Fluid properties and ore-forming process of the giant Jiajika pegmatite Li deposit, western China

The world class Jiajika lithium deposit in the Songpan-Ganze orogenic belt, western China is related to Late Mesozoic S-type granitic pegmatite. More than 500 pegmatite veins have been discovered in the Jiajika deposit and they show well defined zonal mineral pattern, including microcline pegmatite zone (I), microcline-albite pegmatite zone (II), albite pegmatite zone (III), albite-spodumene pegmatite zone (IV), and albite-lepidolite pegmatite zone (V) from the center outward. Pegmatite Li orebodies are primarily hosted in zones III, IV and V. Barren samples of zones I and II were collected from the recent Jiajika Scientific Drilling (JSD) project performed in the years of 2020-2021 and ore samples of zones III, IV and V were collected from outcrop pegmatite veins No. 308, No. 134, and No. 528, respectively. Fluid inclusions in quartz from the Jiajika deposit include intermediate density CO2-bearing inclusions (type A), high density aqueous inclusions (type B), and low density CO2-rich inclusions (type C). Type A inclusions are predominant in zones I and II, whereas coexisting type B and C inclusions are prevailing in zones III, IV and V. Fluid inclusion microthermometry indicates that type A inclusions in zones I and II show homogenization temperatures ranging from 328 to 362 degrees C and salinities from 1.2 to 4.5 wt%. Type B inclusions and coexisting type C inclusions in the zones III, IV and V show homogenization temperatures of 278 to 336 degrees C and 301 to 358 degrees C, respectively. Their salinity values are 8.1 to 11.6 wt% and 0.8 to 3.6 wt%, respectively. The estimated pressures of type A inclusions are 310 to 424 Mpa, and those of immiscible type B and C inclusions are 151 to 239 Mpa. The change of fluid types with decreasing pressure indicates that extensive fluid immiscibility occurred during the formation of Li pegmatite veins in zones III, IV and V. Moreover, the LA-ICP-MS microanalysis demonstrates that chemical composition of fluid inclusions in this deposit is relatively simple, mainly containing Li, Na, K, Rb, Cs, B, and As. Such enrichment of alkaline elements (Li, Na, K, Rb and Cs) and volatile elements (B and As) is a typical geochemical index for lithium pegmatite. We suggest that the high-degree fractionated of parent magma and strong fluid immiscibility promoted the formation of the giant Jiajika Li deposit. This study highlights that immiscible fluid inclusions and strong enrichment of alkaline elements in inclusions can be an excellent tool for explorations of lithium pegmatites like Jiajika.