Earth Sciences Seminar: Emeritus Professor David Groves

Presenter

Emeritus Professor David Groves
National Geoscience Champion

Abstract

Distribution of iron-oxide copper-gold (IOCG) deposits through earth history with implications for deposit associations and tectonic setting

The iron oxide-copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential endmembers or analogs are included, with over 100 significant deposits listed in reviews up until 2010 alone, and many more incorrectly-ascribed added since.  The broader group comprises essentially low-Ti iron oxide-associated deposits that include iron oxide (p), iron oxide (F, REE), skarn Fe or Cu-Au, high-grade Au ± Cu, and IOCG sensu stricto deposits.  Consideration of this broad group as a whole obscures the critical temporal distribution and tectonic environments of the IOCG deposits, leading to proposals of multiple tectonic settings of formation.

A more robust definition of IOCG deposits, consistent with Olympic Dam as the type example, is that they are magmatic-hydrothermal deposits that contain economic Cu ± Au ± U grades; are structurally controlled, commonly with breccias; have abundant low- Ti iron oxides or iron silicates intimately associated with Fr-Cu sulfides; have LREE enrichment and low- S sulfides (pyrrhotite, chalcopyrite, bornite, chalcocite); lack quartz veins or silicification; and show a clear temporal, but not spatial, relationship to causative intrusions.  These intrusions have highly anomalous, commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic compositions with strong evidence for mantle derivation of at least the mafic endmembers of the suite.  In conjunction the giant size of the deposits, breccias pipes and surrounding alteration zones, occurrence of a pipe and maar at Olympic Dam that strongly resembles those of kimberlite or lamproite pipes, highly saline ore fluids and both stable and radiogenic isotope data, indicate release of deep, high energy, volatile-rich magmatic fluids through devolatilization of causative mantle-derived magmas.

Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources.  The 12 IOCG deposits with >100 Mt resources are situated in intracratonic settings close to the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 – 200 my after assembly of the supercontinents Kenorland, Columbia and Rodinia.  Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle upwelling or plumes.  Low-degree partial melting of volatile-rich and metal-enriched metasomatized early Precambrian sub-continental lithospheric mantle (SCLM), fertilized during earlier back-arc subduction, is considered to have produced basic to ultrabasic melts which then melted overlying continental crust and mixed with resultant felsic melts, with devolatilization to produce the IOCG deposits.  Preservation of even near-surface deposits such as Olympic Dam was possible due to their formation in buoyant SCLM.

Precambrian iron oxide (P) or magnetite-apatite deposits have a totally different temporal distribution, forming in convergent margin settings following or prior to supercontinent assembly, negating their classification as endmembers of the IOCG deposit group.  It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are pene-contemporaneous in convergent margin setting during similar time brackets relative to the supercontinent cycle as the giant Precambrian magnetite-apatite deposits.  The Phanerozoic IOCG deposits such as Candelaria occur in anomalous extensional to trans-tensional zones in the Coastal Cordillera which are also the site of mantle-derived mafic to felsic intrusions that are anomalous in a Phanerozoic context.  Combined with the timing of mineralization at ca 115 Ma, broadly coincident with the giant Cretaceous mantle plume in the Pacific, these factors imply that special conditions, probably detached slabs of metasomatized SCLM, are required in convergent margin settings to generate world-class IOCG deposits.

It appears most likely that formation and preservation of giant IOCG deposits was largely a Precambrian phenomenon related to heightened activity of mantle plumes that impacted on buoyant  metasomatized SCLM at that stage in Earth history, with Phanerozoic IOCG deposits forming only rarely in tectonic settings where conditions similar to those in the Precambrian were replicated.