Minerographic study of the ore samples collected from Ingaldhal copper mines shows that the ores are largely massive, banded and disseminated in structure. Pyrite, arsenopyrite, cobaltite, sphalerite, pyrrhotite, galena and chalcopyrite as primary minerals and pyrrhotite, chalcopyrite, stromyerite, bornite, chalcocite and tennantite as secondary minerals are present. The ores exhibit inclusion, panidiomorphic, mutual boundary, exsolution and replacement textures. From the mineralogical assemblage and the textures it is inferred that the ore deposits were formed inthe temperature range of 350°–500° C

Naganna, C.

Pathan, A.M.

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MINEROGRAPHIC STUDY OF SOME OF THE S U L P H I D E  ORES FROM INGALDHAL, CHITALDURG DISTRICT, MYSORE STATE BY A. M. PATHAN AND C. ]~AGANNA (Department of Geology, Bangalore University, Bangalore) Received March 28, 1970 (Communicated by Dr. B. P. Radhakrishna, r.A.sc.) ABSTRACT Minerographic study of the ore samples collected from Ingaldhal copper mines shows that the ores are largely massive, banded and dis- seminated in structure. Pyrite, arsenopyrite, eobaltite, sphalerite, pyrrhotite, galena and chalcopyrite as primary minerals and pyrrhotite, chaleopyrite, stromyerite, bomite, chaleocite and tennantite as secon- dary minerals ate present. The ores exhibit inclusion, panidiomorphic, mutual boundary, exsolution and replacement textures. From the minera- logical assemblage and the textures it is inferred that the ore deposits were formed in the temperature rafige of 3500-500 ° C. INTRODUCTION OCCURRENCE of sulphide ores in Ingaldhal Oat. 14 ° 12', long. 76 ° 27')has been known for a long t i m e .  A brief account of the exploratory work done in this area is given by Radhakrishna (1967). Though sulphide ores occur in different parts of  Mysore State, a systematic study of  these has not  so far been attempted. As a part of  the minerographic study of  the sulphide ores of the Mysore State, the authors have completed the ore microscopic studies of  the sulphide ores collected from adit 2 and 3 of  Ingaldhal copper mines. The result of this study is reported in this paper. Mode of Occurrence.--The sulphide ores are seen confined to narrow brecciated and sheared zones of trap rocks with intercalations of  chert bands, Mineralisation in some places appears to move across the sheared zone increasing the thickness of  mineralised zone to a width of about a metre. Where the mineralised zone has greater width the adjoining trap rocks are also impregnated wi th  sulphide ore minerals. The sulphide ores occur as bands, veins and veinlets. n4 41 42 A . M .  PATHAN AND C. NAGANNA The mineralised zones occurring in schistose trap appear to swell and pinch giving an en-echelon pattern of ore distribution. The epidiorite trap ranges in colottr from ash-grey to green and varies in texture from coarse-grained to almost glassy. Description of ores under the microscope.--The ores from Ingaldhal can be grouped into four categories namely: the massive ore, the brec- ciated ore, the disseminated ore and the banded ore. The massive ore is made up mostty of chalcopyrite; some massive ores contain an assemblage of three to four minerals, namely chalcopyrit~, pyrrhotite, pyrite and sphalerite. The brecciated ore is differentiated from the massive ore by the presence of angular fragments of gangue minerals. The disseminated ore is largely made up of euhedral crystals of pyrites. In the banded ore allotriomorphic grains of pyrites occur as continuous streaks alternating with chert bands. The microscopic descriptions of the ore minerals are summarized in Table I. Textures.--Sulphides under study are characterised to a great extent by massive and banded structures. Pyrite and arsenopyrite show banded structure. Chalcopyrite and sphalerite show massive structure. Besides these, the presence of the follovfing textures have been established. Inclusion texture.--This texture is best shown by the inclusion of euhedral to rounded crystals of pyrite, arsenopyrite and cobaltite within chalcopyrite. Few anhedral grains of pyrrhotite are seen enclosed in sphalerite and also chalocpyrite (Plate V, Fig. 1 a, b). Panidiomorphic texture.--This texture is very well developed by pyrite and arsenopyrite. Even in hand-specimen these minerals show euhedrism. Cubes of pyrite range in size from 0-25 cm. to 1.3 cm. and arsenopyrite from one ram. to a few ram. These minerals are embedded in chert and trap rocks (Plate V, Fig. 2a,  b). Mutual boundary texture.--This texture is very well observed in chalcopyrite and pyrrhotite. Contacts of both the minerals are smooth and not ragged; protuberances of one mineral in another is very well seen. All these factors are evidences of simultaneous fotmation of two minerals producing mutual boundary texture (Plate V Fig. 3). Exsolution texture.--This texture is observed between sphalerite- chalcopyrite, sphalerite-pyrrhotite and between chalcopyrite-stromyerite. Description Sulphide Ores from Ingaldhal, Chitaldurg District TABLE I of individual minerals as identified under ore microscope 43 Mineral Pyrite Chalcopyrite Arsenop) rite Sphalerite Pyrrhotite Description White in colour and iso- tropic Textural descriptions Ye l low in eolour with slight cream coloura- tion. I t  is non-pleo- chrcic and twinning is sometimes observed. Anltotroplsm is weak to moderate. White in eolour; anlso- tropism weak. Grey in colour and iso- tropic Cream-coloured, non- pleochroic, e x h i b i t s  strong a~sotropism. Galena White in colour and iso- tropic. Cobaltite . .  Pinkish in colour and iso- tropic Stromyefite . . i  Pinkish-violet in colour. Reflection pleochroism is weak in pink colours. Anisotropism moderate. Chalcocite . .  Bluish in colour, anise- tropism moderate. Bornite . .  Pinkish-brown, isotropic. Tennantite (?) . .  Bluish-green, isotropic I Occurs in two forms. (i) Banded ] and (if) disseminated. Banded I pyrite is anhedral granular aggre- gate and disseminated pyrite is coarse and fine euhedral crystals. Some grains are seen enclosed in chaleopyrite and pyrrhotite with corroded outlines. Majority of the minerals aremassive and anhedral--also occurs as dis- creet grains, irregular in shape fill- ing the spaces between quartz. Occasionally it fills the openings in pyrite. I t  encloses p y r i t e ,  arsenopyrite and cobaltite. It is limited in occurrence, It either forms anhedral wedge-shaped crystal forming bands in chert and schist or  it occurs as massive aggregate of coarse subhedral- anhedral grains. The euhedral crystals are short and stumpy. At many places mineral shows frac- turing healed by quartz. Occurs as anhedral granular aggre- gate in chert with inclusions of pyrrhotite. Mineral occurs as massive anhedral granular aggregate in association with chalcopyrite and sphalerite. I t  shows numerous inclusions of angular fiagmentsofcountry rock. Occurs as anhedral grains with per- fect 3 sets of cleavages forming triangular pits. Occurs as inclusion in chalcopyrite, It is anhedral to euhedral with corroded borders. It occurs in chalcopyrite as wedges and lenses which show variation in size. Occurs as secondary mineral along fractures and grain boundaries re- placing chalcopyrlte. do. Occurs as secondary minerals replac- ing arsenopyrite I . . Micro- hardness in kg./mm3 Etch test 1023 153 977 Except HNOa negative to all other reagents. HN08 tarnishes v e r y  slowly, aquarogia stains brown. Negative to all other reagents. H N O ,  slow effervescence. P o s i t i v e  to HgCla. Negative to other re- agents. o *  228 HNO,  b rown,  J o  1 )20 70 , *  o *  I o  KOH brown and negative to other reagents. HC1 tarnishes, FeCls tarnishes and negative to other reagents, Negative to all reagents. HNOs stains brown, HCI fumes, FeCb stains irid, K O H  nega. tive, HgCl2 stains and aquaregia turns grey. l e  44 A . M .  PATHAN AND C. NAGANNA In the case of sphalerite-chalcopyrite combination string-like blebs or globules of chalcopyrite occur in sphalerite. These globules and blebs are very small in size and in some cases many parallel strings are noticed (Plate V, Fig. 4). Minute globules and small wedges of pyrrhotite are seen distributed or dispersed throughout sphalerite, but crystallographic direc- tion is not apparent. In chalcopyrite-stromyerite, stromyerite occurs in chalcopyrite as wedges and lenses. These wedges range in size from one rnm. to one era. Cgmm~nly stromyerite shows crenulated boundaries to chalcopyrite, but boundary between the two is continuous. These wedges and lenses become so wide that they can no longer be termed as lamellae. Replacement texture.--This texture is very well seen in almost all the minerals. In most of the cases replacement appears to have taken place along grain boundaries, cleavages, and original bedding planes. Some of the specimens particularly those containing pyrite show beautiful banding even in hand-specimen and the banding here resembles banding noticed in banded ferruginous cherts. The pyrite-chert banding is apparently caused by the sulfurisation of the iron minerals present in banded ferruginous cherts. Chalcopyrite is replaced by chalcocite and bornite along grain boundaries and fractures. The mineral tennantite appears to replace arseno- pyrite. PARAGENESIS AND ORIGIN OF THE SULPHIDE ORES From the study of the textures and the mutual relationship existing between the various ore minerals it is inferred that the minerals like pyrr- hotite, chalcopyrite, pyrite, cobaltite, sphalerite and arsenopyrite are primary, since they occur either as euhedral crystals with mutual boundary relations or occur as host minerals in which secondary minerals are housed. Chalco- pyrite and pyrrhotite in sphalerite and stromyerite in chalcopyrite, tennan- tite in arsenopyrite and bornite and chalcocite in chalcopyrite are secondary since they clearly show indications of either replacement or exsolution. Based on the textures the following paragenetic scheme for the sulphide ore minerals is proposed. ~ The presence of mineral assemblage like pyrite-arsenopyrite-pyrrhotite suggests that the primary minerals are folmed below 491 ° C. as this assemb- lage is considered to be unstable above this temperature (Clark, 1960). Further the presence of exsolution textures particularly sphalerite-chalco- Sulphide Ores from Ingaldhal, Chitaldurg District 45 pyrite and sphalerite-pyrrhotite indicate the temperature of formation to range from 350 ° to 500 ° C. (Edward, 1954). TABLE II Time Minerals Pyrite Arsenopyrite Cobaltite Sphalerite Pyrrhotite Galena Chalcopyrite Strornyerite Bornite Chalcocite Tennantite (?) Primary Secondary (in sphalerite) (in sphalerite) (in chalcopyrite) (in chalcopyrite) (in chalcopyrite) (in arsenopyrite) Bagchi (1970) has suggested sedimentary-syngenetic origin to pyrrhotite and pyrite recording the presence of slump and load-cast structures. He has proposed hydrothermal origin for the other sulphide ore minerals. The authors have also noted the presence of structures indicative of sedi- mentary origin. The sulphide ores particularly pyrite occurs in all the lithofacies--that is they are seen in chert bands, in trap rocks and in schistose rocks as bands, veins and veinlets. It is difficult to explain how through a sedimentary-syngenetic origin this mineral can occur in such disconnected manner in various lithotypes. The sedimentary structures like banding exhibited by pyritic cherts in this area can be explained through the process of " sulfurization" of the banded ferruginous cherts, where hydrothermal fluids containing H~S or divalent sulphur can convert ferrous minerals into pyrite without altering the original texture or structures. Lovering (1963) has indicated that minerals formed through such a process are not to be considered as syngenetic in the strict sense of the teim. He has suggested the name " diplogenetic" for such ores of dual origin. The authors are therefore inclined to believe that the entire sulphide ore minera- lisation in Ingaldhal area hasbeen brought about by hydrothermal processes, the pyrite ores exhibiting the sedimentary features being diplogenetic, 46 A . M .  PATHAN AND C. NAGANNA ACKNOWLEDGEMENT The authors sincerely thank Dr. B. P. Radhakrishna, Director, Depart- ment of Mines and Geology, for the encouragement he has given to take up this work and for giving permission to collect the samples in Ingaldhal copper mines. One of the authors (A. M.P.) thanks the C.S.I.R. for providing a Fellowship to undertake this research. 1. Bagchi 2. Brett, R. 3. Cheney, E. S. and Lange, I . M .  4. Clark, L. A. 5. Edward, A. B. 6. Lovering, T. S. 7. Radhakrishna, B. P. REFERENCES O t 0 Q Abstract, 57th Session, Ind. ScL Cong., 1970. "Experimental data from the system on Cu-Fe-S and their bearing on exsolution in ores," Eco. Geol., 1964, 59, 1241-69. Evidence of Sulphurization and Origin of Some of  Sudbury. Type Ores, Min. Dept., 1967, pp. 80-94. "The Fe-As-S system: phase relation and application," Eco. Geol., 1960, pp. 1345-81. Textures of Ore Minerals and Their Significance. Aust. Inst. Min. Met.., Melbourne, 1954. "Epigenetic, diplogenetic, syngenetic and lithogenetic deposits," Eco. Geol., 1963, pp. 315-31. .. Copper in Mysore, 1967, 2076-70. Printed at The Bangs.lore Press, Bangalore-18, by V. J. F. Jesudason, L.P.T., Superintendent. Published by B. S. Venkatachar, Editor, "Proceedings of the Iadiaa Academy of 8¢ieaces", Baagalor¢ A. M. Pathan and C. Naganna Proc. Ind. Acad. Sci., B, Vol. LXXII, Pl. V FIG. 1 a. Inclusion Texture: Rounded grains of cobaltite in chalcopyrite, ×80. FI6. 1 b. Inclusion Texture: Rounded pyrite grain in chalcopyfite, ×80. FI~. 2 a. Panidiomorphic Texture: Euhedral and subhedral crystals of pyrite in chert, ×50. FI~. 2 b. Panidiomorphic Texture: Euhedral and subhedral crystals of arsenopyrite in chert, ×80. FIo. 3. Mutual Boundary Texture: Chalcopyrite (dark grey) and pyrrhotite (light grey) showing mutual boundary texture, × 80. FtG. 4. Exsolution Texture: Globules and strings of chalcopyrite (light grey) in sphaierite (dark grey), x 1,260. 

Minerographic study of some of the sulphide ores from ingaldhal, chitaldurg district, mysore state

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Minerographic study of some of the sulphide ores from ingaldhal, chitaldurg district, mysore state

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