松辽盆地钱家店铀矿床中碳酸盐胶结物赋存状态及其与铀成矿关系

Occurrence of Carbonate Cements and Relationship between Carbonate Cementation and Uranium Mineralization of Qianjiadian Uranium Deposit, Songliao Basin

砂岩型铀矿床中碳酸盐胶结物往往被作为研究成矿流体的示踪剂,然而它普遍存在种类多样化及沉积-成岩多阶段性等特点,判断其是否属于铀成矿阶段产物存在一定的挑战性.以松辽盆地钱家店铀矿床为对象,利用岩心观察、偏光显微镜、扫描电镜、XRD等方法,系统研究了铀储层中碳酸盐胶结物种类、赋存状态及生成序列,在此基础上,结合碳酸盐胶结物与铀矿体空间配置规律探讨了其与铀成矿的关系.取得的主要成果和认识如下:(1)铀储层中碳酸盐胶结物类型主要包括白云石、菱铁矿、铁白云石和方解石.白云石以接触式胶结为主,常见镶嵌粒状结构,其平均含量在部分氧化带中最高.菱铁矿以接触式胶结为主,常见残留的自形晶骸,其平均含量在还原带中最高.铁白云石以接触式、孔隙式胶结为主,常见镶嵌粒状结构,其平均含量在过渡带中最高.方解石常见孔隙式胶结、接触式胶结,以镶嵌粒状结构为主,其平均含量在微弱氧化带中最高.(2)碳酸盐胶结物由早到晚生成序列为:白云石→菱铁矿→铁白云石→方解石.在薄片中观察到:1?菱铁矿、铁白云石被方解石包裹;2?菱铁矿被铁白云石包裹;3?铁白云石在白云石表面生长;4?菱铁矿充填白云石溶蚀孔等现象.(3)推断菱铁矿为成岩期产物,白云石为准同生或早成岩期产物,铁白云石属于铀成矿期产物,方解石为成矿期后产物.综上所述,钱家店铀矿床中与铀成矿密切相关的碳酸盐胶结物为铁白云石.

Carbonate cements from sandstone-hosted uranium deposits have generally been considered as a tracer for ore-forming fluids. However, it is difficult to confirm whether carbonate cements occurred in the ore-forming period of the sandstone-hosted uranium deposits since they generally have characteristics of variable minerals and multiple depositional and diagenetic stages. The purpose of this study is to characterize the mineral species, occurrence, generation of the carbonate cements in the Qianjiadian uranium deposit, southern Songliao basin, based on a series of approaches such as core description, micropolariscope and scanning electron microscope (SEM). According to these data in combination with the spatial allocation of carbonate cements and uranium ore bodies, relationship between carbonate cementation and uranium mineralization are revealed.The key conclusions are summarized as follows: (1) Carbonate cements in the Qianjiadian uranium deposit are primarily composed of dolomite, siderite, ankerite and calcite. The dolomite cements with crystalline granular texture and contact cementation have the highest average content in the partial oxidation zone. Siderite cements with a large quantity of residual euhedral crystals often show as contact cementation, which have the highest average content in the reduction zone. Ankerite cements with mosaic granular structure generally present contact cementation and porous cementation. The average content of ankerite is the highest in the transition zone. Calcite cements with porous cementation and contact cementation mainly have mosaic granular texture, which have the highest average content in the weak oxidation zone. (2) The dolomite, siderite, ankerite and calcite are successively generated from early to late stages. Four evidences have been found in the thin sections. Firstly, the siderite and ankerite cements are included by the calcite. Secondly, the siderite is included by the ankerite. Thirdly, the dolomite is coated with ankerite. Finally, the dissolved pores of dolomite are filled by siderites. (3) It can be inferred that the siderite was formed during the diagenetic stage, and the dolomite may have occurred in penecontemporaneous or early diagenesis stages. It can be concluded that ankerite is associated with uranium mineralization. The calcite may have occurred after the metallogenic period. Therefore, the ankerite of carbonate cements is closely related to uranium mineralization in the Qianjiadian uranium deposit.