The U-Pb chronology and trace elements fingerprinting of cassiterite from the Leeuwpoort tin deposit in the Bushveld district, South Africa

<p indent="0mm"> The exposed igneous rocks of the Bushveld large igneous rock province consist mainly of the Rustenburg mafic-ultramafic bedded complex, Rooiberg felsic volcanic rocks, Rashoop granitic porphyry and Lebowa granite. There are Zaaiplaats, Rooiberg, Union, Bela-Bela, Marble Hall and Moloto tin orefields developed in the granite intrusion at the top of the Bushveld complex. The Leeuwpoort tin deposit is one of the representative deposits in the Rooiberg tin orefield. At present, the precise age of its cassiterite, the compositions of its cassiterites in different stages, the source and characteristics of its ore-forming fluid are still unclear. In this paper, taking the cassiterite from the Leeuwpoort tin deposit in the Bushveld district, South Africa as the research object, we have undertaken the cathodoluminescence (CL) microstructural study, in situ U-Pb geochronology, and trace element geochemistry of cassiterites. The LA-ICPMS U-Pb age of cassiterites in the Leeuwpoort deposit is <sc>(2052.7±6.3) Ma,</sc> which is close to the time of the second-stage magmatic activity in the Bushveld igneous province and the age of the Lebowa granite <sc>(~2054 Ma).</sc> The early-stage cassiterites of the Leeuwpoort deposit contain relatively high contents of Nb and Ta, with relatively low brightness of their CL images. The relatively high brightness of CL images of the late-stage cassiterites is believed to be mainly related to the decrease of their Fe and Nb contents. The internal ion substitution mechanisms of cassiterite crystals are 2(Nb, Ta)⁵⁺+(Fe, <sc> Mn)²⁺ </sc>=3Sn⁴⁺ and Sn⁴⁺=Fe³⁺+H⁺. The Nb/Ta ratios of cassiterites in the Leeuwpoort deposit have characteristics of the magmatic hydrothermal cassiterite. The decrease of Ti/Zr ratios and the increase of Mn/(Fe+Mn) values of cassiterites from the early stage to the late stage are believed to have been controlled by the decreased temperature of the ore-forming fluid at late stage. In addition, cassiterites are characterized with enriched Nb, high Ti and low Hf, reflecting that the ore-forming fluid was acidic oxidized fluid.