The magmatic origin of propylitic alteration of the Zhengguang epithermal Au-Zn deposit, Heilongjiang, China: evidence from mineral compositions and H–O-Sr isotopes

The origin of propylitic fluids in intermediate sulfidation mineralization has not been investigated in detail. Here, we present an extensive petrographic, geochemical, and isotopic (O-H-Sr) study of propylitic epidote, chlorite, and calcite from the Zhengguang intermediate sulfidation epithermal Au-Zn deposit, NE China. Propylitic minerals can be divided into three main types based on their different textural occurrences, namely interstitial cement of clasts of hydrothermal breccia, replacement of primary plagioclase or hornblende, and vein infill of cracks, with late, minor calcite as amygdules in vesicles of andesite representing a fourth textural occurrence. The H-O isotope compositions and mass balance calculations suggest that most propylitic epidote records a dominant (> 50%) contribution of magmatic fluids. The decrease of the average δ¹⁸ \({\mathrm O}_{{\mathrm H}_2\mathrm O}\;\) ${\mathrm{O}}_{{\mathrm{H}}_2\mathrm{O}}$ values equilibrated with different types of epidote (cement 6.8 ± 0.7‰, replacement 5.1 ± 1.1‰, vein 4.5 ± 1.4‰, 1 SD), and the decreasing content of high-temperature elements (e.g., Cu-Mo) from cement, through replacement to vein epidote and chlorite, collectively indicates a decreasing role of magmatic fluids. Replacement epidote and chlorite are enriched in Sr-Mn-Y-Sb, whereas replacement epidote and calcite record similar (⁸⁷Sr/⁸⁶Sr)_i values to the andesitic host rock, suggesting that replacement minerals inherit certain elements from plagioclase and hornblende, and the Sr isotope signature of the wall rocks. We highlight that propylitic alteration in epithermal deposits can involve significant proportions of magmatic fluids and texturally different alteration mineral types should be considered when using mineral isotopic or chemical compositions to track fluid sources or to vector towards the location of intrusive centers.