Mississippi Tialley-type zinc-lead deposits and ore occurrences in the
San Vicente belt are hosted in dolostones of the eastern Upper Triassic
to Lower Jurassic Pucara basin, central Peru. Combined inorganic and
organic geochemical data from 22 sites, including the main San Vicente
deposit, minor ore occurrences, and barren localities, provide better
understanding of fluid pathways and composition, ore precipitation
mechanisms, Eh-pH changes during mineralization, and relationships
between organic matter and ore formation. Ore-stage dark replacement
dolomite and white sparry dolomite are Fe and rare earth element (REE)
depleted, and Mn enriched, compared to the host dolomite. In the main
deposit, they display significant negative Ce and probably Eu anomalies.
Mixing of an incoming hot, slightly oxidizing, acidic brine (H2CO3 being
the dominant dissolved carbon species), probably poor in REE and Fe,
with local intraformational, alkaline, reducing waters explains the
overall carbon and oxygen isotope variation and the distributions of REE
and other trace elements in the different hydrothermal carbonate
generations. The incoming ore fluid flowed through major aquifers,
probably basal basin detrital units, with limited interaction with the
carbonate host rocks. The hydrothermal carbonates show a strong regional
chemical homogeneity, indicating access of the ore fluids by
interconnected channelways near the ore occurrences. Negative Ce
anomalies in the main deposit, that are absent at the district scale,
indicate local ore-fluid chemical differences. Oxidation of both
migrated and indigenous hydrocarbons by the incoming fluid provided the
local reducing conditions necessary for sulfate reduction to H2S,
pyrobitumen precipitation, and reduction of Eu3+ to Eu2+. Fe-Mn
covariations, combined with the REE contents of the hydrothermal
carbonates, are consistent with the mineralizing system shifting from
reducing/rock-dominated to oxidizing/fluid-dominated conditions
following ore deposition. Sulfate and sulfide sulfur isotopes support
sulfide origin from evaporite-derived sulfate by thermochemical organic
reduction; further evidence includes the presence of C-13-depleted
calcite cements (similar to-12 parts per thousand delta(13)C) as sulfate
pseudomorphs, elemental sulfur, altered organic matter in the host
dolomite, and isotopically heavier, late, solid bitumen. Significant
alteration of the indigenous and extrinsic hydrocarbons, with absent
bacterial membrane biomarkers (hopanes) is observed. The light
delta(34)S of sulfides from small mines and occurrences compared to the
main deposit reflect a local contribution of isotopically light sulfur,
evidence of local differences in the ore-fluid chemistry.