Study on Occurrence of Noble Mental Elements in Black Shale Series in Zhangjiajie, Hunan Province by Heavy Placer Classification-SEM-EDS and Other Techniques

BACKGROUND Black shale series is an important carrier of many useful elements. Cu, Pb, Zn, Ni, Mo, V, U, Au, Ag, rare and REE elements were abnormally enriched in the black rock series. The noble metal elements (PGE, Au, Ag) in some black shale series are also anomalously enriched, even partially exceeding the basic and ultrabasic rocks. However, the occurrence of PGE in such black shale series is still an unsolved problem. OBJECTIVES In order to clarify the occurrence of noble metal elements, Mo-Ni deposits in the black shale of the Niutitang Formation of the Cambrian system in Zhangjiajie, Hunan Province, were collected as research samples. METHODS The heavy placer classification, chemical analysis, X-ray diffraction analysis, scanning electron microscopy and energy disperse spectroscopy were used to study the occurrence of noble metal elements. RESULTS Chemical analysis showed that:Pt 531×10-9, Pd 514×10-9, Os 185×10-9, PGEs 1259.43×10-9, Au 332×10-9, Ag 12.2×10-6. These precious metals were concentration in ore-formation. CONCLUSIONS 1) Silver exists mainly as independent minerals, a small amount exist as isomorphism in other minerals. Independent minerals include:novakite, polybasite, andorite, dyscrasite, canfieldite, marrite, hocartite, argentite, Ag-Sn-Sb intergrowth compounds. The isomorphism is mainly freibergite-Ag-tennantite, silver-bearing galena and silver-bearing domeykite. 2) Native gold doesn't exist as independent mineral and is also not adsorbed by carbon. Meanwhile, pyrite was an important gold-bearing mineral. 3) PGEs do not exist as independent minerals and are no longer enrichment in different classification sample. It is speculated that PGE minerals are likely to exist in nanometer form. It is suggested that the application of nanomaterials can be considered in the future study of mineral processing.