The influence of fluid pressure, redox potential and crystal growth characteristics in Mississippi-Valley-Type (MVT) ore formation - lessons from a modern geothermal scale

Studying the formation of ore deposits is normally only indirectly possible. However, scales in geothermal power plants provide a rare opportunity to study mineral formation and element enrichment almost in-situ under monitored conditions within a short time period. The studied geothermal system in the Upper Rhine Graben (Germany) provides physical and chemical fluid parameters similar to ore-forming fluids of MVT deposits. Varying temperatures and pressure conditions during an injection test with a perforated screen caused the growth of a carbonate-dominated scale with sulfide minerals. Major growth occurred during periods with reduced fluid pressure, allowing for CO₂ effervescence and carbonate super-saturation. Operation conditions with minor variations led to a very slow growth and a switch in redox conditions, causing sulfide mineralization and visible lamination. Under standard operation conditions (injection without perforated screen) no scale formation is observed. Specific scale zones show element associations characteristic for MVT deposits that include domains with significantly elevated Zn and Pb concentrations (enrichment by up to a factor 100 for Pb). The existence, concentration and interplay of redox agents determine the mineral to be precipitated and related element enrichment. Transferring the observation to natural systems, MVT ore mineralization can be triggered by a rapid decrease in pressure in the order of several bars and related degassing/effervescence with changes in the redox potential, resembling the suction pump model combined with a hydrocarbon-assisted redox change.