Geochemical behavior of Au in terrigenous and pelagic marine sediments

Abstract

A total of 224 marine sediments of terrigenous and pelagic origin were collected from the Obama Bay, Suruga Bay, south of Kii Strait, off Northeast Japan and the Japan Trench (terrigenous sediments), and the Central Pacific (pelagic sediments). These samples were analyzed for Au by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation. The average Au content of the terrigenous (2.4 ± 1.2 ppb, n = 85) and the pelagic (1.4 ± 1.3 ppb, n = 139) sediments were similar to crustal abundance. A significant correlation ( r = 0.82) was found for Au and Cu in terrigenous sediments, while no such trend was found in the pelagic sediments. Sediments collected around the Au mineralized regions are known to exhibit high Au/Cu ratios. Therefore, the Au/Cu ratio should serve as a useful indicator for the geochemical exploration of Au deposits. With respect to the content of Au and organic carbon in the pelagic sediments, surface samples had higher values, whereas deeper sediments were depleted. There was no such vertical concentration trend for Au and organic carbon in the terrigenous sediments. The vertical profile of Au in the pelagic sediments is thought to be a result of upward migration of Au-organometallic complexes during early diagenesis. Since the most probable form of Au in the terrigenous sediments is elemental, significant migration during diagenesis would not occur, explaining the lack of a vertical trend for Au.