ABSTRACT

   The Altai-Sayan orogenic system, located in the core of the Altaid orogenic collage of central Asia, hosts numerous iron oxide deposits, many of which contain substantial gold, copper, apatite, fluorite, barite, REE and uranium concentrations. These deposits represent several styles of mineralisation that belong the broad IOCG association, namely, (1) proximal to distal metasomatic (skarn) and more distal scapolite-albite to chlorite-amphibole replacement; (2) stratabound magnetite-apatite occurring within volcanic sequences (Kiruna-type); and (3) carbonatites. Each of these styles corresponds to a different tectonic terrane and metallogenic epoch.
  The skarn and other replacement-type iron oxide (±copper, gold) deposits are especially numerous in the Caledonian orogenic terranes of the Middle to Late Cambro-Ordovician, and occasionally Silurian, with minor mineralisation re-deposited(?) in the Devonian. They include deposits associated with (1) proximal contact skarns, localised in the immediate vicinity of related intrusives; (2) proximal to distal skarns, frequently well above large plutons, but occurring in zones where intrusive apophyses, small stocks and dyke swarms are present, in some cases related to breccia pipes or diatremes, or as flat-lying skarns and orebodies in shallow-dipping host lithologies; (3) proximal to distal albite-scapolite-rich alteration in fault zones, with only minor skarn; and (4) distal chlorite-amphibole-dominated accumulations, also within fault zones. Deposits of the two latter sub-types host the largest iron oxide concentrations, often containing as much as 500 to 700 Mt of iron ore.
  Stratabound magnetite (-hematite) and magnetite-apatite deposits are mostly found in Hercynian orogenic terranes, although older concentrations may locally occur in Caledonian domains. Devonian volcanic sequences exert a distinct stratigraphic control on these deposits, with gradual transitions from iron oxide-enriched alkalic volcanics (trachytes, etc.) to semi-massive and massive iron oxide mineralisation. Multi-stage formation of these deposits appears to be the most plausible explanation of their occurrence, with local re-deposition and enrichment of mineralisation related to the emplacement of younger plutons.
  Mesozoic (Cretaceous?) iron oxide-rich carbonatites emplaced in Caledonian and older terranes are of particular interest due to their possible relationships with potassic granites and strong enrichment in hematite, REE (bastnaesite, monazite, etc.), uranium (uraninite), barite, fluorite, apatite, locally magnetite and chalcopyrite.

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