Diagenesis of sabkha-related, sulphate nodules in the early Proterozoic Gordon Lake formation, Ontario, Canada

Abstract

Unusually old (c. 2.4 Ga) loosely-packed anhydrite nodules and replacement nodules of gypsum, quartz and jasper are well preserved in transgressive marine siliciclastic sabkha sediment of the Huronian Gordon Lake Formation. There is no evidence of a gypsum precursor. The nodules are commonest in mud chip breccia at the base of sandstone, siltstone and mudstone upward-fining storm cycles. Anhydrite nodules are composed of a mosaic of blocky crystals. An earlier lath form is preserved in silicified outer rims. Its texture is similar to that of Recent displacive sabkha anhydrite nodules. Siliceous nodules are composed of megaquartz, some calcite-cored, or of jasper. Megaquartz is normally anhedral with strongly undulose (flamboyant) extinction. Crystals may be columnar or crudely spheroidal. Some megaquartz crystals are lath-shaped with the oblique extinction of lutecite. Rarer euhedral megaquartz in calcite nodule cores may have simple extinction but was probably not formed by cavity filling. Megaquartz crystals and anhydrite inclusions may preserve the texture of pre-existing early anhydrite laths. Smaller “jasper” nodules are composed of hematite-bearing fibrous fine-grained quartz, that is coarser than true jasper. Fibrous structure in this jasper may have oblique, length fast or length slow extinction, suggestive of recrystallization after lutecite, length fast and length slow chalcedony. True chert and chalcedony (<20μ) are absent. The jasper of the smaller nodules may contain ghost laths, after anhydrite? and scattered anhydrite and carbonate crystals. The jasper of larger nodules is granular. The jasper nodules contain rare gypsum crystals as well as anhydrite crystals which have been replaced to varied degrees by pseudomorphous quartz mosaics and carbonate rhombs. Some jasper nodules have an outer rim of megaquartz with anhydrite inclusions and associated carbonate. Some larger jasper nodules may have been formed in subaqueous sediments. Some weakly-silicified anhydrite nodules have been hydrated to gypsum nodules that are connected with subhorizontal gypsum veinlets. The hydration postdates silicification and conversion of anhydrite laths to blocky anhydrite. The veinlets, probably related to late unroofing, are a possible access route for water of hydration. They are later than host-rock lithification and calcite formation in the anhydrite nodules. Where the Gordon Lake Formation is metamorphosed megaquartz replacement nodules can be recognized by inclusions of anhydrite and interstitial calcite patches. These nodules are a useful indicator of vanished evaporites in such rocks. If the evaporite nodules of the Gordon Lake Formation, and the replaced evaporites of the Kona Dolomite of Michigan and the Peribonca Formation of Quebec are stratigraphically equivalent they indicate a cupriferous sabkha with an original extent of at least 1400 km.