Tethyan Tectonics and Metallogeny (Main theme on SEG – MJD Conference 2016, hosted in Turkey, September 25-28, 2016)
The Tethyan belt is one of the world’s most extensive tectonic and metallogenic belts, extending from Europe across Turkey and Asia Minor, through Iran and Tibet to the Malay peninsula, a distance of 12,000+ km. The main mineral deposit types associated with the Tethyan ocean basins and their destruction are seafloor massive sulfide deposits, porphyry Cu±Mo±Au, and epithermal Au±Cu deposits.
Porphyry formation in the Tethyan orogen can be broadly divided into four main episodes: early Mesozoic (Triassic–Jurassic), late Mesozoic (Cretaceous), Paleogene, and Neogene
Figure of Paleogeographic reconstruction of the Neotethyan region and the location of major porphyry depostits formed during each general period (black circles) / Source : www.osdn.de
No significant Paleozoic porphyry deposits related to Paleotethyan subduction are known in the belt, but a few such deposits formed in the Mesozoic. These include the Jurassic Xietongmen porphyry Cu-Au deposit in the Lhasa terrane of Tibet, and the Pulang and Yangla porphyry Cu deposits in Indochina (Fig. 1A). The rarity of Paleotethyan deposits is likely explained by erosional loss, because there is no reason to suppose that such deposits did not form in response to Paleotethyan subduction.
The Cretaceous featured a number of porphyry and related epithermal deposits in the Balkans, the Lesser Caucasus, and the Qiangtang terrane of Tibet (Fig. 1B). The Balkan deposits such as Majdanpek, Elatsite, ssarel, and Chelopech are related to subduction of the Vardar ocean, which may have been a remnant of the Paleotethys or a Neotethyan back-arc basin. The geological setting of mid-to-late Mesozoic porphyry Cu-Mo deposits such as Tekhut in the Lesser Caucasus is not well understood, but these deposits broadly relate to convergence between the South Armenian block and Eurasia. Paleogene porphyry Cu-Mo deposits in the same region such as Agarak likely elate to more advanced collisional processes.
Many of these Paleogene deposits are either only indirectly related to subduction processes (e.g., in back-arc extensional settings such as in Turkey and Iran), or are collision-related (as in the Carpathians, Balkans, and eastern Tibet). Recognition of these atypical tectonic settings for porphyry formation, where previously such deposits were all assumed to be subduction related, has come with better geochronological constraints and paleotectonic reconstructions, which have demonstrated that in many cases these deposits could not have been formed by active subduction, either because subduction had ceased or was located elsewhere at the time.
Porphyry formation continued into the Neogene, by which time active subduction had ceased along almost the entire length of the orogen. The only Neogene porphyry deposits that can be directly related to subduction formed in the Makran of eastern Iran and western Pakistan, and include the large Saindak and Reko Diq Cu-Au deposits.
Post-subduction and collision-related porphyry–epithermal deposits are almost indistinguishable from normal subduction-related systems, except that the associated magmas tend to be slightly more alkaline (high-K calc-alkaline to shoshonitic), and some are distinctly gold enriched. The sources of these magmas are thought to reside in previously subduction-modified lithosphere, with asthenospheric melt involvement only evident in extension-related or transtensional systems. Recycling of subduction-modified lithosphere (and in rare cases, asthenosphere) explains the similarity in magmatic and isotopic compositions to prior arc magmatism. Metals may be remobilized from small amounts of residual or cumulate sulfide phases in lower crustal arc cumulates or metasomatized subcontinental mantle lithosphere. These sulfides may be highly siderophile element-enriched (e.g., Au and PGE) where abundances of sulfide are low, leading to the formation of porphyry deposits with Au and PGE enrichments (e.g., Skouries) and epithermal Au deposits (e.g., Sari Gunay). Larger proportions of residual sulfides will dilute this Au-enrichment with Cu, leading to more normal porphyry Cu±Au deposit formation (e.g., Tibet), or may prevent deposit formation altogether if sulfide volumes are sufficiently large to prevent total sulfide dissolution during lower crustal partial melting.
Jeremy P. RICHARDS, 2014. A Review of Tectonics and Metallogeny of the Tethyan Orogen. Acta Geologica Sinica (English Edition), 88(supp. 2): 923-925
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