甘肃省龙首山牛角沟铀矿点钾钠混合交代作用研究

On Potassium-sodium Mixed Metasomatism of the Niujiaogou Uranium Occurrence in Longshou Mountain,Gansu Province

牛角沟铀矿点是甘肃省龙首山铀成矿带中段东部重要的钠交代型铀矿点,该矿点外围新发现的钾钠混合交代花岗岩在该区难得一见。为更细致的解析钾钠混合交代作用,本文从岩石学、矿物学、地球化学、同位素年代学等方面开展研究,认为牛角沟中粗粒花岗岩是早古生代(445.2±7.8Ma)祁连山—龙首山造山作用晚期,后碰撞伸展环境中产出的中硅、富碱弱过铝、低铁和钙而贫钛、镁、磷的高钾钙碱性花岗岩。钾钠混合交代作用形成蚀变岩从中心向外依次为孔洞状钾钠混合交代花岗岩→雪花状方解石化钾钠混合交代花岗岩→少石英无暗色矿物紫红色钾钠混合交代花岗岩→假象绿泥石化花岗岩逐渐过渡为正常花岗岩。钾钠混合交代作用的热液溶蚀萃取了流经花岗岩中的SiO2、TiO_2、FeO、Fe_2O_3、MnO、MgO、CaO和P2O5,以及REE、Ba、Sr、Zr、Sc、Cr、Sr、Co、Mo、Hf、Bi、Sb、Pb等化学组分,迁入孔洞状钾钠混合交代花岗岩中的组分有Na_2O、K_2O、MnO、Th、U、Rb、Nb、Ta、W、Sr等。CaO、MnO、Na_2O、Th、U、Nb、Ta、Rb、HREE等组分与方解石一起迁入雪花状方解石化钾钠混合交代蚀变带中,而TiO_2、FeO、Fe_2O_3、MgO、CaO、P_2O_5、V、U、Se、Zr等化学组分在紫红色钾钠混合交代花岗岩中析出富集。钾钠混合交代花岗岩性脆、易碎、多孔、"疏松",为之后钠交代铀成矿作用提供了良好的围岩基础。其侵位时代进一步印证了龙首山钠交代型铀矿成矿作用起始于444～442Ma前后的可靠性。

Niujiaogou uranium occurrence is one important Na-metasomatic uranium occurrence in Longshou Mountain,Gansu Province.The potassium-sodium mixed metasomatic rock,recently discovered at the periphery of the occurrence,is pretty rare t in this area.In order to investigate the potassium-sodium mixed metasomatism in details,this study carried out petrology,mineralogy,geochemistry,and isotope chronology analyses for the Niujiaogou medium-coarse grained granite and altered rocks.Geochemical study shows that the Niujiaogou medium-coarse grained granite is high-K calc-alkaline granite formed in the post-collision setting of Qilian-Longshou orogeny in early Paleozoic(445.2±7.8 Ma),from which the granite formed characterized by medium Si,rich alkali and weak peraluminous,low Fe and Ca,and poor Ti,Mg and P.Potassium-sodium mixed metasomatism gives rise to the formation of porous altered granite→snowflake calcilization granite→claret-colored granite with poor quartz and no dark minerals→ pseudochloritization granite and finally to primary granite successively from inner to outer.The potassium-sodium mixed hydrothermal fluid extracted iO2,TiO2,FeO,Fe2O3,MnO,MgO,CaO,P2O5,REE,Ba,Sr,Zr,Sc,Cr,Sr,Co,Mo,Hf,Bi,Sb and Pb from granite.Some components such as Na2O,K2O,MnO,Th,U,Rb,Nb,Ta,W and Sr precipitated in porous altered granite.CaO,MnO,Na2 O,Th,U,Nb,Ta,Rb and HREE migrate with calcite into snowflake calcitization potassium-sodium mixed alteration zone,while TiO2,FeO,Fe2O3,MgO,CaO,P2O5,V,U,Se and Zr are enriched in claret-colored potassiumsodium mixed metasomatic zone.The potassium-sodium mixed metasomatic medium-coarse grained granite is characterized by brittleness,fragility and porosity,which provide favorable basis for Na-metasomatic uranium mineralization.Their emplacement age further confirms that Na-metasomatic uranium mineralization started around 444 Ma^442 Ma.