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Mineralogy & Texture of the Askot Sulfides

Mineralogical and textural studies of the sulfides are based on the microscopic examination of 120 polished sections in reflected light. Replacement is the dominant process in the formation of sulfide deposits of Askote. Replacement has affected not only the minerals of the host rock, but also the earlier formed ore and gangue minerals. The resulting textures are varied. Only a few minerals, chiefly the hard and early formed ones, e.g. pyrite and arsenopyrite have developed well defined crystal outlines. Vein replacements are quite common in hard minerals like quartz and earlier formed pyrite, magnetite and arsenopyrite. These minerals, shattered by small movements contemporaneous with mineralization, are commonly invaded along the fractures by younger minerals. The invading mineral penetrates the host laterally from the fractures into the mineral. In advanced stages of vein replacement, only ragged residuals of the invaded mineral remain.

Mineralogy of the sulfides is complex - there being a considerable variation in the quantities, form, and textures of the individual minerals. Two phases of mineralization are recognised. The first generation sulfides, viz., pyrite, arsenopyrite, sphalerite, galena and chalcopyrite, occur in minor amounts and are replaced by the second generation arsenopyrite, sphalerite, galena, chalcopyrite, cubanite, gudmundite, and pyrrhotite. In the massive sulfide zone all the sulfide minerals are intimately intergrown whereas in the disseminated zone, only pyrite, arsenopyrite, sphalerite, galena and chalcopyrite are found.

1. Arsenopyrite
Arsenopyrite commonly occurs as euhedral to subhedral crystals with a characteristic rhomb shape. Cataclastic texture is commonly observed. The mineral is replaced by the younger sulfides along fractures and grain boundaries. Growth twins with irregular twin boundaries are very commonly observed (fig. 1).

Fig 1: Twinned crystal of arsenopyrite (lower left corner).  Note that the twin boundary is irregular and hence the twin is a product of the primary growth of the crystal. (+Nic, 250X, imm.)

 The crystals vary in size from tiny specks to more than 20 mm in length. The size of crystals is largest in the centre of orebody and gradually decreases towards the fringe. The mineral shows a tendency towards euhedral crystallization regardless of its age relations. Its paragenetic sequence is determined on the basis of replacement along fractures. Acicular shape of arsenopyrite is indicative of selective rapid growth in one direction indicating the direction of flow of mineralizing solutions.

Porphyritic texture is exhibited by arsenopyrite. The growth of large crystals of arsenopyrite is due to their enhanced force of crystallization. 

Fig 2: Intergrowths of arsenopyrite (white) and pyrrhotite (grey).  Such intergrowths are interpreted to be products of decomposition of early formed Fe- and As-rich fahlores. (+Nic, 250X, imm.).

Intergrowths of arsenopyrite with pyrrhotite (fig. 2) are interpreted to be decomposition products of the early formed Fe- and As-rich fahlores (tenantite).

2. Sphalerite
Sphalerite occurs as irregular and anhedral masses ranging in size from tiny specks to crystals more than 5 mm across. The mineral is intimately intergrown with pyrite, galena, chalcopyrite, and pyrrhotite. The early formed sphalerite is often fractured and is replaced by the later sulfides. Cellular island shaped forms of sphalerite in chalcopyrite and galena that are in contact with sphalerite are evidence of significant replacement of sphalerite by these minerals. Penetration of galena and chalcopyrite along grain boundaries and fractures in sphalerite are quite common. The replacement of sphalerite by chalcopyrite has also given rise to shredded textures. 

Fig 3: Tiny islands of sphalerite (med. grey) in chalcopyrite (white) adjacent to bigger crystals of sphalerite are evidence of peripheral replacement of the latter. (125X).

Graphic textures resulting from the replacement of sphalerite by chalcopyrite and galena are rare. Tiny islands of sphalerite in chalcopyrite and galena adjacent to large sphalerite grains are evidence of perepheral replacement of the latter (fig. 3). The distribution of the sphalerite islands are indicative of the shape and size of the original sphalerite grain.

Inclusions of galena and chalcopyrite of first generation in sphalerite of second generation are quite common. Phenocrysts (the term merely indicates large mineral grains in the midst of a fine grained surrounding) of sphalerite are generally to be seen. More commonly, however, it occurs as fine grained undeformed aggregates.

Fig 4: Beautifully developed sphalerite stars in chalcopyrite.  The sphalerite has exsolved along the (111) direction of chalcopyrite. (400X, imm.).

 Small inclusions of dust like and especially star shaped particles and crosses of sphalerite are contained in chalcopyrite. These are generally interpreted as products of exsolution. The sphalerite stars are generally branched (fig. 4) following the cleavage planes of chalcopyrite.

3. Galena
Galena occurs as anhedral to subhedral masses showing perfect cleavage visible as triangular pits. Inclusions of first generation galena in second generation sphalerite, and phenocrysts of second generation galena in a matrix of fine grained chalcopyrite are quite common. It also occurs as fine grained aggregates. The grain size of galena increases from the Centre of the orebody towards the outer zone of massive sulfide mineralisation and then again decreases in the zone of disseminated sulfides. Interfingering texture with sphalerite and chalcopyrite are also to be seen. Cellular island shaped relics of first generation galena in second generation sphalerite are frequently encountered. 

Fig 5: Blebs of bismuth (white) in later formed sulfide minerals.  Such inclusions are interpreted as trapped droplets of bismuth in the host. (200X).

Second generation galena penetrates sphalerite along fractures and also replaces the latter along grain boundaries. Graphic textures resulting from the replacement of galena by other minerals are rare. Spheroidal texture exhibited by inclusions of bismuth in second generation galena (fig. 5) are interpreted as inclusions of molten bismuth in a growing crystal of galena. Bismuth is molten above 2800^o C.

Argentite occurs as tiny cubic or triangular crystals about 5-10 microns in diameter in galena. These are interpreted to be products of exsolution. The tiny argentite crystals are generally to be found at the periphery of the galena crystal or outside it (fig. 6), indicating thereby that they have been expelled from the galena lattice by diffusion. 

Fig 6: Tiny crystals of argentite (white) in galena (light grey).  The argentite is migrating towards the grain boundaries of galena. (250X, imm.).

Argentite can make up as much as 10 percent of the mixed crystal with galena at high temperatures. Below a temperature of 179^o C the symmetry of argentite, which is cubic at high temperature with a galena structure, inverts to a monoclinic symmetry (Ramdohr, 1969). Hence, the expulsion of exsolved argentite is logical in view of the great difference between the symmetries of the two minerals at lower temperatures. Exsolution segregation at grain boundaries has frequently been reported (Ramdohr, 1969). In very fine grained aggregates, and with great velocity of migration and very slow cooling, all unmixed substances can migrate outwards, and therefore the diagnostic texture can disappear entirely.

4. Chalcopyrite
Chalcopyrite occurs as aggregates of medium to coarse grained anhedral crystals. The first generation chalcopyrite occurs as inclusions in galena and sphalerite.

Fig 7: Oriented intergrowths of chalcopyrite (light grey) and pyrrhotite (white and dark grey) are products of exsolution of a former high temperature chalcopyrite (chalcopyrrhotite). (+Nic, 250X, imm.)

 Oriented intergrowths of chalcopyrite and pyrrhotite (fig. 7) are interpreted as a decomposition product of a high temperature chalcopyrite (chalcopyrrhotite). Second generation chalcopyrite is one of the most abundant sulfide minerals and replaces almost all other sulfides. Xenoblats of chalcopyrite are quite common, and the mineral occasionally forms interfingering textures with galena and sphalerite. Cellular island shaped forms of first generation chalcopyrite are quite common in later minerals. Pseudoeutectic intergrowths between idiomorphic to subidiomorphic gudmundite and chalcopyrite are also common (fig. 7). First generation chalcopyrite penetrates into first generation sphalerite and galena, replacing these minerals along fractures. Shredded textures are produced by the replacement of sphalerite by chalcopyrite. Graphic textures resulting from the replacement of earlier minerals by chalcopyrite are rare.

Fig 8: Exsolution lamellae of cubanite (dark grey) oriented along (111) planes of chalcopyrite (light grey). (175X, imm.).

Cubanite lamellae occur as exsolution bodies in second generation chalcopyrite. The lamellae are oriented parallel to the crystallographic planes (111) (fig. 8). Small inclusions of dust-like and especially star-shaped particles and crosses of sphalerite are contained in chalcopyrite (fig. 4). The sphalerite stars are generally branched, following the cleavage planes of the host. They are generally elongated in the (111) planes of chalcopyrite. These star-shaped skeletal crystals of sphalerite are interpreted to be products of exsolution. At high temperatures chalcopyrite can dissolve up o 17 percent sphalerite, whereas chalcopyrite and cubanite are miscible in all proportions. Pyrrhotite is often found in the form of Stringers and plates in chalcopyrite (fig. 7). In some cases the pyrrhotite stringers may be the decomposition products of exsolved cubanite lamellae in chalcopyrite.

Spindle-shaped inversion twins are quite common in second generation chalcopyrite (fig. 9). The boundaries of the twin lamellae are not parallel all over the mineral grain. 

Fig 9: Spindle shaped inversion twins in a former high temperature chalcopyrite. (+Nic, 175X, imm.).

They commonly form intergrowth networks and are hardly accompanied by strain and translation. These twins are irregular in shape and are strongly interwoven and unevenly distributed. Such twins are termed inversion twins (Ramdohr, 1969) and are the result of cooling of high temperature minerals.

5. Cubanite
Cubanite occurs in intimate association with chalcopyrite as medium sized subhedral to anhedral grains. Phenocrysts of cubanite in a matrix of fine-grained chalcopyrite are common. Replacement of cubanite along grain boundaries by chalcopyrite is fairly widespread. Growth twins are quite common in cubanite. Considerable amounts of cubanite occur as exsolution lamellae in chalcopyrite (fig. 8). The individual lamellae have varying thickness, often pinching and swelling. In unmixed crystals, chalcopyrite is generally in excess; in some instances, chalcopyrite and cubanite occur in equal proportion. The mineral also occurs in myrmekite like eutectoid intergrowths with fine vermicular gudmundite. Graphic replacement texture, resulting from replacement of gudmundite by cubanite, which encloses tiny crystals of gudmundite having the same optical orientation, are occasionally seen. Pyrrhotite stringers are occasionally seen in cubanite and are interpreted as exsolution products. Shreds of gudmundite with concave replacement relicts in cubanite are often seen and are considered a product of replacement.

Fig 10: Magnetite (dark gray) replaced by second generation sphalerite (light gray) and chalcopyrite (white) along-grain boundaries and fractures. (175X).

6. Gudmundite
Gudmundite occurs as aggregates of tiny euhedral crystals. The mineral is seen replaced by cubanite and chalcopyrite along grain boundaries. Clusters of tiny crystals of gudmundite occur in a matrix of cubanite and chalcopyrite (fig. 7). The gudmundite crystals show an optical continuity, indicating a replacement by cubanite and chalcopyrite along fractures. Gudmundite also occurs in fine vermicular myrmekite-like intergrowths with cubanite. The two minerals are present in comparable amounts. The grain boundaries are mutually rounded. In some cases the myrmekitic intergrowth grades into lamellar intergrowth where both components show a uniform optical orientation throughout the grain. Some gudmundite has also formed as tiny idioblastic grains by decay of fahlore. Besides gudmundite, the products of such decay are chalcopyrite, pyrrhotite and arsenopyrite. Seetharam (1981) has also reported the occurrence of gudmundite as a substitution product in cubanite and as a reaction rim around breithauptite (?).

Graphic intergrowths of gudmundite and cubanite are quite common. Serrate boundaries of gudmundite, tongues of cubanite penetrating into gudmundite, and transition from the graphic association to the unquestioned peripheral replacement indicate that such graphic intergrowths are products of replacement of gudmundite by cubanite, rather than simultaneous precipitation.

7. Pyrite
Pyrite occurs as euhedral cubes and pyritohedra. The mineral shows a tendency to euhedral crystallization regardless of its age relations. Pyrite is very abundant in the zone of disseminated sulfides. Its paragenetic sequence is determined on the basis of replacement. In the zone of massive sulfides, pyrite is replaced by the second generation sulfides along grain boundaries and fractures.

8. Pyrrhotite
Pyrrhotite usually occurs as granular masses of anhedral grains and in veins replacing second generation chalcopyrite along fractures. It occurs as exsolution lamellae oriented along the crystallographic directions in chalcopyrite (fig. 7) and cubanite. In chalcopyrite, it is sometimes a decomposition product of exsolved cubanite. Pyrrhotite also occurs in minor amounts in association with gudmundite and is a product of decomposition of fahlore. Intergrowths of arsenopyrite and pyrrhotite (fig. 2) are interpreted to be decomposition products of early formed Fe- and As-rich fahlores.

9. Bismuth
Bismuth occurs as blebs and droplets in second generation galena and chalcopyrite (fig. 5) and in thin veins in the gangue minerals. These inclusions are interpreted as trapped droplets of molten bismuth. Bismuth is molten above 280o C. Spindle-shaped and lance-like twining is quite common in bismuth. The twin boundaries are not parallel all over the grain.

10. Other Minerals
Magnetite occurs as medium sized euhedral crystals in minor amounts. It is replaced by second generation sphalerite and chalcopyrite along grain boundaries (fig. 10). Marcasite occurs in minute quantities as tiny subhedral crystals. Bornite and chalcocite occur as small irregular polycrystalline aggregates in second generation chalcopyrite. Tiny crystals of argentite occur as inclusions in galena (fig. 6), and have commonly migrated to the periphery of the galena grain by diffusion. The argentite is considered a product of exsolution.

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