Geochemistry And Geochronology Of Volcanics And Subvolcnics Associated With Meishan Polymetallic Iron Deposit

The Meishan polymetallic iron deposit is one of the representative deposits in Ningwu Basin, with its Fe-Cu-Au-Mo mineralization spatially, temporally, and genetically associated with the gabbro-diorite porphyry and related Dawangshan Formation shoshonitic volcanic rocks. The origin of the host rocks in this region has been studied intensively but still remains controversial. U–Pb dating of zircons from the magenta biotite andesite, the ore–bearing gabbro–diorite porphyry and the fresh gabbro–diorite porphyry under the orebody in the Meishan mining area using laser ablation inductively coupled plasma mass spectrometry(LA–ICP–MS) yields ages of 138~139 Ma and 138~141 Ma respectively; Ar–Ar dating of biotites from biotite andesite of the Dawangshan Formation yields a plateau age of(138.0±1.0) Ma. The host rocks of of the Meishan polymetallic iron deposit formed in 138~141Ma, and the accompanying Fe–Cu–Au–Mo mineralization might have taken place later, both occurred in Early Cretaceous, earlier than any other deposits in this basin. The formation age of host rocks could be considered as a lower limit age of the magmatic-hydrothermal metallogenic system. Depending on the chronology and geological features, we also proposed a new viewpoint that the volcanic-magmatism of the Ningwu basin began from the north and lasted between 7~10 Ma at least. The host rocks in the Meishan mining area, consisting of the gabbro-diorite porphyries and the associated Dawangshan Formation volcanic rocks, are assigned to shoshonite suite. The mineral geochemistry of the rock-forming minerals from the Meishan host rocks suggests that their parental magmas are mantle-derived with high oxygen fugacity, and the parental magmas were probably separated into silicate-rich melts and Fe-oxide-rich melts by liquid immiscibility. These sobvolcanic rocks and associated volcanic rocks are characterized by light rare earth element(LREE)–enriched patterns, depletions in high field–strength elements(HFSE). They experienced the fractional crystallization of plagioclases and clinopyxenes. Significant negative Nb, Ta and Ti anomalies suggest the derivation from subduction–modified mantle, while very weak Eu anomalies indicate the high oxygen fugacity environment under which sobvolcanic rocks and associated volcanic rocks evolved. The geochemistry and Sr–Nd–Hf isotopic characteristics probably indicate that the Meishan sobvolcanic rocks and associated volcanic rocks are derived from an EMⅠenrichment mantle, probably metasomatized by the subducted oceanic sediments of the palaeo–Pacific plate in Cretaceous period(Yanshanian). The parental magma probably originates from a hornblende–bearing spinel peridotite stability field mantle source. The mineral geochemistry, petrogeochemistry and zircon trace elements characteristics manifest that sobvolcanic rocks and associated volcanic rocks probably formed in a continental marginal arc setting associated with the subduction of palaeo–Pacific plate. The subduction–related melts metasomatism implys that the crucial factor(the addition of P) of liquid immiscibility mechanism for the Fe mineralization may be due to the marine sediments in mantle source region rather than the assimilation of phosphorous beds in the crust during magma ascent. The favorable factors for the Meishan Fe–Cu–Au deposit include the basic parental magma which are rich in volatiles, the high oxygen fugacity, fractional crystallization in the high level magma chamber and the shallow emplacement depth.