华南晚古生代碳酸盐岩建造中层控铀矿床的地质和成因探讨

A PRELIMINARY DISUSSION ON GEOLOGY AND GENESIS OF STRATABOUND URANIUM DEPOSITS IN LATE PALEOZOIC CARBONATE FORMATION OF SOUTH CHINA

矿床多出现于晚古生代海盆边缘凹陷中。矿床的产出和分布受泻湖相潮坪碳酸盐沉积所控制。矿体呈似层状、透镜状,与上下围岩整合-准整合接触,沿层分布。矿石即为含铀不纯碳酸盐岩,其特征是色较深,含泥碳质及星散黄铁矿。矿石成分、结构构造、微量元素含量、δ^(14)S、δ(18)O值与围岩近于一致,唯矿石中铀含量高,达工业采冶要求。近矿围岩蚀变微弱。沥青铀矿U-Pb年龄测定多为57—96Ma,主要反映矿石的后生改造年龄,但部分沥青铀矿年龄(380Ma)与所在地层时代(D_1)相近,表明成岩成矿期矿石仍有存在。且后生改造、叠加富集作用主要发生在原有矿层内,使矿体(层)仍保留原地-准原地特征,因而,应属沉积成岩成矿为主兼有后生改造的层控铀矿床。矿床的形成经历了沉积、成岩成矿和改造富化三个阶段。

Stratabound uranium deposits in carbonate formation of South China mostly occur in arginal epressions f the Late Paleozoic seabasin adjacent to the post-Caledonian upwarping rea. The uraniferous carbonate formation s a suite of tidal flat sediments of lagobn facies haracterized by richness in uranium and existence of pelitic and arbonaceous materials as well s disseminated pyrite.Ore bodies, stratoid and lenticular in form, show conformable r nearly onformable contact with overlying and underlying strata; an ore body (bed) and its wall rocks ake up a equence of ore-bearing rocks, mdicating that ore bodies still preserve autochthonous r semi-autochthonous haracters. Ores are simply uraniferous micritio bioclastic limestone, icritic limestone containing bioclastic aterials, imestone of breccia fragments, fine-rystalline dolo mite and argillite. Uranium in ores occurs mainly as isseminations adsorbed by elitic and carbonaceous materials, and partly as independent minerals such as itchblende. The ear-ore wall rocks have only suffered weak alteration. Ore-forming temperatures are in the ange of 50—89℃, δ^(34)S values of pyrite are +10——24‰, and δ^(18)O_(SMOW) values of arbonate nerals are +8.81—+17.31‰. Ores are basically consistent with wall rocks in mineral and hemical composition, ^(34)S values and δ^(18)O values. U-Pb ages of pitchblende are mostly 96—57 a with rather emarkable time difference between ore and rock, indicating obvious epigenetic ransformation which, however, ainly ook place within the original ore beds. It is therefore onsidered that these ore deposits are stratabound uranium eposits in carbonate formation.The re-forming process of these uranium deposits consists of the following three tages: (1) ynsedimentary stage, forming uraniferous lime mud layer; (2) diagenetic-metallogenic stage, at hich ediments were consolidated to form primitive ore bed first, and then the adsorbed uranium n this bed was partly rystallized to form microgranular pitchblende due to the deep-buried ffects; (3) epigenetic enrichment stage, when uperimposition and enrichment of uranium took lace in the original ore bed as a result of transformation by nderground water or heated ater.