Mineralization processes in the Suoluogou orogenic gold deposit, Sichuan Province, China: evidence from Au occurrence and trace elements

Abstract

Clarifying the occurrence of Au, characteristic mineral assemblages, and trace element contents in key minerals like pyrite and rutile is crucial for constraining the mineralization processes and elucidating the ore genesis of the Suoluogou Au deposit. Electron probe microanalysis and laser ablation–inductively coupled plasma–mass spectrometric analysis indicate that Au in this deposit exists as invisible Au within pyrite and arsenopyrite. There are four generations of pyrite; three generations of quartz and dolomite; and two generations of rutile, sericite, arsenopyrite, and florencite-(Ce). The first- to fourth-generation pyrite has average Au contents are 26.2, 107, 87.8, and 5.1 ppm, respectively. The second- and third-generation arsenopyrite has average Au contents of 260 and 414 ppm, respectively. There exists a positive correlation between Au and As within pyrite, and the average Au content in arsenopyrite surpasses that in pyrite, suggesting that As plays a pivotal role in the enrichment of Au. The Au mineralization process, as traced by Ti, Nb, W, V, and Zr, can be divided into four stages, with the second and third stages being the main mineralization stages. The first stage is the pyrite and A1 vein stage. The second stage is the pyrite and A2 vein stage, during which fluid rich in ore-forming elements such as Ti, Nb, W, V, Zr, Co, Ni, and Au was injected along the nearly E–W-trending main fault. Due to the strong fluidity of the fluid, the minerals formed during this stage are fine-grained texture. The third stage is the pyrite-arsenopyrite-A3 vein-B1 vein stage. As the fluidity of the fluid weakened, the environment for mineral crystallization stabilized, resulting in the formation of numerous medium- to coarse-grained minerals. The fourth and final stage is marked by the carbonate-B2 vein-B3 vein stage. The water–rock reactions involved in the Au mineralization includes: the first-generation rutile underwent dissolution and recrystallization, giving rise to anatase and the second-generation rutile; similarly, the first-generation florencite-(Ce) experienced dissolution and recrystallization, giving rise to the second-generation florencite-(Ce); K-feldspar + H₂O → altered K-feldspar; the quartz recrystallized by hydrothermal processes is mixed with Al, Mg, Ca, K, Ti, Fe, and H₂O. Furthermore, carbonatization led to a drastic reduction in CO₂ within the ore-forming fluid. As sulfides crystallized, the Au(HS)²⁻ complex in the hydrothermal fluid became destabilized, facilitating the precipitation of Au. The metamorphic-hydrothermal origin of the first-generation rutile found within the ore suggests that the Suoluogou Au deposit is an orogenic gold deposit with a metamorphic-hydrothermal genesis. The presence of anatase, a mineral typically formed at low temperatures, indicates that the main mineralization stage occurred in a low-temperature environment.