Geology of the Gushan iron oxide deposit associated with dioritic porphyries, eastern Yangtze craton, SE China

摘要

The major Gushan iron oxide deposit, typical of the Middle-Lower Yangtze River Valley, is located in the eastern Yangtze craton. Such deposits are generally considered to be genetically related to Yanshanian subvolcanic-volcanic rocks and are temporally-spatially associated with ca. 129.3-137.5 Ma dioritic porphyries. The latter have a very narrow ~(87)Sr/~(86)Sr range of 0.7064 to 0.7066 and low epsilon_(Nd)(t) values of —5.8 to -5.7, suggesting that the porphyries were produced by mantle-derived magmas that were crustally contaminated during magma ascent. The ore bodies occur mainly along the contact zone between dioritic porphyries and the sedimentary country rocks. The most important ore types are massive and brecciated ores which together make up 90 vol.- percent of the deposit. The massive type generally occurs as large veins consisting predominantly of magnetite (hematite) with minor apatite. The brecciated type is characterized by angular fragments of wall-rocks that are cemented by finegrained magnetite. Stockwork iron ores occur as irregular veins and networks, especially with pectinate structure; they are composed of low-temperature minerals (e.g. calcite), which indicate a hydrothermal process. The similar rare earth element patterns of apatite from the massive ores, brecciated ores and the porphyries, coupled with high-temperature fluids (1000 deg C) suggest that they are magmatic in origin. Furthermore, melt flow structure commonly developed in massive ores and the absence of silicate minerals and cumulate textures suggest that the iron ores formed by the separation of an immiscible oxide melt from the silicate melt rather than by crystal fractionation. Combined with theoretical and experimental studies, we propose that the introduction of phosphorus due to crustal contamination during mantle-derived magma ascent could have been a crucial factor that led to the formation of an immiscible oxide melt from the silicate magma.