b'CommentaryMinerals geophysicsThe search for favourable environments Minerals geophysics for pegmatite development might reasonably focus on the location and mapping of parent magmas, most typically manifest as granite batholiths Terry Harveywith contact metamorphism rims. In Associate Editor for Minerals geophysics some environments, granitic rock types will have locally lower densities, and E terry.v.harvey@glencore.com.au the hornfels margins, particularly those derived from basic igneous and volcanic Lithium exploration fromresistivity and electromagnetics, includingcountry rocks, can be magnetically rst principles MT/AMT. Re\x1cection seismics should o\x1feranomalous. Regional gravity and the most detailed resolution, but expensemagnetic surveys would appear to o\x1fer Based on present day technology, lithiumand vehicle access limitations on boggythe best chances of success in batholith is seen as essential for the proposedsalt lake surfaces may be considerations. identi\x1ecation and mapping.transition to renewables as a greenGeophysical location and mapping of source of the worlds energy needs. NotMapping of the extent and thickness ofindividual pegmatites is less straight-surprisingly, given its strategic value,lithium brine aquifers is rendered moreforward. Given their typically felsic, lithium exploration and production haveamenable to geophysical explorationnon-magnetic and resistive nature, already attracted a body of literature.by the expected strong electricalpegmatites may be identi\x1eable within Nevertheless, as a mental exerciseconductivity of the concentrated brine.some ma\x1ec, magnetic or conductive I thought it might be instructive toElectromagnetics would appear to becountry rock as areas of little or no postulate from \x1erst principles howthe method of choice here, particularlyresponse. But where the pegmatites have geophysics could contribute to lithiumin mapping the limits of the brine whereno signi\x1ecant contrasts in petrophysical exploration. As a \x1erst step in this process,it interfaces with fresher water, althoughproperties with their environment, it is necessary to understand thethe generally conductive environmentgeophysics just wont be e\x1fective. Some geological nature of lithium deposits andof salt lakes might prove troublesome. pegmatites do feature the development their constituent mineralisation.For relatively shallow aquifers, groundof potassium feldspar, and some carry penetrating radar may also have a role.radioactive minerals such as uraninite, Much of the worlds production of lithiumOnce drill-testing has commenced, is sourced from two strikingly di\x1ferentgeophysical drill-hole logging,which o\x1fer the possibility of direct styles of depositssoft rock evaporitesparticularly with resistivity/conductivitydetection with radiometrics, but only for and hard rock pegmatites. and porosity tools, would contribute tothose bodies that outcrop, or are covered the characterisation of the aquifers. with residual soils or a very thin veneer of Soft rock - lithium brines transported sediments. Lithium brine deposits typically occurHard rock - lithium pegmatites As with any situation, the selection and within a closed basin, where lithiumapplication of geophysical exploration salts derived from weathering ofPegmatites represent the \x1enal phasetechniques will depend on the nature of surrounding lithium source rocks haveof magma melts, more typically felsic,the target sought and the environment been concentrated in saline watersoften injected into the adjacent countryin which it is located.There is no single contained within suitable basin aquifersrock, and can be highly enriched in tracesuits-every-case approach.(Bradley et al. 2013). Because the miningelements and volatiles (Bradley et al. 2017). In some cases, pegmatites can beSo, in no way is the above intended to process requires prolonged evaporationspatially relatable to their parent magmabe prescriptive. Rather, it is aimed at of the brines to concentrate the highlybody, in other cases, no such parentalhighlighting the thought processes that soluble lithium salts prior to extraction,body is evident from surface mapping.might be used to select geophysical these basins necessarily are in aridFrom a lithium extraction viewpoint,techniques appropriate to an exploration environments. Typically, then, the surfacespodumene(LiAl(SiO 3 ) 2 ) is the preferredtarget and its environment, in this expression of the basin will incorporatelithium-bearing mineral in pegmatites; itcaselithium.a dry salt lake. The playas and salars ofis a non-magnetic pyroxene silicate with a the Atacama Desert in Chile and adjacentdensity of 3.03-3.23. Lithium pegmatites high Andean areas of Argentina andare typically also enriched in other traceReferencesBolivia typify this environment. elements; the Greenbushes lithiumBradley, D.C., McCauley, A.D., and L.M. Stillings, Geophysical exploration for lithiumpegmatite deposit for instance is also one2017. Mineral-deposit model for lithium-caesium-brines might reasonably then compriseof the worlds largest tantalumdeposits.tantalum pegmatites. U.S. Geological Survey two phases: basin delineation, andScienti\x1dc Investigations Report 20105070O, 48 p. aquifer mapping and characterisation. Geophysical exploration for lithiumhttps://doi.org/10.3133/sir20105070Opegmatites might then comprise twoBradley, D., Munk, L., Jochens, H., Hynek, S., and Basin delineation is a common aim inphases: identi\x1ecation of favourableK. Labay, 2013. A preliminary deposit model for soft rock geophysical exploration, andenvironments for pegmatitelithium brines. U.S. Geological Survey Open-File techniques typically brought to bearoccurrence, and pegmatite locationReport 20131006, 6 p.might include re\x1cection seismics, gravity,andcharacterisation.APRIL 2024PREVIEW 33'