Evolution of the uranium mineralisation in the Zoujiashan deposit, Xiangshan ore field: Implications for the genesis of volcanic-related hydrothermal U deposits in South China

Abstract

Volcanic-related hydrothermal U deposits from the Xiangshan ore field are situated within the Gan-Hang Metallogenic Belt in South China. These deposits are dominantly hosted in late Yanshanian felsic volcanic rocks that were emplaced during the Yanashanian post-orogenic extension. The host volcanic rocks are moderately fractionated high-K calc-alkaline A2-type volcanic rocks and are one of the possible U sources for U ore genesis in the district. Porphyroclastic rhyolite is the main host facies and contains an average of 12 ppm U, which can be released through volcanic glass devitrification and mobilised by oxidised fluids for the formation of hydrothermal mineralisation. However, REE patterns of the U mineralisation suggest multiple U sources with U mobilised from additional felsic basement rocks subjacent to the Xiangshan caldera and/or from magmatic fluids related to a late peralkaline intrusion.At the Zoujiashan deposit in the Xiangshan ore field, structurally controlled albitised zones with brannerite-dominated U mineralisation are constrained by a maximum age of 116 ± 1 Ma on hydrothermal zircon and a minimum crystallisation age of 109 ± 1 Ma on pitchblende. This constitutes the early stage of mineralisation that occurred during the early Cretaceous (ca. 120–105 Ma). The Th-Mo-P-REE-Y-Ti-Zr whole-rock geochemical signature and disseminated Th-brannerite associated with a complex mineral assemblage including hydrothermal rutile, apatite, zircon, uranothorite, sphalerite, pyrite, xenotime and REE-carbonate indicated temperatures >250 °C for the hydrothermal system. The Th-REE-Y-Nb-Zr signature of the U mineralisation concomittent with the intrusion of alkaline mafic dykes suggest that a peralkaline intrusion located at depth, postdating the Xiangshan volcanic-intrusive complex by at least 10 Ma, provided the heat source and U-Th-REE-Y-Nb-Zr-rich fluids to the hydrothermal system for the formation of the early U mineralisation. In contrast, the structurally controlled disseminated- to vein-type pitchblende-dominated U mineralisation that yielded a minimum crystallisation age of 81 ± 4 Ma represents the main stage of mineralisation (ca. 100–80 Ma) and formed during the late Cretaceous crustal extension. The mineralisation occurred at low- to moderate-temperatures (150–250 °C) as Ti-Th-Zr-rich pitchblende associated with extensive illite and fluorite alteration. Here, pitchblende that overprinted the early mineralisation stage most likely inherited the primary chemical signature of this early mineralisation, especially the high Th contents. During this episode of crustal extension, uranium was leached from the host volcanic rocks and subjacent felsic basement rocks by surface-derived oxidised fluids and transported through basement faults as carbonate and/or fluoride complexes in solution. These U-bearing solutions mixed with deep-seated magmatic-derived fluids, and the resulting destabilisation of metal complexes in solution together with pressure decrease in opening fractures promoted U deposition. Therefore, the volcanic-related hydrothermal U mineralisation in the volcanic belt formed during two major stages related to different genetic conditions.