四川拉拉铁氧化物-铜-金-铀矿床的铀成矿时代与成矿环境

Geochronology and environment of uranium mineralization in the Lala iron oxide-copper-gold-uranium deposit,Sichuan Province

位于扬子地块西南缘康滇地轴的拉拉矿床是我国典型的铁氧化物-铜-金-铀(IOCG)矿床。该矿床主要产出铁铜矿石,并共(伴)生有一定规模的铀矿化。本文在野外地质调查的基础上,综合利用微区X射线荧光光谱法(μ-XRF)、扫描电镜(SEM)、电子探针(EPMA)和激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)等测试技术,针对拉拉矿床中的铀矿化开展了详细的矿相学、地球化学和同位素年代学研究,并进一步探讨了铀成矿构造背景和成矿环境。研究表明,晶质铀矿、钛铀矿和铀石是拉拉矿床中主要的铀矿物,晶质铀矿同时产于铁铜矿石和铀矿化样品中,但钛铀矿和铀石仅产于铀矿化样品中。晶质铀矿主要呈立方体晶形产出,具有高U、Pb和Y,低Th、Ca、Si、Ti和P的地球化学组成,属于低钍晶质铀矿。U-Pb同位素定年表明拉拉矿床发育两期铀矿化,早期铀矿化产于铁铜矿石中,形成年龄为990±4 Ma,与早期铁铜矿化(~1.05 Ga)同步,晚期为后期叠加的热液铀矿化,以晶质铀矿-方解石-萤石脉的形式穿插于铁铜矿石中,形成年龄为~850 Ma,与晚期铁铜矿化(~850 Ma)同步,早晚两期矿化分别形成于大陆裂谷环环境和活动大陆边缘的岛弧背景下。综合前人研究资料,本文认为早晚两期铀矿化均形成于高温热液环境中,早期矿化的形成可能与非岩浆流体(如大气降水)的加入有关,而晚期矿化的形成与区域变质作用有关。

The Lala deposit located at the Kangdian region in the southwest margin of the Yangtze block is a representative iron oxide-copper-gold-uranium deposit in China.This deposit was produced mostly Fe-Cu mineralization;however,it was also associated with considerable U mineralization.In this study,the uranium mineral,geochemistry and U-Pb geochronology of the Lala deposit have been investigated usingin-situX-ray fluorescence spectroscopy(μ-XRF),scanning electron microscope(SEM),electron probe microanalysis(EPMA),and laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)analysis to constrain the metallogenic tectonic setting and environment.The main uranium minerals found in the Lala deposit are uraninite,brannerite,and coffinite.The uraninite occurs in both Fe-Cu and U ores,whereas the brannerite and coffiniteare exclusively found in U ores.The uraninite typically occurred as cubic crystal with high U,Pb and Y contents,low Th,Ca,Si,Ti and P contents.Our U-Pb dating results suggested that there are two stages of U mineralization events in the Lala deposit.The first U mineralization,which was found in Fe-Cu ores,developed at 990±4 Ma in sync with the regional early Fe-Cu mineralization(ca.1.05 Ga)during the continental rift setting.The second U mineralization,which evolved as late U-bearing quartz-calcite-fluorite veins in Fe-Cu ores,formed at ca.850 Ma maintaining pace with the regional late Fe-Cu mineralization(ca.850 Ma)under the island arc setting of active continental margin.Combined with previous research results,this study suggests that both the U mineralization formed at the high temperature hydrothermal environment,and were linked to the addition of non-magmatic fluids(e.g.,meteoric water),as well as regional metamorphism,respectively.