Petrochemical and geodynamic correlation of the porphyry-copper systems in the Eurasian copper belt: the segment Romania – Pakistan

pp. 3-33
Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia
Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia

The porphyry-copper systems are represented by ore-bearing magmatic complexes which to a greater or lesser extent are the host rocks of mineralization, and the processes involved in the differential formation of intrusive and subvolcanic bodies are similar to those leading to the release and concentration of useful components during some of the subsequent postmagmatic processes.
   Producers of porphyry-copper ore are mostly the intermediate and slightly acid rocks, the majority showing some tendency towards increased potassic alkalinity. The magmatic systems involved usually manifest several pulses of different compositions and/or fabric grouped into larger and more complex intrusive centres, sometimes called "batholiths". Copper, however, concentrates in separate, small and relatively late magmatic differentiates of increased acidity. Porphy.ry-copper mineralizations are characteristic of magmatic systems of homodromous evolution during subduction. Porphyry-copper deposits associated with the bimodal basalt-rhyolite magmatism of the epicontinental riftogenesis or with MORB are not known.
   In the Eurasian copper belt, the porphyry-copper systems are typically associated with massive and disseminated copper (polymetallic) - pyrite (±Au) mineralizations in propylites and secondary quartzites and with copper-gold (±Ag) base-metal veins and skarns. All three types are closely associated in space forming, along with the productive volcano-intrusive centres, an integral polygenic and polyfacial ore-magmatic system.
   The segment discussed in this study includes 65 deposits located exclusively within the Eurasian Plate active margin and associated with the formation of mature magmatic arcs during the period from the Upper Cretaceous till the Neogene inclusive. The most typical feature of the geodynamic setting is that a number of small continental blocks (micro-continents) have been also involved in the process of ocean (Neotethys) closing accompanying the collision of the major lithospheric plates (the Afro-Arabian and Eurasian ones). These blocks probably broke off the Afro-Arabian Plate and migrated to the north where they stuck to the Eurasian Plate southern margin forming an accretionary collage. The formation of porphyry-copper systems in the magmatic arcs is exclusively a post-accretion process at advanced subduction and collision. An especially characteristic feature is the localization of the largest and best defined deposits inside the accreted exotic blocks. Deposits formed in the back-arc thrust zone of the Eurasian Plate active margin are comparatively less widespread.
   In Romania, the porphyry-copper systems formed during the Upper Cretaceous and the Neogene are located in a block collage traced back, rather guestionably, to the Gondwana. The porpltyry-copper systems in East Serbia occur in a similar setting. As to the Tertiary deposits in North Macedonia, Krusha Mountain and Halkidiki Peninsula, they definitely belong to the Serbo-Macedonian accreted terrane. On the territory of Bulgaria, the majority of the Upper Cretaceous porphyry-copper deposits are lo;atd in the Central Srednogorie, Sakar Strandja and East-Thracia terranes. Separate deposits are situated (possibly as parts of allochthons) in the Balkaoid's thrust-nappe – back-arc belt. In Turkey (Strandja Mt.), a group of complex copper-molybdenum-tungsten porphyry deposits are located not far away from the Bulgarian ones, again in the East Thracia terrane. Anotller, less conspicuous group of Upper Cretaceous (and m1y be even younger) age is located in the active southern margin of the Eurasian Plate. Further east, all porphyry-copper deposits of the USSR, Iran and Pakistan occur in accreted exotic terranes including those of Lesser Caucasus, Northwest Iran, Central Iran and Hilmend.
   Local petrochemical correlation is based on the K2O+Na2O/SiO2 diagram on which fields of porphyrycopper deposits from the Circum-Pacific metallogenic belt and from the Caribbean have been plotted prior to analysis. Compared to these plots, the porphyry-copper systems in the Eurasian belt show analogous acidity of magmatism. Alkalinity is variable but generally the Eurasian (Alpine, Mediterranean) belt as compared to the Pacific one is characterized by a pronounced potassic alkilinity (high·potassic calc-alkaline series). This is consistent with the overall Mediterranean petrochemistry. Outside it, e.g. further east in Pakistan, the ore-bearing magmatic complexes are of normal calc-alkaline character.
   A general petrochemical correlation shows that compared to the Pacific belt the porphyry-copper systems in the Romania-Pakistan segment of the Eurasian copper belt are formed in a mixed geodynamic setting involving both continental and island arcs. It is suggested that the present-day mixing of geodynamic settings is a secondary phenomenon caused by Late Alpine collision processes which have deformed the primary tectonic features and have pushed to the north many of the magmatic arcs involved.