Fluid evolution and connections between porphyry-type and skarn-type ore in an intracontinental porphyry-skarn Cu-Au system: Evidence from apatite textures, composition, and oxygen isotopes at Dongguashan Cu-Au(-Mo) deposit, eastern China

Abstract Porphyry-skarn deposits contain significant copper (Cu)-gold (Au) resources that combine the characteristics of both porphyry and skarn systems. The connections between porphyry-type and skarn-type ore in intracontinental porphyry-skarn Cu-Au systems, and whether meteoric water influences porphyry and skarn alteration and ore formation, are not well understood. Dongguashan, eastern China, was selected as a representative porphyry-skarn Cu-Au deposit for this study. We present in situ textural, compositional, and oxygen isotope variations of magmatic and hydrothermal apatite to elucidate the ore-forming processes and to establish a comprehensive ore system for porphyry-skarn Cu mineralization. Magmatic apatite is common in intrusions, and hydrothermal apatite is widespread in porphyry and skarn alteration zones. Hydrothermal apatite can be recognized by its distinctive internal cathodoluminescence texture and its association with hydrothermal minerals (K-feldspar, biotite, muscovite, epidote, skarn minerals, and sulfide), and all apatites studied at Dongguashan are fluorapatite. The in situ oxygen isotopes of hydrothermal apatite indicate that the ore-forming fluids involved in both porphyry and skarn mineralization were dominantly magmatic in origin, with only minor amounts of meteoric water (~14% ± 10%) present during propylitic alteration (δ18O of 3.85‰–8.19‰). Similar apatite compositions present in the potassic alteration and prograde skarn suggest that the early magmatic-hydrothermal fluid was enriched in volatile (F, Cl, and S) and metallic (rare earth elements, Mn, Fe, and Na) elements. The Cl contents in the parental fluids show a systematic decrease from the early exsolved magmatic-hydrothermal fluid (0.83–4.18 wt%) to the later hydrothermal fluids (0.06 ± 0.03 wt%) associated with potassic alteration and prograde skarn, which suggests the occurrence of early-phase separation. The negative Eu anomaly of apatite from the porphyry alteration zone indicates oxidized conditions, but changes in the Eu anomaly in apatite from the prograde skarn (negative) to the early skarn sulfide stage (positive) reflect a shift to increasingly reducing conditions. The elevated S, REE, Mn, Fe, and Na concentrations in apatite in the late skarn sulfide stage probably resulted from the injection of a late magmatic-hydrothermal pulse, and the presence of oscillatory cathodoluminescence rims reflects fluctuating fluid compositions. Our results indicate that prograde skarn can be formed contemporaneously with potassic alteration, whereas the late phyllic stage does not correspond to the retrograde skarn stage. The ore-forming process in the skarn was more complex than in the porphyry stage, and developed from multiple magmatic-hydrothermal pulses. We emphasize that meteoric water was not a significant component of the porphyry-skarn Cu system, with pulses of magmatic fluids being the dominant contributors to both porphyry ore and hydrous skarn ore formation. Detailed textural observations, combined with in situ isotope and chemical analyses of apatite, can provide high-resolution data about multiple hydrothermal overprinting processes in porphyry-skarn Cu systems worldwide. The occurrences of stratabound, skarn, and porphyry ore in eastern China highlight the potential for porphyry Cu exploration beneath stratabound skarn or skarn Cu deposits globally.