Zn-clay minerals in the Skorpion Zn nonsulfide deposit (Namibia): Identification and genetic clues revealed by HRTEM and AEM study

Abstract

Zn-clays worldwide occur in various supergene nonsulfide (Zn-Pb) ores, where they can be the prevailing economic minerals or represent minor concentrations in the mineral assemblage. The world-class Zn smectite-rich Skorpion mine (Namibia) is considered one of the most important supergene nonsulfide zinc deposits in the world. At Skorpion, the trioctahedral Zn-bearing smectite predominates over the other Zn-oxidized minerals. This work is focused on microtextural observation and chemical analyses on the clay nano-particles of the supergene nonsulfide ores from Skorpion, carried out for the first time using TEM/HRTEM and AEM investigations. This approach helped to better understand the formation mechanism of the Skorpion Zn-clays and related phases down to the nanoscale. The microtextures of the Skorpion Zn-clays suggest they formed from fluids, meteoric and/or hydrothermal in nature, in two textural contexts: smectites can grow on previously deposited phyllosilicates (mica) (CCP texture), and/or directly nucleate from Zn-rich solutions (PCA texture). The Skorpion sauconite is chemically characterized by a greater homogeneity if compared with natural sauconites from other occurrences; it is quite stoichiometric, with Ca as interlayer cation and limited quantities of Mg and Fe, with an average composition of Ca0.14K0.02(Zn2.7Mg0.09Al0.14Fe0.10)(Si3.4Al0.6)O10(OH)2·nH2O. Contrary to Peru nonsulfide ores (Accha and Yanque), at the micro- and nanoscale the occurrence of Zn-beidellite at Skorpion is very subordinated. Chlorite and baileychlore have been also detected. Detrital micas are commonly the template for epitaxial sauconite growth. Typical supergene processes at ambient temperatures should be considered for the genesis of the Skorpion sauconite-bearing deposit, with some local contribution of low-T hydrothermal fluids. The micro- and nano-features of the Skorpion mineral assemblage confirm the complex mineralogical nature of the smectite-rich nonsulfide (micro)systems, with remarkable implications for mineralogical evaluation and processing.