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
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).