Origin of ore-forming fluids of Longtan Au-Sb deposit in western Guangxi: insight from mineralogy and geochemistry of magnetite

<p indent="0mm"> The Youjiang Basin is a significant concentration area for Carlin-type Au deposit and related Sb, Tl, and Hg low-temperature hydrothermal deposits in China. To investigate the source of ore-forming fluids in these deposits, this paper takes the magnetite vein first discovered in the Longtan Au-Sb deposit in western Guangxi as the research object, and carries out mineralogical, electron microprobe, and inductively coupled plasma mass spectrometry analyses. The results show that the Longtan magnetite is closely associated with arsenopyrite, exhibiting high As (average <sc>461 μg/g)</sc> and high Sb (average <sc>427 μg/g)</sc> contents, and has a right-skewed rare earth element (REE) distribution pattern with relatively enriched light REE (average La_N/Yb_N value of 8.4), similar to the element enrichment characteristics and REE distribution patterns of the Au-bearing pyrite in the Carlin-type Au deposits of the Youjiang Basin, suggesting that the Longtan magnetite is a product of the same hydrothermal event as the gold-antimony mineralization. In addition, the Longtan magnetite is also enriched in trace elements such as Al, Pb, V, Ti, Zn, with average contents of <sc>6205 μg/g,</sc> <sc>1454 μg/g,</sc> <sc>877 μg/g,</sc> <sc>630 μg/g,</sc> and <sc>201 μg/g,</sc> respectively. Elemental geochemical diagrams show that the genesis of magnetite is related to magmatic hydrothermal fluids and has a higher precipitation temperature <sc>(&gt;500 ℃).</sc> Based on the above analysis, this paper believes that the ore-forming fluid of the Longtan gold-antimony deposit in western Guangxi is related to deep magmatic activity, suggesting that the ore-forming fluid of the Carlin-type gold deposits and related antimony, thallium, mercury, and other low-temperature hydrothermal mineral deposits in the Youjiang Basin may have a magmatic origin.