Evidence for dissolution-reprecipitation of apatite and preferential LREE mobility in carbonatite-derived late-stage hydrothermal processes

Abstract

The Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an "original" euhedral apatite, "turbid" apatite, and "overgrowths" of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F-and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REEfluorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl 2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals.

Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, 87Sr/86Sr) data from Kangankunde, Malawi

“…However, combining the field and mineralogical evidence outlined in previous contributions (e.g. Garson and Campbell Smith 1965;Wall and Mariano 1996;Duraswami and Shaikh 2014;Broom-Fendley et al 2016a) with the new isotope data presented herein, it is possible to outline the order of events and postulate two major fluid generations. These different fluid generations are divided into (1) mineralising fluids, responsible for altering burbankite to the strontianite-baryte-monazite mineral assemblage; and (2) subsequent altering fluids, which modify the isotopic composition of the host carbonates and can erroneously be associated with the mineralising event.…”

“…A range of rare earth minerals also occur within the latestage quartz-rich rocks, including florencite-goyazite and apatite. The latter mineral displays evidence for multiple stages of dissolution-reprecipitation related to late alteration (Wall and Mariano 1996;Broom-Fendley et al 2016a). The most common REE mineral, however, is monazite occurring in a similar assemblage to that found in the main carbonatite, but without associated strontianite.…”

“…Inset map indicates the position of Kangankunde in southern Malawi. Maps redrawn after Garson and Campbell Smith (1965) and Broom-Fendley et al (2016a) Figure 2) 4. Monazite-rich quartz rocks from within the main intrusion 5.…”

“…Chloride is currently commonly favoured as the most important ligand for transporting the REE (e.g. WilliamsJones et al 2012;Downes et al 2014;Trofanenko et al 2016;Broom-Fendley et al 2016a, 2017a) but there is not yet any fluid inclusion evidence at Kangankunde to support this. Given the circumstantial nature of the evidence for different fluid compositions, it is difficult to conclude how the REE are transported at Kangankunde.…”

“…The processes behind the formation of these deposits are increasingly of interest due to the recognition of the REE as a 'critical metal' in terms of its supply security (e.g., Chakhmouradian and Wall 2012;European Commission 2014;Chakhmouradian et al 2015). Only certain carbonatites, however, are REE-rich [e.g., Mountain Pass and Bear Lodge, USA (Castor 2008;Moore et al 2015;Verplanck et al 2016); Kangankunde, Tundulu and Songwe, Malawi (Ngwenya 1994;Wall and Mariano 1996;Broom-Fendley et al 2016a, 2017a; Wigu Hill, Tanzania (Mariano 1989); Khibiny, Russia (Zaitsev et al 1998(Zaitsev et al , 2002(Zaitsev et al , 2015; Barra do Itapirapuã, Brazil (Andrade et al 1999;Ruberti et al 2008); and the Mianning-Dechang and Qinling belts in China (Kynicky et al 2012)]. It is almost always during the last stages of Abstract Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic.…”