| dc.description.abstract | As the world shifts to clean energy technologies, to reduce greenhouse gas emissions, there is increasing demand for the metals and minerals needed to produce them. Rare earth elements (REE) are one of the key raw materials in the clean energy sector. Most of the global REE are sourced from China, however, more deposits are urgently needed to satisfy the increasing needs of industry. Carbonatites host the world’s largest resources of REE. This research is focussed on carbonatites found in south-eastern Zambia to better understand their nature and petrogenetic evolution.
Historically, seven carbonatite complexes have been identified in Zambia: four in the Rufunsa valley near the border with Mozambique, Kesya and Mkwisi to the south-east of Lusaka, the capital of Zambia, and Nkombwa Hill in the north near the border with Tanzania. The carbonatites in Rufunsa are Chasweta, Mwambuto, Nachomba and Kaluwe Hills. Although Kaluwe is the biggest carbonatite body in Zambia, the source of this vast apron of volcaniclastic material has never been established. To address this, field studies, petrography and electron beam mineral chemical studies, whole-rock chemistry and stable isotope analyses are used to determine the nature of these igneous rocks and to construct a new geomodel for the Rufunsa valley carbonatites. In the past, there has been debate as to whether the Kesya and Mkwisi carbonatites are bona fide carbonatites; addressing this is one of the main aims of the current study.
The Rufunsa carbonatites were intruded at the intersection of NE-SW trending mid-Zambezi – Luangwa rift and the SE-NW trending lower-Zambezi Transform Fault. In the Rufunsa valley, the five main carbonatite generations identified are: dolomite carbonatite, calcite carbonatite, ferroan dolomite carbonatite, juvenile dolomitic melt and silicified carbonatites. Intrusive dolomite carbonatite magmatism is considered to be the earliest carbonatite generation. Calcite carbonatite, like dolomite carbonatite, is considered to be magmatic in origin. It is this calcite carbonatite that ultimately constitutes the calcite carbonatite lavas at Kaluwe Hill and Chasweta carbonatite volcano. This study is the first to identify the presence of calcite carbonatite lavas at Rufunsa carbonatite volcanoes, where previously the only form of effusive rocks reported were dolomitic agglomerates. This research has provided the first field, petrographic and geochemical evidence showing that calcite carbonatite at Rufunsa crystallised from parental dolomitic magma. These findings support experimental models that indicate that parental magmas for carbonatites can be dolomitic in composition.
The third type of carbonatite generation is ferroan dolomite carbonatite which is the predominant carbonatite in the Rufunsa volcanic carbonatite centres. This carbonatite is considered to be hydrothermal in origin.
The juvenile dolomite carbonatite which was the last to be intruded is thought to have been directly derived from the mantle. It was intruded during a series of fluidised diatreme eruptions that were laden with mantle-derived chromium spinel. The juvenile dolomite carbonatite is regarded here as closest in composition to parental mantle melt. The silicified carbonatites that occur in all the carbonatite centres were extensively modified by weathering.
This research has for the first time provided evidence for the Uma depression in the Rufunsa valley being a large, weathered carbonatite caldera, and that eroded material from this was deposited to form the Kaluwe Hill carbonatite.
The petrogenesis of Kesya and Mkwisi carbonatites was a matter of debate because of their unusual mineralogical, geochemical and Sr isotopic compositions. Stable C and O isotope data for these two carbonatites plot outside the ranges of what is considered typical for primary igneous carbonatites. In addition, these carbonatites have the lowest δ18O values reported for any carbonatite in the world. The extreme low values of δ18O have been attributed to the interaction of the carbonatites with externally infiltrating H2O-rich fluids during regional metamorphism associated with the Neoproterozoic Pan African orogeny. Apatite U-Pb geochronology of the two carbonatites indicate Neoproterozoic ages, Kesya is 535±16 Ma while Mkwisi is 569±19 Ma. This is the first time that stable C and O isotope and geochronological studies have been conducted on the Kesya and Mkwisi carbonatites. The Kesya and Mkwisi carbonatites are proposed here to be part of the regional Neoproterozoic kimberlite and carbonatite tectono-magmatic events associated with supercontinent assembly and/or fragmentation. | en_GB |
