Geita Greenstone Belt

GEITA GREENSTONE BELT

Introduction

The Geita greenstone belt (Fig.22) is marked by high relief in the form of hills, ridges, and a plateau, underlain by steeply dipping BIF. This greenstone belt has been the most productive in Tanzania with a nearly continuous history of activity from 1932 to the present. Three airborne geophysical surveys were made within the Geita greenstone belt during 1959 - 1960, by McPhar, Lundberg and Hunting. Each of them produced maps of magnetic, electromagnetic and radioactivity surveys. Ground follow up work was then conducted by the British Overseas Geological Survey, including some ground geophysics and diamond drilling targets directed toward base metals discovery. This work resulted in the discovery at Samena Hill (Fig.19) of large tonnages of massive iron sulphides and minor amounts of zinc. STAMICO has drilled additional holes at Samena without enhancing the base metal potential. In 1984, the government granted prospecting licenses for five large areas, including Geita, to Dar Tadine el Umma Ltd. An affiliate of the group, Geosurvey International GmbH, had earlier made a helicopter borne geophysical survey of the Geita area, apparently on a speculative basis. Results of this work were not compiled. The UNDP returned to the Geita area in 1985 to conduct geochemical and geophysical surveys over the strike projections of the known ore zones at the Ridge 8 and Northeast Extension deposits of the Geita Mine. This work continued in the form of diamond drilling by the UN Revolving Fund, which stopped at the end of 1994. In 1988, the Allway prospect (now Mgusu) became the site of a rush by artisanal miners. Results of recent geophysical work are described by Batterham (1983). Other recent work is reported by the UNDP (1983).Other recent work is reported by the UNDP (1986 and 1987). Hester (1990) supplies detailed information on some individual prospects. Recent (post 1990) exploration at Geita has focused primarily on areas of prior mining activity.Large, prospective area of the belt however, are covered by lake ï¿½mbugaï¿½ sediments and other regolith forms which to date remain unexplored. The discovery of the ï¿½blindï¿½ Nyankanga deposit under cover, but proximal to the Geita Gold Mine, clearly highlights the exploration potential of the Lake Victoria Goldfields in general, and the Geita greenstone belt in particular. Since 1994, Cluff Minerals,POangea Minerals, Samax Gold, Ashanti Goldfields and Anglogoilg have explored the area around the Old Geita Mine and the surrounding prospects. Geology. The Geita greenstone belt consists of two WNW and ESE trending Banded Iron Formation (BIF) ridge complexes within a poorly exposed sequence of Nyanzian Formation. The latter is thought to underlie much of the surrounding lower ground but is largely obscured by ferricrete and Mbuga cover. It comprises tholeiitic basalt lavas commonly containing pillows, mafic tuffs. The Geita greenstone belt, which hosts the Geita Trend ( Nyankanga Block), is dominated by upper Nyazian (2.5½ and 2.8 Ga) sediments composed of intermediate to felsic volcaniclastic rocks and BIF volcaniclastic sequence series are poorly exposed mafic tuffs and quartz porphyry which grades into felsic tuffs. The Geita Trend (Nyankanga Block), is dominated by upper Nyanzian. BIF is closely associated with all the known gold occurrences of economic significance. Other rocks associated with gold mineralisation include felsic tuffs, sheared mafic volcanic rocks, volcanogenic massive sulphides such as the Samena pyrite deposit and ferricrete. A coarse grained facies of the igneous rock is now interpreted as pyroclastic rather than intrusive, thus changing the interpretation of ore controls, which was favoured while the mines were operating. Underlying and to the south of the BIF Volcaniclastic sequence series are poorly exposed mafic volcanic rocks. These are bounded on the south by granites, which presumably intrude the volcanic rocks. No comprehensive interpretation of the structural geology of the Geita District has ever been published. However, recent exploration work in the Geita greenstone belt has revealed that two main structural trends of northwest and northeast dominate. The latter is commonly associated with the regional quartz gabbro dykes.

Striking north to north northwest is commonly intruded by dolerite dykes of Karoo age. Subsidiary faults are mapped from magnetic interpretation. Laterite is well developed at surface, particularly over mafic volcanic rocks. A few prospects show compelling evidence that migration and secondary enrichment of gold has occurred due to weathering processes, as described from Western Australia by Mann (1984).

Mineralisation. Gold mineralisation in the Geita area was first discovered in the early 1930s. Between 1936 and 1966, the Geita Gold Mines Ltd., constituted the largest gold operation in East Africa that produced 5.5 million tones of gold ore at an average grade of 5.3 g/t gold sourced from five deposits including Geita, North and East Extension, Lone Cone, Prospect 30 and Ridge 8 Mineralisation resembles in general style that described in the literature from BIF gold occurrences such as in Western Australia and the Geraldton area of Ontario, Canada. Gold mineralisation and distribution in the Geita greenstone belt is controlled by a complex interplay between structural and lithological features. The Geita Trend (Nyankanga Block deposits occur over a strike length of some 5km from Nyankanga in the west through Lone Cone to Geita Hill (Northeast Extension in the east). All deposits occur along a major dislocated thrust surface which form a semi-continuous east-northeast mineralised trend near the hinge of a principal regional west-plunging synform. All deposits dip north- to northwest, sub-parallel to the stratigraphy. Total gold resource from these deposits accounts for more than 70% of the Geita Gold Mine total resource inventory.Gold occur in native form, commonly in close association with pyrite and pyrrhotite, in fractures following the bedding of BIF units. Formerly it was believed that the BIF and its associated mineralisation would not persist into the supposed intrusive rocks so on exploration was undertaken in that direction. These areas are now of great interest, as the gold mineralisation may persist along strike within the BIF into the pyroclastic volcanic rocks.

An unpublished study of the geomorphological condition in the Geita area in 1986 by D.Sutherland concluded that placer concentrations of gold at Geita are unlikely to be well developed. This conclusion agrees with discouraging results from work in streams by early prospectors. The major mineralisation targets in Geita area fall into the following groups Mineralisation within BIF located on crests of ridges. It occurs either at intrusive contact within the main BIF sequence or close to felsic tuff contacts. Mineralisation associated with shear zones in mafic volcanic rocks. Classical volcanogenic massive sulphide deposits such as at Samena Hill pyrite deposit

Mineralisation in ferricrete either as enrichment over bedrock or from erosion of mineralized BIF ridges.

Geita Gold Mine.

In 1994, Cluff Resources (a UK company) acquired two prospecting licences covering much of east and west of the Geita area including the old Geita Mine and surrounding prospects (fig.19). Cluff conducted exploration work at Lone Cone, Samena, Nyamonge and Prospect 30 prospects. Significant discoveries were made, including large quantities of low - grade gold bearing material in and around old mine workings. In early 1996, Cluff Resources was taken over by Ashanti Goldfields (a UK and Ghana company), which continued exploration work with a discovery of additional high - grade mineralised zone at Nyankanga west of lone cone. IN 1997, SAMAX Resources acquired licences to the north and northeast (Kukuluma Hill) of the old Geita Mine.

In the area of the Geita Gold Mine, NW trending deformation corridors separate the Geita Greenstone into three distinct sub terrains, which have been named Nyamulilima in the west, Nyankanga - Geita in the central part and kukuluma to the north - east (Fig.22).

The Geita Gold Mine comprises distinct deposits, which include Nyankanga, Lone Cone, Geita (Geita Project), which have been explored by Cluff Resources and Ashanti Goldfields, Kukuluma Project consisting of Kukuluma, Mtandani and Area 3 west deposits (Fig.22), which have been explored by SAMAX Resources since 1997 and Nyamulilima Hill which was explored by Anglogold.

Gold Mineralisation throughout the Geita area can be related closely to the nature of the host rock lithologies (Fig.20). Mineralisation is best developed in areas where there is interbedded BIF/andesite, or BIF /tuff sequence and is poorly developed in andesite dominated sequences. Intensive small scale folding is locally present of sulphides within the BIF. The mineralisation is closely associated with the presence of sulphides, predominantly pyrite. It is also present as free gold in carbonate veins commonly with high but variable grades. Quartz veins commonly with high but variable grades. Quartz veins containing high grades are present locally and may attain up to 2m width. They are partly concordant with the enclosing host rocks, commonly in association with felsic intrusions.

The Geita Trend ( Nyankanga Block) is situated on the southern limb of the principal, westerly plunging synformal fold with a west - north - west axial planar trend and includes the Geita - Lone cone hills and Nyankanga (Fig.21).

At Geita and Lone Cone Hills, gold minelisation is strongly linked to sulphides.It is controlled by major shear zones trending north. Higher - grade mineralisation in preferentially located in BIF units close to the contacts with andesites or tuffs. Nyankanga ore body was discovered in 1996 following RAB follow up of a strong soil anomaly. It lies 0.6km west of Lone Cone and is thought to be the extension of the main North East Extension - Geita Lone cone shear zone. The principal structures are associated with a major northwest trending fault. The mineralised zone trends northeast and extends over more than 1 km strike and to a vertical depth of 150 m.

The kukuluma Trend comprises five deposits within a five kilometre long east - southeast mineralisation trend cutting obliquely across a northwest trending horseshoe ridge of BIF. The five areas comprise three on the eastern limb of the horseshoe in Area 3 and two close to the apex on the western limb now referred to the apex on the western limb now referred to as the Kukuluma and Matandani deposits (Fig.22).

The kukuluma Hill and Mtandani deposits are located 6 km northeast of the Nyankanga - Geita deposits on a separate ridge system. The two deposits are hosted by two shear zones that are discordant with the strike of the bedding. The mineralised shear zones trend WNW to NW. Mineralisation is preferably developed in BIF - Chert units. Favourable host lithologies for mineralisation are BIF, chert and interbedded tuff and chert units. Eight percent of the mineralisation is in oxidized rocks amenable to open pit mining. The mineralisation is open at depth. Grades and widths indicate potential for underground mining. The Mtandani deposit is also made up to two parallel shear zones.

The Nyamulilima Hill deposit is situated in the northwestern portion of the Greenstone belt, 15 km west of Geita. The hill rises approximately 300m above the surrounding plains and comprises an oxide facies banded ironstone sedimentary sequence flanked by felsic volcanics that have been intruded by silicified quartz - feldspar porphyry (QFP).

Two major northwest trending shears on either side of the hill have given rise to subsidiary north - south and northwest trending structures cross - cutting the ridge. Fold axial structures and predominantly northwest southeast trending dextral shearing control mineralisation. Most of the ore bodies are hosted in BIF and at QFP contacts. The auriferous zones are normally typified by silicification with pyrite and / or pyrrhotite mineralisation.

The Geita Gold Mine operates a conventional open pit mine consisting of six distinct deposits whose total proven and inferred resources are 14.6 Moz at an average grade of 4.0 g/t. The Geita Gold mine became operational in 2000. joint venture basis with estimated annual production of 0.6 Moz.

Mgusu Deposit (Saragura - Allways).

Mgusu deposit is located on Saragura Hill, west of Geita Gold Mine. This hill is part of the ridge entire Saragura - Prospect 30 to the southeast. The entire Saragura - Prospect 30 Ridge is an intriguing target for prospecting.Mgusu deposit is hosted by BIF and quartz porphyry. The UNDP drilled three diamond holes in 1985 and located modest gold values. The area is being intensively worked by artisanal miners. Gold occurs in banded iron formation at the contact with tuffs.

The area has been explored by Pangea Minerals / Ormonde and five mineralised structures have been delineated. Two of the structures were subjected to detailed exploration involving trenching, RC drilling and diamond drilling. Ground magnetics and soil sampling have identified a 5.2 Km long gold - in - soil overlying a major shear structure which has been inferred to continue on strike from the nearby Geita Gold Mine.

Stockwork mineralisation has been discovered within quartz - porphyry and increases towards a contact with BIF. Pangea reported gold values of 14.9 g/t gold over 9m width, 5.89 g/t gold over 21 m and 21.09 g/t gold over 5 m contained in mineralised saprolite porphyry. A resource of 1.83 million tonnes at an average grade of 4.6 g/t gold was calculated for a total inferred resource of 271,000 oz gold occurring from the surface to a vertical depth of 150 m.