Ore Geology Reviews 61 (2014) 175–191

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Ore Geology Reviews

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Age and petrogenesis of the Neoproterozoic Chon-Ashu alkaline complex, and anew discovery of chalcopyrite mineralization in the eastern Kyrgyz Tien Shan

Dmitry Konopelko a,⁎, Georgy Biske a, Reimar Seltmann b, Sergey V. Petrov a, Elena Lepekhina c

a Geological Faculty, St. Petersburg State University, 7/9 University Embankment, St. Petersburg 199034, Russiab Center for Russian and Central EurAsian Mineral Studies (CERCAMS), Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UKc Center of Isotopic Research, Russian Geological Research Institute (VSEGEI), 74 Sredny Pr., St. Petersburg 199106, Russia

⁎ Corresponding author.E-mail address: konopelko@inbox.ru (D. Konopelko).

http://dx.doi.org/10.1016/j.oregeorev.2014.02.0040169-1368/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 September 2013Received in revised form 6 February 2014Accepted 7 February 2014Available online 14 February 2014

Keywords:Tien ShanKyrgyzstanCryogenian alkaline complexU–Pb zirconChalcopyrite mineralization

Neoproterozoic volcanics and granitoids formed at Rodinia margins within a time span of 880 Ma–700 Ma, arewell-documented in many terranes of the southern Central Asian Orogenic Belt (CAOB). Ages younger than550 Ma corresponding to the opening of the Terskey Ocean are also common. However, so far, there were veryfew published ages in the range 700 Ma–550 Ma from the Kyrgyz Tien Shan. In this paper we present newdata for the alkaline Chon-Ashu complex emplaced at the end of the Cryogenian Period of the Neoproterozoic(850–635 Ma, Gradstein et al., 2012). The alkaline complex intrudes the Precambrian metamorphic rocksnorth of the Nikolaev Line which separates the Northern and Middle Tien Shan terranes in the easternKyrgyzstan. The undeformed shallow level alkaline rocks range from olivine gabbro to nepheline and cancrinitesyenites and leucosyenites. The differentiated rock assemblage can be explained by fractional crystallization ofhigh-silica mineral phases which drives nepheline-normative melts away from the silica saturation boundary.The alkaline rocks of Chon-Ashu are enriched in LILE and HFSE indicative of their origin from lithospheric mantle.An age of 678 ± 9Ma (U–Pb, SHRIMP) was obtained for a protolith of country gneiss, and an age of 656 ± 4Mawas obtained for the crosscutting alkaline rocks of the Chon-Ashu complex. Seven zircon grains recovered fromgneiss and alkaline rocks had bright overgrown rimswhich yielded a cumulative age of 400± 8Ma. Ametamor-phic event, followed by uplift and emplacement of shallow level alkaline complex, constrains the geodynamicsetting. Alkaline rocks usually form in an extensional setting and originate from lithospheric mantle. The690Maxenoliths ofmafic granulite from theNWTarimhave been interpreted to originate bymafic underplating.Thismafic underplatingmay have been responsible formetamorphism in themiddle crust prior to emplacementof the Chon-Ashu complex. The 670 Ma–630Ma period of extension and emplacement of enriched alkaline rockscan be also traced on a regional scale through southernKazakhstan and the northern Tarim.We tentatively inter-pret these events as a result of mafic underplating and subsequent rifting related to the break-up of Rodinia.During field work at Chon-Ashu, rich chalcopyrite mineralization has been discovered in carbonate veinlets inleucosyenite alkaline dikes and has also been found in the adjacent Cambrian gabbro and granites shown onthe map as undivided Devonian–Silurian. Stockwork mineralization predominates though disseminated miner-alization is also present. The Cu content reaches 16,184 ppmand is associatedwith elevated concentrations of Pb,Zn and Ag. The polyphase structural evolution of the area suggests that mineralization could have formed in sev-eral genetically unrelated stages. Based on structural andmineralogical evidencewe tentatively relate the earlieststage of chalcopyritemineralization to the latemagmatic CO2-rich fluids emanating from the Cryogenian alkalinecomplex. The Early Devonian thermal event registered by growth of new zircon at 400 Ma has importantmetallogenic implications on a regional scale. However the origin of two zones of alteration in the undividedSilurian–Devonian granites is ambiguous because their age was not determined geochronologically. The 522 ±4 Ma Cambrian gabbro of the Tashtambektor Formation is strongly foliated along the splays of the NikolaevLine, indicating a Hercynian origin of the fabric. Superimposed mineralized stockwork postdates the foliationand suggests a late-Hercynian age of mineralization in gabbro. The new data enable a reassessment of themetallogenic potential of the Eastern Kyrgyz Tien Shan. Presence of not eroded high-level mineralizedNeoproterozoic alkaline intrusions points to a previously underestimated metallogenic potential ofpre-Hercynian granitoids whichmay host preserved porphyry systems, skarns and shear-relatedmineralization.Finally, the Devonian magmato-metamorphic event which caused formation of a number of ore deposits incentral Kyrgyzstan and Kazakhstan could also create potential exploration targets in eastern Kyrgyzstan.

© 2014 Elsevier B.V. All rights reserved.

176 D. Konopelko et al. / Ore Geology Reviews 61 (2014) 175–191

1. Introduction

Fig. 2. Tectonic scheme of the eastern Tien Shan in Kyrgyzstan. Abbreviations as in Fig. 1.

The easternmost part of the Kyrgyz Tien Shan is formed by a com-plex collage of Precambrian and Paleozoic tectonic slices. This complex-ity originatedwhen the Tarimmargin indented the Tien Shan structuresto the north as a result of the India–Asia collision. This created an area ofintensive crustal shortening and drove the lateral escape of large crustalblocks (Burtman et al., 1996; Buslov, 2011; De Jong et al., 2009). Thewhole terrane of the Kyrgyz Middle Tien Shan thins out in this areaand does not continue farther east to China (Figs. 1–3). The recent com-pilation for the Khan–Tengry region (Mikolaichuk and Buchroithner,2009) presents evidence for major Precambrian, and Early and LatePaleozoic complexes (Fig. 3). However the geology and metallogenicpotential of this region are still poorly understood. The present studywas focused on the alkaline Chon-Ashu complex. This undeformedalkaline intrusion cutting Precambrian metamorphic rocks was shownon a map as Permian by Grishchenko et al. (1985) despite a knownNeoproterozoic U–Pb zircon age (Kiselev, unpublished). The determina-tion of the age and petrogenesis of the enriched alkaline complex wasthe initial aim of this work. However the area sampled was increasedbecause rich chalcopyrite mineralization was discovered in the Chon-Ashu rocks and has also been found in the adjacent deformed gabbroand granites shown on the map as undivided Silurian–Devonian. Thiswork resulted in new Neoproterozoic U–Pb zircon ages for the alkalinerocks of Chon-Ashu (4 samples), a slightly older age for a gneissic gran-ite from surrounding metamorphic rocks and a Cambrian age for themineralized gabbro. The petrogenesis of the Chon-Ashu complex isinterpreted using new chemical analyses of a range of alkaline rocks.The probable geodynamic setting of the Chon-Ashu complex is consid-ered in relation to magmatic complexes of similar ages in adjacentterranes. The characteristic contents of base metals, Au and Ag in fourmineralized zones that were mapped and sampled in the field weredetermined by the analysis of 11 selected samples. The mineral compo-sition of these samples has been determined using SEM. The newfindings enable the reassessment of the metallogenic potential of theeasternmost Kyrgyz Tien Shan and suggest that exploration targetsexist within Precambrian and Early–Middle Paleozoic intrusions.

2. Geology of the eastern Kyrgyz Tien Shan

The Tien Shan (Tianshan) orogen formed during the Late Paleozoic(Hercynian) collision between the Precambrian microcontinents ofKarakum and Tarim in the south and the Early Paleozoic Kazakhstancontinent in the north (Bakirov and Maksumova, 2001; Biske andSeltmann, 2010; Charvet et al., 2011; De Jong et al., 2009; Zonenshainet al., 1990). The Kyrgyz Tien Shan is composed of three tectonic units(Fig. 1): theNorthern Tien Shan, theMiddle Tien Shan and the SouthernTien Shan. However, due to crustal shortening theMiddle Tien Shan ter-rane in the easternmost Kyrgyz Tien Shan thins out in an easterly

Fig. 1. Principal terranes and tectonic lineaments of the Tien Shan in Kyrgyzstan. Abbreviationsshow locations of Figs. 2, 3 and 4.

direction and does not continue farther east to China (Figs. 1–3). TheChon-Ashu alkaline complex is situated in the Northern Tien Snan im-mediately north of the “Nikolaev Line”, a tectonic boundary separatingthe Northern and Middle Tien Shan terranes (Figs. 2 and 3). Becausethe Chon-Ashu rocks yielded Neoproterozoic ages, the description ofthe geology of the Tien Shan terranes given here places emphasis onthe Precambrian rocks.

The Northern Tien Shan in Kyrgyzstan consists of an early Paleozoiccontinental arc and its Precambrian basement (Fig. 2). Paleoproterozoicgneisses with U–Pb ages 1.79 and 2.79 Ga were reported from theadjacent region of Kazakh Northern Tien Shan (Alexeiev et al., 2011)while in the Kyrgyz Northern Tien Shan recent geochronological workrevealed numerous fragments belonging to a Mesoproterozoic domain

: NTS— Northern Tien Shan, MTS—Middle Tien Shan, STS — Southern Tien Shan. Frames

Fig. 3. Granitoid intrusions and Precambrian basement complexes with known ages extracted from digital geological map of eastern Kyrgyzstan and Kazakhstan (Mikolaichuk andBuchroithner, 2009). Note the intrusions of Devonian magmatic front spreading south from Kazakhstan.New data from this study and data of Kröner et al. (2009, 2011) are added.

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with Grenvillian ages of 1.3 and 1.1 Ga (Kröner et al., 2013).Neoproterozoic ages of 844 and 778 Ma were obtained for metamor-phosed granites from the Aktyuz and Kemin complexes while apost-tectonic granite from Aktyuz complex yielded an age of 692 Ma(Kröner et al., 2012). Further evolution of the Northern Tien Shan wascontrolled by the development of the Terskey Ocean to the south andthe formation of an Andean type active margin in the Northern TienShan. The beginning of this process ismarked by intrusion of Early Cam-brian granitoids and by Early Cambrian–Ordovician ophiolites (Kiselev,1999; Kiselev and Maksumova, 2001; Lomize et al., 1997; Mikolaichuket al., 1997). Progressive subduction to the north and subsequent clo-sure of the Terskey Ocean in the Late Ordovician with accretion of theMiddle Tien Shan to the Northern Tien Shan is marked by continuousAndean type magmatism during which voluminous subduction-related Ordovician–Early Silurian granitoids were emplaced (Fig. 3)(Ghes, 2008; Kiselev and Maksumova, 2001; Konopelko et al., 2008).The boundary between the Northern and Middle Tien Shan terranes isknown as the Nikolaev Line (Nikolaev, 1933) or Terskey suture(Lomize et al., 1997). The Nikolaev Line formed as the Late Ordovician(Caledonian) suture marked by ophiolites which was transformed intoa regional sinistral shear zone during the Late Carboniferous(Hercynian) collision and was later affected by Cenozoic deformations(Biske, 1995; Lomize et al., 1997).

The Precambrian basement of the Middle Tien Shan east of theTalas–Fergana fault consists of Paleoproterozoic gneisses of the Kuilyu

complex cropping out in the Saryjaz river basin and in the Ak-Shijrakridge (Figs. 2 and 3). Discordant U–Pb ages of ~2 Ga were obtained forthese rocks by previous multigrain zircon dating (Kiselev, 1999) andages of ca 2.3 and 1.8 Ga were determined recently by SHRIMP dating(Kröner et al., 2011) (Fig. 3). Neoproterozoic magmatic rocks are repre-sented by the granites of the Saryjaz complex (Fig. 3) and by rhyolites ofthe Bolshoi Naryn Formation dated at 830 Ma and 764 Ma respectively(Kröner et al., 2009, 2011). Similar SHRIMP ages of 778 and 728 Mawere reported recently by Konopelko et al. (2013) for deformed gran-ites in the Talas–Fergana fault zone. The Neoproterozoic magmaticrocks are overlain by Cryogenian and Ediacarian (Vendian) sandstonesand diamictites, which are transitional upsection to Cambrian shalesand carbonates, Ordovician cherts and turbidites. Docking of theMiddleTien Shanwith the Northern Tien Shan in the Late Ordovician led to theformation of an unconformity and uplift. Marine sedimentation re-sumed in this terrane in Middle–Late Devonian and continued untilMiddle Carboniferous. As shown by Mikolaichuk and Buchroithner(2009), the easternmost parts of the Northern and Middle Tien Shanterranes were affected by Devonian granitoid magmatism formed in aback arc setting relative to the Devonian arc in Kazakhstan (Fig. 3).

The Southern Tien Shan terrane is a fold and thrust belt formedduring the closure of the Paleo-Turkestan ocean which separated thePrecambrian continents of Karakum and Tarim in the south and theEarly Paleozoic Kazakhstan continent in the north during the Late Paleo-zoic. In eastern Kyrgyzstan the Southern Tien Shan was traditionally

178 D. Konopelko et al. / Ore Geology Reviews 61 (2014) 175–191

considered as a stack of folded nappes that were thrust southward ontopassive margin sediments and continental basement of the Tarim dur-ing the Late Carboniferous (Hercynian) collision between Tarim andPaleo-Kazakhstan (Biske, 1995; Biske and Seltmann, 2010; Zonenshainet al., 1990). However, there is a growing evidence that the evolutionof the Southern Tien Shan in eastern Kyrgyzstan and adjacent regionsof China was more complex and the southward thrusting representsthe final stage of the collision while prior to the Early Carboniferousthe subduction zone probably was south-dipping and the northernmargin of Tarim was an active margin during this period (Charvetet al., 2011 and references therein). After the final closure of thePaleo-Turkestan ocean in the Late Carboniferous, the suture zoneknown as the Atbashi–Inylchek fault separating the South and theMiddle Tien Shan terranes in eastern Kyrgyzstan has been formed(Figs. 1–3). On a post-collisional stage the Atbashi–Inylchek fault, aswell as the Nikolaev Line, has been transformed into a regional sinistralshear zone (Biske, 1995). All three terranes of the eastern Kyrgyz TienShan were affected by post-collisional uplift and intrusion of EarlyPermian granitoids (Fig. 3) (Konopelko and Eklund, 2003; Konopelkoet al., 2011; Seltmann et al., 2011).

Descriptions of the Precambrian basement of Tarim have been givenin a number of works in which several stages of crust formation withages similar to those characteristic of the Middle and Northern TienShan terranes have been recognized. Paleoproterozoic ages werereported for gneisses from the Kurukhtag block in the northern Tarim(Shu et al., 2011) and from the eastern part of the Kyrgyz SouthernTien Shan (Mikolaichuk and Buchroithner, 2009 and references there-in). Mesoproterozoic ages of 1250–950Mawere obtained formetamor-phic and magmatic rocks in the Kurukhtag block (Shu et al., 2011;Zhang et al., in press) and were reported for detrital zircons fromthe Chinese Tien Shan which were presumably derived from theTarim basement (Ma et al., 2012 and references therein). Finally ca.820–750 Ma bimodal volcanic assemblages, intrusive granitoids anddike swarmswere described in several areas of the northern and centralTarim and interpreted as evidence of rifting (Guo et al., 2005; Xu et al.,2005; Zhang et al., 2009; Zhu et al., 2008). The marked similaritybetween the structures of the Precambrian basements of the NorthernandMiddle Tien Shan terranes and the Tarim led several authors to sug-gest that these terranes formed a single Precambrian continent, at leastuntil the latest Neoproterozoic when they were rifted off and becamePrecambrian microcontinents in the evolving Central Asian OrogenicBelt (Kröner et al., 2013 and references therein).

3. Geology of the Chon-Ashu area and strategy of sampling

The study area approximately 4 × 4 km in size is situated south ofChon-Ashu mountain pass (3822 m) on the southern slope of theTerskey–Alatau ridge. A schematic geological map of the study areaaccompanied by cross section (Fig. 4) was compiled by the authorsbased on the 1:50,000 scale map of Grishchenko et al. (1985). Thearea shown on the schematic map is situated within 1 km distancenorth of the Nikolaev Line which is usually positioned in this regionalong the northern contact of the large Neoproterozoic granite intrusionof the Saryjaz complex (Figs. 2 and 3). Following Grishchenko et al.(1985) we consider the granites in the southern part of the study areaas undivided Silurian–Devonian granites (Fig. 4) and we believe thatthe contact with Neoproterozoic granites, corresponding to theNikolaev Line, is situated within 1 km distance to the south and is cov-ered by Quaternary deposits. As seen in Fig. 4 the study area is dividedinto a series of tectonic slices by several NE striking faults which maybe considered as splays of the Nikolaev Line. The rocks within tectonicslices are strongly foliated along the strike of the Nikolaev Line whichreflects the latest post-Carboniferous sinistral motions along the Line(Fig. 4). The contacts between various rock types are mostly tectonicand even contacts shown by Grishchenko et al. (1985) as stratigraphicand intrusive are strongly sheared. However larger tectonic lenses

probably preserved pre-Hercynian structural fabrics at least locally.One example of a tectonic lens less affected by Hercynian deformationsis represented by gneisses, amphibolites and migmatites of the Chon-Ashu Formation shown on the map as Precambrian (Grishchenkoet al., 1985). The rocks are variably deformed but their deformation pat-terns do not demonstrate alignment with the NE striking Nikolaev Line(Fig. 4). A gneissic granite from themetamorphic Chon-Ashu Formationsampled for SHRIMPdating is shown in Fig. 5a. The undeformed alkalinerocks of the Chon-Ashu complex intrude the metamorphic rocks of theChon-Ashu Formation and form a wide aureole of intensely fenitizedmetasomatic rocks. Three main rock types forming the alkalineChon-Ashu complex have been described by previous investigators(Grishchenko et al., 1985 and references therein) and observed in thefield; (1) ultramafic and mafic olivine pyroxenites, troctolites and oliv-ine gabbro form relatively small outcrops in the upper part ofMontoitorriver; (2) coarse-grained nepheline and cancrinite syenites comprisethe main volume of alkaline rocks and consist of biotite ± hornblende(up to 15%) and variable amounts of albite, nepheline and/or cancrinite.Accessory phases include zircon, sphene, fluorite and opaques. A typicalcancrinite syenite is shown in Fig. 5b; and (3) ankerite nephelineleucosyenites and alkali feldspar syenites are the third most commonrock type and form dikes and small bodies several meters thick. Twophotos of ankerite leucosyenites containing mineralized carbonateveinlets are shown in Fig. 5c and d. The contacts between these threerock types have not been observed in thefield. Themetamorphic and al-kaline rocks form a tectonic lens elongated in a NE direction. Howeverleucosyenite dikes cutting the metamorphic rocks generally strikeacross the regional structures in a N–S direction indicating that pre-Hercynian structural pattern was preserved within the tectonic lens(Fig. 4). The rocks of the Early Paleozoic (Caledonian) orogenic stagein the study area are deformed gabbro, basalts and tuffs of the CambrianTashtambektor Formation (Grishchenko et al., 1985) and sedimentaryrocks of Middle Ordovician age. A foliated gabbro with chalcopyrite-bearing stockwork vein crosscutting foliation is shown in Fig. 5e. Thefoliation in gabbro strikes in a NE direction parallel to the NikolaevLine. The undivided Silurian–Devonian granites occur as relativelysmall bodies in the northern part of study area and as a larger body inits southern part (Fig. 4). The rocks comprise variably foliated alkali-calcic medium-grained red granites. Small bodies of granites in thenorth are strongly foliated in the NE direction while the southernbody preserved pre-Hercynian structural elements comprising two 20and 50 m thick zones of alteration striking in the N–S direction. Theyoungest rocks in the study area are a tectonic sliver of Early Carbonif-erous sandstones (Fig. 4) formed in a back arc extensional basin duringthe Late Paleozoic (Hercynian) orogenic stage (Biske, 1995).

Sampling was originally focused on the alkaline rocks of the Chon-Ashu complex from which 4 samples for SHRIMP dating and 7 samplesfor geochemical analysis were collected. The chalcopyrite mineraliza-tion was first discovered in leucosyenite dikes of the Chon-Ashucomplex and then found in the adjacent deformed gabbro andSilurian–Devonian granites (Fig. 4). In order to understand the regionalgeology and to characterize the mineralized rocks we extended thesampling area and collected 2 additional samples for SHRIMP dating(a gneissic granite from the metamorphic Chon-Ashu Formation and adeformed mineralized gabbro from the Tashtambektor Formation) aswell as 11 mineralized samples from 4 zones of mineralization. In addi-tion the base metal, Au and Ag contents of 19 samples of barren rocksfrom the study area and alkaline rocks from the Chon-Ashu complexhave been analyzed for purposes of comparison.

4. Geochronological results

Two samples ofmineralized leucosyenite dike (416305 and 416306)and 2 samples of coarse-grained nepheline and cancrinite syenite(416307 and 417001) from the alkaline Chon-Ashu complex werechosen for U–Pb geochronology. In addition, a deformed mineralized

Fig. 4. Simplified geological map of Chon-Ashu area and schematic cross-section A–B. Quaternary deposits and glaciers are not shown on the cross-section.Modified by the authors after Grishchenko et al. (1985).

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gabbro from the Tashtambektor Formation (416200) and a gneissicgranite from the metamorphic Chon-Ashu Formation (416301) havealso been dated. The U–Pb analytical data and the coordinates of thesampling sites are given in Table 1, CL images of analyzed zircons anddiagrams with concordia are shown in Figs. 6 and 7, respectively. Agesare also shown on a schematic geological map in Fig. 4.

4.1. Analytical procedure

Selected zircon grains were hand-picked and mounted in epoxyresin together with chips of standard zircon grains. The grains were ap-proximately sectioned in half and polished. Prior to analysis, the zircongrains were investigated in transmitted and reflected light and under ascanning electron microscope equipped with cathodoluminescence

(CL) and back-scattered electron (BSE) detectors. The U–Th–Pb isotopeanalyses were made using Sensitive High-Resolution Ion Microprobe(SHRIMP-II) in the Centre for Isotopic Research, VSEGEI, St. Petersburg,Russia. Each analysis consisted of four scans through the mass range.The diameter of the spot was about 30 μm, and the primary beamcurrent was about 4 nA. Every fourth measurement was carried out onthe zircon standard TEMORA 1, with an accepted 206Pb/238U age of416.75 ± 0.24 Ma (Black et al., 2003). The Pb/U ratios have been nor-malized relative to a value of 0.0668 for the 206Pb/238U ratio of theTEMORA 1 standard. The zircon standard 91500, with a U concentrationof 81.2 ppm and an accepted 206Pb/238U age of 1065 Ma (Wiedenbecket al., 1995) was used as a “U-concentration” standard. The data werereduced in a manner similar to that described by Williams (1998) andreferences therein, using the SQUID Excel Macro of Ludwig (2000).

Fig. 5. Principal rock types and styles of mineralization: a — gneissic granite of the metamorphic Chon-Ashu Formation, b — even-grained cancrinite syenite of the Chon-Ashu alkalinecomplex, c and d — mineralized carbonate veinlets and disseminated mineralization in ankerite leucosyenite dikes of the Chon-Ashu alkaline complex from Mineralized zone 1 inFig. 4, e — deformed gabbro of Tashtambektor Formation with chalcopyrite in quartz veinlet from Mineralized zone 3 in Fig. 4.

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Corrections for common Pb were made using the 204Pb isotope (mea-sured 204Pb/206Pb) and the present day terrestrial average Pb-isotopiccomposition (Stacey and Kramers, 1975). Uncertainties given for indi-vidual analyses given in Table 1 (ratios and ages) and Fig. 5 are at the1σ level, however the uncertainties in calculated concordia ages(Fig. 6) are reported at 2σ level. The concordia plots were constructedusing the ISOPLOT/EX macro (Ludwig, 1999). Ion-microprobe datingof zircons younger than about 1000 Ma is best achieved by using206Pb/238U-ages (Black and Jagodzinski, 2003), while the 207Pb/206Pbisotopic system is suitable for dating of zircons older than 1000 Mabecause of the short half-life of 235U producing 207Pb (Black et al.,2003). In this paper we generally followed this practice when reportingthe ages obtained.

4.2. Results

Zircon grains separated from the samples of the alkaline Chon-Ashucomplex are large crystal fragments with oscillatory zoning (Fig. 6a–c).From 5 to 9 spots have been analyzed in samples 416306, 416307 and417001 but due to technical problems only one spot was analyzed insample 416305. Analytical data for samples 416306, 416307 and417001 plot as tight clusters for which 206Pb/238U concordia ages of661 ± 7 Ma, 654 ± 11 Ma and 653 ± 7 Ma have been calculated(Fig. 6 a–c). The ages obtained agree within error and if all 21 analysesof 4 samples of alkaline rocks are calculated together they yield a cumu-lative 206Pb/238U concordia age of 656±4Ma (MSWD= 0.21) (Fig. 7d)which is a good estimate of the age of emplacement of the Chon-Ashualkaline complex. In 2 grains bright overgrowth rims have been identi-fied in CL images and analyzed. The ages obtained were around 380Ma(Table 1). Fig. 6c is a CL image showing an example of a rim of youngovergrowth dated at 388 Ma.

Zircon grains separated from the gneissic granite of the metamor-phic Chon-Ashu Formation (416301) do not display structures typicalof metamorphic zircons (Fig. 6d). They may well be zircons from the

igneous protolith of this rock. Eight analyses plot on concordia as a rel-atively tight cluster and yield a 206Pb/238U concordia age of 678 ± 9Ma(MSWD = 0.05) (Fig. 7e). However 2 older analyses which probablyrepresent inherited or xenogenic components yield a concordia age of743±26Ma. However, if the age of the igneous protolith of the gneissicgranite is about 680 Ma, it was emplaced and then transformed into agneiss within the 20–25 Ma interval of time preceding emplacementof the alkaline complex at 654 Ma. An interesting feature of the zircongrains from the gneissic granite are their well-defined bright rims.Five analyses of these yielded ages around 400Ma (Fig. 6d). Being calcu-lated together with 2 analyses of overgrowth rims on zircons from alka-line rocks, all 7 analyses yield a 206Pb/238U concordia age of 400± 8Ma(MSWD = 0.95) (Fig. 7f). An age of 400 Ma registers an Early–MiddleDevonian magmato-metamorphic event and matches well with agesof Devonian granites on a map of Mikolaichuk and Buchroithner(2009) (Fig. 3).

Finally, a uniform population of magmatic zircon grains has beenrecovered from sample 416200 of the deformed gabbro from theTashtambektor Formation (Fig. 6e). Twelve spots in 9 grains were ana-lyzed. The data are concordant and plot as a tight cluster yielding a206Pb/238U concordia age of 522±4Ma (MSWD= 0.025) (Fig. 7g) pro-viding a good estimate of the crystallization age of the deformed gabbrowhich is similar to the earliest ages of Caledonian intrusions andophiolites from the Northern Tien Shan (Kiselev, 1999; Kiselev andMaksumova, 2001).

5. Geochemistry and petrogenesis of the Chon-Ashualkaline complex

Chemical compositions of 7 samples from the Chon-Ashu rocks havebeen determined by XRF and ICP-MSmethods at VSEGEI, St. Petersburg,Russia (Table 2). In addition major and trace elements of several Chon-Ashu rocks together with mineralized samples analyzed by ICP-MSwere also plotted on variation diagrams. The Chon-Ashu rocks show a

Table 1U–Pb analytical data and calculated ages.

Concentrations Isotope ratiosa Age (Ma)

Sample-spot #b U Th Th/U 206Pbc f206d 207Pbc/206Pbc ±1σ 207Pbc/235U ±1σ 206Pbc/238U ±1σ Err.e corr. 206Pbc/238U ±1σ 207Pbc/206Pb ±1σ Disc.f

ppm ppm ppm % % % % %

Sample 416200— deformed gabbro (N 42° 23.012, E 79° 03.636)416200.1.1 63 26 0.44 4.5 0.00 0.0592 3 0.684 3.3 0.0839 1.3 .412 519 6.7 573 65 10416200.1.2 151 55 0.38 11.1 0.16 0.0575 3.2 0.679 3.4 0.08557 1.1 .318 529 5.5 511 70 −3416200.2.1 100 32 0.33 7.22 0.07 0.0572 2.4 0.663 2.7 0.0841 1.2 .433 521 5.9 500 54 −4416200.3.1 56 18 0.33 4.09 0.43 0.0592 6.7 0.695 6.8 0.0851 1.5 .215 526 7.4 576 140 10416200.3.2 84 19 0.24 5.92 0.58 0.0578 5.9 0.649 6 0.0814 1.3 .216 505 6.3 523 130 4416200.4.1 188 83 0.46 13.5 0.17 0.0574 2.9 0.662 3.1 0.08362 1 .337 518 5.1 508 63 −2416200.4.2 55 14 0.26 3.88 0.67 0.0562 6.5 0.629 6.6 0.0812 1.5 .219 503 7.0 459 140 −9416200.5.1 77 19 0.26 5.64 0.38 0.0566 6.1 0.66 6.2 0.0845 1.3 .212 523 6.6 477 130 −9416200.6.1 109 36 0.34 8.18 0.37 0.0565 5.8 0.675 5.9 0.0868 1.2 .203 536 6.2 471 130 −12416200.7.1 107 32 0.31 7.78 0.37 0.0571 5.8 0.663 5.9 0.0842 1.2 .203 521 6.0 496 130 −5416200.8.1 67 16 0.25 4.89 0.44 0.0583 6.8 0.676 6.9 0.0841 1.4 .202 521 7.0 541 150 4416200.9.1 76 19 0.26 5.59 0.17 0.0585 4.2 0.689 4.4 0.0854 1.3 .289 528 6.5 548 92 4

Sample 416305— leucosyenite dike (N 42° 22.22, E 79° 63.321)416305.1.1 113 47 0.43 10.4 0.30 0.0611 2.6 0.9 3.1 0.1068 1.6 .511 654 ±9.8 644 ±57 −2

Sample 416306— leucosyenite dike (N 42° 22.22, E 79° 63.321)416306.1.1 133 39 0.30 12.8 0.00 0.0628 2 0.968 2.6 0.1117 1.7 .638 683 11 703 43 3416306.2.1 240 182 0.78 21.7 0.08 0.0618 1.5 0.897 2.2 0.1053 1.6 .718 645.2 9.7 667 33 3416306.3.1 39 14 0.36 3.7 0.57 0.0608 6.7 0.923 7.1 0.1101 2.1 .292 673 13 632 150 −6416306.3.2 27 3 0.12 2.6 0.63 0.0601 4.8 0.906 5.2 0.1094 1.9 .375 669 12 606 100 −10416306.4.1 295 385 1.35 27.3 0.00 0.06157 1.2 0.918 1.9 0.1082 1.5 .790 662.2 9.6 659 25 0416306.5.1 193 99 0.53 17.6 0.11 0.061 1.8 0.894 2.4 0.1063 1.5 .640 651.5 9.5 639 40 −2416306.6.1 240 114 0.49 22.0 0.09 0.06096 1.2 0.895 1.9 0.1065 1.5 .793 652.1 9.4 638 25 −2416306.7.1 95 49 0.53 8.9 0.16 0.0625 2.6 0.93 3 0.1079 1.6 .534 661 10 691 55 5

Sample 416307— syenite (N 42° 22.22, E 79° 63.321)416307.2.1 30 429 14.76 1.6 3.33 0.061 26 0.51 26 0.0604 3.1 .116 378 11 631 560 67416307.5.1 182 51 0.29 16.6 0.20 0.0611 3 0.894 3.5 0.106 1.7 .493 650 11 644 66 −1416307.1.1 29 6 0.20 3.1 13.53 0.065 29 0.95 29 0.1063 3 .102 651 19 762 620 17416307.4.1 327 180 0.57 30.0 0.02 0.0624 1.8 0.918 2.4 0.1067 1.6 .671 653 10 688 38 5416307.3.1 98 42 0.45 9.0 0.11 0.0622 2.5 0.918 3 0.107 1.7 .555 655 10 680 53 4

Sample 417001— syenite (N 42° 22.22, E 79° 63.321)417001.2.2 4 1 0.33 0 6.42 0.064 52 0.55 52 0.0621 5.8 .111 388 22 730 1100 89417001.1.1 355 182 0.53 31 0.19 0.06104 1.3 0.853 2 0.1013 1.5 .763 622.2 8.9 641 27 3417001.2.1 192 35 0.19 17 0.17 0.06276 1.5 0.908 2.1 0.105 1.5 .718 643.4 9.3 700 32 9417001.4.1 150 42 0.29 14 0.21 0.0612 2.2 0.89 2.8 0.1054 1.6 .588 646.1 10 647 48 0417001.3.1 242 142 0.61 22 0.28 0.0619 2 0.914 2.6 0.1072 1.6 .613 656.4 9.8 669 43 2417001.6.1 167 42 0.26 15 0.24 0.0602 2.5 0.892 3 0.1074 1.6 .544 658 10 611 54 −7417001.5.1 177 64 0.37 16 0.09 0.0614 1.7 0.913 2.3 0.1077 1.6 .681 660 10 655 37 −1417001.7.1 187 51 0.28 17 0.09 0.0624 2.1 0.935 2.8 0.1086 1.9 .658 665 12 688 46 4417001.5.2 21 2 0.11 2 1.32 0.0659 10 1.02 10 0.1123 2.2 .214 686 14 803 210 17

Sample 416301— gneissic granite (N 42° 22.22, E 79° 63.321)416301.1.2 200 7 0.04 11 0.00 0.053 3.3 0.454 3.9 0.0622 2.1 .548 389 8 329 74 −15416301.10.1 187 9 0.05 10 0.57 0.0543 4.1 0.477 4.6 0.0638 2.1 .452 399 8 381 93 −4416301.7.1 42 19 0.48 2 0.63 0.0552 8.6 0.499 8.9 0.0656 2.5 .281 410 10 420 190 3416301.11.1 72 62 0.89 4 3.89 0.053 21 0.5 21 0.0676 5.4 .253 422 22 340 470 −19416301.9.1 30 11 0.36 2 1.35 0.0492 13 0.459 13 0.0677 2.9 .220 422 12 155 300 −63416301.4.1 898 501 0.58 78 0.56 0.06318 1.4 0.874 2.4 0.1003 2 .817 616 12 714 30 16416301.4.2 165 78 0.49 15 0.64 0.0604 3.6 0.892 4.1 0.107 2.1 .497 655 13 619 77 −5416301.10.2 83 59 0.74 8 0.00 0.0596 3.3 0.883 4 0.1075 2.3 .570 658 14 588 72 −11416301.6.1 558 239 0.44 53 0.09 0.0619 1.1 0.935 2.2 0.1096 2 .881 670 12 671 23 0416301.3.1 367 147 0.41 34.9 0.14 0.0629 1.5 0.959 2.7 0.1106 2.2 .834 676 14 705 31 4416301.11.2 1268 435 0.35 121.0 0.02 0.06206 0.82 0.952 2.1 0.1112 2 .924 680 13 676 17 −1416301.5.1 322 78 0.25 31.3 0.24 0.06222 1.6 0.967 2.5 0.1127 2 .786 689 13 682 33 −1416301.2.1 145 29 0.21 14.2 1.12 0.0602 4.2 0.937 4.7 0.1129 2.1 .445 690 14 609 90 −12416301.9.2 389 127 0.34 38.0 0.14 0.06362 1.5 0.995 2.5 0.1134 2 .795 692 13 729 32 5416301.12.1 89 41 0.47 9.4 0.01 0.0626 2.8 1.052 3.6 0.122 2.3 .622 742 16 693 60 −7416301.1.1 25 3 0.13 2.7 1.77 0.061 21 1.05 21 0.1245 3.6 .170 756 26 648 450 −14

a Corrected for 204Pb.b The last two digits denote number of grain and number of analytical spot within the grain.c Radiogenic Pb.d f206 denotes 100 ∗ (common 206Pb) / (total measured 206Pb).e Error correlation 207Pb/235U − 206Pb/238U.f Disc. % denotes 100 ∗ ((1 − (age206Pb / 238U) / (age207Pb / 206Pb)).

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range of compositions with SiO2 contents ranging from 45 to 62 wt.%.On a total alkalis versus silica (TAS) classification diagram(Middlemost, 1994), two samples of mafic rocks plot in the gabbrofield (Fig. 8a). The rocks are silica saturated and contain normative

olivine. Five samples of alkali syenites and leucosyenites plot in thefields of foid monzodiorite, foid monzosyenite, foid syenite, and syeniteand quartz monzonite and define an alkaline trend (Fig. 8a). Four out of5 samples contain normative nepheline. On the K2O vs. Na2O diagram

Fig. 6. CL images of analyzed zircon grains: a–c — rocks of the alkaline Chon-Ashu complex, d — gneissic granite from the metamorphic Chon-Ashu Formation, e — deformed gabbro ofTashtambektor Formation. Ovals define analytical spots. Sample and spot numbers are as in Table 1. Ages are in Ma, errors at 1σ level.

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for alkalinemagmatic rocks (McBirney, 1993) the Chon-Ashu rocks plotin thefield of theNa-series and just cross the boundary of thefield of theK-series (Fig. 8b). The syenites and leucosyenites are enriched in FeOrelative to MgO and their Fe-indexes shown on the FeOtot / (FeOtot +MgO) vs. SiO2 diagram of Frost and Frost (2008) increase withincreasing SiO2 contents (Fig. 8c). On the AI vs. FSSI diagram of Frostand Frost (2008) in which alkalinity index AI = Al − (Na + K) mol.and FSSI is the feldspathoid silica-saturation index expressed as norma-tive Q − [Lc + 2(Ne + Kp)] / 100 where Q — quartz, Ne — nepheline,Kp— kaliophilite, and Lc — leucite, the Chon-Ashu rocks define a trendfrom mafic silica saturated rocks to silica undersaturated syenites andleucosyenites in which increasing normative nepheline is correlatedwith decreasing AI index (Fig. 8d). Trends of this type can be explainedby fractional crystallization of high-silica mineral phases such asaegirine and Ca-rich plagioclase which drives nepheline-normativemelts away from the silica saturation boundary (Frost and Frost,2008). Trends on the variation diagrams are mainly consistent withthis scenario. Because no regular trends were found on variation dia-grams against silica we plotted major and trace element concentrationsof the Chon-Ashu rocks against Mg# = MgO / (MgO + FeOtot) (Fig. 9).

Decrease of CaO with decreasing Mg# is consistent with fractionalcrystallization ofmineral phases rich in SiO2 and CaO such as Ca-rich py-roxene and plagioclase. Following Ca-rich phases extracted from themelt, Sr also demonstrates compatible behavior. Fractionation of pyrox-ene and olivine is illustrated by the compatible behavior of Cr and Ni(not shown). The alkali content illustrated in Fig. 9 by Na2O generallyincreases with decreasing Mg#. Enrichment in K2O is associated withincompatibility of Ba and Rb (not shown). Phosphorus accumulates inthe most evolved compositions. Finally high-field strength elements(HFSE) and rare earth elements (REE) also show incompatible behaviorwhich is illustrated by the Nb and Ce vs. Mg# diagrams in Fig. 9. REEpatterns are also consistent with fractionation of Ca-rich plagioclase.General enrichment in light REE is seen in Fig. 10a. It is also evidentthat mafic rocks display positive Eu anomalies indicating accumulationof plagioclase whereas the most evolved syenites show a pronouncednegative Eu anomaly which is probably the result of separation of

plagioclase from the melt. The primitive mantle-normalized traceelement abundances of the of Chon-Ashu rocks (Fig. 10b) show thatonly the most evolved syenites and leucosyenites are moderatelyenriched in large ion lithophile elements (LILE) and HFSE thus illustrat-ing the differentiated character of the Chon-Ashu complex.

Some of the leucosyenite dikes contain several modal percents ofcarbonateminerals, predominantly calcite and ankerite (this is reflectedin the high LOI values of samples 416305 and 416306 in Table 2). Re-placement of nepheline by cancrinite is common in the coarse-grainedsyenites and is also indicative of the activity of CO2-rich residual fluidswhich reactedwith nepheline to produce cancrinite in an evolving alka-line series (Henderson and Gibb, 1983). Finally, mineralized carbonateveinlets in leucosyenites also indicate that CO2-rich fluids were present.These late magmatic CO2-rich fluids can react with the rocks and causedisturbance of geochemical trends of P2O5, Sr and Bawhich is character-istic of the most evolved rocks with Mg# b 0.15 (Fig. 9). No evidence ofcarbonatitemelt has been found in theChon-Ashu rocks so far. Howeverthe differentiated character of the pluton and the wide aureole offenitization suggest that unexposed alkaline intrusions containing highlydifferentiated melts including carbonatites may be present at depth.

6. Chalcopyrite mineralization associated with theChon-Ashu complex

6.1. Descriptions of mineralized zones

The chalcopyrite mineralization was first discovered in carbonateveinlets in leucosyenite dikes of the Chon-Ashu complex and thenfound in adjacent deformed gabbroic rocks shown on the map as theCambrian Tashtambektor Formation (Grishchenko et al., 1985) andalso in undivided Silurian–Devonian granites. In total 4 mineralizedzones have been identified andmapped (Fig. 4) fromwhich 11mineral-ized grab sampleswith visible chalcopyritewere collected for investiga-tion. In addition 19 samples of barren Early Paleozoic rocks and alkalineChon-Ashu rocks were collected for comparison. The weight of each ofthe mineralized samples was 2–4 kg. In order to determine the

Fig. 7. Concordia diagrams for zircon U–Pb SHRIMP data of the rocks from Chon-Ashu area. Sample numbers as in Table 1.

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Table 2Chemical compositions of the rocks of the Chon-Ashu alkaline complex.

Rock-type Mafic rock Mafic rock Syenite Syenite Syenite Leucosyenite dike Leucosyenite dike

Sample 417003 417004 417001 416307 417007 416305 416306

SiO2 48.50 48.50 55.60 55.70 62.20 45.80 53.70TiO2 0.26 0.12 0.26 0.14 0.36 0.22 0.31Al2O3 20.60 25.10 21.40 23.10 17.90 17.00 22.80FeOtot 4.37 3.54 4.84 2.22 2.83 7.40 5.61MnO 0.09 0.07 0.08 0.06 0.06 0.27 0.12MgO 8.01 5.30 0.39 0.57 0.12 1.50 0.48CaO 14.00 12.30 1.15 0.98 2.38 7.61 1.48Na2O 1.89 2.40 9.82 11.40 6.12 7.03 6.71K2O 0.28 0.42 4.13 3.94 5.48 1.86 3.55P205 b0.05 b0.05 0.23 0.05 0.09 0.07 0.15LOI 0.84 1.57 1.48 1.53 1.75 10.30 4.21Cr 626.0 91.3 9.0 9.9 18.4 12.4 21.0Ni 39.9 33.8 3.9 4.6 1.6 8.0 7.6V 73 19 5 7 8 9 5Rb 9 12 65 44 20 28 55Ba 78 148 434 461 272 274 611Sr 499 729 388 335 118 444 326Ga 14.2 14.9 25.5 27.5 31.4 21.1 27.0Zr 10 3.6 84 315 429 394 392Hf 0.38 0.12 1.97 7.41 8.69 7.95 5.20Y 5.6 2.7 4.8 4.2 27.0 5.7 5.5Nb 2.0 0.87 46.4 29.0 29.3 34.4 51.7Ta 0.11 b0.05 0.67 0.76 0.54 0.26 0.53U 0.17 0.21 0.29 0.85 0.26 0.69 0.35Th 0.6 0.41 0.52 1.5 0.66 2.5 1.2Cu 24.1 4.9 4.4 6.6 22.2 676 7.3Pb 5.1 12.7 2.18 2.7 4.2 28.3 1.7Zn 39.0 29.0 76.6 41.7 34.9 210 66.3La 3.17 .35 13.4 11.0 83.9 11.3 16.3Ce 6.32 2.54 31.0 21.8 197.0 22.7 34.1Pr 0.79 0.32 3.79 2.42 23.2 2.60 3.93Nd 3.59 1.39 13.5 7.9 82.7 8.7 13.6Sm 0.97 0.34 2.47 1.36 13.8 1.81 2.20Eu 0.50 0.42 0.84 0.54 1.39 0.65 0.90Gd 1.01 0.30 1.90 1.33 10.50 1.57 1.89Tb 0.15 0.06 0.26 0.18 1.48 0.25 0.28Dy 0.98 0.33 1.11 0.95 6.89 1.18 1.21Ho 0.21 0.09 0.17 0.16 1.19 0.23 0.21Er 0.51 0.26 0.42 0.41 2.91 0.56 0.46Tm 0.08 0.04 0.05 0.05 0.38 0.08 0.07Yb 0.47 0.25 0.33 0.35 2.45 0.56 0.36Lu 0.07 0.05 0.06 0.05 0.44 0.09 0.07

SiO2 — LOI wt.%; Cr — Lu ppm; b0.05— below detection limit.FeOtot — total Fe as FeO.

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character of the mineralization, all samples were analyzed for 29elements, including base metals and Ag, and by FA for Au, at the AlexStewart Assay and Environmental Laboratories LTD in Kara-Balta,Kyrgyzstan using the ICP-MS method. Concentrations of base metals,Ag, and Au in themineralized samples are given in Table 3. The mineralcompositions of ore and gangueminerals frommineralized zone 1weredetermined by energy-dispersive x-ray spectrometry (EDS) using aJEOL 5700LV scanning electron microscope (SEM) equipped with alight element detector, Be to U (in the IAC Laboratory of The NaturalHistory Museum, London). EDS spectra were acquired for 50 s (livetime) with an accelerating voltage of 20 kV, a beam current of 2 nAand a varying electron-beam diameter. Well-characterized syntheticcompounds and natural minerals were used as standards. The spectrawere processed using Oxford Instruments INCA software. Backscatteredelectron (BSE) images ofmineralization in carbonate veinlets are shownon Fig. 10a and b.

6.1.1. Mineralized zone 1The richest copper mineralization was discovered in carbonate vein-

lets a few cm thick forming an irregular stockwork in the ankeriteleucosyenite dikes of the Chon-Ashu complex. The dikes of leucosyeniteseveral meters thick cut the metamorphic rocks of the Chon-AshuFormation and generally strike in a N–S direction (Fig. 4). The most

intensivelymineralized samples have been found in boulders and blocksof leucosyenite which probably fractured along mineralized carbonateveinlets. As a result, 1–7 cm thick mineralized veinlets form the surfaceof many of the “ore” boulders in mineralized zone 1 (Fig. 5c). The oreminerals in the carbonate veinlets are generally chalcopyrite and pyritebut variable amounts of galena, sphalerite, arsenopyrite, ilmenite, rutileand magnetite were also identified using the SEM. The BSE image inFig. 11 a shows the intergrowth of chalcopyrite, pyrite and carbonateminerals in the feldspathic matrix of a leucosyenite while ilmenite,chalcopyrite and galena in an ankeritic matrix form the inner part of asulfide-rich carbonate veinlet illustrated in Fig. 11b. The Cu content inthe veinlet samples reaches 16,184 ppm and is associated with elevatedconcentrations of Pb (182 ppm), Zn (1856ppm) andAg (37 ppm). How-ever some leucosyenites host disseminatedmineralization in aureoles ca.10–20 cmwide around veinlets. The disseminated mineralization is alsocharacterized by elevated Cu contents up to 1626 ppm (Table 3).

6.1.2. Mineralized zone 2Mineralized zone 2 is situated on a small ridge NE of the Chon-Ashu

mountain pass (Fig. 4) in a deformed gabbro of the Tashtambektor For-mation. The mineralization forms a stockwork of small veins ca. 10 cmthick and 1–2 cm thick quartz-carbonate veinlets crosscutting the folia-tion in gabbrowhich strikes NE along the Nikolaev Line (Fig. 5e). All the

Fig. 8. Rocks of theChon-Ashu alkaline complex on the total alkali vs. silica (TAS) diagram (a). Fields afterMiddlemost (1994): 1— foidolite, 2— foid gabbro, 3— peridotitic gabbro, 4— foidmonzodiorite, 5 — monzogabbro, 6 — gabbro, 7 — foid monzosyenite, 8 — monzodiorite, 9 — gabbroic diorite, 10 — monzonite, 11 — diorite, 12— foid syenite, 13— syenite and quartzmonzonite, 14 — granodiorite, 15 — granite. (b) K2O versus Na2O diagram showing that Chon-Ashu alkaline rocks mostly plot into the fields of Na-, and K-series (McBirney, 1993),(c) Felsic rocks of Chon-Ashu complex (SiO2 N 50%) on classification diagramof Frost and Frost (2008): FeOtot / (FeOtot+MgO)vs. SiO2,wt.%., (d)—Chon-Ashu rocks onAI vs. FSSI diagramof Frost and Frost (2008) where AI=Al− (Na+K)mol. and FSSI is the feldspathoid silica-saturation index as normative Q− [Lc+ 2(Ne+Kp)] / 100where Q— quartz, Ne— nepheline,Kp— kaliophilite, Lc — leucite. Major oxides plotted on these diagrams are recalculated on a volatile-free basis.

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primary oreminerals (chalcopyrite and pyrite)were apparently leachedout and the copper is now located in secondaryminerals. Three samplesfrommineralized zone 2 have been analyzed. The copper content in the3 samples varies from 213 to 1516 ppm but concentrations of othermetals are insignificant (Table 3).

6.1.3. Mineralized zone 3Mineralized zone 3 was sampled from talus debris accumulated on

the southern side of the small ridge NE of the Chon-Ashu pass. Themin-eralized samples comprise deformed gabbroic rocks with quartz-carbonate veinlets a few cm thick containing chalcopyrite, pyrite andsubordinate secondary Cu minerals. The mineralized quartz-carbonatestockwork veinlets crosscut the foliation in gabbro (Fig. 5e). The Cu con-tents of two samples from this zone that were analyzed are 1005 and1211 ppm. Anomalous Au concentrations (up to 0.03 ppm) werefound only in these unoxidised samples (Table 3). We believe that theleached stockwork identified in mineralized zone 2 is the surficial partof this mineralized system which, in talus debris, is almost unaltered.The abundance of themineralized debris in talus shows thatmineraliza-tion is probably more widespread and that other mineralized zonescould be exposed at surface on top of the ridge.

6.1.4. Mineralized zone 4Mineralized zone 4 comprises two zones of alteration 50 and 20 m

thick found in Montoitor river canyon. The zones cut through the undi-vided Silurian–Devonian granitoids and strike in a N–S direction acrossthe regional structures. The rocks in these zones are completely leachedand without any traces of mineralization. The material which was

sampled consisted of quartz, hydrated phyllosilicates and clay minerals.It is believed that this oxidation zone transforms at depth into a zone ofsecondary enrichment and further into unoxidized copper mineraliza-tion. Sample 1100, with a Cu content of 239 ppm (Table 3), collectedat the mouth of Montoitor river may be representative of the minerali-zation underlying this leached zone.

6.2. Structural and lithological constraints on the age and genesis ofmineralization

The mineralization is associated with 3 rock types: ankeriteleucosyenite dikes of the alkaline Chon-Ashu complex, deformedCambrian gabbro of Tashtambektor Formation, and undivided Silurian–Devonian granites.Most of the rock units in the area of study occur as tec-tonic slices separated by faults (Fig. 4). Consequently, mineralizationzones cannot be traced across tectonic boundaries, hampering correla-tions. Besides, the polyphase structural evolution of the area suggeststhat chalcopyrite mineralization could form in several geneticallyunrelated stages.

Several lines of evidence lead to an assumption that the earlieststage of mineralization probably associated with the ankeriteleucosyenite dikes of the Cryogenian Chon-Ashu complex. The dikesstrike in theN–S direction and preserve a structural pattern that formedprior to the SW–NE oriented tectonic slicing which occurred during theLate Carboniferous (Hercynian) collision (Biske, 1995). The dikes cross-cut the metamorphic Chon-Ashu Formation and are not observed else-where outside the metamorphic rocks (Fig. 4). The dikes emplaced atshallow crustal level and demonstrated evolved character containing

Fig. 9.Variation diagrams for non-mineralized rocks of the alkaline Chon-Ashu complex.Major and trace elements are plotted versusMg#= 100(MgO / (MgO+FeOtot)).Major oxides arerecalculated on a volatile-free basis. See Fig. 8 for legend.

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several percents of carbonate in both barren and mineralized varieties.The amount of carbonate is growing within 10–20 cm zones aroundmineralized carbonate veinlets, probably indicating influx of late

magmatic CO2-rich fluids emanating from the alkaline complex. Basedon these features, including disseminated mineralization in 10–20 cmaureoles around richly mineralized veinlets and diverse ore mineral

Fig. 10. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element abundances (b) of the rocks of Chon-Ashu complex. See Fig. 8 for legend.Normalizing values after Sun and McDonough (1989).

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composition lacking in other mineralized zones, we tentatively relatethe first stage of mineralization to the late magmatic processes in theCryogenian Chon-Ashu complex.

Two zones of leaching in undivided Silurian–Devonian granites alsostrike N–S across regional structural pattern. However, age and signifi-cance of these zones remain unclear because a geochronological age ofthe granite is yet to be determined. Assuming a Devonian age of the gran-ite onemay conclude that the leaching zones formed in Devonian or later.The growth of zircon rims in Early Devonian, established in this study, hasan important regional metallogenic significance which is discussedbelow.However in the Chon-Ashu area it is unlikely that aDevonianmag-matic pulse formed all four mineralized zones prior to intensive sinistralmotions which affected the area during Late Carboniferous–Permian.

The foliation in Cambrian gabbro of the Tashtambektor complex isaligned to the NE–SW striking splays of the Nikolaev Line and probablyformed as a result of post-Carboniferous sinistralmotions along the Line.The chalcopyrite sits in the irregular quartz-carbonate stockwork veinswhich clearly crosscut the foliation in gabbro (Fig. 5e). The associationof chalcopyrite mineralization with deformed Cambrian gabbro is not

Table 3Concentrations of ore metals in mineralized samples.

Sample number Coordinates Rock-type

Mineralized zone 16100 N 42° 22.202′

E 79° 03.848′Mineralized carbonate veinlets in leucosyen

6101 N 42° 22.202′E 79° 03.848′

Leucosyenite dike with carbonate veinlets

6103 N 42° 22.202′E 79° 03.848′

Mineralized leucosyenite dike

Mineralized zone 23200 N 42° 23.436′

E 79° 03.867′Deformed gabbro

3600 N 42° 23.436′E 79° 03.868′

Deformed gabbro

3400 N 42° 23.436′E 79° 03.869′

Deformed gabbro

Mineralized zone 34100 N 42° 23.163′

E 79° 03.848′Deformed gabbro from talus

4200 N 42° 23.137′E 79° 03.722′

Deformed gabbro from talus

Mineralized zone 41100 N 42° 20.349′

E 79° 03.578′Mineralized granite

7400 N 42° 20.352′E 79° 03.581′

Altered granite

unique. Shear-related auriferous metasomatites at Morennoye occur-rence (Fig. 2) are also hosted by andesites and basalts of Cambrian–Ordovician age (Nikonorov et al., 2007). Finally, the super-large Kumtorgold deposit, situated immediately south of the Nikolaev Line (Fig. 2),has a similar geological structure. At Kumtor the irregular auriferousstockwork veins, dated at 284Ma (Ar–Ar,mica), crosscut strongly foliat-ed in the NE direction Neoproterozoic black shales (Mao et al., 2004). Byanalogy with Kumtor and other similar deposits we believe thatstockwork type chalcopyrite mineralization was superimposed onfoliated Cambrian gabbro of the Tashtambektor complex in the LatePaleozoic.

7. Discussion

7.1. Neoproterozoic and Cambrian events: implications of the newgeochronological and geochemical data

Neoproterozoic bimodal volcanics and granitoid intrusions with ages880–700 Ma are well documented in many terranes of the southern

Cuppm

Pbppm

Znppm

Agppm

Auppm

ite dike 16,184 172 1856 37 0.005

1034 18 181 3 b0.005

1626 24 185 1 b0.005

213 b3.5 86 b1.0 b0.005

1516 9 80 b1.0 b0.005

697 b3.5 17 b1.0 0.005

1211 b3.5 43 b1.0 0.022

1005 b3.5 58 b1.0 0.028

239 15 21 b1.0 b0.005

98 b3.5 62 b1.0 b0.005

Fig. 11.Mineralized zone 1: BSE images ofmineralized leucosyenite dike: a— development of chalcopyrite, pyrite and carbonateminerals in feldspathicmatrix, b— ilmenite, chalcopyriteand galena in a larger carbonate veinlet.Abbreviations of mineral names after Whitney and Evans (2010).

188 D. Konopelko et al. / Ore Geology Reviews 61 (2014) 175–191

Central AsianOrogenic Belt (CAOB) fromKazakhstan to Tarim (Degtyarevet al., 2011; Guo et al., 2005; Kheraskova et al., 2003; Kiselev, 1999;Kröner et al., 2009, 2011; Shu et al., 2011; Xu et al., 2005). They are usuallyexplained by rifting related to break-up of Rodinia however the discoveryof 730 Ma old blueschists in northern Tarim (Nakajima et al., 1990; Zhuet al., 2011b) may indicate a more complex geological history.

The next major event that affected the terranes of Kazakhstan andTien Shan was the opening of the Terskey Ocean that took place duringthe Early Paleozoic (Caledonian) orogenic cycle. This event is recordedin a number of published ages of ophiolites and associated intrusions(e.g. Kiselev, 1999; Kiselev and Maksumova, 2001; Lomize et al., 1997;Mikolaichuk et al., 1997) and by the ages of UHP metamorphism (e.g.Dobretsov and Shatsky, 2004; Konopelko et al., 2012). Thoughophiolites and subduction-related intrusions formed continuouslyuntil Middle–Late Ordovician, the earliest ages in the Tien Shan andKazakhstan usually range from 550 Ma to 500 Ma corresponding toEarly–Middle Cambrian (Kiselev, 1999; Kiselev and Maksumova,2001; Konopelko et al., 2008; Lomize et al., 1997; Mikolaichuk et al.,1997). The age of 522 Ma obtained in this study for the gabbro fromthe Tashtambektor Formation fits well in this time interval.

However ages from 700 Ma to 550 Ma are relatively rare and havebeen obtained from granitoid intrusions of unclear geodynamic setting.Previous U–Pb multigrain zircon dating (Kiselev, 1999) has shownthat only 3 intrusions in the Northern Tien Shan and southernKazakhstan yielded ages in the time slice from 700 Ma to 550 Ma: apost-kinematic granite in the Aktyuz metamorphic complex (692 ±15Ma), a monzodiorite in the basement of the Early Paleozoic magmat-ic arc (629±11Ma) and a post-kinematic quartz porphyry in the Talas–Karatau terrane (685 ± 10 Ma). Similar ages of 730–690 Ma have beenestablished for composite intrusions in the Moyunkum and Issyk-Kulblocks of the Northern Tien Shan and Kazakhstan (Kiselev, 1999).

The new SHRIMP ages of 678 Ma and 656 Ma corresponding to theend of the Cryogenian Period of the Neoproterozoic (850–635 Ma,Gradstein et al., 2012) bracket the formation of the gneiss protolith,metamorphism of amphibolite facies, uplift and the emplacement ofthe alkaline intrusion at a relatively shallow crustal level and providenew information about the geochronologically poorly studied periodbetween 700 Ma and 550 Ma. Amphibolite facies metamorphismfollowed by the emplacement of an alkaline complex within a relativelyshort time span 678–656 Ma places certain constraints on thegeodynamic environment of these events. Alkaline rocks usually formin an extensional setting and their enriched geochemistry points to anorigin from lithospheric mantle. Though direct equivalents of theserocks are not known in the Kyrgyz Northern and Middle Tien Shan thepieces missing from this jigsaw-puzzle can be found in northern Tarimand southern Kazakhstan. Xenoliths of mafic granulites from theTuoyun basalts in the NW Tarim with youngest ages of around

690 Ma (which is only slightly older than the 678 Ma age inferred formetamorphism in Chon-Ashu) have been interpreted as a result ofmafic magmatic underplating corresponding with a major period ofrift-related volcanism (Zheng et al., 2006). The mafic underplatingcould have been responsible for the formation of zones of amphibolitefacies metamorphism in themiddle crust which were rapidly exhumedinto the upper crust between 678Ma and 656Ma prior to emplacementof the alkaline Chon-Ashu complex. This extension and involvement oflithospheric mantle in the production of enriched alkaline rocks canalso be traced on a regional scale as indicated by the age of 635 ±6 Ma obtained for a spessartite dyke from northern Tarim (Zhu et al.,2011a) and an age of 673 ± 2 Ma reported by Degtyarev et al. (2011)for alkaline syenites forming the Karsakpay ring intrusion and thesouthern part of the Ulutau massif in southern Kazakhstan.

The new dates determined in our investigation thus define aCryogenian metamorphic event and a period of regional extension lead-ing to emplacement of enriched alkaline rocks. Both events can beinterpreted as a result of mafic magmatic underplating (Zheng et al.,2006) related to the break-up of Rodinia.We accept that this is a tentativescenario which may change when more data for Neoproterozoic rocks ineastern Kyrgyzstan are available. It should be noted that alkaline rocksand carbonatites with similar ages 730 Ma–650 Ma are known in thesouthernmargin of the Siberian cratonwhere they are also thought to in-dicate the break-up of Rodinia (Yarmolyuk et al., 2006) and in the west-ern margin of Siberia in the Enisey ridge where they are interpreted tohave formed in a back-arc extension zone (Vernikovsky et al., 2008).

7.2. Devonian magmatism

Devonian granitoids in the Eastern Kyrgyz Tien Shan were oftenoverlooked during mapping because they are similar in compositionto post-collisional Silurian and/or Permian granites. For examplethe Devonian age of the intrusions hosting the Aktyuz REE depositin the Northern Tien Shan, previously considered to be Permian(Djenchuraeva et al., 2008), was recently dated at 414 ± 7 Ma(Seltmann et al., 2011). Using various previously published agesMikolaichuk and Buchroithner (2009) showed that Early–Middle Devo-nian granites are much more numerous in the easternmost Kyrgyzstanand adjacent Kazakhstan territory (Fig. 3) than is shown on regionalmaps (e.g. Tursungaziev and Petrov, 2008). Biske et al. (2013) suggestedthat Early Devonian granites formed in a back-ark or intraplate settingrelative to the active margin of southern Kazakhstan (Balkhash–Yilibelt). They also showed that a belt of Early Devonian intrusions can betraced farther east to the Chinese Tien Shan. In the present study, Devo-nian overgrowth rims have been identified in 7 zircon grains fromgneissic granite and alkaline rocks from the Chon-Ashu complex. All 7analyses yield a 206Pb/238U concordia age of 400 ± 8 Ma. This age fits

189D. Konopelko et al. / Ore Geology Reviews 61 (2014) 175–191

well within the 414 Ma–371 Ma time span reported for Early–Middle Devonian granites in Kyrgyzstan (Fig. 3) (Apayarov, 2010;Mikolaichuk and Buchroithner, 2009; Seltmann et al., 2011). The newzircon growth at 400 Ma may indicate a magmato-metamorphic eventproducing metamorphism at depth and granite intrusions at highercrustal level.

7.3. The newly discovered chalcopyrite mineralization and implications forthe metallogenic potential of eastern Kyrgyzstan

Apart from deposits in sedimentary formations, the mineralizationin the Northern and Middle Tien Shan east of the Kumtor Au depositconsists of small stockwork type Mo–W deposits and shear-relatedAu ± Cu, base metals occurrences (Bakirov et al., 2001; Nikonorovet al., 2007). The latter includes the Morennoye Au occurrence situatednorth of the Nikolaev Line ca. 20 km east of the Chon-Ashu pass (Fig. 2)(Nikonorov et al., 2007). Both types are normally associated with smallhigh level stocks of the Early Permian granitoids indicating a well de-fined metallogenic potential of late Hercynian stage (Nikonorov et al.,2007). The Kumtor gold deposit (Fig. 2) is an eye-catching exampleof an Early Permian mineralized stockwork superimposed on stronglyfoliated Neoproterozoic shales in a tectonic slice (Mao et al., 2004).By analogy with Kumtor, based on structural relationships betweenthe NE–SW striking foliation in the Cambrian gabbro of theTashtambektor complex and superimposed mineralized stockwork,we tentatively propose an Early Permian age for the mineralizationin gabbro, supporting significance of the metallogenic potential ofHercynian stage.

Despite the obvious importance of Hercynian ore deposits in easternKyrgyzstan, prior to the present study, there was little to indicate themetallogenic potential of pre-Hercynianmagmatic complexes in this re-gion. The metallogenic potential of the Devonian magmatic rocks wasrecognized recently when an age of 414 Ma was obtained for the hostintrusion of the Aktyuz REE deposit, and a groupof adjacent ore depositsin central Kyrgyzstanwas also proved or assumed to be of Devonian age(Seltmannet al., 2011). The Early–MiddleDevonian granites formed in aback-arc or intraplate setting relative to the active margin of southernKazakhstan (Biske et al., 2013). Being emplaced at a relatively highcrustal level, the Devonian intrusions host a variety of mineralizationincluding base metals and Au–Bi skarns (Mironovskoye, Boordu),Au-sulfide stockworks in volcanic rocks (Taldybulak Levoberezhny)and REE and Zr in evolved granites (Aktyuz) (Djenchuraeva et al.,2008). The Early–Middle Devonian magmatic front spreading fromKazakhstan to eastern Kyrgyzstan may be seen on the map ofMikolaichuk and Buchroithner (2009) (Fig. 3). The new zircon growthat 400 Ma, established in this study, indicates that a thermal event pro-ducing metamorphism at depth and granite emplacement at highercrustal level affected the easternmost Kyrgyzstan as well. This ca400 Ma event could initiate circulation of fluid cells making Devonianintrusions in eastern Kyrgyzstan potential targets for discovery of avariety of ore deposits.

Finally, we investigated the 656 Ma shallow level alkaline Chon-Ashu complex including highly differentiated rock types whichmay in-clude carbonatites presently not exposed at the surface. Our findingsprovide evidence that preservation of shallow level Precambrian rocksin the extremely complex Tien Shan orogen was possible. If our tenta-tive suggestion that the earliest stage of chalcopyrite mineralization as-sociated with evolved dikes of the Cryogenian Chon-Ashu complex iscorrect, then ore occurrences may be looked for in voluminous EarlyPaleozoic batholiths which may host preserved skarns and porphyrysystems similar to the Ordovician Au–Cu porphyry deposit atTaldybulak in western Tien Shan (Yakubchuk et al., 2010). Thus ourdata show that in eastern Kyrgyzstan there may be new Devonian andpre-Devonian targets and a reassessment of the metallogenic potentialof this region is timely.

8. Conclusions

In the easternmost Kyrgyzstan north of the Nikolaev Line the unde-formed shallow level alkaline rocks of the Cryogenian Chon-Ashu com-plex cut across and fenitize slightly older metamorphic rocks of theChon-Ashu Formation. The alkaline rocks range from olivine gabbro tonepheline and cancrinite syenites and ankerite leucosyenite dikes. Thedifferentiated rock assemblagemay be explained by fractional crystalli-zation of high-silica mineral phases which drives nepheline-normativemelts away from the silica saturation boundary. The alkaline rocks areenriched in LILE and HFSE. Some of the leucosyenite dikes contain sev-eral percent of carbonate minerals. This suggests that highly differenti-ated melts including carbonatites may be present at depth.

The magmatic protolith of a gneiss from the Chon-Ashu metamor-phic formation produced an age of 678 Ma. Four samples of the Chon-Ashu alkaline rocks yielded an age of 656 Ma. An adjacent deformedgabbro of the Tashtambektor Formation gave an age of 522 Ma. Finally7 zircon grains recovered from gneiss and alkaline rocks had brightovergrown rims which yielded a cumulative age of 400 Ma. The meta-morphic event at 678 Ma, shortly followed by uplift and emplacementof a high-level alkaline complex at 656 Ma, places certain constraintson the geodynamic setting of these events. Alkaline rocks usually formin an extensional setting and originate from lithospheric mantle. Xeno-liths of mafic granulites from the NW Tarimwith youngest ages around690 Ma have been interpreted to be a result of mafic underplating(Zheng et al., 2006). The mafic underplating may have created zonesof amphibolite facies metamorphism in the middle–upper crust priorto emplacement of the Chon-Ashu complex. These events are tentative-ly interpreted as a result of mafic underplating and subsequent riftingrelated to the break-up of Rodinia.

In the course of the field work a rich chalcopyrite mineralization hasbeen discovered in the ankerite leucosyenite dikes of the alkaline Chon-Ashu complex and also has been found in the adjacent deformed gabbroof the CambrianTashtambektor Formation and also in theundividedDe-vonian–Silurian granites. Stockwork type mineralization predominatesbut disseminated mineralization is also present. The Cu content in grabsamples reaches 16184 ppm and is associated with elevated concentra-tions of Pb, Zn and Ag. The polyphase structural evolution of the areasuggests that mineralization could form in several genetically unrelatedstages. Based on structural, lithological and mineralogical evidence wetentatively relate the earliest stage of chalcopyrite mineralization in an-kerite leucosyenite dikes to an influx of late magmatic CO2-rich fluidsemanating from the Cryogenian alkaline complex. The Early Devonianthermal event registered by growth of new zircon at 400Ma has impor-tantmetallogenic implications on a regional scale. However the origin oftwo zones of leaching in theundivided Silurian–Devonian granites is un-clear because an age of the granites and composition of unoxidized oresare unknown. Finally the Cambrian gabbro of the Tashtambektor Forma-tion is strongly foliated along the splays of the Nikolaev Line indicatingthe Hercynian origin of the fabric. Superimposedmineralized stockworkpostdates the foliation suggesting late-Hercynian age of mineralization.

The new findings enable the metallogenic potential of the EasternKyrgyz Tien Shan to be reassessed. Previously pre-Hercynian intrusiveswere not considered to be prospective targets. Presence of unerodedhigh-level mineralized Cryogenian alkaline intrusions point to the pre-viously underestimated metallogenic potential of Precambrian andEarly Paleozoic granitoids whichmay host uneroded porphyry systems,skarns or shear-related mineralization. Finally, the Devonian thermalevent which formed a number of deposits in central Kyrgyzstan andKazakhstan could also create interesting exploration targets in easternKyrgyzstan.

Acknowledgments

This study was supported by a research grant from Saint PetersburgState University (DK and SVP). Dr. Chris Halls (NHMLondon) suggested

190 D. Konopelko et al. / Ore Geology Reviews 61 (2014) 175–191

improvements to the English text of the paper. We appreciate theconstructive reviews of Jacques Charvet and Koen de Jong that helpedto improve the first version of the manuscript. This is a contribution toIGCP Project 592 funded by IUGS and UNESCO.

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