Tourmaline as an indicator for late-magmatic to hydrothermal fluid evolution of the Neoproterozoic Baotan tin deposit, South China

Abstract

Neoproterozoic tin mineralization in South China was closely related to the highly-evolved peraluminous granites in the southeastern and western margins of the Yangtze Block. The Baotan (23 Mt @ 0.43 % Sn) is a largest tin deposit in the Jiuwandashan–Yuanbaoshan tin district, northern Guangxi, and the orebodies are mainly hosted in the Lower Neoproterozoic Sibao Group metasedimentary rocks and Early Neoproterozoic mafic rocks intruding the Sibao Group as well as the apex of the Pinying granitic pluton. This deposit shows widely important tourmaline alteration. Based on petrographic observations, four generations and eight occurrences of tourmalines are identified in the Baotan tin deposit. The first generation is late-magmatic tourmalines and occurs as disseminated (Tur1a) and tourmaline–quartz nodule (Tur1b) in the Pingying biotite granite. The pre-ore hydrothermal tourmalines (Tur2) occur as tourmaline–quartz veinlet in the biotite granite and can be subdivided into both the early Tur2a (second generation) and the late Tur2b (third generation) according to microscopic observation and BSE image. The fourth generation is syn-ore hydrothermal tourmalines (Tur3), and they occur as disseminated in mafic- (Tur3a) and metasedimentary (Tur3c) host rocks, or as vein-type tourmaline–quartz–cassiterite–chlorite ores in mafic- (Tur3b) and metasedimentary (Tur3d) host rocks.The different generations of tourmalines have both similarities and distinct differences in their compositions. Both the late-magmatic Tur1a and Tur1b show similar chemical compositions, with high Fe/(Fe + Mg) (0.73 ∼ 0.96, avg. 0.89) and Na/(Na + Ca) (0.87 ∼ 0.98, avg. 0.95) ratios, and Li, Zn, Ga, Nb and Ta contents, and belong to alkali group and schorl tourmaline. Based on mineral textural and compositional features and Mössbauer spectroscopic analyses, it can be speculated that the Tur1b in the tourmaline–quartz nodules and Tur1a are mainly sourced from a late-magmatic immiscible B–Na–Fe-rich hydrous melt in a reduced environment. The pre-ore Tur2a has similar SiO2, Al2O3, TiO2, Na2O, Li, Be, V, Sn and Zr contents, Fe/(Fe + Mg) and Na/(Na + Ca) ratios and total Al (apfu) with the late-magmatic Tur1a and Tur1b, indicating that it also originates from the Pingying granitic magma. The lower compatible elements (e.g., Fe, Mn, Co, Ni and Zn) and higher LILEs (e.g., Sr, Rb, Ba and Cs) in the pre-ore Tur2a and Tur2b suggest that these tourmalines precipitate from the early evolved magmatic hydrothermal fluids. The wide variations of Fe/(Fe + Mg) and Na/(Na + Ca) ranges, elevated Sr and V contents for the syn-ore Tu3a to Tur3d are attributed to the compositional differences of the host rocks and different degrees of interaction between the ore-forming fluids and the host rocks. The increasing Sr content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 suggests an involvement of the meteoric water. The Mössbauer spectral features of tourmalines from the late-magmatic Tur1 and syn-ore Tur3 confirm that they form under the different redox environments. The early fluids are relatively reduced magma-derived fluids. The addition of recycled meteoric water causes the fluids at the Tur3 stage to become relatively oxidized and also to be Sr-rich. The significant increase of Sn content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 also reflect a change from the early reduced fluid to the late oxidized fluid. The textural and compositional changes of Tur1 to Tur3 reflect the evolution of the ore-forming fluids. Fluid mixing between an acidic, reduced, Sn-rich magmatic hydrothermal fluid and cooler, oxidizing meteoric water and as well intensive water–rock reaction are considered as the main processes that cause extensive cassiterite precipitation.