Hutti Greenstone Belt: Host to a world class gold deposit (Fig.1)
Gold is being mined by The Hutti Gold Mines Ltd (HGML), a Government of Karnataka undertaking, near Hutti village in northern Karnataka. The mine is developed on a classic Archaean lode gold deposit, similar to those in Australia, Canada and South Africa. Gold mining activity in the belt has been known since pre-Ashokan time, about 3000 years ago. Modern gold mining commenced in the Hutti area around 1947.
The Hutti Mine has so far produced about 2 million ounces while remaining ore reserves (proven and probable) are believed to be 6,400,000 tonnes of ore grading 6.48g/t upto 750 metres depth (i.e. 1.33 million ounces). The average recovered grade from modern operations is approximately 6.8 g/t.
Exploration was carried out by DGML under two Reconnaissance exploration Licences (RPs) covering a total area 851 sq km of the most prospective part of the Hutti belt.
Our exploration comprised Geophysical surveys by geophysical techniques involving about 9 line km of Induced Polarization(IP) and 145 line km of magnetic surveys in selected areas. Surface geochemical exploration involved the collection and analysis of 347 samples of stream sediments, 85 samples of termite mound, 819 soil, 4933 bedrock chip samples and 1646 channel profile samples. Based on results from the geophysical and geochemical investigations, reconnaissance reverse circulation drilling and diamond core drilling have been carried out. A total of 2296 drillhole samples were analysed for gold. These studies led to defining 21 potential gold bearing blocks for detailed prospecting under Prospecting Licenses (PLs) for which applications have been filed with the Government of Karnataka. The P.L. blocks identified are shown in (Fig.1).
Among the 21 gold prospects so far recognised, the most important are the Hutti Mine North prospect located immediately north of the currently operating Hutti mine, Hirenagnur Prospect, Yathkal Prospect, Uti south prospect, Chinchergi-Topaldoddi prospect, Bullapur prospect, Buddini old mine prospect, Sanbal-Maski prospect. Highlights of these prospects are presented in the following:
A. Hutti Mine North prospect (Fig.2)
This block is located immediately north of the currently operating Hutti Gold Mine. Eight parallel gold bearing quartz-sulphide veins known as Reefs or lodes are known to exist in the currently operating Hutti Mines. These lodes were speculated to extend northwards into our exploration ground for about 2.0 km beyond the northern boundary of the Hutti mines.
The objective of our exploration was to prove the extension of the lodes. Since most parts of this prospect is under cover of black cotton soil, IP geophysical survey was carried out which revealed chargeability anomalies. Follow up drilling in the eastern side revealed the possible extension of the Strike Reef with intersections of 5.3g/t gold for 2.05m width, 3.19g/t for 2.71m width and 0.65g/t for 6.3g/t width. The geophysical anomalies taken together with the drill intersections including the earlier GSI's drilling data confirmed the possibility of the northward continuity of the gold-bearing reefs.
Follow up, prospect-wide bed rock profile sampling revealed anomalous gold values between 20 to 90 ppb over a 180m wide zone along the inferred northern extension of the Main Reef of the Hutti Mines. Few samples of exposed quartz veins gave up to 16 g/t gold on the possible extension of this reef. The most significant observation was the possible continuation of the New East Reef with 9.4 to 10 g/t gold over a width of 1m as revealed by channel sampling over the geophysically anomalous zone. The extension of the Zone I Reef was also recognised in drilling but it did not give a significant grade at the point of intersection. All the inferred extensions are shown in (Fig.2).
A PL application has been lodged over the prospect, grant of which will enable us to take up a detailed drilling programme needed to define the subsurface extensions and grades of mineralized zones.
One of the highlights of our exploration work has been the establishment of a 2 km long and 50 m wide gold bearing zone in the Hirenagnur Prospect, located 5 km SE of the Hutti Gold Mines. This prospect is an open ended N-S trending, zone revealed initially by a single rock-chip sample and subsequently by systematic bed-rock sampling and ground magnetic surveys along eleven profiles.
Analysis of 473 bedrock samples collected at 10 m intervals along E-W profiles spaced 40m apart revealed presence of highly anomalous gold values of 56 to 461ppb in banded iron formation. Further follow up rock chip samples showed values of up to 7.78 g/t gold. Channel sampling across the 50m wide zone, identified sub-parallel mineralized zones of 2 to 15 m width with gold grades of 1 to 3 g/t.
These coincident geophysical and geochemical anomalies encouraged drilling. A total of 1078 m of RAB and RC drilling were successful in defining 4 parallel gold bearing zones having a cumulative strike length of 1030 m (Fig.3b). They are:
a) Footwall magnetite-gold lode: 500m long with 7.5m av. width @ 3.34g/t av. gold.
b) Central magnetite's-gold zone: 280m long with 6.23m av. width @ 2.12g/t av. gold.
c) Hanging wall sulphide-gold zone: 250m long with 8.63m av. width; 4.34g/t av. gold.
d) A single intersection lode west of (c): 25m long with 6m av. width; 3.52g/t av. gold grade.
The current shallow recconnaissance drilling has tested the depth persistence of lodes up to 60 m from the surface. Extension of the lodes both along strike and depth are open for future detailed drilling.
Hirenaganur prospect is a structurally controlled mineralization. Therefore, the continuity of gold bearing structures is expected up to several hundred metres. Applications have been filed for both PL as well as a Mining Lease.
Ore beneficiation tests are underway at the Indian Bureau of Mines. A Prospecting Licence application has been lodged, upon grant of which Deccan Gold will undertake further detailed drilling to test the deeper extensions of the zones.
b) Uti mine lodes- Southern and Northern continuity (Fig. 4)
The Uti Mine is owned and operated, as an open pit mine, by the Hutti Gold Mines Ltd. GSI's drilling helped to trace the southern continuity of the gold-bearing reefs beyond the lease hold of the Uti mine. Structural mapping revealed that the Uti mine is controlled by a major discordant structure. This structural control encouraged us to investigate the southern and northern continuity of the Uti mine. Preliminary drilling was directed at establishing the southern continuation.
Bed rock sampling at 20 m intervals along 9 profiles southwest of Uti mine and along 3 profiles north of the mine have revealed gold anomalous zone in strike continuity of Uti mine lodes for about 3.5 km to the SW and also towards north.
609m of reconnaissance drilling confirmed the strike continuity of gold lodes to the south. One of the lodes showed gold in the range of 0.82 to 1.4 g/t over widths ranging from 1 to 5m; a second lode yielded 1.34 to 1.72 g/t gold over 1 to 2m width; one more intersection analyzing 3.49 g/t gold over 4m was traced to yet another lode. Detailed drilling is called for to establish the full potential of the Uti south and North prospects.
c) Uti Temple Prospect
Old diggings by ancient miners and our recent channel sampling have revealed existence of three sub-parallel mineralized zones in an area 1.2 sq km west of the HGML's Uti mine. Surface samples have given high gold values of up to 22.94 g/t and 40.38 g/t gold. Bedrock geochemical sampling has indicated an anomalous gold bearing zone which is 1200m long and 80m wide.
Chinchergi prospect located some 15 km east of Hutti Gold Mine is an area of extensive ancient mining. Reconnaissance rock chip samples analyzing 5.59 g/t, 8.33 g/t, 9.92 g/t and 12.73 g/t were met within this prospect. Channel sampling revealed 1 to 2m wide narrow lodes analyzing 1.18 g/t to 4.38 g/t. Systematic bedrock sampling involving collection of 1500 samples helped to define a 80 to 200m wide anomalous zone of quartz veins with alteration haloes south of Chinchergi. Also observed was another similar vein- alteration anomalous zone, 75 to 100m wide, west of Chinchergi. Payable reefs are however, narrow; eg. 0.63 to 4.4 g/t gold over a maximum width of 4m for 400m strike length.
Eight widely spaced reconnaissance reverse circulation bore holes were drilled covering a strike length of 3.7 km. One borehole (DCH-1) intersected an ore zone with an average gold value of 1.41 g/t over 8m width. Important intersections in two other boreholes include 4m x 1.1 g/t (DCH-7), 5m x 2 g/t (DCH-2). These results are considered encouraging to warrant more further drilling on close spaced grids.
e) Bullapur Prospect (Fig.6)
This is one of the recently discovered prospects in the Hutti belt located outside of the GSI-explored area of ancient gold workings. In the course of geological traversing along the major structural discontinuity south east of Chinchergi village, a new zone of mineralization was located close to Bullapur village. The mineralized zone comprises a series of quartz veins disposed as N-S plays from a narrow ENE trending shear zone. One of the veins showed a number of visible gold grains on panning. A rock chip sample yielded a good value of 6.53 g/t gold. The area deserves to be investigated in detail under a Prospecting Licence.
f) Yatkal Prospect (Fig.7)
Yatkal Prospect is situated along the southern margin of the North Huttti R.P. Block and gold is found in the granites intruding the Hutti volcanic belt. Stream sediment sampling initially drew attention to the area as a possible source of gold anomaly. In several termite mound samples, more than 100 ppb gold, a highly anomalous geochemical value, was noted. This prompted a systematic soil grid geochemical survey involving collection of 819 soil samples at 20 m interval along profiles 40m apart. Gold content of greater than 50 ppb was taken to define the anomaly.
Two distinct zones of gold mineralization emerged following the soil sampling. A 900 m long, 150 m wide NE trending anomalous zone in the western part and a NW trending 400 m long narrow zone in the eastern part. The western anomalous zone shows sulphide rich granodiorites with 5 parallel zones of quartz veins. The NW trending eastern zone is a zone of highly silicified pink granite with amphibolite patches traversed by quartz veins, disseminated with chalcopyrite and pyrite. Rock chip sampling has revealed anomalous gold content in several samples with best values ranging from 1 to 32.05 g/t. Detailed sampling by trenching and drilling is warranted under a Prospecting License.
B. Prospects In South Hutti RP block (Fig.8)
Ancient gold workings dating back to Ashokan time dot a 20 km long tract in the South Hutti belt which is marked by a major N-S trending shear zone designated as the "Central Shear Zone (CSZ)". The tract was explored by us under a Reconnaissance Permit. This tract contains several promising prospects shown in Fig.8. The following targets have been chosen for intensive exploration based on highly encouraging gold values obtained during reconnaissance geological mapping and geochemical sampling which included analysis of 930 outcrop rock chip samples, 287 stream sediments and 389 bed rock samples. The exploration was successful in identifying four prospects worthy of detailed investigation, hence, these have been covered under PL applications which are in various stages of processing.
a) Tuppadhur-Buddini Prospect
A series of ancient pits and 6 inclined shafts are spread over a strike length of about 2.6 km in this prospect. Gold mineralisation is confined to the central shear zone and is generally found in quartz-ankerite veins with the wall rock showing intense chlorite and carbonate alteration. At Buddini, four parallel lodes have been identified. Extensive British time old workings occur along two of them - the Main and Mopla Lodes. Samples from mine dumps assayed up to 19.3 g/t gold. Detailed drilling will be undertaken once PL is granted for which our application is pending with the Government.
b) Maski Prospect
This prospect has all the signs of an important gold field. Thirteen old workings were reported in this prospect in the early 1900s by the Hyderabad Geological Survey. M/S John Taylor and Sons developed one of these workings with a 35 m deep shaft, drove levels north and south and mined the ore of an average grade of 30 g/t gold. All of the diggings are now buried under black cotton soil. Rock chip sampling, in the vicinity of these old diggings, has given some high gold assays, including 8.93 g/t from the sludge of a ground water borehole; up to 11.6 g/t from a pit to the bed rock and up to 33.86 g/t from float samples. Based on these results, an auger-bedrock sampling programme was undertaken which revealed five broad gold anomalies. These results are to be investigated in detail under a Prospecting Licence.
c) Ashoka Prospect
The Ashoka Prospect (named after the great Indian emperor whose inscriptions are seen near Maski town) was a discovery happened during one of our stream sediment geochemical survey programmes. The prospect is defined by brecciated quartz-carbonate-haematite veins traversing fractured pink potassic granite. Rock chip sampling gave assays of 4.9 g/t gold and 0.63% copper. Reconnaissance geological mapping has revealed that the mineralized zone trends WNW, but the zone is covered under thick black cotton soil. Detailed trenching and geochemical soil sampling is planned prior to preliminary drilling.
d) Sanbal Prospect
Five sub-parallel zones of mineralisation have been identified in this prospect. Among them, Zone I consisting of highly folded quartz veins assaying up to 500 g/t Au. An open pit was developed by the Hutti Gold Mines Limited on Zone I, and about 2000t to of ore at a grade of 7.5 g/t gold was mined and processed. We plan to carry out detailed prospecting involving drilling to find the depth extensions of the high grade gold ore shoots mined in the past and existence of additional ore shoots.