Sulfarsenides and arsenides are found in almost all samples and include arsenopyrite, cobaltite, clinosafflorite and nickeline in decreasing order of abundance. Typically, arsenopyrite and cobaltite occur as clusters and aggregates of large prismatic grains within the sulfide matrix, whereas clinosafflorite and nickeline are characterized by exsolution textures.
6.2.1 Arsenopyrite
Arsenopyrite is generally idiomorphic; individual crystals are prismatic, and in cross section perpendicular to the elongation of the prism are rhombic in shape (Figure 6.3a). Single arsenopyrite crystals are found dispersed in the sulfide matrix or are concentrated in aggregates of numerous intergrown individuals. Single crystals commonly range in size between 0.1 and 0.5 mm, but may locally attain lengths up to 5 mm. It is white with a very weak yellow tint and is easily distinguishable by the high reflectance. Under crossed polarizers is strongly anisotropic having an array of interference colors from light to dark blue.
Both optically homogenous and distinctly zoned arsenopyrite grains are present. In backscattered electron (BSE) images most of the small arsenopyrite grains (<150 m) are optically homogenous and devoid of inclusions. Larger individuals however, present a well- developed, oscillatory zoning parallel to crystal faces, which is indicative of primary growth zoning. They display commonly a BSE-bright core that turns to fine, oscillatory zoning with
72 slightly increasing BSE-brightness in the outer parts of the crystals (Figure 6.3b and 6.4). The arsenopyrite crystals are often partly corroded and embayed at rims or showing extensive replacement and overgrowth textures by pyrrhotite and chalcopyrite. Composite grains of arsenopyrite and clinosafflorite are typified by the presence of native Bi, Au and Bi-Au-Ag-Te mineral inclusions (Section 6.3; Figure 6.11).
Figure 6.3 Backscattered electron images of (a) coarse, prismatic arsenopyrite grains enclosed in sulfide-gangue matrix, and (b) single arsenopyrite grains with embayed grain boundaries exhibiting the typical oscillatory zoning pattern observed in several samples.
Figure 6.4 (a) High-contrast backscattered electron image of a lath-shaped arsenopyrite displaying well developed, growth zoning. A BSE-bright core turns to fine, oscillatory zoning with progressively increasing BSE-brightness in the outer parts of the crystal. The dots (1-10) correspond to EMP analyses shown in Figure b, and (b) compositional variation diagram along traverse: darker areas correspond to iron-rich arsenopyrite, whereas lighter areas are Co- and Ni-rich (An. No. 79-88; Table 12.34).
EMP analyses along traverses show that the compositional heterogeneities in arsenopyrite are mostly related to the variation of the As/S atomic ratio and Fe, Co and Ni contents. The BSE-bright zones of the crystals have the highest As/S ratios and Co and Ni amounts, whereas the BSE-dark zones are characterized by higher Fe and S amounts (Figure 6.4b).
73 The proportions of As and Fe in the composition of the arsenopyrite vary widely within single crystals or between individual grains ranging from 31.4 to 41.9 at.% and from 20.6 to 33.0 at.%, respectively (Table 12.34). Co and Ni exhibit elevated contents up to 11.4 and 2.0 at.%, respectively. There is a high positive correlation between the proportion of Co (and Ni) and As, and respectively between Fe and S (Figure 6.5). Generally, the observed micro-scale variation in metal and As/S composition, describes a trend where Fe replaces the Co and Ni mixture.
Figure 6.5 Correlation between Co and
As in the arsenopyrite. Correlation
coefficient is 0.71 (n=158).
The linear trend in the composition of the arsenopyrite can be seen in the Fe-Co-Ni(AsS) ternary system, which includes also the minerals glaucodot, alloclasite, cobaltite and gersdorffite (boundaries after Petruk et al. 1971 and Gammon 1966; Figure 6.6). The composition of the arsenopyrite from the ore ranges from stoichiometric arsenopyrite up to nearly glaucodot composition. According to Gammon (1966) glaucodot contains a minimum of 9 wt. % Co.
6.2.2 Cobaltite
Individual cobaltite grains are found either disseminated in the sulfide matrix or rimming arsenopyrite grains. It forms coarse (0.2-2 mm), idiomorphic grains usually having a triangular, prismatic or cubic shape. Optically, cobaltite closely resembles in habit the arsenopyrite from which is hardly distinguishable. However, cobaltite is devoid of concentric zoning or mineral inclusions, which are fairly typical in arsenopyrite.
The As contents in cobaltite exhibit a limited variation from 31.2 to 34.6 at.% (Table 12.35). In contrast, Co contents vary widely from 17.7 to 28.7 at.%, whereas Fe and Ni range between 2.5 to 7.2 at.% and 2.2 to 8.9 at.%, respectively. The relative wide
74 elemental variations occur mainly between individual grains and rarely within single grains that are usually devoid of any zoning. Like arsenopyrite, the compositional variation in cobaltite describes linear trend in the Fe-Co-Ni(AsS) ternary system, where Fe replaces the Co and Ni mixture (Figure 6.6).
Figure 6.6 Composition of the analyzed arsensulfide (n=158) and cobaltite (n=29) grains on the NiAsS-CoAsS-FeAsS ternary diagram. Compositional fields are after Petruk et al. (1971) and Gammon (1966).
6.2.3 Clinosafflorite
Clinosafflorite occurs sporadically and always restricted to the interior of coarse, idiomorphic arsenopyrite grains. It forms small (10-100 m), irregular inclusions occupying the core of the arsenopyrite grain. Where present, it is always riddled with minute Bi-Au-Ag-Te-bearing minerals (Figure 6.11). Under plane polarized light it is pure white and can be distinguished from the host arsenopyrite by the higher reflectance and the lower relief.
Chemically, clinosafflorite is a cobalt-rich species of löllingite. The As amount varies slightly between 65.4 and 67.1 at.%, and S is constantly present with an average amount of 1.6 at.% (Table 12.36). It presents a limited variation in Fe, Co and Ni contents that average at 17.2, 11.2 and 4.0 at.%, respectively. The compositional heterogeneity resembles that of the sulfarsenides, i.e., lineal variation in the amounts of Fe, in respect to Ni and Co (Fe replaces Co+Ni). The total Co and Ni content in clinosafflorite decreases with increasing Fe content. The strong negative correlation between Fe and Co+Ni can be seen in the Fe-(Co+Ni) scattergram (Figure 6.7). The composition of clinosafflorite can be expressed in the Fe-Co-Ni(As2) ternary system of the diasenides, which includes the
75 minerals löllingite, safflorite, clinosafflorite, rammelsbergite and pararammelsbergite (Figure 6.8). Compositional fields used here are after Radcliffe and Berry (1968) and Petruk et al (1971).
Figure 6.7 Correlation between (Co+Ni) and Fe in clinosafflorite. Correlation coefficient is very high at 0.97 (n=26).
According to Roseboom (1963) and Radcliffe and Berry (1968) there is a continuous solid solution series between FeAs2 and CoAs2, where löllingite is close to pure FeAs2 (less than 3 mol% CoAs2) and safflorite-clinosafflorite composition extending from 3 to 100 mol% CoAs2, with up to 30 mol% NiAs2. Radcliffe and Berry (1968) classify safflorites into 5 groups (I-V), based on crystallographic and compositional data (Figure 6.8). All of the analyzed clinosafflorites fall in to the compositional field II of Radcliffe and Berry (1968).
Figure 6.8 Composition of analyzed clinosafflorite grains on the NiAs2-CoAs2-FeAs2 ternary diagram. Compositional fields are after Radcliffe and Berry (1968; n=26).
76
6.2.4 Nickeline
Nickeline appears to be largely restricted along the grain boundaries or fractures in coarse arsenopyrite grains. It exhibits exsolution textures forming micrometer scale (1-10 m) brush- or flame-like bodies (Figure 6.9). More rarely forms granular (10-20 m) exsolved inclusions within clinossaflorite. One analyzed nickeline grain has Ni amount at 38.4 wt.%, As at 43.0 wt.%, and shows high Fe and Co contents (9.5 wt.% and 2.2 wt.%, respectively; Table 12.39).
Figure 6.9 Backscattered image of flame- like nickeline developed in arsenopyrite along fractures and grain boundaries.