HYDROTHERMAL DOLOMITIZATION: NEW TECHNIQUES SHED LIGHT ON AN OLD PARADIGM

Since the turn of the century, hydrothermal dolomite (HTD) has been of great interest because it is easily recognised in outcrop. It records evidence of paleo-fluid flow at multiple scales (centimeter to kilometer) and has perceived value as hydrocarbon reservoirs or as hosts for low temperature economic mineralisation. Over the last two decades, there has been a marked shift in our understanding of the processes that govern the genesis of HTD bodies. For example, several studies have shown that HTD can form at very shallow depths in many sedimentary basins, soon after deposition, so HTD is not necessarily indicative of ‘deep burial’ conditions. The tectono-stratigraphic framework of the basin in which HTD is encountered is critical to prediction of how the dolomitizing fluid(s) flowed and reacted with the succession. There is still some uncertainty regarding the source of Mg²⁺ and the dolomitizing fluid, but fluid source(s) can now be robustly fingerprinted through the integration of geochemical proxies. Therefore, it is possible to do more than simply attribute dolomitization to generic and poorly defined ‘basinal brines’. Empirical data and numerical modelling has shown that seawater is a far more important agent in the formation of HTD than previously recognized, particularly when mixed with crustal fluids that have increased potency due to fluid-rock interaction (e.g., carbonation of mafic and ultramafic rocks).

In this presentation, the Western Canadian Sedimentary Basin is used as an exemplar to demonstrate how HTD bodies form during multiple phases of dolomitization, cementation, and recrystallization. The interpretations are underpinned by basin-scale field mapping, hyperspectral imaging, high-resolution petrography and geochemistry, coupled with reactive transport modelling. By the integration of experimental geomechanical data and multi-proxy geochemical data, we also show how the timing and mechanism of dolomitization influences the occurrence of rock textures that are often considered to be diagnostic of HTD, such as zebra textures and dolomite breccias. These new ideas are highly relevant to our understanding of hydro-geochemical processes in sedimentary basins, which are of critical importance to mineral exploration, carbon sequestration, and geothermal heat production.