Timing and Controls on Gold Mineralization in the Abitibi Greenstone Belt

Timing and Controls on Gold Mineralization in the Abitibi Greenstone Belt

And an update on the Mineral Exploration Research Centre (MERC), Department of Earth Sciences, Laurentian University

John Ayer, PhD, PGeo; Associate Director, MERC

Much of the Abitibi's gold production is from orogenic deposits controlled by late regional faulting and synorogenic sedimentary and volcanic units. These deposits young southward across the Abitibi. In the north; late tectonic assemblages consist of the ca. 2700 Ma Caopatina assemblage turbiditic sediments, unconformably overlain by sediments and alkalic volcanics of the ca. 2692 Ma Opemisca assemblage. In the NE Abitibi, gold mineralization associated with D1 is tightly constrained at 2697±1 Ma by mineralized and post-ore porphyry dikes. At the Detour Mine, D1 resulted in ca. 2725 Ma Deloro assemblage volcanics thrust over the Caopatina assemblage to the south along the Sunday Lake deformation zone (SLDZ). Subsequent oblique-slip deformation overprinted D1 with both dextral and sinistral components of displacement. Gold occurs in the hanging wall of the SLDZ in narrow high grade zones next to flexures along the faulted contact between the sediments and volcanics, and as broad, stockwork-disseminated lower grade zones in the hanging wall mafic flows. The age of a barren albitite dike cutting mineralization indicates that gold occurred around 2697 Ma.

South-verging auriferous thrusts extend south to the Pipestone deformation zone (PDZ). Here, 2720-10 Ma Kidd Munro assemblage volcanics were thrust over the 2690-80 Ma Porcupine assemblage turbidites. Gold occurs in the hanging wall of the PDZ and in intrusions within the fault zone. The timing of D1 and gold mineralization is younger (i.e. <2680 Ma) than in the northern Abitibi. The Porcupine-Destor and Larder Lake-Cadillac deformation zones (PDDZ & LLCDZ) differ from more northerly structures in spatial association with younger sediments and alkalic volcanics of the Timiskaming assemblage (2676-70 Ma), and north-verging thrusting. Oblique-slip deformation overprinted the thrusting with sinistral followed by dextral displacements. Truncation of the PDZ by the PPDZ indicates the former is an older structure. Significant gold mineralization occurred at around 2665-60 Ma along the PDDZ and LLCDZ. Further to the south, in the Shining Tree area, molybdenite associated with gold mineralization in a breccia pipe has been dated as young as ca. 2630 Ma, further supporting the progressive southward younging and/or telescoping of gold events.     

Controls on southward younging of tectonic events and gold mineralization are best explained by successive episodes of accretion of terranes proceeding from collision of the Abitibi with the Opatica in the north and culminating in collision with the Minnesota River Valley terrane to the south. Southward younging accretionary-related fault structures are thus considered the pathways for fluids bringing gold to the presently exposed crustal level.

A smaller but significant amount of gold production is associated with earlier base metal  mineralization in VMS deposits and high level porphyry intrusions. The oldest is the Cote Gold Deposit associated with the 2740 Ma Chester Intrusive Complex in the Swayze belt.

Bio: John Ayer, Associate Director at Mineral Exploration Research Centre, Laurentian University

Education: 

-BSc in Geology from Carleton University (1976)

-MSc from Carleton University (1979)

-PhD, University of Ottawa (1999). 

Experience: 

-Project geologist in exploration for a variety of mining companies across Canada (1979-1984).

-Ontario Geological Survey (1984-2011); 

-1:20 000 and 1:50 000 scale mapping of the Lake of the Woods greenstone belt, Wabigoon Subprovince (1984-1990).

-1:20 000 and 1:50 000 scale mapping of the Northern Swayze greenstone belt, Abitibi Subprovince at (1990-1995).

-Group Leader of the Precambrian Section Mapping Program in NE Ontario (1996 to 2011). Work included production of seven 1:100 000 scale compilation maps covering the Abitibi Greenstone belt in Ontario; OGS Representative on the Discover Abitibi Initiative and coordinator for the TGi-3 Abitibi Project in collaboration with the Geological Survey of Canada and Geology Quebec (2007-2011).

-Associate Director of the Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University (2012-present).