SGS MINERALS SERVICES TECHNICAL BULLETIN MMI-TB18 2008

RARE EARTHS AND MMI GEOCHEMISTRY

INTRODUCTION

Rare earth elements appear commonly and strongly in partial digestions from a wide range of “unexpected” mineral deposit styles and geological assemblages – komatiitic nickel sulphides, veintype Au deposits, and kimberlites for example. Rare earths commonly are thought of as being associated with felsic and granitic rocks and carbonatite mineralization. Soils over these rock types exhibit concentrations of rare earths commonly of the order of hundreds of ppb in MMI digestions.

The rare earths are in fact, not rare: they are more correctly referred to as the It has been used, usually in a “chondrite Investigations of several komatiite-type Lanthanides, although the term “Rare normalised” form, to investigate the NiS deposits from Western Australia Earths” is widely used and accepted. history and origin of magmas and crustal (e.g. Nepean, Miitel North) have shown That term probably arose because their rocks. The light rare earths are among similar high concentrations of Ce, Nd and very similar chemistry delayed their the “incompatible” elements such as other rare earths from MMI digestions of separation and discovery as 14 discrete Cs, Rb, U, Th, K, Ti, Li and Be which soils in close proximity to sulphide ore, elements. Cerium is more common in do not readily enter into the early rock even when felsic rock types are absent the earth’s crust than copper, and even building minerals. Cerium, the most or laterally displaced by some distance. the least abundant “rare earth” is more abundant rare earth, occurs principally Comparison with whole fresh-rock common than elements such as As as the minerals bastnasite, a carbonate, geochemistry and geological models and Cd, commonly used as pathfinders and allanite and monazite, both silicates. suggests that in these cases the rare in geochemistry. The chemistry of All are accessory minerals in granites, earths are probably derived from felsic the Lanthanides is dominated by the grano-diorites, monzonites and syenites. plumes incorporated into the basal part trivalent oxidation state except for Ce Allanite also occurs in some limestone of the massive sulphides. As such the which has a tetravalent state as well, skarns and pegmatites, whilst monazite rare earths from partial digestions of and for Eu which has a divalent ion. is of course also a detrital mineral soils can be a very good vector to ore The Lanthanides have characteristically in mineral sand deposits where it – at the very least they are an accurate relatively insoluble sesqui-oxides, but sometimes constitutes an ore for Ce indicator of where the base of the importantly good mobility in weathering and other rare earths (and Th). Typical adcumulate ultramafic has undergone profiles and ability to be complexed e.g. contents of Ce in various rocks of the thermal erosion. in partial extractions. The trivalent cations crust are (data source Bernard Gunn, are large (e.g. Ce+++ ionic radius 1.11 Ao www.geokem.com): c.f. Fe+++=0.64Ao and Al+++ =0.50Ao) and possibilities for substitution in silicate N-TYPE MORB EPR FERRO-BASALT MAUNA KEA HAWAIITE ISLAND PHONOLITE crystal lattices is probably limited. The so-called Lanthanide contraction (shown 0.5-1.5 5-21 44-67 100-330 in Figure 1) is the very small reduction Table 1. Typical Ce contents of Oceanic Crust (ppm). in ionic radius which occurs from light to heavy rare earths from La+++=1.15Ao to Lu+++=0.93Ao is as a result of imperfect shielding of each 4f electron, similar outer electron shells and steadily increasing atomic number. SGS MINERALS SERVICES TECHNICAL BULLETIN MMI-TB18 2

Kimberlites, whilst having some a wider range of mineral deposit types two elements). However the differences similarities with ultramafics, also appear than previously recognized. in the ionic radii across the series result to contain concentrations of rare earths in subtle differences in the behaviour along with high Ni and Fe as would In country rocks it may well be that the of the rare earths during cooling and be expected from magmas with deep limited opportunities for substitution precipitation from magmas. As shown in seated origins. It is also possible that result in the Lanthanides along with Figure 1, the heavy rare earths are more in the ejection process, reaction occurs other “incompatibles” such as Cs, Rb, compatible than the light rare earths between the ejecting magma and the U, Th, K, Ti, Li, Be being “left” in high due to their decreased ionic radius. In crustal rocks through which it is being level fractionating fluids, and thence fact, the light to heavy rare earth ratios ejected and rare earths are incorporated an associate of feldspar in granites, in rocks provide a very good method for or at least “mobilized” around the syenites, monzonites and pegmatites identifying and categorizing magmatic reaction front. Irrespective of these two i.e. in felsic rock types. It is this context history. Soil up until recently have not possibilities, a “reaction rim” of rare that certain Lanthanides may prove to been studied for similar behaviour. Figure earths is a very important, consistent be useful as diagnostic of certain rock 1 below shows the Ni and Ce values and recognizable feature of kimberlite types, and certain alteration zones, obtained from MMI digestion of soils pipes. This can, and is easily detected in and have potentially wide application in across a BIF/Felsic and ultramafics, part soil geochemistry e.g. with MMI. mineral exploration. In this document of a komatiite sequence at Kooldesak, on several examples will be mentioned – the western edge of the Yilgarn Craton. Similar observations have been made Nickel Exploration, Diamond (Kimberlite) on a limited number of vein-type gold Exploration, “Inferred Geology” using Ni The ultramafics in the komatiitic deposits. Partial (MMI) digestions often and Ce as indicators of basic Archaean central section of the transect can be show high values for rare earths such geology, and La, Nd, and Ce as indicators readily identified by the Ni>2000ppb as Ce, Nd, La, Yb coincident with the of various porphyry alteration zones. soil samples. Clearly Ce is lower in high Au and Ag signals from soils above this (central) section and higher in the the mineralized part of the vein system. LIGHT TO HEAVY RARE EARTH adjacent BIF/Felsic terrain. In Figure 3 Again, comparison with the hard rock RATIOS (a-c), the histograms for other rare earths geochemistry shows that the rare earths Eu and Yb are shown along with Ce, are present in the mineralized part of As shown on Figure 1 above, there is as well as a figure (d) showing various the vein system, and probably indicative not a great deal of difference in ionic ratios, Ce/Yb, Yb/Eu and Ce/Eu. of the source of sulphur. Whilst the radius between one rare earth and the exact reasons are not yet clearly known, next. For many years it was thought that The heavier rare earth Yb, shows it appears that rare earth elements the very similar chemical configuration a much lower contrast in absolute have a geochemical mobility which is of the Lanthanides would result in very concentrations between the ultramafic both conducive to excellent vertical constant ratios for light e.g. La, or Ce and felsic/sediment samples (n.b. there migration (and spatial discrimination), and versus heavy e.g. Yb, or Lu elements in is one exceptional very high value). compatibility with weak extractions and the sequence in a variety of rock types This same phenomenon is observed in ICPMS analysis. Accordingly, they may (the ratio Ce/Yb is often used because of hard rock geochemistry - ytterbium is play a very useful role in exploration for the greater absolute abundance of these considered “conservative” and cerium “non-conservative”. Thus the Ce/Yb ratio also shows a marked change between the komatiite lavas and felsics/ sediments. This could be important and useful, particularly for applying this geochemistry to areas with cover, where absolute concentrations will not be directly relevant. The anomalous soil sample is very interesting; it also contains very high Fe, Eu, Cr, Sc, Al, U and Ti and is skeletal soil over a uniquely mapped sediment. Soil samples demonstrably komatiitic, either sheet flow ultramafic or mafic (MMI Ni>1500) were selected from the above data set; likewise soil samples demonstrably derived from BIF and containing high Fe (MMI Fe>15ppm) were selected from the above data. Table 2 shows relevant data for the rare earths Ce and Yb for these two data sets. Figure 2. Ni and Ce concentrations in soils after MMI-M extraction. SGS MINERALS SERVICES TECHNICAL BULLETIN MMI-TB18 3

These figures are compelling. For the in rocks – not surprising in one sense Ce move towards the high temperature komatiite group the Ce/Yb ratio is close since the soils are derived from them, aqueous (residual) phase much more to one - some of thes oils had a Ce/Yb but also comforting to know that at least readily than HREE when equilibrated ratio of less than unity, i.e. Yb greater in this instance, no major perturbations with rock material. It is no surprise to than Ce. The BIF group had only one soil have been introduced in the creation of find LREE enriched in late stage felsics, with a ratio of less than ten. These soils the soil imprint. Experimental evidence granites and sediments. show similar behaviour to that observed also shows that light rare earths such as

FACTOR KOMATIITE SOILS BIF + FeOx SOILS Ce mean 94.8 819.4 S. Dev. (n) 161 (23) 327 (27) Yb mean 34.9 67.4 S. Dev. (n) 24.8 (23) 47 (27) Ce/Yb ratio 1. 6 14 S. Dev. (n) 2.03 (23) 3.8 (27)

Table 2. MMI Ce and Yb data for komatiite and BIF samples. SGS MINERALS SERVICES TECHNICAL BULLETIN MMI-TB18 4

APPLICATION TO A RANGE OF POTENTIAL USES ROCK TYPES Nickel Applications Soils over ten different rock types at Diamond Applications “classic” or definitive locations in the Gold Applications Yilgarn Craton have been analyzed for Geological Mapping Applications the rare earths. The Ce/Yb ratios for these samples are shown in the graph in Figure 4.

Soils over granite, BIF and felsic rocks have Ce/Yb ratios over 20 (and they also have very high Ce). Mafic and ultramafic rocks have ratios less than 5.

Figure 4. Ce/Yb ratios for soils over various rock types.

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