STRUCTURAL, MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS IN NONSULFIDE SUPERGENE ORES (PERU AND NAMIBIA)

Zn-clay minerals are worldwide associated with a variety of base metal ores, as the supergene nonsulfide supergene deposits [1]. In these occurrences, sauconite (the Zn-bearing triocthaedral smectite) occurs either the prevailing economic minerals or minor components of the weathering-derived mineral assemblage. The best example of supergene nonsulfide zinc deposit – where sauconite predominates over the other Zn-oxidized minerals - is the world-class Skorpion mineralization (Namibia). Skorpion ore is hosted in Neoproterozoic rocks and sauconite predominates over the other Zn-oxidized minerals [2]. Among other worldwide occurrences, the Yanque (Zn)-Pb nonsulfide supergene deposit is also sauconite-dominated [3]. This study deals with a detailed characterization of sauconite from Skorpion and Yanque by means of combined XRPD, WDS, FTIR and TEM-HRTEM, aimed to determine composition, structural features and micro- and nano-textures of the Zn-clays occurring in the two deposits. Taking into account that the mineralogical data on sauconite are few [i.e. 4,5], this information allow to better identify their properties, to constrain the genesis of the ore deposits, as well as to plan a correct metallurgical processing. Chemical data on the studied sauconite samples indicate that octahedral sites are occupied by Zn cations with minor contents of Al, as well as Fe and Mg, while the interlayer contains predominally Ca2+ cations. The IR spectra show a distinctive band at about 3644 cm-1, assigned to the Zn(OH)3 stretching vibrations. For the Skorpion sample it has been possible to separate a small fraction of pure smectite, whereas the Yanque sample contains small amounts of quartz, illite, kaolinite, chlorite, K-feldspar and goethite. The XRDP pattern of the Skorpion sample shows the (001) basal reflection at about 14.72 Å, characteristic of di-hydrated Ca2+ rich-smectite, and the diagnostic peak d060 at 1.533Å, indicative of the trioctahedral nature of this smectite. The pattern exhibits broad and asymmetric diffraction maxima that are typical effects of disorder, stacking faults and/or fine grained crystallite dimension. To overcome these issues a suite of crystallographic programs are employed for a thorough microstructural characterization of these mineral phases.The TEM-HRTEM study shows that the microtextures of the Zn-clays observed at size below 10 μm are of two types, i.e. compact clay packages and porous clay aggregates. The first type is represented by straight and almost isoriented packets, as an epitaxial growth onto mica crystals or rarely as void fillings. The packets can locally pass to slightly curved fibres coexisting with the straight packets. The second type consists of very fine-grained packets, with sizes lower than the compact clay packages types, ca. < 400 nm. Electron diffraction patterns show that sauconite has a turbostratic arrangement. As reported in literature, it is tricky to obtain lattice-fringe images of very hydrated clays, as sauconite, due to the structural damage caused by the electron beam and the vacuum of the TEM environment and/or electron irradiation, which cause dehydratation and collapse of smectite. For the investigated samples, the measured smectite spacing is from 10 Å, in the case of a complete collapse, to ≈14 Å in case of an incomplete collapse (for example d001 of 13 Å in the Skorpion sauconite). TEM-AEM study demonstrated that the Skorpion sauconites are mainly Ca-rich varieties, in comparison to the Peruvian samples, which contains also significant quantity of K. In Skorpion, the dioctahedral smectite (beidellite) is very subordinate respect to its Zn-triocthaedral counterpart, and finely intergrown with sauconite (as in Peru samples, where beidellite is clearly more abundant). The occurrence of Zn-beidellite and other phyllosilicates (chlorite, baileychlore, etc.) at the micro- and nanoscale in the studied samples confirms the complex mineralogical nature of the Zn-nonsulfide smectite-rich (micro)systems, with remarkable implications for mineralogical evaluation and processing. [1] Boni M. & Mondillo N. (2015) The "Calamines" and the "Others": the great family of supergene nonsulfide zinc ores. Ore Geology Reviews, 67, 208-233 [2] Borg, G., Kärner, K., Buxton, M., Armstrong, R., van der Merwe, S.W. (2003) Geology of the Skorpion non-sulphide deposit, southern Namibia. Economic Geology, 98, 749–771.. [3] Mondillo N., Nieto F., Balassone G. (2015) Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru. Am. Min., 100, 2484–2496 [4] Ross, C.S. (1946) Sauconite—a clay mineral of the montmorillonite group. American Mineralogist, 31, 411–424. [5] Choulet, F., Buatier, M., Barbanson, L., Guégan, R., Ennaciri, A., (2016) Zinc-rich clays in supergene non-sulfide zinc deposits. Mineral. Deposita 51, 467–490.