A CO 2 -rich porphyry ore-forming fluid system constrained from a combined cathodoluminescence imaging and fluid inclusion studies of quartz veins from the Tongcun Mo deposit, South China

Genesis of the Xishadegai Mo deposit in Inner Mongolia, North China: Constraints from geology, geochronology, fluid inclusion, and isotopic compositions

“…They have similar homogenization temperatures, implying synchronous entrapment from the immiscible fluid. Such phase separation conforms to the phase diagram and has been observed in recent studies on the H 2 O–CO 2 –NaCl system (Duan, Møller, & Weare, ; Ni, Pan, et al, ). The boiling process is supported by the following evidence: (a) Type‐I inclusions with different vapour to liquid ratios (from <10% to 65%), Types II and IVb inclusions coexist in the ore‐stage quartz; (b) microthermometric data show that Types I and IVb inclusions homogenized at similar temperatures, but contrasting salinities, that is, Type‐I inclusions with various vapour/liquid ratios (from <10% to 65%) and Type‐IVb inclusions within a single fluid inclusion assemblage in smoky grey quartz homogenized at similar temperatures between 275 and 315 °C, whereas that of dark grey quartz homogenized at temperatures in the range of 230 to 270 °C.…”

“…As the ore fluids ascend buoyantly further along fractures into the shallower parts of the magmatic system, fluid pressures may fluctuate from lithostatic to hydrostatic and the fluids may undergo phase separation by boiling to generate hypersaline liquid and vapour (Bodnar, ; Hedenquist, Arribas, & Reynolds, ; Muntean & Einaudi, ). CO 2 escape and dramatic change in salinity during fluid immiscibility or boiling was likely to result in Mo precipitation, which were suggested in previous studies (Ni et al, , ). Therefore, phase separation may be the most effective mechanism for the ore deposition at Xishadegai, as evidenced by fluid inclusions of the ore‐stage quartz.…”

“…Petrography and microthermometry of fluid inclusions trapped in hydrothermal veins provide insights into the nature of ore‐forming fluids and the formation of ore deposits (Lu, ; Roedder, , ). Three major types of fluid inclusions developed in the Xishadegai deposit, including two‐phase aqueous inclusions, CO 2 ‐H 2 O inclusions, and daughter mineral‐bearing inclusions, which are commonly identified in hydrothermal minerals of many porphyry deposits worldwide (Chen et al, ; Lecumberri‐Sanchez et al, ; Ni et al, , ; Pirajno, ; Wang, Zhou, et al, ; Wu, Chen, & Zhou, ; Zarasvandi et al, ), such as the Bajo de la Alumbrera Cu deposit, Argentina (Ulrich, Gunther, & Heinrich, ), the Red Mountain Cu deposit, Arizona (Lecumberri‐Sanchez et al, ), the Elatsite Cu‐Au deposit, Bulgaria (Stefanova et al, ), the Shuijiadian Cu deposit, China (Wang, Zhou, et al, ), and the Dalli porphyry Cu‐Au deposit, Central Iran (Zarasvandi et al, ). Containing abundant CO 2 ‐H 2 O inclusions is a remarkable feature at Xishadegai, whereas brines are typically less common.…”

“…Phase separation (immiscibility or boiling) is widely developed in geological systems, including immiscibility of basic and felsic magmas, magma and hydrothermal fluids, saline aqueous and CO 2 fluids, and boiling of aqueous fluids (Lecumberri‐Sanchez et al, ; Lu, ; Roedder, ; Ulrich et al, ; Wang, Zhou, et al, ; Zarasvandi et al, ). As an effective mechanism of ore deposition, it is involved in many magmatic‐related ore deposits, especially in porphyry–skarn polymetallic systems (Bodnar, ; Chi & Lu, ; Lu, , ; Ni et al, , , ; Roedder, ; Roedder & Bodnar, ; Stefanova et al, ; Wang, Zhang, et al, ; Zhang, Gu, Liu, et al, ). Immiscibility and boiling most likely happened in the Xishadegai deposit based on the criteria outlined by Roedder ().…”

“…China is among the largest Mo‐producing countries in the world (Fan et al, ; Zeng et al, ). The Mo deposits in China are spatially distributed over several metallogenic belts, including the southern and northern margins of the North China Craton (NCC), South China, Tibet Plateau, and Tianshan–Beishan districts (Fan et al, ; Ni, Pan, et al, ; Ni, Wang, et al, ). They are controlled by the Central Asian Orogenic Belt (CAOB), Circum Pacific Tectonic Belt, and Tethyan Tectonic Belt and thus are considered to be indicators of tectonic settings and associated evolutionary trends (Chen, Zhang, Wang, Pirajno, & Li, ; Fan et al, ).…”