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Chapter 5

MINERALISATION

Chapter 5 Mineralisation

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5.1 Gold mineralisation in Dharwar Craton

Gold mineralization is associated with a major event of accretion of volcano-

sedimentary basins during a convergent tectonic regime in Dharwar Craton (Fig. 5.1).

More than 100 occurrences, prospects and depoists of gold are identified in

Neoarchean greenstone belts of of Dharwar Craton spread over 320,000 sq kms in

parts of Karnataka, Andhara Pradesh, Goa, Tamil Nadu and Kearala states. The

Craton comprises a Foreland with its volcano-sedimentary basins classified under

Dharwar Supergroup and their basement Peninsular Gneiss containing older

Supracrustals rocks in the western half of the Craton and, an Accretionary complex

dominated by polyphase calc-alkaline plutonic rocks interspersed with relics of intra-

arc basins (schist belts) in the eastern half (Chadwick et al. 1996). Kolar, Hutti,

Ramagiri, Jonnagiri, Gadwal, Raichur, Kushtagi-Hungund and Sandur belts form part

of the Accretionary complex.

The Foreland basins are characterized by shallow marine or fluvial orthoquartzites,

bimodal basaltic and rhyolitic suites, banded iron formation, polymict conglomerates,

greywackes and limestone. The schist belts represent Intra-arc and marginal basins,

which are also of mixed mode type with dominant bimodal volcanics, subordinate

sedimentary suites comprising greywackes, psammopelitic sediments and polymict

conglomerates. The geochemical characteristics of the metabasalt of the greenstone

belts support the Island arc tectonic setting of schist belts (Table 5.1). Gold and

sulphide gold lodes in the Dharwar Craton are hosted by metabasalt (i.e. Kolar,

Ramagiri, Hutti belts, Bellara in Chitradurga belt and Hosur in Gadag schist belt),

metagabbro (Manigatta Sygattur in North Kolar belt), metaultramafites (e.g.

Kempinkote in Nuggihalli belt), meta psammopelites and associated polymict

conglomerates (Chigargunta, Maharajagadai and Surapalli in Kolar belt), banded iron

hirenagnur in Hutti belt), contact of metabasalt and graywckes/phyllite (Sangli,

Attikatti, Kabulyatkatti in Gadag belt, C.K. Halli, G.R. Halli in Chitradurga belt),

contact of metabasalt and felsic volcanic rock (Hira-Buddini in Hutti belt) and

granodiorites and K-rich granites (Dona East and South Blocks in Jonnagiri belt,

Yatkal in Hutti belt, Boksampalli in Ramagiri belt, Honnemardi in Chitradurga belt,

margins of Hagari and Kushatagi belts).


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Fig. 5.1 Geology of Dharwar Craton showing Gold occurrences (yellow circles)

within the respective schist belts (green colour), (map adapted from Field Guide to

Selected gold prospects in Karnataka and Andhra Pradesh, Vasudev.V.N, GSI,2009).

As in other Cratons of the world the most common style of mineralization in the

Dharwar greenstone belts is structurally controlled, epigenetic, ‘orogenic’ or ‘lode

gold’ style most commonly associated with quartz ± carbonate veining and

sulphidation of high strain zones or certain litho units such as BIFs or carbonaceous

phyllites.


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Gold mineralization is defined by schistocity parallel as well as discordant fracture

controlled quartz-carbonate veins, folded in places. The timing of the event of gold

mineralization in the Dharwar Craton is placed at ~2,547 Ma (Srinivasa Sarma et. al.,

2009) and is relatable to the timing of emplacement of voluminous granitoids in the

eastern Dharwar Craton. The HT assemblages were variably retrogressed along high

strain zones to green schist facies assemblages as a result of hydrothermal activity.

These gold fields are located in the Neoarchaean Greenstone belts of the Dharwar

Craton.

The Gadag schist belt is composed dominantly of metabasalt in the western half of the

belt and sediments in the eastern half (Fig. 5.2). The sediments include greywacke,

chlorite actinolite schists, BIFs and quartz sericite schists. Felsic volcanics and thick

units of conglomerates occur in the southern part of the known gold field. Gold

mineralization within the belt is associated with a number of prominent shear zones,

striking NNW-SSE but with important deviations at places. The western group is

hosted in metabasalt whereas the middle block is mostly confined to metasedimentary

rocks. The eastern group is also hosted by the greywacke suite of rocks. In addition,

gold mineralisation is also known from the many units of BIF and their contact with

tuffaceous rocks. All known mineralisation are structurally controlled vein systems.

In the Nagavi study area, there is a strong and highly significant correlation between

gold values and the abundance of ore related pyrite. In general the extended

sulphidation process is more important during mineralisation. One likely process

involves mixing of an invading, mineralizing fluid containing sulfur and gold with a

wall rock fluid containing iron derived from adjacent rocks. Recognition of the

source(s) for this iron and fluid flow pathways responsible for introducing it to the ore

zone could provide useful guidance in exploration.


105

\

Fig. 5.2 Regional Geology map of Gadag schist belt

5.2 Gold mineralisation in Gadag Schist Belt

Gold mineralization within the belt is associated with a number of prominent

auriferous shear zones, striking NNW-SSE but with important deviations at places.

The known gold prospects could be classified into three groups viz.,

a) The western group comprising the Hosur-Champion, Yelisirur and Venkatapur

Prospects

b) The middle group comprising Kabulayatkatti-Attikatti, Mysore Mine-Sangli

Mine.

c) The eastern group comprising Sankatadodak Zone.


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The western group is hosted in metabasalts whereas the middle block is mostly

confined to the metasedimentary rocks. The eastern group is also hosted by the

greywacke suite of rocks. In addition, gold mineralisation is also known from the

many units of BIF and their contact with tuffaceous rocks. All known mineralisation

are structurally controlled vein systems (Curtis and Radhakrishna, 1993; Sawkar R. H.

and Shankarappa, 2007).

Sangli mine block is situated 24km SSE of Gadag city and south of the Mysore mine.

It was worked by Sangli Gold Mining Co. Ltd. The main development was carried out

on three parallel reefs by two shafts and several adits. Considerable development was

carried out to a depth of 152m at various levels. No production details are available.

More recent work has revealed that the mineralisation occurs in three sub-parallel

reefs along the sheared contact between the western metabasalt and the eastern

greywacke. The three mineralised zones are designated as Western Lode, Temple East

lode and New East Lode. They extend for a strike length of 2100m, 820m and 680m

respectively, of which the Temple Lode has proved the most promising. The first two

mineralised zones are in fine grained banded sediments. The gold bearing vein quartz

in the Western Lode forms stock work style mineralization. As indicated above the

Sangli prospect has an open pit reserve of 1.8 million tonnes of ore of an average

grade of 2.8 g/t gold and an underground resource of 2.2 million tonnes of 3.7 g/t

grade (V.N. Vasudev 2009).

5.3 Gold mineralisation in Nagavi area

Nagavi area consists of different litho-units like BIFs, Metabasalt, Shale, Schist,

Argillites and Quartz. BIFs are the prominent linear bands, trending approximately

NW-SE direction (Fig. 5.3) and dipping towards NE. These BIFs are located on top of

the hill mound and these bands are dissected along 5km length within study area and

further and width varies from 0.5 to 15m. The metabasalt situated on both sides of the

hill (small mound) and quartz veins are cross cut across the various lithounits. The

Nagavi area BIFs comprise of Banded Hematite Quartzite (BHQ), Banded Chert

Quartzite (BCQ), Banded Magnetite Quartzite (BMQ) with the general trend of BIF

being N 250 W and S 25

0 E.


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Table 5.1 Gold occurrences in Karnataka

Dharwar Shimoga

Belt Nuggihalli Belt Chitradurga Belt

c. Ajjanahalli-

Bellara Tract

Karajgi Yelavari-Gollarhalli a. Gadag region Bodimardi/Iplara

Chinmulgund Jalagaranhalli Hosur-Shirunj Dindivara

Kudrekonda Aladahalli-

Honnenahalli

Yelisirur-

Venkatapura Anesdri

Palavanahalli Tagadur Nagavi-Beladadi-

Nabhapur Javanahalli

Chorana Edehalli Kempinkote Kabulayatkatti-

Attikatti Ramenahalli

Honnuhatti-hosur-

Tambadhihalli

Mysore and Sangli

Mines Ajjanahalli

Jalagaragundi Holenarasipur Belt Bellara

Shiddarhalli Anekere -Kallenhalli b. Chitradurga

Sulphide zone Honnebagi

Nandi Konganhosur

Hanni-Bukkambudi K.R.Pet Belt Honnemaradi Nagamangala

Tract

Honnayakanahalli Pura-Bellibetta-

Katargatti Chikkannanahalli Kalinganahalli

Karthikere-Kalasapura G.R.Halli Nagamangala

Devrukal Kushtagi Belt Gonnur-Kotemardi Hunjanakere-

Naranhalli Madakeripura Tittanamangala

Sandur Belt Kilarhatti Kunchinganhal

Lingadahalli Ingaldhal Hutti Belt

South of Vibhutigudda Mangalur Belt Kallehadlu Hutti Gold Mine

Mangalur Gold Mine Halekallu Virapur-Yathkal

Tuppadur

Deodurga Belt Buddini

Chikhonnakuni Southern High

Grade Migmatic

Terrain

Maski

Nadapanhalli Sanbal

Kolar Belt Karimadanahalli Ramaldinni

Manighatta Sonhalli(Honhalli) Udbal

Patna-Tambahalli Woolagiri (Volagere) Kadoni

Betrayanbetta-

Kamandahalli Amble Uti

K.G.F western Lode Wandali

K.G.F Champion Reef Chinchergi

Mallappanakonda Hira Buddini

Bullapur


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The gold mineralization in the Nagavi is associated with a deformed iron formation

hosted in a polydeformed paragenesis sequence. The gold mineralization is

characterized by strong sulphide mineralization, silica and auriferous quartz veins

developed within a sulphidic chert unit. The gold mineralization is epigenetic in

nature but strata bound because it is confined to the cherty iron formation. Fractures

are filled with remobilized silica or quartz carbonate veinlets. The amount of gold is

directly proportional to the amount of sulphides. Mineralization dominantly occurs as

dissemination with small amount in fine veinlets.

The source of Iron in BIF could be due to submarine volcanism and associated

hydrothermal circulation by exhalite model proposed by Gross (1965, 1960). The gold

has close association with pyrrhotite suggesting that mineralisation occurs under

conditions of relatively low sulphur activity. Gold is associated with sulphide

commonly occurs as inclusions in pyrite grains along boundaries of pyrrhotite and

gangue minerals as micro-crack fillings in pyrite or as discrete grains.

The sulphide oxide replacement textures are observed in the ore zones, the occurrence

of gold mineralisation. The gold values varies from <25 ppb to 0.04 ppb (Table 5. 2).

All samples were analysed for Au by GTA - AAS method. The BIF samples were

collected from different location over a width of 0.2 to 0.3m with intrusion of shear

quartz vein. NNB 14 and NNB 15 samples collected from shear quartz vein with BIF

contact. This vein quartz emplacement is showing a brownish colour due to oxidation

of sulphides, the quartz vein shows prominent pinching and swelling structures.

NNB 16 and NNB 27 samples showing gold values of <25 and these samples

collected in old working trail pit (trail pit 1 and pit 2) at different places in the contact

of auriferous shear zones. NNB 17 and NNB 24 samples collected from old working

trench. BIF hosted gold mineralisation forms an integral part of many Archaean

greenstone belts in countries such as Australia (Vielreicher et al. 1994), Brazil

(Morro Velho; Ladeira 1991), Canada (Lupin; Lhotka and Nesbitt 1989), South

Africa (Fumani; Pretorius et al. 1988) and Zimbabwe (Vubachikwe; Saager et al.

1987).


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Fig. 5.3 Geological and mineralisation map of Nagavi

The varying styles of gold mineralisation in BIF have led to the proposition of three

different genetic models, namely, syngenetic, epigenetic and a hybrid of both the

epigenetic and syngenetic (multistage) models for its deposition (Fripp 1976; Phillips

et al. 1984; Groves et al. 1987; Saager et al. 1987. The syngenetic model proposes

that sulphide minerals, carbonate minerals, chert and gold were deposited from the


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hydrothermal fluids on the seafloor during chemical sedimentation (Kishida and

Kerrich 1987).

Table 5.2 Details of analytical results of BIF hosted shear zone samples from

Nagavi study area

Sl.

No.

Sample

No.

* Au by GTA-

AAS (in ppb) Sample description

1 NNB 14 <25

Shear quartz vein zone with

BIF contact

2 NNB 15 <25 Shear quartz vein on top of hill

3 NNB 16 <25

Auriferous shear zone (trail Pit

No.1)

4 NNB 17 <25 Ancient working trench

5 NNB 24 <25 Ancient working trench

6 NSB 27 <25

Shear auriferous zone (trail Pit

No.2)

7 NNB

1343 0.04

Pale greenish Metabasic

carbonated sample from pit

*GTA-AAS testing carried out by GSI Lab, Chemical Division, Bangalore

Gold mineralisation in these settings is discussed as of epigenetic (Phillips et al. 1984;

Browning et al. 1987) versus syngenetic origin (Fripp, 1976). The mineralisation is

clearly epigenetic because: 1) it occurs in a zone of deformed rock associated with a

large regional shear-zone structure; 2) it is restricted to zones of intense fracturing

with discordant quartz– sulphide veins; 3) textural evidence indicates replacement of

primary magnetite by pyrite.

The auriferous veins are made up of fine grained milky quartz veins, which display

granoblastic textures. Moderate to strong undulate extinction is also evident together

with deformation lamellae. Quartz grains form polygonal aggregates of variable size,

which show no lattice-preferred orientation. These gold bearing quartz veins also

contain a distinct suite of sulphides and less commonly, chlorite. Pyrite is the

dominant sulphide. It occurs mainly as a replacement of magnetite (±hematite) in the

wall rock immediately adjacent to the quartz veins. It displays sutured or irregular

surfaces in contact with magnetite bands but subhedral to euhedral shape in contact

with quartz layers.


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5.4 Ore Microscopic Study

Microscopic study of the polished specimens of sulphides bearing carbonated sheared

anorthosite samples shows sulfide assemblages of pyrite and arsenopyrite. They show

mutual boundary, replacement, inclusion, panidiomorphic and exsolution textures.

Some grains show dotted appearance due to exsolved blebs of chalcopyrite.

Chalcopyrite shows brass yellow colour and occurs as blebs in sphalerite and as

growth crystals in arsenopyrite and as replacement in pyrrhotite.

The carbonated sheared anorthosite rock exposure near Mallasamudra and the rock

sample (sample no. 10) has been examined under megascopic and it shows compact

and light greenish grey in colour. The rock is very fine to medium grained and

inequigranular. The mineral assemblages of simple to polysynthetically twinned

subhedral (100-200 by 400-600 μm) laths (~40-45%) set in a groundmass of irregular

anhedral grains of secondary carbonate (~35-40%), chlorite (~8-10%), opaque (~5-

7%) and accessory epidote and quartz. Plagioclase is slightly sausuaritised. Carbonate

appears to be after plagioclase. Carbonate also occupies fracture in plagioclase and

rarely forms bands parallel to tectonic foliation. Opaque mostly occurs as euhedral to

subhedral, square to rhomb shaped 100 by 100 to 150 by 250 μm sized randomly

distributed discrete grains. Small sized aggregates of opaque occurring parallel to the

tectonic foliations in association with chlorite are also noticed. Opaque are mostly

magnetite and rarely pyrite. Tectonic foliation is defined by presence of recrystallized

feldspar, bent plagioclase lamellae and tapering deformation twins in plagioclase

(Plate 5.1 and Plate 5.2).

Pyrite is the dominant mineral and it occurs as individual or aggregates of subhedral

to all otriomorphic grains. It is fine to medium grained and creamy to whitish yellow

coloured. It also occurs as inclusions in the pyrite grains. It shows strong yellowish to

greyish anisotropism and brownish cream to reddish brown bi-reflectance. Pyrite

occurs as perfect cubes (panidiomorphic) with slightly corroded margins and as

aggregates. It shows yellowish white reflectance. It also occurs as fine to coarse

grained, euhedral to subhedral crystals and as fracture fillings along with pyrrhotite

and arsenopyrite. Thus, pyrrhotite (FeS) is the first sulfide to appear from which other

sulfides-pyrite (FeS2) and arsenopyrite (AsFeS) are formed.


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Plate 5.1 Microphotograph exhibiting chlorite and pyrite (as opaque)

Plate 5.2 Microphotograph exhibiting secondary carbonates with Pyrites

The greenish color shows the chlorite and opaque / black color indicating the pyrites.

These pyrites are in cubic shape and irregular share at places.


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5.5 Wall rock alterations and sulphidation

The prominent wall rock alterations were observed in the study area. The

chloritisation and sericitisation occurs at places near Nagavi and Mallasamudra area

and carbonization and silicification is also observed. Based on the field observations

of petrography and analytical data, the gold mineralization in Nagavi area is of BIFs

hosted auriferous shear zones. The gold mineralization appears to be closely

associated with BIFs. The intensity of mineralization is not strong enough to consider

this as a commercial gold mineralisation. However, further detailed study is essential

to identify the auriferous mineralized shear zones characteristics. Chloritisation,

sericitisation, carbonatisation and silicification suggests that the hydrothermal

alterations in the ancient workings as seen in the pit. Good amount of sulfides in the

form of pyrite are seen metabasalt. The auriferous shear zones are clearly showing

gold mineralisation.

The banded iron formation (BIFs) hosted gold mineralisation with quartz vein

permeations which contain sulfides such as pyrite, pyrrhotite and arsenopyrite. The

concentration of sulfides is more pronounced at the contact between BIF and quartz

veins and also in the inter fragmentary spaces which indicate that the sulfides have

formed as a result of fluid permeation consequent to intrusion of quartz veins. The

quartz veins are also fractured and fragmented suggesting that they are Syntectonic.

The original BIF is of oxide facies and it shows sulfides. Almost all the samples

analysed from this zone show auriferous nature with Au values varying from < 25 to

0.04 ppm. Thus, the fluids accompying the quartz veins have interacted with the iron

rich BIF resulting in the formation of iron sulfides due to ‘sulphidation’.

5.6 Ancient workings in auriferous shear zones at Nagavi area

There are old exploratory trenches in the Nagavi area and these trenches are made

across the auriferous shear zone on top of hill near Nagavi. All the trenches have

intersected BIFs and fractured quartz veins. The weathered zone is restricted to the

top 0.2 to 0.5m only. The average length of the trench is about 6 to 8m, width is 1m

and depth is 1m. The exploratory trenches were made to confirm the width of

auriferous shear zone. Plate 5.3 and plate 5.4 showing the exploratory trenches for


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gold exploration. Plate 5.5 shows that the trial pits were made in the contact of BIF

and metabasalts where the auriferous sheared zones found. The old Adit was found in

reef and it indicates the sheared zones are at top of hill also in the form of BIF hosted

gold mineralisation (Plate 5.6).

There are exploratory pits in auriferous shear zones in western part of hill which

contains metabasalt and BIF contact. These pits trend NW-SE direction. The distance

between two pits is about 15m. The average length of the pit 3 to 4m, width is about 1

to 2m and depth is 2 to 3m. There are two mineralised zones are delineated in this

area (gold values < 25ppb to 0.04 ppb). The one shear zone is observed at reef and

another one is found at NW part of the hill in the contact of BIF and metabasalt where

ancient working pit made in shear zones. Another shear zone occurs near

Mallasamudra. The pyritiferous metabasalts were found near Mallasamudra (Plate

5.7).

Plate 5.3 Photograph showing old working trench


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Plate 5.4 Photograph showing old working trench

Plate 5.5 Photograph showing exploratory trail pits in auriferous shear zones (Pit

1 and Pit 2) in western side hill

Pit 2

Pit 1


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Plate 5.6 Photograph showing ancient Adit on top of hill

5.7 Structural Controls

The mineralisation is confined to a narrow shear zone of 0.5m to 2m (inclusive of the

quartz vein). Shearing has affected the host rock-argillites and the quartz vein

(fragmented) which suggests that the quartz vein emplacement was Syntectonic,

which was concomitant with the main folding event, whose anticlinal closure lies in

Nagavi gold mineralisation. There are major and minor faults in the study area and

these faults might have facilitated the upward channelization of the mineralizing

fluids.

5.8 Stages of fluid channelization and mineralisation

As the shearing progressed in the deformation zones in the high strain zones in the

hinge portion (closure portion) of the anticlinal fold at Nagavi north, the mineralizing

fluids accompying the quartz vein emplacement got focused and channelized along

the shear zones in stages. Quartz and carbonates were deposited in the shear zone at

an early stage. During this stage, fluid-wall rock alteration took place resulting in

silicification and carbonatisation in the enclosing rocks. Simple sulphides such as

pyrrhotite and pyrite also formed at this stage.


117

Plate 5.7 Photograph showing pyritiferous metabasalt

The second stage was marked by an episodic shearing and fracturing as a result of

which, the earlier intruded quartz and carbonate veins got fractured resulting in

increased permeability for the ascending ore bearing solutions. At this stage,

additional silica and metals were deposited in the altered and structurally prepared

shear zone and crushed wall rock zones and finally resulting in the formation of

arsenopyrite, chalcopyrite and Pyrite with the availability of As, Cu, Zn and Pb. The

third stage is marked by renewed fracturing /fragmentation resulting in breakup of

early formed sulfides and alterations of early formed sulfides such as pyrrhotite to

arsenopyrite and pyrrhotite to chalcopyrite. Gold seems to have been occluded in the

sulfide minerals (as micro grains or as invisible gold in the lattice) during the second

and third stage rather than the first stage. This observation confirms with the three

stage model of auriferous metallogenesis in shear zones (Bonnemaison and Marcouz,

1990).

5.9 Gold mineralisation model

The Nagavi gold mineralisation displays many similarities to other Archaean lode-

gold deposits such as Mt. Morgans and Cleo in Western Australia (Vielreicher et al.

1994; Brown et al. 2003, respectively), Hollinger-McIntyre in Ontario (Walsh et al.


118

1988), Fumani and Kalahari Goldridge in South Africa (Pretorius et al. 1988;

Hammond and Moore 2006). A feature that is common to most of these areas,

mineralisation is the tectonic control of vein formation and the inferred metamorphic

origin of the fluids. The high CO2 content and low salinity of the fluids, together with

traces of CH4 and N2 found in the inclusions, suggest a similar origin for the Au-

bearing fluids at Nagavi, i.e. that they were probably produced by devolatilisation

reactions during regional metamorphism ( Kerrich and Fyfe 1981; Wyman and

Kerrich 1988; Mikucki and Ridley 1993; Kerrich et al. 2000).

The fact that the mineralised veins crosscut the BIF layering but are parallel to

regional schistocity is consistent with origin of the fluid. The abundant BIFs within

the Nagavi and surrounding area most likely provided the source of gold to these

hydrothermal fluids. The BIFs rheological characteristics and high Fe content were

probably the principal cause for Au deposition. Firstly, quartzite is more competent

than the schistose wall rocks. Shearing caused it to fracture thus allowing fluid

migration. Secondly, the high Fe content provided a good chemical trap. The sulphide

oxide replacement textures observed in the ore zones, the occurrence of gold within

pyrite and the presence of H2S gas detected in the fluid inclusions together with the

low salinity of the fluid suggest gold transport by sulphide complexing (Seward 1973;

Zotov and Baranova 1989; Benning and Seward 1996; Gibert et al. 1998 and Tagirov

et al. 2005). It follows that wall-rock sulphidation could have been a potential

mechanism for gold precipitation, through the destabilization of gold thio-complexes.

In addition, the fact that pyrite occurs as replacement of magnetite indicates that the

fluid was not in equilibrium with the host rock but in the pyrite stability field, where

Au solubility is much higher. We believe that this was likely the primary factor in

causing Au mineralisation; however, a further drive to Au precipitation would be the

loss of sulphur that took place during fluid phase separation due to preferential

partitioning of H2S into the exsolving CO2 phase. The mineralisation consists of gold

bearing quartz sulphide veins crosscutting BIF hosted in the Nagavi area.

Investigation of fluid inclusions hosted in Au-bearing vein quartz indicates

heterogeneous trapping of a low-salinity aqueous fluid and a carbonic fluid containing

traces of H2S.


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The transport of gold in solution can occur in a number of ways, some of which have

been investigated in detail both theoretically and experimentally. The two most

important and experimentally demonstrated systems involved the gold-litho complex

species and the gold chloride complex species. The solubility of gold in relatively low

temperature bisulfide solutions (1500 – 300

0 C) has been investigated by Ogryzlo

(1935), Weissberg (1970) and Seward (1973). Gold solubility is unaffected by NaCl

concentration, increases with increasing NaHS concentration and temperature and

decreases markedly with increasing alkalinity. Seward further concluded that greater

hydrogen fugacities associated with pyrite pyrrhotite buffered assembles considerably

depress gold solubility, and that thio gold complexes of the type Au (HS)2 are present

in the system. He also noted that association of gold with As and Sb in low

temperature deposits suggests the arseno-thio and antimino-thio gold complexes such

as Au (As Sb)2

and that may be an important gold transport mechanism.

5.10 Genesis of BIF hosted gold mineralisation

BIF is restricted to later structures (quartz veins / shear zones) in vein type gold

mineralisation or iron sulphide rich zones adjacent to such structures. BIF hosted gold

mineralisation are characterized by the following factors;

1) A close association between gold and iron sulfide minerals.

2) The presence of gold bearing quartz veins and/or shear zones;

3) Structural complexity of the host area

4) Paucity of lead and zinc in the two principal verities of BIF hosted gold

mineralisation can be defined can be based on the dominant style of gold distribution

(Kerswill, 1986, 1993, Mac Donald, 1990): Stratiform and non-strati-form or vein

type (Table 5.3).


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Table 5.3: Characteristics of BIF hosted gold deposits (J. Kerswill, 1993)

Features common to all deposits

There is a very strong spatial association between native gold and iron sulphide minerals

Gold bearing quartz rich veins and (or) shear zones are present and locally abundant

Deposits occur in structurally complex settings

Ores contain only background contents of lead and zinc

Features diagnostic of non-

stratiform deposits

Features diagnostic of sediment-hosted ( Lupin-

like stratiform deposits)

Deposits are non-stratiform Deposits are stratiform

Gold is commonly not restricted to

sulphide BIF or veins that crosscut

BIF

Gold is commonly restricted to sulphide BIF or to

veins that crosscut sulphide BIF

Sulphide BIF does not occur in

laterally continuous units

Sulphide BIF occurs in several thin but laterally

continuous units that are conformably interlayered

with barren silicate BIF and (or) carbonate BIF and

clastic sedimentary rocks

Sulphide BIF is not well laminated:

iron-sulphide minerals are commonly

massive

Sulphide BIF is well laminated and chert-rich; iron

clearly controlled by veins and (or) late structures

Orebodies are typically less deformed

than associated rocks

Orebodies are as deformed as or more deformed

than associated rocks.

Iron-sulphide minerals tend to be

relatively undeformed and

unmetamorphosed

Iron sulphide minerals show effects of deformation

and metamorphism

Deposits are not restricted to, but are

most abundant in greenschist facies.

Deposits occur in both greenschist and amphibolite

facies terranes

Sulphidation textures are ubiquitous Sulphidation textures are absent in stratiform ore

Oreboby scale alteration exists Orebody scale alteration is lacking; localised vein

related alteration does occur.

Alteration products are generally

similar to those in mesothermal vein

gold deposits

Vein related alteration is commonly a typical of

mesothermal vein gold deposits

Oxide BIF is typically the principal

BIF lithology in the deposit.

Oxide BIF is lacking in the deposits, irrespective of

metamorphic grade

Pyrite is commonly the dominant iron

sulphide mineral

Pyrrhotite is typically the dominant iron sulphide

mineral; in some cases early pyrrhotite has been

replaced by pyrite.

Arsenic, if present, is

characteristically directly correlated

with gold

Arsenic is generally abundant adjacent to late

quartz, veins but is not well correlated with gold.

Silver contents of gold grains are

typically low (Au/Ag ratios > 8.0)

Silver contents of gold grains are moderately high

(Au / Ag ratios - 3.0 - 7.0)

Deposits are relatively common,

generally small and difficult to

evaluate and mine

Deposits are rare, can be very large and easy to

evaluate and mine, relative to no-stratiform deposits.


121

A strong positive correlation exists between gold and arsenic. The gold is uniformly

disseminated in thin but laterally extensive units of cherty, pyrrhotite rich iron

formation that are conformably interlayered with sulfide and oxide poor formation.

In the mineralisation area, the sulfide rich in iron formation that is associated with

carbonate facies and black carbonaceous shale relatively close to volcanic centers. In

hydrothermal systems in the earth’s crust, mineral solution equilibria give rise to ore

forming fluids which are characterized by low oxidation potentials (Seward, 1984;

Henley et al. 1984) such that the dominant oxidation state of dissolved gold.

chapter 5 mineralisation - inflibnetshodhganga.inflibnet.ac.in/bitstream/10603/84120/6/15. chapter...

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