the koshrabad granite massif in uzbekistan: petrogenesis, metallogeny, and geodynamic setting

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The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

D.L. Konopelko a,*, Yu.S. Biske a, K. Kullerud b, R. Seltmann c, F.K. Divaev d

a St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia b Department of Geology, University of Tromsø, N-9037 Tromsø, Norway

c CERCAMS, Department of Mineralogy, NHM, Cromwell Road, London SW7 5BD, UK d SE “Tsentral’naya Geological and Geophysical Expedition” of the Uzbekistan State Geology Committee,

ul. Gagarina 148, Samarkand, 103030, Uzbekistan

Received 11 April 2011; accepted 31 May 2011

Abstract

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) maficand quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, oftenrimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the stillunconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized byhigh f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product offractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. Inhigh f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocksconfirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acidmelts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusionof compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronousmelting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter inthe dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulationin commercial concentrations.© 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Keywords: geochemistry; tectonics; metallogeny; Koshrabad massif; Tien Shan

Introduction

The Koshrabad massif in western Uzbekistan is one of themost interesting Hercynian intrusions in the Tien Shan. Themassif is famous as one of scarce Phanerozoic rapakivi granitecomplexes (Yudalevich et al., 1973). Moreover, it bears amagmatic gold deposit, the largest one in the Tien Shan(Abzalov, 2007). The massif is localized in the North Nurataustructure resulted from the regional dilatation at the postcolli-sional stage. Granitoid intrusions in the structure are diversein composition and are an example of specific graniteformation in this geodynamic setting. The Koshrabad massifand associated deposits are described elsewhere (Abzalov,

2007; Bortnikov et al., 1996; Golovanov, 2001; Khamrabaevet al., 1973; Kotov, 1993; Proskuryakov et al., 1979), but theearlier papers on the massif petrogenesis (Izokh et al., 1975;Yudalevich et al., 1991) were based on scarce data on therock composition. In this work we consider the petrogenesisof the Koshrabad massif, taking into account new data on therock petrography and composition, the field correlationsbetween rocks, and modern concepts of the formation ofintricate intrusions. The established geochemical evolutiontrends of the massif rocks supplement the existing conceptsof the formation of gold deposits in the massif. The geody-namic setting of formation of the North Nuratau Ridgeintrusions is discussed with invoking new geochemical andgeochronological data. Thus, this is the first paper presentingall these materials.

Russian Geology and Geophysics 52 (2011) 1563–1573

* Corresponding author.E-mail address: [email protected] (D.L. Konopelko)

doi:10.1016/j.rgg.201 . 001068-7971/$ - see front matter D 201 IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.V S.. S bolev1,

1o

11+. 9

Available online at www.sciencedirect.com


The geologic structure of the region

The pre-Mesozoic deposits exposed within the present-dayTien Shan uplift are subdivided into the North, Central, andSouth Tien Shan (Fig. 1). The North and Central Tien Shanare parts of the Kirghiz–Kazakh continent, or Paleo-Kazakhstan, whose amalgamation terminated by the lateOrdovician (Ges’, 1999). In the middle Paleozoic, the upperstructural stage of Paleo-Kazakhstan formed. In the CentralTien Shan, this stage is composed of Upper Devonian–LowerCarboniferous marine shelf carbonate facies, whereas in theNorth Tien Shan it is observed as incomplete sections withred-colored sandstones (Biske, 1996). The South Tien Shanresulted from the closure of the Turkestan paleoocean andcollision of Paleo-Kazakhstan with more southern continents(the Karakum–Tajik continent in the Uzbekistan sector) in theMiddle–Late Carboniferous, which were caused by the sub-duction of oceanic lithosphere northward and the formation ofthe Bel’tau–Kurama magmatic arc on the southern margin ofPaleo-Kazakhstan (Biske, 1996). The South Tien Shan iscomposed of ophiolites and marine, including bathyal, depositsof the Early–Middle Paleozoic Turkestan ocean. The depositsof the North Nuratau Ridge, where the Koshrabad massif islocalized (Fig. 2), are the northern margin of the South TienShan domain and are regarded (Biske and Usmanov, 1981;Porshnyakov et al., 1991) as an ensemble of nappes thatmoved from the collision suture southward and then werefolded. There are distinct nappes of thin Devonian–MiddleCarboniferous carbonate deposits. However, the exfoliation ofnappes also took place at deeper stratigraphic layers, includingthick Ordovician–Silurian terrigenous series, up to LowerPaleozoic and, probably, Upper Precambrian carbonate-sili-ceous and black-shale deposits, which led to a significantthickening of the dislocated complex. The North Nuratau

folded nappes are cut by a regional shear, which is atransformed suture zone (South Tien Shan suture) stretchingalong the southern margin of Paleo-Kazakhstan (Porshnyakovet al., 1991). It is generally accepted that the regional shearsformed at the postcollisional stage after the closure ofpaleooceanic basins and the collision of Paleo-Kazakhstanwith more southern continents (Laurent-Charvet et al., 2003).In the North Nuratau Risge, the South Tien Shan suture iscomplicated by feathering faults and is a large dilatation shearstructure formed at the postcollisional stage. It is intruded bygranitoids, including the Koshrabad massif. The intrusionsextend in the northwestern direction concordantly to theHercynide strike, though they are often cut by the enclosingrocks. Because of the compositional difference, the NorthNuratau granitoid intrusions were assigned to different com-plexes on mapping (Izokh et al., 1975) and later wereconsidered to be representatives of different geodynamicsettings (Abzalov, 2007). The formation conditions and settingare discussed below with regard to new geochronological andgeochemical data.

The geologic structure of the Koshrabad massif

The Koshrabad massif is 196 km2 in area, wedge-like, andof W–E strike (Fig. 3, a). It is composed of rocks with a SiO2content varying from 50 to 75 wt.%. In this work we arbitrarilysubdivided all rocks into mafic rocks (50–62 wt.% SiO2) andmain intrusion phase including quartz monzonites and rapakivigranites (62–73 wt.% SiO2). There are also their dike varieties.Mafic rocks occur in the central part of the massif andcompose 3–4% of its area. About 94% of the area are occupiedby ovoid quartz monzonites and granites, and 2%, by dikesand stocks of aplites and fine-grained granites. Dikes are

Fig. 1. Schematic tectonic map of the Tien Shan. 1, Cenozoic cover; 2, Paleozoic deposits; 3, areas of granitoid magmatism (320–270 Ma); 4, gold deposits.NTSh, North Tien Shan, CTSh, Central Tien Shan, STSh, South Tien Shan, NL, Nikolaev line, TFS, Talas–Fergana shear, STShS, South Tien Shan suture.

1564 D.L. Konopelko et al. / Russian Geology and Geophysics 52 (2011) 1563–1573


present mainly in the eastern part of the massif, where theyform two crisscross bunches, of NE and W–E strikes. Themassif endocontact lacks a well-expressed quenching zone.The exocontact abounds in garnet-quartz-mica and quartz-cor-dierite-mica hornfelses developed after the 50–200 m thickclastic host rocks. The carbonate rocks at the exocontact aretransformed into garnet skarns.

The mafic rocks are amphibole-pyroxene (sometimes, witholivine) K-feldspar-containing gabbros, essexites, monzonites,amphibole-pyroxene and biotite-amphibole syenites, andquartz monzonites. The quartz monzonites of the main phasediffer from those of the mafic rocks in having higher SiO2contents (>62 wt.%) and coarser-grained textures. Smallbodies of aplites and fine-grained granites, including ultraacidgranites, are also composed of amphibole-biotite varieties. Aspecific feature of the main-phase rocks is the presence ofalkali K-feldspar ovoids up to 6× 3 cm in size, often with anoligoclase rim, bearing concentrically arranged fine (≤2 mm)inclusions of dark colored minerals (Fig. 4, a). By thearrangement of ovoids, flow structures (trachytoid) and struc-tures typical of feldspathic cumulates are recognized. Alkali-feldspar and andesine-labradoritic cumulate bodies were alsorevealed on the massif mapping (Yudalevich et al., 1991). Therelationship between the mafic and the main-phase rockspoints to the simultaneous intrusion of basic and acid melts.The formation of specific “pillows” (Fig. 4, b) is explainedby the immiscibility of melts of different compositions(Cantagrel et al., 1984). We also established the disintegrationof cumulates and mobilization of feldspar megacrysts into thebasic melt with the formation of hybrid rocks (Fig. 4, c).Similar relationships have been established between the ovoid

granites of the main phase and late aplites and fine-grainedgranites. The shapes of the latter two rocks point to the meltintrusion into the nonconsolidated rocks of the main phase(Fig. 4, d). One can also see the mobilization of ovoids intothe injecting melt, which sometimes causes their breaking(Fig. 4, e). All researchers who studied the massif noted itsintricate structure. Three homodromous rhythms of rocks wererecognized, and both sharp intrusive contacts and gradualtransitions between the rock varieties were observed (Yuda-levich et al., 1991). We think the massif structure and rock

Fig. 2. Schematic geological map of the North Nuratau Ridge, after Abzalov (2007), Izokh et al. (1975), and Seltmann et al. (2011). 1, Cenozoic cover; 2, carbonatesand fletzes (S–C); 3, volcanics (S–D); 4, metasediments (Pr3–O); 5, gold deposits (a) and gold occurrences (b); 6–8, granitoids of: 6, A-type, 7, S-type, and 8, I-type.Other designations follow Fig. 1.

Fig. 3. Schematic geologic structure of the Koshrabad massif (a) and map of theGuzhumsai gold deposit (b), after Abzalov (2007), with some changes. 1, gran-ites of the main phase; 2, mafic rocks; 3, enclosing rocks; 4, granite dikes;5, orebodies; 6, faults.

D.L. Konopelko et al. / Russian Geology and Geophysics 52 (2011) 1563–1573 1565


relationships are due to the repeated injections of mafic meltinto the incompletely consolidated granitoids of the mainphase. Probably, different types of rocks formed nearlysynchronously in the geologic-time scale.

The mineral composition of the rocks is as follows: alkalifeldspar, weakly zonal plagioclase with An32–45 in the phe-nocryst cores, high-Fe biotite-lepidomelane, and hastingsiteamphibole. The mafic rocks contain clinopyroxene partlyreplaced by aegirine (augite). Both the mafic and the main-phase rocks sometimes bear grains of ferruginous olivine-fay-alite. The most abundant accessory minerals are ilmenite(sometimes together with magnetite), apatite, zircon, andorthite.

By a set of rocks, textures, and structures the Koshrabadmassif is a close analog of Precambrian rapakivi granites. But

the latter usually associate with tholeiitic gabbroids, whereastheir Phanerozoic analogs, including the Koshrabad massif,associate with alkaline rocks (Haapala et al., 2005). Thegeochemical similarity of the massif rocks to A-type granitesis discussed below.

Geochemical features

The chemical composition of rocks was determined byXRF, ICP-MS, and ICP-BF-ESMS (Table 1). The earlieranalytical data on the massif rocks (Izokh et al., 1975;Yudalevich et al., 1991) were also included into the databaseand processed together with the new results. On the (Na2O +K2O)–SiO2 (TAS) diagram (Fig. 5, a), the compositions of

Fig. 4. Photomicrographs of the Koshrabad massif rocks. a, Alkali feldspar ovoids in granitoids of the main phase; b, formation of “pillows” as a result of the intrusionof melts of different compositions; c, intrusion of mafic melt into nonconsolidated rocks of the main phase and disintegration of feldspathic cumulates; d, intrusion ofresidual acid melts into nonconsolidated rocks of the main phase; e, mobilization and breaking of alkali feldspar ovoids in residual acid melt.



the Koshrabad massif rocks fall into the field of both alkalineand subalkaline series. The mafic rocks correspond in compo-sition to monzogabbro, monzodiorite, and monzonite, and therocks of the main phase lie mainly in the field of quartzsyenites and monzonites (Middlemost, 1994). On the(Na2O/K2O)–SiO2 diagram (Fig. 5, b), the massif rocks forma trend from sodic to potassic compositions as the SiO2 contentincreases. The diagram in Fig. 5, c shows that the mafic rockshave medium contents of Al2O3, whereas part of the main-

phase rocks and compositionally similar dikes are oversatu-rated with it. On the (Na2O + K2O–CaO) vs. SiO2 diagram(Fig. 5, d), the mafic rocks of the Koshrabad massif form asteep trend running through all fields from calcic to alkalineseries as the SiO2 content increases. Such trends are typicalof the rock series resulted from the fractionation of maficminerals, e.g., augite (Frost and Frost, 2008). The rocks of themain phase are enriched in CaO as compared with the maficrocks and do not form a common trend on the (Na2O +

Table 1. Contents of main (wt.%) and trace (ppm) elements in rocks from the Koshrabad massif

Compo-nent

420500 T6-043 420400 T6-041 420700 420600 420900 401700 420800 T6-042 421000 11

1 2 3 4 5 6 7 8 9 10 11 12

SiO2 50.70 53.26 56.00 56.73 60.55 61.05 61.90 64.65 65.00 65.42 65.70 72.47

TiO2 1.30 1.20 1.13 0.59 0.80 0.48 0.70 0.54 0.59 0.49 0.57 0.22

Al2O3 14.50 17.94 14.70 17.00 17.45 19.65 17.10 15.92 15.95 15.21 16.05 14.42

FeOtot 12.10 9.42 10.04 6.98 5.21 4.36 5.43 4.85 5.08 4.14 4.87 3.42

MnO 0.19 0.16 0.16 0.12 0.08 0.06 0.10 0.06 0.07 0.06 0.07 0.02

MgO 0.98 0.42 0.73 0.41 0.10 0.65 0.48 0.75 0.75 0.64 0.70 0.55

CaO 12.15 7.24 8.62 3.91 2.66 5.39 3.71 3.15 3.45 2.87 3.23 2.24

Na2O 3.75 5.52 4.20 4.98 5.56 5.32 4.43 4.31 3.72 3.31 3.66 3.80

K2O 2.25 1.97 3.66 4.84 5.84 2.19 5.44 4.09 3.89 4.57 4.33 2.26

P2O5 0.68 0.24 0.46 0.17 0.07 0.15 0.18 0.19 0.20 0.17 0.18 0.15

Rb – 37 – 112 – – – 157 – 198 – 104

Ba 1831 1127 1589 1685 828 918 1862 1477 1401 1465 1580 920

Sr 1058 566 880 507 537 461 474 224 – 199 – 222

Ga – 23.6 – 23.1 – – – 26.9 – 23.3 – –

Zr 221 306 163 151 120 276 532 229 326 244 305 305

Hf 4.6 8.0 4.0 4.0 2.3 6.9 10.9 6.0 8.3 8.0 7.6 7.6

Y 36.7 33.4 38.2 25.2 14.3 38.4 31.2 38 46.2 39.1 42.1 42.1

Nb 24.8 24.0 29.2 17.5 19.8 21.2 36.2 19.0 24.8 21.2 23.0 23.0

Ta 1.9 1.4 2.4 0.9 1.0 1.5 2.3 1.3 1.6 2.4 1.6 1.6

U 2.0 2.1 2.8 1.4 0.6 2.1 2.8 6.5 5.6 3.7 6.8 6.8

Th 5.9 6.6 6.7 10.1 1.2 9.5 10.1 16.3 16.7 15.5 16.1 16.1

La 50.5 39.3 47.9 31.6 20.2 45.6 35.4 48.2 42.9 48.3 49.2 34.0

Ce 98.1 79.3 94.8 63.1 37.0 83.6 64.4 93.8 82.3 94.3 91.0 57.0

Pr 10.9 9.7 10.9 7.6 4.1 9.4 7.1 11.0 9.3 10.7 10.0 –

Nd 43.8 35.5 44.4 28.6 16.4 38.7 29.5 40.4 39.4 38.2 40.1 –

Sm 8.6 7.1 8.9 5.3 3.5 8.1 6.2 8.4 8.9 7.8 8.6 6.0

Eu 2.2 2.1 2.0 2.2 2.5 2.2 2.0 1.7 1.6 1.5 1.7 1.7

Gd 6.8 6.2 7.0 5.1 2.8 6.9 5.3 7.8 7.5 7.3 7.4 –

Tb 1.1 1.0 1.2 0.8 0.5 1.2 0.9 1.3 1.4 1.2 1.3 0.9

Dy 6.4 5.4 6.7 4.2 2.6 6.7 5.3 6.9 7.9 6.3 7.3 –

Ho 1.2 1.2 1.3 0.9 0.5 1.3 1.1 1.4 1.6 1.3 1.4 –

Er 3.7 3.1 3.8 2.6 1.5 3.7 3.2 3.8 4.6 3.6 4.0 –

Tm 0.5 0.5 0.6 0.3 0.2 0.5 0.5 0.6 0.7 0.5 0.6 –

Yb 3.5 3.2 3.6 2.3 1.6 3.2 3.1 3.4 3.9 3.2 3.5 2.6

Lu 0.5 0.6 0.6 0.3 0.2 0.5 0.5 0.5 0.6 0.5 0.5 0.4

Note. 1–7, mafic rocks: 1, monzogabbro, 2, monzodiorite, 3, 4, monzonite, 5–7, quartz monzonite; 8–11, coarse-grained ovoid quartz monzonites of the mainphase; 12, granite, after Yudalevich et al. (1991). Dash, Element was not analyzed.



K2O–CaO) vs. SiO2 diagram; they fall into the fields ofcalc-alkalic and alkaline series (Fig. 5, d). The CaO enrich-ment of the most acid varieties is usually explained by theassimilation of crustal matter, mainly of quartz-feldspar com-position (Frost and Frost, 2008). All rocks of the Koshrabadmassif are strongly enriched in Fe relative to Mg. On theFeOtot/(FeOtot + MgO) vs. SiO2 diagram (Fig. 5, e), the rockcompositions lie in the fields of tholeiitic series and A-typegranites. The high FeOtot/(FeOtot + MgO) value is the mainspecific feature of the massif granitoids, making them similarto A-type granites. Other geochemical characteristics, first ofall, the relative enrichment in CaO, do not permit theKoshrabad rocks to be referred to as A-type granites.

Figure 6 shows variation diagrams of main and tracecomponents relative to SiO2 in the massif rocks. The bend of

the trend on the Al2O3–SiO2 diagram illustrates the transitionfrom the fractionation of alumina-poor dark colored mineralsin mafic rocks to the fractionation of feldspars in the rocks ofthe main phase. The drastic increase in Na2O and K2Ocontents with increasing SiO2 contents in the mafic rocksemphasizes a minor participation of feldspars in the fractiona-tion. In the rocks of the main phase, the contents of Na2O andK2O, on the contrary, decrease with an increasing SiO2content, which is specific for feldspathic cumulates. A slightdiscordance between the compositions of the mafic andmain-phase rocks is also observed on the CaO, FeOtot, andP2O5 vs. SiO2 diagrams; the rocks of the main phase areenriched in these oxides relative to the mafic rocks. All massifrocks are characterized by high contents of Ba (800–1800 ppm) and Sr (200–800 ppm), medium contents of Rb

Fig. 5. Classification diagrams for rocks of the Koshrabad massif. a, (Na2O + K2O–CaO) vs. SiO2, fields after Middlemost (1994): 1, foidolite, 2, feldspathoid gabbro,3, gabbro-peridotite, 4, feldspathoid monzodiorite, 5, monzogabbro, 6, gabbro, 7, feldspathoid monzosyenite, 8, monzodiorite, 9, gabbro-diorite, 10, monzonite,11, diorite, 12, feldspathoid syenite, 13, syenite and quartz monzonite, 14, granodiorite, 15, granite; dashed line separates the fields of alkaline and subalkaline series,after Irvine and Baragar (1971); b, Na2O/K2O–SiO2, c, Al–(Na + K) vs. Al/(Ca–1.67⋅P + Na + K); d, (Na2O + K2O–CaO) vs. SiO2; e, FeOtot/(FeOtot + MgO) vs. SiO2,the field of S- and I-granitoids of the Temirkobuk massif is given for comparison after Izokh et al. (1975). Other designations of the fields in c–e are given after Frostand Frost (2008). 1–3, intrusive rocks; 4–6, dikes. 1, 4, granitoids of the main phase, SiO2 > 62 wt.%; 2, 3, 5, 6, mafic rocks: 2, 5, 52 < SiO2 < 62 wt.%; 3, 6, SiO2 <52 wt.%.



(100–200 ppm), and high contents of HFSE. The latter featuremakes them similar to A-type granites, but high contents ofBa and Sr are more specific for rocks of a postorogenicassociation. On the Rb–SiO2 variation diagram (Fig. 6), themassif rocks form a trend with a sharp bend, similar to theNa2O and K2O trends. The trend on the Sr–SiO2 diagram issimilar to the CaO and P2O5 trends and also demonstrates arelative Sr enrichment of the main-phase rocks. The decreasein Sr contents with increasing SiO2 contents in the mafic rockscorrelates with a decrease in P2O5 contents, which suggeststhe removal of Sr from the melt as a result of the apatitefractionation. The REE patterns are moderately fractionatedand show an LREE enrichment and slight positive or negative

Eu anomalies or their absence (Fig. 7). In general, the REEpatterns are almost identical for all types of the massif rocks.

Petrogenesis

The fact that all rocks of the Koshrabad massif belongto the same genetic complex is confirmed by the samemineral composition and geochemical characteristics: highFeO/(FeO + MgO) values and similar REE patterns. But studyof their geologic and geochemical properties showed that therocks formed, most likely, in several stages as a result ofdifferent processes. The compositions of mafic rocks evidence

Fig. 6. Variation diagrams of main and trace components relative to SiO2 in the rocks of the Koshrabad massif. Designations follow Fig. 5.



that they formed from an alkali-basaltic melt during the intensefractional crystallization of mafic minerals (e.g., augite)without the participation of feldspars. The relative enrichmentof these rocks in Fe and the presence of olivine in them pointto the reducing conditions of their formation and the relativedryness of the melt (Frost and Frost, 1997). The rocks of themain phase also formed in the reducing conditions, but theirgeochemical features (enrichment in CaO, P2O5, and Srrelative to the mafic rocks) indicate that the compositionchanged as a result of some additional process. It is assumedthat the composition modification is the result of the assimi-lation of crustal rocks in an intermediate magma chamber. Thecompositional variations of the main-phase rocks are duemainly to the feldspar fractionation. This is confirmed by fielddescriptions of andesine-labradorite and alkali-feldspathic cu-mulates (Yudalevich et al., 1991). Thus, all rocks of the mainphase, composing 94% of the massif area, are a series offeldspathic cumulates and acid residual melts mixed indifferent proportions. The cumulates formed, most likely, insitu or during the massif intrusion. The outcrops of mafic rocks

occur locally in the central part of the massif (Fig. 3, a). Thefield structures point to the simultaneous intrusion of basicand acid melts (Fig. 4). Probably, the area of mafic rocks inthe central part of the massif formed as a result of the injectionof mafic magma into the nonconsolidated massif of rocks ofthe first phase. The repeated intrusion of mafic melt led to theformation of hybrid rocks and a series of mafic dikes. Theacid dikes are probably of a different genesis. The leucogranitedikes might be residual melt portions filtered off during theformation of feldspathic cumulates. The produced portions ofleucogranitic melts might have intruded as dikes into thealready consolidated parts of the massif and the enclosingrocks or might have injected nonconsolidated feldspathiccumulates, which led to the formation of joint-intrusionstructures and mobilization of alkali-feldspar ovoids into themelt (Fig. 4). The role of these late leucogranitic melt portionsin the formation of gold mineralization is considered below.

The genesis of gold mineralization

The Koshrabad massif encloses three proximal gold depos-its: Guzhumsai, Promezhutochnoe, and Charmitan, knownunder the common name Zarmitan, with the total gold reservesof 300 tons and an average Au content of 9.8 ppm (Abzalov,2007). Orebodies 1–3 m in thickness, localized within themassif and, partly, in the enclosing rocks, are a single swarmof quartz veins persistent in strike and dip. In the west of thedeposit, quartz veins are of the same orientation as the dikesof fine-grained granitoids of the Koshrabad massif (Fig. 3, b).Gold mineralization in the quartz veins is associated withsulfides (mainly pyrite and arsenopyrite; the rest are scheelite,polymetal sulfides, tellurides, and stibnite (Abzalov, 2007;Golovanov, 2001). Note that the alteration zones in the veinselvages are no wider than a few centimeters (Fig. 8).

In magmatic systems, Au is, most likely, an incompatibleelement in relatively reducing conditions (Thomson et al.,1999). This is confirmed by the fact that all magmatic golddeposits are genetically related to ilmenite-containing intru-

Fig. 7. Chondrite-normalized (Sun and McDonough, 1989) REE patterns of theKoshrabad massif rocks. Designations follow Fig. 5.

Fig. 8. Gold-bearing quartz veins of the Guzhumsai deposit in the weakly altered granites of the main phase of the Koshrabad massif. Alkali-feldspar ovoids are 5 cmin average size.



sions (Baker et al., 2005), i.e., intrusions formed in relativelyreducing conditions, which favored the accumulation of goldin residual melts and hydrothermal solutions. The rocks of theKoshrabad intrusion are significantly enriched in gold relativeto its clarke; its maximum concentrations were established insome late leucocratic dikes (Fig. 9). The magmatic origin andspecific features of the Zarmitan gold mineralization areconsidered elsewhere (Abzalov, 2007; Bortnikov et al., 1996;Kotov, 1993; Proskuryakov et al., 1979; Teslenko and Polyk-ovskii, 1988; Teslenko and Polykovskiy, 1994). Our data onthe geochemical evolution of the massif rocks are additionalevidence for the genetic relationship of the gold mineralizationwith the Koshrabad intrusion. Apparently, the magmas thatgenerated the intrusion rocks were initially enriched in gold.The reducing conditions and the existence of appropriatecomplexing agents ensured the accumulation of gold duringthe magmatic evolution instead of its deposition at the earlystages of crystallization. This resulted in high concentrationsof gold in late dikes and hydrothermal solutions. The intrusionof late dikes and the formation of quartz veins were controlledby W–E-striking faults and, probably, took place nearlysynchronously with the crystallization of the Koshrabadintrusion. The absence of distinct aureoles of altered rocksfrom the ore vein selvages points to their rapid formation. Thegold deposition might have been caused by a drastic decreasein the temperature of hydrothermal solutions or a change ofthe redox conditions. Elucidation of the detailed chronologyof the deposit formation calls for subsequent studies of latedike series and orebodies.

There are several large gold deposits in the Tien Shansector of western Uzbekistan (Fig. 1). In the South Tien Shan,in particular, in the North Nuratau Ridge, gold mineralizationis localized mainly in Lower Paleozoic black-shale strata andin places is spatially associated with granitoid intrusions(Fig. 2). The problem of a mantle or crustal source of goldwas discussed elsewhere (Chiaradia et al., 2006; Golovanov,2001). A mantle source was not revealed in the rocks and oresof the Muruntau and Zarmitan deposits (Graupner et al., 2010).

The regional association of the deposits with black-shale stratapoints more likely to the crustal origin of gold. For theKoshrabad massif, this is confirmed by higher gold contentsin the rocks of the main phase, which, in contrast to the maficrocks, are contaminated with crustal matter. At the same time,melts and fluids of mantle genesis might have been the sourcesof heat and solutions necessary for the mobilization of goldfrom black-shale strata.

The geodynamic setting of intrusion formation in the North Nuratau Ridge

The large-amplitude shears formed along the main tectonicTien Shan sutures at the postcollisional stage, beginning fromthe Early Permian (Laurent-Charvet et al., 2003). The NorthNuratau Ridge is a dilatation structure complicating the mainSouth Tien Shan suture passing in its northern part (Fig. 2).A specific feature of the ridge is abundant granitoid intrusionsdiffering significantly in composition. Amphibole-containingI-type granitoids are the most widespread; they compose theeastern part of the Temirkobuk massif, the Aktau massif, andother intrusions of the North Nuratau Ridge. The western partof the Temirkobuk massif is composed of garnet-containingtwo-mica S-type granites, which were also mapped as individ-ual bodies within the Aktau and other massifs (Fig. 2). Ingeochemistry the Koshrabad massif is similar to A-typegranites. A strong difference in FeO/(FeO + MgO) valuesbetween the Koshrabad massif rocks and the I- and S-typegranites of the Temirkobuk massif is seen from Fig. 5, e. Thecompositionally different types of granites might have formedin different geodynamic settings and thus at different time.However, U-Pb zircon dating of the North Nuratau intrusionsshowed their close ages (Fig. 2); moreover, the I- andS-granites of the Temirkobuk massif and granites of theKoshrabad massif formed nearly synchronously, at 281–287 Ma (Seltmann et al., 2011). Thus, the North Nurataustructure exhibits all main features of the Hercynian postcol-lisional granitoid magmatism in the Tien Shan: a relativelynarrow age interval (280–295 Ma), confinement to the regionalshear zones, and the presence of spatially and temporarilyassociated granitoids of different types and alkaline rocks(Konopelko et al., 2007, 2009; Nenakhov and Belov, 1996;Seltmann et al., 2011). To explain these peculiarities, Kono-pelko et al. (2007) proposed a model, based on the construc-tions by Teyssier and Tikoff (1998), for the San Andreas Faultin California, which is the best studied modern intracontinentaltransform fault with a vertical shear. According to this model(Fig. 10), the large-amplitude shears that appeared at thepostcollisional stage are transcrustal and disturb both thelithospheric mantle and the asthenosphere. Shear stresses resultin dilatation and the appearance of subhorizontal detachmentzones in the crust basement. The dilatation causes the ascentof hot asthenospheric material along the shear zone and intothe subhorizontal detachment zones. The asthenospheric sub-stance is a source of heat and fluids, which can cause meltingin the shear zone and subhorizontal zones in the crust

Fig. 9. The Au–SiO2 diagram for the dike series of the Koshrabad massif. Forcomparison, a field of intrusive rock varieties is given, after Yudalevich et al.(1991). Designations follow Fig. 5.



basement. Melting of the lithospheric mantle and crustalprotoliths can lead to the appearance of melts of differentcompositions, including alkaline rocks, and crustal cells ofhydrothermal-solution circulation. The intrusion of postcolli-sional granitoids and formation of ore deposits as a result ofthis circulation can be caused by the same processes but notbe related to each other, though both are controlled by thesame zones of transcrustal shears. A subsequent expansion ofthe detachment zones in the crust basement can lead to thedelamination of the lithospheric mantle and induce newmagmatism pulses.

Thus, the Hercynian postcollisional magmatism and metal-logeny of the North Nuratau Ridge were probably determinedby the processes running within the large-amplitude transcrus-tal shear and feathering faults. The mafic rocks of theKoshrabad massif resulted from the fractionation of initiallymantle alkali-basaltic melt, and the granitoids of the mainphase bear signs of crustal-matter assimilation. The synchro-nous formation of the I- and S-granites of the Temirkobukmassif was probably caused by the synchronous melting ofdifferent crustal protoliths at different depths along the shearzone. The wide spread of gold-ore occurrences associated withgranitoid intrusions evidences that the postcollisional magma-tism was accompanied by the intense circulation of hydrother-mal solutions, which led to the mobilization of ore elementsfrom crustal rocks and the formation of their commercialconcentrations.

In terms of metallogeny, the proposed model explains onlythe main factors determining the localization of gold miner-alization in the region. Explanation of other metallogenicfeatures calls for additional studies. For example, in the NorthNuratau zone, only A- and I-type granites are gold-bearing,whereas S-granites are barren, even if they are intruded intoblack-shale strata. Moreover, the proposed geotectonic modelwas elaborated for the Hercynian postcollisional magmatism

in the Tien Shan and is not universe. The voluminous literatureon Paleozoic and Mesozoic granitoids of other Central Asiaregions gives other scenarios of the postcollisional magmatismand other ways of granitic-magma generation.

Conclusions

The Koshrabad massif is assigned to Hercynian postcolli-sional intrusions, which are characterized by a narrow ageinterval (280–295 Ma), confinement to regional shear zones,and the presence of different types of spatially and temporarilyassociated granitoids and alkaline rocks.

The massif is composed of two series: mafic rocks andquartz monzonites and granites of the main phase. Theporphyritic granitoids of the main phase contain alkali-feldsparovoids and are a series of feldspathic cumulates. The maficrocks developed in the central part of the massif resulted fromthe injection of mafic magma into the nonconsolidated rocksof the main phase with the subsequent formation of hybridrocks and various dike series.

All massif rocks are characterized by high FeO/(FeO +MgO) values and the presence of fayalite, which evidencesthe reducing conditions of their formation. The mafic rocksresulted from the fractional crystallization of alkali-basalticmantle melt, and the granites of the main phase show signsof contamination with crustal substance. The high FeO/(FeO +MgO) values and HFSE contents make the massif rockssimilar to A-type granites.

The data on the geochemical evolution of the massif rocksconfirm the genetic relationship of the gold deposits localizedwithin the massif with magmatic processes and suggest theaccumulation of gold in residual acid melts and the rapidformation of ore quartz veins in the same structures thatcontrolled the intrusion of late dikes.

The formation of coeval but compositionally differentgranitoid intrusions in the North Nuratau Ridge is explainedby the synchronous melting of various crustal protoliths in thezone of transcrustal shear as a result of the ascent of hotasthenospheric matter in the dilatation setting. The circulationof hydrothermal solutions caused by this process favored themobilization of ore elements from crustal rocks and theformation of their commercial concentrations.

Acknowledgements. We thank R. Armstrong for accessto the regional geochemical database and N.N. Kruk andA.S. Borisenko for constructive reviews.

This study was supported by Federal Grant-in-Aid Program“Human Capital for Science and Education in Innovative Rus-sia” (Governmental Contract No. 14.740.11.0187) and aresearch grant from Saint Petersburg State University (DK).

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Fig. 10. Model for the formation of Hercynian postcollisional intrusions in theTien Shan. Sinistral shear is given after Biske (1996). For explanation, see thetext. Designations follow Fig. 1.



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the koshrabad granite massif in uzbekistan: petrogenesis, metallogeny, and geodynamic setting

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