白云岩风化剖面元素地球化学特征:对黔中九架炉组“三稀金属”富集机制的启示

黔中早石炭世九架炉组铝土矿含矿岩系富集Ti、Li、Sc、V、Ga、Nb、Ta、Zr、Hf、Th和稀土(REEs)等"三稀金属",具有成为独立矿床或伴生资源的潜力。这些元素大部分与九架炉组共有同一母岩,且富集程度受母岩的成分和风化作用控制。本研究选取九架炉组母岩乌当娄山关群白云岩和纳雍牛蹄塘组泥质白云岩的现代风化剖面为研究对象,研究元素在风化作用过程中的迁移特征及分布规律,进而为九架炉组微量元素的富集机制提供启示。研究获得以下认识:1)依据剖面结构、ZrHf、Nb-Ta、Y-Ho二元图特征及REE配分曲线和Eu/Eu*值的相似性表明研究区土层主要来源于基底或母岩的风化; 2)白云岩风化成土过程中Si、Fe、Cr、As、Sb、Ti、Nb、Ta、Zr、Hf、Th、REEs等元素化学性质相对稳定,富集程度较高,而Ca、Mg、Na、K、Sr、P、Mo、Cd等元素化学性质活泼,容易淋失亏损; 3)纳雍剖面REEs~(3+)和Ca~(2+)半径差与REEs富集系数相关性良好,表明碳酸盐岩风化过程中,含钙矿物磷灰石是稀土元素分配的重要控制因素; 4)九架炉组的母岩也是Ti、Li、Sc、V、Ga、Nb、Ta、Zr、Hf、Th等微量元素的主要物质来源,母岩风化过程中,这些微量元素首先在副矿物、黏土矿物、铝矿物及磷灰石等矿物相中初步富集,之后随风化碎屑物一起沉积形成微量元素超常富集地层; 5)纳雍剖面地表和地下水提供了部分P、Be、Zn、Sb、Pb、Y及REEs来源,指示水体迁入作用也是九架炉组REEs富集的重要原因。研究表明黔中九架炉组微量元素的来源较复杂,风化-沉积过程中,化学性质稳定的元素残留在风化碎屑物中并被搬运-沉积在负地形中,而化学性质活泼的元素首先被带入水体,在沉积-成岩过程中特定条件下发生二次富集作用(例如次生矿物的形成及吸附)形成微量元素富集的地层。

Geochemical characteristics of dolomite weathering profiles and revelations to enrichment mechanism of trace elements in the Jiujialu Formation, central Guizhou Province

The Lower Carboniferous Jiujialu Formation is rich in Ti, Li, Sc, V, Ga, Nb, Ta, Zr, Hf, Th and Rare Earth Elements (REEs) and has the potential of comprehensive utilization as an independent trace elements deposit or associated resources in the central Guizhou Province. Most of those share the same parent rocks with the Jiujialu Formation. The element enrichment degree was controlled by chemical composition and weathering degree of the parent rocks. Therefore, the weathering process of the precursor rock provides an important object to study the mechanism of trace element enrichment in the Jiujialu Formation. In this study, the Wudang and Nayong dolomite weathering profiles, which were parent rock of the Jiujialu Formation, were selected to study the elements migration and distribution characteristics during chemical weathering. Consequently, it is to reveal the enrichment mechanism of some trace elements, REEs, and dispersed elements in the Jiujialu Formation. This study gained the following understanding:1) Characteristic of profile structure and Zr-Hf, Nb-Ta, Y-Ho bivariate-plots, combined with the similarity of rare earth elements distribution curves and Eu/Eu^* ratios reveal that these two profiles were mainly derived from weathering of base/parent rocks; 2) During the processes of dolomite weathering, Si, Fe, Cr, As, Sb, Ti, Nb, Ta, Zr, Hf, Th, and REEs showed chemical stability and result in enrichment, whereas Ca, Mg, Na, K, Sr, P, Mo, and Cd showed chemical activity thus lead to depleted; 3) REEs especially MREEs were leached out and enriched during leaching and accumulation stages, respectively, in the Nayong profile rather than the Wudang profile. The difference of ionic radius between REEs³⁺ and Ca²⁺ showed good correlation with REEs enrichment ratios, indicate that Ca-bearing minerals such as apatite was the most important control factors of REEs distribution during carbonate weathering processes in the Nayong profile; 4) Essential features of some elements were hided by homogenization effects such as mineral decomposition-formation, allochthonous materials addition, groundwater leaching etc. during carbonate rock weathering. The Nayong profile that located at -170m underground, was less influenced by homogenization effects, some elements embodied its essential characteristics such as REEs mentioned above; 5) The parent rocks of the Jiujialu Formation were major materials of Ti, Li, Sc, V, Ga, Nb, Ta, Zr, Hf, and Th. During weathering processes, these elements were preliminary enriched in accessory minerals, clay minerals, aluminum minerals, and apatite, then sedimentation together with weathering detritus forming the trace elements enrichment stratum. In addition, surface/groundwater supplies part of P, Be, Zn, Sb, Pb, Y and REEs sources in the Nayong profile, implies that the water addition effect was another important factor for REEs enriched in Jiujialu Formation. As a consequence, the sources of trace elements in Jiujialu Formation are complicated. During weathering and sedimentation, immobile elements remained in weathered detritus and be deposited in depression, whereas mobile elements (e.g., REEs) were first dissolved in water and then can be uptake into sediments by secondary enrichment process to form trace elements enrichment strata.