Summary:
Pegmatites form the host rock to the Li2O and Ta2O5 mineralisation at Mt Cattlin. It is generally accepted that pegmatites form by a process of fractional crystallisation of an initially granitic composition melt. The fractional crystallisation concentrates incompatible elements, such as light ion lithophile elements and volatiles (such as B, Li, F, P, H2O, and CO2) into the late-stage melt phase. The volatiles lower the viscosity of the melt and reduce the solidification temperature to levels as low as 350 °C to 400 °C. This permits fractional crystallisation to proceed to extreme levels, resulting in highly evolved end-member pegmatites. The fluxing effect of incompatible elements and volatiles allows rapid diffusion rates of ions, resulting in the formation of very large crystals characteristic of pegmatites.
The less dense pegmatitic magma may rise and accumulate at the top of the granitic intrusive body. However, typically the more fractionated pegmatitic melt phases escape into the surrounding country rock along faults or other structures to form pegmatites external to the parent intrusive, which is the case at Mt Cattlin.
Based on apparent mineral assemblages and textures, Mt Cattlin has been categorised as an albitespodumene type with the LCT classification. Moreover, the relatively highly coarse nature of spodumene at Mt Cattlin compared to that of other LCT pegmatites in WA suggests that these pegmatites crystallised from a high-fluxing agent melt (Wells et al., 2022). Whole rock geochemistry and mineralogy at Mt Cattlin indicate a broad fractionation trend to the northeast. The broad change in mineralogy from spodumene only to spodumene + lepidolite towards the northeast may represent a residual concentration of volatile and incompatible elements in this direction (cf.Sweetapple, 2010).
Various types of internal zonation from the footwall to the hanging wall of the pegmatites, based on variations in mineralogy, grain size and fabric, are reported in the literature (London, 2008). While zonation is not strongly developed in the Mt Cattlin pegmatites, changes in mineralogy and grain size are recognised across the pegmatite in places. In addition, the characteristics of the Mt Cattlin pegmatites vary to some extent laterally and between the different pegmatite sheets. Late stage metasomatism of lithia, in proximity to both the later dykes and post-emplacement faults is recognised.
Mt Cattlin lies within the Ravensthorpe Terrane, with host rocks comprising both the Annabelle Volcanics to the west and the Manyutup Tonalite to the east. The contact between these rock types transects the Project area.
The pegmatites, which comprise the orebody, occur as a series of sub-horizontal sills hosted by both volcanic and intrusive rocks. These are of the albite-spodumene subtype (Wells et al., 2022). Several dolerite or quartz gabbro dykes trending roughly east-northeast and north-south crosscut all lithologies including the pegmatite sills and are believed to be Proterozoic in age.
Mt Cattlin hosts spodumene-rich, Ta-bearing pegmatites. They occur as a series of sub-horizontal to shallowly dipping horizons that have intruded both the Annabelle Volcanics and the Manyutup Tonalite in areas close to the contact between these two sequences.
In places, the pegmatites occur as stacked horizons that overlap in section. Pegmatite mineralisation defined to date covers an area of approximately 1.6 km east-west and 1 km northsouth. The main pegmatite units are generally between 30 m and 140 m below the surface, and outcrop in some locations.
The Mt Cattlin pegmatites have diverse mineralogy with major minerals comprising of the following (Grubb, 1963, Sweetapple, 2010):
• Quartz;
• Albite;
• Cleavelandite (platy albite);
• Microcline;
• Perthite;
• Spodumene;
• Eucryptite;
• Muscovite;
• Lepidolite.
Minor minerals include the following (Grubb, 1963, Sweetapple, 2010):
• Tourmaline;
• Schorlite;
• Elbaite;
• Beryl;
• Microlite;
• Columbite-tantalite;
• Sphalerite;
• Amblygonite-montebrasite;
• Triphylite;
• Apatite;
• Spessartite;
• Fluorite.
Spodumene is the dominant Li2O ore mineral. Several varieties of spodumene are recognised including light green and white varieties. Ta2O5 occurs as the manganese-rich end members of the columbite-tantalite series including Ta-rich manganotantalite, and as microlite (Sweetapple, 2010).
Various lithium minerals have been observed within the pegmatites and include the following:
• Spodumene LiAl(SiO3)2 containing 4% to 8% Li2O;
• Amblygonite, LiAl(F,OH)PO4, contains 8% to 10% Li2O;
• Lepidolite, (lithium mica) contains 2% to 4% Li2O;
• Cookeite, (lithium-bearing chlorite).
The mineralogy within the pegmatites varies laterally and displays a crude zonation oriented perpendicular to the margins, which are identified by changes in mineralogy and grain size. Northeast portions of the now depleted deposit contain the Li2O-bearing mica lepidolite. The lepidolite-rich zones contain higher Ta2O5 grades, which are mainly microlite, and display more pronounced zonation perpendicular to the margins of the pegmatite. Zonation within the pegmatites include an aplitic rock comprising mainly quartz-albite-muscovite near the contacts with the country rocks, and zones of predominantly light green, and predominantly white spodumene. Lepidolite is generally associated with white spodumene. Quartz-tourmaline veins related to pegmatite emplacement are observed in the country rock up to tens of metres away from the pegmatite.