“Garnierite” in weathering crust of ultramafic blocks from Cao Bang area, north Viet Nam

Abstract

"Garnierite" is an important group of ore minerals in the supergene nickel deposits. Garnierite group minerals are Ni-bearing hydrous magnesium silicate formed due to the lateritic weathering of ultramafic rocks under humid tropical conditions. In the weathering crust on ultra-mafic blocks Ha Tri and Phan Thanh (Cao Bang province), garnierite group minerals have been found in the saprolite zone or as a coating on the surface of the fissures. Analytical results show that garnierite consists of a mixture of three solid solutions: (i) serpentine-like - 7Ao (lizardite-nepouite), (ii) talc-like - 10 Ao (pimelite-willemseite) and (iii) chlorite-like - 14 Ao (clinochlore-minite). EPMA analysis shows that garnierite in Cao Bang area is characterized by high Ni content (25.50-40.06%), low Fe content (0.09-0.9%) and almost no Al (<0.02%), similar to garnierite in the famous nickel mines in New Caledonia, Dominican Republic, etc... Notable, Ni and Mg contents in garnierite show a clear negative correlation, indicating the replacement for each other in the phases. Under the scanning electron microscope (SEM), garnierite has a raspberry shape forming continuous zones covering the unfinished weathered rock surface. These zones represent different stages of garnierite formation. The results of the study allow proposing a model of garnierite formation in the lateritic weathering zone in ultramafic blocks in the Cao Bang area.