Palaeogene Sediment-hosted Pb–Zn deposits in SE Asia: the Uragen example

The strata-bound Pb–Zn deposits in western China share many similarities and are controversial in genesis. The large Uragen Pb–Zn deposit is located in the northwestern part of the Kashgar sag, southwest of Tarim Basin, Xinjiang, NW China. With the 980.9 Mt tons of Pb–Zn ores at 0.45%Pb and 2.61%Zn, it is the third largest known Pb–Zn deposits in China. The orebodies are stratiform and stratabound and are predominantly controlled by the Uragen syncline that has an E–W axial trend. Mineralization mostly occurs in sandstones and conglomerates of the Lower Cretaceous Kezilesu Group (K1kz), with a small amount in the Palaeocene limestone. The main ore types consist predominantly of disseminated ore with minor massive ore, veined ore, and breccia ore. The primary metal minerals are composed of sphalerite, galena, pyrite, and minor arsenopyrite and chalcopyrite, and the supergene metal minerals include smithsonite, cegamite, beudantite, jarosite, limonite, and minor hemimorphite. The gangue minerals are composed of dolomite, calcite, quartz, celestite, and gypsium. Our new Rb–Sr isotopic analyses on the separated sphalerite, galena yielded an isochron age of 55.1 ± 1.6 Ma, coeval to an isochron age of 55.4 ± 2.2 Ma by Sm–Nd isotopic data. This age is much younger than the youngest ore-bearing strata (E11, 65.5–61.7 Ma), arguing for an epigenetic origin. The calculated initial 87Sr/86Sr ratio of sulphides is 0.710322, which is much lower than those of basement formation, regional bituminous sandstone, and even the ore-bearing strata, but higher than the regional mantle-derived, alkaline volcanic rocks and marine carbonate. The calculated initial 143Nd/144Nd ratio of calcite and galena is 0.512081. These data suggest that the metals may be chiefly derived from crust, possibly from the minor contribution of mantle materials. Our new-age data, in combination with the previous data, suggest that there probably is a huge medium-low-temperature epigenetic stratabound Pb–Zn belt, which is possibly correlated to the India–Asia collision event.

中国西部的层控铅锌矿床（strata-bound Pb–Zn deposits）具有诸多共性，但其成因却长期存在争议。位于中国西北新疆塔里木盆地西南部喀什凹陷西北部的乌拉根（Uragen）大型铅锌矿床，以980.9百万吨铅锌矿石、品位0.45%铅与2.61%锌的规模，成为国内已知第三大铅锌矿床。矿体呈层状、层控产出，主要受轴向近东西向的乌拉根向斜构造控制。矿化主要赋存于下白垩统克孜勒苏群（K1kz）的砂岩与砾岩中，少量见于古新统灰岩内。主要矿石类型以浸染状矿石为主，次为块状矿石、脉状矿石及角砾状矿石。原生金属矿物以闪锌矿、方铅矿、黄铁矿为主，伴生少量毒砂与黄铜矿；表生金属矿物则包括菱锌矿（smithsonite）、塞加迈石（cegamite）、臭葱石（beudantite）、黄钾铁矾（jarosite）、褐铁矿（limonite），以及少量异极矿（hemimorphite）。脉石矿物由白云石、方解石、石英、天青石及石膏组成。本次对分离得到的闪锌矿、方铅矿开展的铷-锶（Rb–Sr）同位素分析显示，其等时线年龄为55.1±1.6 Ma；钐-钕（Sm–Nd）同位素数据给出的等时线年龄为55.4±2.2 Ma，二者时代一致。该年龄远晚于含矿地层的最新沉积时代（E11，65.5~61.7 Ma），佐证其为后生成因。计算得到的硫化物初始87Sr/86Sr比值为0.710322，显著低于基底地层、区域沥青砂岩乃至含矿地层的锶同位素比值，但高于区域幔源碱性火山岩与海相碳酸盐岩。计算得到的方解石与方铅矿初始143Nd/144Nd比值为0.512081。上述数据表明，成矿金属主要源自地壳，可能有少量幔源物质参与其中。结合本次新获得的年代学数据与既往研究资料，研究团队认为区内可能存在一条大型中低温后生层控铅锌矿带，其形成或与印度-亚洲碰撞事件存在关联。