Contents of major and noble chemical elements in phosphorites and bottom sediments

Ag and Au are typically concentrated in phosphorites; they genetically related to organic matter of bottom sediments that extract these elements from seawater or interstitial water. Consequently, the phosphorites inherit Ag and Au from host sediments that are not always enriched in them. In contrast to other organic-rich sediments, analyzed sample of recent diatom ooze from the Namibian shelf is not enriched in Ag and Au, although some sediments from this region are enriched in Au. In addition to authigenic Au, allochthonous Au associated with quartz grains and micrograins can also be present in shelf phosphorites. This was observed in oceanic phosphorites of various types. Anomalous Au and Fe contents recorded in one seamount phosphorite sample can be related to extraction of Au and nonferrous metals by ferromanganese hydroxides from seawater. This process can serve as one of major mechanisms of Au supply to ferromanganese crusts on seamounts. Phosphorites and sediments are enriched in Ru simultaneously with U. Author's data show that U content varies from 17 (seamount phosphorite) to 887 ppm (Pleistocene phosphorite nodule from the Namibian shelf). This is probably caused by different types of behavior of light and heavy PGEs in the marine environment.银（Ag）与金（Au）通常富集于磷块岩中；二者在成因上与底沉积物有机质密切相关——这类有机质可从海水或间隙水中萃取上述元素。因此，磷块岩中的银和金继承自宿主沉积物，而宿主沉积物并非总能富集这两种元素。与其他富有机质沉积物不同，本次分析的纳米比亚陆架现代硅藻软泥样品并未富集银和金，尽管该区域部分沉积物富含金。除自生金外，陆架磷块岩中还可存在与石英颗粒及微颗粒伴生的异地金，这一现象已在多种类型的大洋磷块岩中被观测到。某海山磷块岩样品中记录到的金与铁（Fe）含量异常，该现象可能与铁锰氢氧化物从海水中萃取金及有色金属有关，这一过程可作为海山铁锰结壳的主要金补给机制之一。磷块岩与沉积物可同时富集钌（Ru）与铀（U）。笔者的实测数据显示，铀含量范围为17（海山磷块岩）至887ppm（纳米比亚陆架更新世磷块岩结核）。该含量差异或源于海洋环境中轻、重铂族元素（Platinum Group Elements，PGEs）的地球化学行为差异。