云南昆阳磷矿黑色页岩微量元素特征及其地质意义

Trace element distribution in black shales from the Kunyang phosphorite deposit and its geological significances

昆阳磷矿是我国下寒武统黑色页岩中赋存的最大的磷块岩矿床。为了探讨磷块岩及上覆黑色页岩的源区性质及古海洋的氧化还原环境,本文对梅树村组中谊村段的磷块岩、石岩头段的黑色页岩以及筇竹寺组玉案山段的砂质黑色页岩的地层剖面进行了岩石地球化学研究。发现昆阳剖面磷矿区的下寒武统黑色页岩Th-Zr和Th-Y/Ho比值以及Y-Y/Ho比值与陆源碎屑物质更为接近,石岩头段黑色页岩样品的稀土元素总量平均为174×10-6,与大陆壳稀土总量的平均丰度基本一致。稀土配分模式也十分类似,说明陆源碎屑物质来源比海水自生来源占的比重更大。受陆源碎屑物质的影响,Ni/Co,V/Cr和V/(V+Ni)等氧化还原敏感元素比值并不能很好的指示古海洋的沉积环境。但石岩头段黑色页岩的氧化还原敏感元素相对大陆壳具有一定程度的富集,并且富集程度不强烈,Ce具有轻微的负异常,反映了黑色页岩可能形成于次氧化环境。

The Kunyang phosphorite deposit is the largest phosphorite deposit hosted by Early Cambrian black shale in Yunnan Province, Southwest China. In order to better understand its origin and paleo-redox conditions of phosphorite and overlying black shales, we conduct detailed geochemical study of the Kunyang profile, which spans the phosphorite of Zhongyicun member, black shale of Shiyantou member, and sandy black shale of Yuanshan member. Th-Zr, Th-Y/Ho and Y-Y/Ho plots indicate that metals in black shales were dominantly originated from terrestrial sources. Average ~ REE of the Shiyantou black shales is 174 × 10 -6, similar with that of the continental ernst. REE distribution patterns of the Shiyantou black shales are also generally parallel with that of the continental curst. The REE geochemistry indicates that terrestrial origin is greater than the seawater authigenic origin. Due to little proportion of authigenic origin redox sensitive element, Ni/Co, V/Cr and V/（ V ＋ Ni） ratios are not suitable to study paleo-redox conditions of seawater. However, slight enrichment of redox sensitive element and weak Ce negative anomalies indicate that the Shiyantou member black shales might formed under suboxic condition.