Apatite (Ca10(PO4)6(OH, F, Cl)2) is a very useful accessory mineral that is found in a variety of ore deposits (IOCG, ISCG, IOA, Carbonate Replacement, Skarn). Recent advances in analytical techniques coupled with detailed experimental investigations into its geochemical behaviour has highlighted the ability of apatite to provide a unique insight into the timing, origin and evolution of ore-bearing fluids. In this blog we will briefly cover two unique properties of apatite that allow it to be such a useful mineral in ore deposit studies.
Geochronological Versatility
Apatite is in a league of its own when considering its versatility regarding geochronology. Apatite can be dated using three separate geochronological methods, which all provide a unique insight into the formation/history of the mineral. Apatite can be routinely dated using U-Pb geochronology, which has been made possible through recent analytical advances (Chew et al. 2014). Due to its ability to incorporate common Pb when crystallising, a common-Pb line is commonly plotted through the discordant apatite U-Pb data to produce a lower-intercept age. The one caveat when interpreting apatite U-Pb data is that it has a low thermal resetting temperature of ~375–550°C (Cochrane et al. 2014), which means that if your deposit has been metamorphosed above this temperature, or exhumed after ore deposition, and if your apatite is small (<100 µm), it is unlikely to record the timing of primary crystallisation. Recent advances pioneered at The University of Adelaide now allows for in-situ Lu-Hf geochronology to be routinely performed on apatite (Simpson et al. 2021). Unlike U-Pb system, the Lu-Hf geochronometer has a relatively high resetting temperature of ~700°C and is therefore more likely to record a primary crystallisation age (Barfod et al. 2005). However, the caveat is that the apatite must be sufficiently large (>70 µm diameter) to be analysed using this method (Simpson et al. 2021). The last of the geochronological methods is Sm-Nd geochronology. This method can be completed in-situ, and similar to Lu-Hf geochronology, it has a high resetting temperature of >700°C and is therefore likely to record the primary crystallisation age (Fisher et al. 2020). However, as the half-life of the 147Sm to 143Nd decay scheme is so large (~106 billion years; (Tavares and Terranova 2018)), the uncertainties associated with this method are often large (<100 Ma), limiting the ability to make precise temporal correlations. We acknowledge that we have not presented any information regarding the use of apatite as a low-temperature thermochronometer, and its ability to record orogenic cooling trends using fission-track analyse. However, we promise this will be covered in a future blog post.
Stable Isotopes & Metasomatism
As apatite often contains abundant Nd and an extremely low Rb/Sr ratio, it can be a useful isotopic tracer, allowing for the 143Nd/144Nd ratio and 87Sr/86Sr ratios to be determined (Yang et al. 2014). With well characterised reservoirs for each of these isotopic systems, it allows for the origin of the ore-bearing fluids to be assessed. The elements/halogens F, Cl, H2O and S can all play a significant role in the transport and deposition of metals. As apatite is a carrier of these elements, the proportion of these elements within apatite can be directly related to the fluid from which it precipitated (assuming other minerals that contain these elements are accounted for) (Harlov 2015). Moreover, the abundance of halogens within altered portions of apatite can be used to assess the composition of ore-bearing or post-ore fluids. A variety of experiments have been conducted on apatite which use a variety of conditions and fluid compositions (Harlov et al. 2002; Harlov and Förster 2003). In these experiments the altered unaltered portions of the apatite are often compared to the altered, or metasomatized portions of the apatite, which in turn relate to the fluid parameters. Regions of apatite that has been altered by a fluid containing H2O, H2O–CO2, KCl, H2SO4, and HCl all result in the formation/precipitation of secondary xenotime and monazite (Harlov et al. 2002; Harlov and Förster 2003; Harlov 2015). Except for in extreme conditions (700–900 °C and 700–2000 MPa), the presence of Na in the fluid inhibits the formation/precipitation of secondary xenotime and monazite. Moreover, as apatite is a dominant REE-carrier, comparing the REE composition of altered and unaltered portions of the apatite can provide valuable information regarding the composition of the altering fluid (such as S- & OH abundances) and/or the presence of co-genetic REE-bearing minerals.
In conclusion, apatite is a very useful mineral to have in your ore deposit. Its unique features allow it to be dated using three (U-Pb, Lu-Hf & Sm-Nd) separated geochronological methods of which have the possibility to provide unique information on ore-deposition and post-ore processes. Furthermore, the Sr & Nd stable isotope composition of apatite can provide valuable information regarding the source of ore-bearing fluids. Furthermore, the mineral inclusions and the REE abundance of altered apatite can provide invaluable information regarding the composition of fluids associated with ore-deposition or post-ore processes.
References
Barfod GH, Krogstad EJ, Frei R, Albarède F (2005) Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite. Geochim Cosmochim Acta 69:1847–1859
Chew DM, Petrus JA, Kamber BS (2014) U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb. Chem Geol 363:185–199
Cochrane R, Spikings RA, Chew D, et al (2014) High temperature (> 350 C) thermochronology and mechanisms of Pb loss in apatite. Geochim Cosmochim Acta 127:39–56
Emproto, C., Alvarez, A., Anderkin, C., & Rakovan, J. (2020). The crystallinity of apatite in contact with metamict pyrochlore from the Silver Crater Mine, ON, Canada. Minerals, 10(3), 244.
Fisher CM, Bauer AM, Luo Y, et al (2020) Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite–Improvements, potential reference materials, and application to the Archean Saglek Block gneisses. Chem Geol 539:119493
Harlov DE (2015) Apatite: A fingerprint for metasomatic processes. Elements 11:171–176
Harlov DE, Förster H-J (2003) Fluid-induced nucleation of (Y+ REE)-phosphate minerals within apatite: Nature and experiment. Part II. Fluorapatite. Am Mineral 88:1209–1229
Harlov DE, Förster H-J, Nijland TG (2002) Fluid-induced nucleation of (Y+ REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite. Am Mineral 87:245–261
Simpson A, Gilbert S, Tamblyn R, et al (2021) In-situ LuHf geochronology of garnet, apatite and xenotime by LA ICP MS/MS. Chem Geol 577:120299
Tavares OAP, Terranova ML (2018) Toward an accurate determination of half-life of 147Sm isotope. Appl Radiat Isot 139:26–33
Yang Y-H, Wu F-Y, Yang J-H, et al (2014) Sr and Nd isotopic compositions of apatite reference materials used in U–Th–Pb geochronology. Chem Geol 385:35–55