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.
No abstract is available for this article.
St. Boyadjiev. Comparative petrologic-geochemical eharacterization of the Pre-Mesozoic granitoids of the Sredna Gora zone. Pre-Hercynian (Late Proterozoic!) and Hercynian (South Bulgarian) types of granitoids are described. Grounds are given for their palingenetic origin and crustal level of magma generation.
On the basis of the analysis performed, the South Bulgarian granitoids are determined as a bimodal superformation, respectively presented by: a) Formation of basic and ultrabasic rocks of a probable Precambrian age- mainly embedded in single layers among the Precambrian crystalline complex; regarding them as a single gabbro-granite formation is not grounded. b) Formation of the batholitic granitoid complex which is distinguished for its polyphase-polyfacies structure. It is presented by three temporarily and laterally defined granitoid phases (impulses): first - marginal facies built by contaminated (a result of assimilation of basic and ultrabasic rocks) rocks which are gradually and centripetally transformed into granitoids up to granites; second – predominantly medium- up to coarse-grained, porphyroid biotite to two-mica granites, partially to granodiorite; third – medium-grained, predominantly biotite to two-mica granites, rarely granodiorites. On the basis of the relationships presented and on the basis of geochemical criteria as well, the development of a fourth granitoid phase - aplite-pegmatoid biotite to two-mica subalkaline granites to leucogranites of not elucidated, probably Alpine age - is grounded.
The granitoids are referred·to the K-Na series. They are characterized by a very high aluminity, weak ferreous and magnesium properties, which determine the low content of mafites and the strongly expressed leuco-rate frequency of rocks.
Ba, B. Pb, Th have contents below the clarke, while the other elements studied have contents about or under the clarke; the granitoids of fourth phase are enriched in So, W, Mo, Ag which determines their metallogenic specialization. They are characterized by a deficiency of REE, Eu and a negative europium anomaly; in the fourth granitoid phase REE is two times higher.
A complete petrologic and phase-facies analogy is established between the pre-Mezozoic granitoids from the Sredna Gora zone and from the Rhodope Massif.
The 70 m thick Pliocene lacustrine marly limestone at Agios Ilias has been sampled in detail and analysed for grain size, clay mineralogy, CaCO3 and Corg. Two types of terrigenous fraction are seen: one with a substantial amount of coarse silt, the other with little sediment coarser than 6 φ. Sometimes there is an inverse relationship between abundance and grain size ofterrigineous fraction. The higher silt and sand content of the less terrigenous marly limestones indicate wind-blow transport of sediment; the clay rich, more terrigenous marly limestones may reflect stream input of suspended material. The illite, montmorillonite and kaolinite comprising the clay fraction are probably of detrital origin. The marty·limestones consist of > 80% CaC03. Calcium carbonate precipitates inorganically resulting from photosynthetic increase of the pH. The very low organic carbon and general lack of stratification show a well oxygenated bioturbated lake floor. Variation in Corg ret1ect proximity to the lake margin. There is no evidence of evaboritic playa conditions. The marly limestooes are contrasted with other Neogene lacustrine marly limestopes in the Peloponesse in terms of source area, clay mineralogy and fossil content.
B. Monov. Old Kimmerian Structural Plan in the Western Fore-Balkan. Palaeotectonic analysis of the structures from the Early Kimmerian deformation stage in the eastern part of the West Fore-Balkan between the village of Yinitza and the lskar river has been carried out on the basis of more than 80 deep wells. Palaeotectonic and palaeostructural maps and palaeotectonic profiles of the Early Kimmerian structural plan have been made. Four faults have been traced – Vinitzian, Oghostian, Kotljansko-Krivodolski and Mramorian. They divide the region in 4 blocks with a specific lithological stratigraphic and tectonic structure. Two positive Early Kimmerian structures – Oghostian and Mramorian, ana one negative – Belovish-Gaganish have been distinguished. They are linearly oriented in WNW-ESE direction and have a complex internal structure. On these grounds they may be characterized as anticlinorium and synclinorium, the predecessors of the present Alpine structurees. The rise gradients of the Early Kimmerian anticlinorium is above 1800 m, which is almost equal to the thickness of the denudated during the Early Kimmerian tectonic stage Middleand Upper Triassic sediments.
GEOLOGICAL INSTITUTE “Strashimir Dimitrov”
Acad. G. Bonchev Str., Bl. 24
1113 Sofia
Bulgaria
Phone: +359 (02) 979 2250
Fax: +359 (02) 8724 638
E-mail: editorial-office@geologica-balcanica.eu
The construction of this website is
financed by the
Bulgarian “Scientific Research” Fund, Ministry of Education and Science.