Characterization and origin of low-T willemite (Zn2SiO4) mineralization: the case of the Bou Arhous deposit (High Atlas, Morocco)

Abstract

International audienceWillemite (Zn2SiO4) usually reported in hypogene non-sulfide deposits is described as the main ore mineral in the carbonate-hosted Bou Arhous zinc deposit. This deposit is located in the High Atlas intracontinental range that formed during the Tertiary. Based on a set of microscopic observations, it was possible to establish that willemite replaces primary sphalerite. On the basis of cathodoluminescence imaging, three successive generations of willemite are distinguished, with evidence of dissolution–reprecipitation processes. Willemite is also variably enriched in Ge (up to 1000 ppm), while Ge contents lower than 100 ppm are reported in the primary sulfide minerals. Depending on the willemite generation, this substitution was positively or negatively correlated to the Zn-Pb substitution. According to the nature of zoning (sector versus oscillatory), the incorporation of Ge was either controlled by crystallographic factors or by the nature of the mineralizing fluids. Willemite is associated with other oxidation-related mineral species, like cerussite (PbCO3) but is not in isotopic equilibrium and therefore not considered to be cogenetic. Oxygen isotope compositions support the formation of willemite at temperatures below 130 °C, from mixed meteoric and deeper, hydrothermal fluids. The formation of the High Atlas Belt during the Tertiary has contributed to the exhumation of the sulfide minerals and the development of vertical conduits for percolation of meteoric water and ascending hydrothermal fluids. In addition to a local contribution of silicate minerals of the host limestone, hydrothermal fluids probably transported Si and Ge that are incorporated in willemite

Critical elements in non-sulfide Zn deposits: a reanalysis of the Kabwe Zn-Pb ores (central Zambia)

“…Germanium incorporation in willemite is not unusual, having already been observed in various Zn-silicate deposits, e.g. Franklin (Sheffer, 1966), Tsumeb (Lombaard et al , 1986), Beltana (Groves et al , 2003), Bou Arhous (Choulet et al , 2017) and Tres Marias (Saini-Eidukat et al , 2009, 2016). In particular, Saini-Eidukat et al (2016) have described the structural occurrence of Ge in the Tres Marias willemite in detail, and have shown that “Ge in willemite occurs as Ge 4+ and is four-fold coordinated with oxygen, as expected because the ionic radius of Ge 4+ (0.44 Å) is close to that of Si 4+ (0.39 Å) and isomorphism of Ge 4+ with Si 4+ is most probable”.…”

“…Comparison of the δ 18 O compositions of the Kabwe willemite with willemites from other deposits of the world is hampered by lack of data. At present, δ 18 O compositions of willemite have been only measured at Sterling Hill (New Jersey, USA) (7.4–11.4‰ V-SMOW; Johnson et al 1990), at Vazante (Minas Gerais, Brazil) (10.9–13.8‰ V-SMOW; Monteiro et al , 1999), and at Bou Arhous (L'oriental Region, Morocco) (5.3–7.8‰ V-SMOW; Choulet et al , 2017), where willemite is considered to be precipitated from hydrothermal fluids (Johnson et al 1990; Monteiro et al , 1999; Choulet et al , 2017). Looking at the W1-water oxygen isotope fractionation curves, we see that for a limited range of temperatures only (~0 to 40°C), the precipitating fluids would be characterized by the negative or slightly positive δ 18 O compositions generally indicative of a relatively unaltered surface fluid origin in tropical latitudes.…”

“…Willemite with textures similar to W2 (i.e. euhedral hexagonal habit, with chemical oscillatory zoning) has been observed at Star Zinc (willemite II; Terracciano, 2008; Boni et al , 2011), at Tres Marias (Saini-Eidukat et al , 2009, 2016), and at Bou Arhous (Choulet et al , 2017), where the mineral was considered to be of hydrothermal origin. If this was the case here, then clearly it was a distinctly different hydrothermal event compared to W1, given that the W2 precipitating fluid O isotope compositions, preserving a meteoric signature, are distinctly different from W1.…”

“…This was GSD-1g glass (USGS) for goethite and hematite, and NIST 610 for smithsonite and descloizite. Following the procedure in Choulet et al (2017), the external calibration standard for willemite and hemimorphite was NIST 612. More complicated was the analytical approach to the sphalerite analysis and the choice of the appropriate external standard (see Appendix 1 for details).…”

Origin of the Moroccan Touissit-Bou Beker and Jbel Bou Dahar Supergene Non-Sulfide Biomineralization and Its Relevance to Microbiological Activity, Late Miocene Uplift and Climate Changes

“…However, the mineralogy and origin of the non-sulfide ores have been neglected and have only recently become the subject of ongoing investigations [30][31][32].…”