The F’derik-Zouérate Iron District: Mesoarchean and Paleoproterozoic Iron Formation of the Tiris Complex, Islamic Republic of Mauritania

Abstract

High-grade hematitic iron ores (or HIF, containing 60–65 wt% Fe) have been mined in Mauritania since 1952 from Superior-type iron deposits of the F’derik-Zouérate district. Depletion of the high-grade ores in recent years has resulted in new exploration projects focused on lower-grade magnetite ores occurring in Algoma-type banded iron formation (or BIF, containing ca. 35 wt% Fe). Mauritania is the seventeenth largest iron producer in the world and currently has about 1.1 Gt of crude iron ore reserves. The main host for Algoma-type iron ore in the district is magnetite quartzite layers (formerly BIF) within Mesoarchean granulite-facies rocks of the Tiris Complex. Superior-type iron ores are restricted to the allochthonous Paleoproterozoic sequences of the Kediat Ijil and Guelb El Mhaoudat, which overlie the Tiris Complex. Paleoproterozoic BIF are present in the Sfariat belt that hosts at least three known iron occurrences, and at Guelb Zednes, all of which are interpreted as fragments of Superior-type BIF that were imbricated with, or allochthonously deposited on, the Mesoarchean-Paleoproterozoic suture zone during the Birimian orogeny. Prominent linear, high-amplitude magnetic anomalies associated with BIF are characteristic of the region. Paleoproterozoic rocks in the Kediat Ijil, a klippe of metasedimentary rocks including ferruginous chert capped by a distinctive conglomerate unit, and in the El Mhaoudat Range, produce very prominent, broad, high-amplitude magnetic anomalies that can be extended at least 150 km along strike projected beneath the Taoudeni Basin to depths of >2000 m. New petrographic, geochemical, and geochronologic data presented here elucidate several features of the iron deposits, their tectonic history, and possible processes of enrichment of BIF protore to HIF. The Zouérate district iron ores are remarkably pure, consisting almost wholly of hematite and quartz. Contents of all major elements other than iron and silica are well below global averages for hematitic iron ores; minor and trace element contents are similar to those of other deposits worldwide. However, Zouérate HIF shows a general depletion of REE, a positive Eu anomaly, and a preferential enrichment of HREE compared to LREE. U–Pb ages of detrital zircons in the Tazadit and Seyala Conglomerate Formations of the Kediat Ijil are consistent with their derivation from the Tiris Complex. This interpretation suggests a passive margin depositional environment on the northeastern edge of the Mesoarchean Rgueïbat Shield, unaffected by clastic input from early Birimian tectonic elements. In contrast, ages of detrital zircons from rocks faulted against the Mhaoudat Formation show depositional peaks consistent with input from Birimian and Neoproterozoic sources. These age data imply that tectonic emplacement of Guelb El Mhaoudat could have occurred as a result of Pan African orogenic events; such a Pan African record represents a previously unrecognized tectonic element in this part of the Rgueïbat Shield. Timing of enrichment of protore BIF to HIF is poorly constrained and may have resulted from multiple metamorphic-hydrothermal events from 2.83 to 1.6 Ga. Geochemical trends are consistent with interaction by a relatively high-temperature, oxidizing, and possibly alkaline fluid. A supergene weathering profile exists in the Zouérate district and began forming during uplift related to opening of the Atlantic Ocean at about 160 Ma, based on a new apatite fission track age. However, effects of this supergene enrichment are relatively minor, being superimposed on one or several metamorphic-hydrothermal events responsible for the transformation of BIF to HIF.