Abstract
Stratiform, sediment-hosted, base metal deposits in the Aravalli-Delhi orogenic belt of northwestern India are hosted by the sediment-dominated Bhilwara, Aravalli and Delhi sequences of Proterozoic age. Pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information. Model Pb ages using a newly developed ‘Proterozoic model’ are ∼ 1800 Ma for the Rampura-Agucha, Rajpura-Dariba and Saladipura deposits, ∼ 1700 Ma for the Zawar deposits and ∼ 1100 Ma for the Ambaji and Deri deposits. Mineralization in this orogenic belt thus occurred at three stages in Middle to Upper Proterozoic times. The oldest of these, reflected by deposits in the Bhilwara sequence, was of broad regional extent. The Rampura-Agucha deposit is thus hosted by highly metamorphosed rocks that belong to the Bhilwara belt and not to an older supracrustal component of the Archean Banded Gneissic complex. The host rocks to the Saladipura deposit are Bhilwara-equivalent and are not part of the Delhi Supergroup, as previously mapped, and therefore stratigraphic assignments in the area must be re-examined. Model ages of ∼ 1100 Ma for the Ambaji-Deri deposits are the only evidence obtained in this study regarding the age of the Delhi Supergroup, and are in apparent conflict with much older ages indicated by previous studies. Rocks of such late Upper Proterozoic age may be restricted to the southwestern segment of the orogenic belt, or, alternatively, thrust stacking and/or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available. All the data can be readily explained by the extraction of Pb from sediments derived from an upper-crustal basement. The isotopic compositions of the Zawar deposits provide the clearest evidence for an ancient, U-enriched, upper-crustal source. The Ambaji-Deri Pb data suggest a mature status of the Delhi are when these deposits were formed.
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