Geology and wall rock alteration at the Hercynian Draa Sfar Zn–Pb–Cu massive sulphide deposit, Morocco

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Abstract

Draa Sfar is a siliciclastic–felsic, volcanogenic massive sulphide (VMS) Zn–Pb–Cu deposit located 15 km north of Marrakesh within the Jebilet massif of the western Moroccan Meseta. The Draa Sfar deposit occurs within the Sarhlef series, a volcano-sedimentary succession that hosts other massive sulphide deposits (e.g., Hajar, Kettara) within the dominantly siliciclastic sedimentary succession of the lower Central Jebilet. At Draa Sfar, the footwall lithofacies are dominated by grey to black argillite, carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes, associated in situ and transported autoclastic deposits, and lesser dykes of aphanitic basalt and gabbro. Thin- to thick-bedded, black carbonaceous argillite, minor intercalated siltstone, and a large gabbro sill dominate the hanging wall lithofacies. The main lithologies strike NNE–SSW, parallel to a pronounced S1 foliation, and have a low-grade, chlorite–muscovite–quartz–albite–oligoclase metamorphic assemblage. The Draa Sfar deposit consists of two stratabound sulphide orebodies, Tazakourt to the south and Sidi M'Barek to the north. Both orebodies are hosted by argillite in the upper part of the lower volcano-sedimentary unit. The Tazakourt and Sidi M'Barek orebodies are highly deformed, sheet-like bodies of massive pyrrhotite (up to 95% pyrrhotite) with lesser sphalerite, galena, chalcopyrite, and pyrite. The Draa Sfar deposit formed within a restricted, sediment-starved, fault-controlled, anoxic, volcano-sedimentary rift basin. The deposit formed at and below the seafloor within anoxic, pelagic muds.

The argillaceous sedimentary rocks that surround the Draa Sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns. In the more proximal volcanic area to the south, the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the Tazakourt orebody. Chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of Fe and Mg and a loss of K and Na. In the volcanically distal and northern Sidi M'Barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite. A calcite–quartz–muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire Draa Sfar deposit. The sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow/domes within the footwall succession.

Introduction

The newly opened (2005) Draa Sfar massive sulphide deposit, located in the southeastern margin of the Hercynian Jebilet massif, is one of two principal Zn producers in Morocco (Fig. 1). The Draa Sfar deposit is a sheet-like, massive sulphide deposit hosted within the Sarhlef series, a terrigeneous, sediment-dominated, volcano-sedimentary succession consisting of carbonaceous argillite, argillite, siltstone and sandstone with subordinate volcanic and volcaniclastic rocks (Huvelin, 1977). The massive sulphide deposit occurs near the top of an argillite–siltstone succession that conformably overlies a rhyodacite dome complex and associated volcaniclastic rocks and is conformably overlain by carbonaceous argillite. The Draa Sfar deposit is overturned, dips steeply to the east and faces west (Fig. 2). The massive sulphide deposit consists of two, highly deformed, sheet-like orebodies, Tazakourt and Sidi M'Barek, that combined have a total strike length of 1.5 km. Only Tazakourt has been developed and is currently in production. It has an average thickness of 20 m and an N–S strike length of 1 km (Fig. 2). Boudinage of the Tazakourt massive sulphide lens has resulted in several, en echelon, moderately north-plunging lenses.

In this paper we present the results from mapping, petrographic, mineralogical (X-ray diffraction), electron microprobe (EPMA) and geochemical studies of the sedimentary and volcanic lithofacies and alteration assemblages that host the Draa Sfar deposit. Our objectives are: (1) to provide the first reconstruction of the volcano-sedimentary environment in which the Draa Sfar deposit formed; (2) to describe the mineralogy, composition and distribution of hydrothermal alteration assemblages, particularly those that developed within the sedimentary lithofacies as they are only sparsely documented in the literature; and (3) to compare the pyrrhotite-rich Draa Sfar deposit to the more common and larger pyritic massive sulphide deposits of the Iberian Pyrite Belt, which share a similar age, sedimentary host rocks, and tectonic setting. This paper is complemented by a second contribution that focuses on the mineralogy, composition and origin of the sulphide ores at Draa Sfar (Marcoux et al., 2008).

Section snippets

Geological setting

The Draa Sfar mine is located along the southern margin of the Jebilet massif, in the southern part of the west Moroccan Meseta (Fig. 1a). The Jebilet massif, which extends for 170 km E–W and 40 km N–S, has been interpreted as a Devono-Carboniferous, intra-continental, rift basin (Huvelin, 1977, Beauchamp, 1984, Aarab and Beauchamp, 1987, Piqué and Michard, 1989, Beauchamp et al., 1991). The Jebilet massif consists of three lithotectonic domains, the western, central, and eastern domains (Fig. 1

Geology of the Draa Sfar deposit

The Central Jebilet succession has been subdivided into a lower volcano-sedimentary succession, the Sarhlef series and an upper sedimentary succession, the Teksim series (Huvelin, 1977, Bordonaro, 1983). The Draa Sfar deposit is located in the upper part of the Sarhlef series (Fig. 2, Fig. 3) and is underlain by more than 500 m of strata consisting of carbonaceous argillite, argillite, intercalated siltstone and a subordinate, but locally dominant coherent rhyodacite and associated

Geochemistry and hydrothermal alteration

One hundred and twenty-five drill core samples (7 cm in length) were collected from three representative drill holes, DS110, DS132, and DS125. These, plus fifteen surface samples were selected to provide coverage of the entire Draa Sfar deposit, its enclosing host rocks, and related hydrothermal alteration (Fig. 2). The drill core samples were powdered using an agate shatter box at the sample preparation laboratory, Marrakech—Cadi Ayyad University, Morocco.

Major elements (oxides), trace

Mineralogical characteristics and zonation

Alteration was examined using samples collected from outcrops and the three drill holes mentioned previously. Chemical, petrographic, microprobe, and X-ray diffractometry (XRD) analysis (Rietveld method) were performed on the samples (see Fig. 2; Supplementary Tables 1a,b and c). The principal minerals formed by hydrothermal alteration are chlorite and muscovite, with minor quartz and calcite. These minerals are also present in the least-altered volcanic and sedimentary rocks, but the

Discussion and conclusions

The dominance of more than 1 km of black, carbonaceous argillites within the footwall and hanging wall strata to the Draa Sfar deposit suggests that it formed within a restricted, sediment-starved, anoxic basin within the upper part of the Sarhlef Formation (e.g., Beauchamp et al., 1991). The argillites formed through background suspension sedimentation within the basin whereas the siltstones, which noticeably lack carbon, are distal turbidites that were derived from the basin margins. This

Acknowledgements

This research would not have been possible without the financial support of REMINEX. In particular we would like to thank Solange Brunet for her time, effort and guidance. Dr Andrew McDonald, MERC, Laurentian University, provided the excellent XRD analysis and modal analysis using the Rietveld method. Thorough and helpful reviews by Ore Geology Reviews reviewers Rodney Allen and Fernando Tornos and a subsequent review by Philips Thurston improved the manuscript. H.L Gibson gratefully

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