MINERAL COMPOSITIONS AND PHASE RELATIONS (1)

MINERAL COMPOSITIONS AND PHASE RELATIONS OF Ni–Co–Fe ARSENIDE ORES FROM THE AGHBAR MINE, BOU AZZER, MOROCCO

F ernando 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

I sabeL FanLo, V anessa CoLÁs and I GnaCIo sUbÍas

Universidad de Zaragoza, Departamento de Ciencias de la Tierra, Cristalografía y Mineralogía, c) Pedro Cerbuna 12, E–50009 Zaragoza, Spain

a bstraCt

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.

I ntrodUCtIon like those located in the Cobalt–Gowganda district in Canada and those from the Bou Azzer district in

Arsenides and sulfarsenides of the system Ni–Co– Morocco. Fe–As–S show complex and incompletely established

Our aim in this paper is to contribute to the knowl- phase-relations, both in nature (Radcliffe & Berry edge of mineral assemblages, mineral compositions

1968, Petruk et al. 1971, Oen et al. 1984, Gervilla & and phase relations of the Aghbar arsenide ores; we Rønsbo 1992, Gervilla et al. 1996, Hem et al. 2001, compare the extent of the solid-solution series, the phase

Fanlo et al. 2004, Gritsenko et al. 2004, Parviainen relations and the conditions of formation of the natural et al. 2008, among others) and in experiments (Clark

arsenides and sulfarsenides with the experimental data 1960, Roseboom 1962, 1963, Yund 1962, Klemm 1965,

in the system Ni–Co–Fe–As–S. This study was carried Barton 1969, Maurel & Picot 1974, Kretschmar &

out on a set of samples donated by M. Leblanc, collected Scott 1976, Hem & Makovicky 2004, Hem 2006). One

during his Ph.D. studies.

of the reasons for this poor knowledge is the scarcity of mineralogical studies of natural occurrences and

b aCkGroUnd I nFormatIon deposits of these minerals. Some of them (Co–Fe and, to a lesser extent, Ni arsenides) constitute the main

The Bou Azzer mining district contains over sixty ore assemblages of many small vein-type Ni–Co–Ag– Co–Ni–Fe arsenide orebodies located in the central As–Bi deposits as well as of important cobalt deposits part of the Anti-Atlas Mountains of Morocco. They

the CanadIan mIneraLoGIst

have contributed as much as 8% of the world’s cobalt followed by hydrothermal activity at 330 and 300 Ma output in recent years (Anonymous 2004). At present, (Leblanc 1975). Managem S.A. continues to mine cobalt ore. The irst

The timing of mineralization in the Bou Azzer studies were conducted by geologists of the Ministry of

mining district is still controversial; radiometric ages Mines of Morocco and the “Techno Export Company” ranging between ca . 685 and 215 Ma have been (1935 to 1971). Mineralogical, petrological and struc- proposed by various authors. Thus, whereas Cheilletz tural studies were carried out by Besson & Picot (1978)

et al. (2002) and Levresse et al. (2004) considered that and Leblanc and coworkers (Leblanc 1975, Leblanc the silver ores of the world-class Imiter silver deposit & Billaud 1982, Leblanc & Lbouabi 1988, Leblanc &

formed at ca. 550 Ma, its gold mineralization was dated Fischer 1990). Later, En-Naciri (1995), En-Naciri et al.

at 301 ± 7 by Gasquet et al. (2005). Leblanc (1981) (1997), Maacha et al. (1998), Lebedev et al. (1999), proposed a multistage model of ore formation starting Essarraj et al. (2005), Dolansky (2007) and Ahmed et

with a late Pan-African (~685–580 Ma) event followed al. (2009) mainly focused their research on the char- by remobilization during the Hercynian. Ledent (1960) acterization of the hydrothermal luids responsible of

determined an age of 240 ± 10 Ma (Pb–Pb) for synmin- the precipitation of various Bou Azzer ores through eralization brannerite. This age is slightly younger than

the study of luid inclusions and stable isotopes. The that reported by Dolansky (2007), who constrained the depositional sequences of the ore deposits tend to follow

timing of mineralization on the basis of in situ dating of

a similar pattern, characterized by the early crystal- brannerite included in skutterudite, from 383 ± 7 to 257 lization of diarsenides followed by triarsenides and ± 8 Ma. In good agreement with these results, Oberthür sulfarsenides (Besson & Picot 1978, En-Naciri 1995,

et al. (2009) estimated the age of the Co–Ni–As–(Au) En-Naciri et al. 1997, Ahmed et al . 2009). However,

mineralization of the Bou Azzer district as 310 ± 5 Ma Dolansky (2007) disagreed with this interpretation, using carbonates and brannerite coexisting with molyb- arguing that triarsenides formed earlier than diarsenides

denite. In contrast, En-Naciri et al. (1997) reported a on the basis of textural relationships.

SIMS U–Pb age of 550 Ma for brannerite. En-Naciri et al. (1997) suggested that the Bou

G eoLoGy Azzer deposits likely formed from percolating basinal brines at <300°C. El Ghori et al. (2008) indicated that

The Bou Azzer mining district occurs within the basinal brines may have penetrated into the Neoprotero- Moroccan Anti-Atlas belt, located on the northern edge

zoic basement along major faults. According to these of the Eburnian (2000 Ma) West African Craton (Fig. authors, metals, especially Ni, were probably provided 1A). This belt is separated from the High Atlas and

from the serpentinization of olivine, and As and S were the Mesetian domains to the north by the South Atlas probably scavenged from metasediments bordering the Fault (Gasquet et al. 2005). Several Precambrian inliers

ophiolite belt. In contrast, Ahmed et al. (2009) consid- (Bas Drâa, Ifni, Kerdous, Akka, Igherm, Sirwa, Zenaga,

ered the serpentinites the source of As. Bou Azzer, Saghro, and Ougnat) are distributed along the South Atlas Fault and the Central Anti-Atlas fault

t he a Ghbar d eposIt (Gasquet et al. 2005). These inliers are remnants of

a Pan-African suture zone (685–580 Ma) represented The Aghbar deposit is located in the western central by a dismembered ophiolite (697 ± 8 Ma: El Hadi et

area of the Bou Azzer inlier (Fig. 1A). It consists of al. 2010), unconformably overlain by late Ediacaran

a 600 3 150 m mineralized body, 2 to 10 m thick, to Cambrian rocks and then by mostly sedimentary surrounding a diapir of serpentinite (Leblanc 1975,

Paleozoic rocks. Leblanc & Billaud 1982) almost buried under the As Leblanc (1981) and Saquaque et al. (1989, 1992)

ignimbrites of the Ediacaran–Cambrian cover. The pointed out, the Bou Azzer inlier is the structurally most

Aghbar structure is connected to the east with the complex portion of the Anti-Atlas. A Paleoproterozoic

main serpentinite massif and with the so-called Ambed basement, including gneisses, amphibolites, and schists,

Formation crust (Fig. 1B; Leblanc & Billaud 1982). is intruded by granites and overlain by Cryogenian According to these authors, the Ambed Formation is

formations (Bou Azzer Group). The Cryogenian forma-

a silica–carbonate crust developed over weathered tions consist of epicontinental sedimentary and volcani- ophiolitic rocks. clastic rocks as well as an ophiolite complex interpreted

The mineralized structure as a whole is a contact as a slice of upper Proterozoic ocean crust obducted

orebody with a concave-down form draped over the onto the continental margin of the West African Craton.

serpentinite massif (Fig. 1B). The Aghbar deposit has The end of subduction and the beginning of obduction been commonly described as a complex shell, owing have been dated as 655–635 Ma (El Hadi et al. 2010).

to numerous subvertical lame-like bodies (Dolansky The reactivation of the major Eburnean and Pan- 2007). Much of the complex shell is poorly mineral- African faults during Variscan times promoted the ized; however, a subvertical, lode-shaped orebody injection of felsic dykes at 470 and 400 Ma (Huch 1988)

~600 m long, located along the southern border of the ~600 m long, located along the southern border of the

(Leblanc 1986). Nickel and cobalt ore minerals occur sulides) followed by the Co–Fe ores (Co–Fe arsenides in massive lenses and are disseminated in gangue and Co–Ni–Fe sulfarsenides), and the late Cu ores (Cu minerals within veins. Mineralized veins commonly sulides and sulfosalts). The Ni ores invariably crystal- display crack-seal-style textures or contain breccias lize in veins, concentrated in a narrow band within of ignimbrite fragments cemented by cobalt ores and the lode-shaped orebody (Leblanc 1975, Leblanc & gangue minerals.

Billaud 1982). They mainly consist of nickeline (NiAs), rammelsbergite (NiAs 2 ), members of the rammels- s ampLes and a naLytICaL m ethods

bergite–safflorite solid-solution series (Ni,Co)As 2 , members of the rammelsbergite – saflorite – löllingite

Dr. Leblanc provided all samples investigated in solid-solution series (Ni,Co,Fe)As 2 , members of the this study. They were collected at different depths in löllingite–rammelsbergite solid-solution series (Fe,Ni) the active Aghbar mine during his Ph.D. ieldwork.

As 2 , and Ni-rich skutterudite (Co,Ni,Fe)As 3 . Late Forty of these samples were studied by relected-light

sulfarsenides and sulides ill cracks in the above arse- microscopy, X-ray diffraction using the powder method

nide assemblage and include Co- and Fe-rich gersdorf- (PXRD), back-scattered-electron (BSE) images and

ite (NiAsS), millerite (NiS), siegenite (Ni 3 S 4 ) and minor electron-probe microanalysis (EPMA). The XRD

galena (PbS), sphalerite (ZnS) and greenockite (CdS). studies were carried out using a Philips PW1729 diffrac- The Co–Fe ores occur in veins associated with calcite tometer with a monochromatic CuKa radiation and and dolomite, but mainly replace the host serpentinite. equipped with an X-ray-diffraction analysis program

They show a mineral assemblage made up of members (Martin 2004). We made use of a JSM 6400 scanning

of the cobaltite–gersdorffite solid-solution series electron microscope (SEM) at the Universidad de Zara- (Co,Ni)AsS, members of the löllingite–(clino)saflorite goza to obtain back-scattered electron (BSE) images. solid-solution series (Fe,Co)As 2 , löllingite (FeAs 2 ), Electron-probe micro-analyses (EPMA) of ore minerals

skutterudite (Co,Fe,Ni)As 3 and arsenopyrite (FeAsS). were performed with a CAMECA SX–50 instrument at

The coexistence of the monoclinic and orthorhombic the Universidad de Barcelona. The ore minerals were polymorphs of CoAs 2 , although indistinguishable under analyzed for As, S, Fe, Co, Ni, Cu, Zn, Pb, Cd, Au,

the ore microscope, has been veriied by PXRD. This is Bi and Mo; we monitored the following peaks: AsLa,

the reason why we use the term (clino)saflorite here- SKa, FeKa, CoKa, NiKa, CuKa , ZnKa, PbMa, BiLa,

after, as En-Naciri (1995) did in descriptions of other AuLa, MoLa and CdLa. Operating conditions included

deposits from Bou Azzer. The late Cu ores consist of

a beam diameter of 3 mm, an accelerating voltage of 20 bornite (Cu 5 FeS 4 ), chalcopyrite (CuFeS 2 ), tennantite kV and a beam current of 20 nA. The counting times (Cu,Ag) 10 (Fe,Zn) 2 As 4 S 13 , wittichenite (Cu 3 BiS 3 ) and were 20 s on TAP/PET and 30 s on LiF crystals. The

molybdenite (MoS 2 ) with a quartz gangue. ZAF corrections were performed using the program

Although the existence of extensive solid-solution supplied by CAMECA. Pyrite, GaAs, NiO, as well as

in the system NiAs 2 –CoAs 2 –FeAs 2 under equilib- pure Co, Cu, Au, Zn, Bi, Pb, Cd and Mo metal were

rium conditions (Yund 1962, Roseboom 1963, Hem used as primary standards. Maximum, minimum, mean

& Makovicky 2004, Hem 2006) is well known, and representative results of point analyses are given in

the textural relations and mineral compositions of Tables 1 to 8.

the arsenide ores studied require naming separately the portions of this system, simply for purposes of

m IneraLoGy and t extUres

description. Thus, intermediate compositions of the rammelsbergite–safflorite subsystem were grouped

The Aghbar deposit displays noticeable mineral- under the rammelsbergite–safflorite solid-solution ogical variations depending on the type of mineraliza- series, intermediate ternary compositions in the system tion: illing veins or replacing the host serpentinite.

CoAs 2 –NiAs 2 –FeAs 2 were named as rammelsbergite – Dissolution and replacement of serpentinite account saflorite – löllingite solid-solution series, members of for the abundance of serpentine and chlorite inclusions

the löllingite–rammelsbergite subsystem were grouped in diarsenides and triarsenides (commonly following as löllingite–rammelsbergite solid-solution series, the growth planes of these arsenides), as well as by the

Ni-poor members of the löllingite–saflorite subsystem presence of chromite grains total or partially included were named as löllingite–saflorite solid-solution series, in arsenides (Fig. 2). Grains of chromite are zoned

and only diarsenides containing Co and Ni but having (owing to alteration) and cracked, and become partially

a composition close to the FeAs 2 apex of the system dissolved where included in arsenides.

were named löllingite.

The mineral assemblages identified allow us to group them into three different ore types: Ni ores, The Ni ores Co–Fe ores and Cu ores. Furthermore, their textural relations provide evidence of a three-stage depositional

The crystallization sequence of arsenides in these

450 the CanadIan mIneraLoGIst 450 the CanadIan mIneraLoGIst

F IG . 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.

rammelsbergite, rammelsbergite – saflorite – löllingite Azzer district. However, X-ray-diffraction analyses solid solution, löllingite–rammelsbergite solid solution

confirm the occurrence of krutovite in the Aghbar and Ni-rich skutterudite (Fig. 3), revealing a continuous

samples. In places, rammelsbergite appears as small increase in arsenic fugacity concomitant with a decrease

pods or remnants of crystals, or aggregates of crystals, in Ni content in the ore-forming environment.

all of them partly replaced, overgrown by or intergrown

Nickeline has only been found in one sample as with rammelsbergite–saflorite or rammelsbergite – remnant inclusions in rammelsbergite. It shows a saflorite – löllingite, all of them enclosed in Ni-rich distinct (0001) parting emphasized by alteration. Minute

skutterudite (Figs. 3D, 3E, 3F). These textures clearly inclusions of gold (Au) are found as fine, rounded provide evidence of destabilization and re-equilibration and discrete grains included in nickeline and along processes. the contact between nickeline and rammelsbergite

Crystals of rammelsbergite–saflorite solid solution (Fig. 3A).

occur intimately associated with those of rammelsber-

Rammelsbergite occurs as massive aggregates occa- gite and rammelsbergite – saflorite – löllingite solid sionally in association with nickeline and gold, exhib- solution. Whereas the crystals of rammelsbergite–saflo- iting oscillatory zoning as a result of Co substitution for

rite display a strong anisotropy in pale blue and brown Ni (Fig. 3B). Massive rammelsbergite occurs as centi- colors, those of rammelsbergite – saflorite – löllingite metric masses and exhibits anisotropic colors in various

show deeper blue to purple colors (Fig. 3F). These shades of purple, blue and brown, with twinning in very

diarsenides occur as: (i) small lath-shaped crystals of pronounced lamellae or as inversion polysynthetic twins

rammelsbergite–saflorite overgrowing the last remnants oriented in two directions, which intersect almost at 90°

of minute crystals of rammelsbergite that have modiied (Fig. 3C). These complex patterns of twinning suggest their original composition (as we will see in the next

that at least part of the massive rammelsbergite forms sections) and, in turn, are partially replaced or over- by inversion of earlier krutovite (ideally NiAs 2 , which grown by rammelsbergite – saflorite – löllingite (Fig. has a trimorphic relationship with rammelsbergite and 3D), and (ii) irregular patches or small pods, overgrown pararammelsbergite). This conclusion notably contrast or partially replaced by rammelsbergite – saflorite – with En-Naciri’s (1995) observations of the existence

löllingite (Figs. 3E, 3F, 3D). Both types of assemblages of pararammelsbergite in other deposits of the Bou 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. F IG .

1. A) Geological map of the Anti-Atlas belt, southern The complex twinning showed by rammelsbergite led Morocco, showing the location of the main ore deposits

to differences in the path followed by the mineralizing and the Bou Azzer study area (rectangular area). B) The

luids through this phase, which may account for the complex shell of the Agbar mine (from Leblanc & Billaud

irregular replacements of one phase by another, as is 1982).

observed in Figures 3D and 3G. Most contacts between

the CanadIan mIneraLoGIst

diarsenides and triarsenides suggest partial dissolution Skutterudite II forms idiomorphic single crystals or of the former and precipitation of the latter (Figs. 3E, 3F,

clusters of crystals disseminated in serpentinite. The 3G), although the local coprecipitation of members of

main difference relative to Ni-rich skutterudite is the the rammelsbergite – saflorite – löllingite solid-solution

oscillatory zoning (Fig. 3O) caused by slight varia- series and Ni-rich skutterudite cannot be ruled out.

tions in the S:As ratio. In some cases, this skutterudite Finally, crystals of the löllingite–rammelsbergite

overgrows löllingite and löllingite–(clino)safflorite solid-solution series appear as small pods or euhedral masses (Fig. 3K). crystals, illing fractures in rammelsbergite – saflorite –

Arsenopyrite occurs as prismatic crystals over- löllingite solid solution and partially replaced by Ni-rich

growing most of the previously formed Co–Fe arsenides skutterudite. The anisotropy colors are strong blue and and sulfarsenides. Locally, it is intergrown with bornite. light brown (Fig. 3H).

Nickel-rich skutterudite is the last and most abundant

The Cu ores

arsenide in the Ni ores. It occurs as millimetric masses and as euhedral, unzoned crystals, enclosing previously

The Cu-rich sulide assemblage forms late in the formed diarsenides (Figs. 3E, 3F, 3G).

depositional history of the Aghbar ores (Fig. 3P).

Late alteration and partial dissolution of rammels- Bornite (Bn) ills fractures and cracks and occasionally bergite led to the deposition of gersdorfite followed

replaces previous arsenides, mainly in serpentinite- by the sulide assemblage cited above. Gersdorfite

hosted ores. Chalcopyrite (Cp) precipitates simultane- partially replaced rammelsbergite and illed veins in

ously with bornite, whereas tennantite ills cracks in massive rammelsbergite (Fig. 3I) Sulides formed in

the aforementioned minerals. Wittichenite (Wtc) ills association with quartz, illing cracks and fractures in

voids and fractures in bornite (Fig. 3L), chalcopyrite the arsenide ores (Fig. 3J).

and cobaltite–gersdorfite. Finally, molybdenite (Mo) occurs as small grey rosettes, included in serpentinite

The Co–Fe ores or illing voids in skutterudite II. On the basis of the above mineral textures and asso-

The crystallization sequence of arsenides in Co– ciations, the paragenetic sequence of the Aghbar mine Fe ores starts with the formation of members of the is shown in Figure 4.

cobaltite–gersdorfite solid solution followed by löllin- gite–(clino)saflorite solid solution, löllingite, a new

t he C omposItIon oF m IneraLs In the n I o res generation of skutterudite and arsenopyrite (Fig. 3). Members of the cobaltite–gersdorfite solid-solution

Nickeline

series occur in two distinct textural positions in serpen- tinite-hosted ores: (i) aggregates of idiomorphic crys-

EPMA results (Table 1) of nickeline reveal an almost tals partially replaced by skutterudite II and löllingite

stoichiometric composition, with only a negligible (Fig. 3K), exhibiting perfect cleavage and commonly,

content of Co (below 0.24 wt.%): Ni 0.99–1.01 As 0.99–1.01 . concentric or sector zoning characterized by variations in Ni and Co contents; (ii) idiomorphic crystals enclosed

Diarsenides

in, and selectively replaced by bornite (Fig. 3L). The latter sulfarsenide crystals display growth-bands due to

Rammelsbergite shows a compositional variability chemical zoning from a Ni-rich core to a Co-rich rim. depending on its textural type. As cited above, this Most Ni-rich cores exhibit decomposition processes mineral occurs as large masses enclosing minor nick- leading to a symplectite-type texture made up of inter- eline and gold. Its chemical composition shows a very grown bornite and cobaltite–gersdorfite (Fig. 3M).

limited substitution of Co for Ni (<5.77 wt.% Co; Members of the löllingite–(clino)saflorite solid- Table 1, Fig. 5A) with compositions close to stoichi- solution series only occur in mineralized serpentinite. ometry [(Ni 0.80–1.00 Co 0–0.21 )As 1.84–2.01 S 0–0.16 ]. These data These crystals show a characteristic spindle-like shape are similar to those obtained by En-Naciri (1995) and arranged in the typical star-like twins. They are also Ahmed et al. (2009) in arsenide ores from other deposits characterized by rhythmic compositional zoning with of the Bou Azzer district, but differ from those obtained Fe-rich and Co-rich bands (Fig. 3N). They form aggre- by Dolanski (2007), who reported smaller chemical

gates surrounded by skutterudite II, which also ills variations in samples from the Aghbar deposit. spaces among the crystals of löllingite–saflorite solid

The members of the rammelsbergite–saflorite solid- solution.

solution series contain up to 2.63 wt.% Fe, 15.82 wt.%

The crystals of löllingite, unlike those of löllingite– Co and 6.47 wt.% S (Table 2, Fig. 5A). Their general (clino)saflorite solid solution, are small and unzoned.

formula corresponds to (Ni 0.39–0.79 Co 0.22–0.56 Fe 0–0.10 ) They occur as isolated grains in serpentinite or as aggre- As 1.61–1.98 S 0.02–0.39 . Nickel shows a strong negative gates enclosing crystals of cobaltite–gersdorfite solid

correlation with Co and Fe contents, and no correlation solution (Fig. 3K) or, most commonly, forming massive

with S or As (Figs. 5B, 5C). Cobalt mainly substitutes with S or As (Figs. 5B, 5C). Cobalt mainly substitutes

saflorite – löllingite solid-solution crystals. Moreover, No correlation between anions and cations has been as the replacement proceeds, an increase in Fe content found. Sulfur contents are the highest of all diarsenides,

occurs from center toward the edge of the masses in slightly exceeding the range established experimentally

contact with Ni-rich skutterudite. This skutterudite by Yund (1962). These data are similar to those obtained

shows a decrease in Fe content compared with that by En-Naciri (1995), Dolansky (2007) and Ahmed et al.

far from these remnants of diarsenides. As is shown (2009). Figure 3D and Table 2 show that rammelsber- in Figure 5A, most of these compositions were not gite–saflorite crystals overgrow and replace crystals of

reported previously in the Bou Azzer district. rammelsbergite, which show the highest Co contents,

The analyzed members of the löllingite–rammelsber- suggesting re-equilibration with the mineralizing luids,

gite solid-solution series represent a restricted portion whereas the rammelsbergite–saflorite crystals display

of the solid solution along the Fe–Ni join, with up to the lowest Fe contents. As replacement proceeds, the

12.17 wt.% Ni and 3.26 wt.% Co (Table 4, Fig. 5A): Fe contents slightly increase at a distance from the (Ni 0.12–0.43 Co 0.02–0.12 Fe 0.48–0.86 )As 1.84–2.01 S 0–0.14 . There rammelsbergite.

is a negative correlation between FeAs 2 and NiAs 2 Crystals of rammelsbergite – saflorite – löllingite

[NiAs 2 = –0.927(FeAs 2 ) + 90.350; Fig. 5C] and, to solid solution show the highest compositional varia-

a lesser extent, between FeAs 2 and CoAs 2 [CoAs 2 = tion in metal content (Table 3). They follow a trend –0.073(FeAs 2 ) + 9.654; Fig. 5D]. The sulfur content is very different from the other solid solutions, centered low (up to 2.31wt.%) and is negatively correlated with along the Co-richer side of the ½Co + ½Ni = Fe trend Co and Ni. From core to rim of single crystals, there is a (Fig. 5A). Notwithstanding, they display a narrow Ni increase and a Fe decrease. Neither En-Naciri (1995) variability in As and S contents (Fig. 5B): (Ni 0.06–0.48

nor Dolansky (2007) reported compositions similar to Co 0.10–0.54 Fe 0.10–0.83 )As 1.89–2.03 S 0–0.10 . These diarsenides

those reported here. In contrast, Besson & Picot (1978) show a strong negative correlation between the NiAs 2 reported one composition of nickeliferous löllingite,

and FeS 2 [NiAs 2 = –0.447(FeAs 2 ) + 42.232; Fig. 5C], and Ahmed et al. (2009) presented similar results, but

showing quite different chemical trends (Fig. 5A). + 54.770; Fig. 5D], but no correlation between cations and anions. The extent of Ni + Co replacement by Fe Triarsenides also correlates with the modal abundance of skutteru- dite: the higher the amount of skutterudite in the sample,

and between CoAs 2 and Fe As 2 [CoAs 2 = –0.553(FeAs 2 )

Nickel-rich skutterudite is characterized by a the higher the extent of Ni + Co substitution by Fe in broad range of compositions (Table 5) with very high diarsenides. As can be seen in Figures 3E and 3G and

Ni and Fe contents: (Co 0.28–0.75 Ni 0.17–0.43 Fe 0.06–0.40 ) in results of point analyses (Table 3), there is a clear As 2.85–3.01 S 0–0.11 . There is a substantial replacement of sequence of mineral replacement: rammelsbergite is

Co by Ni and Fe, and a limited replacement of As by S, replaced by crystals of rammelsbergite–saflorite solid

with Ni and Fe positively correlated with As. The arrow solution, and the latter, in turn, by rammelsbergite – in Figure 6A shows the compositional trend exhibited

F IG . 3. Back-scattered electron images and microphotographs in relected light, showing representative textures of the Aghbar ores. The numbers indicate compositions referred to in Tables 1 to 8. Symbols: Nc: nickeline; Rmb: rammelsbergite; Au: gold; Ni–Sk: Ni-rich skutterudite from Ni ore; RS: rammelsbergite–saflorite solid-solution crystals; RSL: rammelsbergite – saflorite – löllingite solid-solution crystals; LR: löllingite–rammelsbergite solid-solution crystals; Cal: calcite; Fe–Co– Gdf: Fe- and Co-rich gersdorfite from Ni ore; As–Gdf: As-rich gersdorfite from Ni ore; Mlr: millerite; Sp: sphalerite; Gck: greenockite; Qtz: quartz; Lol: löllingite; Sk II: skutterudite from Co–Fe ore; CG: cobaltite–gersdorfite solid solution crystals from the Co–Fe ore; Bn: bornite; Wtc: wittichenite; Bn–CG: intergrown of bornite and cobaltite–gersdorfite; Cp: chalcopyrite; Tn: tennantite. A) Minute inclusions of nickeline and gold hosted by rammelsbergite. B) Rammelsbergite showing oscillatory zoning and numerous gold inclusions. C) Massive rammelsbergite exhibiting anisotropic colors, lamellae and inversion-induced polysynthetic twins. Crossed nicols, relected light. D) Remnants of idiomorphic crystals of rammelsbergite partly replaced or overgrown by RS crystals. E) Minute patches of rammelsbergite partly overgrown by RS crystals, which, in turn, is overgrown by RSL crystals. F) Ni-rich skutterudite enclosing a small pod of rammelsbergite overgrown by RS and RSL crystals. Crossed nicols, relected light. G) RS crystals overgrown by RSL crystals, and all them enclosed by Ni-rich skutterudite. H) Crystals of löllingite–rammelsbergite solid-solution series illing fractures in masses of rammelsbergite – saflorite – löllingite solid solution. Crossed nicols, relected light. I) Crystals of rammelsbergite partly replaced by As-rich gersdorfite, which, in turn, is overgrown by idiomorphic crystals of Fe–Co-rich gersdorfite. J) Cracks in rammelsbergite illed with quartz and sulides. K) Crystals of cobaltite–gersdorfite solid solution partially replaced by skutterudite II and löllingite. L) Idiomorphic crystals of cobaltite–gersdorfite enclosed in and selectively replaced by bornite. Plane-polarized relected light. M) Chemical zoning in cobaltite–gersdorfite solid-solution crystals; the Ni-rich cores exhibit decomposition processes. N) Rhythmic compositional zoning in löllingite–saflorite solid-solution crystals. O) Idiomorphic crystal of skutterudite II showing oscillatory zoning. P) Bornite, chalcopyrite and tennantite illing cracks and partly replacing previous arsenides. Plane-polarized relected light.

454 the CanadIan mIneraLoGIst 454 the CanadIan mIneraLoGIst

the CanadIan mIneraLoGIst

F IG . 4. Paragenetic sequence of the mineralization at the Aghbar mine.

TABLE 1. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF NICKELINE AND RAMMELSBERGITE FROM THE AGHBAR MINE, BOU AZZER, MOROCCO ___________________________________________________________________________________

___________________________________________________________________________________ Nickeline (n = 4)

Rammelsbergite (n = 84)

___________________________________________________________________________________ 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#.

by Ni-rich skuterrudite. The regression lines describing triarsenides determined by Roseboom (1962), which the composition of this skutterudite are: NiAs 3 = does not extend to either pure Ni or Fe end-members.

–0.287(CoAs 3 ) + 42.969 and FeAs 3 = –0.713(CoAs 3 )

+ 57.036 (Figs. 6B, C). The composition of Ni-rich Co- and Fe-rich gersdorfite skuterrudite extends toward compositions richer in Ni and Fe than previously documented at Bou Azzer. This

Two chemically different types of Co- and Fe-rich substitution is well supported by naturally occurring gersdorffite have been identified in Ni ores, corre- skutterudite compositions and also broadly describes sponding to the two textural varieties. Gersdorffite the experimentally determined miscibility region of illing veins and replacing rammelsbergite shows the

narrowest range in metal contents (As–Gdf in Fig. 7A) narrowest range in metal contents (As–Gdf in Fig. 7A)

TABLE 2. STATISTICAL RESULTS OF ELECTRON-MICROPROBE ANALYSES OF RAMMELSBERGITE–SAFFLORITE SOLID SOLUTION FROM THE AGHBAR MINE, BOU AZZER, MOROCCO ___________________________________________________________________________________

___________________________________________________________________________________ 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

___________________________________________________________________________________ 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 ___________________________________________________________________________________

___________________________________________________________________________________ Rammelsbergite – safflorite – löllingite (RSL) solid solution (n = 83)

___________________________________________________________________________________ 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 Figures 3E and 3G.

and the highest As contents: (Ni 0.70–0.82 Co 0.15–0.29 euhedral gersdorfite (Fig. 3I) with a core rich in As and Fe 0–0.02 )As 1.34–1.45 S 0.57–0.66 . It does not contain signii-

a rim with a As:S ratio close to 1. It is relatively rich cant Fe (<0.45 wt.%), but Co reaches up to 9.43 wt.%.

in both Fe (up to 10.63 wt.%) and Co (11.87 wt.%): Gersdorfite replacing rammelsbergite is overgrown by

(Ni 0.54–0.58 Co 0.13–0.35 Fe 0.05–0.32 )As 1.07–1.20 S 0.82–0.94 (Fe–

the CanadIan mIneraLoGIst

F IG . 5. A) Plot of compositions of diarsenides from Ni ore in the system CoAs 2 –NiAs 2 –FeAs 2 . 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 NiAs 2 versus FeAs 2 of diarsenides from Ni ore. Regression lines for each phase are shown. D) Binary plot of CoAs 2 versus FeAs 2 showing regression lines.

Co–Gdf in Fig. 7A). These two textural types of gers- bution. En-Naciri (1995) did not report compositions of dorfite show signiicant positive correlation between

sulfarsenides from Bou Azzer. Ni and As (As/S = 4.497Ni – 1.235; Fig. 7B), and Fe correlates negatively with As in euhedral gersdorfite.

Sulides

The trend described by these sulfarsenides does not it any of the trends established by Hem et al. (2001).

The structural formulae of sulides in Ni ores are:

A comparison with previously published data shows millerite (Ni 0.98–1.02 Co 0.01 )S 0.97–1.02 , siegenite (Ni 2.32–2.46 that Ahmed et al. (2009) only found Ni end-member Co 0.55–0.61 )S 3.96–4.03 As 0.03 , sphalerite (Zn 0.96–0.98 compositions, and Dolansky (2007) reported composi- Ni 0.02–0.04 )S 0.98–1.00 , and greenockite (Cd 0.89–0.93 tional ranges narrower than those shown in this contri- Zn 0.07–0.10 Ni 0.01–0.05 )S 0.99–1.00 .

n i – C o – F e arsenIde ores From the aGhbar mIne , boU azzer , moroCCo 459 t he C omposItIon oF m IneraLs 3.29 wt.%, Table 7, Fig. 8A): (Fe 0.06–1.03 Co 0–0.90 Ni 0–0.12 )

In the C o –F e ores As 1.76–2.01 S 0–0.23 . This igure also shows the reciprocal substitution of Fe and Co. As shown in Figures 8B

Gersdorfite–cobaltite and 8C, there is a poor correlation between metals and anions; however, the Fe-rich members of the löllingite–

The members of the cobaltite–gersdorfite solid- (clino)saflorite solid-solution series (>0.5 apfu Fe), solution series occurring in the Co–Fe ores replacing tend to be richer in S than the Co-rich members (<0.5 serpentinite exhibit large compositional variations apfu Fe) (Fig. 8C). in the Fe-poor region of the system CoAsS–NiAsS–

The composition of löllingite is rather limited FeAsS (Table 6; CG in Fig. 7A): (Co 0.18–0.94 Ni 0.03–0.76

(Table 7) clustering near the Fe apex of the Co–Ni–Fe Fe 0.02–0.17 )As 0.92–1.20 S 0.78–1.08 . The Co contents tend to ternary field (Fig. 8A): (Fe 0.67–1.03 Co 0–0.23 Ni 0–0.16 ) increase from core to rim of single crystals, whereas As 1.82–2.01 S 0–0.18 . Some crystals are somewhat richer in the As:S ratio decreases. The variation in the As:S ratio

Ni than those of the löllingite–saflorite solid solution, shows a weak, positive correlation with Ni (As/S = but the S contents are lower. Both Ni and Co correlate

0.556Ni + 0.960; Fig. 7B). This variability is consistent

negatively with As.

with the precipitation of cobaltite–gersdorfite crystals in an environment with increasing sulfur fugacity. As Triarsenides cited above, comparable data have not been reported by Ahmed et al. (2009), Dolansky (2007) or En-Naciri

Skutterudite II is characterized by high Fe and Ni (1995).

contents (somewhat lower than Ni-rich skutterudite) and relatively high S contents (Table 8): (Co 0.56–0.97

Diarsenides Fe 0.03–0.28 Ni 0–0.20 )As 2.73–3.01 S 0–0.22 . Its compositional trend (Fig. 6A) is similar to that exhibited by Ni-rich

Diarsenides in the Co–Fe ores are represented skutterudite, that is, Ni and Fe substitute for Co (Ni/Fe by members of the löllingite–(clino)saflorite solid- trend). The regression lines describing the composition solution series and löllingite. The composition of the

of skutterudite are NiAs 3 = –0.437(CoAs 3 ) + 42.321 former tends to cluster along the Co–Fe join, with and FeAs 3 = –0.563(CoAs 3 ) + 57.672 (Figs. 6B, 6C). comparatively few examples containing some Ni (below

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 ___________________________________________________________________________________

___________________________________________________________________________________ Löllingite–rammelsbergite solid solution (n = 28)

___________________________________________________________________________________ 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 ___________________________________________________________________________________

___________________________________________________________________________________ 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

___________________________________________________________________________________ Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right.

the CanadIan mIneraLoGIst

indicate that relative decreases in Co and S are accom- Arsenopyrite panied by relative increases in Ni, Fe and As, suggesting that paired substitution of Co for Ni + Fe, as well as

The arsenopyrite (Apy) composition is almost S for As, is common. In fact, the S content is directly stoichiometric, with very few grains containing small related to the compositional zoning observed in some amounts of Co (Table 8) but neither nickel nor anti- crystals of skutterudite (Fig. 3O), as the dark bands mony: (Fe 0.93–1.05 Co 0–0.07 ) As 0.92–1.06 S 0.93–1.05 . have the high S contents. This inding suggests that metal substitution is determined by S and As fugacities.

t he C omposItIon oF m IneraLs In the C U o res

As mentioned above, the Cu ores represent the latest episode in the crystallization history of Aghbar ores.

F IG . 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 FeAs 3 –CoAs 3 –NiAs 3 . 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 NiAs 3 versus

CoAs 3 of skutterudite from Ni and Co–Fe ores. Regression lines for each phase are

also shown. C) Binary plot of FeAs 3 versus

CoAs 3 , showing regression lines.

n i – C o – F e arsenIde ores From the aGhbar mIne , boU azzer , moroCCo 461

F IG . 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

Ni As# ___________________________________________________________________________________

Fe–Co-rich G ersdorffite (n = 3)

As-rich G ersdorffite (n = 12)

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

Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right. The

the CanadIan mIneraLoGIst

Most sulides show limited substitution of metals and

d IsCUssIon nearly stoichiometric compositions. Bornite [(Cu 4.83–4.90 Ni 0–0.15 Co 0.05–0.20 )Fe 0.99–1.05 S 4.01–4.11 ] contains some The sequence of crystallization cobalt and nickel (up to 2.15 wt.% and 1.58 wt.%, respectively) substituting for copper; chalcopyrite

Mineral textures and compositional trends exhibited (CuFeS 2 ) and wittichenite (Cu 3 BiS 3 ) show stoichio- by Ni–Co–Fe arsenide ores at Aghbar provide evidence metric compositions; tennantite shows limited incorpo- of a two-stage depositional history. In fact, Leblanc ration of the tetrahedrite component but contains some

(1975) and Leblanc & Billaud (1982) reported that the

Ni ores concentrated only in a vein-like body indepen- Ni 0–0.15 )(As 3.17–3.44 Sb 0.15–0.31 )S 13.00–13.30 ] and molybde- dent but enclosed in the large lode-shaped, Co–Fe arse- nite shows some substitution of Cu and minor Fe and nide orebody. The irst stage (Ni ores) is characterized

Ni and Co [Cu 9.97–9.98 (Zn 1.23–1.40 Fe 0.31–0.61 Co 0.15–0.45

Co for Mo: (Mo 0.86–0.98 Cu 0.02–0.18 Fe 0.02–0.06 Co 0–0.02 )S.

by the sequential crystallization of nickeline, rammels- bergite, rammelsbergite–safflorite solid solution,

F IG . 8. A) Plot of the compositions of diarsenides from the Co–Fe ore in the system CoAs 2 –NiAs 2 –FeAs 2 . The compositional

ields of diarsenides analyzed by other authors in various mines of the Bou Azzer district are also shown for comparison, with shading as in Figure 5A. B) Binary plot of Co (in apfu) versus As in weight percent. C) Binary plot of Fe (in apfu) ields of diarsenides analyzed by other authors in various mines of the Bou Azzer district are also shown for comparison, with shading as in Figure 5A. B) Binary plot of Co (in apfu) versus As in weight percent. C) Binary plot of Fe (in apfu)

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 MeAs 1+x S 1–y ) as well as those analyzed by Ahmed et al. (2009) (named NiAs 2–x S x ) 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 ___________________________________________________________________________________

___________________________________________________________________________________ Löllingite–safflorite solid solution (n = 167)

Löllingite (n = 134)

___________________________________________________________________________________ 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 ___________________________________________________________________________________

___________________________________________________________________________________ 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

Arsenopyrite (n = 63)

___________________________________________________________________________________ Compositions are expressed in wt.% on the left and in atoms per formula unit (apfu) on the right.

the CanadIan mIneraLoGIst

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 MeAs 1+x S 1–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(S 2 ),

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