贵州务川瓦厂坪铝土矿床稀土元素地球化学特征及其对成矿过程的指示

Rare-earth element(REE) geochemistry and ore-forming process of the Wachangping bauxite deposit, Wuchuan-Zheng'an-Daozhen area, northern Guizhou Province, China

瓦厂坪铝土矿床是黔北务川-正安-道真铝土矿成矿区内的大型古风化壳沉积型铝土矿床,其含矿岩系位于下二叠统栖霞组(P2q)灰岩和志留系中下统韩家店组(S1-2hj)泥页岩之间的下二叠统梁山组(P2l)地层中。铝土矿石的矿物组成主要以硬水铝石、勃姆石、高岭石、蒙脱石以及赤铁矿为主,其次还含有少量的三水铝石、黄铁矿、针铁矿、锐钛矿、锆石、长石和石英等。矿物学特征表明,铝土矿物质经历了搬运和重新沉积。研究表明铝土矿石及下伏志留系韩家店组泥页岩的稀土元素地球化学特征较相似,暗示韩家店组泥页岩可能为铝土矿主要的成矿原岩。铝土矿床的形成是下伏韩家店组泥页岩通过长期原位风化作用逐步转变而来的。铝土矿剖面上稀土元素的地球化学演化是多方面因素综合作用的结果,这些因素包括:原岩类型、p H值、矿物颗粒的大小、地下水的化学性质、剖面上Fe的浓度变化、所含矿物的种类以及稀土元素的地球化学性质等因素。根据地质学、矿物学和地球化学数据,对务川瓦厂坪铝土矿床提出了4阶段的成矿模式。

The Wachangping Bauxite deposit is one of the largest deposits in the Wuchuan-Zheng'anDaozhen alumina metallogenic province, and belongs to paleo-weathering of crustal sedimentary deposits. Bauxite occurs in sandstone and shale of the lower-middle Silurian Hanjiadian group(S1-2hj), or on the limestone surface erosion of the Upper Carboniferous Huanglong formation(C2h). Ore controlling strata are middle Permian Liangshan Formation(P2l). Diaspore, boehmite, kaolinite, smectite and hematite are the major mineral components in the bauxite ore with small amounts of gibbsite, illite, goethite, pyrite, anatase, zircon, quartz and feldspar. The textural features of the ore suggest that the bauxite has an authigenic origin but locally underwent transportation and re-deposition. We can conclude that the main source of the Wachangping bauxite deposits is the underlying sandstone and shale of the lower-middle Silurian Hanjiadian formation(S1-2hj) in view of their similar chondrite-normalized REE patterns. The weathered Hanjiadian group was partly converted to bauxite by means of progressive in situ lateritic weathering. Factors such as type of parent rock, p H variation in weathering solutions, grain sizes of the minerals, groundwater chemistry, variation of Fe concentration in the weathering profiles, the degree of mineral leaching, and the geochemistry of elements present, play a significant role in the distribution of trace and REE during weathering of lateritic bauxite. Based on the mineralogical, petrographical and geochemical analysis, a four stage ore-forming processes is proposed.