Low-Temperature Fluorocarbonate Mineralization in Lower Devonian Rhynie Chert, UK

: Rare earth element (REE) ﬂuorocarbonate mineralization occurs in lacustrine shales in the Lower Devonian Rhynie chert, Aberdeenshire, UK, preserved by hot spring siliciﬁcation. Min-eralization follows a combination of ﬁrst-cycle erosion of granite to yield detrital monazite grains, bioweathering of the monazite to liberate REEs, and interaction with ﬂuorine-rich hot spring ﬂuids in an alkaline sedimentary environment. The mineral composition of most of the ﬂuorocarbonates is referable to synchysite. Mineralization occurs at the surface, and the host shales subsequently experience maximum temperatures of about 110 °C. Most ﬂuorocarbonate mineralization originates at much higher temperatures, but the Rhynie occurrence emphasizes that low-temperature deposits are possible when both ﬂuorine and REEs are available from granite into the sedimentary environment.


Introduction
Fluorocarbonates are the major source of rare earth elements [1][2][3][4]. It is, therefore, important that their origin is understood. The majority of known occurrences are associated with carbonatites and syenite intrusions [5][6][7][8][9], but they also form in high-temperature ore deposits [10][11][12]. However, there are also examples formed by relatively low-temperature processes, especially by the alteration of carbonatite [13] and granite [14]. This study reports an example in which a fluorocarbonate mineral was precipitated in shale following REE availability in surface conditions, in which biological action played a critical role.
The mineral occurrence is within the Lower Devonian Rhynie chert lagerstätte, which contains exceptionally preserved plants, fungi, and animals, and is considered to represent the world's oldest terrestrial ecosystem. The fossils occur in chert bed layers interbedded with sandstones and shales (Figures 1 and 2). Locally, the sandstones are interbedded with andesitic ash tuffs. The rocks are not exposed, but they are archived in a number of cores at the University of Aberdeen (Figure 3). Petrographic studies of the plant-bearing sediments show that detrital monazite grains exhibit extensive leaching. The alteration is attributable to fungi, whose remains are widespread in the deposit and which would have been pivotal to the supply of phosphorus for plant growth.
The Devonian continental sediments accumulated in a small basin bounded by an active normal fault, marked by a silicified breccia (Figure 1). The rocks are gently folded in the vicinity of the chert outcrop ( Figure 1).
Monazite grains in sandstone beds below the Rhynie chert are rounded and unaltered, as is generally the case in sediments with a low-temperature history. Monazite in the granite is also unaltered. However, monazite grains in the chert-bearing succession are instead strongly altered to form a porous texture ( Figure 4). The host sediment for the altered monazite contains abundant evidence of fungi [19][20][21], which are an expected cause of the alteration. Monazite grains exposed to fungi, either naturally or experimentally, exhibit extensive bioweathering [22][23][24][25][26][27]. Monazite is a source of phosphorus, which fungi release for uptake by plants as part of the fungus-plant symbiosis. In the process, REEs are also liberated from the monazite. An authigenic REE mineral phase recorded here in the Rhynie succession may, therefore, represent the REEs released from the Rhynie monazite. The REE phase occurs in shales which are interpreted as lacustrine deposits, interbedded with fluvial sediments that contain the plant fossils for which the Rhynie chert is internationally important [28].  Monazite grains in sandstone beds below the Rhynie chert are rounded and unaltered, as is generally the case in sediments with a low-temperature history. Monazite in the granite is also unaltered. However, monazite grains in the chert-bearing succession are instead strongly altered to form a porous texture ( Figure 4). The host sediment for the altered monazite contains abundant evidence of fungi [19][20][21], which are an expected cause of the alteration. Monazite grains exposed to fungi, either naturally or experimentally, exhibit extensive bioweathering [22][23][24][25][26][27]. Monazite is a source of phosphorus, which fungi release for uptake by plants as part of the fungus-plant symbiosis. In the process, REEs are also liberated from the monazite. An authigenic REE mineral phase recorded here in the Rhynie succession may, therefore, represent the REEs released from the Rhynie monazite. The REE phase occurs in shales which are interpreted as lacustrine deposits, interbedded with fluvial sediments that contain the plant fossils for which the Rhynie chert is internationally important [28].
The REE phase is characterized here by: (i) Measurement of the chemical composition of the mineral.

Materials and Methods
Shale and sandstone samples were taken from cores 97/2 ( Figure 3) and 19C, in the immediate vicinity of the plant-bearing chert outcrop (Figures 1 and 2). The stratigraphy of the cores and the correlation between them (Figure 2) are described in [26]. The cores were drilled to help delineate the extent of the plant-bearing deposits [17,28].

Materials and Methods
Shale and sandstone samples were taken from cores 97/2 ( Figure 3) and 19C, in the immediate vicinity of the plant-bearing chert outcrop (Figures 1 and 2). The stratigraphy of the cores and the correlation between them ( Figure 2) are described in [26]. The cores were drilled to help delineate the extent of the plant-bearing deposits [17,28].
Petrographic studies were performed at the University of Aberdeen ACEMAC Facility using a Carl Zeiss Gemini 300 VP field emission gun scanning electron microscope (FEG-SEM) on polished blocks of rock. Samples were carbon-coated and analysed at 20 kV with a working distance of 10.5 mm. Samples were analysed using an Oxford Instruments NanoAnalysis Xmax80 Energy Dispersive Spectroscopy (EDS) detector and the AZtec software suite. For routine work, the beam diameter is just a few nanometres, but the X-ray excitation volume is larger than the beam diameter. Spectra were recorded using a live time of either 18 s or 25 s. The standards used were a mixture of natural minerals, metal oxides, and pure metals, as calibrated by the factory. The element weight percent detection limits relevant to this study are 0.28% F, 0.27% Y, 0.32% Gd, and 0.38%-0.44% for other REEs. Carbonate contents were determined by stoichiometry. The compositions of fluorocarbonate grains (Table 1) were identified by comparison with mean compositions for phases interpreted as synchysite and parisite in published case studies ( Table 2).
Fungal activity in the succession was tested using the detection of the biomarker perylene, which records such activity in the geological record [29]. Gas chromatographymass spectrometry was used to measure the mass fragment m/z 252, which records perylene.
The maximum temperature experienced by the host rock was derived from the thermal maturity of organic matter in shale and fine sandstone ( Figure 2), determined by Raman spectroscopy. A Renishaw inVia reflex Raman spectrometer was used, with a backscattering geometry in the range of 700-3200 cm −1 , a 2400 L/mm spectrometer grating, and a CCD detector. Microscopic observations were carried out with a ×50 confocal optical power objective with a numerical aperture (NA) of 0.90. The slit opening was 65 um with a CCD area of approximately 10 pixels (80% of the total signal height hitting the CCD chip) and a confocal hole of 200 µm. A 514.5 nm diode laser was used for excitation with an output of 50 mW. Optical filters (1%) were used to adjust the power of the laser to less than 0.5 mW to avoid alteration [30]. Vitrinite reflectance equivalent values (Ro eq %) were calculated from Raman spectra by using an approach [31] based on an automatic deconvolution in which D and G bands are fitted separately, applying a one-band asymmetrical Gaussian band fitting for each spectral region. Position, intensity, width, and integrated area of the D and G bands were determined and used to calculate Ro eq % by means of a multi-parametric equation (see [31] for more details). Temperatures were calculated by converting vitrinite reflectance equivalent (Ro eq %) by using the Barker and Pawlewicz equation [32].

Results
The lacustrine sediments consist of millimetre-to-centimetre scale beds of black shale (mudstone, siltstone) with interbedded streaks of fine sandstone. The sandstone is calcitecemented and additionally may have sub-millimetre laminae of pure carbonate. At about a 50 m depth in borehole 97/2, sandstone is also cemented by fluorite (Figure 4). The shales consist of detrital quartz, feldspar, micas, chloritoid grains, and fragments of organic plant matter. The organic matter is concentrated in specific layers. Accessory minerals include zircon, garnet, monazite, and xenotime. Among these grains are framboids and solid crystals of pyrite, as well as crystals of REE-fluorocarbonate. The pyrite framboids are mostly 20 microns in diameter or smaller, and they include traces of arsenic and/or manganese. The REE-fluorocarbonate is typically 3 to 10 microns in size and forms irregular shapes in a matrix of microquartz crystals and clay ( Figure 5). The small size of the crystals means they are of mineralogical interest only and do not represent an anomalous REE concentration. Samples containing fluorocarbonate mineralization are located in both cores 19C and 97/2 ( Figure 2). Mineralization was found in five samples, covering a 20 m depth range in core 97/2 ( Figure 2). In the immediate vicinity, several shale samples in the sections ( Figure  2) contain at least 0.5% total organic carbon (TOC) and thus can be classified as black shales [42,43].
The majority of the 41 compositions measured in the Rhynie fluorocarbonates (12 highest totals and lowest contaminations from Si in surrounding silicate are shown in Table 1) are similar to mean published compositions for synchysite ( Figure 6). There are, however, several compositions measured at Rhynie that either plot with published parisite data or have intermediate compositions ( Figure 6). The synchysite is cerium-rich (20-  Figure 2) contain at least 0.5% total organic carbon (TOC) and thus can be classified as black shales [42,43].
The majority of the 41 compositions measured in the Rhynie fluorocarbonates (12 highest totals and lowest contaminations from Si in surrounding silicate are shown in Table 1) are similar to mean published compositions for synchysite ( Figure 6). There are, however, several compositions measured at Rhynie that either plot with published parisite data or have intermediate compositions ( Figure 6). The synchysite is cerium-rich (20-25 wt. % Ce 2 O 3 ), with lower lanthanum (10-15 wt. % La 2 O 3 ) and neodymium (8-10 wt. % Nd 2 O 3 ), and minor contents of yttrium and other REEs (Table 1). Minor deviations from 100% totals are due to contamination from surrounding silicate and variable substitution by the -OH anion.
Minerals 2023, 13, x FOR PEER REVIEW 10 of 17 spectra exhibit both ordered and disordered peaks (i.e., D and G bands), superimposed on a fluorescence background. Roeq% values vary between 0.66% and 0.73%, indicating the mature stage of hydrocarbon generation (oil window). Associated maximum paleotemperatures, calculated by the conversion of Roeq% values [32], range between 100 and 115 °C (Table 3).  Three samples of shale analysed by GC-MS record the distinctive peak of perylene in mass fragment 252 (Figures 2 and 7). Perylene is an aromatic cyclic hydrocarbon with the formula C 20 H 12 .
Analyses of carbonaceous matter by Raman spectroscopy were performed on thinsection samples of shale and fine sandstone (Figure 2). Up to sixteen spectra were acquired in each sample depending on the abundance of carbonaceous matter. Organic matter spectra exhibit both ordered and disordered peaks (i.e., D and G bands), superimposed on a fluorescence background. Ro eq % values vary between 0.66% and 0.73%, indicating the mature stage of hydrocarbon generation (oil window). Associated maximum paleotemperatures, calculated by the conversion of Ro eq % values [32], range between 100 and 115 • C (Table 3).  (Table 2), also showing compositions measured in the Rhynie chert, which are predominantly in the synchysite field. The ideal compositions for fluorocarbonate mineral species are shown.

Requirements for Fluorocarbonate Mineralization
The irregular shape of the fluorocarbonate indicates that it is in a diagenetic phase, and fluorocarbonate minerals in sedimentary rocks are considered to be authigenic rather than detrital grains [44]. The crystal size is comparable with many published examples. Growth during diagenesis is consistent with the deposition of fluorite cement in the interbedded sandstones. The accompanying diagenetic mineralogy of pyrite framboids and calcite implies that the environment was anoxic and alkaline [45]. These characteristics help to show how fluorocarbonate mineralization occurred at Rhynie.
Fluorocarbonate minerals occur in a diversity of settings (Table 2). They are most commonly found in carbonatites, especially carbonatites that have experienced hydrothermal alteration (e.g., [13,38]). Other settings include a range of hydrothermal systems, massive sulphides, laterites/bauxites, and black shales ( Table 2). Occurrences in shales and pelites (e.g., [35,41]) are assumed to reflect a high content of trace elements, including REEs, in the depositional environment, promoted by the low-Eh conditions conferred by organic matter. The REEs were subsequently concentrated during diagenesis and metamorphism. In the case of Rhynie, REEs released during monazite alteration would have entered groundwaters and become fixed by organic matter in the sediment.
The hot springs that caused the Rhynie chert deposit to be preserved by silicification were also the source of the fluorine that contributed to the fluorocarbonate mineralization. Many hot springs contain anonymously high fluorine contents, and they may consequently be a cause of dental problems if the water enters the drinking supply [46][47][48]. Fluorine in the Rhynie hot spring system is evidenced by fluorite-calcite veins cutting the lower part of the chert succession [16,18]. Shales and fine sandstones also show mixed fluorite and calcite cement (Figure 4), showing that fluorine was able to enter the sediment matrix and react with other components.
The precipitation of fluorocarbonates further required an alkaline environment, which is evidenced by the carbonate laminae and calcite cement. Limited stable isotope data for the carbonates (sampling points in Figure 2) implies an early diagenetic origin [15]. The combination of calcite, organic matter, and pyrite constrains the conditions of the peat-like environment [49] to a limited Eh-pH space ( Figure 8) that is appropriate for fluorocarbonate deposition [50].

Role of Granite
The fluorocarbonate mineralization is in sediments, but granite played a critical role. Firstly, the monazite grains that yielded the REEs during diagenesis were derived from underlying Caledonian granites. The attribution of monazite grains in the Devonian sediments to the granite was one of the earliest studies of provenance using heavy mineral analysis [51]. Secondly, the granites are enriched in fluorine [52,53], which caused fluorite mineralization in the Rhynie chert. High fluorine contents are indicated by high lithium contents in the granite micas, as the two elements show positive covariance [54]. A regional input of fluorine-rich fluid is also evident in fluorite mineralization, which occurs where Dalradian Supergroup (late Neoproterozoic) limestones were intruded by Caledonian granites on Deeside, 30 km to the south [55,56]. The Ballater and Mount Battock gran-

Role of Granite
The fluorocarbonate mineralization is in sediments, but granite played a critical role. Firstly, the monazite grains that yielded the REEs during diagenesis were derived from underlying Caledonian granites. The attribution of monazite grains in the Devonian sediments to the granite was one of the earliest studies of provenance using heavy mineral analysis [51]. Secondly, the granites are enriched in fluorine [52,53], which caused fluorite mineralization in the Rhynie chert. High fluorine contents are indicated by high lithium contents in the granite micas, as the two elements show positive covariance [54]. A regional input of fluorine-rich fluid is also evident in fluorite mineralization, which occurs where Dalradian Supergroup (late Neoproterozoic) limestones were intruded by Caledonian granites on Deeside, 30 km to the south [55,56]. The Ballater and Mount Battock granites on Deeside are both dated at 406 Ma [57], within error of the Rhynie chert dated at 407 Ma [58]. Another Caledonian granite in the vicinity contains fluorine-bearing zinnwaldite [59,60]. Further evidence of the role of the granites in mineralizing the Rhynie chert is in the precipitation of tungsten-bearing rutile in the chert [61], which reflects derivation from granites that are also mineralized by tungsten [60,62]. The fluorocarbonates at Rhynie are of mineralogical, rather than commercial, interest. However, the occurrence adds to the evidence for the flux of mineralizing fluids in the region, which could reflect as yet-undiscovered ore deposits.

Identification of Fluorocarbonate Mineral
The most widespread fluorocarbonate minerals in the Rhynie occurrence are synchysite Ca(REE)(CO 3 ) 2 F and parisite Ca(REE) 2 (CO 3 ) 3 F 2 . In both synchysite and parisite, the F/Ca atomic ratios commonly fall below the ideal stoichiometric values, i.e., below 1 and 2, respectively. This is mostly a consequence of hydroxyl (-OH) substitution for fluoride (-F), which can occur up to several 10 s % (e.g., [5,34,41]). Thus, in several deposits, mean F/Ca ratios for synchysite are in the range of 0.5 to 0.6 [10,11,34]. The median value determined for the fluorocarbonate phase at Rhynie is 0.85 (n = 41), which is consistent with synchysite rather than parisite. Similarly, the median REE/Ca atomic ratio of the Rhynie phase is 1.10 (n = 41), which is comparable with values for other synchysite occurrences. The data that show a plot between synchysite and parisite indicate that some grains may not be synchysite. We omit the intermediate rontgenite (rongtenite, röngtenite), as it is not universally used ( Figure 6). Other case studies report fine intermixtures of synchysite and parisite [6,13,38], which would explain the range of the Rhynie data. Synchysite occurs as Ce-, La-, Nd-, and Y-rich forms, and it is Ce-rich in this case. Many of the compositions may therefore be attributed to synchysite-(Ce) [34]. The profile of REEs (Ce > La > Nd >> others) is similar to many other occurrences of synchysite. It is also similar to the profile of REEs in typical monazite [63,64], which is consistent with a source for the fluorocarbonate REEs by the alteration of monazite. The apparent non-stoichiometry at the SEM scale is also explainable in terms of nanoscale disorder, or possibly unnamed phases [65,66]. There are also OH-dominant members of the series, e.g., hydroxylbastnäsite-(Ce), which would influence the stoichiometry if present.

Temperature of Mineralization
The range of settings for fluorocarbonate mineralization results in a range of temperatures for their formation. Most examples were probably deposited at temperatures above 100-200°C (Table 2). However, there are also low-temperature occurrences (Table 2), which implies that REE availability is the critical factor.
The temperature of the silica-rich fluid that formed the Rhynie chert was probably no more than 50°C [18], and bioweathering of the monazite would have occurred in these cool conditions. The fluorite-calcite veins record primary aqueous fluid inclusion temperatures of 88 to 110°C in fluorite [18], but rapid heat dissipation around narrow veins would leave the sediments unperturbed by the minor hydrothermal activity. The maximum temperature of the rock, determined by Raman spectroscopy of carbonaceous matter, is about 110°C ( Table 3), but that is accounted for by post-mineralization heating due to burial, during the late Palaeozoic (Carboniferous-Permian) [67]. The palynological study of samples in core 97/2 is in agreement with Ro eq % values calculated from Raman spectroscopic analyses and confirms that the organic matter has relatively low thermal maturity [68].

Biological Contribution to REE Mineralization
The bioweathering of REE minerals occurs at a scale that is of commercial value. In addition to their role in releasing REE through the weathering of minerals including monazite, fungi and bacteria can then concentrate the REE and are implicated in the formation of so-called regolith-hosted REE deposits [69].
Fluorocarbonates are more amenable to the extraction of REEs than oxide/phosphate minerals. Accordingly, research has been directed to model the hydrothermal evolution of REE phases to precipitate fluorocarbonates [70] and to patent a commercial laboratory version of the process [71]. The occurrence at Rhynie indicates that biological activity can contribute to this kind of transformation of REE in monazite to form desirable fluorocarbonates.
The biomarker perylene, which signifies fungal activity, is present in several shale samples in the studied section. The perylene is unlikely to have formed in the shale but rather has been washed into the lake from the adjacent land surface. The transport of perylene into water bodies is demonstrable today and is also recorded in other Devonian shales [72]. Land plants preserved in chert beds at Rhynie show extensive fungal colonization [19][20][21], so the transfer of perylene to the lake waters could have occurred readily. Samples of plant fossils on the adjacent land surface contain fossilized fungi in borehole 19C, including at the depths where fluorapatite mineralization occurs [73]. The evidence of fungal activity supports the model of biological alteration of monazite implied by its exceptional occurrence in the Rhynie chert succession.

Organic-Rich Sediments
In other cases where fluorocarbonates occur in shales, pelites, and coal, metamorphism was critical to mineralization. Metamorphism could have concentrated REEs from the organic-rich sediment. Most notably, the Columbian emerald deposits, which include gemquality fluorocarbonates, are in metamorphosed black shales [41]. However, the Rhynie shales are not metamorphosed. The role of the shale at Rhynie was instead the occurrence of carbonate, which created the necessary alkaline environment.
Carbonates are common in organic-rich sediments in lacustrine environments, relative to open marine environments [74,75]. The lacustrine carbonates form during early diagenesis at shallow burial depths. The lacustrine shales are therefore a good focus for fluorocarbonate mineralization in continental sequences. As discussed above, ephemeral lakes that precipitate calcite and pyrite have Eh-pH conditions which are suitable for fluorocarbonate minerals.

Conclusions
Fluorocarbonate mineralization occurs in Lower Devonian carbonate-bearing shale within the Rhynie chert. The chert occurrence is spatially related to a granite outcrop. In most cases, the mineral phase has a composition referrable to synchysite. Unlike most examples of fluorocarbonate mineralization elsewhere, the occurrence in the Rhynie chert formed at a relatively low temperature. The mineralization was engendered by: (i) Availability of REEs by bioweathering of monazite grains in the sequence, derived from underlying granite. (ii) Availability of fluorine, evident as fluorite, associated with hot spring activity also related to underlying granite. (iii) Precipitation in the alkaline environment of carbonate-bearing shales.
The rocks achieved a maximum burial temperature of about 110°C but mineralization probably occurred at a lower temperature during shallow burial. This study supports a model for fluorocarbonate mineralization at relatively low temperatures and hence encourages the exploration for fluorocarbonate resources in rocks without a history of high temperatures.

Data Availability Statement:
All data is reported in the paper.