Shallow emplacement of silver-rich rhyolitic magma enhancing enrichment and mineralization of silver

Abstract The felsic magma garners considerable interest due to its high silver (Ag) solubility. However, the factors controlling Ag enrichment, whether related to magma source, evolution, and fluid exsolution, remain debated. Understanding the behavior of Ag and associated ligands in felsic melts is therefore essential. The Lengshuikeng Ag-Pb-Zn deposit, one of the largest independent magmatic-hydrothermal Ag deposits in China, is associated with an oxidized granite porphyry and exhibits Ag-rich, Cu-poor mineralization, making it an ideal natural laboratory for studying these processes. We conducted laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on melt inclusions within early-crystallized quartz phenocrysts from the Jurassic causative granite porphyry. These melt inclusions exhibit highly variable and heterogeneous compositions, with some displaying anomalously high Ag concentrations (> 80 ppb), indicating local pre-enrichment in the primitive melt. Combined with its existing petrogenesis study, we attribute this Ag pre-enrichment to the contribution of ancient crustal material. We compared apatites from causative and barren granites using electron probe micro-analysis (EPMA), examining both apatite inclusions within zircon (interpreted as early-crystallized due to zircon’s high crystallization temperature) and groundmass-hosted apatites. Apatite inclusions within zircon from causative granite show higher sulfur (S) but lower chlorine (Cl) contents than those from barren granites. We interpret the lower Cl content to reflect H2O-rich conditions which limit available lattice substitution sites for Cl in the causative melt. These data indicate that the causative felsic magma was S- and H2O-rich, conditions more favorable for Ag enrichment. This is because S is conducive to sulfide precipitation, and water facilitates fluid exsolution and transport of ore-forming metals. In barren granites, Cl content in apatite decreases progressively from zircon inclusions to groundmass crystals, indicating volatiles saturation and fluid exsolution. In contrast, S and Cl contents in apatite from causative granite show less pronounced temporal variation. We propose that fluid exsolution was incomplete when apatite crystallization ceased in causative melt, consistent with its depressed crystallization temperature (indicated by high fluorine (F) content). Based on structural controls (reverse fault), the presence of crypto-explosive breccias within the granite, vermicular textures in F-rich quartz phenocrysts, and previous fluid inclusion studies in the Lengshuikeng deposit, we suggest that ore-forming fluid exsolution likely occurred at shallow crustal levels. This shallow exsolution significantly contributes to Ag enrichment and mineralization. This study demonstrates that terranes containing ancient crustal materials and hosting shallowly emplaced, oxidized, and H2O-rich rhyolitic magmas are preferred targets for Ag prospecting.