UNDERGROUND MINING METHODS AT KOLAR GOLD FIELDS

The mines and method of stoping may not exist, But will remain in the History and reference books

UNDERGROUND MINING METHODS AT KOLAR GOLD FIELDS. Contributor Author : D. Vidyarthi  (Chief Mining Engineer, Nundydrug Mine)

Mining Engineers Dairy - Metalliferous Mines India

Gold mining is carried on in India at the mines of Kolar Gold Field (Karnataka State), Hutti Gold Mines (Karnataka State), and Yeppamana mine (Andhra Pradesh). One gold mine in Ramagri Gold Field where the present Yeppamana mine is located was worked in the past but is now abandoned.

Kolar Gold Field is known as a major gold producing centre in India has some of the deepest gold mines in the earth’s surface. Systematic gold mining started in this field in about 1880 with the arrival of M/s Jonn Yaylor & Sons, Managing Arents, KGF mines had produced gold from reefs averaging 18.5 g/te/ The average gold content in ores during the days of John Taylor and after nationalization.

 

Decad                                          Average gold content grams / te of ore

 

1.      1881 – 1900                                47.5

2.      1901 – 1910                                28

3.      1911 – 1920                                18.19

4.      1921 – 1930                                19.6

5.      1931 – 1940                                15.4

6.      1941 – 1950                                12.43

7.      1951 – 1960                                7.15

8.      1961 – 1970                                5.35

Nowadays gold content of 2.2 g/te is considered to be the economic pay load for an ore.

The mines of KGF were taken over by the Government of Mysore in 1956 from M/s John Taylor & Sons. Later on the Govt of India took over the mines from the Government of Mysore in 1962 and in 1972 the management was entrusted to Bharat Gold Mines Ltd, a company in the Public Sector under Govt. Hutti Gold Mines Ltd., a Public Sector Company owned by the State of Karnataka. The Yeppammna Gold Mines is worked by a Publictor Company, under the Govt. of India, (BGML)

Substantial gold deposits have been found along the entire Shivalik Formation in Utter Pradesh. The UN-sponsored Kerala Mineral Exploration and Development Project has located 1 million cub. M of gold bearing gravels on the upper reaches of chaliyar and Punnapuzha rives in Wynad district. A total reserve of 30 milion of gold bearing gravels, on a potential 50 km river stretch, has been indicated with a grade of 0.13 g/cub m.

In January 1986, Mineral Exploration Corporation Ltd     has discovered during its drilling in Bihar gold at Kunderkocha, about 40 km from Jamshedpur. Located at only 100m depth the vein reveals 10-15 grams of gold per tonne of ore as against 3 grams per tonne in the depleting resources of KGF. The potential mine site covers an area of nearly 40 sq. km and is close to Bihar-Orissa suggesting possibility of extension of the reef into Orissa. Further exploration by MECL is continuing.

A gold deposit at Chirgranta in A P is under detailed exploration and development by the GSI and the MEC. The potential output from this project is estimated at 300 kg / yr and the shallow depth will keep the consts low. The Hosur block fo Gadag glod field in Dharwar district of Karnataka has reserves estimated at 1.23 Mte of ore with an inferred gold content of 3 g/te.

Geology

The Kolar Gold Field which is known as Kolar Schist Belt is situated in a 100 km long north – south tending ‘Precambrain Green stone Belt’. This belt is in the eastern part of Karnataka Creaton. The width of the belt is about 4-6 km with the rocks having a general westerly dip. The eastern part of the belt consists of champion Gneiss’. The middle part of the belt is occupaied by metabsalts which at palces show typical pillow structures. The western portion is occupied by a prominent band of banded iron formation ( B ( F) associated with graphitic schists. All the above formations occur within the regional “Penisular gneissis complex” and are metamorphosed. The longitudinal dolerite dykes which also intrude the formations, are post-metamorphic in age. Pegmatites as flat and irregular fracture fillings are seen in the deeper parts of the mines, suggesting possible subjacent granitic body.

The above formations, which are within the penisular gueissic complex, have been highly distrurbed and have become tectonites in the areas of mineralization.

The rock formations show moderate dip near the surface which steepens at a depthe of around 1500 meters, becoming near vertical below that depth.

Diagonal faults trending NW-SE have dislocatd the rock formations. The major faults  are:-

1.      The Balaghat north fault, which defines the northern limit of the ore bodies in Nunndroog area.

2.      The Mysore north fault in the Champion Reef – Mysore Mine area and,

3.      The Gifford’s system of faults in champion Reef Mine area.

Ore bodies and mineralization

There are as many as 26 lode in KGF which are running almost parallel to one another. Three of them are prominent, namely:

  1. Champion lode.
  2. Oriental lode.
  3. Mc Taggart’s lode.

The lode in KGF trend N-S and dip towards Wedt. The champion lode is the major lode of KGF. Of economic importance and has been worked in all the mines of KGF over a strike length of 8 km. The KGF mines are confined to a land measuring 8 km long and 2km wide.

Mining of Champion lode which is devoid of sulphides, and free-milling in character is still continuing in Mysore Mine and Champion Feef Mine whereas the entire reserves of Champion lode in Nundydroog mine have been exhausted. The Champion lode has an average width of 1.25 m. The ore body has a westerly dip which is about 30-45 degrees at shallow depths and steepens to near vertical at deeper horizons.

Exploration of Oriental and Mc Taggart’s lode is now being carried out in Nundydroog Mine. These lodes are Sulphidie in nature and refractory in character. They carry sulphides like Phrrohotie, Pyrite, Chacopyrite, Galena, Arsenopyrite etc. The other sulphidie lode in Nundydroog Mine area is Mc Taggart’s East which is not so prominedt, Width of the ore body in these lodes is about 2 km with an average dip of 85 degrees.

The mineralization in KGF is hydrothermal fissure vein type and native gold is a major mineral.

Shafts and development of mine

It sounds strange but it is true ! Karnataka had its own brand of “gold rush” about the same period when the United States of Ameerica had its Caligornian “Gold Rush”, Canada its Ktondike and South Africa its Witwastersrand. That was during the later part of 19th century. Unlike the gold rush in western countries marked by violence and adventure, the gold rush to Karnataka was peaceful and carried out by a silent band of men consisting of geologists, prospectors and fortune hunters. But the organised activities in kolar Gold Field started in 1880. Since then several shafts have been sunk in the field bringing the cumulative depth of shafts to about 64 kms. This includes the deepest shadt in the world – the Giffords shadt of Champion Reef Mine sunk in 1940, to a depth of 2010 m. The other major shafts are: Henry’s Shaft of Nundydroog Mine which extends to 1247 m  below the suface and Edgar’s Shaft of Mysore Mine which was sunk to a depth of 1160m. Although 59 surface shafts and 81 undergound shafts have been sunk in Kolar Gold Fields.

The mines were developed from the shafts by cutting X-cuts to insert the reef. The levels (2.1 m x 2.1 m ) were driven along the reef, at intervals ranging from 30 m to 60 m

The criteria for deciding the level interval was :-

  1. To provide a large number of stoping faces in view of the labour intensive mining, and
  2. To facilitate quick disposal of broken ore from the workings.

X-cuts and drives were driven from shafts at various horizons and winzes / raises were put up to connect up the levels at regular intervals and block out the depost. The connections thus afforded by raises / winzes serve as free faces to start stoping operations, ventilation air ways, passage for men and materials etc.

Drives / X-Cuts

The method of developing drives and X-cuts remains the conventional one, viz drilling, blasting and mucking. Drives are while X-cuts ( 2.1 m x 2.1 m) are the excavations made in barren rock to provide connection between the shaft and the Reef-Drive. They ae generally driven at ¼ % gradient towards the shaft

Two types of Drilling pattern are in vogue at KGF, namely

  1. Burn cut, and
  2. Wedge cut.

Mucking operation has mostly been manual. However at several palces Eimco 12 B pneumatic loader was put in operation. Mucking rate with Eiimco Loader, in conjuction with battery locomotive and 1.4 te. Mine cars, was approximately 20 te / shifts, and was adequate to clear the face within a shift.

STOPING 

Stoping methods in vogue in nundyroog Mine at present are:

  1. Cut-and fill stoping – with back filling of mill tailings.
  2. Granite packed bottom stopes

Nundydroog Mine has four distinct lodes, viz. 

  1. Champion Lode (Main Reef) with an average width of 1.25 m.
  2. Oriental Lode (West Reef) which is to the west of Main Reef, and has an average width of 2 m.
  3. Mc Taggart’s West Lode ( Width – 1.5 to 2 m)
  4. Mc Taggart’s East Lode. (Width – 1.2 to 1.5)

These lodes along with a number of other lodes extend to a strike length of about 4545 m and a depth of about 2420 m with a general westerly dip. While workings on champion lode have been exhausted, the Oriental lode and McTaggart’s West lode continue to be exploited.

The Oriental lode is being worked by Cut and fill stoping with back filling of mill tailings, while extraction on McTaggart’s West lode is by granite packed bottom stopes.

Cut-and-fill stoping

Cut and fill stoping with back-filling of mill tailings has been adopted in Nundydroog Mine due to three main reasons:-

  1. Back filling (Hydraulic sand stowing) ensures about 80% support.
  2. It is much quicker than other methods, and ensures faster extraction of the ore.
  3. Hydraulic sand stowing is more economical than any other type of support

 General guide lines for cut & fill stoping

  1. All methods and procedures are in compliance with the provisions of the Mines Act 1952 and the Metallifarous Mines Regulations 1961.
  2. the standard length of a sand filled back stope is 21.3 m.
  3. Stoping is carried out in a definite sequence. These sequences (referred to as stoping sequences) are well planned giving due consideration to geological and ground control conditions, so that formation of pillars, remnands or promontories avoided to the maximum possible extent.  As far as practicable dykes and faults and tetreants from it.
  4. No stope is left idle for any length of time. A continuous cycle of breaking and support is maintained as far as possible.
  5. Wherever veins branch, stoping begins at the intersection and moves away from it, taking on branch at a time. 

Before beginning the actual stoping operations certain amount of preparatory work has to be carried out. This involves the following: 

1.      Chutty Stpoping chutty stope is made over the entire length of the tope by breaking to a depth of 1.2 m below. After the broken ore is cleared concrete is poured in the chatty stope. The concrete plugs are 3.0 m to 4.5 m long, 1.2 m deep and separted from one another by gaps of about 1.5 m chatty stoping helps in protecting the levels when a stope comes to “Holing-Through” stage.

2.      Stripping of the back of the level to a length of 21.3 m along strike (stoping span)

3.      Erection of arch rails or steel sets at intervals of 1.5 m to cover the entire stoping span as well as the front and back abutments to a distance of 10 m. Steel setts are made out of 90 ibs Rails bent in the form of an arch. Two segments held together with the help of steel shoes, make an elliptical shape – measuring axis (horizontal)

4.      On top of the steel setts, Baby Setts (special steel setts) are fixed. The The special steel setts, also called Baby Setts are circular, 1066 mm inside dia. constructed out of two 63 mm x 13 mm MS flat rings with stiffeners of 8 mm thick MS plates and MS plate ribs equally spaced along the periphery. They are welded construction.

5.      The Main steel setts as well as the special setts (baby setts) are convered all around with lagging poles and tightly packed with dry packings and waste rock.

6.      Above the baby setts ore-pass-foundations are made with concrete. Then, ore-pass 1.5 dia, are raised and the stope is filled with sand (Mill tailings) after fixing a barricade on one side. Ore passes are formed with pre-cast interlocking concrete blocks.

After completing the above preparations the stoping-block is ready for extraction.

Back stoping is practiced in two different ways:

  1. By breasting
  2. By drilling of upper-inclined holes (back-holes)

Breasting:

About 12 horizontal holes are drilled at the face and blasted at a time, the depth of holes varying between 1.5 to 2.0 m. Explosive consumption is about 0.4 kg/te. A special stage called drilling stage has to be fixed for drilling of the horizontal holes. The stage is prepared by fixing three stulls between hanging wall and foot wall close to the face, and covering with lagging poles. Poles used are either casurina or eucalyptus.

A big disadvantage in this method is that in every drilling shift a lot of time is wasted on the following activities.

  1. Dressing of loose
  2. Fixing temporary support.
  3. Fixing Drilling stage.
  4. Removal of Drilling stage towards the end of shift prior to blasting. 

This considerably reduces the drilling time in a shift and caused low productivity

Cycle of operation

 A complete cycle of operations in breasting comprises the following activities. 

  1. Extraction of complete bench 21.3 m along strike and 2.5 m in height.
  2. Simultaneously, clearing / mucking of broken ore
  3. After clearing of stuff (broken ore), raising of orepasses (4Nos) using the ore pass blocks and Hessian cloth. ( It is a specially made impervious cloth., prepared b pasting thick brown paper over jute net with a bitumastic).
  4. Errection of end barricades.
  5. Supporting the stope with mill tailings by hydraulic stowing. Requirement of sand (mill tailings) works out at about 80% of the tonnage broken in the stope.

Back-Hole-Drilling : 

To overcome the disadvantages of Breasting and to ensure a faster rate of extraction and higher productivity, drilling of uppers (inclined back holes) is envisaged. The preparatory work and the stoping span in Back-Hole-Drilling remain the same as that in Breasting.

In this method back holes are drilled at an inclination of 60º - 70º  from horizontal throughout the bench, the last three rows being drilled at 70º, 80º and 90º respectively. Blasting is generally carried out in three stages, taking 7.1m (along Strike) at a time. Explosive consumption is between 0.4 to 0.5 kg/te. Holes are fired in a “diagonal initiation pattern” using milli-second delay detonators which ensures good fragmentation.

Cycle of operations:

  1. Extraction f 21.3 m of bench by drilling & blasting.
  2. Clearing / mucking stuff (broken ore).
  3. Raising of ore-passes and erection of end-barricades
  4. Hydraulic stowing of mill tailings.
  5. Rock bolting before resuming drilling operation

Holing-Through Procedure: 

(This is applicable to both the methods – viz Breasting as well as Back-Hole-Drilling)

In a stoping block under extraction, with the completion of each cycle of operations, the bench moves upwards from lower to upper level, by 2-2-.5 m. Once the ground reduces to 9.0 m, the stope is said to be holing-through the next level above and at that stage the holding-through procedure has to be adopted.

With regular stoping in progress, stresses keep accumulating in the abutment and the last ledge or 9 m is considered to be highly stressed. It is therefore very risky to employ any workmen underneath this ground. That is where the holling through procedure comes in the picture, where all the operations are carried out from one side and no persons are employed directly under the block being extracted. In holing-through stage, at time only 9 m (along dip) x 6 (along strike) bock is extracted and supported. 

Systematic support rules for sand-filled back stopes

A.        Support of Levels:

1.      Top and bottom levels shall be kept supported along the stoe length and in advance to a length of 10 m by means of steel setts (with suitable laggings) erected at intervals of not more than 1.5 m. The remaining portion of top and bottom levels upto the end of the stoping block shall be kept supported by means of stulls not less than 15 cm in diameter. Stulls shall be set at an interval of not more than 1.5 m in the same row and the vertical interval between the rows shall not be more than 1.8 m.

2.      Head boards used with stulls shall have a width and length not less than the diameter of the stull and a thickness not less than 8 cm.

3.      Wedges shall not be less than 20 cm in length and 8 cm in width, and suitably tapered.

4.      Timbers used in the construction of crib setts shall not be less than 1.2 m in length and 12.5 cm in diameter and not be less than 1.2 m in length and 12.5 cm in diameter and shall have the two opposite sides joggled flat to provide suitable bearing surface.

5.      Stulls shall be set on solid footwall in hitches cut not less than 2 cm deep and not on loose ground. They shall be kept tight against the hanging wall. Where stulls are to be set on sand, a flat base piece not less than 5 cm thick, 25 cm wide and 0.5 m long shall be used.

6.      Additional supports shall be provided as and when necessary.

7.      The distance between the top of the sand fill and back of the stope shall not be more than 4 m at any time

Granite walled bottom stopes:  Granite walled bottom stopes differ from sand filled back-stopes to the following extent.

  1. The support material is granite-pack-wall which is quite expensive and a rather slow method.
  2. Granite walled bottom stopes do not require the estensive preparatory work as carried out in sand-filled back stopes
  3. Drilling operation is underhand (downwand), eliminating the use of pusher legs.

In bottom stopes, stopping commences from an upper level and proceeds towards a lower level. Pack-walls are buit at intervals of 1.5 m. the size of the pack wall is 6.0 m along the dip and 3.0 m along the strike. Ladder way is extender between two rows of pack walls such that a stope always maintains two outlets. Pack walls are built on timber beds prepared by fixing three stulls between hanging wall and footwall and covering three with lagging poles. The size of granite blocks used for building packwalls is generally 25 cm x 20.3 cm x 20.3 cm. While constructing the walls, cushioning timbers are inserted in between.

Systematic support rules for granite packed bottom stopes

A.                 Support in the stopes

1.      Row of granite massonary packwalls, shall be carried down in all the bottom stopes. They shall be at intervals of not more than 1.5 m. Two ladderways (exits) shall be provided for each stope.

2.      Bed timbers supporting the granite wall shall not be less than 15 cm ind diameter. Granite walls shall be of masonary type and at least 3 m along the strike and 6 m along the dip with a gap of not more than 1.5 m between adjoining walls, While building granite walls, timber lagging shall be inserted in the wall along the strike to provide cusnioning against pressure. The distance between the adjoining bed timbers supporting a wall shall not be more than 3 m.

3.      Stulls not less than 15 cm indiameter shall be set at an interval of not more than 2 m between stulls in the area under actual extraction, the front row being not more than 2 m from the face.

4.      Stulls shall be kept on solid footwall in hitches cut not less than 2 m cm deep and not on loose packing material. They shall be kept tight against the hangingwall.

5.      Crib setts shall be put as temporary support wherever the distance between the face of the stope and the bottom of the masonary wall above exceeds 4 m.

B.        Support of levels:

    1. Top and bottom levels shall be kept supported along the stope length and in advance to a length of 10 m by means of steel setts with stulls nd suitable laggings erected at intervals of not more than 1.5 m.
    2. Head boards used with stulls shall have a width and length not less than the diameter of the stull and a thickness not less than 8 cm.
    3. Wedges shall not be less than 20 cm in length and 8 cm in width, and suitably tapered.
    4. Timbers used in the construction of crib setts shall not be less than 1.2 m in length and 12.5 cm in diameter and shall have the two opposite sides joggled flat to provide suitable bearing surface.
    5. Stulls shall be set on solid footwall in hitches cut not less than .5 cm deep and not on loose ground. They shall be kept tight against the hanging wall.
    6. Crib setts shall be set on solid footwall and not on  loose ground. They shall be kept tight against hanging wall to ensure maximum control between the timber and the back
    7. Additional supports shall be erected as and when necessary.

 Sampling: The distribution of gold in quartz reef channels is very erratic. Sampling of erratic gold assumes great importance in view of the capricious distribution of the metal and also because no adequate idea of the grade of the ore can be formed at sight. In the old days, nearly 50 years ago, a payload of 30 grams per tonne in the ore was considered economical but nowadays 2.2 grams per tonne is considered as the economical limit for extraction of ore (1 gram – 1.6dwt.) The sampling of the drives along the lode channels during the process of development is very important and is the first step to form and idea of the quantity of the ore. Further developmental and mining activities are based on the results obtained during this phase of mining activity. Careless sampling may lead to erroneous conclusions. Before sampling of the development face, the freshly exposed rock surface is thoroughly washed with water. The band or bands of reef are clearly marked in chalk after tracing the major geological features. They are divided into a number of sections parallel to the walls, the width of which may vary from 76 mm to 600 mm. If the width exceeds 600 m, the samples are divided into two parts. In case of single band of reef less than 600 mm in a drive 2.1 m x 2.1m, the reef divided in to three sections. Chnnels of 75 mm width and depath of 50 mm are cut along the sections and sent for assaying. The character of the lode is recorded by terms such as quartz, mixed quartz, stringers if the quartz is thin formation, dyke, pegmatite instruction, etc. Association with sulphides such as pyrite, pyrrhotite, etc., is also recorded. To allow for dissemination of gold beyond the contracts on either side of the reef channels, at least 75 mm of the wall rock is also covered. Normally the corss cuts with approach the reef are not sampled unless they show evidences of mineralization. The sampling procdure in raises and winzes is basically the same as in the development drives.The reef is sectionalized and channels of dip samples are taken. The sampling procedures in raises thus taken at the respective locations are recorded in the sampling book. Tokens or chits are attached to the bags and  sent to the Assay Office for obtaining the values. In the sampling of stopes, samples are collected two or four times along the strike depending on the rate of progress. The stopes face is divided into equal sections along the face and samples collected at these points. In both development and stope sampling, sample variations are computed for each face position. At intervals of 1.5 m the quartz “in situ” is sampled and the results are made use of in recuating ore reserves. In sampling of drill cores, the core is carefully scrutinized for determination of the rock types, faults, dykes, fissures of quartz reefs, etc. and these are recoreded. The core is then split lengthwise; one half is crushed and assayed and the other retained for cores check. Sludge samples are collected invariably and this assumes great importance in identifying the nature of the lode. In areas where reclamation is carried out to win ore, grab sampling is resorted to. Stuff is collected at random,  coned and quartered before sending the same fro assaying. Check samples are taken as and when required in drives, crosscuts, raises and winzes and stopes with a two-fold objective-either to confirm the previous value or to get a fair average. ( Contributor authors: S. Sambasiva Rao (Mines Manager) And C Annu Rao (Chief surveyor)

 IN Kolar Gold Fields many stoping methods have been in vogue, such as open stopes, timber stopes with heavy stuils, Cribsett stopes, granite packwall stopes in the form of 



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