Tourmaline as a Recorder of Ore-Forming Processes in the Xuebaoding W-Sn-Be Deposit, Sichuan Province, China: Evidence from the Chemical Composition of Tourmaline

Abstract

The Xuebaoding W-Sn-Be deposit located in the Songpan-Ganze Orogenic Belt (Sichuan Province, China) is a hydrothermal deposit with less developed pegmatite stage. The deposit is famous for the coarse-grained crystals of beryl, scheelite, cassiterite, apatite, fluorite, muscovite, and others. The orebody is spatially associated with the Pankou and Pukouling granites hosted in Triassic marbles and schists. The highly fractionated granites are peraluminous, Li-Rb-Cs-rich, and related to W-Sn-Be mineralization. The mineralization can chiefly be classified based on the wallrock and mineral assemblages as muscovite and beryl in granite (Zone I), then beryl, cassiterite and muscovite at the transition from granite to triassic strata (Zone II), and the main mineralized veins composed of an assemblage of beryl, cassiterite, scheelite, fluorite, and apatite hosted in metasedimentary rock units of marble and schist (Zone III). Due to the stability of tourmaline over a wide range of temperature and pressure conditions, its compositional variability can reflect the evolution of the ore-forming fluids. Tourmaline is an important gangue mineral in the Xuebaoding deposit and occurs in the late-magmatic to early-hydrothermal stage, and can thus be used as a proxy for the fluid evolution. Three types of tourmalines can be distinguished: tourmaline disseminations within the granite (type I), tourmaline clusters at the margin of the granite (type II), and tourmalines occurring in the mineralized veins (type III). Based on their chemical composition, both type I and II tourmalines belong to the alkali group and to the dravite-schorl solid solution. Type III tourmaline which is higher in X-site vacancy corresponds to foitite and schorl. It is proposed that the weakly zoned type I tourmalines result from an immiscible boron-rich aqueous fluid in the latest stage of granite crystallization, that the type II tourmalines showing skeletal texture directly formed from the undercooled melts, and that type III tourmalines occurring in the mineralized veins formed directly from the magmatic hydrothermal fluids. Both type I and type II tourmalines show similar compositional variations reflecting the highly fractionated Pankou and Pukouling granites. The higher Ca, Mg, and Fe contents of type III tourmaline are buffered by the composition of the metasedimentary host rocks. The decreasing Na content (<0.8 atoms per formula unit (apfu)) and increasing Fe3+/Fe2+ ratios of all tourmaline samples suggest that they precipitated from oxidized, low-salinity fluids. The decreasing trend of Al content from type I (5.60–6.36 apfu) and type II (6.01–6.43 apfu) to type III (5.58–5.87 apfu) tourmalines, and associated decrease in Na, may be caused by the crystallization of albite and muscovite. The combined petrographic, mineralogical, and chemical characteristics of the three types of tourmalines thus reflect the late-magmatic to early-hydrothermal evolution of the ore-forming fluids, and could be used as a geochemical fingerprint for prospecting W-Sn-Be mineralization in the Xuebaoding district.

Mineralogical classification and crystal water characterisation of beryl from the W–Sn–Be occurrence of Xuebaoding, Sichuan province, western China

“…in pegmatites (Černý et al, 2003;Neiva and Neiva, 2005;Wang et al, 2009) and with the limited Fe 3+ and A-site substitution not being of magmatic origin. These results are further consistent with the understanding that the Xuebaoding W-Sn-Be-F-P mineralisation is primarily hydrothermal in origin and derived from a highly evolved fluid emerging from the Pankou and Pukouling albite leucogranites that are highly fractionated, alkali, peraluminous, Li-F-enriched (Zhu et al, 2020) giving rise to the beryl, cassiterite, scheelite, fluorite and apatite mineral assemblage.…”

“…It is a greisen-type deposit of quartz and muscovite. Albeit locally variable in relative distribution, the joint mineralisation of scheelite, cassiterite and beryl suggests that it is derived from a highly fractionated and evolved magma in the apical section of the granitic body (Cao et al , 2002, 2004; Li et al , 2007; Zhang et al , 2014; Zhu et al , 2020). Highly enriched in volatiles and metallic elements this gives rise to simple mineralisation of large, mostly common, W, Sn, Be, F and P minerals.…”

Hydrothermal fluid evolution in the Cuonadong Sn–W–Be polymetallic deposit, southern Tibet: indicated by the in–situ element and boron isotope compositions of tourmaline

“…Li and Be are incompatible elements in most rock-forming minerals and tend to remain in the melt during fractional crystallization with tourmaline-melt partition coefficients of 0.89 and 0.44, respectively (van Hinsberg, 2011). An increase in the Li and Be contents of the tourmaline can record progressive Li and Be enrichment during magmatic evolution (Zhu et al, 2020). The Li and Be contents will also increase from magma to hydrothermal fluids due to two processes: (1) melt-fluid separation (Liu et al, 2020;Wang et al, 2022) and (2) water-rock interaction (Xiang et al, 2020;Wang et al, 2022).…”