Simulation and sensitivity of isotopic alteration and vein development in carbonate rock formations

Abstract

Isotopic alteration of carbonate rocks during hydrothermal flow is common across a range of geological settings, can be used to identify areas of greater mineralization potential, and is a guide to mineral exploration. To understand potential controls on the variability of this isotopic alteration, we created a series of models to investigate parameter sensitivity. The resulting variograms for δ18O distribution closely matched the variograms for the initial porosity and demonstrate the importance of porosity in controlling isotopic variation. This result shows the utility of variograms for investigating the correlation between hydrologic parameters and isotopic distribution and vectoring toward ore deposits. The mass transfer variables between flowing and stagnant zones within the model were the strongest secondary control on isotopic alteration. The alteration of the carbonate rocks releases 18O, creating a pulse of enriched fluid that flows through the system. The isotopic variation of the solid phase decreases with distance from the inlet because of mixing and the increased size of the 18O-enriched pulse that travels through the system within the fluid phase. The isotopic pulse seen in the model matches isotopic records preserved in zoned calcites from the Upper Mississippi Valley. We also inferred from the model that when sampling isotopic systems, sample intervals taken from large intervals of core can reasonably be used to determine the average alteration but that characterizing the variation is sensitive to sample spacing and frequency.