Depositional Gaps in Abitibi Greenstone Belt Stratigraphy: A Key to Exploration for Syngenetic Mineralization

Models of greenstone belt development are crucial for exploration. Allochthonous models predict belts to be a collage of unrelated fragments, whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals. Superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary “Timiskaming-style” assemblages, and field and geochronological data indicate that the Abitibi greenstone belt developed autochthonously. We describe major revisions to stratigraphy of the Abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization. The Abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous U-Pb zircon ages of pre-2750, 2750 to 2735, 2734 to 2724, 2723 to 2720, 2719 to 2711, 2710 to 2704, and 2704 to 2695 Ma. We present revised lithotectonic and/or stratigraphic nomenclature using these time intervals, including (1) isotopic inheritance in younger episodes which indicates that the older episodes (2750–2735 and 2734–2724 Ma) had greater extent than is presently seen, (2) dikes feeding younger volcanic episodes (2706 Ma) cutting older volcanic units (2734–2724 Ma), and (3) 2710 to 2704 Ma mafic to ultramafic sills intruding the 2719 to 2711 Ma episode. Changes to the nomenclature include the identification of pre-2750 Ma volcanic episode (supracrustal fragments) in the northern and southern Abitibi greenstone belt and subdivision of the 2719 to 2711 Ma, 2710 to 2704 Ma, and 2704 to 2695 Ma episodes into lower and upper parts. We present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and/or lithotectonic map of the Abitibi greenstone belt. Many of the volcanic episodes are intercalated with and capped by a relatively thin “sedimentary interface zone” dominated by chemical sedimentary rocks. Stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m.y. between volcanic episodes. The zones consist of up to 200 m of iron formation, chert breccia, heterolithic debris flows of volcanic provenance, sandstone and/or argillite and conglomerate. Modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years, whereas the time interval between basalt-rhyolite cycles is 106 years. The sedimentary interface zones are therefore interpreted as condensed sections, zones with very low rates of sedimentation in a basinal setting, or zones with negligible rates of sedimentation marked by silicification of existing rock types. The sedimentary interface zones are therefore considered submarine correlative conformities, disconformities, or unconformities separating the equivalent of group level stratigraphic and lithotectonic units. The unconformity-bounded stratigraphic model provides a new regional to deposit-scale interpretive model for use in exploration for syngenetic mineralization.