Elsevier

Ore Geology Reviews

Volume 149, October 2022, 105105
Ore Geology Reviews

Discovery of antiskarn-hosted strategic metal mineralization in the Upper Cretaceous Twihinate carbonatite intrusion (West African Craton Margin, Moroccan Sahara)

https://doi.org/10.1016/j.oregeorev.2022.105105Get rights and content
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Highlights

  • The Upper Cretaceous Twihinate carbonatite is mineralogically and chemically classified as calcite carbonatite (sövite).

  • The metasomatic interaction between the carbonatite melts and the silicate wall-rocks led to development of antiskarn rocks and related Nb-P-Fe-LREE ± U ± Th-bearing mineralization.

  • The antiskarn rocks are interpreted to have formed as a result of silica contamination of the carbonatite melt by the Silurian-Devonian granitic country rocks.

  • The antiskarn paragenesis may be used as reliable exploration metallotect to target prospective occurrences in the search for critical metals.

Abstract

The Upper Cretaceous Twihinate carbonatite in Moroccan Sahara, which is the focus of ongoing exploration for Nb-P-Fe ± LREE ± U-Th resources, consists of a crescent-like intrusion made of a central sövite body encircled by an annular ring of vuggy siliceous breccia. Drill core logging coupled with petrographic observation, whole-rock and mineral chemistry reveal that the metasomatic interaction between the carbonatite melt and the silicate wall-rocks led to development of calc-silicate paragenesis within the sövite itself at depths ranging from ∼60 to >200 m from the surface for which the term antiskarn is attributed. We provide the first petrographic and textural descriptions, and report the whole-rock major- and trace-element geochemistry and chemical compositions of the main antiskarn-forming minerals and related strategic metal-bearing ore. Geochemically, the antiskarn lithotypes are characterized by a compositional range of 7–17 wt% SiO2, 25–72 wt% CaO, 1–2 wt% Al2O3, and 12–26 wt% loss on ignition, and enrichment in large-ion lithophile elements (LILE), particularly Sr (3125–7018 ppm ppm), Ba (393–1300 ppm), U (10–787 ppm) and light REEs (LREEs) (1058–1569 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, Ta and Hf. Nb and V show, however, much higher concentrations ranging from 140 ppm to >1000 ppm, and 603 to 1008 ppm, respectively. Mineralogically, the antiskarn lithotypes consist predominantly of diopsidic (Di66-82 Hd18-34) to aegirine-augite clinopyroxene (Di38-55 Ae21-35) and andradite (And86-94 Gro1-9) with subordinate amounts of micas (biotite/phlogopite) and titanite. The paragenetically later mineral assemblages involve amphibole, chlorite, epidotes, baryte, fluorite, kaolinite, carbonates and quartz. The main Nb-P-Fe ± LREE ± U-Th ore minerals consist of Fe oxides (titanomagnetite principally), pyrochlore group minerals, phosphates (apatite, monazite-(Ce)), and REE-fluorcarbonates (bastnäsite-(Ce)) with subordinate sulfides (chalcopyrite, sphalerite, and galena), fluorite and baryte. Development of the antiskarn calc-silicate paragenesis and related strategic metal-bearing mineralization is consistent with derivation from a carbonatite melt through assimilation of silicate country rocks including the Silurian-Devonian gneissic granitoid of the Laknouk suite. From an economic perspective, the widespread occurrence of high-Ti (0.7–16. 9 wt% Ti) and V- (0.2–0.7 wt% V) magnetite opens new opportunities in regard of exploration for V; an emerging strategic commodity whose presence has to be reassessed in the Twihinate prospect. The currently described antiskarn paragenesis may be therefore used as reliable exploration metallotect to target prospective occurrences in the search for critical metals including V as by product.

Keywords

Carbonatite
Strategic metals
Antiskarn
Silica contamination
Moroccan Sahara

Data availability

Data will be made available on request.

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