Textural and chemical evolution during dedolomitization: A case study of the Benassal Formation, Maestrat Basin, Spain

Abstract

The process of dedolomitization (dolomite calcitization) has been the subject of several studies, but the mechanisms that control it are still not fully understood. Dedolomitization, i.e. the replacement of dolomite by calcite, is an important diagenetic process that affects dolostones, in certain cases decreasing the local porosity and altering the rock mechanical properties, reservoir quality and fluid flow pathways. In this contribution we report the petrographic and geochemical evidence of the progressive dedolomitization and its impact on the Lower Cretaceous dolostones of the Benicàssim area (Maestrat Basin, Spain) that host Mississippi Valley Type (MVT) ore deposits. Textural observations and geochemical data suggest that replacement was induced by the interface coupled dissolution-precipitation mechanism. Moreover, quantitative chemical content measurements with mass balance calculations show that dedolomitization caused a ∼5% gain of mass during the reaction, associated with 5 vol% of solid volume increase. The lack of microfracture development suggests that the compensation of such volume increase is achieved by microporosity being filled during calcite precipitation. Considering the meteoric origin of the dedolomitizing fluids, the mass balance calculation allows estimating the element mass transfer during the reaction. The fact that the fluid was enriched in Zn supports that the fluid was acidic, and likely percolated through the local MVT deposits. Calculation of the required volume of the reactive fluid (102–104 m3/m3 dolostone) also shows that thermodynamic calculations overestimate the volume of fluid. This study shows the importance of quantifying mass transfer in diagenetic conditions to better understand metasomatic processes and constrain the fluid pathway in the basin history. As a societal impact, this diagenetic process releases ∼11% of the carbon mass but seems to preserve all rare earth elements.