Troilite together with pentlandite that exsolved along grain boundaries and fissures in pyrrhotite make up the sulfide assemblage that was formed under low temperature re- equilibrium conditions in pyrrhotite. Troilite lamellae hosted by hexagonal pyrrhotite at the Guelb Moghrein deposit have an almost stoichiometric composition (49.5-49.9 at.% Fe). Troilite, whose name applies only to the polymorph of stoichiometric FeS, is stable below 140°C (Gronvold and Haraldsen 1952; Yund and Hall 1969). According to Kissin and Scott (1982) high temperature hexagonal pyrrhotite itself undergoes structural changes below 300°C with the development of superstructures and the formation of monoclinic pyrrhotite and, at 142°C, the exsolution of troilite from hexagonal pyrrhotite of S-poor composition. Durazzo and Taylor (1982) showed experimentally in the Fe-Ni-S system that flame-like pentlandite can exsolve from pyrrhotite at temperatures below 250 oC. Additional to this, Naldrett et al. (1967) has described similar pyrrhotite-pentlandite associations suggesting that the appearance of pentlandite in sulfur-rich ores is the result of low temperature exsolution below 200°C.
Cubanite (tetragonal) and mackinawite form abundant exsolution lamellae in chalcopyrite. In the Fe-Cu-S system, the development of tetragonal cubanite lamellae in chalcopyrite is indicative of low temperature of formation (below 252 °C) as in higher temperatures cubanite is isometric (Yund and Kullerud 1966). The formation of mackinawite largely by replacement of chalcopyrite and cubanite, apparently postdated the development of cubanite. Generally, mackinawite is found in association either with troilite or with low- temperature pyrrhotites (Evans et al. 1964; Kullerud 1986). Benning et al. (1999) showed experimentally that mackinawite cannot persist structural transformation above 187 °C.
Minute inclusions of native bismuth and gold, maldonite, Bi-tellurides and gold-bearing, Ag-Te complexes occur inside or along the edges of the arsensulfides. These textures indicate that the Bi-Au-Ag-Te minerals were either exsolved from arsenopyrite or that they were co-precipitated. Traditionally the presence of native Bi has been considered as indication of temperatures at or below the melting point of Bi (271°C; Douglas et al. 2000).
81
Table 6.1 Table of the primary, higher temperature assemblage and the exsolution, lower temperature assemblage of the Guelb Moghrein ore mineralization, together with the estimated temperatures.
H
ig
h
e
r
te
m
p
e
ra
tu
re
MagnetiteC
ry
st
a
ll
iz
a
ti
o
n
Co-crystallization with the sulfide assemblages. Clinosafflorite
+ Arsenopyrite
+ Cobaltite
Prior- to co-crystallization with sulfide minerals. Idiomorphic crystals with concentric growth zoning parallel to crystal faces. Temperature estimated at
340-400oC and >380 oC (this study)
Chalcopyrite + Pyrrhotite
Massive aggregates that make up the matrix for all other ore minerals.
<425oC (Yund & Kullerud, 1966)
L
o
w
e
r
te
m
p
e
ra
tu
re
CubaniteE
x
so
lu
ti
o
n
Coarse exsolution lamellae in chalcopyrite <252oC (Yund & Kullerud 1966)
Mackinawite Exsolution lamellae in cubanite
<137-187oC (Benning et al. 1999)
Pentlandite Flame, bleb- or grainy-like types in hex. pyrrhotite <250oC (Naldrett et al. 1967; Durazzo & Taylor 1982) Nickeline Flame-like occurrences along grain boundaries in
arsenopyrite Troilite
Fine exsolution lamellae in hexagonal pyrrhotite <142 oC (Gronvold and Haraldsen 1952; Yund and Hall
1969; Kissin and Scott 1982)
Maldonite Minute inclusions, droplets
116–373 °C (Okamoto & Massalski 1983)
Native Bi Minute inclusions in clinosafflorite
271 °C (Douglas et al. 2000)
Hedleyite Minute inclusions in clinosafflorite
312–266 °C (Elliott 1965) Native Au, petzite
82 This is in agreement by the maldonite inclusions found, as maldonite is stable only in the temperature interval 116–373 °C (Okamoto and Massalski 1983; Afifi et al. 1988). In addition, Bi-tellurides themselves or associations of them offer few temperature constraints. The melting point of pure hedleyite is at 312 °C whereas the association of hedleyite with native bismuth form a melt eutectic at 266 °C (Elliott 1965). However, some studies indicate that native Bi, maldonite and Bi-Au-Ag-tellurides, could have separated from the fluid at higher temperatures, forming melt droplets in the fluid (Sheppard et al. 1995; Tomkins and Maurogenes 2001; Ciobanu and Cook 2003).
6.5 Summary
The principal sulfide minerals include chalcopyrite, hexagonal, and lesser monoclinic pyrrhotite, troilite, pentlandite, cubanite and mackinawite. The sulfide mineral assemblages host all other arsensulfide and arsenide phases which include arsenopyrite, cobaltite, clinosafflorite, and subordinate nickeline. Magnetite is constantly present within the sulfide matrix; however in massive ore is a minor component. The principal metal, gold, is found either in native form, in solid solution with silver (electrum) or bismuth (maldonite), or in a complex association with silver, bismuth and tellurium (Bi-Au-Ag-tellurides). These form irregular, bleb- and drop-like inclusions commonly occupying the boundaries between clinosafflorite and arsenopyrite or riddle the interior of clinosafflorite.
From textural features and comparisons with published phase equilibria data, it is shown that the sulfides are characterized by a primary, higher temperature assemblage and a lower subsolidus, exsolution assemblage. The primary ore mineralogy includes a relative early precipitation of the arsenopyrite, cobaltite and clinosafflorite followed by pyrrhotite chalcopyrite and magnetite. Arsenopyrite geothermometry indicates formation temperature of the primary mineralization above 380oC but under 425 oC, which is in agreement to the calculated temperature of the retrograde metamorphism of 410 ±30 oC (Section 3.10). The textures of troilite, pentlandite, cubanite, mackinawite and nickeline, as well as, the Bi-Au- Ag-Te mineral associations, suggest that these have resulted from exsolution during cooling. Most of these indicate exsolution temperature under 250 oC.
The variation of Fe, Co and Ni in arsenopyrite, cobaltite and clinosafflorite describe linear trends in which Fe replaces a mixture of Co and Ni relative to a increasing S/As ratio. This indicates that the ore initially formed under low fugacities of sulfur.
83
7 GEOCHEMISTRY
The alteration zones defined petrologically in the metacarbonate body and in the Akjoujt Metabasalt unit at Guelb Moghrein (Chapter 5) have been analyzed for major, trace and rare earth element compositions. A total number of 23 samples from the main ore breccia in the metacarbonate and from the biotite-chlorite-grunerite-calcite alteration zone in the Akjoujt Metabasalt unit were analyzed in order to determine and quantify the geochemical changes that occurred during hydrothermal alteration and mineralization, in comparison to their immediate unaltered rocks. The main analytical work is based on core from one drill hole (RCGM88) that intersect both the hanging wall and footwall units as well as the metacarbonate and the ore-breccia zones in their entire extent (Figure 7.1). It was therefore selected to provide a descriptive geochemical profile for the deposit.
Figure 7.1 Schematic geologic log and selected major element geochemical data of the RCGM88 drill core, which intersect the hanging wall and footwall lithologies of the Guelb Moghrein deposit as well as the host metacarbonate and the ore-breccia zones. Some of the major element data i.e. K2O, and Fe2O3 show systematic increase correlated with proximity to the ore bodies.
84
7.1 Geochemistry of the ore breccia
A number of 7 samples from the mineralized breccias zones in the metacarbonate were analyzed for major, and a suite of trace elements. These were selected to be representative of the variably altered and mineralized specimens. The dominant mineral components in the rock are siderite, Fe-Mg clinoamphiboles, magnetite and sulfides whose modal proportions control the bulk geochemistry of the samples. Fe2O3 and MgO show the highest concentrations that range between 33.8-64.7 wt.% and 4.9-16.4 wt.%, respectively (Table 12.46). In contrast, MnO and CaO amounts are low averaging at 0.7 wt.% and 1.9 wt.%, respectively. TiO2 is generally absent; Al2O3 concentrations are very low (avg. 0.9 wt.%), and total alkali (Na2O+K2O) amounts are consistently below 1 wt.% except one sample that shows elevated alkali content up to 3.3 wt.%. The SiO2 content, most of which is incorporated in the Fe-Mg clinoamphiboles, varies between 0.4 wt.% in massive sulfide zones up to 11.5 wt.% in more distal parts.
In respect to least altered metacarbonate, the breccia zones show enrichment in Sr (40-121 ppm), Ba (139-414 ppm), and La (251-381 ppm). Among the transition elements, V and Cr amounts are similar to that in non-mineralized metacarbonate with averages at 161 ppm and 55 ppm, respectively, whereas Ni, Co, As, and Cu are correlatively highly enriched with values commonly >1500 ppm (upper detection limit of XRF) reflecting the high modal proportion of copper-sulfides and Co,Ni-arsensulfides in the rock (Table 12.46). S content reaches up to 6 wt.% in massive ore, whereas the total amount of metals makes up more than 20 wt.% of the composition of the breccia.
Four specimens comprising of massive, both barren and mineralized, Fe-Mg clinoamphibole zones from the matrix of the breccia have been analyzed for major, trace and rare earth elements. Siderite in these zones is absent and the geochemistry reflects in addition to magnetite and sulfides, the chemistry of the Fe-Mg clinoamphibole. Thus, they are typified by high Fe2O3 (avg. 32.3 wt.%), MgO (avg. 15.1 wt.%), and SiO2 (avg. 50.1 wt.%) contents (Table 12.44). Mineralized samples are characterized by very high Au, Cu, Co, Ni, As, REE, Th, U, and S concentrations. In particular, one sample contains up to 247 ppb Au, whereas Cu, Co, As amounts exceed 1500 ppm, which is the upper detection limit of the XRF technique. The rock is typified by very low to negative Loss On Ignition values (LOI) which range between -0.3 and 1.4 wt.%, due to the low water contents and the high values of gain on ignition from the oxidation of Fe+2 to Fe+3 in Fe-Mg clinoamphiboles and magnetite.
85