THE LINK BETWEEN TECTONICS AND CONTINENTAL MARGIN BRINE FACTORIES: A NEW PERSPECTIVE ON THE FUNDAMENTAL CONTROLS ON THE DISTRIBUTION OF CLASTIC-DOMINATED ZN-PB-AG DEPOSITS IN THE ROCK RECORD

Clastic-Dominated (CD) Zn-Pb-Ag deposits (SEDEX) have an uneven distribution in the rock record that reflects secular changes in the oxidation of the Earth and supercontinent cycles that provided favorable tectonic environments for ore deposition. The selective preservation and tectonic recycling of CD deposits in Earth history has undoubtedly had a profound effect on the secular distribution of deposits, but are difficult to quantify. The CD Zn-Pb-Ag deposits became important at about 1.8 Ga, a time when seawater evaporative brines became oxidized and extreme redox contrasts developed in sedimentary sequences creating geochemical conditions favorable for the transport and deposition of the metals.

Traditionally these ores were thought to form from brines that exhaled into a water column in continental rift-sag basins (e.g., North Australia Zn Belt). However, recent observations and studies are providing new evidence that the ores form dominantly by sub-seafloor replacement processes (e.g., sulfide replacement of carbonate or organic matter) in clastic-dominated continental passive margins (e.g. Red Dog deposits, Alaska) or continental back arc basins (e.g. Meggen and Rammelsberg, Germany) but not in typical continental rift basins. The giant North Australia Zn Belt has long been the “type example” of CD ores in a continental rift-sag basin sequence. However, new interpretations of the hosting sequence and basin seismic profiles are consistent with a passive margin sequence or a marine-filled back arc basin. The only known CD deposit in a continental rift-sag basin is the Sullivan deposit in the Proterozoic Belt-Purcell Basin. However, the basin contained the critical CD ore forming elements characteristic of fertile passive margins: it was open to the ocean and was flanked by an evaporitic carbonate platform or brine factory.

The fundamental principle underlying the dominance of CD ores in passive margins is that the world’s greatest brine factories for CD ore fluids are seawater evaporative environments along marine-continental margins. This principle is illustrated by paleoplate reconstructions that show all of the known CD deposits hosted in rocks less than 1 billion years old formed in latitudes that mimic present-day deserts (less than about 40 degrees N and S; absence at equatorial latitudes).