Pulsed magmatic fluid releasing in the formation of the Taoxihu Sn polymetallic deposit, eastern Guangdong, SE China: Evidence from fluid inclusions, cassiterite U–Pb geochronology, and stable isotopes

Abstract

The Taoxihu deposit is a newly discovered Sn-polymetallic deposit in eastern Guangdong, SE China. The mineralization is related to an Early Cretaceous, highly fractionated I-type granite porphyry. The ore-forming process is divided into six stages, i.e., garnet (stage I), quartz–fluorite–biotite (stage II), quartz–biotite–cassiterite–sulfides (stage III, main mineralization), quartz vein with minor sulfides and cassiterite (stage IV), quartz–K-feldspar–biotite–cassiterite–sulfides (stage V), and quartz–K-feldspar–chlorite–sulfides (stage VI). Fluid inclusion study shows that the hydrothermal fluids from stage II (salinity = 5.9 to 13.2 wt%, and Th = 302 to 382 °C) to stage IV (salinity = 0.2 to 5.9 wt%, and Th = 275 to 315 °C) are of H2O–CO2–(CH4)–salt composition and show a trend of systematically decreasing temperature and salinity from early to late stages. In contrast, the fluids in stage V (salinity = 6.5 to 13.9 wt% and Th = 223 to 343 °C) and stage VI (salinity = 6.0 to 11.8 wt%, and Th = 140 to 300 °C) are of the H2O–salt compositional system, and demonstrate also a trend of decreasing temperature and salinity with time, representing a distinct hydrothermal event. H–O isotope data indicate that these two different fluid systems are all initially of magmatic origin. In view of the almost identical cassiterite U–Pb ages of stage III (139.7 ± 1.1 Ma) and stage V (139.8 ± 1.6), the two hydrothermal events are interpreted to represent two pulses of magmatic fluid releasing from the same magma chamber. The first pulse is characterized by an initially single-phase CO2-bearing magmatic fluid that was subsequently subject to unmixing or phase separation in relation to hydraulic fracturing and decompression, which lead to the main phase mineralization, whereas the second pulse is the release of residual, CO2-free magmatic fluid, which mixed with meteoric water and resulted in the second phase of mineralization.