Muruntau gold deposit

Geometry of the Mineralisation

The geometry of the Muruntau deposit was controlled by three main factors: presence of the carbonaceous and sulphidic Besopan 3 unit; proximity to the ENE-trending Muruntau-Daugyztau Fault and presence of an ENE-trending fold axial zone on the north side of the fault. Drew et al. (1996) have argued that another control is the presence of a Carboniferous thrust fault at the top of the Besopan 3 unit. Thus the orebody is to some degree stratabound within the Besopan 3 horizon but is also structurally-controlled by small-scale folding and fractures related to the Muruntau-Daugyztau fault zone and its related folding. Interpretation of the ore geometry is complicated by some post-ore movement on the Muruntau-Daugyztau Fault.

Gold occurs in sub-vertical quartz-sulphide veins and irregular quartz vein stockworks which cut hydrothermally-altered clastic host-rocks. The veins and vein stockworks are most intense within a few hundred metres of the major north-easterly trending Muruntau-Daugyztau strike-slip fault (Fig. 3). Several large sub-vertical quartz-gold veins (the Central Veins) form the core of the mineralisation and contain the highest gold grades seen at Muruntau. These veins are undeformed and trend ENE and easterly. They can be several metres thick.

Mineralised veins such as the Central Veins and stockworks postdate earlier, boudinaged and isoclinally folded quartz veins. The early veins are typically thin (a few cm) and usually parallel to the dominant cleavage of the host phyllites.

Ore Mineralogy and Paragenesis

Gold is associated with pyrite and arsenopyrite, which comprise a few volume percent of the ore. The sulphide minerals often occur as veinlets in altered host-rocks and as isolated clots within quartz veins. Pyrite grains sometimes enclose small pyrrhotite grains suggesting that some pyrite may have replaced pyrrhotite (Gilbert, 1995). Rare chalcopyrite, sphalerite and molybdenite were observed in a number of low-grade stockpiled ore samples. Gold has been observed as minute inclusions in pyrite (Gilbert, 1995) and intimately intergrown with antimony sulphide (Schandl, 1997).

The deposit is anomalous in tungsten which occurs as scheelite. According to Uspenskiy and Aleshin (1993) tungsten is confined to shallow-dipping stratabound zones which are cut by later discordant gold ore zones. Grades are generally sub-economic (typically hundreds of ppm) but can reach 0.5%. Uspenskiy and Aleshin (1993) provide evidence that scheelite occupies fractures in early folded quartz veins and also occurs as veinlets with pyroxene and amphibole selvages.

Tungsten veins are cut by auriferous arsenopyrite veinlets, suggesting that the tungsten pre-dates gold mineralisation. In the open pit, tungsten-bearing zones are observed to be cut by gold ore zones, which are related to the large discordant quartz veins (Central Veins; Uspenskiy and Aleshin, 1993).

A distinctive silver-rich gold and base-metal sulphide assemblage occurs in the region about the Muruntau deposit. Samples from the Cosmanachi silver mine, 16 km west from Muruntau, consist of galena, sphalerite, tetrahedrite, pyrargyrite, millerite, an unidentified silver sulphide, a silver sulphide-telluride mineral, lead silver antimony sulphide and others (Gilbert, 1995). These minerals overprint early arsenopyrite and pyrite (Gilbert, 1995) suggesting that the silver-rich mineralising event is later than the main gold event. Secondary minerals include chalcocite, covellite, lead arsenate and stolzite (lead tungstate).

Hydrothermal Alteration

Gold-coeval hydrothermal alteration at the Muruntau deposit is very extensive. The alteration, which consists of quartz, albite and biotite ("metasomatite") overprinting and replacing the regional and contact metamorphic assemblages, extends in a lens-shaped area with dimensions of approximately 8 km by 2 km about the gold mineralisation at Muruntau, in the so-called "Muruntau Lens" (Fig. 3); (e.g. Marakushev and Kokhlov, 1992; Kotov and Poritskaya, 1992). There is some petrographic evidence that much of this alteration consists of layer-parallel replacement of carbonate-bearing units (Gilbert, 1995; Schwandl, 1997). The modal mineral composition of the alteration is illustrated in Figure 4.

The albite-stable alteration ("metasomatite") is overprinted by sericite and chlorite related to the main ore depositing phase (Kotov and Poritskaya, 1992). Sericite and chlorite is limited in extent relative to the earlier alteration, being confined to narrow vein selvages of a few millimetres extent (Kotov and Poritskaya, 1992) and pervasive replacement of feldspar.

A ubiquitous alteration phase is pale-coloured dravitic tourmaline (Gilbert, 1995). Microprobe analysis reveals that these dravites are rich in V (up to 2.7% V2O3), Mg and Na (Gilbert, 1996). Carbonaceous material is locally abundant in the pit. Petrographic determinations on stockpiled ore samples reveal an average of 4% by volume and locally as much as 50% (Schandl, 1997). Marakushev and Khokhlov (1992) describe carbon "fronts" but the relationship between carbon "fronts" and gold is not well understood at this time.

Age of the Hydrothermal System(s)

One of the most controversial aspects of the Muruntau orebody is its age. Alteration replaces and exploits a preexisting cleavage and also overprints contact metamorphic andalusite and cordierite associated with the buried alaskite pluton beneath the deposit (Kotov & Poritskaya, 1992; Drew et al., 1996). These observations strongly suggest that alteration was considerably later than peak deformation, and post-dates the thermal event accompanying the emplacement of the adjacent granite. Kostitsyn (1993, 1996) provides three Rb-Sr mineralisation ages: 257.6 ± 2.2, 230.2 ± 3.5 and 219.4 ± 4.2 million years (Permian to Triassic). All these ages are significantly younger than the crystallisation age of the Murunski intrusive, 4 km beneath the Muruntau deposit.