A CRYSTAL-CHEMICAL INVESTIGATION ON NATURAL Zn-CLAYS FROM NONSULFIDE ORE DEPOSITS

Zn-bearing clay minerals occur worldwide in several supergene nonsulfide ores, where they may represent an important metal source [2,3,4]. In the present work a crystal chemical investigation, by means of XRPD, WDS, ICP-MS/OES and FTIR, has been undertaken on two trioctahedral Zn-smectites, belonging to the supergene nonsulfides of the Skorpion (Namibia) and Yanque (Peru) deposits. The Skorpion mineralization is the largest supergene nonsulfide zinc deposit in the world. It is hosted in Neoproterozoic rocks that are part of a volcano-sedimentary sequence within the Gariep Belt in the southern part of the country. In this deposit sauconite predominates over the other Zn-oxidized minerals, mainly represented by smithsonite, hemimorphite and Zn-bearing phosphates [5,6]. The Yanque prospect is a (Zn)-Pb nonsulfide concentration located 20 km north of Santo Tomás village in Peru [7]. The deposit consists of several sub-horizontal stratabound bodies that extend in depth to more than 100 m. Yanque is hosted by a sedimentary breccia with lateral facies variations, which stratigraphically comprehends parts of the Mara and Ferrobamba Formations [8]. Sauconite is the most abundant economic Zn mineral in the Yanque deposit. The first studies on the composition and thermal properties of sauconite from different localities were provided by [9, 10]). Recently, mineralogical studies on this Zn-clay were carried out by various authors, i.e. [3, 11-13], but structural data on sauconite are scarce. The XRDP pattern of the Skorpion sample exhibits broad diffraction maxima indicative of the lack of perfect long-range order due to stacking faults and/or fine grained crystallite dimension. The (001) basal reflection is positioned at about 14.72 Å, whereas the diagnostic peak d060 is at 1.533Å. For the Yanque sample XRPD data indicate the presence of minor amount of quartz, illite, kaolinite, chlorite, K-feldspar and goethite. Both sauconites show variable contents of Al and Zn, as well as Fe and Mg. The IR spectra show a distinctive band at about 3644 cm-1, assigned to the Zn(OH)3 stretching vibrations. [1] Boni M. & Mondillo N. (2015) The "Calamines" and the "Others": the great family of supergene nonsulfide zinc ores. Ore Geology Reviews, v. 67, 208-233 [2] Boni M., Balassone G., Arseneau V., & Schmidt P. (2009) The nonsulfide zinc deposit at Accha (Southern Peru): geological and mineralogical characterization. Economic Geology, 104(3), 267-289. [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. [3] 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. [5] Kärner, K. (2006) The metallogenesis of the Skorpion Non-sulphide Zinc Deposit, Namibia. Unpublished Ph.D. Thesis, Martin-Luther-Universität Halle-Wittenberg (Germany), 133 pp. [6] Zincore Metals, Inc. (2013) AZOD Zinc Oxide project, NI 43-101 Technical Report on a Preliminary Feasibility Study: Zincore Metals, Inc., Vancouver, Canada(http://www.zincoremetals.com/_shared/pdf/170848_Zincore_PFS_Tech Report_AZOD_26August2013_Final.pdf). [7] Pecho, V., and Blanco, E.Z. (1983) Geología de loscuadrángulos de Chalhuanca, Antabamba y Santo Tomás: Instituto de Geología, Minería y Metalurgia, Boletín, Lima, Peru, n. 35, 97 p. [8] Mondillo N., Boni M., Balassone G., Villa I.M. (2014) The Yanque Prospect (Peru): from polymetallic Zn-Pb mineralization to a Nonsulfide deposit. Economic Geology, vol. 109, p. 1735-1762. [9]Ross, C.S. (1946) Sauconite—a clay mineral of the montmorillonite group. American Mineralogist, 31, 411–424. [10]Faust, G.T. (1951) Thermal analysis and X‑ray studies of sauconite and of some zinc minerals of the same paragenetic association. American Mineralogist,36, 795–822. [11] Kaufhold S., Färber G., Dohrmann R., Ufer K., Grathoff G. (2015) Zn-rich smectite from the Silver Coin Mine, Nevada, USA. ClayMinerals, 50, 417-430. [12] 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. [13] Buatier M., Choulet, F., Petit, S., Chassagnon, R., Vennemann, T. (2016)Nature and origin of natural Zn clay minerals from the BouArhous Zn ore deposit: Evidence from electron microscopy (SEM-TEM) and stable isotope compositions (H and O). Applied Clay Science, http://dx.doi.org/10.1016/j.clay.2016.07.004, 1-14