Sulfur and lead isotopic compositions of ore sulfides and mining economic potential of the High Atlas Mississippi Valley-type ore province, Morocco

https://doi.org/10.1016/j.gexplo.2021.106765Get rights and content

Highlights

  • The High Atlas MVT Pbsingle bondZn (Ba-Sr-Fe) deposits are hosted in the Jurassic rocks.

  • The ore deposits display regional sulfur isotope variations.

  • The sulfur derived from TSR and/or BSR of the Triassic-Jurassic sulfates.

  • The Paleozoic basement is the primary source of metals.

  • The center of the basin is particularly a potential target for ore exploration.

Abstract

The Moroccan High Atlas metallogenic province hosts numerous Pbsingle bondZn (Ba-Sr-Fe) ore deposits hosted in the Jurassic carbonates. The high Atlas basin was formed during the Triassic-Jurassic rifting and underwent a Cenozoic tectonic inversion during the Alpine orogeny. This work presents a study of Pb and S isotopes of ore sulfides (galena, sphalerite, pyrite) from the Central and Oriental High Atlas of Morocco (CHA, OHA). The investigated ore deposits are Mississippi Valley-type (MVT) ore deposits with a simple paragenesis consisting of galena and sphalerite with subordinate barite, celestine, and pyrite. These ore deposits belong to three geographic zones; the northern border zone (NZ), the southern border (SZ), the central zone also called the diapiric zone (CZ), and the western zone (WZ) in the High Atlas. Based on the sulfur isotopic data, we suggest the Triassic-Jurassic sulfates and/or coeval pore water sulfates as the source of sulfur. Sulfur isotope compositions (δ34S = −18.6 to 22.2‰) are regionally zoned with isotopically heavy δ34S (14.3‰) in the center of the basin, negative δ34S values at the northern border (−16.9‰) and western border (−9.1‰), and intermediate δ34S values (4.4‰) at the southern border. This δ34S zonation suggests different sulfate reduction mechanisms; a low-temperature bacterial sulfate reduction (BSR) that induced the negative δ34S values and a high-temperature thermochemical sulfate reduction (TSR) that yielded positive δ34S values. The intermediate sulfur isotopic composition reflects the mixing of the TSR- derived sulfur and the bacteriogenic sulfur. The western part of the High Atlas near the Precambrian-Paleozoic paleohigh is characteristically more radiogenic (206Pb/204Pb = 18.709 207Pb/204Pb = 15.658 and 208Pb/204Pb = 38.808) than the rest of the basin (206Pb/204Pb = 18.151 to 18.369, 207Pb/204Pb = 15.631 to 15.668 and 208Pb/204Pb = 38.433 to 38.555). The Paleozoic rocks are the primary source of lead and by inference other metals. The metallogenic zone CZ with heavy sulfur isotopic composition and abundant Triassic evaporites (diapiric zone) constitutes a potential zone for future ore exploration. The SZ is also a promising target for ore exploration since it hosts ore sulfides that contain the thermally reduced sulfur (TSR).

Introduction

The Northwest African Atlas extends over a large distance (>2000 km) from the Moroccan Atlantic Ocean to the Tunisian Mediterranean coast. It corresponds to an intra-continental basin filled with Mesozoic to Cenozoic sediments. The Moroccan High Atlas range contains several Pbsingle bondZn (Ba-Sr-Fe) mines and prospects (Ovtracht, 1978), the majority of which are Mississippi Valley-type ore deposits. These ore deposits belong to the Circum-Mediterranean Pbsingle bondZn province (Rouvier et al., 1985). Among the most productive ore deposits are the Touissit Bou Beker and EL Abded in the Moroccan-Algerian confines (Bouabdellah and Sangster, 2016), the Moroccan High Atlas, and the diapiric zones of Tunisia and Algeria. The Pb-Zn-(Ba-Sr-Fe) ore deposits of the High Atlas form an important mining province that consists of medium- to large- size orebodies and small uneconomic prospects (Ovtracht, 1978). Several zones in the High Atlas are not yet explored and investigated for potential ore deposits of economic interest. Hence, the present work addresses the mining economic potential of the High Atlas MVT ore province in Morocco using sulfur and lead isotopic composition of sulfides. Several studies have also used these isotopes for ore exploration (Gulson, 1986; Sangster, 1990; Leach et al., 2005; Leach et al., 2010). Assessing the mining potential of the many Jurassic carbonate-hosted-Pb-Zn (Ba-Sr-Fe) ore deposits of the Moroccan High Atlas (Fig. 1) raises the question of whether size differences reflect (i) different processes of sulfur reduction, (ii) source/availability of metals and sulfur, and/or (iii) the different processes of ore precipitation. The development of large-sized ore deposits of economic interest requires a large amount of sulfur and metals among other factors. The generation of sulfur through TSR and/or BSR depends on the availability of a great quantity of dissolved sulfates. Similarly, a potential metal reservoir is required to supply metals such as Pb, Zn, Fe, Ba, and Sr for the sulfide and sulfate precipitation. In that respect, the analysis of S and Pb isotopes of sulfides provides insight into possible sources of sulfate and metal reservoirs.

A limited number of the aforementioned ore deposits were thoroughly studied (Mouguina, 2004; Adil et al., 2004; Rddad and Bouhlel, 2016; Rddad et al., 2018; Rddad, 2019). Previous sulfur isotope data have been interpreted as a result of the thermochemical sulfate reduction (TSR) from the Triassic-Jurassic sulfates and/or coeval pore water sulfates (Bou Dahar, Rddad and Bouhlel, 2016; Bouabdellah and Sangster, 2016; Ali Ou Daoud, Rddad et al., 2018) or by a combination of both TSR and bacterial sulfate reduction (BSR) (Bou Dahar; Rddad and Bouhlel, 2016; Bouabdellah and Sangster, 2016). Based on the Pb isotope data, the Paleozoic basement was considered the main source of lead and by inference other metals (Rddad and Bouhlel, 2016; Rddad et al., 2018). Fluid inclusion studies reveal that the ore-bearing fluids are basinal brines with a salinity range (3–28 wt% NaCl equiv.) and a temperature range (40° - 180 °C), which fall within those of MVT ore deposits (Mouguina, 2004; Adil et al., 2004; Rddad and Bouhlel, 2016; Rddad et al., 2018). Besides these geochemical attributes, the High Atlas ore deposits are epigenetic and stratabound (ore-bearing faults and fractures are confined to Jurassic carbonates), which further confirm their integration into the MVT ore deposits class. The metalliferous basement-derived fluids ascended through the major faults toward the Jurassic carbonates where they mixed with the reduced sulfur and/or dissolved Triassic-Jurassic sulfates (Rddad and Bouhlel, 2016; Rddad et al., 2018; Rddad, 2019). This fluid-mixing process is proposed to be the primary mechanism for ore precipitation for Bou Dahar (Adil et al., 2004; Rddad and Bouhlel, 2016), Tazoult (Mouguina, 2004), Ali Ou Daoud (Mouguina, 2004; Rddad et al., 2018), and Tigrinine-Taabast (Rddad, 2019). The formation of the ores took place during the Eocene-Miocene time in relation to the Alpine orogeny (Pbsingle bondZn (Basingle bondSr) Bou Dahar ore deposits, Rddad and Bouhlel, 2016; Bouabdellah and Sangster, 2016, Pbsingle bondZn Tigrinine-Taabast ore deposits, Rddad, 2019) and the Late Jurassic-early Cretaceous in relation to the extensional tectonic activity (e.g., Ali Ou Daoud and Tazoult ore deposits, Mouguina, 2004, Ali Ou Daoud, Rddad et al., 2018). Although fluid inclusion and Pbsingle bondS isotopic studies were carried out on few ore deposits in the High Atlas (Mouguina, 2004; Adil et al., 2004; Rddad and Bouhlel, 2016; Rddad et al., 2018; Rddad, 2019), no comprehensive Pb and S isotope study were carried out regionally at different localities of the High Atlas basin. The purpose of this paper is twofold: (i) constraining the source of sulfur and metals through the analysis of S and Pb isotopes of sulfide ores from representative ore deposits and (ii) addressing the question of the determinant factor(s) responsible for the formation of medium- to large-size orebodies of economic importance in this metallogenic province.

Section snippets

Regional geologic background

The Atlas Mountains are subdivided into the NE-SW-trending Middle Atlas and the ENE- WSW-trending High Atlas (Fig. 1). The High Atlas is subdivided into the Western (occidental), Central, and Eastern (oriental) High Atlas (OCHA, CHA, and OHA, respectively). The term High Atlas, thereafter, refers to the Central and Oriental High Atlas, which are the main focus of this study. The High Atlas consists of Mesozoic to Cenozoic sedimentary rocks covering the Precambrian to Paleozoic basement which is

Ore deposit geology

The Moroccan High Atlas Pbsingle bondZn (Ba-Sr-Fe) metallogenic province is one of the most important provinces in North Africa. The ore deposits are hosted in the Jurassic carbonates and occur along the Jurassic ridges intruded by the Triassic evaporites (diapirs) and the Jurassic igneous intrusions (e.g., Tazoult, Fig. 2A), the southern border (e.g., Bou Dahar; Tirginine-Taabast), norther border (e.g., Aoudine, Sidi Belghite), and western border near the Paleozoic paleohigh (e.g., Tadagaste) (Fig. 1,

Analytical methods

For sulfur and lead isotope analyses, mineral separates were prepared under a binocular microscope by handpicking or by the use of a micro-drill. Sulfur isotope analyses were carried out on samples of sphalerite (n = 15), galena (n = 31), and pyrite (n = 4) from different ore styles at the Spectrometry facility of the Department of Engineering, University of Nevada. The sulfur isotope compositions were measured on a Eurovector elemental analyzer interfaced to a Micromass Isoprime stable isotope

Sulfur isotopes

The sulfur isotopic compositions of sphalerite, galena, and pyrite are reported in Table 2 and Fig. 6, Fig. 7. The studied ore deposits in the High Atlas basin show a large variation in δ34S values ranging from −18.6 to 22.2‰. The Aoudine and Sidi Belghite ore deposits, located in the northern part of the High Atlas basin (NZ), show negative δ34S values that range from −18.6 to −15.2‰ (avg. = −16.9‰). Conversely, the ore deposits that are located in the center of the High Atlas basin (CZ)

Source of sulfur

The sulfur isotopic compositions of sulfides in the High Atlas ore deposits are highly variable ranging from −18.6 to 22.2‰ (Fig. 7). Previously published data of sulfur isotopic compositions of barite and celestite in Bou Dahar (δ34S ~ 15–20‰) (Rddad and Bouhlel, 2016) fall within the range of the Mesozoic seawater sulfates (e.g., Triassic seawater: 11 to 20‰; Jurassic seawater: 14 to 18‰) (Claypool et al., 1980). These δ34S values of sulfates also fall within those of the Triassic evaporites

Concluding remarks

On the basis of the geologic study and the sulfur and lead isotopic data of sulfide ores from the ore deposits in different zones (NZ, WZ, SZ, CZ) in the central and oriental High Atlas, salient conclusions can be drawn.

  • 1.

    Sulfide ore samples display concrete sulfur isotopic composition zonation with δ34Savg. values of 14.3‰, −16.9‰, −9.1‰, and 4.4‰ in the CZ, NZ, WZ, and SZ, respectively. This zonation reflects the predominance of the isotopically heavy sulfur (TSR) and the bacteriogenic sulfur

Declaration of competing interest

The authors confirm that all the claims, statements, and conclusions are true and that there is not conflict of interest.

Acknowledgment

The first author would like to thank the Professional Staff Congress of the City University of New York (PSC/CUNY) for the partial funding of this project. We are also grateful to Dr. Francis Albarède for his insightful comments and suggestions that improve the quality of this manuscript. We would also like to express our gratitude to the associate editor and the Editor-in-Chief Dr. Stefano Albanese for handling this paper.

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