SR-ISOTOPE, C-ISOTOPE AND O-ISOTOPE SYSTEMATICS IN THE PUCARA BASIN, CENTORAL PERU - COMPARISON BETWEEN MISSISSIPPI VALLEY-TYPE DEPOSITS AND BARREN AREAS

A combined Sr, O and C isotope study has been carried out in the Pucar a basin, central Peru, to compare local isotopic trends of the San Vic ente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, l imestone and regional replacement dolomite yield Sr-87/Sr-86 ratios th at 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 Hett angian and the Toarcian may extend to lower Sr-87/Sr-86 ratios than pr eviously published values. An Sr-87-enrichment is noted in (1) carbona te rocks from the lowermost part of the Pucara basin, and (2) differen t carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lowermost part of the carbonate seque nce interacted with Sr-87-enriched fluids. The fluids acquired their r adiogenic nature by interaction with lithologies underlying the carbon ate rocks of the Pucara basin. The San Ramon granite, similar Permo-Tr iassic intrusions and their elastic derivatives in the Mitu Group are likely sources of radiogenic Sr-87. The Brazilian shield and its erosi on products are an additional potential source of radiogenic Sr-87. Vo lcanic rocks of the Mitu Group are not a significant source for radiog enic Sr-87; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic Sr-87. T he marked enrichment in Sr-87 of carbonates toward the lower part of t he Pucara Group is accompanied by only a slight decrease in delta(18)O values and essentially no change in delta(13)C values, whereas replac ement dolomite and sparry carbonates at the MVT deposits display a coh erent trend of progressive Sr-87-enrichment, and O-18- and C-13-deplet ion. The depletion in O-18 in carbonates from the MVT deposits are lik ely related to a temperature increase, possibly coupled with a O-18-en richment of the ore-forming fluids. Progressively lower delta(13)C val ues throughout the paragenetic sequence at the MVT deposits are interp reted as a gradually more important contribution from organically deri ved carbon. Quantitative calculations show that a single fluid-rock in teraction model satisfactorily reproduces the marked Sr-87-enrichment and the slight decrease in delta(18)O values in carbonate rocks from t he lower part of the Pucara Group. By contrast, the isotopic covariati on trends of the MVT deposits are better reproduced by a model combini ng fluid mixing and fluid-rock interaction. The modelled ore-bearing f luids have a range of compositions between a hot, saline, radiogenic b rine that had interacted with lithologies underlying the Pucara sequen ce and cooler, dilute brines possibly representing local fluids within the Pucara sequence. The composition of the local fluids varies accor ding to the nature of the lithologies present in the neighborhood of t he different MVT deposits. The proportion of the radiogenic fluid in t he modelled fluid mixtures interacting with the carbonate host rocks a t the MVT deposits decreases as one moves up in the stratigraphic sequ ence of the Pucara Group.