Geochemical alteration of pyrochlore group minerals; pyrochlore subgroup

Abstract

Primary alteration of uranpyrochlore from granitic pegmatites is characterized by the substitutions ADYD-+ ACaYO,ANaYF-+ ACaYO,and ANaYOI-I--+ ACaYO.Alteration occurred at 450-650 °C and 2-4 kbar with fluid-phase compositions characterized by relatively low aNa+,high aeaH, and high pH. In contrast, primary alteration of pyrochlore from nepheline syenites and carbonatites follows a different tre:nd represented by the substitutions ANaYF -+ ADYDand ACaYO-+ ADYD.In carbonatites, primary alteration of pyrochlore probably took place during and after replacement of diopside + forsterite + calcite by tremolite + dolomite :t ankerite at 300-550 °C and 0-2 kbar under conditions of relatively low aHF,low aNa+,low aeaH, low pH, and elevated activities of Fe and Sr. Microscopic observations suggest that some altered pyrochlor1es are transitional between primary and secondary alteration. Alteration paths for these specimens scatter around the trend ANaYF-+ ADYD.Alteration probably occurred at 200-350 °C in the presence of a fluid phase similar in composition to the fluid present during primary alteration but with elevated activities of Ba and REEs. Mineral reactions in the system Na-Ca-Fe-Nb-O-H indicate that replacement of pyrochlore by fersmite and columbite occurred at similar conditions with fluid conpositions having relatively low aNa+,moderate aeaH, and moderate to high aFeH.Secondary alteration « 150 °C) is charactlerizedby the substitutions ANaYF-+ ADYD,ACaYO-+ ADYD,and ACaXO-+ ADXDtogether with moderate to extreme hydration (10-15 wt% H20 or 2-3 molecules per formula unit). Minor variations in the amounts of Mg, AI, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result of secondary alteration. Major cation exchange for K, Sr, and Ba is a feature of samples from laterite horizons overlying carbonatites. In most cases U, Th, and B-site cations remain relatively constant. Radiogenic Pb is typically lost via long-te:rm diffusion, but in some grains of uranpyrochlore 25-90% of the Pb is lost as a result of alteration.