Sr, C and O isotope systematics in the Pucará Basin, central Peru: Comparison between Mississippi Valley-type deposits and barren areas

A combined Sr, O and C isotope study has been carried out in the Pucará 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 ⁸⁷Sr/⁸⁶Sr ratios that fall within or slightly below the published range of seawater ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr ratios than previously published values. An ⁸⁷Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with ⁸⁷Sr-enriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic ⁸⁷Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic ⁸⁷Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic ⁸⁷Sr: however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic ⁸⁷Sr. The marked enrichment in ⁸⁷Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in δ¹⁸O values and essentially no change in δ¹³C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive ⁸⁷Sr-enrichment, and ¹⁸O- and ¹³C-depletion. The depletion in ¹⁸O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a ¹⁸O-enrichment of the ore-forming fluids. Progressively lower δ¹³C 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 ⁸⁷Sr-enrichment and the slight decrease in δ¹⁸O values in carbonate rocks from the lower part of the Pucará 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 Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará 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 Pucará Group.