The Proterozoic eon (c.2500 to 600 million years ago) encompasses more than 40% of Earth's history, a time during which fundamental atmospheric and oceanic changes occurred, biological diversity increased, and modern-style plate tectonics evolved. A disproportionately large share of the world's metallic resources occur in sedimentary basins of this age. World-class lead-zinc-silver deposits found in eastern Australia, British Columbia, and Montana; copper-silver deposits of Zambia, Michigan, and Montana; banded iron formations located in Michigan, Minnesota, and Canada; and iron-oxide copper-uranium-gold-silver-rare earth element deposits like Olympic Dam, Australia are found in these basinal strata. This project aims to improve fundamental understanding of the relationship between the evolution of the Proterozoic basins and the generation of associated metal deposits.
Our approach is to look holistically at the sedimentary basin in which the ore deposits are found. Ultimately we hope to track the origin of the ore deposit components from their site of residence within minerals in the basinal strata into the ore fluids, follow the effects of the migrating ore fluids through the basin aquifers, and understand the processes that resulted in precipitation of the ore deposit from the ore fluids. We will look at this fluid transfer process in the context of the evolving fill of the basin, its depositional unit geometries, their patterns of permeability, and its compaction, cementation and thermal maturation.
Some of the important questions that will need to be answered
- Heat source - (magmatism, geothermal gradient) What is the relative timing of magmatism, fluid migration and ore deposition? What constraints on tectonic setting of the basin and the nature of the underlying crust are provided by the chemistry of basin magmatism ? What was the geothermal gradient and how does that correlate with the tectonic setting of the basin?
- Nature of the basinal fluids - Much has been learned by the petroleum industry about the evolution of basin fluids and its relation to the generation and migration of hydrocarbons. Can we apply similar principles to the mobilization, transport and deposition of metals in sedimentary basins? How do the fluids evolve and what processes produce the metalliferous basinal fluids?
- Source of deposit components- Some of the sources of required components typically called for in both the sedex (intrabasinal evaporates as a source of brine Cl) and the sedimentary rock-hosted copper models (interbedded subaerial mafic volcanics) are not known for the world class deposit examples in the Belt basin. Copper deposits in the mid-continent rift formed in a sulfur deficient environment, leading to different trapping mechanisms than suggested in existing models. What can we learn from the differences in the origins of these deposits from the general models?
- Fluid pathways - In both deposit types, metal-bearing hydrothermal fluids are thought to move through stratabound aquifers and cross-cutting syn-depositional structures. Can we track the pathways of the ore fluids and their components? What features of basin evolution (subsidence or deformation patterns, depositional fill geometries, etc.) governed the fluid pathways? How important were these pathways in the derivation of deposit components and in the precipitation of components from fluids?
- Structural features - Can syndepositional structural features be documented and what role do they play in the movement and focusing of mineralizing fluids? Can available geophysical data (seismic, gravity, magnetic, other) be used to help define deeper tectonic controls and preexisting crustal features, as well as to help restore the effects of later deformational features? What is the influence of growth faults and crustal thickening (folds, thrust faults) in creating fluid flow gradients?
- Ore precipitation processes - Typically movement of basinal ore fluids into contrasting geochemical or geothermal environments results in precipitation of its dissolved metal load. This can result from release of the basinal fluid into air or into standing fresh or salt water, movement of the basinal fluid into an aquifer whose mineralogy changes the geochemistry of the basinal fluid, or mixing of the metal-bearing basinal fluid with other basinal fluids. To what extent are mineralizing fluids buffered by lithology and influenced by flow rates? What are the factors that lead to mineable grades and tonnages of basin-derived ore deposits?