Le minerai de fer d'Hajigak (Afghanistan). Position stratigraphique, cadre géologique et type du gisement

Abstract

Age and grade of metamorphism of the considerable iron deposit of Hajigak (Central Afghanistan) have been controversial until now. A recent field study on the eastern part of the district produced new data concerning the geological environment and paragenesis of the ore. The iron beds are situated in the Upper Kalu Formation (= Awband Suite) of lower Paleozoïc, may be Silurian to Lower Devonian age. Basic lavas and tuffs, converted into green-schists, always are present near the ore bodies; and straight relationship is evident between basic rocks and iron ore. We consider the succession of events at Hajigak to be as follows: — at the bottom of a Paleozoïc sedimentary basin, an andesitic submarine volcanism brought along tuffs, lavas, and sericitic and chloritic sediments of exhalative origin; in the same time, hematite and magnetite precipitated. — afterwards gabbro and dolerite sills were intruded; as a result, iron oxides were remobilized, filling fractures across gabbro boundaries, and replacing neighbouring dolomites. — at last, but before Upper Devonian, Lower Paleozoïc rocks of Hajigak and Turkman area were affected by two kinds of metamorphism: the first one, of green-schist facies, is contemporaneous with magmatic activity; the second one, sericitic dynamo-metamorphism, appeared when the folding took place, mainly in the eastern region (Khesh and Zerok areas). In conclusion, the Hajigak iron ore appears to be a submarine — exhalative deposit, synchronous with a Lower Paleozoïc basic volcanic activity (but non ophiolitic).