Do Volcanism and Mineralization in The Timok Magmatic Complex (East Serbia) Begin Earlier Than Previously Thought?

by M. Banješević¹, N. Vasić ², N. Pantelić³

¹ University of Belgrade, Technical Faculty in Bor, Bor, Serbia, 

² University of Belgrade, Faculty of Mining and Geology, Belgrade, Serbia  

³ Metalfer Group, Belgrade, Serbia

*  Corresponding author: mbanjesevic@tfbor.bg.ac.rs

Abstract

What we know so far about the development of the Timok Magmatic Complex (TMC) after almost 150 years of research and 120 years since the discovery of copper mineralization in Bor (eastern Serbia) is that volcanism in the TMC developed at the beginning of the Upper Cretaceous on the continental crust of Paleozoic to Lower Cretaceous age. The TMC belongs to the Late Cretaceous Apuseni-Banat-Timok-Srednogorje Belt (ABTS) and together belong to the transcontinental Tethyan-Eurasian Metallogenic Belt. The Upper Cretaceous rocks of the TMC consist of volcanic/magmatic, volcanogenic-sedimentary and sedimentary rock packages. Magmatism/volcanism occurred in at least two phases from the Upper Turonian to the Upper Campanian: I volcanic phase – V1 (90-82 Ma), II volcanic phase – V2 (83-78 Ma), with a constant decrease in age from east to west. The most significant epithermal and porphyry mineralization, in the world-famous copper deposits (Majdanpek, Bor, Veliki Krivelj, Cukaru Peki), is related to the volcanic phase V1, which is predominantly developed in the easternmost part of the TMC and developed between 87-80 Ma. However, recent exploration drill holes conducted for the purpose of exploring for strategic mineral resources south of Bor, in their deeper intervals, have drilled, in a normal stratigraphic succession, clastic sediments of Albian-Cenomanian age. These clastics are intruded by hornblende andesites that are believed to belong to the V1 volcanic phase, and that according to preliminary data are about 92-90 Ma old. All of the above data are more or less known to the wider geological public, however, what is unusual in the composition of the Albian-Cenomanian sediments and so far, unrecorded is the presence of fresh to slightly altered andesite and basaltic andesite fragments, which according to their macroscopic characteristics have not been recorded so far on the surface or in exploration drill holes in the TMC. Does the occurrence of these clasts in the sediments that are significantly older than all previously recorded volcanics mean that magmatism/volcanism in the TMC began significantly earlier than previously thought, probably in the Albian-Cenomanian? We will certainly seek answers to these questions in future specialized geological investigations and perhaps initiate new thinking about the geodynamic development of the TMC.

Keywords

volcanism, mineralization, TMC, Cenomanian, andesitic clasts

Note:

Papers published in this special issue of the European Geologist journal have undergone a thorough peer-review process but have not been copy-edited. Authors bear full responsibility for the linguistic accuracy of their contributions.

This work is licensed under a Creative Commons Attribution 4.0 International License.

1. Introduction

The Timok Magmatic Complex (TMC) almost totally overlaps the Bor Metallogenetic Zone (BMZ) which belongs to the Late Cretaceous Apuseni-Banat-Timok-Srednogorje Belt – ABTS [1,2] and together they belong to the Tethyan-Eurasian Metallogenic Belt [3]. The BMZ is known to host some of the largest Cu, Au deposits in Europe and has been the subject of study and exploration for more than 100 years by many geologists or companies. The recent discovery of a world class Cu, Au deposit of Čukaru Peki (southeast of Bor) aroused serious new interests for further investigating the BMZ, both in the exploration and the scientific context [4-10]. This study revises, supplements, classifies, supplements and reports stratigraphy characteristics of the main units and indicated the link between magmatism and mineralization in the BMZ.

2. Stratigraphy of the TMC

The geology of the TMC and location of the major ore deposits are given in Figure 1. The stratigraphy and time space development of the TMC and relationship of published sedimentary formations and volcano-sedimentary unit, magmatic suites and mineralized rocks [10-13] are given in Figure 2. The TMC developed on a continental crust composed of different types of the Proterozoic to the Lower Cretaceous rocks partially overlain by the Miocene and Holocene sediments. The Late Cretaceous is made up of the volcanic/magmatic, volcano-sedimentary and sedimentary rock packages that consists of the following units/formations [12,13]:

Sedimentary formations – Lenovac Formation (LE), Oštrelj Formation (OS), Bor Clastic (BK) and Bukovo Formation (BS);

Volcano-sedimentary unit – Metovnica Epiclastite (ME);

Volcanic/Magmatic suites – Timok Andesite (AT), Osnić Basaltic andesite (AO), Ježevica Andesite (AJ), and Valja Strž Plutonite (VS).

Figure 1: Simplified geological map of the TMC with the locations of major ore deposits; the inset shows the regional geotectonic position. The red circle highlights the location of exploratory drill holes associated with this work.

Figure 2: The stratigraphy and time space development of the TMC, relationship of sediment formations and magmatic suites and their link with mineralization [10-13]. On the left side is schematic stratigraphic chart with epoch and age in Ma. LE – Lenovac Formation, OS – Oštrelj Formation, AT V1 – Timok Andesite volcanic phase, V1A – Timok Andesite sub-phase, V1B – Timok Andesite sub-phase, P1 – Shallow intrusions of V1 and possible source of the alteration and mineralization processes, MP1 – Porphyry and skarn mineralization, ME – Metovnica Epiclastite, V2 – Volcanic phase, AO – Osnić Basaltic andesite, AJ – Ježevica Andesite, VS – Valja Strž Plutonite, MP2 – Porphyry, epithermal and skarn mineralization, BS – Bukovo Formation, BK – Bor Clastic, M – Miocene sediments, Timok EAST – The eastward tectonic block, Timok WEST – the westward tectonic block. Red line highlighted schematic unconformity of porphyry mineralization in the eastward tectonic block.

After almost continuous carbonate sedimentation from the Early Jurassic to the Lower Cretaceous, a new sedimentation period of clastic character commenced with the Albian transgression related to oscillations of the depositional environment during the Albian-Cenomanian age. The unit is built of sandy claystones, sandy marlstones and sandy siltstones topped by sandstones with siderite, with an often-rich content of Upper Albian-Cenomanian planktonic foraminifera [13]. After hiatus the sedimentation re-started in the Turonian with basal conglomerate and continued with clastic to carbonate sediments (OS). These sediments discordantly overlie LE (Figure 2). During the Upper Turonian to the Senonian, the whole TMC area shows a difference in the evolution between the eastward and the westward tectonic block (Timok East and Timok West in Figure 2, [11]).

The volcanism occurred in two phases from the Upper Turonian to the Upper Campanian: V1 (90-82 Ma), V2 (83-78 Ma). In the eastward block, predominate volcanic rocks of biotite-hornblende andesite compositions (AT, V1 in Figure 2). These rocks stratigraphically overlie the LE or Turonian age part of OS (Figure 2). The available U/Pb zircon ages of the TA range 90-82 Ma [5,14,15]. The AT, V1 volcanic phase can be split into two different andesite sub-phases: V1A (older, substantially altered and mineralized) and V1B (younger, fresh and non-mineralized), which systematically differ in their stratigraphic position and petrography, in age and in their link to mineralization (Figure 2, [5,9]). The V1A is feldspar rich andesite and hosted the world-class epithermal – high-sulfidation and porphyry Cu, Au deposits in the BMZ (MP1 in Figure 2) and overlain by V1B. The V1B is hornblende rich andesite, fresh, non-mineralized and overlain by the ME or grey to reddish sediments of the OS.

In the central and western part of the TMC, V2 volcanic phase begins, which is formed by hornblende-pyroxene- to pyroxene andesite and basaltic andesite (AJ, AO, V2 volcanic phase, Figure 2). There are no precise radiometric age data of these rocks, but based on other geological indications, it is assumed that they are in the range of 83-81 Ma. They are also stratigraphically interlayered with the above-mentioned OS and ME [12].

In the western part of the TMC occur plutonic rock from monzodiorite, monzonite, diorite, Q-diorite, granodiorite, syenite and rare gabbro (Valja Strž Plutonite VS [12], P2 in Figure 2). This rock intruded into the AJ and AO volcanics and surrounding sediments and dated at 83-78 Ma [14,16]. Large number of morphogenetic types of mineralization in V2 rocks are mainly concentrated in the vicinity of the discovered plutonite (MP2 in Figure 2).

During the volcanism and after its interruption, V1 and V2 volcanics begin to be resedimented, transported and formed post-eruptive volcano-sedimentary package – epiclastic, Metovnica Epiclastite (ME in Figure 2). The ME are represented by polymictic volcaniclastic breccia, conglomerate and sandstone, with clasts of volcanic and non-volcanic origin. ME are often interbedded with OS [12,17].

The sedimentary series of the Oštrelj Formation (OS in Figure 2) are continuously developed and can be found over the Early Cretaceous of the volcanic basement rock but also as underlying or overlying the V1 and V2 volcanites and their epiclastic products. OS are represented by layered to laminated greyish, rare reddish marl and siltstones, sandy limestones and calcareous sandstones and contain Turonian–Campanian micro fauna [13].

From the Upper Campanian to Maastrichtian, in the Timok West block, reef sedimentation commenced (Bukovo Formation, BS in Figure 2) and at the same time, in the Timok East block, coarse-grained and regressive polymictic clastites were deposited (Bor Clastic, BK in Figure 2, [13]). This was the period when TMC intensively tectonically uplifted, ceased to exist as an active volcanic area, and at the end of this period also as a sedimentation area [11]. The Miocene sediments, after the great hiatus, cover large areas of volcanogenic-sedimentary rock units in the TMC (Figure 2).

3. The succession of the Albian-Cenomanian clastites

During geological exploration works within the framework of the project for the exploration of strategic mineral resources in eastern Serbia, exploration drill holes were drilled in the area south of the city of Zaječar (Figure 1). These holes confirmed the previous knowledge about the stratigraphy of the area but also revealed some new facts that were not known until now. Beneath this package is the Metovnica Formation with all its facies, textural and structural differences and the Miocene sediments. Since these two units are not the subject of this paper, we will not dwell on them in more detail. An extensive sedimentological study [18] conducted by the company as an internal document describes all the details of the stratigraphic development of this area, obtained from the logs of the exploration drillholes. Only some parts of this study were taken for this work.

Figure 3: Simplified succession of the Albian and Cenomanian fine-grained clastites in the exploration drill hole [18].

The oldest and deepest unit in which the exploration hole was stopped is the Albian sandstones with glauconite (Figure 3). The glauconitic fine- to medium-grained sandstones are stratified into thin to medium-thick layers. The components of the Albian sediments are quartz, feldspars, and rock fragments. The rock fragments originate mainly from metamorphic rocks. Microfossils are present, but poorly preserved. The presence of grains or accumulations of glauconite is characteristic of the Albian sediments. The unit of the Cenomanian fine-grained clastics (Lenovac Formation – LE, on Figure 2) continuously develops from the Albian sediments (Figure 3). The epiclastic of the Metovnica lie above. This boundary is sharp, i.e. discordant. The true thickness of the Cenomanian sediments is about 150 m. The unit looks very monotonous because it is dominantly built of fine-grained clastites. The colour is dark grey, almost black. The layering is very difficult to notice, that is, it is observed only sporadically. In addition to fine-grained clastites dominated and sandstones, conglomeratic sandstones and sandy conglomerates are found too, but only as layers and interlayers in fine-grained sediments. Fine-grained clastites regularly contain shells of Cenomanian age microorganisms. Coarse-grained sandstones and conglomerates occur sporadically, as layers and interlayers 10 to several tens of centimetres thick. These layers are composed of fragments of volcanic rocks and limestone. The subject of our greatest interest, as well as the topic of this paper, are volcanic fragments in the coarse-grained fraction in the drill hole interval from 700 m to 591m (Figures 3-5). Andesite fragments correspond to hornblende andesites and basaltic andesite (Figures 4 and 5). Hornblende andesites are completely altered plagioclase, hornblende and rare biotite phenocrysts. Plagioclase is calcitized, hornblende and biotite are chloritized. It is most likely that there are several different andesite facies from which the material came. Basaltic andesite fragments are darker and fresher, i.e. alteration changes on phenocrysts are not as strong. Phenocrysts are plagioclase and pyroxene with rare hornblende, mainly in the groundmass. The limestone fragments correspond to different micritic microfacies. This association of microfacies indicate to the Barremian-Aptian limestones (basement rocks of the TMC). The limestone fragments show significantly worse roundness than volcanic fragments (Figures 4, 5).

Figure 4: Rounded large fragments of dark basaltic andesite (encircled by lines) and small angular fragments of limestone (small L) in the Cenomanian conglomerate.

Figure 5: The andesitic (encircled by lines) and limestone fragments (L) in the Cenomanian conglomerate package. The dotted line in the middle separate parts of the rock with andesitic fragments of different grain sizes (A – finer, B – coarser, [18].

4. Conclusion

The current understanding, supported by data from recent geological explorations of mineralization and ore deposits, and new discoveries of copper and gold deposits in the TMC, indicates that volcanism and magmatism, to which mineralization processes are also related, began around 90 Ma (Upper Turonian) and lasted until the middle Campanian (around 78 Ma). Before the beginning of volcanism, and after its end, there was generally continuous sedimentation over a wide area of present-day eastern Serbia, mainly in relatively shallow marine conditions. However, new data obtained from exploration drill holes, which are the subject of this work, show previously unobserved intervals of the Cenomanian age rocks on the surface, which also contain fragments of hornblende andesite and pyroxene basaltic andesite. These data imply several questions:

1. Did volcanism in the TMC begin earlier than we now know?
2. Was this earlier volcanism related to hydrothermal processes and mineralization and what is the potentiality of these processes?
3. What are the possible implications in terms of geodynamic development of the TMC?
4. Are the volcanic clasts in the Cenomanian sediments found in the wider TMC area or is this related to local development?

In general, this paper does not intend to provide a large amount of scientific data or evidence, but rather the authors’ intention is to point out and inform and encourage the wider scientific community towards further research in this area.

Acknowledgments: Gratitude to the companies Serbia Zijin Mining and Balkan Exploration and Mining, Bor for their understanding and support. Many thanks also to reviewer Gareth Jones for corrections and suggestions.

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This article has been published in European Geologist journal 60 – 5th IPGC Special Edition 1