Alteration Features in Natural Zirconolite from Carbonatites

Abstract

In nature, zirconolite occurs as an accessory mineral in many different rock types, but the majority of reported occurrences are from carbonatites (magmatic carbonates) of geological age varying from a few million years to 2 billion years old. Within these 19 carbonatite occurrences, of which 15 have been studied in some detail, zirconolite displays varying degrees of alteration in six samples. This alteration ranges from incipient minor effects to major corrosion, recrystallization and complete replacement by secondary phases. The degree of alteration broadly correlates with either the age, or actinide content of the zirconolite (or both), and thus the extent and degree of metamictization. Changes in zirconolite composition with alteration include an increase in hydration (H2O), Si, Ba and Pb (possibly radiogenic in origin), and a decrease primarily in Ca and Fe. Th can be remobilized, and of the rare earth elements (REE), there is evidence that the heavy-REE are mobilized more readily than the light-REE. Using backscattered electron images and electron microprobe analyses, this study documents and illustrates the range of alteration features observed in zirconolite from several carbonatites, in terms of both compositional and textural changes, and provides some physico-chemical information on the fluids responsible for the alteration.

Role of geochemical alteration on the formation of secondary Zr- and U-bearing minerals in El Atshan trachyte, central Eastern Desert, Egypt

“…However, some grains were found to have an irregular shape (Fig. 5) A similar unidentified secondary Zr -rich silicate mineral has been identified by Williams et al (2001). They interpreted it as being an alteration product of zirconolite, which occurs at Cummins Range, Sokli, Phalabora, and Sebl'yavr (Table 1).…”

“…In these examples, zirconolite grains have been altered to form unidentified secondary Zr -rich silicate minerals of variable composition, but which are all hydrated, enriched in Ba and Si, and depleted in Ca and Fe. These minerals are more abundant in U than in the precursor zirconolite, suggesting that these elements were mobilized and transported by magmatic -derived fluids enriched in CO 2 , P, F, Na, and K, and subsequently deposited with the alteration products (Williams et al, 2001). Figure 7a shows the REE distribution patterns in the unidentified Zr -rich silicate mineral.…”

“…Several other examples of zirconolite alteration have been presented by Hart et al (1998), but none of these appear to involve significant losses of actinide elements. Williams et al (2001) have studied zirconolite from 19 carbonatite occurrences. Zirconolite displays varying degrees of alteration that ranges from incipient minor effects to major corrosion, recrystallization, and complete replacement by an unidentified actinide -rich (ACT), Ba, Ti, Zr, and Nb silicate phases.…”

Pb/Pb geochronology, petrography and chemistry of Zr-rich accessory minerals (zirconolite, tranquillityite and baddeleyite) in mare basalt 10047

“…terrestrial occurrences, where both minerals are susceptible to alteration to fine-grained secondary zircon by late-stage magmatic fluids or younger metamorphic fluids (Heaman and LeCheminant, 1993;Williams et al, 2001). In contrast, many tranquillityite crystals display evidence of alteration to zirconolite, baddeleyite, ilmenite and fayalite.…”

New insights into the origin of the Evate apatite-iron oxide-carbonate deposit, Northeastern Mozambique, constrained by mineralogy, textures, thermochronometry, and fluid inclusions

“…Nb-rich zirconolite and baddeleyite are minerals typical of phoscorites, although they may also occur in mantlederived mafic magmas and genetically associated metasomatic skarns (e.g. Williams 1996, Williams et al 2000, Zaccarini and Stumpfl 2004, Rajesh et al 2006, Pascal et al 2009). …”