A combined Sr, O and C isotope study has been carried out in the Pucara
basin, central Peru, to compare local isotopic trends of the San Vicente
and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with
regional geochemical patterns of the sedimentary host basin. Gypsum,
limestone and regional replacement dolomite yield Sr-87/Sr-86 ratios
that fall within or slightly below the published range of seawater
Sr-87/Sr-86 values for the Lower Jurassic and the Upper Triassic. Our
data indicate that the Sr isotopic composition of seawater between the
Hettangian and the Toarcian may extend to lower Sr-87/Sr-86 ratios than
previously published values. An Sr-87-enrichment is noted in (1)
carbonate rocks from the lowermost part of the Pucara basin, and (2)
different carbonate generations at the MVT deposits. This indicates that
host rocks at MVT deposits and in the lowermost part of the carbonate
sequence interacted with Sr-87-enriched fluids. The fluids acquired
their radiogenic nature by interaction with lithologies underlying the
carbonate rocks of the Pucara basin. The San Ramon granite, similar
Permo-Triassic intrusions and their elastic derivatives in the Mitu
Group are likely sources of radiogenic Sr-87. The Brazilian shield and
its erosion products are an additional potential source of radiogenic
Sr-87. Volcanic rocks of the Mitu Group are not a significant source for
radiogenic Sr-87; however, molasse-type sedimentary rocks and
volcaniclastic rocks cannot be ruled out as a possible source of
radiogenic Sr-87. The marked enrichment in Sr-87 of carbonates toward
the lower part of the Pucara Group is accompanied by only a slight
decrease in delta(18)O values and essentially no change in delta(13)C
values, whereas replacement dolomite and sparry carbonates at the MVT
deposits display a coherent trend of progressive Sr-87-enrichment, and
O-18- and C-13-depletion. The depletion in O-18 in carbonates from the
MVT deposits are likely related to a temperature increase, possibly
coupled with a O-18-enrichment of the ore-forming fluids. Progressively
lower delta(13)C values throughout the paragenetic sequence at the MVT
deposits are interpreted as a gradually more important contribution from
organically derived carbon. Quantitative calculations show that a single
fluid-rock interaction model satisfactorily reproduces the marked
Sr-87-enrichment and the slight decrease in delta(18)O values in
carbonate rocks from the lower part of the Pucara Group. By contrast,
the isotopic covariation trends of the MVT deposits are better
reproduced by a model combining fluid mixing and fluid-rock interaction.
The modelled ore-bearing fluids have a range of compositions between a
hot, saline, radiogenic brine that had interacted with lithologies
underlying the Pucara sequence and cooler, dilute brines possibly
representing local fluids within the Pucara sequence. The composition of
the local fluids varies according to the nature of the lithologies
present in the neighborhood of the different MVT deposits. The
proportion of the radiogenic fluid in the modelled fluid mixtures
interacting with the carbonate host rocks at the MVT deposits decreases
as one moves up in the stratigraphic sequence of the Pucara Group.