Pyrochlore in Carbonatites, a Key to Understanding Element Mobility in Carbonatite Complexes?

Sammanfattning

Critical raw materials are more relevant now than ever before, and among these, Niobium and rare earth elements have particularly sensitive supply chains. Most of the world’s Niobium originates from the Alto Paranaiba igneous province of Brazil, where it is found in the mineral Pyrochlore. Pyrochlore (A2B2X6Y, ideally NaCaNb2O6F) is an accessory mineral to carbonatites, a class of igneous rock defined by its low silica content. Oldoinyo Dili and Alnö, two different carbonatite complexes, also host pyrochlore, where it is commonly found in intrusive Sövite dykes. The pyrochlore from these locations display two different zonation patterns, one oscillating and one patchy. The pyrochlores from Oldoinyo Dili and Alnö are previously unstudied, but pyrochlore from other similar locations has proven a key indicator for both primary magmatic and secondary hydrothermal processes. This thesis aims to quantify the element composition of both zonation patterns, and thus understand the mechanism creating them, and further compare the zonation across two different carbonatite complexes. Through Electron Probe Microanalysis, the compositional variation of elements was obtained. The oscillation proved to be partly silica based, where the pyrochlore crystals are theorized to sporadically move through a chemically stratified melt, with pH being an important factor, as silica is heterogeneously incorporated from the country rock. This mechanism is consistent across both complexes. The patchy zonation proved to be entirely devoid of Ba, Sr and REE, where the surroundings are completely devoid of Na, Ca and F. The patchy zonation is therefore theorized to originate from hydrothermal alteration, where Ba, Sr and REE rich fluids flush through the pyrochlore crystals, leaching Na, Ca and F. This leads to small patches of its original Na, Ca and F rich composition remaining while their surroundings are altered to a Sr, Ba, REE rich composition. Interestingly, the REE pattern reverses from Oldoinyo Dili to Alnö, where REE is found enriched in its original composition. This is thought to be partly because of lower fluid temperatures, and partly due to the composition of the original melt being richer in REE. These findings provide insight into how elements move through a carbonate intrusion, both during crystallization, and later hydrothermal alterations, making pyrochlore a key component in understanding these systems.