Multiple mineralization events at Paleozoic Sanchakou porphyry Cu deposit, Xinjiang: New insights from geology, geochronology, fluid inclusions, and H-O-C isotopes

The Sanchakou Cu deposit is located in the eastern section of the Dananhu arc belt in Eastern Tianshan, Xinjiang, and recorded multiple magmatic-mineralization events. The porphyry-style mineralization period produced disseminated, sulfide-dominant veined, and patchy sulfides containing chalcopyrite +/- bornite +/- carrolite +/- molybdenite (Mol-1), which was associated with magnetite +/- biotite +/- chlorite +/- sericite alteration in a suite of quartz diorite intrusions. The overprinting mineralization period contributed to minor Cu and Mo mineralization, featured by later disseminated molybdenite (Mol-2), chalcopyrite, and widespread low-temperature (<280 degrees C) vein-type stellerite assemblages. Two distinct geological ages of molybdenite (including Mol-1 and Mol-2) from Sanchakou have been determined in this study. The old Re-187/Os-187 isochron age for euhedral Mol-1 represents the timing of original porphyry-style mineralization (386 +/- 16 Ma), in agreement with LA-ICP-MS zircon U-Pb age of dark diorite dykes (390 +/- 11 Ma). The young Re-187/Os-187 age for fine-grained Mol-2 is interpreted to be a maximum age of sulfide formation or remobilization (364 +/- 15 Ma), most likely associated with the emplacement of Late Devonian gneissic granite intrusions. Fluid inclusion microthermometry, mineralogy, and physicochemical calculations for the porphyry-type mineralization period (including four stages) at Sanchakou suggest that earlier magmatic fluids have evolved from high T (310-460 degrees C), high logf(O2) (-22 to - 26), and alkaline condition during stage I to low T, low logf(O2) ( -3 4), and acidic condition during stage II. The decrease of oxygen fugacity caused by abundant deposition of magnetite in stage I produced large-scale Cu and Mo precipitation. Stage II to IV fluids have gradually decreasing temperatures and salinities, due to involvement of meteoric water that also facilitates the ore deposition. This result is consistent with the evolution of O isotope compositions of the fluids (delta O-18(fluids) = - 9.3 to 4.3), further implying the significance of fluid mixing instead of fluid boiling that caused ore deposition. Locally high-grade copper concentrations at Sanchakou are likely attributed to the overprinting modification event on the original ore bodies. All of these observations indicate that the Paleozoic Sanchakou ore-forming system is similar to other deformed porphyry Cu systems in the Central Asian orogenic belt (CAOB) and worldwide. The Cu mineralization and modification event at Sanchakou is coincident with the second peak productivity in CAOB. We further suggest that the Paleozoic diorite complex and stellerite alteration can be as a significant exploration target for porphyry-style Cu mineralization in the Eastern Tianshan.