The REE composition of the hydrothermal carbonates of the San Vicente Mississippi Valley-type district were used to constrain the path-way of the ore fluid and changes in the Eh-pH conditions during mineralization. The ore-stage dark replacement dolomite and white sparry dolomite are depleted in REE compared to the host dolomite. They display negative Ce anomalies. These results suggest that the incoming ore fluid was slightly oxidizing and acidic and poor in REE, which implies that the fluid flow was through a main aquifer, likely the detrital units at the base of the basin and had limited interaction with the carbonate host rocks. The alteration of the native organic matter by the incoming fluid provides the local reducing conditions for thermochemical sulfate reduction (and reduction of Eu+3 to Eu2+). The REE encrichment and the positive Eu anomaly of the late stage vug-filling carbonates indicate that the post-ore "residual" acidic fluid was again oxidizing due to continiuous influx of fresh basinal waters.