Abstract
The Angolan alkaline–carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected.
Highlights
The Monte Verde complex presents an association of alkaline rocks and carbonatitic and alkaline magmatic breccias
The magmatic breccias draw circular outcrops around a central alkaline plug, which could be indicative of the existence of a collapse caldera structure of nearly 5 km radius, partly covered by colluvial sediments
Based on field and textural evidence, we conclude that the intrusions of the alkaline breccias and dykes were produced later than that of the carbonatite breccias
Summary
Research has been done on the latter as a potential source for critical elements, such as REE [9,10,11,12,13,14,15,16], Nb [17], and Be [18,19,20,21]. This has led to an increased interest in alkaline magmatic terrains due to their association with carbonatites enriched in critical elements (e.g., [8,22]). Magmatic breccias are set into the alkaline rocks, Minerals 2020, 10, 5; doi:10.3390/min10010005 www.mdpi.com/journal/minerals
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