Depositional history of the Ament Bay Assemblage in the Sturgeon Lake Greenstone Belt, Northwestern Ontario: implications for gold metallogeny

Abstract

The Sturgeon Lake greenstone belt makes up the easternmost portion of the western Wabigoon terrane of the Superior craton, and is comprised of mostly Neoarchean volcanic assemblages, minor siliciclastic successions, and subalkalic- to alkalic intrusions. The Ament Bay assemblage is the youngest supracrustal assemblage of the Sturgeon Lake greenstone belt with a newly determined maximum depositional age of 2695.2 ± 7.8 Ma. Ament Bay consists dominantly of polymictic conglomerates, subarkosic- to arkosic arenites and wacke-mudstone sequences, interpreted as a sub-aerial fan delta that is transitional into subaqueous turbidites. These lithofacies are intruded by syenites of the Sturgeon Narrows Alkalic Complex (ca: 2693.2 ± 0.9 Ma), which are also incorporated as clasts in conglomerates of the Ament Bay assemblage, indicating a coeval relationship between alkalic magmatism, uplift erosion and sedimentation. Both the Ament Bay assemblage and Sturgeon Narrows Alkalic Complex are cross-cut by the Sturgeon Lake fault zone. Coeval alkalic magmatism and sedimentation in a fault-controlled basin are features of the 2680-2670 Ma Timiskaming assemblage which controls much of the orogenic-style gold endowment in the Abitibi greenstone belt. The Ament Bay assemblage has similar features but gold mineralization is not recognized in the belt. The lack of gold endowment in the Ament Bay assemblage could be a result of crustal influence on the alkalic melts and shallow-penetrating faults, in contrast to the juvenile nature of alkalic magmatism and deep-penetrating faults associated with the Timiskaming assemblage.