gold in arsenian pyrite, indicating that invisible gold occurs as Au1+ in the pyrite lattice. The extent of invisible gold was incorporated into pyrite depending on the arsenic content, but visible gold mineralization mainly was controlled by vapor phase escape and fO2 increase which attributed to phase separation.