Metallogenic Mechanism Of The Guangshigou Uranium Deposit From The Northern Qinling Orogen,China

Uranium mineralization associated with biotite granitic pegmatites occurs in the Shangdan triangular zone of the northern Qinling Orogen,constituting a significant uraniferous province.The Guangshigou deposit is the the most productive pegamatite type uranium deposit in China.The uranium mineralization occurs at the contact zone between biotite granitic pegmatite and bearing biotite plagioclase gneiss.Combined with structural features,two magmatic pulses are clearly distinguished in the Guangshigou U deposit.The granitic intrusions are composed of the Early Silurian granite?gneissic adakitic rock?and the Late Silurian-Early Devonian high-K granite-pegmatite association?mylonitic syenogranitite and undeformed pegmatites?.The characteristics of element geochemistry and Sr-Nd-Hf isotopic composition suggest these Caledonian granitoids share a common source.The Caledonian intrusions were derived from major anatexis of bassltic lower crust with some addition of mantle-derived materials.The Early Silurian granite pluton formed in a syn-collisional setting with the uplift of terranes,however,the Late Silurian-Early Devonian granitoids formed in a post-collisional setting.It was followed by weakening of convergent squeezing action in the Late Silurian-Early Devonian?420⁴12 Ma?.The granosyenite-granitic pegamatite association in the study area was generated by the comagmatic slow fractional crystallization.In addition,the apparent ages for uraninite mainly range from 405 Ma to 414 Ma,indicating that the Guangshigou pegamatite-hosted uranium mineralization formed in a stable continental evolution stage in late orogenic stage of the northern Qinling terrane.Mineralogy,element geochemistry and isotope geochemistry analasis were employed for the research of AFC process and its contribution to uranium saturation.Three major results about it are listed as below:?1?This work systematically discusses the physiochenmical process of assimilation-fractional crystallization of the pegmatitic magma.The pegmatitic magma emplacement?The incorporation of hydrous mineral-rich?biotite?bearing biotite plagioclase gneiss into the host magma?physical and chemical disintegration of xenolith and xenocryst caused by mealting heat?The incongruent dehydration-melting of country rocks containing biotite,triggering the formation of an anhydrous mineral association coexisting with a xenolith-derived hydrous melt?the mixture between the xenolith-derived hydrous melt and the host magama,producing a mixed melt at the hybrid zone?the formation of new crystalline phases?biotite peritectic crystals,quartz,perthite,microcline,oligoclase?caused by hydration crystallization of the mixed melt at the contact?hybrid zone?.?2?The assimilation of country rock may affect the normal fractional crystallization process of pegmatitic magmaThe relatively higher FeO^T,MgO,TiO₂,MnO,LILE?Rb,Ba,K?and HFSE?Nb,Ta?of uraniferous biotite grantic pegmatite is caused by chemically interacting between the host magma and country rock.Moreover,compositional heterogeneity caused lithology changesin adividual pegmatite.During the assimilation-fractional crystallization process,?¹⁸O and radiogenic ⁸⁷S were introduced in the host pegmatitic magma.However,the contamination by wallrock has not significantly affected the zircon Lu-Hf system.?3?This work interprets saturation mechanism of uranium related to the assimilation-fractional crystallization of the pegmatitic magma.The UF_m⁴-m-m complex was the main transport form of uranium in the high-T?⁶39??pegmatitic melt.The reactive bulk assimilation of bearing biotite plagioclase gneiss strata and related xenolith,give rise to decrease logf_O22 to-24.81^-13.34 and decrease temaperature to 518⁶03?at the hybrid zone.U-F complexes are destabilized by reducing fluorine activities during the formation of biotite peritectic crystals in the hybrid/assimilation zone between host pegmatitic magma and country rock,triggering decrease of U solubility and saturation.