Dolostone-Limestone Transitions in the Hydrothermal Dolomitization System: Origin, Controls, and Evolution

Abstract

The processes governing the position and style of dolomitization fronts has been an enigmatic problem for geologists. Few studies have attempted to discuss their origin and there is still a gap in knowledge as to their geometry or the governing processes behind their morphology. This research utilizes three study locations which are situated in Alberta, Western Canada Basin; the Derbyshire Platform, UK, and the Western High Atlas, Morocco. Each location exhibits fault-controlled dolomitization forming discrete dolostone bodies, with saddle dolomite and hydrobrecciation observed. Combined depleted stable oxygen isotope, Eu enrichment and temperature obtained from fluid inclusion analysis (Th: >110^oC) suggest that dolomitization took place from hot, crustal fluids.Field studies indicate several distinct types of dolostone-limestone contact, associated with the presence of physical, depositional and diagenetic barriers, which are commonly scale independent. In this presentation, stratabound terminations where there are no distinct boundaries (i.e. fracture or stylolite) between the dolostone and limestone are described. Mineralogical and geochemical data from a number of transects across the mapped dolostone-limestone transitions show consistent trends toward the reaction fronts, including: (i) the dolomite becomes less stoichiometric and more calcium-rich; (ii) crystal zonation is better preserved in the replacive dolomite phase; (iii) cooler homogenization temperatures (Th); and (iv) heavier δ¹⁸O values (up to +6‰ compared to values in the centre of the body). The hypothesis is therefore tested that the genesis of dolomitization fronts is mainly controlled by the depletion of magnesium concentration as the dolomitizing fluids moved away from the fluid source. Although fluid temperatures are cooler at the reaction fronts than within the main dolostone body, the change in dolomite crystal texture suggests that this reflects later recrystallization of dolostone bodies inboard of the termination. In this scenario, subsequent fluxes of dolomitizing brine did not reach the termination point of the previous dolostone body, suggesting that the position of the dolomitization fronts retreated through time (i.e. the observed reaction fronts is the oldest one). Ultimately, this study may provide a new insight on how the dolostone bodies evolve and reaction fronts progress, through time in a hydrothermal dolomitization system.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019