By: Mark Arundell, IMEx Consulting and Erik Ronald, Mining Geology HQ
29 August 2016
(This article is modified and expanded from an original publication in LinkedIn on 31 May 2016)
We are currently in the middle of a lithium boom! Whether this boom has been sparked by the increasing global lithium (Li) demand, the limited supply from OECD countries, or just the pure optimism of Li-ion battery technology going forward isn’t clear. What is clear is that prices for lithium carbonate have risen in the past decade (Figure 1) attracting mining finance and fueling global exploration activity.
Global Li production is restricted to either pegmatites, Li-rich brines, and to a lesser extent Li-rich clay deposits. Li-bearing pegmatites can display either a zoned or homogeneous unzoned structure with the common ore minerals being: spodumene, petalite, lepidolite, and amblygonite.
The first movers in the Li exploration space are making strong progress in bringing on new deposits but don’t think you are too late to catch the enthusiasm for a new hard rock lithium discovery. It’s true that over the past few years every old mine, prospect, or known occurrence has been pegged and staked. So what is an exploration geologist to do? “Nearology” seems to be the first reaction of many but that only works in certain instances when you’re in the right terrain and have the right rocks for Li prospectivity.
So what else can you do? Where are we going to find the next Greenbushes deposit (the header image)? The following points are proven exploration techniques that IMEx Consulting has been using with clients to successfully explore for lithium around the globe.
1. Lithium / Cesium / Tantalum Correlation
There is an excellent volume of Elements (v.8 issue 4, August 2012) that discusses the association of Li by using Cesium (Cs) and Tantalum (Ta) in pegmatites as indicators. This insight is useful when the geochemical assay data is predominately X-Ray fluorescence (XRF) as Li is not reported in typical XRF analysis suites and most prospecting for Li is solely focused in Li values!
The presence of anomalous Cs and/or Ta can be indicative of enriched Li mineralisation due to their common direct correlation in pegmatite-type mineralisation. A high correlation of Ta versus Cs is regarded by Möller & Mortenai (1987) as a strong indicator of Ta mineralisation – may not be such a good indicator for Li. It should be noted that Ta associated with tungsten and/or tin can be discriminated from the above, both geologically and geochemically.
2. Potassium / Rubidium Ratio as a Pathfinder
Analysis for Li, Cs and/or Ta is not ubiquitous u and often the digestion and analytical methods are not ideal. Work by Trueman & Cerny (1982) described a number of useful correlations to differentiate rare metal bearing pegmatite from barren pegmatite. The basis of the potassium (K) / rubidium (Rb) ratio is that Rb substitutes for K in micas and K-feldspar in the final stages of crystallisation. They note that a K/Rb <160 indicates increasing fractionation. Ratios of <15 were found in highly fractioned pegmatite and were usually indicative of rare metal mineralisation – Ta, Nb, Be, Cs, and Li.
3. Whole Rock Databases
Many government geological surveys maintain whole rock databases available to the public. These are often a compilation of government and university projects supplemented with company data. The prevalence of Li analysis tends to be low due to whole rock / major element chemistry typically being the focus but these datasets. From points 1. and 2. above, these databases are useful sources of K/Rb and Cs/Ta data for further analysis. In available databases that do contain Li analysis, there are a number of “curious” occurrences that have been identified such as anomalous Li associated with ignimbrites that warrant further investigation.
4. Exploration Databases
In addition to whole rock databases, many government agencies have exploration geochemistry databases containing rock, soil, stream, till and/or drill hole geochemistry information. IMEx Consulting has completed an investigation into one particular State’s geochemistry database (including nearly one million records!) that revealed a spatial cluster of rock samples described as “pegmatites” with anomalous Li. This location was not covered by any mining or exploration tenement and had no previously documented Li mineralisation. This particular case was validated by subsequent rock sampling thus confirmed the original anomalous Li values.
Interrogation of one particular State database revealed that none of the pegmatites identified from field mapping had been geochemically analysed. One client saw this as an opportunity and initiated a program of pegmatite sampling with encouraging results.
This is by no means a definitive list nor is it “the” formula for successful exploration. The occurrence of Li as amblygonite, spodumene, petalite, and lepidolite is worthy of a separate article. Also (because someone will ask), no thresholds have been presented due to client privilege.
Case Study – is it all iTaLiCs?
The graphs presented below are from a regional sampling program from an area known to contain Li bearing pegmatite. Initially, the company had focussed on an area where historic government whole rock sampling (albeit limited) had returned values of +500ppm Li. They had been using Cs & Ta as regional “pathfinders” for the Li mineralisation with what they considered limited success. There was no apparent correlation between Li and Cs and Ta which didn’t assist in anomaly ranking.
An initial review of the data focussed on determining where the most prospective rocks were located. It was identified that the samples were generally coarse grained to pegmatitic granites not true pegmatite, so geochemical values are “subdued”. Thus the goal of interpreting the regional sampling was to identify areas where the granites were highly fractionated. Also, we’ve used this technique on a regional assessment of a number of granite terrains.
In order to define the most fractionated granites, trace element analysis was performed by plotting K/Rb against Cs, Li, and Ta (Figure 3). The data apperas to cluster in three distinct populations below ~150 K/Rb and thus have been subdivided into low, moderate, and highly fractionated.
All the pegmatite samples except two of the Highly Fractionated (HF) samples cluster in the one area – the HF Cluster (Figure 4). Immediately, an area of 50 km2 has been prioritised from a region of 25,000 km2. Not bad for a start! We’ve gone from an area the size of Israel to Liechenstein in one pass of sampling. All samples that are elevated in Ta, Cs & Li occur in the HF Cluster area. Of the two other HF samples, one is elevated in Li and thus has been assigned a second order priority for follow-up investigation and further sampling.
The title of this case study is about iTaLiCs or specifically, are there other pathfinder elements useful in making the next Li discovery? The dataset clearly shows the answer is Yes! It should be noted that these elemental associations may be specific to the Li mineralisation in this particular area so they must be used with caution.
Figure 5 is a plot of Li versus various elements showing some interesting correlations. As expected, Li exhibits a strong association with the HF granites and may be associated with high Cs and Ta. It is not certain because two of the samples display anomalous Li based on the Cs content and another has anomalous Li for its Ta content. These “departure anomalies” are samples that should be prioritised for follow-up since they indicate Li metal accumulation away from what may purely be a fractionation trend (subject of a post to be written soon). The other elements that show correlation with Li are Be, Bi, Nb, Sn, Tl and W. Also of interest to note are the low (possibility depleted) Na, Ca and Sr values.
Many of the government datasets that IMEx Consulting is evaluating in the Great Lithium Hunt do not contain Li, Cs, and Ta so iTaLiCs cannot be used to prioritise areas. However, whole rock data ubiquitously contains K, Na and Ca, and generally has Rb, Sr and may even contain Be, Bi, Nb, Sn, Tl and W that provide further tools to evaluate and analyse.
Mark Arundell is based in Orange, NSW, Australia and has been exploring and developing industrial mineral projects for over 15 years. After university, he spent ten years with RGC as an exploration and mine geologist in various roles in Australia. He completed a Masters Degree at CODES (Univeristy of Tasmania) whilst working with North Ltd in NSW. Since 2000, Mark has worked as a consultant geologist / geochemist and spent three years with Rio Tinto Exploration focussed on Industrial Minerals – primarily potash. During the last eight years, Mark has worked on industrial mineral projects in North America, Europe, Asia, Africa, South America, and Australia.