The Canadian Mineralogist Vol. 50, pp. 447-470 (2012) DOI : 10.3749/canmin.50.2.447
MINERAL COMPOSITIONS AND PHASE RELATIONS OF Ni–Co–Fe ARSENIDE ORES
FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
Fernando GerVILLa§
Departamento de Mineralogía y Petrología (UGR) and Instituto Andaluz de Ciencias de la Tierra (CSIC–UGR), Facultad de Ciencias, Avda. Fuentenueva s/n, E–18002 Granada, Spain
IsabeL FanLo, Vanessa CoLÁs and IGnaCIo sUbÍas
Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Cristalografía y Mineralogía, c) Pedro Cerbuna 12, E–50009 Zaragoza, Spain
abstraCt
In an attempt to contribute to a better knowledge of phase relations, chemical trends and complex depositional history of the Co–Ni arsenide ores at Bou Azzer district, Morocco, (and by extension to the natural arsenides and sulfarsenides of the Ni–Co–Fe–As–S system), forty samples from the Aghbar mine have been studied. The Aghbar mine, located in the western central area of the Bou Azzer inlier, displays noticeable mineralogical variations depending on the type of mineralization: illing veins or replacing the host serpentinite. Nickel ores of vein type, Co–Fe ores of serpentinite-hosted type and late Cu ores are the assemblages that we identiied. Although there is a good agreement with the general sequence established for the Bou Azzer district, meaningful differences in mineral chronology, composition and temperatures of formation are discussed on the basis of the experimental data. The crystallization sequence of Ni ores reveals a continuous increase in As fugacity up to the formation of Ni-rich skutterudite, with formation temperatures itting well the ield of solid solution experimentally established at 650°–625°C. In contrast, the Co–Fe ores seem to form at a somewhat lower temperature (from ~500° to 400°C), and especially at the lower part of this range, during the crystallization of arsenopyrite. These temperature differences between the two main ores of the deposit can be interpreted by assuming either different temperatures in the two ore-forming luids or considering differences in the paths followed by similarly hot mineralizing luids.
Keywords: Ni–Co arsenide ores, phase relations, vein ores, replacement textures, Bou Azzer district, Morocco.
§ E-mail address: gervilla@ugr.es IntrodUCtIon
Arsenides and sulfarsenides of the system Ni–Co– Fe–As–S show complex and incompletely established phase-relations, both in nature (Radcliffe & Berry
1968, Petruk et al. 1971, Oen et al. 1984, Gervilla & Rønsbo 1992, Gervilla et al. 1996, Hem et al. 2001, Fanlo et al. 2004, Gritsenko et al. 2004, Parviainen et al. 2008, among others) and in experiments (Clark 1960, Roseboom 1962, 1963, Yund 1962, Klemm 1965, Barton 1969, Maurel & Picot 1974, Kretschmar & Scott 1976, Hem & Makovicky 2004, Hem 2006). One
of the reasons for this poor knowledge is the scarcity of mineralogical studies of natural occurrences and deposits of these minerals. Some of them (Co–Fe and, to a lesser extent, Ni arsenides) constitute the main ore assemblages of many small vein-type Ni–Co–Ag– As–Bi deposits as well as of important cobalt deposits
like those located in the Cobalt–Gowganda district
in Canada and those from the Bou Azzer district in Morocco.
Our aim in this paper is to contribute to the knowl-edge of mineral assemblages, mineral compositions and phase relations of the Aghbar arsenide ores; we compare the extent of the solid-solution series, the phase relations and the conditions of formation of the natural arsenides and sulfarsenides with the experimental data in the system Ni–Co–Fe–As–S. This study was carried out on a set of samples donated by M. Leblanc, collected
during his Ph.D. studies.
baCkGroUnd InFormatIon
448 theCanadIanmIneraLoGIst
have contributed as much as 8% of the world’s cobalt
output in recent years (Anonymous 2004). At present,
Managem S.A. continues to mine cobalt ore. The irst
studies were conducted by geologists of the Ministry of Mines of Morocco and the “Techno Export Company” (1935 to 1971). Mineralogical, petrological and
struc-tural studies were carried out by Besson & Picot (1978)
and Leblanc and coworkers (Leblanc 1975, Leblanc
& Billaud 1982, Leblanc & Lbouabi 1988, Leblanc &
Fischer 1990). Later, En-Naciri (1995), En-Naciri et al.
(1997), Maacha et al. (1998), Lebedev et al. (1999), Essarraj et al. (2005), Dolansky (2007) and Ahmed et al. (2009) mainly focused their research on the
char-acterization of the hydrothermal luids responsible of
the precipitation of various Bou Azzer ores through
the study of luid inclusions and stable isotopes. The depositional sequences of the ore deposits tend to follow
a similar pattern, characterized by the early crystal-lization of diarsenides followed by triarsenides and
sulfarsenides (Besson & Picot 1978, En-Naciri 1995,
En-Naciri et al. 1997, Ahmed et al. 2009). However,
Dolansky (2007) disagreed with this interpretation, arguing that triarsenides formed earlier than diarsenides on the basis of textural relationships.
GeoLoGy
The Bou Azzer mining district occurs within the Moroccan Anti-Atlas belt, located on the northern edge of the Eburnian (2000 Ma) West African Craton (Fig.
1A). This belt is separated from the High Atlas and
the Mesetian domains to the north by the South Atlas
Fault (Gasquet et al. 2005). Several Precambrian inliers (Bas Drâa, Ifni, Kerdous, Akka, Igherm, Sirwa, Zenaga,
Bou Azzer, Saghro, and Ougnat) are distributed along the South Atlas Fault and the Central Anti-Atlas fault
(Gasquet et al. 2005). These inliers are remnants of
a Pan-African suture zone (685–580 Ma) represented by a dismembered ophiolite (697 ± 8 Ma: El Hadi et al. 2010), unconformably overlain by late Ediacaran to Cambrian rocks and then by mostly sedimentary
Paleozoic rocks.
As Leblanc (1981) and Saquaque et al. (1989, 1992)
pointed out, the Bou Azzer inlier is the structurally most
complex portion of the Anti-Atlas. A Paleoproterozoic
basement, including gneisses, amphibolites, and schists, is intruded by granites and overlain by Cryogenian
formations (Bou Azzer Group). The Cryogenian forma -tions consist of epicontinental sedimentary and volcani-clastic rocks as well as an ophiolite complex interpreted
as a slice of upper Proterozoic ocean crust obducted
onto the continental margin of the West African Craton. The end of subduction and the beginning of obduction
have been dated as 655–635 Ma (El Hadi et al. 2010).
The reactivation of the major Eburnean and
Pan-African faults during Variscan times promoted the
injection of felsic dykes at 470 and 400 Ma (Huch 1988)
followed by hydrothermal activity at 330 and 300 Ma (Leblanc 1975).
The timing of mineralization in the Bou Azzer mining district is still controversial; radiometric ages ranging between ca. 685 and 215 Ma have been
proposed by various authors. Thus, whereas Cheilletz
et al. (2002) and Levresse et al. (2004) considered that the silver ores of the world-class Imiter silver deposit formed at ca. 550 Ma, its gold mineralization was dated
at 301 ± 7 by Gasquet et al. (2005). Leblanc (1981)
proposed a multistage model of ore formation starting
with a late Pan-African (~685–580 Ma) event followed by remobilization during the Hercynian. Ledent (1960) determined an age of 240 ± 10 Ma (Pb–Pb) for synmin -eralization brannerite. This age is slightly younger than that reported by Dolansky (2007), who constrained the timing of mineralization on the basis of in situ dating of
brannerite included in skutterudite, from 383 ± 7 to 257 ± 8 Ma. In good agreement with these results, Oberthür et al. (2009) estimated the age of the Co–Ni–As–(Au)
mineralization of the Bou Azzer district as 310 ± 5 Ma
using carbonates and brannerite coexisting with molyb-denite. In contrast, En-Naciri et al. (1997) reported a
SIMS U–Pb age of 550 Ma for brannerite.
En-Naciri et al. (1997) suggested that the Bou Azzer deposits likely formed from percolating basinal
brines at <300°C. El Ghori et al. (2008) indicated that
basinal brines may have penetrated into the Neoprotero-zoic basement along major faults. According to these authors, metals, especially Ni, were probably provided from the serpentinization of olivine, and As and S were probably scavenged from metasediments bordering the ophiolite belt. In contrast, Ahmed et al. (2009) consid-ered the serpentinites the source of As.
the aGhbar deposIt
The Aghbar deposit is located in the western central area of the Bou Azzer inlier (Fig. 1A). It consists of
a 600 3 150 m mineralized body, 2 to 10 m thick, surrounding a diapir of serpentinite (Leblanc 1975,
Leblanc & Billaud 1982) almost buried under the
ignimbrites of the Ediacaran–Cambrian cover. The Aghbar structure is connected to the east with the main serpentinite massif and with the so-called Ambed
Formation crust (Fig. 1B; Leblanc & Billaud 1982).
According to these authors, the Ambed Formation is a silica–carbonate crust developed over weathered ophiolitic rocks.
The mineralized structure as a whole is a contact orebody with a concave-down form draped over the serpentinite massif (Fig. 1B). The Aghbar deposit has been commonly described as a complex shell, owing
to numerous subvertical lame-like bodies (Dolansky
2007). Much of the complex shell is poorly mineral-ized; however, a subvertical, lode-shaped orebody
serpentinite dome, was exploited to a depth of ~400 m
(Leblanc 1986). Nickel and cobalt ore minerals occur
in massive lenses and are disseminated in gangue minerals within veins. Mineralized veins commonly display crack-seal-style textures or contain breccias of ignimbrite fragments cemented by cobalt ores and gangue minerals.
sampLes and anaLytICaL methods
Dr. Leblanc provided all samples investigated in this study. They were collected at different depths in
the active Aghbar mine during his Ph.D. ieldwork. Forty of these samples were studied by relected-light
microscopy, X-ray diffraction using the powder method
(PXRD), back-scattered-electron (BSE) images and electron-probe microanalysis (EPMA). The XRD studies were carried out using a Philips PW1729 diffrac -tometer with a monochromatic CuKa radiation and
equipped with an X-ray-diffraction analysis program (Martin 2004). We made use of a JSM 6400 scanning electron microscope (SEM) at the Universidad de Zara -goza to obtain back-scattered electron (BSE) images.
Electron-probe micro-analyses (EPMA) of ore minerals
were performed with a CAMECA SX–50 instrument at the Universidad de Barcelona. The ore minerals were
analyzed for As, S, Fe, Co, Ni, Cu, Zn, Pb, Cd, Au,
Bi and Mo; we monitored the following peaks: AsLa, SKa, FeKa, CoKa, NiKa, CuKa, ZnKa, PbMa, BiLa, AuLa, MoLa and CdLa. Operating conditions included a beam diameter of 3 mm, an accelerating voltage of 20 kV and a beam current of 20 nA. The counting times
were 20 s on TAP/PET and 30 s on LiF crystals. The ZAF corrections were performed using the program supplied by CAMECA. Pyrite, GaAs, NiO, as well as pure Co, Cu, Au, Zn, Bi, Pb, Cd and Mo metal were
used as primary standards. Maximum, minimum, mean and representative results of point analyses are given in
Tables 1 to 8.
mIneraLoGy and textUres
The Aghbar deposit displays noticeable mineral-ogical variations depending on the type of
mineraliza-tion: illing veins or replacing the host serpentinite.
Dissolution and replacement of serpentinite account for the abundance of serpentine and chlorite inclusions in diarsenides and triarsenides (commonly following the growth planes of these arsenides), as well as by the presence of chromite grains total or partially included
in arsenides (Fig. 2). Grains of chromite are zoned
(owing to alteration) and cracked, and become partially dissolved where included in arsenides.
The mineral assemblages identified allow us to group them into three different ore types: Ni ores, Co–Fe ores and Cu ores. Furthermore, their textural relations provide evidence of a three-stage depositional history characterized by early crystallization of the
Ni ores (Ni-rich Ni–Co–Fe arsenides and minor, late
sulides) followed by the Co–Fe ores (Co–Fe arsenides
and Co–Ni–Fe sulfarsenides), and the late Cu ores (Cu
sulides and sulfosalts). The Ni ores invariably crystal-lize in veins, concentrated in a narrow band within the lode-shaped orebody (Leblanc 1975, Leblanc &
Billaud 1982). They mainly consist of nickeline (NiAs),
rammelsbergite (NiAs2), members of the rammels-bergite–safflorite solid-solution series (Ni,Co)As2,
members of the rammelsbergite – saflorite – löllingite
solid-solution series (Ni,Co,Fe)As2, members of the
löllingite–rammelsbergite solid-solution series (Fe,Ni)
As2, and Ni-rich skutterudite (Co,Ni,Fe)As3. Late
sulfarsenides and sulides ill cracks in the above arse -nide assemblage and include Co- and Fe-rich
gersdorf-ite (NiAsS), millergersdorf-ite (NiS), siegengersdorf-ite (Ni3S4) and minor
galena (PbS), sphalerite (ZnS) and greenockite (CdS).
The Co–Fe ores occur in veins associated with calcite and dolomite, but mainly replace the host serpentinite. They show a mineral assemblage made up of members of the cobaltite–gersdorffite solid-solution series
(Co,Ni)AsS, members of the löllingite–(clino)saflorite
solid-solution series (Fe,Co)As2, löllingite (FeAs2), skutterudite (Co,Fe,Ni)As3 and arsenopyrite (FeAsS). The coexistence of the monoclinic and orthorhombic polymorphs of CoAs2, although indistinguishable under
the ore microscope, has been veriied by PXRD. This is the reason why we use the term (clino)saflorite here -after, as En-Naciri (1995) did in descriptions of other deposits from Bou Azzer. The late Cu ores consist of bornite (Cu5FeS4), chalcopyrite (CuFeS2), tennantite (Cu,Ag)10(Fe,Zn)2As4S13, wittichenite (Cu3BiS3) and molybdenite (MoS2) with a quartz gangue.
Although the existence of extensive solid-solution in the system NiAs2–CoAs2–FeAs2 under equilib
-rium conditions (Yund 1962, Roseboom 1963, Hem & Makovicky 2004, Hem 2006) is well known,
the textural relations and mineral compositions of
the arsenide ores studied require naming separately
the portions of this system, simply for purposes of description. Thus, intermediate compositions of the rammelsbergite–safflorite subsystem were grouped under the rammelsbergite–safflorite solid-solution series, intermediate ternary compositions in the system CoAs2–NiAs2–FeAs2 were named as rammelsbergite –
saflorite – löllingite solid-solution series, members of the löllingite–rammelsbergite subsystem were grouped as löllingite–rammelsbergite solid-solution series, Ni-poor members of the löllingite–saflorite subsystem were named as löllingite–saflorite solid-solution series,
and only diarsenides containing Co and Ni but having a composition close to the FeAs2 apex of the system
were named löllingite.
The Ni ores
The crystallization sequence of arsenides in these
rammelsbergite, rammelsbergite – saflorite – löllingite solid solution, löllingite–rammelsbergite solid solution
and Ni-rich skutterudite (Fig. 3), revealing a continuous increase in arsenic fugacity concomitant with a decrease in Ni content in the ore-forming environment.
Nickeline has only been found in one sample as remnant inclusions in rammelsbergite. It shows a distinct (0001) parting emphasized by alteration. Minute inclusions of gold (Au) are found as fine, rounded and discrete grains included in nickeline and along the contact between nickeline and rammelsbergite (Fig. 3A).
Rammelsbergite occurs as massive aggregates occa-sionally in association with nickeline and gold, exhib-iting oscillatory zoning as a result of Co substitution for Ni (Fig. 3B). Massive rammelsbergite occurs as centi-metric masses and exhibits anisotropic colors in various shades of purple, blue and brown, with twinning in very pronounced lamellae or as inversion polysynthetic twins
oriented in two directions, which intersect almost at 90°
(Fig. 3C). These complex patterns of twinning suggest that at least part of the massive rammelsbergite forms by inversion of earlier krutovite (ideally NiAs2, which has a trimorphic relationship with rammelsbergite and pararammelsbergite). This conclusion notably contrast
with En-Naciri’s (1995) observations of the existence
of pararammelsbergite in other deposits of the Bou
Azzer district. However, X-ray-diffraction analyses
confirm the occurrence of krutovite in the Aghbar samples. In places, rammelsbergite appears as small pods or remnants of crystals, or aggregates of crystals, all of them partly replaced, overgrown by or intergrown
with rammelsbergite–saflorite or rammelsbergite – saflorite – löllingite, all of them enclosed in Ni-rich
skutterudite (Figs. 3D, 3E, 3F). These textures clearly
provide evidence of destabilization and re-equilibration
processes.
Crystals of rammelsbergite–saflorite solid solution occur intimately associated with those of rammelsber-gite and rammelsbergite – saflorite – löllingite solid
solution. Whereas the crystals of rammelsbergite–saflo -rite display a strong anisotropy in pale blue and brown
colors, those of rammelsbergite – saflorite – löllingite
show deeper blue to purple colors (Fig. 3F). These diarsenides occur as: (i) small lath-shaped crystals of
rammelsbergite–saflorite overgrowing the last remnants of minute crystals of rammelsbergite that have modiied
their original composition (as we will see in the next sections) and, in turn, are partially replaced or
over-grown by rammelsbergite – saflorite – löllingite (Fig.
3D), and (ii) irregular patches or small pods, overgrown
or partially replaced by rammelsbergite – saflorite – löllingite (Figs. 3E, 3F, 3D). Both types of assemblages
occur included in massive Ni-rich skutterudite. In places, all these diarsenides appear approximately as a mixture of patches or inhomogeneous masses (Fig. 3D),
suggesting non-equilibrium with surrounding luids.
The complex twinning showed by rammelsbergite led to differences in the path followed by the mineralizing
luids through this phase, which may account for the
irregular replacements of one phase by another, as is
observed in Figures 3D and 3G. Most contacts between FIG. 2. Microphotographs in relected light, showing grains of chromian spinel hosted in the Ni–Co–Fe mineralization at the
Aghbar mine. A) Spindle-shaped crystals of löllingite–(clino)saflorite (LS) replacing a crystal of chromian spinel (chr) and fragments of serpentinite (srp). B) Zoned crystals of chromite included in a skutterudite crystal (Sk), replacing the serpentinite.
FIG. 1. A) Geological map of the Anti-Atlas belt, southern
452 theCanadIanmIneraLoGIst
diarsenides and triarsenides suggest partial dissolution of the former and precipitation of the latter (Figs. 3E, 3F,
3G), although the local coprecipitation of members of the rammelsbergite – saflorite – löllingite solid-solution
series and Ni-rich skutterudite cannot be ruled out. Finally, crystals of the löllingite–rammelsbergite
solid-solution series appear as small pods or euhedral
crystals, illing fractures in rammelsbergite – saflorite – löllingite solid solution and partially replaced by Ni-rich
skutterudite. The anisotropy colors are strong blue and
light brown (Fig. 3H).
Nickel-rich skutterudite is the last and most abundant arsenide in the Ni ores. It occurs as millimetric masses and as euhedral, unzoned crystals, enclosing previously
formed diarsenides (Figs. 3E, 3F, 3G).
Late alteration and partial dissolution of
rammels-bergite led to the deposition of gersdorfite followed by the sulide assemblage cited above. Gersdorfite partially replaced rammelsbergite and illed veins in massive rammelsbergite (Fig. 3I) Sulides formed in association with quartz, illing cracks and fractures in
the arsenide ores (Fig. 3J).
The Co–Fe ores
The crystallization sequence of arsenides in Co–
Fe ores starts with the formation of members of the
cobaltite–gersdorfite solid solution followed by löllin
-gite–(clino)saflorite solid solution, löllingite, a new
generation of skutterudite and arsenopyrite (Fig. 3). Members of the cobaltite–gersdorfite solid-solution
series occur in two distinct textural positions in serpen-tinite-hosted ores: (i) aggregates of idiomorphic
crys-tals partially replaced by skutterudite II and löllingite (Fig. 3K), exhibiting perfect cleavage and commonly,
concentric or sector zoning characterized by variations in Ni and Co contents; (ii) idiomorphic crystals enclosed in, and selectively replaced by bornite (Fig. 3L). The latter sulfarsenide crystals display growth-bands due to chemical zoning from a Ni-rich core to a Co-rich rim. Most Ni-rich cores exhibit decomposition processes leading to a symplectite-type texture made up of
inter-grown bornite and cobaltite–gersdorfite (Fig. 3M).
Members of the löllingite–(clino)saflorite
solid-solution series only occur in mineralized serpentinite. These crystals show a characteristic spindle-like shape arranged in the typical star-like twins. They are also characterized by rhythmic compositional zoning with Fe-rich and Co-rich bands (Fig. 3N). They form
aggre-gates surrounded by skutterudite II, which also ills spaces among the crystals of löllingite–saflorite solid
solution.
The crystals of löllingite, unlike those of löllingite–
(clino)saflorite solid solution, are small and unzoned.
They occur as isolated grains in serpentinite or as
aggre-gates enclosing crystals of cobaltite–gersdorfite solid solution (Fig. 3K) or, most commonly, forming massive
aggregates associated with skutterudite II.
Skutterudite II forms idiomorphic single crystals or clusters of crystals disseminated in serpentinite. The main difference relative to Ni-rich skutterudite is the oscillatory zoning (Fig. 3O) caused by slight varia-tions in the S:As ratio. In some cases, this skutterudite
overgrows löllingite and löllingite–(clino)safflorite masses (Fig. 3K).
Arsenopyrite occurs as prismatic crystals over-growing most of the previously formed Co–Fe arsenides and sulfarsenides. Locally, it is intergrown with bornite.
The Cu ores
The Cu-rich sulide assemblage forms late in the depositional history of the Aghbar ores (Fig. 3P). Bornite (Bn) ills fractures and cracks and occasionally
replaces previous arsenides, mainly in serpentinite-hosted ores. Chalcopyrite (Cp) precipitates simultane-ously with bornite, whereas tennantite ills cracks in
the aforementioned minerals. Wittichenite (Wtc) ills
voids and fractures in bornite (Fig. 3L), chalcopyrite
and cobaltite–gersdorfite. Finally, molybdenite (Mo) occurs as small grey rosettes, included in serpentinite
or illing voids in skutterudite II.
On the basis of the above mineral textures and
asso-ciations, the paragenetic sequence of the Aghbar mine
is shown in Figure 4.
the ComposItIon oF mIneraLs In the nI ores
Nickeline
EPMA results (Table 1) of nickeline reveal an almost
stoichiometric composition, with only a negligible
content of Co (below 0.24 wt.%): Ni0.99–1.01As0.99–1.01.
Diarsenides
Rammelsbergite shows a compositional variability depending on its textural type. As cited above, this mineral occurs as large masses enclosing minor nick-eline and gold. Its chemical composition shows a very
limited substitution of Co for Ni (<5.77 wt.% Co;
Table 1, Fig. 5A) with compositions close to stoichi-ometry [(Ni0.80–1.00Co0–0.21)As1.84–2.01S0–0.16]. These data are similar to those obtained by En-Naciri (1995) and Ahmed et al. (2009) in arsenide ores from other deposits of the Bou Azzer district, but differ from those obtained by Dolanski (2007), who reported smaller chemical variations in samples from the Aghbar deposit.
The members of the rammelsbergite–saflorite solid-solution series contain up to 2.63 wt.% Fe, 15.82 wt.% Co and 6.47 wt.% S (Table 2, Fig. 5A). Their general
where Fe begins to enter in the structure (Figs. 5C, 5D). No correlation between anions and cations has been found. Sulfur contents are the highest of all diarsenides, slightly exceeding the range established experimentally
by Yund (1962). These data are similar to those obtained
by En-Naciri (1995), Dolansky (2007) and Ahmed et al.
(2009). Figure 3D and Table 2 show that
rammelsber-gite–saflorite crystals overgrow and replace crystals of
rammelsbergite, which show the highest Co contents,
suggesting re-equilibration with the mineralizing luids, whereas the rammelsbergite–saflorite crystals display
the lowest Fe contents. As replacement proceeds, the Fe contents slightly increase at a distance from the rammelsbergite.
Crystals of rammelsbergite – saflorite – löllingite
solid solution show the highest compositional varia-tion in metal content (Table 3). They follow a trend very different from the other solid solutions, centered along the Co-richer side of the ½Co + ½Ni = Fe trend (Fig. 5A). Notwithstanding, they display a narrow variability in As and S contents (Fig. 5B): (Ni0.06–0.48 Co0.10–0.54Fe0.10–0.83)As1.89–2.03S0–0.10. These diarsenides show a strong negative correlation between the NiAs2 and FeS2 [NiAs2 = –0.447(FeAs2) + 42.232; Fig. 5C], and between CoAs2 and Fe As2 [CoAs2 = –0.553(FeAs2) + 54.770; Fig. 5D], but no correlation between cations and anions. The extent of Ni + Co replacement by Fe also correlates with the modal abundance of skutteru-dite: the higher the amount of skutterudite in the sample, the higher the extent of Ni + Co substitution by Fe in
diarsenides. As can be seen in Figures 3E and 3G and
in results of point analyses (Table 3), there is a clear
sequence of mineral replacement: rammelsbergite is replaced by crystals of rammelsbergite–saflorite solid
solution, and the latter, in turn, by rammelsbergite –
saflorite – löllingite solid-solution crystals. Moreover,
as the replacement proceeds, an increase in Fe content occurs from center toward the edge of the masses in contact with Ni-rich skutterudite. This skutterudite shows a decrease in Fe content compared with that far from these remnants of diarsenides. As is shown in Figure 5A, most of these compositions were not reported previously in the Bou Azzer district.
The analyzed members of the löllingite–rammelsber -gite solid-solution series represent a restricted portion of the solid solution along the Fe–Ni join, with up to
12.17 wt.% Ni and 3.26 wt.% Co (Table 4, Fig. 5A):
(Ni0.12–0.43Co0.02–0.12Fe0.48–0.86)As1.84–2.01S0–0.14. There is a negative correlation between FeAs2 and NiAs2 [NiAs2 = –0.927(FeAs2) + 90.350; Fig. 5C] and, to a lesser extent, between FeAs2 and CoAs2 [CoAs2 = –0.073(FeAs2) + 9.654; Fig. 5D]. The sulfur content is
low (up to 2.31wt.%) and is negatively correlated with
Co and Ni. From core to rim of single crystals, there is a Ni increase and a Fe decrease. Neither En-Naciri (1995) nor Dolansky (2007) reported compositions similar to
those reported here. In contrast, Besson & Picot (1978) reported one composition of nickeliferous löllingite,
and Ahmed et al. (2009) presented similar results, but
showing quite different chemical trends (Fig. 5A).
Triarsenides
Nickel-rich skutterudite is characterized by a broad range of compositions (Table 5) with very high Ni and Fe contents: (Co0.28–0.75Ni0.17–0.43Fe0.06–0.40) As2.85–3.01S0–0.11. There is a substantial replacement of Co by Ni and Fe, and a limited replacement of As by S, with Ni and Fe positively correlated with As. The arrow
in Figure 6A shows the compositional trend exhibited
FIG. 3. Back-scattered electron images and microphotographs in relected light, showing representative textures of the Aghbar
456 theCanadIanmIneraLoGIst
TABLE 1. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF NICKELINE AND RAMMELSBERGITE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO ___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Nickeline (n = 4)
min 0.00 55.13 0.01 0.12 43.47 99.55 0.00 0.99 0.00 0.00 0.99 1.00
max 0.02 56.44 0.04 0.24 44.68 100.33 0.00 1.01 0.00 0.00 1.01 1.00
mean 0.01 55.81 0.03 0.17 43.96 100.05 0.00 1.00 0.00 0.00 1.00 1.00
Rammelsbergite (n = 84)
min 0.00 67.41 0.00 0.19 22.49 98.55 0.00 1.84 0.00 0.00 0.80 0.92
max 2.72 72.67 0.77 5.77 28.18 101.63 0.16 2.01 0.02 0.21 1.00 1.00
mean 0.44 70.72 0.11 2.56 25.88 99.86 0.03 1.96 0.00 0.09 0.92 0.99
___________________________________________________________________________________
Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right. In this and the following tables, As# is equal to As/(As + S). In each table and in each category, the value of As# shown in line 1 is calculated using the minimum value of S and the maximum value of As, then in line 2, As# is calculated using the maximum value of S and the minimum value of As. The third line gives the median value of As#.
FIG. 4. Paragenetic sequence of the mineralization at the Aghbar mine.
by Ni-rich skuterrudite. The regression lines describing the composition of this skutterudite are: NiAs3 =
–0.287(CoAs3) + 42.969 and FeAs3 = –0.713(CoAs3)
+ 57.036 (Figs. 6B, C). The composition of Ni-rich
skuterrudite extends toward compositions richer in Ni and Fe than previously documented at Bou Azzer. This substitution is well supported by naturally occurring skutterudite compositions and also broadly describes the experimentally determined miscibility region of
triarsenides determined by Roseboom (1962), which
does not extend to either pure Ni or Fe end-members.
Co- and Fe-rich gersdorfite
Two chemically different types of Co- and Fe-rich gersdorffite have been identified in Ni ores,
and the highest As contents: (Ni0.70–0.82Co0.15–0.29 Fe0–0.02)As1.34–1.45S0.57–0.66. It does not contain signii
-cant Fe (<0.45 wt.%), but Co reaches up to 9.43 wt.%. Gersdorfite replacing rammelsbergite is overgrown by
euhedral gersdorfite (Fig. 3I) with a core rich in As and
a rim with a As:S ratio close to 1. It is relatively rich
in both Fe (up to 10.63 wt.%) and Co (11.87 wt.%):
(Ni0.54–0.58Co0.13–0.35Fe0.05–0.32)As1.07–1.20S0.82–0.94 (Fe– TABLE 2. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES
OF RAMMELSBERGITE–SAFFLORITE SOLID SOLUTION FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Rammelsbergite–safflorite (RS) solid solution (n = 52)
min 0.33 62.03 0.07 5.83 11.34 98.17 0.02 1.61 0.00 0.22 0.39 0.81 max 6.47 71.46 2.63 15.82 23.12 100.93 0.39 1.98 0.10 0.56 0.79 0.99 mean 1.63 68.87 1.03 11.01 16.96 99.70 0.10 1.89 0.04 0.38 0.59 0.95
RS 106 0.37 71.31 1.92 11.09 16.12 100.93 0.01 1.31 0.04 0.26 0.37 0.99 Ram 107 1.73 68.63 0.12 2.26 27.18 100.05 0.07 1.25 0.00 0.05 0.63 0.95 RS 108 0.48 70.49 1.67 13.87 13.07 99.82 0.01 1.31 0.04 0.33 0.31 0.99 RS 109 4.47 65.62 0.41 10.03 20.02 100.88 0.18 1.14 0.01 0.22 0.44 0.86 RS 110 2.14 67.73 0.35 6.55 23.12 99.98 0.09 1.22 0.01 0.15 0.53 0.93 RS 111 1.02 64.40 1.69 15.06 12.23 99.48 0.04 1.27 0.04 0.36 0.29 0.97 RS 112 0.35 70.30 2.63 11.97 14.45 99.97 0.01 1.30 0.07 0.28 0.34 0.99 Ram 113 1.35 69.03 0.11 2.31 26.78 99.88 0.05 1.26 0.00 0.05 0.63 0.96 Ram 114 1.93 68.72 0.29 5.66 23.56 100.25 0.08 1.23 0.01 0.13 0.54 0.94 Ram 115 2.13 68.03 0.31 5.37 23.65 99.58 0.09 1.23 0.01 0.12 0.54 0.93 Ram 116 0.92 70.26 0.13 2.64 26.19 100.14 0.04 1.29 0.00 0.05 0.62 0.97 ___________________________________________________________________________________
Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right. The location of some representative spots analyzed is shown in Figure 3D.
TABLE 3. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF RAMMELSBERGITE–SAFFLORITE–LÖLLINGITE SOLID SOLUTION
FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Rammelsbergite – safflorite – löllingite (RSL) solid solution (n = 83)
min 0.08 69.38 2.65 3.24 1.78 98.69 0.00 1.89 0.10 0.10 0.06 0.95 max 1.71 72.80 22.57 15.43 13.54 101.40 0.10 2.03 0.83 0.54 0.48 1.00 mean 0.53 70.98 9.83 10.09 8.37 99.99 0.03 1.96 0.36 0.35 0.30 0.98
RS 130 0.33 70.53 1.09 8.20 19.57 99.83 0.02 1.96 0.04 0.29 0.69 0.99 Rmb 131 0.45 70.87 0.22 4.97 23.26 99.79 0.02 1.98 0.00 0.17 0.83 0.99 RS 132 0.44 70.27 1.01 8.93 19.23 100.39 0.02 1.94 0.00 0.31 0.69 0.99 RSL 133 0.58 70.61 4.49 13.89 9.93 99.59 0.03 1.30 0.11 0.33 0.23 0.98 Ni-Sk 134 0.50 78.90 2.94 12.13 5.90 100.48 0.06 2.94 0.14 0.59 0.28 0.98 Ni-Sk 135 0.54 78.34 3.55 12.12 5.35 100.00 0.06 2.94 0.17 0.59 0.25 0.98 Ni-Sk 65 1.30 77.28 2.22 14.00 4.91 100.22 0.11 2.88 0.11 0.67 0.22 0.96 Ni-Sk 66 0.67 77.57 1.29 13.97 5.74 99.38 0.06 2.93 0.06 0.68 0.28 0.98 RSL 67 0.59 70.25 3.53 12.86 12.17 99.74 0.04 1.94 0.12 0.46 0.43 0.98 RSL 68 0.61 70.45 3.07 13.65 11.94 99.78 0.04 1.96 0.10 0.48 0.42 0.98 RS 69 0.53 70.91 2.33 14.09 12.00 100.04 0.04 1.97 0.08 0.50 0.41 0.98 RS 70 0.44 71.44 1.47 14.19 12.48 100.15 0.02 1.98 0.06 0.50 0.44 0.99 RS 71 0.61 70.26 0.85 11.54 16.10 99.63 0.04 1.94 0.04 0.41 0.56 0.98 Ni-Sk 93 0.86 77.51 1.81 13.41 5.78 99.61 0.08 2.90 0.08 0.65 0.28 0.97 RSL 94 0.62 70.37 5.36 12.02 10.97 99.55 0.04 1.94 0.21 0.41 0.39 0.98 RSL 95 0.64 71.17 5.66 11.95 10.69 100.42 0.04 1.97 0.21 0.41 0.37 0.98 RSL 96 0.70 71.13 3.36 15.43 9.74 100.42 0.04 1.95 0.12 0.53 0.35 0.98 ___________________________________________________________________________________
458 theCanadIanmIneraLoGIst
Co–Gdf in Fig. 7A). These two textural types of gers
-dorfite show signiicant positive correlation between
Ni and As (As/S = 4.497Ni – 1.235; Fig. 7B), and Fe
correlates negatively with As in euhedral gersdorfite.
The trend described by these sulfarsenides does not
it any of the trends established by Hem et al. (2001). A comparison with previously published data shows that Ahmed et al. (2009) only found Ni end-member compositions, and Dolansky (2007) reported composi-tional ranges narrower than those shown in this
contri-bution. En-Naciri (1995) did not report compositions of sulfarsenides from Bou Azzer.
Sulides
The structural formulae of sulides in Ni ores are:
millerite (Ni0.98–1.02Co0.01)S0.97–1.02, siegenite (Ni2.32–2.46 Co0.55–0.61)S3.96–4.03As0.03, sphalerite (Zn0.96–0.98 Ni0.02–0.04)S0.98–1.00, and greenockite (Cd0.89–0.93
Zn0.07–0.10Ni0.01–0.05)S0.99–1.00.
FIG. 5. A) Plot of compositions of diarsenides from Ni ore in the system CoAs2–NiAs2–FeAs2. The compositional ields
of diarsenides analyzed by others authors in material from various mines of the Bou Azzer district are also shown for comparison. B) Binary plot of Ni versus S in weight percent. C) Binary plot of NiAs2 versus FeAs2 of diarsenides from Ni
the ComposItIon oF mIneraLs In the Co–Fe ores
Gersdorfite–cobaltite
The members of the cobaltite–gersdorfite
solid-solution series occurring in the Co–Fe ores replacing serpentinite exhibit large compositional variations in the Fe-poor region of the system CoAsS–NiAsS–
FeAsS (Table 6; CG in Fig. 7A): (Co0.18–0.94Ni0.03–0.76 Fe0.02–0.17)As0.92–1.20S0.78–1.08. The Co contents tend to increase from core to rim of single crystals, whereas the As:S ratio decreases. The variation in the As:S ratio shows a weak, positive correlation with Ni (As/S =
0.556Ni + 0.960; Fig. 7B). This variability is consistent with the precipitation of cobaltite–gersdorfite crystals
in an environment with increasing sulfur fugacity. As cited above, comparable data have not been reported by Ahmed et al. (2009), Dolansky (2007) or En-Naciri (1995).
Diarsenides
Diarsenides in the Co–Fe ores are represented
by members of the löllingite–(clino)saflorite solid-solution series and löllingite. The composition of the
former tends to cluster along the Co–Fe join, with comparatively few examples containing some Ni (below
3.29 wt.%, Table 7, Fig. 8A): (Fe0.06–1.03Co0–0.90Ni0–0.12) As1.76–2.01S0–0.23. This igure also shows the reciprocal
substitution of Fe and Co. As shown in Figures 8B and 8C, there is a poor correlation between metals and anions; however, the Fe-rich members of the löllingite– (clino)saflorite solid-solution series (>0.5 apfu Fe), tend to be richer in S than the Co-rich members (<0.5
apfu Fe) (Fig. 8C).
The composition of löllingite is rather limited
(Table 7) clustering near the Fe apex of the Co–Ni–Fe
ternary field (Fig. 8A): (Fe0.67–1.03Co0–0.23Ni0–0.16) As1.82–2.01S0–0.18. Some crystals are somewhat richer in
Ni than those of the löllingite–saflorite solid solution,
but the S contents are lower. Both Ni and Co correlate negatively with As.
Triarsenides
Skutterudite II is characterized by high Fe and Ni contents (somewhat lower than Ni-rich skutterudite)
and relatively high S contents (Table 8): (Co0.56–0.97 Fe0.03–0.28Ni0–0.20)As2.73–3.01S0–0.22. Its compositional
trend (Fig. 6A) is similar to that exhibited by Ni-rich
skutterudite, that is, Ni and Fe substitute for Co (Ni/Fe trend). The regression lines describing the composition of skutterudite are NiAs3 = –0.437(CoAs3) + 42.321 and FeAs3 = –0.563(CoAs3) + 57.672 (Figs. 6B, 6C). Compositional variations among skutterudite crystals
TABLE 4. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF LÖLLINGITE–RAMMELSBERGITE SOLID SOLUTION
FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Löllingite–rammelsbergite solid solution (n = 28)
min 0.12 69.23 12.89 0.57 3.44 98.81 0.00 1.84 0.48 0.02 0.12 0.93 max 2.31 71.97 23.70 3.26 12.17 100.65 0.14 2.01 0.86 0.12 0.43 1.00 mean 0.82 70.81 19.09 1.34 7.65 99.88 0.05 1.93 0.70 0.05 0.27 0.97 ___________________________________________________________________________________
Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right.
TABLE 5. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF Ni-RICH SKUTTERUDITE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO ___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Ni-rich Skutterudite (n = 67)
min 0.09 76.36 1.12 6.13 3.74 98.12 0.00 2.85 0.06 0.28 0.17 0.96 max 1.42 79.53 7.85 15.69 8.93 101.22 0.11 3.01 0.40 0.75 0.43 1.00 mean 0.48 78.15 3.96 11.17 5.92 99.92 0.04 2.94 0.20 0.53 0.28 -0.98 ___________________________________________________________________________________
460 theCanadIanmIneraLoGIst
indicate that relative decreases in Co and S are accom-panied by relative increases in Ni, Fe and As, suggesting that paired substitution of Co for Ni + Fe, as well as S for As, is common. In fact, the S content is directly related to the compositional zoning observed in some crystals of skutterudite (Fig. 3O), as the dark bands
have the high S contents. This inding suggests that
metal substitution is determined by S and As fugacities.
Arsenopyrite
The arsenopyrite (Apy) composition is almost stoichiometric, with very few grains containing small
amounts of Co (Table 8) but neither nickel nor anti -mony: (Fe0.93–1.05Co0–0.07) As0.92–1.06S0.93–1.05.
the ComposItIon oF mIneraLs In the CU ores
As mentioned above, the Cu ores represent the latest episode in the crystallization history of Aghbar ores.
FIG. 6. A) Plot of Ni-rich skutterudite from
the Ni ore and skutterudite from the Co– Fe ore (Sk II), compositions (in mol.%) in the system FeAs3–CoAs3–NiAs3. The
arrows indicate the trend for each type of skutterudite, and the shaded area represent compositions previously documented from Bou Azzer. B) Binary plot of NiAs3 versus
CoAs3 of skutterudite from Ni and Co–Fe
ores. Regression lines for each phase are also shown. C) Binary plot of FeAs3 versus
FIG. 7. A) Composition of gersdorfite (As–Gdf and Fe–Co–Gdf) and cobaltite–gersdorfite solid-solution crystals (CG) in
the system NiAsS–CoAs–FeAsS. The arrows represent the Co and Fe trends. B) As/S relationship versus Ni (in atoms per formula unit). The regression lines have been drawn for both types of gersdorfite from the Ni ore, and cobaltite–gersdorfite solid solution from the Co–Fe ore.
TABLE 6. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF GERSDORFFITE AND GERSDORFFITE-COBALTITE SOLID SOLUTION
IN ORE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Fe–Co-rich G ersdorffite (n = 3)
min 15.04 47.66 1.73 4.44 18.72 100.13 0.82 1.07 0.05 0.13 0.54 0.53 max 18.03 51.76 10.63 11.87 19.69 100.66 0.94 1.20 0.32 0.35 0.58 0.59 mean 16.82 49.39 7.45 7.10 19.36 100.34 0.89 1.12 0.22 0.20 0.56 0.56
As-rich G ersdorffite (n = 12)
min 9.85 56.30 0.17 4.99 22.77 100.03 0.57 1.34 0.00 0.15 0.70 0.67 max 11.89 58.83 0.45 9.43 26.65 101.23 0.66 1.45 0.02 0.29 0.82 0.72 mean 10.75 57.61 0.26 6.50 25.32 100.66 0.61 1.40 0.01 0.20 0.78 0.70
G ersdorffite–cobaltite solid-solution (n = 86)
min 12.33 42.31 0.62 5.69 1.32 98.3 0.69 0.92 0.02 0.18 0.03 0.85
max 21.27 53.6 5.59 33.12 25.34 102.03 1.08 1.3 0.17 0.94 0.78 1.89 mean 18.46 46.48 2.13 25.45 7.46 100.4 0.96 1.04 0.06 0.72 0.22 1.09
23 18.03 47.66 10.63 4.98 18.72 100.22 0.94 1.07 0.32 0.13 0.54 0.53 24 17.40 48.74 9.98 4.44 19.69 100.66 0.91 1.09 0.30 0.13 0.57 0.55 25 11.89 56.30 0.34 9.43 22.77 101.05 0.66 1.34 0.02 0.29 0.70 0.67 26 10.87 57.91 0.33 7.52 24.29 101.23 0.61 1.39 0.02 0.23 0.74 0.69
27 9.94 58.83 0.25 6.14 25.64 100.99 0.57 1.45 0.00 0.18 0.80 0.72
28 10.34 57.41 0.20 6.69 25.09 100.03 0.59 1.42 0.00 0.20 0.79 0.71 29 10.13 58.23 0.21 5.66 26.23 100.70 0.58 1.42 0.00 0.18 0.82 0.71 30 9.85 57.79 0.17 6.85 25.12 100.16 0.57 1.42 0.00 0.22 0.79 0.71 31 10.14 57.54 0.21 6.80 25.27 100.25 0.59 1.41 0.00 0.22 0.79 0.71 32 15.04 51.76 1.73 11.87 19.66 100.13 0.82 1.20 0.05 0.35 0.58 0.59
33 0.01 71.24 0.05 0.89 27.26 99.68 0.00 1.99 0.00 0.04 0.97 1.00
34 0.12 71.35 0.00 1.19 27.07 99.77 0.00 1.99 0.00 0.04 0.97 1.00
35 0.03 70.49 0.00 1.26 27.30 99.45 0.00 1.99 0.00 0.04 0.97 1.00
___________________________________________________________________________________
462 theCanadIanmIneraLoGIst
Most sulides show limited substitution of metals and
nearly stoichiometric compositions. Bornite [(Cu4.83–4.90 Ni0–0.15Co0.05–0.20)Fe0.99–1.05S4.01–4.11] contains some
cobalt and nickel (up to 2.15 wt.% and 1.58 wt.%,
respectively) substituting for copper; chalcopyrite (CuFeS2) and wittichenite (Cu3BiS3) show stoichio-metric compositions; tennantite shows limited incorpo-ration of the tetrahedrite component but contains some Ni and Co [Cu9.97–9.98(Zn1.23–1.40Fe0.31–0.61Co0.15–0.45 Ni0–0.15)(As3.17–3.44Sb0.15–0.31)S13.00–13.30] and molybde-nite shows some substitution of Cu and minor Fe and Co for Mo: (Mo0.86–0.98Cu0.02–0.18Fe0.02–0.06Co0–0.02)S.
dIsCUssIon
The sequence of crystallization
Mineral textures and compositional trends exhibited by Ni–Co–Fe arsenide ores at Aghbar provide evidence of a two-stage depositional history. In fact, Leblanc
(1975) and Leblanc & Billaud (1982) reported that the
Ni ores concentrated only in a vein-like body indepen-dent but enclosed in the large lode-shaped, Co–Fe
arse-nide orebody. The irst stage (Ni ores) is characterized by the sequential crystallization of nickeline, rammels -bergite, rammelsbergite–safflorite solid solution,
FIG. 8. A) Plot of the compositions of diarsenides from the Co–Fe ore in the system CoAs2–NiAs2–FeAs2. The compositional
rammelsbergite – saflorite – löllingite solid solution, löllingite–rammelsbergite solid solution, Ni-rich skut
-terudite and Co- and Fe-rich gersdorfite, whereas the second one shows a sequence starting with gersdorfite– cobaltite solid solution followed by löllingite–(clino) saflorite, löllingite, skutterudite II and arsenopyrite. Both sequences resemble those established by En-Naciri
(1995), En-Naciri et al. (1997) and Ahmed et al. (2009) for the whole district, as in all deposits, triarsenides formed after diarsenides and were followed by sulfarse-nides. In contrast, they differ from those established by
Besson & Picot (1978), who also described a crystal
-lization sequence starting with Co-Fe-Ni diarsenides followed by triarsenides, sulfarsenides and sulides. However, textural relations in the samples studied here
do not show any evidence of crystallization of Co-Fe diarsenides prior to the appearance of Ni arsenides. On the other hand, Dolansky (2007) argued that skut-terudite is the earliest arsenide in all deposits studied, including Aghbar. The textural relations of skutterudite
shown by this author in samples from Aghbar provide evidence that Co–Fe diarsenides surround and include
partly corroded crystals of skutterudite. However, the
chemical composition of the latter resembles that of Ni-rich skutterudite reported here, suggesting that Dolansky (2007) studied samples from Aghbar where Co–Fe ores overlap with (and partly replace) the pre-existing Ni ores. The same interpretation could be made on the presence of nickeline inclusions in skutterudite,
reported by Besson & Picot (1978), but the absence of
chemical data on such skutterudite (hosting nickeline) prevents a proper interpretation. Textural relationships described by Dolansky (2007) support our interpreta-tion of the mineral assemblages observed in our study
on the basis of the sequential crystallization of Co–Fe
ores after Ni ores. Furthermore, the Ni- and As-rich sulfarsenides reported by Dolansky (2007) (named as MeAs1+xS1–y) as well as those analyzed by Ahmed et al.
(2009) (named NiAs2–xSx) showing an As:S ratio from 4.88 to 2.28 (approaching the maximum As content of TABLE 7. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES
OF LÖLLINGITE–SAFLORITE SOLID SOLUTION AND LÖLLINGITE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO
___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As#
___________________________________________________________________________________
Löllingite–safflorite solid solution (n = 167)
min 0.05 67.88 1.89 0.06 0.00 98.00 0.00 1.76 0.06 0.00 0.00 0.88 max 3.81 72.73 28.92 26.03 3.29 101.54 0.23 2.01 1.03 0.90 0.12 1.00 mean 0.64 71.11 19.78 7.82 0.42 99.85 0.04 1.95 0.72 0.27 0.01 0.98
Löllingite (n = 134)
min 0.07 68.33 18.59 0.03 0.00 98.34 0.00 1.82 0.67 0.00 0.00 0.91 max 2.80 73.70 28.26 6.71 4.63 101.74 0.18 2.01 1.03 0.23 0.16 1.00 mean 0.77 71.21 25.35 1.86 0.56 99.99 0.05 1.94 0.93 0.06 0.02 0.98 ___________________________________________________________________________________
Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right.
TABLE 8. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF SKUTTERUDITE II AND ARSENOPYRITE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO ___________________________________________________________________________________
S As Fe Co Ni Total S As Fe Co Ni As# ___________________________________________________________________________________
Skutterudite II (n = 190)
min 0.11 74.42 0.53 11.86 0.07 98.08 0.00 2.73 0.03 0.56 0.00 0.93 max 2.48 80.08 5.63 20.61 4.32 101.79 0.22 3.01 0.28 0.97 0.20 1.00 mean 0.66 78.02 3.05 16.14 1.95 100.11 0.06 2.92 0.15 0.77 0.09 0.98
Arsenopyrite (n = 63)
min 17.95 43.27 31.67 0.00 0.00 98.19 0.93 0.92 0.94 0.00 0.00 0.47 max 21.07 47.87 36.11 2.50 0.29 101.53 1.05 1.06 1.04 0.07 0.00 0.53 mean 19.56 45.82 34.04 0.28 0.05 99.96 1.00 1.00 1.00 0.01 0.00 0.50 ___________________________________________________________________________________
464 theCanadIanmIneraLoGIst
gersdorfite: Yund 1962) can be ascribed to the Co- and Fe-rich gersdorfite formed at the later stages of Ni
ores by replacement of previous rammelsbergite. These authors clearly showed such textural relationships. The Fe-rich members of MeAs1+xS1–y have not been identi-ied in our study.
The crystallization sequence of Ni ores reveals a
continuous increase in As fugacity, up to the formation of Ni-rich skutterudite. This trend is followed by the
precipitation of Co- and Fe-rich gersdorfite and, later, by the crystallization of sulides under increasing f(S2), at the end of the stage. Rammelsbergite and rammels-bergite–safflorite solid solution exhibit the same Co-enrichment trend, characterized by Ni replacement
by Co, until rammelsbergite–saflorite crystals reach
Ni:Co = 1; then the trend changes and Ni becomes
replaced by Co and Fe (Fig. 5A). According to Putnis &
Mezger (2004), the replacement reaction between solid rammelsbergite and a solution containing increasing
amounts of Co must take place at near-equilibrium
conditions throughout the reaction. Therefore, there
is a continuous re-equilibration between the luid and
the precipitating phases. Thus, rammelsbergite has to
re-equilibrate with respect to the Co-rich luids. In
view of the continuous mass-transport between the
reaction interface and the luids, a compositional and
irregular zoning is produced. As the Co-rich fluid penetrated throughout the rammelsbergite crystals and the chemical constituents were transported to the reaction site, the crystals tend to reach the composition Co:Ni = 1:1. Further changes in the composition of the
luid, becoming rich in Fe, lead to a modiication of the composition of diarsenides by the solid–luid reaction described, toward the löllingite corner of the system. If so, rammelsbergite – saflorite – löllingite crystals begin to precipitate at the same time as rammelsbergite–saflo -rite crystals are dissolved.
Textural relations suggest that extensive crystalliza-tion of Ni-rich skutterudite also involved partial dissolu-tion of the previously formed diarsenides. During such a dissolution reaction, Co and, to a lesser extent, Ni could be concentrated in Ni-rich skutterudite, whereas Fe entered or remained in diarsenides, at least along the contact between diarsenides and triarsenides. This hypothesis is also supported by the fact that crystals of
löllingite–rammelsbergite solid solution only occur in
close contact with Ni-rich skutterudite having a high Co/(Ni + Fe) ratio.
The few sulfarsenide crystals identiied in Ni ores
replace rammelsbergite, crystallize from late fluids in microcracks, or overgrow rammelsbergite partially
replaced by the irst generation of gersdorfite. In the
former, the Co:Ni ratio mimics that of the replaced rammelsbergite, suggesting that the metal content of
gersdorfite is inherited from the previous rammels
-bergite. Those crystals illing microcracks show the
same composition. This is in agreement with the low S
content of gersdorfite, relecting low f(S2) at the early
stages of formation of sulfarsenides. Later, during the
crystallization of euhedral gersdorfite overgrowing gersdorfite that replaces rammelsbergite, the chemical
trend shows a decrease in As coupled with an increase
in S and Fe, relecting the compositional evolution of the luid. The latest luids in this event gave rise to the formation of sulides.
Phase relations
Although no precise geothermometers are available to calculate the crystallization temperature of these
arsenide ores, the experimental results by Klemm (1965) and Hem & Makovicky (2004), as well as the compilation made by Hem (2006), allow us to approach
such an estimate of temperature. Figure 9A shows that
the composition of Co- and Fe-rich gersdorfite its well with the cobaltite–gersdorfite solid-solution ield at 650°C deined by Hem (2006). In contrast, most compo -sitions plot in the miscibility gap existing between
gersdorfite and cobaltite ields at 500°C (Fig. 9B). On
the other hand, the trend shown by rammelsbergite,
rammelsbergite–saflorite solid solution and rammels
-bergite – saflorite – löllingite solid solution (Fig. 10A) is concordant with the solvus estimated at 625°C by Gervilla & Rønsbo (1992). The composition of the latter diarsenides mostly overlaps the compositional ield of diarsenides determined by Hem & Makovicky (2004) at 650°C in the system Fe–Co–Ni–As–S. These results
suggest that the Ni ores probably formed at
tempera-tures approaching 650°C.
The chemical trend of gersdorfite–cobaltite solid
solution from the Co–Fe ores shows replacement of Ni by Co from core to rim. This trend is opposite to the
Ni trend described by Hem et al. (2001), suggesting that Ni–Co mutual substitution in these sulfarsenides depends not only on anion fugacities but also on the
concentration of metals in luids (as Putnis & Mezger
2004 showed) and, probably, on temperature. Whereas the Ni-rich cores of these sulfarsenide crystals show
compositions outside the solid-solution ields of gers
-dorfite and cobaltite at 500°C (but within the ield of gersdorfite at 650°C; Hem 2006), the composition of the rims tend to plot in the ield determined experi
-mentally by Hem & Makovicky (2004) at 500°C (Fig. 9B). Thus, the observed trend in gersdorfite–cobaltite
solid solution suggests that these sulfarsenides formed under conditions of decreasing temperature recording the initial stages of Co enrichment in the Co–Fe
ore-forming luid.
The extensive crystallization of diarsenides of the
löllingite–(clino)saflorite solid-solution series, locally
surrounding and partly replacing aggregates of
gers-dorfite–cobaltite but mainly replacing the host serpen -tinite, shows a sharp change in the composition of the
ore-forming luid, characterized by an increase in As
fugacity as well as Co and Fe contents. The extensive
explain the scarcity of sulfarsenides in these ores. The
formation of crystals of löllingite–(clino)saflorite with
very variable compositions along the CoAs2–FeAs2 join seems to suggest a crystallization environment
characterized by rapid changes in luid composition. A detailed study of these crystals under ield emission
scanning electron microscope reveals that the crystals
show characteristic oscillatory zoning made up of alternating, very thin (even <1 mm thick) CoAs2-rich bands and FeAs2-rich bands (Fig. 11). As the beam diameter of the electron microprobe is ~3 mm, most
of the compositions plotted in Figure 8A do not reveal
the composition of single bands but mixed
composi-tions. The experimental results of Hem & Makovicky FIG. 9. The concentration of S (atoms per formula unit) versus the compositional
variations in terms of Ni–(Co+Fe) of sulfarsenides from the Ni ores (white diamond) and the Co–Fe ores (black circle). Phase relations along the Co(As,S)2–Ni(As,S)2 join
466 theCanadIanmIneraLoGIst
FIG. 10. A) Compositional plot of diarsenides from the Aghbar mine in the system NiAs2–CoAs2–FeAs2. Solvus lines at
(2004) show almost complete solid-solution between
löllingite and saflorite at 650°C. This solid solution becomes dramatically reduced at 500°C, down to <7.2 at.% Fe and <1 at.% Ni in saflorite and <7.7 at.% Co and <0.8 at.% Ni in löllingite. These solubility limits
are illustrative only, as they refer to the As-poor side
of the solid-solution ield (Hem & Makovicky 2004),
different from the assemblage found in the Co–Fe ores
at Aghbar, where diarsenides become in equilibrium
with skutterudite. This As-rich environment would explain the local Ni enrichment in some of the analyzed
crystals of löllingite–safflorite. Thus, the restricted solubility of Fe in saflorite (it is even lower in clinosaf
-lorite; Hem & Makovicky 2004) and Co in löllingite at temperature below 650°C should favor the develop -ment of small-scale zoning during crystallization from
a supersaturated luid.
No experimental data exist on the temperature
dependence of the saflorite–clinosaflorite pseudomor
-phic transformation. However, as the Pnnm structure
of saflorite can only be stable for Ni- and Fe-rich compositions (Yang et al. 2008), the crystallization of clinosaflorite must take place at the thermal conditions required for low solubility of Fe and Ni in saflorite
(e.g., <500°C: Hem and Makovicky 2004).
The composition of skutterudite overlaps the Co-rich
portion of the ield experimentally determined by Hem & Makovicky (2004) at 650°C, but expands toward
Ni- and, mainly, Fe-rich compositions with respect to
the ield determined at 500°C (Fig. 10B). The expansion
of the compositional ield of skutterudite away from the Fe-rich limit of the ield determined at 500°C agrees
with the coexistence of this mineral, at the end of the stage, with arsenopyrite. In fact, the oscillatory zoning of skutterudite in the Co–Fe ores, characterized by alternating As-rich and S-rich zones, can be considered as the initial stages of a rise in f(S2) that subsequently promoted the formation of arsenopyrite. Such formation
of arsenopyrite could take place from 524°C to 373°C (most calculations indicate temperatures above 400°C), according to the geothermometer of Kretschmar & Scott (1976).
Temperature estimates from phase relations versus homogenization temperatures of luid inclusions
The above estimates of temperature suggest that Ni
ores probably formed at temperatures around 650°C,
and some of the assemblages (mainly coexisting
diar-senides and Ni-rich skutterudite) locally equilibrated
on cooling. In contrast, the Co–Fe ores seem to form at
somewhat lower temperature (from ~500°C), especially
at the late stages of this event, during the crystallization of arsenopyrite. These temperature differences between the two main ores of the deposit can be interpreted by assuming either different temperatures in the two
ore-forming luids or by considering differences in the paths followed by similarly hot mineralizing luids. Thus, luid migration through open fractures (vein-type Ni ores) should result in limited luid:rock interaction. FIG. 11. Oscillatory zoning in crystals of löllingite–(clino)saflorite solid solution, made
468 theCanadIanmIneraLoGIst
The crystallization of the earliest arsenides (nickeline and rammelsbergite, probably at the walls of the veins) prevented heat exchange between fluids and host
rocks. In contrast, intergranular low of luid through
serpentinite (disseminated-type Co–Fe ores) led to high
luid–rock interaction and, as a consequence, extensive exchange of heat between luids and serpentinites. This
should have promoted a temperature decrease in the
mineralizing luids.
The temperatures estimated from phase relations in the Ni and Co–Fe arsenide ores from Aghbar are
signiicantly higher than the homogenization tempera
-tures of luids inclusions in pre- and post-ore quartz,
and post-ore calcite from different deposits of the Bou
Azzer district (<300°C: En-Naciri 1995, En-Naciri et al. 1997). These homogenization temperatures are lower than those measured by Dolansky (2007) in
pre-ore quartz from Aghbar (374°C at 1.6 to >2 kbar)
at the onset of deposition, but similar to those reported
(299°–209°C at 880–1400 bars) during the subsequent
crystallization of the ores.
Nevertheless, we must keep in mind that homogeni-zation temperatures are minimum values, which in addi-tion strongly depend on pressure. Assuming pressures
and, consequently, depths of deposition higher than
those estimated by En-Naciri (1995), En-Naciri et al. (1997) and Dolansky (2007), the temperatures obtained
it better with those deduced here. Furthermore, the luid-inclusion data of Dolanski (2007) support the
decreasing thermal gradient estimated from the
crystal-lization of Ni ores to Co–Fe ores. However, one should
recall the complete absence of any gangue mineral in the Ni ores from Aghbar and the strong deformation that affected the arsenide lodes in the Bou Azzer district, consisting of three episodes of brecciation with associ-ated remobilization and recrystallization (Leblanc 1975,
Leblanc & Billaud 1982). The latter processes preclude the existence of true primary luid inclusions. In addi -tion to these arguments, it is worth noting that micro-thermometry data obtained by infrared microscopy in different deposits worldwide reveal major discrepancies in homogenization temperatures and salinities between
luid inclusions in ore minerals and associated gangue (even >150°C in ore minerals: Campbell & Panter 1990, Giamello et al. 1992, Lüders 1996).
aCknowLedGements
With this contribution, we express our acknowledge-ment to Emil Makovicky, who initiated and guided one
of us (F.G.) through the study of the phase relations in
the poorly known arsenide ore systems. For this project and collaboration, he was always ready (and happy) to
host F.G. in his institution, the Institute of Mineralogy
of the University of Copenhagen. We cannot forget his
attitude in promoting and maintaining fruitful scientiic
discussions, not only about mineralogy but also many different branches of science (e.g., botany). His person
-ality and expertise have impacted our life, making us better as scientists. The more important conclusion of this long-lived collaboration is that we maintain a close friendship.
The authors also acknowledge the kindness of Marc Leblanc for providing his complete collection of
samples made in connection with his Ph.D. thesis. This paper is a contribution to the project CGL2010–15171.
We are grateful for the suggestions for improvement
provided by guest editor Tonci Balić-Žunić, Skage Hem
and an anonymous referee.
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