Hanns Gully is located only 11km south-east of the Croydon gold mining district, however, Boadicea has identified it primarily as an exploration project for greisen-style lithium mineralisation. The licence has a total area of 296km2.
The key focus of Boadicea’s lithium exploration in the region is the association of lithium with extensive high grade tin mineralisation and previously identified greisen granite within the Esmeralda Granite that covers almost 100% of the area covered by the application (see Figure 2).
Figure 1 Hanns Gully Location Map
Hanns Gully Geology
The application is centred on historic tin workings known as the Stanhills tin field within the Esmeralda granite (Figure 2). Between 1900 and 1936 it is estimated that 260 tonnes of cassiterite concentrates were mined from numerous small but high-grade lodes. Previous explorers reported that tin mineralisation is commonly associated with a strong alteration of the granitic rocks. These greisen zones are fine-grained and composed principally of quartz and muscovite in approximately equal quantities. Lithium micas zinnwaldite and lepidolite are commonly associated with altered granites.
Figure 2 Geology of the Hanns Gully Tenement
The Stanhill field tin lodes are typically associated with chloritic and/or greisen (quartz-muscovite-topaz) alteration of the host rocks. The lodes commonly contain cassiterite, quartz and fluorite as well as sulphide minerals including chalcopyrite, pyrite, sphalerite, galena and arsenopyrite. Initial exploration will be focussed on the potential for large greisen bodies that could contain lithium mineralisation.
Features of Greisen style lithium mineralisation
There are about 120 different lithium bearing minerals. The most common is presented in Figure 3. The target of exploration at Hanns Gully is greisen igneous formation, which is hydrothermally altered rocks that is associated with granitic intrusions.
Figure 3 Geological Framework for Lithium Deposits (after MinEx Consulting 2019)
Greisens correspond to some parts of a granite intrusion transformed by hot hydrothermal fluids into an assemblage mainly composed of muscovite and quartz. Greisens are porous rocks and in these pores can be found apatites, cassiterite and sulphides. Greisens are located at the roofs of granite intrusions as well as along faults intersecting the granites. Greisens developed in crustal granites are typically associated with Sn-W deposits.
As an example of a world class greisen-style deposit is the Cinovec deposit in the Czech Republic (European Metals Holdings) which contains an indicated resource of 372.4 Mt of mineralised greisens grading 0.4% Li2O mainly in the form of zinnwaldite (See Figure 4). Cinovec is the largest lithium resource in Europe and also the world’s fourth-largest non-brine deposit.
Figure 4 Greisen Style Li Mineralisation Geological Model
 Source: Deliverable D5.2: Develop and / or review models for the formation of natural graphite, lithium and Cobalt in Europe. Havard Gautneb et al April 2021