Depressurization and boiling of a single magmatic fluid as a mechanism for tin-tungsten deposit formation

Abstract

Tin (Sn) and tungsten (W) mineralization are often associated with each other in relation to highly evolved granites, but economical ore grades are restricted to rare global occurrences and mineralization styles are highly variable, indicating different mechanisms for ore formation. The Sn-W Zinnwald deposit in the Erzgebirge (Germany/Czech Republic) in the roof zone of a Variscan Li-F granite hosts two contrasting styles of mineralization: 1) cassiterite (Sn) in greisen bodies and 2) cassiterite and wolframite (W) in predominantly sub-horizontal quartz-rich veins. The relative timing and causes for ore formation remain elusive. Studies of fluid inclusion assemblages in wolframite, cassiterite and quartz samples from greisen and veins by conventional and infrared microthermometry and LA-ICP-MS analyses have revealed compelling evidence that all elements required for the formation of the Zinnwald Sn-W deposit were contained in a single parental magmatic-hydrothermal fluid that underwent two main processes: 1) fluid-rock interaction during Sn-greisen formation and 2) depressurization and vapor loss leading to ore precipitation in quartz-Sn-W veins. The results also show that fluid inclusion assemblages in ore minerals can document fluid processes that are absent in the fluid inclusion record of gangue minerals. The study further highlights the role of phase separation in the formation of W-rich Sn-deposits and indicates that W-deposits in distal parts of evolved granites may be restricted to fluids derived from deeper-seated plutons.

The H/F ratio as an indicator of contrasted wolframite deposition mechanisms

“…Therefore, each corresponds to a specific type of W deposition environment, as reflected by the composition of wolframite, hübnerite in the former and ferberite in the latter (Fig.3). In the case of intragranitic vein systems, a magmatic origin for the ore fluid has been generally proposed (e.g., Korges et al, 2018;Smith et al, 1996). However, one major difference between Argemela and these systems (Cligga Head and Cínovec respectively) is the temperature and mechanism of fluid-rock equilibration.…”

“…Hübnerite is a tracer of proximal highly evolved igneous bodies such as RMG and/or pegmatite with potential for commodities such as quartz and feldspar, kaolin and disseminated Li-Sn-Nb-Ta ores (e.g., Aubert, 1969; Fig.3), in addition to W. Wolframite with intermediate H/F ratios is characteristic of medium temperature rock-buffered hydrothermal systems with a strong magmatic signature, both for the fluids and the metals (Fig.3, Smith et al, 1996;Korges et al, 2018). These compositions are typical of greisen-type (both vein and massive) deposits where mostly Li-Sn-Nb-Ta are economically interesting and of intragranitic vein type deposits that follow a rock-buffered path.…”

Genesis and hydrothermal evolution of the Tiantangshan tin‐polymetallic deposit, south‐eastern Nanling Range, South China

“…Within the same deposit, the contemporaneous or successive precipitation of cassiterite, wolframite, chalcopyrite, and other ore minerals indicates that several geologic controls can trigger ore deposition (Korges, Weis, Lüders, & Laurent, ). Greisenization in stage I is one of the typical hydrogen ion metasomatism processes, which is a good catalytic reaction for the precipitation of cassiterite.…”

“…Greisenization in stage I is one of the typical hydrogen ion metasomatism processes, which is a good catalytic reaction for the precipitation of cassiterite. Fluid immiscibility is seen as a significant ore‐forming mechanism in the majority of tin–tungsten polymetallic deposits (e.g., Higgins, ; Korges et al, ; Polya, ; Xiong et al, ; Yu, Chen, & Zhao, ). During fluid immiscibility (phase separation), volatile elements (e.g., SO 2 , CO 2 , HCl, Cl − , H + ) will preferentially partition into the vapour phase (Candela & Piccoli, ), which can readily leave the fluid system into the shallower part due to its low density.…”

“…For instance, Sn can be transported as Sn 2+ or Sn 4+ by complexation with Cl − in the fluid (e.g., NaSnOHCl 2 0 , NaSnCl 3 0 , KSnCl 3 0 , K 2 SnCl 4 0 ; Wilson, 1986). Therefore, cassiterite (SnO 2 ) can be precipitated by lowering the HCl activity in the fluid system (Schmidt, 2018;Taylor & Wall, 1993), and the common reactions may include Within the same deposit, the contemporaneous or successive precipitation of cassiterite, wolframite, chalcopyrite, and other ore minerals indicates that several geologic controls can trigger ore deposition (Korges, Weis, Lüders, & Laurent, 2018). Greisenization in stage I is one of the typical hydrogen ion metasomatism processes, which is a good catalytic reaction for the precipitation of cassiterite.…”