Diamondiferous, Neoarchean fan-delta deposits, western Superior Province, Canada: Sedimentology and provenance

Most fluvial diamond placers occur in non-aggrading braided systems where the diamonds are preferentially trapped by very rough substrates. The recognition that some of the Neoarchean, diamond-bearing breccias near Wawa, Ontario, Canada, are actually fluvial conglomerates, deposited by a mixture of traction current and mass-flow processes, provided the opportunity to add information on aggradational diamond placer systems. The 454 m thick Leadbetter Conglomerate was studied to ascertain the depositional processes that led to the development and provenance of the placer. The Conglomerate is composed of a series of weakly sheared and highly viscous debris-flows interbedded with boulder to pebble conglomerates representing longitudinal bars and channels. Pebble conglomerate and sandstone channels become more dominant upwards, with a decrease in mass-flow processes. An abrupt marine transgression flooded this fan-delta assemblage ending deposition of the coarse-grained lithofacies. The lowest 100 m of the conglomerate is composed of clasts derived solely from the underlying mafic volcanic assemblage. Above this zone the clast types become more diverse and felsic volcanic clasts become progressively more important upwards. Whole rock geochemistry performed on coarse-grained sandstone samples indicates that major sediment contributions came from mafic rocks and nearby diamondiferous lamprophyres and related breccias, though the possible existence of a localized kimberlitic source cannot be excluded. Geochemistry also indicates that minor enrichments of heavy minerals formed on the alluvial fan-delta surface but the primary cause of the conglomerate's diamondiferous nature was a nearby diamond-bearing source rock.

► This is the first Archean placer diamond occurrence to be described.
► It is also one of the most thorough descriptions of an alluvial diamond occurrence.
► Good exposures allowed detailed investigation of the fan-delta system.
► The work explores sediment geochemistry of sandstones from an unusual source.
► The paper raises awareness of possible Precambrian alluvial diamonds in other areas.