近(超)临界状态铀溶解性状实验研究

Experimental Research on Dissolution Property of Uranium in Near(Supercritical)State

近(超)临界流体在产铀岩体的岩浆期后自交代作用、岩浆部分熔融、熔离作用以及“前提蚀变”等岩浆流体作用过程中起着至关重要的作用。以诸广山地区高昔矿床产铀花岗岩为固体标本,采用多种介质溶液对其开展热液系统中铀溶解模拟实验,探讨近(超)临界流体能否导致铀溶解,并促使热液型铀矿的形成。实验结果表明,在近(超)临界恒压变温(pH_(2)O=50 MPa,t=350~480℃)条件下,花岗岩中的铀溶解在Na_(2)CO_(3)溶液中呈现上升趋势。当温度为400℃时,铀溶解度在NaCl、H_(2)SiO_(3)溶液中出现最高值,在H_(2)O溶液中出现微上升趋势;当温度为480℃时,铀溶解度在H_(2)O溶液中出现次高值。NaF、NaCl、H_(2)SiO_(3)、H_(2)O、“644”、“100”和“300”等溶液明显酸化,仅KCl和碳酸盐溶液碱化。在恒温(t=300℃)但pH_(2)O变化时,压力效应表现不明显,如升高pH_(2)O时,H_(2)O和非碳酸盐溶液对铀溶解影响很小。在不断补充岩浆熔融自变质阶段实验中,铀遵守UO_(2)(ar)+H_(2)O=HUO_(3)^(-)+H^(+),最终以晶质铀矿UO_(2)(aq)形式存在于酸性环境。在早期钠长石化、硅化以及粉末状赤铁矿化过程中,成矿溶液酸化对形成富铀矿至关重要。花岗岩熔体结晶时有相当部分的铀以晶质铀矿形式晶出,故酸性环境下晶质铀矿可作为花岗岩副矿物,且其铀溶解贡献非常大。晶质铀矿在花岗岩中晶出,以及近(超)临界状态岩体产生的自交代蚀变,不但是一种潜在铀源,而且可以为评价花岗岩岩体成矿远景提供重要信息。

Near(super)critical fluids play a crucial role in magma-fluid processes such as post-magmatic autometasomatism,partial magma melting,and"premium alteration"of uranium-producing plutons.The uranium-producing granite from the Gaoxi deposit in Zhuguangshan area is taken as the solid standard,and a simulation experiment of uranium dissolution in hydrothermal system is carried out using various types of solutions.Based on the experimental results,it came to the conclusion that the near(super)critical fluids can effectively lead to uranium dissolution,and promote the formation of hydrothermal uranium deposits.Under constant pressure(pH_(2)O=50 MPa)and variable temperature(t=350~480℃),the uranium dissolution of granite in Na_(2)CO_(3) solution dramatically increased.At 400℃,NaCl and H_(2)SiO_(3) solution reach the highest values,respectively,and H_(2)O,which secondary peak appears at 480℃,slightly raises.At 400℃with constant pressure,most solutions including NaF,NaF,NaCl,H_(2)SiO_(3),H_(2)O,"644","100"and"300"appearently acidify,while solutions including KCl and H_(2)CO_(3) alkalify.At 300℃(constant temperature)but variable pressures,the dissolution of uranium remains invarible.At 300℃(constant temperature)with increasing pressure,H_(2)O and non-carbonate solutions have little influence on the dissolution of uranium.The experiment of magma melting-autometamorphic stage show that the uranium abide by the reaction fomula UO_(2)(ar)+H_(2)O=HUO_(3)^(-)+H^(+)and the uraninite UO_(2)(aq)ultimately formed in acid environment.When granite melt crystallized,a considerable quantity of uranium formed as uraninite.Thus,the uraninite commonly occurs as accessory minerals in the acidic environment and contribute a lot to the uranium dissolution.The crystallization of uraninite in granite and autometasomatism in near(super)critical state,are not only regarded as a potential source of uranium,but also provide important information for evaluating uranium prospecting potential in granitoids.