CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Image: View of the Tien Shan Mountains.
Porter GeoConsultancy continued its International Study Tour series of professional development courses by visiting a representative selection the giant orogenic Au, meso- to epithermal Au and porphyry Cu-Au deposits of the Tien Shan gold belt of Central Asia, one of the most heavily gold endowed segments of the Earth's crust.
in Tashkent, Uzbekistan,
Muruntau orogenic Au deposit, Uzbekistan,
Amantaytau & Vysokovoltenoye Au/Ag deposits, Uzbekistan,
Zarmitan orogenic Au deposit, Uzbekistan,
Almalyk porphyry Cu-Au deposits, Uzbekistan,
Kochbulak & Kyzykalma epithermal Au deposits, Uzbekistan,
The tour commenced in Tashkent, Uzbekistan on the evening of Sunday 10 September and ended back in Tashkent on the afternoon of Tuesday 19 September. Participants were able to take any 4 or more days, up to the full tour, as suited their interests or availability.
The main components of the itinerary were:
A one day workshop was run in Tashkent at the beginning of the tour to provide a context to the tectonic, geological and metallogenic setting of the Tien Shan Belt and the deposits within it.
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Overview - Gold mineralisation occurs in two principal settings within the Tien Shan Mineral Belt, namely as i). orogenic-type gold deposits that are structurally controlled, and temporally and spatially associated with late Palaeozoic, syntectonic to early post-collisional, highly evolved, I-type granodioritic to monzonitic intrusives in fore- and back-arc terranes, and ii). porphyry and epithermal systems developed within magmatic arcs (Cole and Seltmann, 2000; Yakubchuk et al., 2002; Mao et al., 2004).
Although belonging to two different terrane settings, the giant orogenic Au mineralisation hosted by the black-shale series of the Southern Tien Shan, and the giant Cu-Au porphyries of the Almalyk district, in the Valerianov-Beltau-Kurama magmatic arc, of the Middle Tien Shan, have some striking similarities. These suggest a crust-mantle interaction and dominance of a deep-seated regime during emplacement (I.M. Golovanov, pers. comm.; Dalimov et al., 2003). They are temporally close (315 to 285 Ma, Seltmann et al., 2004), their isotope signatures reveal the incorporation of a moderate mantle component (Chiaradia et al., 2005), and geophysical patterns from the middle crust in the region exhibit zones of low reflection indicating the existence of extended mafic bodies just beneath both giant ore-magma systems (S. Cherkasov, pers. comm.).
The orogenic gold deposits of the Tien Shan Mineral Belt include some of the largest economic gold accumulations in the world. These deposits are spread across the belt in Russia, Uzbekistan, Tajikistan, Kyrgyzstan, Kazakhstan and western China, and span the time scale from Lower to Late Palaeozoic. The greatest concentration of significant orogenic gold deposits however, is in the southwestern part of the belt, in the South and Middle Tien Shan of Uzbekistan and Kyrgyzstan. These deposits are located at mesozonal crustal levels, associated with and either within Late Palaeozoic granitoid intrusives, or their contact metamorphic aureoles. The mineralisaiton yields radiometric dates coincident with Permian magmatism emplaced during the final- to early post-collisional stages of orogenesis, within a sutured back-arc setting containing carbon-rich sedimentary sequences (Cole and Seltmann, 2000; Yakubchuk et al., 2002; Mao et al., 2004). However, few of the deposits can be shown to have a direct genetic link with the associated intrusives. Never-the-less, geochemical, isotope and fluid-structural models have implicated highly evolved Late Palaeozoic, syntectonic I-type granitoids as the source of metals and/or fluids for spatially associated orogenic gold deposits within the belt
The orogenic gold deposits of the South Tien Shan are controlled by structures related to the Southern Tien Shan Suture Zone that separates the Carboniferous Valerianov-Beltau-Kurama magmatic arc to the north, and the Altai-Tarim-Karakum micro-continent to the south. They are hosted by the back arc accretionary complex deposited in the basin that had separated these two tectonic elements. The suture zone is defined by ophiolites and borders the strongly deformed fold and thrust belt of the Southern Tien Shan that has been extensively intruded by Permian granitoids and hosts most of the significant orogenic-style gold deposits (Mao et al., 2004).
The gold-quartz vein systems of the orogenic gold deposits appear to represent only part of a larger magmatic-hydrothermal system that often includes earlier scheelite (±Au) skarn mineralisation (e.g. Zharmitan in Uzbekistan and Jilau in Tajikistan, while Muruntau, also in Uzbekistan, exhibits some similarities). In these examples, Au and W occur together with characteristic enrichments of As, Bi, Mo and Te deposited from CO2-rich fluids at temperatures of up to 400ūC and pressures of approximately 2 Kbar (Cole and Seltmann, 2000). Cole and Seltmann, (2000) note a general trend in these granitoid related systems, where W, in the form of scheelite, dominates in mesozonal, more reduced settings, while Cu substitutes for W in the paragenesis of epizonal, more oxidised systems. They suggest a continuum which would encompass classic Cu-Mo-Au porphyry, Cu-Au skarn and Au-Ag epithermal deposits in epizonal crustal environments/levels, passing down into W-Mo-Au with associated Bi-As-Te associations in skarn, lode and stockwork deposits (i.e. orogenic-style Au) at mesozonal depths.
This overview is summarised from Seltmann and Porter, 2005.
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The Muruntau gold deposit, which is located within the Kyzyl Kum desert in central-western Uzbekistan is the largest gold resource in Eurasia. It is some 400 km to the west of the capital, Tashkent, and is served by the adjacent town of Zarafshan (#Location: 41° 29' 45"N, 64° 34' 36"E).
The deposit was discovered in 1953, although ancient gold mines have since also been revealed in the vicinity of the ore deposit. Detailed exploration took place between 1960 and 1965. Construction commenced in 1964, and mining in 1967, with the gold plant beginning operation two years later.
The deposit originally contained more than 5400 tonnes (175 Moz) of gold at an open pit recovered grade of 3.4 g/t Au.
The deposit occurs within a pile of imbricated thrusts that was deformed into west-east trending synforms and antiforms exposed in the Tamdy mountains near the western extremity of the South Tien Shan tectonic province (Drew et al., 1996). It lies to the south west of a major regional suture zone (marked by the occurrence of mafic rocks of an ophiolitic association in a zone of intense deformation) which separates the Middle Tien Shan tectonic province to the north east (represented by the Carboniferous Valerianov-Beltau-Kurama magmatic arc), from the South Tien Shan fold and thrust deformed accretionary complex overlying older Meso- to Neoproterozoic basement of the Karakum micro-continent. The South Tien Shan has been interpretted to comprise four gross nappe units, each of which is composed of more than one interleaved thrust slices.
In the Muruntau area, the South Tien Shan consists of tectonically superimposed lithologies (Savchuk et al., 1991; Drew et al., 1996), which represent early-middle Palaeozoic oceanic to accretionary and fore-arc complex rocks thrust onto Meso- to Neoproterozoic to middle Palaeozoic passive margin sedimentary rocks whose late Neoproterozoic (Vendian) to lower Paleozoic section was metamorphosed to amphibolite to greenschist facies.
The host sequence to the Muruntau mineralisation is the Cambrian to Silurian Besapan Formation. This sequence is overlain by a structurally emplaced unit of Devono-Carboniferous carbonates. The Besapan Formation lies on the northeastern flank of a granite gneiss dome, in whose axial core highly metamorphosed Proterozoic rocks are exposed outside of the ore field. These comprise retrograde biotite-garnet gneisses, garnet amphibolites and migmatites. Metamorphism in the region increases towards the south west, from the almost un-metamorphosed upper member of the Besapan Formation, to meta-siltstones of the middle sequence, to the grey schists of the basal unit, which are underlain by more strongly metamorphosed biotite-muscovite schists further to the south-west. On the whole however, the metamorphism of the Besapan Formation is relatively slight, expressed mainly in the pelitic matrix of siltstones and sandstones. The matrix has become schistose with the development of chlorite and sericite in the less affected rocks, to muscovite and biotite in the most strongly metamorphosed bands, with associated albite and quartz. This pattern is overlain by local zones of brecciation, banding and phyllonites, and the superposition of alteraton characterised by biotite, plagioclase, K-feldspar, quartz, graphite and carbonaceous, accompanied by magnetite and sulphides. This alteration has led to the development of an envelope of carbonaceous biotite rocks around the Muruntau deposit that are often brecciated, either conformably or transgressively, with anomalous gold levels (Marakushev and Khokhlov, 1992).
The stratigraphy within the immediate Muruntau district can be summarised as follows, from the base:
Meso- to Neoproterozoic - Taskazgan Suite
Retrograde Gneiss - exposed at the base of the succession to the west of the Muruntau ore field, comprising Mesoproterozoic retrograde gneisses, dated at 1750 ±80 Ma.
Two Mica Schists - predominantly biotite-muscovite schists in the single small exposure found in the Muruntau area, where they are unevenly enriched in carbonaceous material. They contain beds of meta-pelite, siliceous rocks (cherts), mafic volcanics, dolomites and meta-sandstones, and grade upward into the phyllites of the lower sections of the Grey Besapan unit (Marakushev and Khokhlov, 1992).
Late Neoproterozoic to Silurian - Besapan Formation
Grey Besapan - The rocks of the Grey Besapan occupy vast areas of the western and central sections of the ore field. They are uniform, carbonaceous phyllites, consisting of chlorite, muscovite, biotite, albite and quartz. The lower section, sometimes referred to as bS1, is a meta-siltstone, while bS2, the upper part of the unit, is a mixed meta-siltstone and meta-sandstone. The fragmental material in the sediments is composed of quartz and feldspar, with clasts of intermediate and silicic volcanics in isolated horizons. The bedding is especially marked by gritty meta-sandstones. Late Upper Proterozoic to lower Cambrian and Ordovician fauna have been described from this unit. The phyllitic rocks of the Grey Besapan grade gradually into the underlying phyllites, which differ only in their greater abundance of biotite.
Variegated Besapan - This unit hosts the largest of the Muruntau orebodies, and is predominantly composed of carbonaceous, meta-siltstone, meta-sandstone, meta-volcanic rock and minor radiolarian chert. The volcanics are predominantly intermediate and siliceous tuffs. The unit is both carbonaceous and pyritic and is referred to as 'variegated' because of its variable red and green colouration in outcrop (Berger, et al., 1994). It is referred to as bS3, and has been subdivided into three members, as follows (Marakushev & Khokhlov, 1992):
Lower Member - is the most heterogeneous in composition, including quartz-mica meta-siltstones, sometimes containing carbonaceous material, meta-sandstones, phyllites and isolated beds of gritty and calcareous meta-sandstones and meta-siltstones
Middle Member - is characterised by thin beds and lesser lenses of metatuffs, within a mixture of the lithologies described for the 'Lower Member'.
Upper Member - which consists of greenish-grey and varicoloured carbonaceous-micaceous phyllites, polymict meta-siltstone and isolated beds of meta-sandstone.
The Variegated Besapan has been tentatively assigned to the Ordovician to lower Silurian period (Marakushev & Khokhlov, 1992).
Green Besapan - approximately 1000 m thick - This is the uppermost and least metamorphosed unit of the Besapan Formation. It is predominantly a sandstone, composed mainly of quartz, with clay material replaced by chlorite and sericite, giving it a greenish colouration. The base is slightly metamorphosed, or epigenetically altered, but bears the trace of submarine hiatuses and ripple marks. Greenish grey sandstones and siltstones with green argillite are the dominant lithologies with beds and lenses of grits up to 1.5 m thick. Graptolites indicate a Silurian age (Marakushev and Khokhlov, 1992).
Carbonates - up to 3000 m thick - composed predominantly of sandstones and dolomitised limestones. The contact is sometimes an angular unconformity and at other places a structural plane (Marakushev and Khokhlov, 1992).
Granitoids - The closest exposure of a significant granitoid body is some 7 km to the south-east of Muruntau. Here it is largely concealed by Cenozoic cover, but is seen to comprise a medium grained, slightly porphyritic granodiorite-adamellite (Kotov & Poritskaya, 1992). Other igneous rocks have been developed, mainly on the periphery of the ore field. They comprise leucocratic dykes which have been concentrated in several differently oriented zones, in different parts of the field. In the northern section of the field, a 7 km long, east-west striking belt of 34 dykes is recorded, while to the east 44 dykes strike to the north-east. To the south-east a further east-west striking set of 50 dykes are found with two associated stock-like bodies which are around 120x300 m across. These dykes are plagiogranite porphyries, syenite porphyries and spherulitic syenite porphyries (Smirnov, 1981). They have been largely emplaced along regional shear zones (Berger, et al., 1994) and also include lamprophyres, quartz-diorite, syenite-diorite and granosyenite (Kotov and Poritskaya, 1992). Granitoids have also been intersected in deep drilling 4000 m below the deposit.
Kotov and Poritskaya (1992) summarise the tectonic evolution of the South Tien Shan Belt in the Muruntau area into the following stages:
i). The development of regional nappes which dip northwards in the north, and to the south in the south. This thrusting has led to duplication of the sequence in the district, with four major thrust slices being represented;
ii). Formation of conjugate, sub-latitudinal regional fold structures to form a series of parallel antiform and synforms. According to Berger, et al., (1994), following the compression that formed the nappes, subsequent oblique convergence resulted in transpressional deformation and the formation of a broad, sinistral shear zone, the Sangruntau-Tamdytau Shear zone which accompaniedthe folding. This shear zone, which is believed to represent a splay of the major suture zone to the north east, in turn bifurcates and splits into a number of splays to the north west and west of the mine;
iii). Compression of these folds and the Sangruntau-Tamdytau Shear zone to form a number of regional 'Z-shaped' structures, especially at Muruntau where the deposit is near the elbow of one such structures;
iv). Formation of deep seated faults as the regional deformation surface was over-printed by brittle fracture which also influenced the location of Permo-Carboniferous granitoids. The Muruntau deposit lies near the intersection of the brittle north-east trending Muruntau-Daugyztausk fault zone, and the coincident antiform and Sangruntau-Tamdytau Shear zone, where the latter structures are deformed into a regional 'Z-shaped' structure (Kotov and Poritskaya, 1992).
Alteration and Mineralisation
The orebodies at Muruntau essentially constitute a megastockwork (Kurbanov, 1999) of quartz-dominant veins and associated quartz-albite-phlogopite and sericite-chlorite-(K feldspar)-carbonate alteration of two generations.
The ore grade mineralisation at Muruntau is developed within a characteristic massive, light pink to yellow, biotite-plagioclase-quartz-orthoclase rock. The compositional range of these rocks is generally: 25 to 50% orthoclase, 25 to 40% quartz, 15 to 25% plagioclase (albite and albite-oligoclase), and 20 to 40% biotite, representing an enrichment in alkali metals. The gold content of this alteration type is typically 1 to 3 g/t, locally increasing to 20 to 30 g/t Au (Marakushev and Khokhlov, 1992).
These more highly mineralised, two feldspar-biotite-quartz rocks replace, and are found within the core of a larger envelope of black banded rocks rich in carbon and biotite, which contain low grade disseminated gold mineralisation, commonly of around 1 ppm. The mineralised carbon-biotite rocks replace carbonaceous meta-siltstones of the Besapan Formation, preserving their original banded nature and surrounding relicts of un-altered siltstone. In general the replacement is bedding controlled above the base of the Variegated Besapan and is predominantly manifested in a north-east plunging synclinal structure (Marakushev and Khokhlov, 1992).
Prior to the alteration, and the emplacement of both the ore and the regional granitic bodies, the host siltstones had been metamorphosed to greenschist facies to produce siltstones composed of 40 to 50% detrital quartz, with the remainder being chlorite, sericite, oligoclase and K-feldspar in variable amounts (Kotov and Poritskaya, 1992).
Deformation prior to the emplacement of the gold mineralisation is considered important in the development of a high permeability within the Besapan Formation. Schistosity and axial plane cleavage were developed during regional metamorphism, folding and shearing, while brittle fracturing and brecciation accompanied later faulting. The brecciation is commonly sufficiently developed to classify the rocks as phyllonites which exhibit shattering, mylonitisation and crumpling, with the development of films of biotite, muscovite, chlorite and graphite or carbonaceous material along directions that often cut the bedding planes. The schistosity and cleavage are the controls on the largest proportion of the mineralisation which is interpreted to have taken place during the late Carboniferous to early Permian, coincident with a change from compressional to transpressional tectonics (Berger, et al., 1994; Marakushev and Khokhlov, 1992).
Drew et al., (1996), recorded the following paragenesis of alteration and gold mineralisation:
i). Quartz + albite + biotite + chlorite + oligoclase alteration forming linear, subparallel zones of quartz veins and veinlets. Oligoclase was an early phase, overprinted by quartz, albite and later K feldspar. This alteration phase in general has weak associated gold and overprinted the original spotted schists which resulted from regional and contact metamorphism;
ii). The second stage is characterised phlogopite and some pyrite (±arsenopyrite), in en echelon veins with selvages of muscovite, magnesian chlorite, quartz, phlogopite, K feldspar and minor iron-magnesium carbonate, with associated weak gold mineralisation to at least several hundred ppb;
iii). Quartz + K feldspar + muscovite veinlets with ankeritic carbonate and sulphide cut all of the above. This phase is associated with the Central Veins which contain the highest grade gold with grades averaging 3.5 to 11 g/t Au. Siliceous dykes intrude all of thepreceding and follow the emplacement of the Central Veins;
iv). Quartz + K feldspar + dolomitic carbonate + tourmaline (dravite) ± sulphide (pyrite) veinlet set which cuts the siliceous dykes and adjacent wall rocks, being apparently related to brittle fracture and brecciation; and
v). Calcite veinlets and pervasive calcite with rare pyrite (brookite?) phase which is the last hydrothermal event and destroys all earlier phases. Rare earth minerals, including monazite and bastnaesite occur in clusters in some calcite veins.
vi). A late quartz-sericite alteration has been observed, which may be intense, followingbrittle faults that offset the Central Veins.
Gold has apparently been introduced with each of the alteration assemblages detailed above.
Four principal types of veins are recognised at Muruntau, namely:
i). Flat Q1 veins which are mostly gently dipping, controlled by the foliation-parallel Tamdytau-Sangruntau shear zone (Drew et al., 1996). The host rocks carry grades of 0.03 to 0.3 g/t Au with higher concentrations of 1.5 to 2.5 g/t Au in the alteration halo (Kurbanov, 1999). The Q1 veins also host subeconomic scheelite in stratabound zones that formed before the introduction of gold mineralisation (Uspenskiy and Aleshin, 1993) and revealed Sm-Nd ages of 279 ± 18 Ma (Kempe et al., 2001).
ii). Stockwork-type Q2 veins which form an extensive, gently dipping, overall concordant zone of mainly small to microscopic veinlets that have been mined to a depth of 300 m (Berger, et al., 1994). The zone was generated by several stages of mineralisation formed under different rheological conditions, ranging from plastic to brittle (Smirnov, 1981; Sokolov, 1995; Berger, et al., 1994). The zone exhibits an irregular distribution of mineralisation, although Kurbanov (1999) mentioned grades of 3.5 to 5 g/t Au.
iii). Steep Central Veins (Q3), which generally cross-cut the sedimentary layering and schistosity and trend near east-west. They may reach thicknesses of 15 to 20 m in the bulges, are lensoid and traceable for up to 160 m (Graupner et al., 2001), with average grades of 15 to 20 g/t Au (Kurbanov, 1999).
iv). Silver-rich steep veins (Q4) which dip at 60 to 70° and are discordant and also strike near east-west. They are poorly auriferous, quartz-sulphide and sulphide veins which host silver and lead sulphides (Zairi and Kurbanov, 1992).
The discordant portions of the vein systems are the richest in gold. The conformable veins generally contain lesser, and more complexly distributed gold and are only significantly auriferous where discordant veinlets of gold bearing quartz-sulphide have been developed along with the concordant veining (Smirnov, 1981). However, gold is generally found in each of the vein types, namely quartz, quartz-sulphide and sulphide veins, especially where they intersect (Sokolov, 1995).
The principal ore mineral is native gold which occurs in the megascopic to microscopic quartz veins. The gold forms unevenly disseminated fine inclusions and veinlets, and sometimes small nests confined to places where sulphides have accumulated, or to rock fragments, and to the boundaries of quartz grains. The main vein mineral is quartz, with minor amounts of K-feldspar, biotite, ankeritic carbonate or calcite, tourmaline and albite.
The principal sulphide is pyrite with significant arsenopyrite, some marcasite and pyrrhotite. Other minerals include scheelite, gold and bismuth tellurides and selenides, galena, sphalerite, chalcopyrite, molybdenite, wolframite, magnetite and ilmenite. The sulphide minerals are generally associated with quartz, ankeritic carbonate, phlogopite, K feldspar and muscovite, with accessory apatite (turquoise is reported in the Muruntau area), monazite and TiO2 (as brookite?) (Berger, et al., 1994).
In addition to the free native gold, veins and veinlets contain gold in association with pyrite, arsenopyrite, chalcopyrite, sphalerite, bismuthinite, native Bi, ankeritic carbonate and sulpho-salts of silver. In sulphides there are fine (1 to 100 µm) segregations and thin short veinlets of gold from 5 to 10 µm thick. The gold segregations are localised in the cataclased portions of the sulphides or on boundaries of grains, particularly on the mutual contact of pyrite and arsenopyrite. On the whole the gold is fine and partially dispersed. The fineness averages 880 to 910, with traces of Ag, Cu, Bi, Pb, As, and Fe all having been identified (Smirnov, 1981).
Visible gold is rare and its presence may generally only be determined by sampling. The grade of the deposit apparently continues down dip un-diminished to at least 1000 m. The ore is oxidised to an average depth of 300 m below the surface. Silver values are generally low and comparable to those of gold.
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The Amantaytau (Amantaitau) gold deposit is located 30 km south east of the city of Zarafshan in the central Kyzylkum region of western Uzbekistan (#Location: 41° 19' 2"N, 64° 18' 2"E).
The deposit occurs within a pile of imbricated thrusts that was deformed into east-westŠtrending synforms and antiforms exposed towards the western limit of the southern Tien Shan tectonic province. It is hosted by a sequence of carbonaceous flyschoid rocks of the Cambrian to Silurian Besapan Formation, which have been complexly deformed and metamorphosed in the compressional thrust-fold belt. The deposit lies within a linear zone, known as the Amantay-Daugyz-Vysokolt'noye Zone, which is tens of kilometres long and is internally zoned. The Amantaytau orebodies, which comprise gold and sulphide ores accompanied by sericite alteration, lie at one end of this zone, while silver-sulphosalt mineralisation in quartz-chlorite-albite metasomatites is found at the other (Zakharevich, 1993). The Amantaytau and nearby Daugystau (see separate record) deposits represent parts of a single system which have been dextrally offset by 10 km.
The orebodies at Amantaytau constitute a system of stockworks within north-west trending faults, which are truncated by 'through-going' north-east trending faults. The individual stockworks form relatively narrow zones which are steeply dipping, generally at >60°. Ore has been outlined to a depth of 600 m. The host rocks were originally slightly carbonatic and slightly ferruginous, arkosic, polymict, greywacke and tuffaceous sandstone; polymict greywacke siltstones; and, silty and pelitic mudstone. The ore lies in the area of greatest alteration where the rocks have lost their bedded structure, are hydrothermally altered, and have cataclastic textures. The gold and silver mineralisation is predominantly localised within the meta-pelites, which have been very slightly de-silicified in parallel with an enrichment in Al. K was introduced, while Na was removed for a net decrease in the alkalis. Alteration appears to largely take the form of sericitisation (Zakharevich, 1993). Mineralisation is believed to have been emplaced between 260 and 270 Ma (Yakubchuk et al., 2002).
The known resource at Amantaytau is of the order of 700 t of Au at a grade of 7.5 g/t Au in the oxide ore and 14.2 g/t Au in the primary sulphides. These resources/reserve are based on Soviet era testing patterns and reserve classifications, and include 250 t of Au in class C1 and 450 t in class C2. The deposit was discovered in 1975, with detailed exploration beginning in 1980. The results of subsequent JORC compliant testing and reclassification of the reserves and resources by Oxus Gold plc are:
Sulphide Ore - Amenable to underground mining via decline access, using trackless mining equipment.
Reserve - 60 tonnes of Au @ 12g/t Au;
Total resource - 84 t of Au @ 12 g/t Au.
Oxide Ore - Amenable to open pit mining.
Proven + probable reserve of 5.47 Mt of ore @ 11.54 g/t Au (63 t Au);
Total resource of 7.15 Mt of ore @ 11.68 g/t Au (83 t Au).
with significant potential to expand the reserves and resources based on more widely spaced drill intersections.
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The Zarmitan Goldfield is located in the mountainous Samarkand Province of south eastern Uzbekistan, about 70 km north of the regional centre of Samarkand. The Zarmitan Project is composed of two main mineralised zones which are ~1.5 km apart, Charmitan (to the east) and Guzhumsai (to the west) (#Location: Charmitan - 40° 20' 8"S, 66° 44' 40"E).
The host rocks comprise quartz-mica and carbon-mica sandstones, siltstones and andalusite shale, with rare lenses of carbonates and granosyenite. The main deposits were developed near the contact zone of a late Palaeozoic (269 ± 4.2 Ma) intrusive complex which includes phases of gabbro, syenite, tonalite and granosyenite as well as granite and aplite.
The ore field occurs in a region where the strata are boudinaged in major shear zones. The deposit is composed of a series of fault controlled WNW trending, tabular bodies of linear stockwork and sheeted 'crack-seal' quartz rich veins which contain gold, silver, pyrite, arsenopyrite, scheelite and pyrrhotite, with a gangue of quartz, ankeritic carbonate and sericite, and minor amounts of W, Bi, Pb, Zn and Sb bearing sulphides (Berger, etal., 1994). Mineralisation persists to a depth of 2000 m.
On the basis of mineralogical, fluid inclusion, and stable isotope studies, Bortnikov et al. (1996) proposed that mineralisation resulted from the rapid mixing of magmatic and metamorphic fluids in the aureole of the intrusive complex, deposited at temperatures up to 400° C and pressures up to 2.8 Kbars.
The resulting gold deposit, encompassing both the Charmitan and Guzhumsai orebodies, comprises approximately 30 Mt of ore containing 300 t of gold at a grade of around 10 g/t Au as free milling sulphide bearing laminated quartz veins and, to a minor extent, in refractory arsenious high sulphide ores at the eastern end of the Charmitan deposit which average 8 g/t Au.
The Charmitan deposit alone contains reserves of 210 tonnes of gold, grading 8.7 g/t Au, within a total resource of at least 245 t Au. Using the Uzbekistan reserve classification, as of January 2000, the reserves/resources comprised: C1 - 11.344 Mt @ 10.9 g/t Au, 9.4 g/t Ag for 123.4 t Au; and C2 - 12.863 Mt @ 9.4 g/t Au, 12.1 g/t Ag for 121.805 t Au.
These reserve figures are based on more than 7000 surface drill holes, close spaced sampling of over 133 km of exploratory underground development and >884 000 metres of underground diamond drilling at both Charmitan and Guzhumsai since the discovery of the deposit in the 1960's.
Mining commenced at the Charmitan deposit in 1970 under the control of the State-owned Uzalmazzoloto as a series of open pits, continuing until 1997. Underground mining commenced in 1989 at the rate of approximately 0.1 Mt per annum, with the ore treated at the resin-in-pulp gold plant at Marjanbulak, a separate property located some 85 km to the east.
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The Almalyk porphyry Cu-Au system, located adjacent to the town of Almalyk, 65 km to the southeast of Tashkent in eastern Uzbekistan (#Location: 40° 48' 44"N, 69° 38' 49"E).
The complex encompasses the ore deposits at Kal'makyr (2.5 Gt @ 0.38% Cu, 0.5 g/t Au) and Dalnee (2.8 Gt @ 0.36% Cu, 0.35 g/t Au). The Sarycheku orebody (200 Mt @ 0.5% Cu, 0.1 g/t Au) is part of the Saukbulak porphyry Cu-Au system, some 18 km to the south (Seltmann & Porter, 2005). Seltmann, 2012) quotes a total resource for the complex of 8 Gt @ 0.58% Cu, 3 g/t Ag, 0.5 g/t Au, 0.005% Mo.
Both systems are associated with the second, Middle- to Late-Carboniferous, pulse of magmatic activity within the Devono-Carboniferous Valerianov-Bel'tau-Kurama volcano-plutonic belt that is the main element of the Middle Tien Shan terrane in Central Asia. Previous K-Ar dating of the ore-related porphyry intrusive and the mineralisation has returned ages in the range of 310 to 290 Ma, whereas recent U-Pb zircon dating reported for the intrusive sequence in the Almalyk district partially overlaps in the range of 320 to 305 Ma, with Re-Os ages of ore-related porphyries of 315 to 319 Ma (U-Pb and Re-Os age dating from Seltmann & Porter, 2005).
Mineralisation at both Kal'makyr and Dalnee is predominantly in the form of stockworks with lesser disseminations, and is associated with Late Carboniferous quartz monzonite porphyry plugs intruding earlier dioritic and monzonitic intrusive rocks of the same magmatic complex. The orebodies take the form of a cap like shell developed above and draped over the flanks of the related quartz monzonite porphyry stock.
The dominant hosts to ore are the monzonite and diorite wall rocks, with the quartz monzonite porphyry only containing ore in its outer margins, surrounding and/or overlying a barren core. The focus of stockwork development is fracturing related to both the intrusive contact of the porphyry stock and to crosscutting faulting.
Alteration comprises an early K-silicate phase followed by albite-actinolite and peripheral epidote-chlorite-carbonate-pyrite propylites, overprinted by an abundant phyllic episode which is closely related to the final distribution of the ore.
Associated mineralisation commenced with barren quartz-hematite veining, followed by quartz-magnetite, quartz-pyrite-molybdenite-chalcopyrite with the bulk of the contained gold, quartz-carbonate-polysulphide with lesser gold, then by zeolite-anhydrite, and finally carbonate and barite veining. Subsequent oxidation and uplift developed a layer of oxide ore, a limited leached cap and supergene sulphide enrichment, largely in zones of fault related fracturing.
The Kal'makyr deposit is distributed around and within the outer margins of a central plug of Late Carboniferous quartz monzonite porphyry (QMP) intruding earlier Carboniferous monzonite and diorite. The deposit straddles a major fault and extends southwards toward a second fault zone. The main volume of the block defined by these two faults is occupied by monzonite and diorite, although remnants of Devonian volcanic and carbonate rocks are locally preserved.
Approximately 65 to 75% of the ore at Kal'makyr occurs in the form of stockwork veins and veinlets, while 30 to 35% occurs as disseminations. The distribution of the ore stockwork and the intensity of veining is controlled by the density of fracturing and brecciation related to both the intrusion of the QMP stocks and dykes, and to linear fracture zones associated with the Kal'makyr and Karabulak faults. As a result, the stockwork is represented by a downward expanding cone surrounding and capping the quartz monzonite porphyry plug, with a barren core corresponding with the centre of the plug.
The stockwork is represented by a network of fractures which have been healed by quartz veinlets, and less frequently by calcite or anhydrite, which contain large segregations of the ore sulphides, including pyrite, chalcopyrite, chalcocite, pyrrhotite, molybdenite and tetrahedrite. The veinlets vary in thickness from fractions of a millimetre to 3 or 4 cm, and are from a few to a few tens of cm in length. The interval between the veinlets if occupied by lesser disseminated pyrite, chalcopyrite, magnetite and occasionally other sulphides.
The stockwork zone is elongated in a northwest direction, with maximum surface dimensions of approximately 3520 x 1430 m and a maximum vertical extent of 1240 m. The inner annulus of high-grade ore is substantially smaller, with outer dimensions of approximately 1740 x 500 m and a maximum vertical thickness of 450 m. The most intense fracturing and the highest grade ore are related to the intersections of porphyry contacts with the east-west and northeast trending faults and tend to form a broken annulus within the monzonite and diorite wall rocks immediately surrounding the QMP. The grade rapidly decreases inwards to the barren core in the QMP plug. In contrast the high grade annulus is surrounded by a broad halo of medium grade ore before passing into a lower grade periphery. At depth the ore stockwork becomes less continuous and lenses-out downwards via a series of tongues. The primary Kal'makyr ore contains Cu, Mo, Au, Ag and admixtures of Se, Te, Re, Bi, and In.
The upper sections of the ore deposit were subjected to oxidation and supergene enrichment, best developed in areas of more intense fracturing on the QMP contacts and along zones of fault related fracturing. The degree of oxidation, leaching and supergene enrichment varied across the deposit, from oxide to secondary sulphide to mixed oxide-sulphide zones, although the supergene sulphide enrichment was only weakly developed. Oxidation was developed to a maximum depth of 65 m, averaging around 20 m, while leaching, where it replaced in situ oxidation, persisted over a similar thickness. The principal mineral within the oxide zone was malachite, with chrysocolla and turquoise being locally important. Where present, supergene sulphide enrichment, principally as chalcocite and covellite, had a maximum thickness of 70 m, averaging 19 m, while a mixed 'complex' oxide-supergene sulphide zone, where developed, also averaged 19 m in thickness.
The Dalnee group of deposits are a west to northwest, down-plunge continuation of Kal'makyr at deeper levels. The >0.1% Cu outline unites these deposits into a common ellipsoidal sub-economic mineralised mass in plan view. However, post-mineral normal and sinistral strike-slip displacement on both the Karabulak and Kal'makyr faults dissected the ellipse into three blocks. The northern Karabulak block is displaced by 2 km to the west and the Central block (the main Dalnee deposit) for 0.5 km to the west of the southern block and Kal'makyr. The northern block encompassing Karabulak is the least eroded, while the deepest exhumation has affected the southern block and Kal'makyr.
The main Dalnee deposit in the Central area is located within a downward widening tectonic wedge between the Karabulak and Kal'makyr faults which have truncated the mineralised system and displaced ore on the flanks of the underlying QMP. Approximately 58% of the ore in the deposit is hosted by the monzonite, 35% by the diorite and 7% by the QMP. The stockwork is elongated parallel to the east-west direction of faulting and widens downward due to the outward dip of the bounding faults, while the base of mineralisation closely follows the apical surface of the underlying QMP stock. The average vertical extent of mineralisation is about 700 m with a maximum of 1200 m on some sections. The higher grade Cu mineralisation occurs below a depth of 150 to 200 m, while the highest grade core is at a depth of 500 to 600 m. The Karabulak deposit to the north is the least economically important of the Dalnee deposits. Drill holes intersected 'high-grade' Cu mineralisation extending along the Karabulak Fault at depth, associated with a small stock of quartz monzonite porphyry (QMP).
The Sarycheku deposit is related to the hanging wall of the Miskan Fault, with the ore grade mineralisation being confined to the tectonic wedge between the Miskan and Sargalam faults. The host rocks are represented by Devonian rhyolite porphyry, which is cut by Late Paleozoic porphyry intrusion. The oxidised and chalcocite ores have virtually been exhausted, while the underlying hypogene sulphide ore is concentrated within a zone which is 1160 m long and has been traced to a depth of 340 m.
This summary is drawn from Seltmann & Porter (2005).
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The Kochbulak meso- to epithermal Au-Ag deposit is located in Uzbekistan, 30 km northeast of the Kalmakyr-Dalnee porphyry copper deposits and 55 km southeast of the capital, Tashkent. It was emplaced within the Carboniferous Valerianov-Beltau-Kurama magmatic arc at approximately 290 to 280 Ma, and prior to mining contained approximately 135 tonnes of gold at an average grade of 12 g/t Au, 120 g/t Ag.
Kochbulak is hosted by the same magmatic arc that has produced the giant Kalmakyr and Dalnee deposits a few tens of kilometres to the southwest in the Almalyk district, and is less than 20 M.y. younger than the 315 to 290 Ma age of mineralisation at Kal¹makyr (Seltmann et al., 2004, Golovanov et al.,2005).
The Kochbulak gold deposit is located within the Karatash caldera at the intersection of the South Angren and Lashkerek-Dukent fault zones. The caldera is filled by:
i). The Middle to Upper Carboniferous Akcha Formation which comprises more than 1000 m of andesitic and dacitic lavas, and pyroclastic rocks.
ii). The unconformably overlying Nadak Formation, which has been divided into ten units and commences with a basal volcani-mictic conglomerate and sandstone, overlain by andesitic and dacitic lavas and tuffs. The relatively thick units of lava and tuff are separated by thin interlayers of tuffite, sandstone and siltstone.
iii). The Upper Carboniferous Oyasai and Upper Permian to Lower Triassic Kyzylnura formations which comprise rhyolitic lava and pyroclastics confined to the southern part of the caldera (Islamov et al., 1999).
The volcanic succession of the caldera, which represents a calc-alkaline to sub-alkaline, high potassic latite series, is cut by dykes, sub-volcanic intrusions and associated extrusives. The sequence is also cut by Middle Carboniferous pre-mineral granodiorite and monzodiorite porphyry which are comagmatic with the Akcha Formation at the base of the caldera, and by minor rhyolite intrusions related to the Oyasai Formation. Pre-mineral basic dykes of Early Permian age are widespread, while rhyolite, granosyenite, syenite, monzodiorite porphyry and late basic dykes are post-mineral (Islamov et al., 1999).
The deposit area is cut by four large, near north-south trending faults which dip steeply to the west and southwest. Further sets of intervening fractures parallel to the main trend are found in the deposit area, as are intra-formational detachments along the contacts between massive lava units (Islamov et al., 1999).
The Kochbulak mineralisation is restricted to volcanics of the Middle to Upper Carboniferous Nadak Formation on the northern flank of the caldera, close to the Shaugaz Fault. The setting corresponds to the near vent facies of a strato-volcano which was rimmed by sub-volcanic intrusives. Approximately 120 orebodies have been tested, controlled by 32 mineralised structures within a volume of some 4500 x 3000 x 550 m (Kovalenker et al., 1997; Islamov et al., 1999; Yakubchuk et al., 2002).
Alteration and Mineralisation
Three types of orebody are recognised, as follows:
i). Steeply dipping, north to northeast aligned veins (40% of the reserve) controlled by the major and intervening faults described above. Some 45 of these steep veins are recognised;
ii). Moderately dipping, (20 to 40°) near east-west veins (20% of reserves) which are concentrated where the north-south fault set intersects the intraformational detachments, also mentioned above, and
iii). Pipe-like orebodies (40% of the reserves), which are composed of mineralised explosion breccia and which terminate the steeply dipping vein set. There are some 14 pipes, each with a small diameter, but high grade (Islamov et al., 1999).
Mineralisation occurs as massive, banded, brecciated and breccia like textures, with festoon and incrustate structures. Quartz is the dominant gangue mineral, varying from coarse-grained to meta-colloidal to drusy, chalcedonic and amethyst, accompanied by subordinate carbonates and barite. The sulphide content of the two vein types is generally <10%, while in the breccia pipes it may reach 20%. Gold is mainly present as microscopic inclusions, occurring as sheeted, dendritic and cloddy grains in the upper levels and as spongy and drusy gold lower in the deposit. The finest gold is within meta-colloidal quartz, calaverite, sylvanite and altaite, while that in goldfieldite, chalcopyrite and galena is of lower fineness. Electrum accompanies sulphosalts and sulphostannites (Islamov et al., 1999).
The gold mineralisation is present in three associations, namely:
i). Gold-telluride, which occurs as calaverite, petzite, sylvanite, hessite, stutzite, empessite, goldfieldite and a wide range of other tellurides, and is particularly well developed in the upper level veins and in shallow-formed breccia pipes.
ii). Gold polysulphide comprises the association of native gold with sulphides of Cu, Pb, Zn, Bi and Sb, and is most frequently found in the upper levels of both the steep and flat veins.
iii). Gold-pyrite, which is found to varying degrees throughout the system, but is best developed and mineralised with increasing depth. It predominantly occurs as disseminated, uneconomic mineralisation with finely dispersed gold in pyrite, generally only averaging 4 g/t Au (Islamov et al., 1999). In general, the explosive breccia pipes are found in the upper levels of the deposit, passing through a transition zone to steeply dipping mesothermal veins at depth. Mineralisation is known to extend a depth of more than 2000 m.
The pattern of development of the three gold mineralisation associations is zoned both vertically (as described above), and laterally, with the gold-telluride association being the most proximal, within and immediately adjacent to the veins, flanked by the gold-polysulphides, passing out into the lower grade quartz-sulphide association. The distribution is also complicated by the telescoping and resultant superposition of the three zones from different episodes of mineralisation as the deposit evolved (Islamov et al., 1999).
The host volcanics underwent a mild propylitic alteration forming chlorite-carbonate and epidote prior to mineralisation. Alteration related to mineralisation within both the steeply dipping and shallow veins is evident as a regular zonation, with a progressive outward gradation from the ore vein to: i). hydrosericite; ii). adularia-sericite; and iii). chlorite-carbonate, to iv). the 'unaltered' country rock. All of the altered rock contains pyrite, which decreases from around 30% in the hydrosericite to 10% in the chlorite-carbonate zone. Pervasive sericite-hydromica dominates in the exploited parts of the deposit, while the chlorite facies was only penetrated in drilling at depths of >1200 m. The breccia-pipe bodies are accompanied by an intense silicification of the hosts, accompanied by variable amounts of sericite, alunite and diaspore (Islamov et al., 1999).
The Kairagach gold deposit is hosted by similar rocks within the same caldera, some 3.5 km to the northeast of Kochbulak. It has a potential resource of 50 t of Au and 150 t of Ag at a comparable grade to Kochbulak and is similar in many aspects, but with variations in detail (Islamov et al., 1999).
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available soon from the TienShan 2006 Tour options page.
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