Microstructure and Trace Elements of Pyrite from Sanshandao Gold Deposit in Jiaodong District: Implications for Mechanism of Gold Enrichment

The Sanshandao gold deposit, located in the northwest of Jiaodong district, is a super-large altered-rock type gold deposit. The lack of detailed studies of mineralogy and element geochemistry in this deposit limits the understanding of Au enrichment mechanism and process. Based on field work and mineralogical observation in the deposit, the Au mineralization is divided into four stages: the quartz-sericite-pyrite stage (I), quartz-gold-pyrite stage (II), quartz-gold-polymetallic sulfide stage (III) and carbonate-quartz stage (IV). The main gold-bearing mineral is pyrite. Py1 in stage I is characterized by euhedral grains with no deformation and low contents of As and Au. In stage II, the Py2 can be divided into two sub-generations: coarse Py2a with no deformation and Py2b with massive plastic and brittle deformation. The contents of Au and As are high in Py2a and low in Py2b.In stage III, the Py3 includes fine-grained euhedral Py3a coexisting with quartz and Py3b coexisting with polymetallic sulfide, both with weak deformation and medium Au and As contents. In-situ trace element analysis of pyrite indicates Co, Ni and Ag entering pyrite lattice by forming solid solution, whereas Pb, Zn and Cu mainly presenting as sulfide inclusions. The main occurrence state of gold is visible Au that occurs in the cracks and intergranular space of pyrite as electrum. The invisible Au in pyrite is lattice gold, and its enrichment is closely related to As. To be specific, As- replacing S- entered into pyrite, resulting in lattice distortion of pyrite, which prompted the Au+ into the pyrite lattice. The decompression boiling of ore-forming fluids in stage II resulted in Au precipitation and intergrowth with Py2a as visible inclusions and intergranular gold. The plastic deformation of Py2b in stage II induced by syn-activation of ore-controlling fault, such as dislocation creep, lattice rotation, promoted the formation of intracrystalline "fast pathways", and further remobilized invisible Au through intragrain diffusion or fluid-mediated liberation. The remobilized Au re-concentrated as visible gold in microfractures and boundaries of pyrite grains, which contributes to the formation of high-grade gold ores in Sanshandao. © 2022, Editorial Department of Earth Science. All right reserved.