The Connection between Evaporites and Iron Oxide-apatite Deposits, Yangtze River, China - Laser Ablation Inductively Coupled Plasma Mass Spectrometry Analysis of Na, S, Cl and Br in Fluid Inclusions

摘要

The Middle and Lower Yangtze River metallogenic belt is one of the most important polymetallic resource areas in China. Iron oxide-apatite deposits, mainly distributed in Ningwu and Luzong volcanic basins, are a distinct type of iron deposits in this region. These deposits have a close temporal and spatial relationship with Mesozoic dioritic porphyries and are characterised by magnetite-apatite-pyroxene/actinolite ± anhydrite assemblage and large-scale sodic-calcic alteration.Widespread multiphase inclusions in pre- to main-mineralisation stage pyroxene and garnet in Meishan (in the Ningwu basin), Luohe and Nihe (both in the Luzong basin) iron deposits were trapped at ~780°C in extremely saline brines (-90 wt per cent NaCl equivalent), consisting of Na (13-24 wt per cent), K (7-11 wt per cent), Ca (~7 wt per cent), Fe (~2 wt per cent), Cl (19-47 wt per cent) and SO, (3-39 wt per cent), based on petrologic, microthermometric, Raman spectroscopic studies and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of natural fluid inclusions and synthetic analogues. The high Cl/Br, Na/B and S/B ratios in brine inclusions and the heavy sulfur isotopic values of anhydrite and pyrite (delta~(34)S_(Anh) = +15.2 to +16.9‰, delta~(34)S_(Py) = +4.6 to +12.1 ‰) suggest a significant contribution from both halite- and anhydrite-bearing sedimentary rocks. The involvement of evaporites caused the production of large amounts of high-salinity brines with apparently enhanced capacity to extract iron from the dioritic intrusions.According to their geologic features and the above characteristics, these iron oxide-apatite deposits are considered to be a special tvpe of iron oxide copper gold (IOCG) deposits characterised by high contents of Na, Ca, Cl and SO_4 in the ore-forming fluids. Moreover, we believe that evaporite-bearing sedimentary basins are more prospective than evaporite-free ones for these kinds of deposits.