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.

References :

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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