The carbonate-hosted willemite prospects of the Zambezi Metamorphic Belt (Zambia)

Abstract

Zambian willemite (Zn(2)SiO(4)) deposits occur in the metasedimentary carbonate rocks of the Proterozoic Katangan Supergroup. The most important orebodies are located around Kabwe and contain both sulphides and willemite in dolomites of low metamorphic grade. The Star Zinc and Excelsior prospects (Lusaka area), discovered in the early 1920s, occur in the metamorphic lithotypes of the late Proterozoic Zambezi Supracrustal sequence, which were deposited in a transtensional basin formed during the oblique collision of the Kalahari and Congo cratons. The deposits are hosted by the limestone and dolomitic marbles of the Cheta and Lusaka Formations. Structural analysis indicates that several fracture sets host the deposits, which may be genetically related to the Pan-African Mwembeshi dislocation zone (a major geotectonic boundary between the Lufilian Arc and the Zambezi Belt). In both prospects, willemite replaces the marbles and is found along joints and fissures with open-space filling textures and locally may develop colloform and vuggy fabrics as well. Silver as well as traces of germanium and cadmium have been detected within the willemite ore, and lead or zinc sulphides are scarce or absent. Calcite locally replaces willemite. Willemite is associated with specular hematite and franklinite and post-dates the Zn-spinel gahnite in the paragenesis. Genthelvite [Zn(4)Be(3)(SiO(4))(3)S] occurs as a minor phase in irregular aggregates. The willemites from the Lusaka area, though Mn-poor, show green cathodoluminescence colours and bright green fluorescence in short-wave UV (as the high-temperature willemites in USA). Thermometric analyses of primary fluid inclusions in willemite yield homogenization temperatures that range from 160A degrees C to 240A degrees C and salinities of 8-16 wt.% equiv. NaCl. The homogenization temperatures suggest a hypogene-hydrothermal origin for the willemite concentrations. The geochemistry of fluid inclusion leachates suggests that the hydrothermal fluids were brines derived from highly evaporated seawater. Precise age constraints are currently lacking for the Lusaka area deposits, though the deposits are not deformed, indicating that they post-date the Lufilian orogeny (similar to 520 Ma). The possibility of precursor ores exists; the gahnite-franklinite-willemite deposits could have been derived from a metamorphosed primary sulphide (or even nonsulphide) concentration that has subsequently been completely destroyed. However, there is no real evidence of such a primary source for the willemite mineral association. The Lusaka zinc ores may have been produced by an extensive hydrothermal system, with fluids discharging along basinal fracture zones controlled by the pre-Pan-African rifting stage. A paragenesis similar to that of the Lusaka prospects has been proposed to be a vector towards massive sulphide ores in several parts of the world; therefore, it is possible that these small willemite showings in Zambia may be part of a much bigger, and still unexplored, zinc province

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

“…Considering that the present data confirm that W1 at Kabwe is of hypogene origin, we agree with the latter authors on a possible post-'Lufilian' age for this phase. Boni et al (2011) proposed the same age for another Zambian willemite mineralization, located at Star Zinc (willemite I; Terracciano, 2008;Boni et al, 2011), near Lusaka. The age of W2 is more difficult to constrain, but considering the 'meteoric' signature of the precipitating fluids, it probably formed when the sulfide deposit was already exhumed, possibly in association with, or after, major Cretaceous uplifts (De Putter et al, 2015 and references therein).…”

Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin

“…Willemite is traditionally considered of hydrothermal origin (Hitzmann et al, 2003) and mostly forms at high temperatures (Roy andMumpton, 1956, Ingwersen, 1990;Brugger et al, 2003). Remarkable paradigms of such ore deposits are Franklin in USA (Frondel and Baum, 1974), Beltana in Australia (Grove et al, 2003), and Berg Aukas in Namibia (Cairncross, 1997;Boni et al, 2011b). However, the Bou Arhous ore deposit bears structural and paragenetic characteristics (e.g.…”

The Proterozoic Vazante Hypogene Zinc Silicate District, Minas Gerais, Brazil: A Review of the Ore System Applied to Mineral Exploration

“…The Vazante zinc silicate deposits share many similarities with other willemite deposits [2,[42][43][44][45][46], as summarized in Table 1 of Slezak et al [8], suggesting that these deposits were formed by similar processes. The guidelines for exploration discussed below are based mainly on the attributes of the Vazante District, because it is the focus of this review and hosts the world largest The willemite ore formed in zones along major faults in the dolomite unit interbedded with siliciclastic rocks of the Serra do Poço Verde Formation during the Brasiliano orogenic event [5,6,8].…”