Summary:
Deposit Type
The Wodgina pegmatite deposit is hosted within the Wodgina Greenstone Belt of the Pilbara Craton. The Pilbara Craton consists of intrusive granitic batholiths into mostly metamorphic greenstone terranes with associated tin-tantalum-lithium- beryllium pegmatites, ironstone (iron ore) formations, and gold mineralization.
The Wadgina Greenstone Belt is largely a north-to-northeast plunging synformal to monoclinal structure that is approximately 25 km long and 5 km wide. It is comprised principally of interlayered mafic and ultramafic schists and amphibolite, with subordinate komatiite, clastic sediments, band iron formation (BIF) and chert.
The pegmatites which form the Mineral Resources are interpreted to be relatively un-zoned albite-spodumene pegmatites in the upper portions, with increased fractionation at depth of the LCT (Li-Cs-Ta) type. It is generally accepted that pegmatites form by a process of fractional crystallization of an initially granitic composition melt. The fractional crystallization concentrates incompatible elements, such as light ion lithophile elements and volatiles (such as B, Li, F, P, HzO and COs) 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 crystallization 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 peqmatites.
The Wodgina pegmatite field lies immediately to the east of the axial plane of the synform in the Wodgina Greenstone Belt and adjacent to and within splay structures related to a major craton-scale NE-SW trending lineament. The Wodgina pegmatite field contains three major pegmatite groups, each hosted within a different lithology and subject to different structural/rheclogical contrals:
• A complex zoned group, belonging to the lepidolite sub-class of the complex pegmatite type. This pegmatite type encompasses the Wodgina main-lode, Rockhole and Camp pegmatite bodies, hosted by meta-komatiites and meta-basalts of the Kunagunarrina Formation.
• Variably altered, weakly zoned to internally homogeneous pegmatites of dyke and stacked-sheet morphology, belonging to the albite-spodumene pegmatite class. This pegmatite type encompasses the Mt Cassiterite and Mt Tinstone bodies as well as the Eastern Pegmatites (most probably part of the same stacked sequence of sheets); hosted within the psammitic to pelitic interbedded metasediments of the Leilira Formation.
• Simple zoned albite-muscovite-quartz pegmatites, with pale green beryl and columbite mineralization. They are usually of limited thickness and extent, occurring on the margins of the greenstone belt ina sheared metavolcanic to ultramafic unit.
The pegmatites that have been mined in Wodgina’s history are the Wodgina main-lode pegmatite and the Mt Cassiterite and Mt Tinstone pegmatites. A major regional shear zone separates the two main pegmatite groups. Both pegmatite groups have been emplaced syntectonically into fault/shear zones, with a predominantly reverse sense of movement. The Wodgina main lode pegmatite appears to be related to a major inclined fold hinge, while the pegmatites of the Mt Cassiterite group appear to be sheets joined by a number of parasitic fold hinges.
The Wodgina main-lode pegmatite strikes essentially north-south and dips 40° to 45° to the east, it is exposed over a strike length of 670 m and varies in width generally from 3 to 15 m, although at one place on the north end, it reached 91 m in width. Lithium mineralization at Wodgina is concentrated in the Mt Cassiterite-Tinstone Pit area, which contains the in situ Mineral Resources.
The Mt Cassiterite and Mt Tinstone pegmatites consist of a group of subparallel, interfingered, un-zoned albite-spodumene pegmatites that intrude the mafic volcanic and meta- sedimentary host rocks of the surrounding greenstone belt. Individual pegmatites vary in thickness, with an average dip of 22° to the southeast. These pegmatites are abundant in albite and primary spodumene with subordinate K-feldspar and minor muscovite in near-homogeneous sheeted bodies and lepidolite. The pegmatite sheets display a massive to comb-textured internal structure, which is regarded as being characteristic of albite-spodumene type pegmatites.
The pegmatites can be grouped into an upper thinner swarm (10-30 m in thickness), a middle thicker swarm (30-80 m in thickness), and a thick basal unit (120-200 m in thickness) and are typically exposed prior to mining) over an area 1,100 x 800 m. The upper sheets are generally hosted by weathered and oxidized meta-greywacke, whereas the lower pegmatite sheets intrude fresh pyrrhotite/pyrite-rich meta-greywacke.
In addition to the dipping pegmatites, a number of vertical to sub-vertical pegmatite dykes that trend northwest- to-southeast and northeast-to-southwest occur. These dykes vary in width from 10 to 50 m and have been interpreted to extend 600 m along strike and up to 250 m in depth. The pegmatite sheets usually have acoarse- grained (up to 1 cm) massive biotite alteration selvage up to 1 m thick along the footwall and hangingwall contacts where the contact is conformable with the country rock. However, where the contact is structural (generally along thrust-faulted contacts), this selvage zone is absent.
Mineralization
The Mt Cassiterite-Tinstone upper pegmatite sheets are mostly un-zoned, with mineralogy dominated by phenocrysts of spodumene (10-30 cm long) and K-feldspar in a matrix of fine- to medium-grained albite, quartz, and muscovite. Zonation caused by fractionation appears to increase with depth, and varies between the three main domains used in the Mineral Resource estimate. Veins of quartz up to 10 cm thick are common, as are 1 mm thick veinlets of green sericite-albite. These secondary features often occupy parallel fractures adjacent to the main dyke swarms. Texturally the pegmatite is extremely complex, showing evidence of multiple silicification and albitization events.
Some mineralized zoning of the pegmatites has been observed, with higher concentrations of spodumene occurring close to the upper contact, and near-perpendicular alignment of crystals to the pegmatite contact exhibiting distinctive ‘pull apart’ structures. In the massive basal pegmatite, the spodumene is distributed within fine-grained quartz, feldspar, soodumene and muscovite matrix. A weak zonation is evident in the development of finer-grained border units and occasionally in areas rich in microcline crystals.