An apatite for rare earth elements

As the hunt for critical minerals continues to ramp up, exploration geologists need to be on the lookout for phosphate minerals like apatite in a way that they probably haven't been before. Apatite would have always seen as just an accessory mineral, rarely as a target mineral itself. But all that is changing.

Drill core photo of coarse, pink/red apatite intergrown with magnetite, quartz and minor pyrite from drill hole TI009, Gawler Craton, South Australia

In many, if not all deposits, accessory minerals will be found along with the major commodities such as copper, gold or uranium. In such cases, REE could become be part of the inventory of a multi-element ore deposit, if the economics stack up.

Metallurgical flow sheets just got a whole lot more complicated...

The Geological Survey of Western Australia just released a new map showing the WA's rare earth element projects and prospects.

These projects include REE associated with carbonatites, mineral sands, clay-hosted, and unconformity-related. But what it doesn't seem to show is REE associated with one of the key phosphate minerals, apatite. Proterozoic rocks in Australia seem to be more prospective for the apatite-rich deposits, associated with the extensive metasomatic iron oxide-akali-calcic alteration systems, of regions like the Gawler Craton (SA), Tennent Creek (NT), Mt Isa (Qld). But, WA seems to have everything else, so it's probably just a matter of time!

We will also have a brief look at a couple of examples of iron oxide-apatite deposits in which apatite is becoming one of the minerals of interest, where in the past it would probably have been treated as waste material.

Apatite

Apatite is a group of minerals that vary in composition depending on the abundance of OH, Cl, or F in them, hence the general formula for apatite is Ca5(PO4)3(OH,Cl,F). This variation means you will sometimes hear people talking about hydroxyapatite, chlorapatite or fluoraparite.

Apatite is one of the most common phosphate minerals and forms in a range of geological environments including in igneous rocks crystallising directly from a magma, sedimentary rocks as a result of diagenetic alteration, as well as in hydrothermal alteration systems.

Apatite can contain REE in trace amounts (hundreds of parts per million) up to several weight percent, depending on the processes responsible for its formation.

Mineral deposits that contain apatite

There is a class of mineral deposits that have significant amount of apatite in them, so much so that apatite is in the name. These are the iron oxide-apatite (IOA) deposit class. Two of the best known examples of this deposit type are Pea Ridge in Missouri, US, and Kiruna in Norrbotten, Sweden. There are examples of deposits like this in Australia, such as Cairn Hill, in the Gawler Craton.

Both Pea Ridge and Kiruna deposits are dominated by iron oxide in the form of magnetite, while apatite forms the most volumetrically abundant gangue mineral.

Pea Ridge IOA deposit

At Pea Ridge, the main iron ore bodies formed by hydrothermal alteration of the host volcanic rock packages around 1.46 billion years ago. Apatite co-precipitated with the magnetite in the core of the alteration system. Other minerals such as amphibole, K-feldspar and epidote also being part of the broader metasomatic system.

Schematic figure showing principal characteristics and inferred setting of iron oxide ± apatite (IOA)- and iron oxide-copper-gold (IOCG)-type mineral deposits in Missouri iron metallogenic province (based on available drill core and surface information). From Day and others, 2016, doi:10.2113/econgeo.111.8.1825. (Public domain.)

One of the really interesting aspects of Pea Ridge is that the highest-grade REE mineralisation is confined to breccia pipes steeply that crosscut the magnetite orebody. REE minerals in the vuggy , highly porous breccias include monazite, xenotime and the REE concentrations average 20 wt.% total REE oxide (a great summary of the deposit is at https://www.911metallurgist.com/rare-earth-element-gold-geology-breccia-pipes/). These breccia bodies are younger than the main magnetite-apatite hydrothermal event. The key implication is that younger geological processes were able to "harvest" the REE that was originally present in the apatite (and other less abundant phosphate minerals) and concentrate them into the younger structures.

Kiruna and Per Geijer IOA deposit

The Kiruna orebody consists of banded magnetite-apatite ore that more or less follows the contact between two contrasting volcano-sedimentary units and formed at around 1.87 billion years ago. Kiruna produces over 80% of the iron mined in Europe and is the type example of iron oxide-apatite ore deposits. The Kiruna district contains some 40 or so deposits of similar IOA type.

Model of the Per Geijer Deposit and location adkecent the Kiruna magnetite mine, Sweden. Image from https://lkab.com/en/press/europes-largest-deposit-of-rare-earth-metals-is-located-in-the-kiruna-area/

Recent exploration near the main Kiruna deposit, at the Per Geijer deposit, has identified over one million tonnes of REE oxides associated with apatite-rich magnetite orebodies. This is now the largest known REE deposit of its kind in Europe containing both heavy and light REE.

Most of the REE is in apatite and the apatite is generally intergrown with magnetite, but it's interesting that at Per Geijer, hydraulic breccia is also described. These breccias overprint the main deformation fabrics and metasomatic minerals but also contain coarse grained apatite and other phosphate minerals. The redistribution of REE into concentrated packages of fault rock seems to be a theme in these IOA deposits (see https://pubs.geoscienceworld.org/segweb/economicgeology/article/116/8/1981/599879/Structural-Evolution-of-the-Central-Kiruna-Area for a great paper on the deposit structure).

Minerals in the Per Geijer deposit. Mineral distribution maps from reflected light (optical microscopy) and processed SEM-MLA and Raman analysis. Figure from Krolop et al. 2019, https://www.mdpi.com/2075-163X/9/9/544

Implications for REE exploration targeting

IOA deposits are currently mined for their iron ore, in the form of magnetite and occasionally also associated hematite. Generally, the relatively high phosphorus content of these ores compared to the banded iron formations that are the world's main source of iron, require specific metallurgical processing, and consequently the lower phosphorus zones in IOA systems are prioritised for mining.

The fact that the mineral that apatite which is otherwise is deleterious to processing can be itself a valuable mineral now that the REE are a sought after commodity means that there are probably a whole series of IOA deposits can be reassessed.

Apatite is also a mineral that is relatively easily modified in hydrothermal and weathering environments and as a result the REE can be concentrated into structures that are later in the development of the deposit geology. Good structural geological mapping and core logging will help identify the key orientations of younger structures and these can be possible candidates for more targeted mine development, which stands in contrast to the high tonnage bulk mining that is usually undertaken in an iron ore deposit.

I hope this piece has whet your 'apatite' for these intriguing types of ore deposits. Feel free to get in touch if there's something you would like to discuss.