The Lower Palaeozoic Besopan Formation basement outcrops as a number of inliers, comprising perhaps 15% of the Kyzyl Kum area (Fig. 1). These inliers form low ranges of hills which trend west-north-west. T h e Besopan Formation consists of metamorphosed mudstones, siltstones and sandstones. It has been divided into four units, termed Besopan 1, 2, 3 and 4, based on colour variation and the ratio of coarse to fine-grained clastic sedimentary rocks. Besopan 1 is the oldest, and Besopan 4 the youngest. The lithological succession in the Besopan Formation is difficult to define precisely, because of repetition and omission of units caused by isoclinal folding and major thrust faults (e.g. Drew et al., 1996).
Besopan 3 ("Variegated Besopan") is the main gold host at Muruntau. It is represented by green to red (hematitic) phyllites in surface exposures but is extremely carbonaceous at depth (Y. Savchuk, pers. comm., 1996). The Besopan 2 consists of similar lithologies to the overlying Besopan 3, namely phyllite and meta-sandstone. Marakushev and Khoklov (1992) show that the two units are identical petrochemically. The main difference may be caused by variation in the abundance of carbon and in the sulphide minerals between Besopan 2 and Besopan 3.
A distinctive black siliceous layer occurs within the Besopan 3. The layer is a few metres thick and can be traced as discontinuous surface exposures extending more than 15 km west from the deposit. A sample of the siliceous layer (collected 10 km west from the Muruntau open pit) contains elevated palladium (0.1 ppm), V and Hg as well as abundant fine-grained carbon as patches and veins. Inclusions of carbonate in quartz suggest in the sample that this horizon may be a siliceous replacement of a carbonate-bearing shale horizon (Gilbert, 1995).
Unmetamorphosed Devonian to Carboniferous carbonate-dominant rocks overlie the meta-clastic rock of the Besopan Formation with angular unconformity. Comparable sequences are exposed in the Karatau Range of southern Kazakhstan about 200 km north of Muruntau (Fig. 1). Here the sequence consists of a variety of Devonian and Carboniferous carbonate rocks and breccias interpreted as having an evaporitic origin modified by karstic collapse (Lapointe et al., 1997). The depositional environment is interpreted to be a migrating shelf to platform transition reflecting fluctuating sea levels (Fig. 2; Cook et al. , 1997, Lapointe et al., 1997; Cook pers. comm., 1998) .
The late Carboniferous of the Karatau Range is marked by a major change to a shallow-marine terrestrial (evaporitic) facies (Fig. 2). Petroleum exploration drill-holes north-west of Muruntau intersected folded Late Permian to Early Triassic red-brown sandstones and mudstones, black mudstone and tuffaceous sandstones which overlie gray-colored porphyritic andesite lavas (Burshtein, 1998).
The Jurassic of Cover Sequence 2 comprises unfolded terrestrial sediments which include sandstone, siltstone, mudstone and coal. The environment of deposition is interpreted as fluvial, alluvial and lacustrine (Burshtein, 1998). The Cretaceous of Cover Sequence 2 comprises hematitic siltstone, mudstone and conglomerate. Lenses of anhydrite occur in the upper parts. The Early Cretaceous rocks are terrestrial in origin but a marine influence is apparent in the upper part of the section (Burshtein, 1998).
Granites comprise less than 10% of the Palaeozoic bedrock exposures in the Kyzyl Kum region. Aeromagnetic data indicate that they could be more extensive beneath sedimentary rocks of Cover Sequence 2 (Savchuk et al., 1991). Savchuk et al. (1991) recognise an older Bokalinsk Suite and younger Nuratinsk suite. The Bokalinsk Suite outcrops rarely and comprises only 5% of outcropping intrusions. The Nuratinsk Suite is predominantly composed of granite and granodiorite and comprises 95% of exposed intrusions in the Kyzyl Kum region. It is spatially and probably temporally associated with tin mineralisation in the region. Granites of this suite intrude folded sedimentary rocks of the Basement and Cover Sequence 1.
Two intrusive bodies and dykes of several compositions are known in the vicinity of the Muruntau deposit. One of the intrusive bodies, the Sardarin Pluton, is 12 km south of the Muruntau deposit, but is apparently not exposed. Its extent has been determined from drilling and from interpretation of airborne magnetic data. The other intrusive body, the Murunski Pluton, is composed of a medium-grained alaskite, and was intersected beneath the deposit in an exploratory diamond drill hole at a depth of 4 km (Kostitsyn, 1996). Contact metamorphism about the pluton is manifested as biotite porphyroblasts and locally, as andalusite and cordierite porphyroblasts (Kotov and Poritskaya, 1992; Drew et al., 1996). Hornfels has been observed in the drill holes beneath the Muruntau deposit.
The Sardarin Pluton yielded a Rb-Sr isochron of 286 ±
1.8 million years and initial ratio of 0.7078 (Kostitsyn, 1996). The Murunski Pluton yields an identical isochron of 287 ± 1.8 million years (Permo Carboniferous) and initial 87Rb/87Sr ratio of 0.716.