Relationship between silicification and gold mineralization and evolution of hydrothermal systems in Nibao Carlin-type gold deposits, SW China: Insights from rock geochemistry and trace elements of quartz and pyrite

Hydrothermal alteration is a crucial factor in curbing the grade and size of Carlin-type gold deposits, and fluid evolution reflects this alteration. This study investigates the correlation between silicification and Au mineralization in Carlin-type gold deposits in southwestern Guizhou Province, China, and the physical-chemical evolution of hydrothermal fluids in Nibao deposits. The geochemical analysis of the rock indicate that metallogenic elements and sulfur migrated considerably into the ore, while the major elements CaO, MgO, and Na2O were carried out from the host rock. When the silicification degree was lower than 10 wt%, the Au content was both high and low, and the relationship between the two was not apparent; when it was higher than 10 wt%, there was a negative correlation between the silicification degree and the Au content, and the excessive silicification was not conducive to the Au enrichment. Cathodoluminescence (CL) was used to analyze the texture features and luminescence intensity of the quartz, and four quartz generations were identified at the Nibao deposit: Q1 generation quartz (CL-white), Q2 generation quartz (CL-light gray) intergrowth with arsenian pyrite (Q2a and Q2b), Q3 generation quartz (CL-dark gray or black), and Q4 generation quartz (CL-bright white). Trace element variations from Q1 to Q4 (Li, Al, Ti, Ge, Sb, and Sr) reflect the evolution of hydrothermal fluids, and variations in Al concentration reveal decarbonation and argillization during mineralization, which in turn lead to variations in Li concentration. The addition of meteoric water during mineralization induced an enhanced charge-balancing effect of H+, and the Ti concentration of the quartz indicated that the temperature of possible Sb mineralization was slightly lower than that of Au mineralization. According to the petrography and chemical composition of pyrite, three types of pyrite (Py1-Py3) were confirmed: coarse-grained pyrite (Py1), fine-grained pyrite (Py2), and zoned pyrite (Py3), with Py3 subdivided into a core (Py3a) and a rim (Py3b). From Py3a to Py3b, S decreases with increasing As, while As-Au-Cu-Sb increases remarkably. The trace element variation characteristics of quartz and pyrite were used to determine the hydrothermal system evolution of the Nibao deposit. Mineralizing fluids enriched in Au-As-Cu-Sb contributed to increased variations in the Py3a pore structure and ore-forming elements, while CO2 from the dissolution of carbonate rocks caused a rapid decrease in Al concentration in Q2. Fluid-rock interaction eliminated Fe reacts with Au(HS)2- in the fluid to overgrow Py3a to Py3b and precipitate Py2. The Al and Li concentrations of Q2 and Q3 increased during the ore-stage owing to CO2 escape and weakened decarbonation. The hydrothermal fluid temperature was slightly lower in the late-ore-stage, and As-Sb-rich late-ore minerals (stibnite-orpiment-realgar) were precipitated along with high Sb concentration in Q4. The conjoint study of quartz and pyrite has important theoretical and practical implications for constraining the evolution of mineralizing fluids in Carlin-type gold deposits.