DEPOSITIONAL COMPONENTS AS SOURCES OF METALS IN SEDIMENTARY BASINS: THE PROTEROZOIC EXAMPLE

Formation of economic mineralization in basins must be placed in a regional context because the source of the traps and metals can be from within the basin itself and the fluids that transport the metals tend to be widespread and chemically distinct as a result of interactions over large areas and significant time with specific lithologies. This is the case for giant U, Pb-Zn-Ag and Cu deposits in Proterozoic basins in Canada and Australia. Fluids in all basins originate and flow as a result of sedimentological and tectonic events, so that fluid histories should reflect the control of both lithology and tectonism on ore deposition.

Burial diagenesis in Paleoproterozoic basins that host giant ore deposits results in the early occlusion of primary permeability in some facies (diagenetic aquitards) and the enhancement of permeability in other facies (diagenetic aquifers) as secondary porosity develops due to the dissolution of unstable grains. For diagenetic aquifers to develop, unstable grains must have been in contact with basinal fluids in the pore spaces. Diagenetic aquifers, particularly distal fluvial poorly-sorted siliciclastic units containing detrital uraniferous phases or those spatially and temporally associated with volcanic units are the most favorable sources of metals whereas well-cemented sandstones, shales, siltstones and carbonates transmit very small quantities of basinal fluids, and are thus unlikely source rocks for metals but are potential traps. The basinal brines in the basins that produced the deposits were highly saline, 150-250ºC and evolved from seawater during burial to depths of 4 to 8 km. Ages of diagenetic phases extracted from aquifer lithologies reveal that fluid migration from the diagenetic aquifers effectively covers the period for Zn-Pb, U and Cu mineralization. Thus, diagenetic aquifer lithologies were being drained at the same time as the deposits were forming, linking diagenesis and fluid events within the basin to the formation of the deposits. Exploration for deposits in basins can benefit from the regional aspects of fluid-rock interactions provided strategic fluids can be characterized and the importance of specific geologic environments and lithologies recognized.