The rare earth element (REE) composition of the hydrothermal carbonates
of the San Vicente Mississippi Valley-type district, hosted in the
Triassic-Jurassic dolostones of the Pucara Group (central Peru), 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 Eu+2). The REE enrichment 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 continuous influx
of fresh basinal waters.