The mylonitic hanging wall and footwall contacts to the metacarbonate are represented by intensively hydrothermally altered biotite-actinolite schist of the Akjoujt Metabasalt unit. The biotite-actinolite schist shows increasing development of the S2 foliation and alteration towards the metacarbonate, forming a ca. 40 m wide biotite-grunerite-chlorite-calcite alteration halo peripheral to the metacarbonate (Figure 5.1). Although mineralogically similar to the biotite-actinolite schist, this zone contains a pronounced fine-grained, lenticular-banded structure and increased proportion in phyllosilicates and carbonate.
Biotite, grunerite, chlorite and calcite are the main alteration minerals and together with quartz make up more than 90 wt.% of the rock mineralogy. Magnetite, ilmenite, apatite, plagioclase and primary hornblende may occur in appreciable amounts. These are all stable along the S2 foliation, which is mainly defined by very thin (20-100 m), alternating biotite-chlorite and quartz layers (Figure 5.7a-d). Chlorite forms fine (20- 50 m), dark green and strongly pleochroic grains. It is intergrown with biotite and grunerite and in places seems to replace primary hornblende. Based on the classification of
64 Hey (1954), chlorite is ripidolite in composition with XFe varying between 0.51 and 0.58 (Figure 5.6a; Table 12.20). Biotite flakes are very fine (10-50 m), strongly pleochroic and replaces primary hornblende. The composition of biotite shows a wide range of XFe from 0.53 to 0.73 (Table 12.17). According to the biotite classification diagrams of Rieder et al. (1998) it plots in the fields of siderophyllite and annite (Figure 5.6b). Grunerite dominates in the most altered portions of the zone forming small pods, discontinuous lenses, to banded layers (up to 3 cm thick) that intercalate the foliation of the biotite and chlorite (Figure 5.7a,b). Toward the mineralized metacarbonate, the grunerite occurrences in the rock appear to increase rapidly in frequency and thickness, usually within 15 m laterally, and host locally abundant sulfides (Figure 5.7c,d). In grunerite the XMg averages at 0.45 whereas the Al2O3 content is relative high (up to 1.0 wt.%), (n=25; Table 12.23). Calcite accompanies the alteration irregularly occurring either as small veinlets within the biotite- chlorite matrix or filling spaces between grunerite blades. Quartz forms very fine-grained (10-30 mm), equigranular matrix in the layers. Magnetite and ilmenite form disseminated, idiomorphic to irregular grains associated mainly with biotite and chlorite. Relicts of primary amphibole and plagioclase are scarce and, where present, are granulated forming rounded or flattened out grains (Figure 5.2b). The amphibole has a ferro/an pargasite composition as in amphibolite (n=39; Table 12.14) and plagioclase is of albitic composition with anorthite content below 7.5 (n=48; Table 12.11). Apatite occurs as small grains in the biotite-chlorite matrix and has the nominal Ca5(PO4)3(OH,F,Cl) composition; it can be distinguished from apatite occurring in the metacarbonate by the higher F contents (2.92-4.22 wt.%; n=9; Table 12.26).
Figure 5.6 (a) The chlorite composition from the biotite-chlorite-grunerite-calcite alteration plots in the field of ripidolite (n=68; classification of Hey 1954), and (b) the biotite composition plots in the field of annite (n=52; classification of Rieder et al. 1998).
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Figure 5.7 (previous page) Photographs of drill core hand specimens and thin sections showing the (a) biotite-chlorite-grunerite-calcite paragenesis, (b) detail of the the biotite- chlorite-grunerite-calcite alteration zone showing the thin biotite, chlorite, grunerite and quartz banding that wraps around relicts of pargasitic amphiboles, (c) grunerite veinlet accompanied by sulfides from the biotite-chlorite-grunerite-calcite alteration zone proximal to the metacarbonate, (e) Fe-Mg clinoamphibole-chlorite phyllonite from the ore breccia accompanied by abundant sulfides, (f) detail of the very thin chlorite and grunerite banding and the enclosed idiomorphic magnetite and apatite grains in the Fe-Mg clinoamphibole- chlorite phyllonite, (g) brecciated siderite of the metacarbonate within a matrix of a Fe-Mg clinoamphibole, magnetite, graphite and arsenide-sulfide-gold assemblage, and (h) detail of the breccia matrix composed of massive pyrrhotite, chalcopyrite and abundant idiomorphic arsenopyrite grains.
The texture of the biotite-grunerite-chlorite-calcite alteration zone is ultramylonitic defined by small clasts of relict amphibole and plagioclase (< 10% modal proportion) enveloped by the closely-spaced foliation of the chlorite and biotite. Quartz forms ribbons made up of fine, recrystallized grains that have wavy/serrated boundaries and display undulatory extinction (Figure 5.7b-d). The direct replacement of the primary mineralogy (hornblende-plagioclase) by biotite, chlorite, and grunerite along S2 foliation and S2-C' planes indicate that hydrothermal alteration in the Akjoujt Metabasalt unit was contemporaneous with the development of the D2 mylonitic zones in the amphibolite and consequently the breccia zones in the metacarbonate. Where present, the sulfides exhibit primary, undeformed textures (i.e. idiomorphic porphyroblasts) attesting to their syntectonic emplacement.
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5.5 Summary
Shearing during the northeastward (D2) deformational event was partitioned into ductile deformation in the Akjoujt Metabasalt unit and brittle deformation of the metacarbonate. Hydrothermal alteration and Fe oxide-copper-gold-cobalt mineralization in the metacarbo- nate during D2were pervasive, extending into the footwall and hanging wall units.
In the metacarbonate the D2 shear zones have a characteristic geometry of a central, ductile Fe-Mg clinoamphibole-chlorite phyllonite surrounded by a progressively developed breccia of brittle deformed siderite. The Fe-Mg clinoamphibole-chlorite phyllonite comprises of iron-rich chlorite, grunerite, magnetite and accessory apatite, ilmenite and monazite. The phyllonite forms small, puzzle-like breccia in a matrix of massive sulfides and grunerite. In the metacarbonate and toward the massive ore bodies increasingly permeable crackle breccia of siderite give way to breccia with pebble-like siderite fragments and low clast/matrix ratio. The siderite is extensively altered and records an increase in Fe, Zr, Y, and rare earth elements as well depletion in Rb, Sr, and Ba comparatively to least altered metacarbonate. Magnetite, Fe-Mg clinoamphiboles, graphite, apatite, REE-phosphates and an arsenide-sulfide-gold assemblage replaces siderite, along cleavage cracks, grain boundaries and fissures. Magnetite is Ti- and V-poor, and the Fe-Mg clinoamphiboles comprise of composite grunerite-cummingtonite grains. The quartz-free Fe-Mg clinoamphibole-magnetite-graphite paragenetic assemblage is mainly the result of the reaction of the siderite with Si-undersaturated hydrothermal fluids, whereas the abundance of graphite is explained by subsequent decarbonation of siderite during metamorphism.
Peripheral to the metacarbonate in the Akjoujt Metabasalt unit a ca. 40 m wide biotite- chlorite-grunerite-calcite alteration halo developed contemporaneously to the brecciation and hydrothermal mineralization in the metacarbonate. The bulk of the alteration assemblage occurs in the dynamically recrystallized feldspar matrix. These minerals form pseudomorphs after hornblende, replacing the original minerals parallel to the S2 foliation. The degree of alteration increases toward the ore bodies in the metacarbonate; the modal proportion of phyllosilicates and grunerite increases substantially, and proximal to the contact sulfide veinlets along the foliation planes and shear bands become abundant. The sulfides exhibit primary, undeformed textures, attesting to their syn- to early post-tectonic emplacement.
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6 ORE MINERALOGY
The ore at the Guelb Moghrein deposit is a polymetallic assemblage featuring a wide mineralogical variety. In addition to the principal metals copper, gold and cobalt, several elements such as Ni, Bi, Ag, Te, As, Y, REE, Th etc. are subordinate enriched and incorporate in the mineralogy of the ore bodies. Sulfides, sulfarsenides and arsenides are found in almost all samples in decreasing order of abundance and contain the bulk of the ore, whereas Bi-Au-Ag tellurides are relative scarce. In this section, an analytical description of the petrographic features, chemical composition, mineralogy, and phase relations is given for all documented ore minerals. The investigation is based on drill core samples that have been selected from depths underneath the oxidation zone of the deposit; in such, the samples lack of supergene alteration and thus, secondary oxide mineralogy.
6.1 Sulfides
The predominant sulfide mineralogy in the breccia is fairly simple, pyrrhotite and chalcopyrite being the most abundant sulfide species in all of the mineralized environments in the breccia. The modal amount of pyrrhotite and chalcopyrite in massive ore zones typically varies from 50 to 85 percent, and in some samples up to about 95 percent. They occur approximately in equal proportions, typically forming massive aggregates cementing the breccia. Cubanite, mackinawite and pentlandite occur commonly as exsolution lamellae in either chalcopyrite or pyrrhotite, but are volumetrically minor.
6.1.1 Pyrrhotite
Pyrrhotite typically occurs in massive aggregates forming a grayish-brownish matrix of mosaic grain fabric. It forms sub-idiomorphic to xenomorphic, optically continuous twinned grains with average size that varies from 0.05 to 0.2 mm. Its major occurrence is massive and granular however, some ore veins are often rimmed with a zone of oriented, outward- radiating, strongly elongate pyrrhotite grains. It is also found as roundish to oval inclusions in chalcopyrite or overgrowing arsenopyrite in xenomorphic patches. Locally pyrrhotite is sheared along the (0001) basal planes. Pyrrhotite is strongly pleochroic ranging in color from red to creamy brown. It is mostly inhomogeneous in composition typified by the abundance of lamellae (30-50 m) of a second pyrrhotite phase which has exsolved along two structural dimensions (Figure 6.1a).
Electron microprobe analyses (EMP) have shown the lamellae-hosting pyrrhotite to be hexagonal pyrrhotite (Fe11S12) and the lamellae to be troilite (FeS). In some troilite-bearing samples the ratio hexagonal pyrrhotite/troilite exceeds 0.6. Monoclinic pyrrhotite (Fe7S8) has also been identified but is volumetrically less; it occurs in samples in which the sulfides
69 are deformed, and devoid of troilite lamellae. The distinction of the three pyrrhotite types is based on the compositional ranges after Desborough and Carpenter (1965). Both hexagonal pyrrhotite and troilite are iron-rich varieties exhibiting only small variations in iron content; in the hexagonal pyrrhotite Fe ranges from 47.3 to 47.7 at.% (n=14; Table 12.31) and in troilite from 49.5 to 49.9 at.% (n=18; Table 12.31). The monoclinic pyrrhotite is sulfur-rich type and shows a fairly wide compositional variation in Fe from 45.7 to 46.8 at.% (n=39; Table 12.31). Trace elements detected in pyrrhotite include Co, Ni, Cu, Ag and Au of which Co and Ni are most frequent and occur at significant concentrations. Co presents a constant concentration in all pyrrhotite types averaging at 0.06 at.%. Ni shows maximum average content in hexagonal pyrrhotite (0.15 at.%), lower in monoclinic pyrrhotite (0.05 at.%) and is totally absent in troilite. The Au and Ag contents are constantly below the detection limits.
Figure 6.1 Backscattered electron images of (a) troilite exsolution lamellae (light gray) in hexagonal pyrrhotite (dark grey) developed along two directions, and (b) granular and bleb-like pentlandite exsolved along a fracture in troilite-bearing, hexagonal pyrrhotite. 6.1.2 Pentlandite
Pentlandite is associated with troilite and is characterized by exsolution textures (Figure 6.1). It occurs in a wide variety of forms including flame- or bleb-like exsolutions (10-50 m in length) and euhedral, granular exsolved grains (40-150 m). The granular pentlandite is usually found at the triple junctions between the pyrrhotite grains. The flame and bleb types are located along grain boundaries, partings and fractures in pyrrhotite, radiating inward and being pseudo-orthogonal to the fracture boundaries (Figure 6.1b). More rarely pentlandite emanates into the chalcopyrite from the grain boundaries of the coexisting pyrrhotite. Pentlandite is relative rich in Co with average value at 2.9 at.%. The Ni and Fe amounts are relative equal averaging at 24.5 and 25.5 at.%, respectively (n=2; Table 12.33).
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6.1.3 Chalcopyrite
Chalcopyrite is the principal Cu-bearing phase and together with cubanite makes up the bulk of the copper ore. It is as much abundant as pyrrhotite forming massive, brassy golden aggregates interstitial, or disseminated inside pyrrhotite. It forms xenomorphic grains with average size from 100-250 m. The composition of chalcopyrite deduced from EMP analyses appears to be constant and nearly stoichiometric with Cu, Fe and S average contents at 25.1, 25.3 and 49.5 at.%, respectively (n=11; Table 12.32). The total proportion of Ni and Co does not exceed 0.2 at.%. Only two analyses on chalcopyrite grains showed Au contents above the detection limit (0.01 wt.%); one at 0.05 wt.% and the other at 0.1 wt.%.
6.1.4 Cubanite
Cubanite is commonly observed as exsolution lamellae in chalcopyrite particularly where troilite dominates in the samples. In some specimens cubanite dominates proportionally over chalcopyrite with only some chalcopyrite remaining at grain rims. The cubanite lamellae consist of relatively broad, parallel-sided units traversing the entire width of the chalcopyrite host. Commonly, the lamellae of cubanite make intersecting sets at ca. 90o to one another (Figure 6.2a). The average thickness of individual lamella is about 100 μm and maximum length being about 1 mm. The mineral exhibits a weak reflections pleochroism from very light grey-brown to cream-white, and has bright blue, to dark blue and green interference colors, being tetragonal. Very commonly the lamellae have a well-developed twinning that is readily apparent with crossed-nicols.
Figure 6.2 Reflected-light photomicrographs (crossed-polarizers) of (a) cubanite lamellae with twinning developed in two, almost perpendicular directions, and (b) skeletal and brush-like mackinawite associated with cubanite, both exsolving from chalcopyrite.
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6.1.5 Mackinawite
Some exsolution lamellae of cubanite are endowed with minute skeletal-shaped exsolution bodies of mackinawite. It forms thin (10-20 m), nearly pure white and weak pleochroic exsolution lamellae. It is highly anisotropic exhibiting bright blue, pinkish to brownish-white and reddish-brown colors, reflecting the high Co and Ni proportion in the chemical composition. A particular occurrence of mackinawite is exsolution lamellae of several microns wide, lying transverse to the elongation of the exsolution lamellae of cubanite in chalcopyrite (Figure 6.2b). This implies that chalcopyrite has exsolved cubanite on cooling and the cubanite has further exsolved mackinawite. Evans et al. (1964) proposed the name mackinawite for a relatively-rare tetragonal mineral of near-FeS composition; however in the mackinawite described here Ni, Co and Cu are always present in the composition. Fe and Ni show relative constant concentrations averaging at 45.0 and 4.6 at.%, respectively, whereas Co and Cu vary between 0.3-0.7 at.% and 0.1-0.2 at.%, respectively (n=3; Table 12.33).