Apatite as a tracer of the source, chemistry and evolution of ore-forming fluids: The case of the Olserum-Djupedal REE-phosphate mineralisation, SE Sweden

Abstract

This study explores the suitability of apatite as a tracer for the source(s), chemistry, and evolution of oreforming hydrothermal fluids. This is tested by analysing the halogen (F, Cl, Br, and I), stable Cl isotopic, and trace element compositions of fluorapatite from the regional-scale Olserum-Djupedal rare earth element (REE) phosphate mineralisation in SE Sweden, which is dominated by monazite-(Ce), xenotime-(Y), and fluorapatite. The primary hydrothermal fluid flow system is recorded in a sequence from proximal granite-hosted to distal metasediment-hosted fluorapatite. Along this sequence, primary fluorapatite shows a gradual increase of Cl and Br concentrations and in (Gd/Yb)N, a decrease of F and I concentrations, a decrease in δ 37 Cl values, in (La/Sm)N, and partly in (La/Yb)N and (Y/Ho)N. Local compositional differences of halogen and trace element concentrations have developed along rims and in domains adjacent to fractures of fluorapatite due to late-stage partial reaction with fracture fluids. These differences are insignificant compared to the larger deposit-scale zoning. This suggests that apatite can retain the primary record of the original ore-forming fluid despite later overprinting fluid events. The agreement between Br/Cl and I/Cl ratios of apatite and those of co-existing fluid inclusions at lower temperatures indicates that only a minor fractionation of Br from I occurs during apatite precipitation.The halogen ratios of apatite can thus be used as a first-order estimate for the composition of the oreforming fluid. Taking the small fractionation factors for Cl isotopes between apatite and co-existing fluid at high temperatures into account, we propose that the Cl isotopic composition of apatite and the halogen ratios derived from the apatite composition can be used jointly to trace the source(s) of ore-forming fluids. By contrast, most trace elements incorporated in apatite are affected by the host rock environment and by fluid-mineral partitioning due to growth competition between co-crystallising minerals.Collectively, apatite is sensitive to changing fluid compositions, yet it is also able to record the character of primary ore-forming fluids. Thus, apatite is suitable for tracing the origin, chemistry, and evolution of fluids in hydrothermal ore-forming settings.

Apatite at Niutoushan U–Pb–Zn deposit in Xiangshan ore field: A tracer for hydrothermal fluid

“…Hydrothermal‐affected and hydrothermal apatite (characterized by abundant hydrothermal mineral inclusions, patchy texture, cracks, and chemical composition, etc.) has been widely applied to revealing evolution of the ore‐forming fluids and insights into the genesis of the deposit (Andersson et al, 2019; Krneta, Ciobanu, Cook, Ehrig, & Kontonikas‐ Charos, 2016; Mao, Rukhlov, Rowins, Spence, & Coogan, 2016).…”

“…Thus, apatite is regarded as an ideal mineral to record the nature of melts and hydrothermal fluids from which it precipitates and track the evolution history of fluid during metasomatic processes. Numerous studies have focused on apatite as an indicator of mineralizing processes in Iron‐oxide‐Apatite deposits (Bonyadi, Davidson, Mehrabi, Meffre, & Ghazban, 2011; Harlov et al, 2002; Jonsson, Harlov, Majka, Högdahl, & Persson‐Nilsson, 2016), IOCG deposits (Belousova, Walters, Griffin, & O'Reilly, 2001; Ismail et al, 2014; Kontonikas‐Charos, Ciobanu, & Cook, 2014), porphyry Cu (Bouzari, Hart, Bissig, & Barker, 2016), rare earth element (REE) phosphate mineralization related to granitic magmatism (Andersson, Wagner, Jonsson, Fusswinkel, & Whitehouse, 2019) and other magmatic‐hydrothermal systems (Cao, Li, Qin, Seitmuratova, & Liu, 2012; Streck & Dilles, 1998).…”

Anion Composition of Apatite in the Au-Cu Epithermal Deposit of Palai-Islica (Almería, SE Spain) as an Indicator of Hydrothermal Alteration

“…The apatites from the sericitic-chloritic and propylitic alteration, and, more so, those from the orebody, are rich in F. Other studies [47] that analyze both magmatic and hydrothermal related apatites do not show a fluorine enrichment in hydrothermal phases in respect to primary apatite, but just the opposite. Nevertheless, this fact occurs in different geological contexts in respect to the precious metal epithermal volcanic-hosted deposit of Palai-Islica, like REE-phosphate mineralization [47] or in a U volcanic-hosted hydrothermal deposit [48].…”

Evidence on hydrothermal dissolution and replacement of apatite: a review and factual data on the example of metasomatized rocks of Devonian-Carboniferous intrusions of the West Magnitogorsk zone

“…Апатит -широко распространенный акцессорный минерал в большинстве известных типов магматических пород. Индикаторная роль апатита для петро-и рудогенеза заключается, главным образом, в специфических особенностях химического состава этого минерала, зависящих от физико-химических условий минералообразования [Watson, 1980;Холоднов, Бушляков, 2002;Belousova et al, 2002;Brassinnes et al, 2005;Chelle-Michou, Chiaradia, 2017;Andersson et al, 2019;Zhang et al, 2020 и др. ].…”