ARTICLE

Tectonic configuration of the Apuseni–Banat––Timok–Srednogorie belt, Balkans-South Carpathians,constrained by high precision RE–OS molybdenite ages

Aaron Zimmerman & Holly J. Stein &

Judith L. Hannah & Dejan Koželj & Kamen Bogdanov &

Tudor Berza

Received: 24 February 2007 /Accepted: 8 June 2007 / Published online: 11 August 2007# Springer-Verlag 2007

Abstract The Apuseni–Banat–Timok–Srednogorie mag-matic–metallogenic belt (ABTS belt), forms a substantialmetallogenic province in the Balkan-South Carpathiansystem in southeastern Europe. The belt hosts porphyry,skarn, and epithermal deposits mined since pre-Romantimes. Generally, the deposits, prospects, and occurrenceswithin the belt are linked to magmatic centers of calc-alkaline affinity. Fifty-one rhenium-osmium (Re–Os) agesand Re concentration data for molybdenites define system-atic geochronologic trends and constrain the geochemical-metallogenic evolution of the belt in space and time. From

these data and additional existing geologic-geochemicaldata, a general tectonic history for the belt is proposed.Mineralization ages in Apuseni-Banat, Timok, and Panagyur-ishte (the central district of the larger E–WSrednogorie Zone)range from 72–83, 81–88, and 87–92 Ma, respectively, andclearly document increasing age from the northwesterndistricts to the southeastern districts. Further, Re–Os agessuggest rapidly migrating pulses of Late Cretaceous mag-matic–hydrothermal activity with construction of deposits in~1 m.y., districts in ~10 m.y., and the entire 1,500 km belt in~20 m.y. Ages in both Timok and Panagyurishte showsystematic younging, while deposit ages in Banat andApuseni are less systematic reflecting a restricted evolutionof the tectonic system. Systematic differences are alsoobserved for molybdenite Re concentrations on the belt scale.Re concentrations generally range from hundreds tothousands of parts per million, typical of subduction-relatedCu–Au–Mo–(PGE) porphyry systems associated with thegeneration of juvenile crust. The geochronologic andgeochemical trends are compatible with proposed steepeningof subducting oceanic slab and relaxation of upper continen-tal plate compression. Resulting influx of sub-continentalmantle lithosphere (SCML) and asthenosphere provide afertile metal source and heat, while the subducting slabcontributes connate and mineral dehydration fluids, whichfacilitate partial melting and metal leaching of SCML andasthenosphere. Cu–Au–Mo–(PGE) porphyry deposits maydevelop where melts are trapped at shallow crustal levels,often with associated volcanism and epithermal-style depos-its (South Banat, Timok, and Panagyurishte). Mo–Fe–Pb–Znskarn deposits may develop where felsic melts are trappedadjacent to Mesozoic limestones at moderate crustal levels(North Banat and Apuseni). Systematic spatial variations indeposit style, commodity enrichment, Re–Os ages, and Re

Miner Deposita (2008) 43:1–21DOI 10.1007/s00126-007-0149-z

Editorial handling: B. Lehmann

A. Zimmerman (*) :H. J. Stein : J. L. HannahAIRIE Program, Department of Geosciences,Colorado State University,Fort Collins, CO 80523-1482, USAe-mail: aaron.zimmerman@colostate.edu

H. J. Stein : J. L. HannahNorges geologiske undersøkelse (NGU),Leiv Eirikssons vei 39,7491 Trondheim, Norway

D. KoželjSouth Danube Metals,Milentija Popovica 9, Sava Centar,11 070 Novi Beograd, Serbia

K. BogdanovDepartment of Mineralogy, Petrology and Economic Geology,Sofia University,15 Tsar Osvoboditel. Bd.,1504 Sofia, Bulgaria

T. BerzaGeological Institute of Romania,Caransebes Str. 1,78344 Bucharest, Romania

concentrations support specific tectonic processes that led toore formation. In a post-collisional setting, subduction ofVardar oceanic crust may have stalled, causing slab steepen-ing and rollback. The slab rollback relaxes compression,facilitating and enhancing orogenic collapse of previouslythickened Balkan-South Carpathian crust. The progression ofcoupled rollback-orogenic collapse is evidenced by the widthof Late Cretaceous extensional basins and northward young-ing of Re–Os ages, from Panagyurishte (~60 km; 92–87 Ma)to Timok (~20 km; 88–81Ma) to Apuseni-Banat (~5 km; 83–72 Ma). Generation of a well-endowed mineral belt, such asthe ABTS, requires a temporally and spatially restrictedwindow of magmatic–hydrothermal activity. This window isquickly opened as upper plate compression relaxes, therebyinducing melt generation and ingress of melt to higher crustallevels. The window is just as quickly closed as upper platecompression is reinstated. The transient tectonic stateresponsible for economic mineralization in the ABTS beltmay be a paleo-analogue to transient intervals in the present

subduction tectonics of SE Asia where much mineral wealthhas been created in the last few million years.

Keywords Re–Os, molybdenite . Copper porphyry .

Romania . Serbia . Bulgaria . Slab rollback .

Orogenic collapse

Introduction

The Apuseni–Banat–Timok–Srednogorie (ABTS) belt is acomponent of the global Tethyan Eurasian MetallogenicBelt (TEMB) spanning the eastern hemisphere from theWestern Alps to Southeast Asia (Janković 1997). All stagesof the Wilson Cycle, from continental rifting to subductionto continent–continent collision, along with mineralizationstyles typical of the various settings, are present in theTEMB. The ABTS belt (Fig. 1) forms a relatively con-tinuous chain running N–S from the Apuseni Mountains

Fig. 1 The Apuseni–Banat–Timok–Panagyurishte districts anddistribution of deposits in the Balkan–South Carpathian region,southeastern Europe. Inset places the ABTS Belt within the larger

Alps–Balkan–Carpathian–Dinaride (ABCD) orogenic–metallogenicbelt (Heinrich and Neubauer 2002). Modified from Ciobanu et al.(2002)

2 Miner Deposita (2008) 43:1–21

(Romania) through Banat (Romania) to Timok (Serbia) andthen E–W through the Srednogorie Zone (see inset Fig. 1for location) of which Panagyurishte is the central district(Bulgaria). The ABTS belt hosts Cu–Au–Mo porphyry(some with notable PGE abundances), Mo–Fe–Pb–Znskarn, and Cu–Au–Ag epithermal deposits of Late Creta-ceous age. In Apuseni, Neogene events and ore deposits aresuperimposed on Cretaceous deposits (e.g. Neubauer et al.2005). Some deposits have a mining history extending backto pre-Roman times. Elatsite, Chelopech, Majdanpek, Bor,Moldova Nouă, and Băiţa Bihor are presently in produc-tion. Current tectonic models broadly propose metallo-genesis linked to rifting (Popov 1987; Popov et al. 2003),subduction (Hsü et al. 1977; Vlad 1997; Ciobanu et al.2002), or (post)collisional processes (Berza et al. 1998;Nicolescu et al. 1999; von Quadt et al. 2005). Generally,these models lack the necessary chronologic precision anddetail to link mineralization to magmatic-tectonic processes,although limited high precision work was carried out byvon Quadt et al. (2005). Use of thermally susceptiblechronometers that record cooling rather than crystallizationhas resulted in ambiguous models for regional metallo-genesis (Lilov and Chipchakova 1999). Recent high-precision U–Pb and 40Ar/39Ar data have improved andrefined tectonic models linked to resolvable magmaticactivity (Clark and Ullrich 2004; Handler et al. 2004; Lipset al. 2004; von Quadt et al. 2002a, b; 2004, 2005). In thispaper, we introduce highly precise and accurate Re–Os agesfor mineralization to constrain ore deposition and thetectonic model.

Applying the Re–Os system to molybdenite that crys-tallized during main-stage mineralization in skarn andporphyry environments in the ABTS belt provides bothaccurate and highly precise ages (less than ±0.40% 2σexternal error) and Re concentration data. Concentrationdata, along with deposit commodities and melt geochem-istry, aid in constraining the source for ore metals (Stein etal. 2001, 2004; Stein 2006). Trends in both the Re–Os agesand Re concentration data reflect components of thetectonic environment and its evolution. Re–Os data providefirsthand information on the timing of ore deposition at thedeposit, district, and belt scales. These data enablecomparisons between deposits and between districts,thereby tracking the evolution of metallogenesis and relatedtectonic processes. The large Re–Os dataset presented inthis study (51 ages) defines the timing of mineralizationalong a 1,000+ km mineral belt. The ability to compareages for major ore deposits and mineral occurrences using asingle analytical method provides continuity and control inaddressing the geochemistry and geochronology of anevolving tectonic system.

Multiple geochronometers that vary in precision andrecord different geologic phenomena are used to unravel

the full tectonic history. U–Pb chronology pins magmaticand volcanic events, whereas argon-based chronology isuseful for defining cooling histories. K–Ar and Rb–Sr agesare easily disturbed in the ore-forming environment wherepotassic alteration is present and may provide bothimprecise and inaccurate age information. The poorprecision commonly associated with K–Ar and Rb–Sr agesis an indication of isotopic disturbance. Likewise, highprecision geochemical data tracing the genesis and evolu-tion of melts and fluids must also be utilized. Nd, Hf, Sr,and REE data provide important tracer information forconstruction of a tectonic model. Re concentration dataparallel deposit commodities, reflecting the genetic historyof metals, melts, and fluids. Geophysical data are helpful inrestoring the entire region to its Late Cretaceous paleoge-ographic position. These collective data are used to proposea tectonic–metallogenic history for the region. OverprintingCenozoic tectonics greatly reconfigured the region, suchthat Late Cretaceous tectonic models depend on theaccuracy of data used to restore the region to its LateCretaceous configuration.

This large compilation of Re–Os data, with supplemen-tary high precision geochronology and geochemistry,affords the opportunity to speculate on the position andamalgamation of small continental blocks, the closure andsubduction of restricted oceanic basins, and the overallconvergent history of the Tethyan region from the mid-Mesozoic to the K–T boundary.

Geologic setting

The region encompassing the ABTS belt is the site ofmultiple generations of Mesozoic rifting followed bycollisional events in the Early Cretaceous (Austrian Phase),Late Cretaceous (Laramide or Early Alpine Phase), andCenozoic (Alpine Phase). At present, the active tectonicsystem is located to the south at the Hellenic trench, yetlimited seismic activity in Srednogorie occurs today(Shanov et al. 1992). As shown in Fig. 2, the region aboutthe ABTS belt may be defined by seven tectonic units: (1)the Moesian platform; the superimposed (2) Nish-Trojantrough and (3) Balkan-South Carpathian mountains (Dacia-Srednogorie Units); (4) the ABTS belt; (5) the Rhodopianand Serbo-Macedonian massifs (alternatively named theThracian Island by Minkovska et al. 2002, or Drama blockby Ricou et al. 1998); (6) the Vardar ocean; and (7) theDinaride and Hellenide mountains (alternatively named thePelagonian block; Ricou et al. 1998). A classic accordion-like rift margin is illustrated by a schematic cross-section ofthe tectonic units at their maximum extension in the mid-Mesozoic (Fig. 2). The middle Mesozoic tectonic setting,and assumptions therein, sets the stage for Late Mesozoic

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convergent tectonics that ultimately resulted in the richendowment of metals in the ABTS belt. At present day, thetectonic units form an ‘L’ about the Moesian platform as aresult of Cenozoic translation and rotation of the westernhalf of the ABTS belt in response to late Alpine closure ofthe Tethys (Pătraşcu et al. 1990, 1992, 1993, 1994).

Tectonic units

The tectonic units described below are integral to trackingthe evolution of the ABTS belt from the Mesozoic toCenozoic. Much like Tethyan tectonics in general, thedescriptions, locations, and interpretations of each unit arestill debated as is the tectonic significance of each. Tocomplicate matters, many fundamental field observationsand geochemical/geochronological data are published inlocal journals by local geologists in their native language.Their observations and interpretations have only recentlyreceived international recognition, criticism, and affirmation.Descriptions herein reflect generally accepted thinking fortectonic units, but the reader is encouraged to consult theprovided references for more detailed information anddiscussion of controversial aspects of tectonic interpretations.

Cardinal directions stated herein refer to present-dayorientations, as Cenozoic translation and rotation oftectonic units have altered original trends. Comparing thepresent-day orientation (Fig. 1) with the pre-collisional andpre-metallogenic orientation (roughly E–W; see Fig. 2 inNeubauer 2002) shows the western half of the ABTS belthas been ‘bent’ around the Moesian platform.

The Moesian platform, composed of thick Mesozoicsedimentary sequences deposited on crystalline basement,is a crustal unit sutured to the European platform before theLate Cretaceous. The relative and absolute positioning ofMoesia relative to Europe and other tectonic units is basedon the paleomagnetic compilation of Neugebauer et al.(2001). In this model, the platform was fixed to Europe

from 290.0 to 175.0 Ma, rifted away, and was again fixedfrom 141.2 Ma to the present. The Dobrogea region to thenorth may represent the paleo-rift and subsequent suturezone between Moesia and the European platform (Burchfiel1976). Neubauer (2002) posits a westward translation of theMoesian Platform in the Cretaceous. Along the southernand western (and northern) margin of the Moesian platformare the Balkan and South Carpathian mountains, respec-tively. The Moesian platform acts as a backstop for allnorthward migrating lithotectonic units.

The Nish-Trojan trough is an elongate E–W to NW–SE-trending basin initially rifted from the European platform inthe Mesozoic (Hsü et al. 1977; Minkovska et al. 2002). Thegeometry of the trough is poorly understood and, given thatit is fully inverted and strongly overprinted by Cretaceousand Eocene tectonics, support for the basin is sparse andopen to interpretation. Nonetheless, the trough may bedivided into two components: (1) a southern zone, boundedby the Drama block (see below), that underwent terrigenoussedimentation and (2) a northern zone, bounded by theMoesian platform, that underwent carbonate deposition(Minkovska et al. 2002). Sedimentation reached a maxi-mum in the Jurassic and waned before the Cretaceous. Theextensional basin never reached sufficient width to initiate theformation of oceanic crust. Austrian (~110 Ma) compressionresulted in the cessation of sedimentation (Minkovska et al.2002) and basin inversion. Basin inversion is a characteristicprocess in which middle Mesozoic basins created during theopening of the Tethys ocean are subsequently consumed(inverted) during closure of the Tethys (e.g. Cloetingh andVan Wees 2005; Nielsen et al. 2005; Wang et al. 2005).Continued compression led to mountain building (e.g.Balkans-South Carpathians).

The Balkan-South Carpathian mountains form a rela-tively continuous orogenic chain superimposed on theformer Nish-Trojan trough, between the Moesian platformand the nascent ABTS belt. Generally, the mountains

Fig. 2 Schematic cross section,roughly N–S, through theTethyan system at its mostextended state in the mid-Mesozoic. Subsequent closureinduces subduction, basin inver-sion, orogeny, and metallogeny.Conceptual synthesis of crosssections by Boccaletti et al.(1974a, b), Aiello et al. (1977),Hsü et al. (1977), and Ricouet al. (1998)

4 Miner Deposita (2008) 43:1–21

consist of nappe stacks emplaced in the Early to MiddleCretaceous (Burchfiel 1976; Dupont et al. 2002). InRomania and Serbia, the South Carpathians are composedof the Danubian, Getic, and Severin nappe systems. InBulgaria, the nappe structures are less defined, althoughNeubauer (2002) linked the nappes forming the Balkanmountains in Bulgaria to the Danubian nappes of Serbia.Timing of nappe emplacement is determined by uncon-formably overlying sedimentary sequences and intrusions(banatites) that pierce multiple nappes thereby providingminimum thrusting ages. The mountains were integral toLate Cretaceous tectonics as they represent a gravitationalpotential instability that led to orogenic relaxation andextension (in the Late Cretaceous, Berza et al. 1998; or inthe Eocene, Schmid et al. 1998). Additionally, the mountainbuilding thrust faults present zones of weakness that werelater exploited by melts and metal-bearing fluids.

The ABTS belt is a 1,500-km-long, 10- to 70-km-widebelt containing four major ore districts (Apuseni, Banat,Timok, and Panagyurishte). The belt is dominated bymagmatic and volcanic units of medium- to high-K calc-alkaline affinity with alkaline and shoshonitic trends (Berzaet al. 1998; Dupont et al. 2002). Structurally, both Timokand Srednogorie are characterized by rift-like extensionalfeatures with major deposits clearly positioned along deepnormal faults bordering Early Cretaceous nappe systems. Incontrast, Banat and Apuseni are structurally characterizedby major basin-bounding normal faults that either reactivateor parallel Early Cretaceous thrust faults (e.g. Gosau basins;Willingshofer et al. 1999). Mineralization in both Apuseniand Banat is focused along major nappe boundaries.

The Drama block is a highly controversial continentalunit probably belonging to the European Platform. Initialmodels suggest the block was migratory throughout theTethyan system until docking to the Moesian platform in theMiddle to Late Jurassic. Other models place the Dramablock peripheral to the European margin for much of itshistory (Zagorchev 1998). The comment and reply betweenRicou et al. 2000 and Zagorchev 2000 exemplifies thecontroversy surrounding this unit. Similarly, linking theRhodopean and the Serbo-Macedonian massifs as the Dramablock is also controversial. Given the debate concerning theDrama block, the Rhodopian and Serbo-Macedonian massifsare modeled as an elongate continental fragment rifted fromthe European margin in the early to middle Mesozoic andresutured in the late Mesozoic. The massifs are collages ofmixed continental and oceanic material subsequently de-formed during Jurassic to Cretaceous suturing to theMoesian platform to the north and by Vardar subductionand Pelagonian amalgamation from the south.

The Vardar ocean represents a restricted Mesozoic oceanbasin formed during rifting of Pangea. Primary support forthe now consumed Vardar ocean are ophiolites found (1)

between Banat and Apuseni (Mureş ophiolites) and (2) onthe Pelagonian block. Additionally, geophysical studies andseismic activity suggest Vardar crust is still sinking belowthe Srednogorie Zone in Bulgaria (Shanov et al. 1992).Alternatively, Hellenic oceanic crust may be located belowthe region and is responsible for recent seismic activity (deBoorder et al. 1998; Wortel and Spakman 2000). Pelago-nian ophiolites may be linked exclusively to the Vardarocean (Ricou et al. 1986) or may be of mixed lineage fromthe Vardar and Pindos oceans (Fig. 2; Knipper et al. 1986).In either case, the timing of obduction is generallyambiguous due to a lack of unconformable sedimentarysequences or piercing intrusions. Radiometric age dataplace Vardar oceanic crust between 180 and 150 Ma. ThePelagonian ophiolites provide information on the tectonichistory in the region as they track the location and timing ofVardar subduction. The Mureş ophiolites between Banatand Apuseni are an extension of the Vardar ocean entrainedin Jurassic suturing of Apuseni and Banat.

The Pelagonian block represents thinned continentalcrust rifted from the African margin (Stampfli et al. 1991;Golonka 2004). Incomplete rifting created thinned crustalbasins, which became sites for major carbonate platforms inthe middle Mesozoic. A predominantly African origin forthe Pelagonian block is suggested by algal species(Salpingoporella dinarica) found within Mesozoic carbon-ate platform sequences (Carras and Georgala 1998). Boththe Dinaride and Hellenide orogenic systems are thecollisional remnants of the Pelagonian block. Suturing ofthis continental block to the European Platform inducedVardar slab breakoff and initiation of subduction in theHellenic Arc.

Metallogenic districts

Much like the previously described tectonic units, descriptionsof metallogenic districts herein reflect generalized conclusionsdrawn by numerous authors based on detailed studies ofindividual districts. Local ore geology studies published inless accessible literature are summarized in the internationalliterature. Key references are named in each district’sdescription, and should be consulted for additional detail.

The northernmost metallogenic district of the ABTS beltis the Apuseni Mountains of Romania (Fig. 3, upper left).The Apuseni Mountains form a seemingly anomalousorogenic high within the Neogene Pannonian and Transyl-vanian Basins. While appearing disconnected from the mainAlps–Carpathian–Balkan orogenic system, the ApuseniMountains form the westernmost extension of the ABTSbelt before Cenozoic reconfiguration. In addition to aCretaceous metallogenic history, the Apuseni Mountainsalso host Neogene deposits of notable Au–Cu enrichment(i.e. the Gold Quadrilateral or Quadrangle; e.g., Neubauer

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et al. 2005; Manske et al. 2006). The only Late Cretaceousmolybdenite samples from Apuseni come from the BăiţaBihor Mo–Cu skarn and associated Băiţa Plai granodioriteintersected by drill core at 500 to 1,000 m depth. The BăiţaBihor skarn is located along the Blidar thrust fault, whichlikely served as the main conduit allowing ore-formingfluids access to Mesozoic limestone above the Băiţa Plaiintrusion (Vlad, personal communication, 2003). Twoadditional molybdenite samples from the Mureş ophiolitesin the South Apuseni Mountains constrain the timing ofApuseni–Banat suturing to Late Jurassic (Oxfordian).Geochemical (Ciobanu et al. 2002; Dupont et al. 2002),geophysical, and field-based observations also link the two

districts as a single tectonic unit before Late Cretaceousbanatitic magmatic activity.

South of Apuseni is the Banat district, a N–S lineamentof skarn and porphyry deposits (Fig. 3, upper right). Skarndeposits dominate north Banat whereas porphyry depositsdominate the south. The differences in deposit type are afunction of erosional level, proximity to Mesozoic lime-stone units, and crustal control on the level of meltemplacement (Dupont et al. 2002). Mineralization in Banat,like Apuseni, is structurally controlled with depositsconcentrated along boundaries between crustal nappe stacksthat were overthrust in the Early Cretaceous (Csontos andVörös 2004). Thrust faults helped to localize the ascent of

Fig. 3 Upper left: District scale geologic map of the Apuseni district,Apuseni Mountains, Romania. Modified from Burchfiel 1976. UpperRight: District scale geologic map of the Banat district, SouthCarpathians, Romania–Serbia. Deposit localities closely follow nappeboundaries. Modified from Dupont et al. 2002. Lower Right: Districtscale geologic map of the Timok magmatic complex (TMC), South

Carpathians, Serbia. Late Cretaceous polyphase calc-alkaline volcanicand magmatic activity characterize an elongate, rift-like, basinparalleling the Getic Nappe-Danubian Nappe boundary. Modifiedfrom Lips et al. 2004. Lower Left: District scale geologic map of thePanagyurishte district, Srednogorie Zone, Bulgaria. Modified fromStrashimirov et al. 2002. All sampled deposits are marked with a star

6 Miner Deposita (2008) 43:1–21

metal-bearing fluids to higher crustal levels. The thrust faultseparating the Getic and Supragetic nappes was a majorchannel for ore-forming fluids and focused numerous Banatdeposits along its N–S strike. Late Cretaceous volcanic activityis notably absent from Banat. High precision U–Pb geochro-nologic data from Nicolescu et al. (1999) provide magmaticages that agree well with Re–Os mineralization ages.

The Timok district (Timok magmatic complex, TMC,Fig. 3, lower right) in eastern Serbia is transitional betweenthe thrust nappe-dominated Apuseni–Banat districts and theextension-dominated setting of the Panagyurishte district(see below). Focused along the Getic-Danubian nappeboundary, both skarn- and porphyry-style mineralizationare known in Timok. Majdanpek, in northern Timok,contains both skarn and porphyry mineralization (Jankovićet al. 1998), similar to Banat, whereas Bor, in southernTimok, contains porphyry and epithermal mineralization,similar to Panagyurishte (Lips et al. 2004). Three stages(phases) of Late Cretaceous magmatic and volcanic activityare identified within the TMC, spanning ~30 Ma based onK–Ar ages (Janković et al. 2002). Recent higher precision40Ar/39Ar laser probe age determinations constrain thecooling history and temporal evolution of Timok (Lips etal. 2004). Magmatic activity generally progresses from eastto west. Briefly, after the work of Karamata et al. (2002),the main characteristics of each phase are defined. Phase Iin eastern Timok is characterized by stratavolcanos com-posed of hornblende-biotite-pyroxene andesite. Contempo-raneous sub-volcanic to hypabyssal diorite and quartz-dioriteintrusions were emplaced; some of these are associated withporphyry-style mineralization. The Bor volcano and itsassociated porphyry and epithermal Cu–Au–Mo depositformed during this phase, as did the Veliki Krivelj Cu–Moporphyry deposit. Phase II, the most voluminous andwidespread event, is characterized by andesitic basaltsaccompanied by minor andesitic volcanism and subvolcanicmonzonite, granodiorite, and diorite intrusions. The largeValja Strž monzonite complex, and associated DumitruPotok Cu porphyry, formed during Phase II. Phase IIIactivity is restricted to small latite to quartz latite volcanicbodies and quartz-diorite to tonalite dikes. No mineralizationis associated with Phase III. Recent special volumes (localpublications) celebrating Bor’s 100th anniversary provideextensive, detailed information on the Timok magmaticcomplex (Janković et al. 2002, Koželj 2002; Koželj andJelenković 2002).

The Panagyurishte district is the central component ofthe larger encompassing Srednogorie Zone trending E–Wthrough Bulgaria (Fig. 3, lower left). The eastern andwestern districts in the Srednogorie Zone host additionalCu–Pb–Zn–Fe skarn and vein-type mineral deposits(Fig. 1) but rarely contain molybdenite. The Panagyurishtedistrict, in contrast, is composed exclusively of porphyry

and coeval epithermal deposits in a volcanic setting(Strashimirov et al. 2002). Deposits formed in discrete‘ore centers’ localized about volcanic activity. WithinPanagyurishte, the deposits are found in a rift-like structurebetween the Balkan and the Rhodopean lithotectonic units.Falling along a NNW–SSE lineament, the deposits are faultcontrolled, but are not influenced by major E–W EarlyCretaceous thrust faults similar to those found in the otherthree districts. Additionally, support for a component ofstrike-slip motion is present between the bounding shearzones separating north Pangyurishte and the Balkans andsouth Panagyurishte and the Rhodopes (Stoykov et al.2004; Chambefort and Moritz 2006). The Panagyurishtedistrict is the most well-studied of the four districts (seeBogdanov and Strashimirov 2003; Fanger et al. 2001;Strashimirov et al. 2002; Tarkian et al. 2003; von Quadtet al. 2002b, 2005; Zimmerman et al. 2003, 2005).

Analytical methods

Re–Os molybdenite ages in this study were determinedusing established, systematic analytical procedures createdand modified by workers associated with the AIRIEProgram, Colorado State University (Markey et al. 1998,2003; Stein et al. 2001, 2003). Five to one hundredmilligrams of molybdenite mineral separate from entirecrystals or multiple crystals, necessary to ensure intactRe–187Os systematics (Stein et al. 2003; Stein 2006), werecreated using a slow speed drill and diamond-tipped drillbit.Dilution by quartz or co-existing sulfides was minimal anddoes not affect the resultant age because of the extremelyhigh Re and 187Os concentrations in molybdenite relative toall other phases. In addition, minimal dilution means thatreported concentrations of Re are representative of Relevels in the ore-forming fluid. The molybdenite powder,spikes of isotopically enriched 185Re and 190Os for earlysamples (labeled CT, ‘Carius Tube’, in the data tables) ormixed double 185Re-188Os-190Os spikes for later samples(labeled MD, ‘Mixed Double’, in the data tables), andinverse aqua regia are transferred to a thick-walledborosilicate Carius tube (Shirey and Walker 1995). Theuse of a mixed-double spike has the advantage of (1)negating weighing errors as the Re/Os ratio of the spikesolution is fixed (i.e. evaporation has no effect), (2) trackingvery small contributions of 187Oscommon to the 187Ostotalconcentration, and (3) correcting for Os fractionation usingthe fixed 190Os/188Os ratio of 0.13174. The sealed tube isheated for ~12 h at 275°C to achieve sample-spikeequilibration. Os is isolated by distillation and furtherpurified by microdistillation. Re is isolated by anionexchange chromatography using cleaned Biorad AG 1-X8,200–400 mesh, Cl− form resin. The mass spectrometry

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measurements were performed at AIRIE using two NBSTIMS machines after the procedure of Creaser et al. (1991)and Völkening et al. (1991). Approximately 1–2 ng ofdissolved Re or Os is loaded and dried on outgassed Ptfilaments. Measurements were carried out using a singleFaraday cup collector in peak jumping mode with back-ground and interfering isotopes monitored. The Re ismeasured as ReO�

4 and analyses are corrected for baselinecontributions and oxygen isotopic composition (Nier 1950),whereas the Os, measured as OsO�

3 is corrected for (1)baseline contributions, (2) W, Re, and Pt isobaric interfer-ences, (3) oxygen isotopic composition, (4) Oscommon

contribution to the total 187Os and 188Os peak intensities,and (5) mass fractionation using the measured 190Os/188Osratio (Markey et al. 2003). Re and Os concentrations aredetermined by the isotope dilution equation (Heumann 1988)and the Re–Os age is determined by the age equation,187Os=187Re (e l t � 1) with l=1.666×10−11 year−1±0.31%(Smoliar et al. 1996). Repeat analyses of AIRIE in-housemolybdenite standards HLP-5, Aittojärvi, and Ylöjärvi alongwith reproducible and consistent blank measurements assureage accuracy (Markey et al. 1998, 2003). Errors are fullypropagated using standard techniques including isotopedilution error magnification (Crouch and Webster 1963).Ages reported include 2σ standard errors with the decayconstant contribution so that comparisons with othergeochronologic systems account for all internal and externalerrors.

Re–Os data and interpretations

Re–Os data consist of 187Re and 187Os concentrations usedto determine ages and total Re and total Os concentrationdata. The ages and the Re concentration data provideinformation critical to the model derived from this study.Common Os was found to be negligible in all themolybdenite analyses; thus, the total Os is reported asradiogenic 187Os. These Re–Os data provide a uniqueopportunity to investigate the evolution of the ATBS Belton all scales, from the deposit to the district to the belt.

Re–Os geochronology

Re–Os data for each district are presented in Tables 1, 2 and3 and are plotted in Fig. 4. Re–Os ages show importanttrends on the deposit, district, and belt scales. The lifespanof a deposit is between 0.5 to 2.0 m.y. The lifespans areminimum durations given that the exact first and lastmolybdenite-forming pulse is unlikely to have beensampled. Nevertheless, the molybdenites sampled for thisstudy are associated with stages representative of the ore-forming history. Elatsite in Panagyurishte and Băiţa Bihor

in Apuseni are two contrasting deposits, representingexposed porphyry versus skarn mineralization types, re-spectively. Nonetheless, Re–Os ages indicate similar life-spans. Main-stage porphyry-style mineralization at Elatsiteoccurred over ~550,000 years, whereas multiple pulses offluid generation and mineralization at Băiţa Bihor occurredover ~1,940,000 years, with main-stage mineralization ofthe main Băiţa ore body and the Antonio North bodyoccurring in ~1,100,000 years. Thus, taken together, oreformation varied from ~0.5 to ~2 m.y, with main-stage oredeposition developing in ~1 m.y. For other deposits in theABTS belt, Re–Os ages provide similar age spans, againsuggesting that important main-stage mineralization wasindeed geologically short-lived.

On the district scale, the complete construction of agiven district occurs over 5 to 11 m.y. These are minimumlifespans for a district because not all deposits in a districtcould be analyzed in this study. However, most of the majordeposits were sampled and dated, providing an estimate forthe construction of an economically significant miningdistrict. For Apuseni–Banat, skarn- and porphyry-styledeposits formed over 11 m.y., whereas 7 m.y. was neededto form Panagyurishte and 5 m.y. for Timok. Such restrictedand characteristically short mineralization epochs suggestthat fundamental tectonic processes leading to mineraliza-tion must also be short-lived. The construction andtemporal evolution of individual districts and the ABTSbelt itself are best expressed by noting district-scale agetrends (Fig. 4). Mineralization in Panagyurishte is remark-ably systematic with unequivocal younging of ages south-ward. Timok is marginally systematic with east to west(present-day orientation) younging of metallogenic ages.The Apuseni–Banat district is the least systematic, althoughone could argue for a present-day north to south decrease inmineralization age, with exceptions.

Finally, on the belt scale, the entire metallogenicendowment, stretching 1,500 km, was emplaced in only20 m.y. In addition to observed systematic age migrationson the district scale, migration of mineralization on the beltscale is also demonstrated (Fig. 4). With marginal overlap,metallogenesis migrates from east to west, from Panagyurishte(92 to 87 Ma) to Timok (88 to 81 Ma) and finally toApuseni–Banat (83 to 72 Ma). Rapid construction of this1,500 km belt with rich metal endowment is remarkable andsuggests that local transient tectonic states are required,rather than long-lived steady state systems.

Other high-precision geochronology supports the timingrelationships determined by Re–Os. Figures 5, 6 and 7display available U–Pb, 40Ar/39Ar, K–Ar, and Rb–Srchronology for each district (compiled from: U–Pb—Nicolescu et al. 1999, Kamenov et al. 2003, von Quadt etal. 2001, 2002a, b; 2004, 2005; 40Ar/39Ar—Kamenov et al.2003, Clark and Ullrich 2004, Lips et al. 2004; K–Ar—

8 Miner Deposita (2008) 43:1–21

Table 1 Re–Os data for the Apuseni Mountains and Banat District, Romania, S. Carpathians

AIRIE

Run no.

Sample

number

Deposita Deposit Type MoS2 Descriptionb Re,

ppm

187Os,

ppb

Age,

Mac

Apuseni Mountains

MDID-362 RO03-BB1 Băiţa Bihor Mo–Pb–Zn–Fe Skarn Mo clots and nests in diopside skarn 28.17 (4) 23.4 (1) 79.45±0.4

MDID-367 RO03-BB2 Băiţa Bihor Mo–Pb–Zn–Fe Skarn Mo concentrated along qtz vein

cutting calcite, diopside skarn

58.59 (7) 48.8 (2) 79.51±0.4

MDID-499 RO03-BB3 Băiţa Bihor Mo–Pb–Zn–Fe Skarn Large mo clots/nests interstitial to

massive andradite garnet skarn

61.94 (7) 51.6 (2) 79.49±0.4

MDID-361 RO03-BB5 Băiţa Bihor Mo–Pb–Zn–Fe Skarn Large mo clots in grossular-

andradite skarn

47.96 (5) 40.0 (2) 79.56±0.4

MDID-513 BBH 15A.2 Băiţa BihorCC Mo–Pb–Zn–Fe Skarn Disseminated mo, native Cu in

slightly metamorphosed limestone

8.61 (1) 7.10 (3) 78.69±0.4

MDID-75 BB1 Băiţa BihorTB Mo–Pb–Zn–Fe Skarn Mo clots in impure diopside skarn 97.18 (3) 81.70 (3) 80.24±0.3

MDID-91 BB1d Băiţa BihorTB Mo–Pb–Zn–Fe Skarn Mo clots in impure diopside skarn 135.87 (5) 114.79 (5) 80.63±0.3

CT-363 No. 1291BP Băiţa PlaiCC Mo occurrence Small disseminated mo clots

in granodiorite

16.60 (4) 13.83 (2) 79.50±0.4

CT-361 No. 3Cer CerbiaCC Mo occurrence Mo in granite from drillcore 97.70 (7) 163 (1) 159.1±0.5

MD-423 DVM25 SăvărsinCC Mo occurrence Mo–py qtz vein cutting granite 438.4 (5) 735 (3) 159.8±0.7

Banat

MDID-421 CPR3 Valeă CapişoaraCC Cu Porphyry Disseminated mo in relatively

fresh diorite

29.03 (4) 22.7 (1) 74.68±0.4

MDID-508 TNC1 TincovaCC Pb–Zn–Fe Skarn Thin smear of mo along shear plane

within propylitic andesite

82.2 (1) 66.0 (3) 76.59±0.4

MDID-512 TNC1bd TincovaCC Pb–Zn–Fe Skarn Thin smear of mo along shear plane

within propylitic andesite

86.6 (1) 69.4 (3) 76.52±0.4

MDID-501 CAL1 CalovaCC Cu Porphyry Mo intergrown with cp in qtz–

calcite vein cutting fresh diorite

111.3 (1) 84.2 (4) 72.20±0.4

CT-212 OdFD.99G1 DogneceaCC Fe Skarn Large, cm-sized, mo plates in

qtz-rich skarn

1524 (1) 1222 (2) 76.51±0.3

MDID-505 ORV1 OraviţaCC Cu–Mo–Pb–Zn Skarn Marginal mo–cp in qtz vein cutting

altered granitoid

19.77 (4) 15.4 (3) 74.1±1.6

MDID-511 ORV1bd OraviţaCC Cu–Mo–Pb–Zn Skarn Marginal mo–cp in qtz vein cutting

altered granitoid

5.882 (9) 4.55 (2) 73.87±0.4

MDID-500 ORV2 OraviţaCC Cu-Mo–Pb–Zn Skarn Mo clots in vuggy qtz vein cutting

banded, altered hornfels

918 (3) 719 (3) 74.74±0.5

CT-368 No. 2Cic CiclovaCC Mo occurrence Sub-cm mo books in vuggy qtz

vein cutting weakly altered diorite

80.31 (4) 69.61 (4) 82.71±0.3

MDID-404 ORG4 Moldova NouăCC Cu–Au–Mo–Pb–Zn

Porphyry

Inward oriented mo laths along

vein margin cutting highly latered diorite

340.2 (4) 258 (1) 72.42±0.4

MDID-506 OGR8 Moldova NouăCC Cu–Au–Mo–Pb–Zn

Porphyry

Mo clots along qtz vein selvege in

altered andesite

531.7 (6) 403 (2) 72.36±0.4

MDID-516 OGR9 Moldova NouăCC Cu–Au–Mo–Pb–Zn

Porphyry

Fine mo intergrown with cp in qtz

vein cutting altered andesite

158.0 (2) 120.4 (5) 72.70±0.4

MDID-507 FVP2 Moldova NouăCC Cu–Au–Mo–Pb–Zn

Porphyry

Coarse, randomly oriented mo

books in massive qtz

1326 (3) 1007 (4) 72.50±0.4

Number in parentheses followingRe andOs concentration reflects 2σ error of the last digit given. Re andOs analytical blanks range from 2 to 10 pg and valuedoes not affect the final age. Initial 187/188Os composition is assumed to be 0.2, although the estimated value does not significantly affect the final age.a Samples contributed by local geologists to support my personal collection are noted by a two letter superscriptb Representative hand specimen and thin section photographs are presented in Zimmerman 2006 (Appendix A)c Full 2σ external error includes weighing, calibration, measurement, and decay constant errorsd Replicate analysis of a given sample using a new mineral separate, assuring geological reproducibilityCC—Cristiana Ciobanu and Nigel Cook; TB—Tudor Berza

Miner Deposita (2008) 43:1–21 9

Janković et al. 1981, Lilov and Chipchakova 1999,Ciobanu et al. 2002; Rb–Sr—Kamenov et al. 2003). Similarplots are presented in Ciobanu et al. (2002) with K–Ar agesfrom non-economic locations. In general, Re–Os, U–Pb,and 40Ar/39Ar ages cluster at the proposed time ofmagmatic–volcanic–metallogenic activity whereas K–Arand Rb–Sr ages scatter about the same period but arerelatively imprecise. Resolution of mineralization pulses isonly possible using high precision Re–Os ages. U–Pbzircon (von Quadt et al. 2002a, b; 2005) ages for magmaticand volcanic units are in good agreement with Re–Osmineralization ages. 40Ar/39Ar ages (Clark and Ullrich2004; Handler et al. 2004; Lips et al. 2004) generallyoverlap mineralization and magmatism ages, although some40Ar/39Ar ages, not surprisingly, are notably youngerrecording disturbance by later Alpine events and/or doc-

umenting prolonged cooling times during which the datedminerals remained isotopically open systems. In contrast tovariably younger ages for closure of argon-based chronom-eters, the Re–Os and U–Pb systems are noted for their hightemperature of isotopic closure (>500°C, Stein et al. 2001).

Re–Os geochemistry

Rhenium concentrations in molybdenite (Tables 1, 2 and 3;Fig. 8) provide information on the source of Re and, byassociation, other metals (Stein et al. 2001, 2004; Stein2006). High, >250 ppm, to very high, >1,000 ppm, Reconcentrations suggest metal sources that involve fertilemantle or juvenile crustal, whereas low, <100 ppm, andespecially Re concentrations <10 ppm, indicate an evolvedcrustal metal source. Concentrations <10 ppm and espe-

Table 2 Re–Os data for the Panagyurishte district, Srednogorie zone, Bulgaria, Balkans

AIRIE Run no. Sample

Number

Deposita Deposit Type MoS2 Descriptionb Re, ppm 187Os,

ppb

Age,

Mac

MDID-370 BU03-EL1 Elatsite Cu–Au Porphyry Qtz–mo–cp veinlets in hornfels 2330 (1) 2243 (10) 91.88±0.5

MDID-94 Ela99LFX1 ElatsiteLF Cu-Au Porphyry Altered granodiorite with cross-cutting

qtz–mo stockwork veins

1287.1 (2) 1246.7 (5) 92.43±0.3

MDID-100 Ela00LF110 ElatsiteLF Cu–Au Porphyry Mo clots in vuggy qtz–cp–mo vein

cutting highly altered granodiorite

1407.8 (3) 1362.7 (8) 92.37±0.3

MDID-116 Ela00LF85 ElatsiteLF Cu–Au Porphyry Qtz–mo–cp vein cutting highly

altered diorite porphyry

272.88 (6) 263.76 (9) 92.24±0.3

CT-360 1150 level ElatsiteCC Cu–Au Porphyry Pure mo vein cutting altered granodiorite 2499 (2) 2407 (2) 91.94±0.3

MDID-119 1150b leveld ElatsiteCC Cu–Au Porphyry Pure mo vein cutting altered granodiorite 2329.3 (5) 2249.6 (9) 92.16±0.3

MDID-117 ELS1 ElatsiteCC Cu–Au Porphyry Qtz–mo stockwork veins in

altered granodiorite

2740.1 (6) 2644.2 (9) 92.09±0.3

MDID-118 ELS2 ElatsiteCC Cu–Au Porphyry Qtz–mo stockwork veins in altered

granodiorite

1877.3 (6) 1810.6 (6) 92.03±0.3

MDID-502 BU03-MD1 Medet Cu–Mo Porphyry Platy, radial mo from center of

1 cm qtz vein

1162 (1) 1103 (5) 90.55±0.5

MDID-365 BU03-MD2 Medet Cu–Mo Porphyry Mo intergrown with cp in qtz vein 553.0 (6) 527 (2) 91.05±0.5

MDID-184 MDT1 MedetCC Cu–Mo Porphyry Coarse mo clots and laths in vuggy qtz

vein cutting altered diorite porphyry

827.5 (1) 789.0 (3) 91.00±0.3

MDID-188 MDT2 MedetCC Cu–Mo Porphyry Qtz–mo vein in mildly altered diorite 516.3 (1) 494.0 (3) 91.31±0.3

MDID-189 ASR1 AssarelCC Cu Porphyry, Epithermal Hairline qtz-mo vein in massive quartz 692.8 (2) 662.5 (2) 91.25±0.3

MDID-183 ASR2 AssarelCC Cu Porphyry, Epithermal Hairline qtz–mo vein in massive quartz 784.8 (2) 753.5 (4) 91.62±0.3

MDID-185 VV-46/99 Vlaykov VruhKK Cu Porphyry Nests and clots of coarse, disseminated

mo in vuggy qtz

2055.2 (6) 1872 (1) 86.95±0.3

MDID-190 VV-46/99be Vlaykov VruhKK Cu Porphyry Nests and clots of coarse, disseminated

mo in vuggy qtz

1674.9 (3) 1527.7 (7) 87.04±0.3

MDID-371 BU03-VV1 Vlaykov VruhKB Cu Porphyry Disseminated, coarse mo in vuggy qtz 161.0 (3) 148.0 (7) 87.70±0.5

MDID-514 BU03-VV1bd Vlaykov VruhKB Cu Porphyry Disseminated, coarse mo in vuggy qtz 341.9 (4) 311 (1) 86.77±0.5

Number in parentheses following Re and Os concentration reflects 2σ error of the last digit given. Re and Os analytical blanks range from 2 to10 pg and value does not affect the final age. Initial 187/188 Os composition is assumed to be 0.2 although the estimated value does not significantlyaffect the final agea Samples contributed by local geologists are noted by a two-letter superscript.b Representative hand specimen and thin section photographs are presented in Zimmerman 2006 (Appendix A)c Full 2σ external error includes weighing, calibration, measurement, and decay constant errorsd Replicate analysis of a given sample using a new mineral separate, assuring geological reproducibilitye Replicate analysis of a given sample using the same mineral separate assuring analytical reproducabilityLF—Lorenz Fanger; CC—Cristiana Ciobanu and Nigel Cook; KK—Kalin Kouzmanov; KB—Kamen Bogdanov

10 Miner Deposita (2008) 43:1–21

cially sub-ppm level Re are particularly characteristic ofmolybdenite produced during metamorphism (Stein 2006).Dehydration melting of biotite is one mechanism forproducing assemblages containing metamorphic molybde-nite (Bingen and Stein 2003; Stein 2006). Such extremelylow Re concentrations are not found in the ABTS, althougha few samples were highly diluted by host matrix, generallyquartz, resulting in apparently low concentrations.

In general, the porphyry deposits in the ABTS belt showhigh to extremely high Re concentrations (Fig. 8). Thehighest concentrations are associated with Au–PGE–enriched deposits (Elatsite, Majdanpek, and Bor) associatedwith less evolved melt chemistries (e.g. diorite). Skarndeposits show a more variable range of Re concentrations.For example, Re concentrations for abundant clots andnests of molybdenite at the Mo–Pb–Zn–Fe skarn at BăiţaBihor are generally low while the Re concentration for raremolybdenite platelets within vuggy quartz at the Fe skarn atDognecea exhibits extreme Re enrichment suggestingenhanced Re with vapor transport (Hannah et al. in press).Further, the marked difference in Re concentration betweenBăiţa Bihor and Dognecea skarns may reflect an inherent

difference in the Mo:Re ratio. Dognecea is an Fe skarn withminimal Mo enrichment whereas Băiţa Bihor is a Mo–Cuskarn with notable Mo enrichment and abundant molybde-nite. The Re budget is therefore diluted by more abundantmolybdenite at Băiţa Bihor. Nonetheless, the overall Reconcentration data support a subduction-dominated tectonicsetting with increasing crustal involvement with time.

Rhenium concentrations are notably systematic relativeto metallogenic district (Fig. 8a) and deposit type (Fig. 8b).Generally, Re concentrations decrease markedly fromPanagyurishte to Timok to Apuseni-Banat. Re concentra-tions for the four Cu–Mo–Au porphyry deposits (with notablePGE abundances) in Panagyurishte range from ~200–2,000 ppm. Molybdenite hosted in quartz veins containshigher Re and disseminated molybdenite contains lower Reconcentrations. Similarly, Re concentrations in molybdenitefor Cu–Au–Mo porphyry deposits in Timok range from ~100to ~1,000 ppm. One exception from Majdanpek (Ziv1,Table 2), has a Re concentration of ~45 ppm, but themolybdenite habit is markedly different from the rest ofTimok samples. The molybdenite is dull, massive, and veryfine grained, and Re concentration may be diluted over a

Table 3 Re–Os data for the Timok Magmatic complex, Serbia, S. Carpathians

AIRIE Run

no.

Sample

Number

Deposita Deposit Type MoS2 Descriptionb Re,

ppm

187Os,

ppb

Age,

Mac

MDID-369 SE03-MJ1 Majdanpek Cu–Au–Mo Porphyry,

Skarn

Thin, whispy veinlets and disseminated

mo in massive qtz

124.0 (1) 108.4 (5) 83.44±0.5

MDID-364 SE03-MJ2 Majdanpek Cu–Au–Mo Porphyry,

Skarn

Thin, whispy mo veinlets in massive qtz 943 (1) 828 (4) 83.77±0.5

MDID-510 MJD1 MajdanpekCC Cu–Au–Mo Porphyry,

Skarn

Qtz–mo vein cutting altered diorite 931 (1) 813 (4) 83.37±0.5

MDID-77 Ziv1 MajdanpekPZ Cu–Au–Mo Porphyry,

Skarn

Massive, very fine mo with large

floating cp, py aggregates

44.7 (1) 39.08 (2) 83.38±0.4

MDID-89 Ziv1bd MajdanpekPZ Cu–Au–Mo Porphyry,

Skarn

Massive, very fine mo with large

floating cp, py aggregates

44.61 (1) 39.00 (2) 83.43±0.3

MDID-115 Koz1 MajdanpekDK Cu–Au–Mo Porphyry,

Skarn

Mo surfaces in massive qtz vein cutting

altered andesite

608.7 (4) 534.0 (4) 83.73±0.3

MDID-368 SE03-DP1 Dumitri PotokDK Cu Porphyry Thin qtz–mo veinlet in altered andesite 202.3 (2) 171.3 (7) 80.82±0.5

MDID-509 SE03-DP1 Dumitri PotokDK Cu Porphyry Thin qtz–mo veinlet in altered andesite 281.1 (3) 238 (1) 80.69±0.4

MDID-363 SE03-DP1 Dumitri PotokDK Cu Porphyry Thin qtz–mo veinlet in altered andesite 139.6 (1) 118.0 (5) 80.69±0.4

MDID-420 VK140m Veliki KriveljDK Cu–Mo Porphyry Grungy mo–cp in qtz vein cutting altered

diorite porphyry

301.8 (8) 278 (1) 87.88±0.5

MDID-422 96Bor01 BorDK Cu–Au–Mo Porphyry,

Epithermal

Large mo book in altered diorite porphyry 930 (2) 840 (4) 86.24±0.5

MDID-515 96Bor01bd BorDK Cu–Au–Mo Porphyry,

Epithermal

Large mo book in altered diorite porphyry 846 (1) 761 (3) 85.94±0.4

Number in parentheses following Re and Os concentration reflects 2σ error of the last digit given. Re and Os analytical blanks range from 2 to10 pg and value does not affect the final age. Initial 187/188 Os composition is assumed to be 0.2, although the estimated value does notsignificantly affect the final agea Samples contributed by local geologists to support my personal collection are noted by a two letter superscript.b Representative hand specimen and thin section photographs are presented in Zimmerman 2006 (Appendix A)c Full 2σ external error includes weighing, calibration, measurement, and decay constant errorsd Replicate analysis of a given sample using a new mineral separate, assuring geological reproducibilityCC—Cristiana Ciobanu and Nigel Cook; PZ—Persa Živkovi; DK—Dejan Koželj

Miner Deposita (2008) 43:1–21 11

larger volume of molybdenite, whereas the other analyzedTimok molybdenites are discrete crystals in quartz veins. Reconcentrations for skarn deposits in Apuseni–Banat rangefrom ~10–1,000 ppm, although most are between ~10 and100 ppm. Porphyry deposits have Re concentrations between~50 and 1,000 ppm with most between 200 and 1,000 ppm.These results correlate with deposit-commodity type in eachdistrict. Panagyurishte is uniformly characterized by Cu–Mo–Au–(PGE) porphyry deposits, Timok is mixed Cu–Auporphyry with minor skarn deposits, and Banat–Apuseni ismixed Cu–Au–Mo porphyry (South Banat) and Mo–Fe–Pb–Zn skarn (North Banat and Apuseni) deposits.

The systematic progression of Re concentrations mayreflect district-scale interactions (or lack thereof) betweenmelts or fluids and the surrounding environment (mantle,lower crust, and upper crust) as they migrate from source totheir final site of emplacement. It is postulated that crustalthickness increases from east to west as a result of LateCretaceous back-arc thinning or as an inherited crustal settingbefore Late Cretaceous tectonism (Shanov et al. 1992; vonQuadt et al. 2005), although Cenozoic tectonics also accountfor much crustal thinning (Schmid et al. 1998). Meltsgenerated below Panagyurishte were able to access highcrustal levels rapidly with little crustal assimilation resultingin voluminous volcanic activity, notably high Re (and PGE)concentrations, and rich Cu–Au metallogenesis. Stackedthrust sheets in Timok and more importantly in Apuseni–Banat created a thicker crust that may have sequestered

melts, increased assimilation, and resulted in a localpredominance of skarn (versus porphyry) deposits character-ized by lower Re concentrations. Again, the tectonic under-pinnings are reflected in Re concentrations thereby showingRe to be a useful geochemical tracer for tectonic regime.

Other geochemical tracers support conclusions drawnfrom the Re data. Hf isotopes from deposits in Panagyur-ishte support melting of enriched lithospheric mantle withminor crustal contribution (von Quadt et al. 2002b). LateCretaceous intrusives from Banat show ЄNd(t) valuesbetween +3.9 and −0.2 with most analyses between 0 and+2, suggesting material sourced from a variably depletedmantle or young mafic lower crust (Dupont et al. 2002). Amantle or juvenile melt source has been suggested byvarious authors based on initial 87Sr/86Sr that rangebetween 0.705–0.709 for Apuseni, 0.703–0.706 for Banat,0.706–0.710 for Timok, and 0.704–0.705 for Panagyurishte(Janković and Jelenković 1997; Karamata et al. 1997;Berza et al. 1998; Dupont et al. 2002; von Quadt et al.2001, 2002b). Expanded trace element spider diagramsshow remarkable similarities for magmatic units throughoutthe belt. LREE and LIL enrichment, Nb and Ta depletion,and no Eu anomalies characterize intrusions from Pana-gyurishte to Banat (Nicolescu et al. 1999; Dupont et al.2002; von Quadt et al. 2005). Such characteristics arecompatible with subduction-derived melts sourced from themantle. In summary, most geochemical data, including ourRe concentration data, support melting of variably depleted

Fig. 4 Re–Os molybdenite agesorganized N–S geographicallyby district. With few exceptions,ore mineralization occurs inrapid (0.5 to 2.0 m.y.) pulses onthe deposit scale. On the districtscale, resolvable pulses of mag-matic–hydrothermal activity areobserved. The ABTS Belt ischaracterized by a clear migra-tion of mineralization fromPanagyurishte to Timok toApuseni–Banat

12 Miner Deposita (2008) 43:1–21

Fig. 5 Available geochronologyfor Apuseni–Banat illustratesoverlap of precise Re–Os withthe few available U–Pb ages andthe less precise and scatteredRb–Sr and K–Ar ages(references in text)

Fig. 6 Plotting Re–Os ages andavailable U–Pb ages with40Ar/39Ar, K–Ar, and Rb–Srdata for the Timok districtshows general agreement ofages, while still emphasizing thescatter and poor precision of theAr- and Sr-based chronologicdata (references in text)

Miner Deposita (2008) 43:1–21 13

mantle with contributions from juvenile mafic lower crustand lesser involvement of upper crust as the source regionfor ABTS magmatism and metallogenesis.

Tectonic model

The first tectonic models for the ABTS belt and associatedunits followed the zeitgeist of the time—geosynclinaltheory. Most geosynclinal models were only published inlocal literature, although an overview of Bulgaria by Fooseand Manheim (1975) provides a sound introduction to themodel. Following the paradigm shift to plate tectonics,numerous workers re-evaluated the ABTS, especially theSrednogorie Zone and the Balkans (Rădulescu and Săndulescu1973; Boccaletti et al. 1974a; Hsü et al. 1977; Dabovski et al.1991). Thus, newer models generally interpreted theSrednogorie Zone as a back-arc rift (Aiello et al. 1977)and the Balkans as a back-arc thrust belt (Burtman 1986).Subduction processes dominated original plate tectonicmodels (Boccaletti et al. 1974b; Hsü et al. 1977), althoughintra-continental rifting also had proponents (Popov 1987).For the next few decades, studies of the region focused ondelineation of tectonic units and their relationships throughtime. The introduction of in-depth field observationscoupled with high-precision geochemical and geochrono-logical data led to the identification of discrete crustal unitsthat were translated, rotated, and amalgamated, and ocean

basins that were created and consumed. These observationsfurther refined tectonic models (e.g. Kazmin et al. 1986;Ricou et al. 1986; von Raumer et al. 2003). Most recently,more evolved and specific models have been proposed,including those favoring slab rollback (Lips 2002; vonQuadt et al. 2005; Zimmerman et al. 2005; Chambefort andMoritz 2006), slab tear (Neubauer 2002), and orogeniccollapse (Berza et al. 1998; Bojar et al. 1998; Willingshoferet al. 1999; Iancu et al. 2005). While still operating in aconvergent setting, these models identify specific processesin space and time that formed the unique features of thebelt. These Re–Os data continue this legacy of identifyingand refining the key tectonic processes operating in theregion.

Mureş ophiolites and Re–Os ages

Molybdenite samples from the Săvărsin and Cerbiaintrusions in the South Apuseni mountains yielded ages of159.8 and 159.1 Ma (Table 1), respectively, rather than the100–70 Ma ages characteristic of ABTS banatites. Thesemolybdenite samples are from the Mureş ophiolites andrecord Late Jurassic magmatic–hydrothermal activity beforebanatitic magmatism. Both samples represent mineraliza-tion related to granitoid intrusions and associated quartz–molybdenite–chalcopyrite veining. Thus, the intrusions arelikely also of Jurassic age, as suggested by previousworkers.

Fig. 7 Full geochronology forPanagyurishte shows the generaloverlap of Re–Os mineralizationages with U–Pb magmatic ages,and some of the 40Ar/39Ar ages(references in text). Other40Ar/39Ar ages are notablyyounger. K–Ar and Rb–Sr agesare similar but are highlyscattered

14 Miner Deposita (2008) 43:1–21

The Mureş ophiolites are characterized by highlydismembered, upper ophiolitic sequences without themetamorphic sole and mantle rocks typical of otherophiolites from the Tethys realm. Bortolotti et al. (2002,2004) documented formation of the ophiolites at a mid-

ocean ridge based on geochemical evidence. Radiolarianchert intercalated with pillow basalts record Callovian toOxfordian ages (164 to 154 Ma). Limited K–Ar ages forpillow basalts range from 138.9±6 to 167.8±5 Ma (Nicolaeet al. 1992). The ophiolite sequence is covered by up to

Fig. 8 Rhenium concentrationin molybdenite roughly corre-lates with both a district and bdeposit type reflecting juvenilemelt and metal sources

Miner Deposita (2008) 43:1–21 15

1,000 m of calc-alkaline volcanic products. Granites togranodiorites intrude both the ophiolite sequence andoverlying volcanics. Bortolotti et al. (2002) suggest agenetic relationship between calc-alkaline volcanics andgranitic intrusions. Furthermore, they state “K–Ar radio-metric datings for the calc-alkaline volcanics are generallyunreliable...” (Bortolotti et al. 2002, p. 943). Based ongeochemical characteristics of both the calc-alkaline vol-canics and the ophiolite, Bortolotti et al. (2002, 2004)linked the Mureş ophiolites to the Vardar ocean and similarophiolites obducted onto the Pelagonian block. Theirconclusions are compatible with the Csontos and Vörös(2004) reconstruction and evolution of the Vardar, in whicha sliver of Vardar ocean crust is entrained in the core of adeforming Bihor-Getic microcontinent (see Figure 24 inCsontos and Vörös 2004), although the timing of deforma-tion must be older than Aptian.

The Re–Os ages from this study restrict the timing ofobduction, volcanism, and associated granitoid plutonismto >159 Ma. Although K–Ar ages for pillow basalts are asyoung as 139 Ma (Nicolae et al. 1992), the molybdeniteages suggest that ophiolite obduction occurred before159 Ma followed closely by volcanism, coeval magmatism,and associated hydrothermal activity. The new molybdeniteages push back the timing of calc-alkaline activity,corroborate microfossil ages from overlying sedimentaryunits, and suggest events in the Mureş progressed rapidlyfrom obduction to volcanism–magmatism to sedimentation.Additionally, Late Jurassic suturing of the Apuseni Moun-tains with Banat must have occurred before UpperCretaceous banatitic magmatic activity.

ABTS belt tectonics

Two general models for ABTS belt evolution dominatecurrent thought: (1) a slab rollback model and (2) a slab tearmodel. Both models are derived from observations of morerecent processes operating in the Eastern and WesternCarpathians during Cenozoic consumption of the PenninicOcean (Csontos et al. 1992; Linzer 1996; Wortel andSpakman 2000). The slab rollback model (Lips 2002; vonQuadt et al. 2005; Zimmerman et al. 2005; Zimmerman2006), based on the work of Royden (1993), proposes thatsubducting Vardar oceanic slab began steepening (i.e.rolling back) in the Late Cretaceous. The rollback led toupper plate extension, corner flow of sub-continentalmantle lithosphere and asthenosphere, partial melting, andmigratory magmatic–hydrothermal centers. The geochemi-cal signature of magmatic products (calc-alkaline signature,REE systematics, Nd and Sr isotopic composition, and Reconcentration) supports subduction-derived melts. Exten-sion in the upper crust gave melts access to high crustallevels, ultimately leading to volcanism. The rollback model

is not restricted to a specific time in the subduction process.It may happen syn-subduction, syn-collision, or post-collision. All that is needed is enhanced vertical gravitationalforce relative to lateral tectonic force acting on the slab.

The slab tear model (Neubauer 2002), based on Worteland Spakman (2000), states that in a post-subduction, post-collisional regime, the subducting slab tears away from itshigh buoyancy continental counterpart. The tearing slabinitiates asthenospheric upwelling into the slab windowcreated as the lithotectonic units separate. The release ofgravitational pull by the slab results in upper crust reboundand localized uplift. Support for this model is derived fromthe Cenozoic volcanism of the Western and EasternCarpathians where subduction and slab tear of the PenninicOcean lithosphere resulted in systematic, migratory volca-nism of calc-alkaline character (Linzer 1996).

Based on trends in both Re–Os ages and Re concentra-tion data, supported by other high precision geochronologyand geochemistry, it is possible to make coherent inter-pretations linking the timing of mineralization and sourcingof metals to the tectonic history of the ABTS belt. As aresult, it is possible to evaluate slab rollback and slab tearmodels in light of a comprehensive Re–Os dataset. Thetectonic model proposed here incorporates age trends,magmatic–volcanic–hydrothermal activity at temporallyand spatially distinct ore centers, and geochemical evidencefor melts and metals.

Tectonic reconstruction of the ABTS belt requires thatCenozoic tectonic overprints first be removed. Paleomag-netic data comparing Cretaceous to Miocene intrusive andvolcanic units in the ABTS belt show that the belt waslinear and oriented E–W during the Mesozoic. Both theApuseni Mountains and the Banat district rotated ~80°clockwise during Neogene deformation (Pătraşcu et al.1990, 1992, 1994; Rosu et al. 2004). As such, theseworkers suggested a shared tectonic history for Apuseni–Banat since the Late Cretaceous (see Mureş ophiolitesection for Re–Os support of Jurassic amalgamation ofApuseni–Banat). Timok was rotated ~30° clockwise duringthe same deformational period, whereas the SrednogorieBelt has remained in an E–W orientation since the LateCretaceous. Continued convergence between Africa andEurope caused ‘bending’ of the ABTS around the Moesianplatform in the Cenozoic. As such, the conspicuous ‘L’-shaped bend seen today was imparted on the belt after LateCretaceous mineralization. Therefore, the proposed tectonicmodel utilizes an E–W oriented ABTS, similar to orienta-tions used by other workers (Lips 2002; Neubauer 2002;von Quadt et al. 2005).

As schematically shown in Fig. 2, the Drama andPelagonian continental units were south of the Europeanmargin in middle Mesozoic time, separated by the Nish-Trojan and Vardar basins. The Nish-Trojan trough was an

16 Miner Deposita (2008) 43:1–21

intracontinental basin formed on thinned crust that filledrapidly with terrigenous sediments (Minkovska et al. 2002),whereas the Vardar basin developed oceanic crust withassociated marine sedimentation (Ricou et al. 1998). Themodel for subsequent tectonic events propagated from thisinitial setting, such that inaccuracies at this stage mayinfluence later tectonic interpretations.

Far-field tectonic effects dramatically influenced theinitial stages of ABTS construction. Tethyan separation ofAfrica and Europe ended as the Atlantic basin opened in themiddle Mesozoic. As a result, in the Tethys, newly riftedcontinental ribbons (i.e. the Drama block) reversed courseand migrated northward towards the European margin.First, the young, mobile Nish-Trojan basin inverted as the

Drama block translated northward. Basin inversion endedas the Balkan and South Carpathian mountain systemsdeveloped with northward nappe stacking and suturing ofthe Drama block to the Moesian platform. Synchronouswith suturing, continued Africa–Europe convergence initi-ated northward subduction of relatively hot, buoyant Vardaroceanic crust. Initial slab dip was shallow because of itsbuoyancy and rapid rate of subduction, resulting in upperplate compression. The shallowly dipping oceanic slab didnot reach melt generating depths (Fig. 9a). If Vardarsubduction below the Drama block occurred contempora-neously with Nish-Trojan inversion, lithospheric tractionbetween the newly subducting oceanic crust and the Dramablock would have provided the upper plate compression

Fig. 9 a Schematic cross-section through the Tethyansystem during the EarlyCretaceous. Net tectonic trans-port is north at this time withrapid Vardar closure leading toshallow subduction and Balkansand South Carpathian mountainbuilding. Arrows above tectonicunits represent schematic direc-tion and magnitude of net mo-tion towards the fixed Moesianblock. b Schematic cross-sectionthrough the Apuseni and Banatdistricts in the western half ofthe ABTS Belt during metallo-genesis (83–72 Ma). Vardaroceanic slab has not rotated norretreated compared to theeastern half of the belt (see c)resulting in only minor exten-sion and stagnant SCML andasthenosphere. c Schematiccross section through the Timokand Panagyurishte districts inthe eastern half of the ABTSBelt during metallogenesis(92–81 Ma) linked to therotation and trenchward retreatof Vardar oceanic crust. Nettectonic motion is vertical due togravitational pull on the sub-ducting slab. Note enhancedcorner flow of SCML andasthenosphere to maintainneutral mass balance, therebyproviding heat and juvenilecomponents to the system

Miner Deposita (2008) 43:1–21 17

required to invert and close the Nish-Trojan basin.Alternatively, the rate of subduction may have been slowerthan the rate of Africa-Europe convergence. In this situation,the subducting slab would have ‘bulldozed’ the Europeanmargin keeping the upper plate under strong compression(possibly leading to Nish-Trojan inversion and orogenesis).The rationale for shallow subduction and upper platecompression is ambiguous (hot crust, fast convergence,and/or lithospheric coupling). An alternative explanationfor the lack of Early Cretaceous intrusive and volcanicactivity is that upper plate compression sequestered melts atdeep to moderate crustal depths and may have producedequigranular intrusions rich in hydrous minerals.

Subsequently, a Late Cretaceous major plate reorganiza-tion occurred in response to changes in the Atlantic system(Lips 2002). Convergence between Africa and Europestalled and became dramatically less than the rate ofsubduction. Proportionally more vertical gravitational forcerather than lateral tectonic force acted on the slab. As aresult, the slab steepened, and compressional upper plateforces were replaced by tensional stress. The slab penetrat-ed ~90–100 km depth, initiating melting of the SCMLwedge with variable asthenospheric involvement. Rollbackof the slab initiated asthenosphere and mantle lithosphereflow necessary to satisfy mass balance considerations(Kincaid and Griffiths 2003). Restricted relaxation ofApuseni–Banat created normal faulting (Fig. 9b) whereasenhanced relaxation of the Moesian-Drama crust formedextensional structures in Panagyurishte and Timok(Fig. 9c). Fluids derived from the subducting crust fluxedhot, fertile mantle lithosphere and asthenosphere. Newlycreated melts reached high crustal levels, some formingassociated ore deposits. Continued slab steepening shiftedthe locus of melt generation southward, resulting in theunequivocal and distinct age trend identified in Timok andSrednogorie (Fig. 4). Re concentration data support a melt-metal source in fertile mantle and/or juvenile lower crust.

After this transient tectonic reconfiguration, convergencebetween the Pelagonian and Drama fragments renewed.This re-established upper plate compression, shut off meltgeneration, limited ascension of components to high crustallevels, and terminated Late Cretaceous mineralization.Alternatively, the locus of melt generation in the mantlewedge may have migrated southward far enough to betrapped under thicker continental crust (i.e. Drama block),in which case melts would be unable to significantlypenetrate upper crustal levels. In addition, restoration ofupper plate compression may have limited the ability ofgenerated melts to ascend, restricting them to deeper levelswhere fluids were unable to exsolve.

A note on orogenic collapse and post-collisional pro-cesses is warranted. The proposed model is consistent withthe suggestion of orogenic collapse (Berza et al. 1998;

Bojar et al. 1998; Neubauer 2002) in which slab rollbackenhances orogenic collapse. Integrating the orogenic col-lapse model of Selverstone (2005), the ABTS belt mayrepresent a mixed fixed-boundary/free-boundary system inwhich the northern Moesian platform is fixed, and thesouthern Drama block is free. An asymmetrical extensionalregime extending to the south is predicted by the mixedboundary model and may be reflected in ABTS extensionalbasins south of the Balkans-South Carpathians. The ABTSbelt provides a unique opportunity to look at mixed-boundaryorogenic collapse at various stages from the initiation(Apuseni-Banat) to slightly extended (Timok, 20-km-widebasin) to moderately extended (Panagyurishte, 60-km-widebasin). Tracing the system eastward, one could speculate thatthe Black Sea represents the highly extended (100-km basin)segment (Zonenshain and Le Pichon 1986). The belt forms anextensional wedge, which propagated westward as slabrollback and associated extension progressed. Although slabrollback may occur at any time during subduction, it mayhave occurred post-collision in the ABTS system andfacilitated orogenic collapse.

In contrast, the slab tear model is far less supported bythe Re–Os ages, Re geochemistry, and integrated high-precision geochronology and geochemistry for the ABTSbelt. The slab tear model predicts a systematic decrease inages as the tear migrates laterally; pulses of magmatic–hydrothermal activity should parallel the strike of the tear.The Re–Os ages do show systematic younging fromPanagyurishte to Timok and Apuseni to Banat, but theages converge at the Danube River rather than continuingfrom Timok to Banat (Fig. 1). Age progressions parallel tothe strike of the belt in Apuseni and Banat are consistentwith the slab tear model, but those perpendicular to thestrike of the belt in Panagyurishte and Timok are counter tothe predicted slab tear age trend. Additionally, a slab tearmodel requires oceanic crust to be subducted between theMoesian and the Drama blocks as slab tear magmatismoccurs above a near vertically subducting slab (see Figure 4in Wortel and Spakman 2000). It is generally accepted thatthe Nish-Trojan basin was never floored by oceanic crust(Minkovska et al. 2002). Therefore, while processes similarto slab tear may be progressing in the Late Cretaceous, thedominant tectonic process leading to rich metallogenicenrichment in the ABTS was most likely slab rollback andorogenic collapse.

Conclusions

The Re–Os system applied to molybdenite representative ofmineralization from ore deposits and mineral occurrences inthe ABTS belt places precise time constraints on a slabrollback metallogenic–tectonic model for the evolution of

18 Miner Deposita (2008) 43:1–21

this 1,500-km-long belt. Trends in both Re–Os ages and Reconcentrations suggest that short-lived, migratory pulses ofmagmatic–hydrothermal activity are sourced from fertilemantle or juvenile crustal material and were emplaced atvarying crustal levels in a restricted 20 m.y. window in theLate Cretaceous. Individual deposits representing porphyry-style mineralization have lifespans <1 m.y. and generally<500,000 years. Cu porphyry deposits with high Au and PGEhave extraordinarily high Re concentrations (>1,000 ppm).Utilizing Re–Os data as a means to track the tectonicevolution of a mineral belt is valuable for exploration andfor understanding the relation of commodity endowment totectonic environment. It remains clear that tectonic processesgenerating this type of mineral wealth require subduction-related models. Tectonic analogs for both far-field and localprocesses include southeast Asia and the Andes. However,given the remarkably brief period to produce such a vast andvaried metallogenic endowment and the evidence for ribbonsof paleo-oceanic crust, the ABTS belt may be a closeranalogue to present day Southeast Asia.

Acknowledgments This paper constitutes the M.S. thesis work ofthe first author under the AIRIE Program at Colorado State University.Student research grants from the Society of Economic Geologists(SEG Foundation), the Geological Society of America, and Geo-sciences-CSU are gratefully acknowledged. The thesis and theopportunity to visit the ABTS belt to collect molybdenite samples infield context were also funded in part by a US National ScienceFoundation grant (EAR-0087483) to Stein. The project was enhancedby supplementary samples along with logistical and intellectualsupport from Serban Vlad, Christiana Ciobanu, Nigel Cook, LorenzFanger, Kalin Kouzmanov, Albrecht von Quadt, Christoph Heinrich,Persa Žiković, and personnel from visited mines. A constructivereview from Franz Neubauer improved the manuscript. Editorialsupport from Bernd Lehmann is much appreciated.

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