b'Don Emersons best of Exploration GeophysicsFeatureInfluence of country rocks Mount Isa Inlier and the Pine Creek Inlier. Thus a rock unit that is The nature of the country rock is crucial in the case ofstrongly magnetic, indicative of high magnetite content, may be magmatic-hydrothermal skarn deposits, which developa favourable site for deposition of Au-Cu mineralisation sourced in carbonate rocks that have been metamorphosed andfrom a nearby granitoid.metasomatised by the mineralising intrusion. In most cases emplacement of the intrusion into non-carbonate rocks wouldConclusionsnot have resulted in economic mineralisation. The review of Einaudi, Meinert, and Newberry (1981) contains much usefulRelationships between magnetic properties of igneous information relevant to magnetic petrology of skarn deposits.intrusions and their mineralogy, chemical composition, Magnetite contents of magnesian skarns developed in dolomitegeological setting and history are complex. However, clear are generally higher than those of calcic skarns developedpatterns can be discerned and much progress has been made in in limestone, because Fe-rich calc-silicates are not stable in arecent years in understanding the geological factors that control high-Mg system. However both island arc type calcic skarnstheir magnetic properties. Although magnetic properties are (associated with gabbros and diorites in volcano-sedimentarynot predictable with any reliability from first order rock names, sequences) and Cordilleran-type magnesian skarns (associateda more detailed classification of intrusive igneous rocks does with quartz monzonites or granodiorites intruding dolomites)correlate well with magnetic properties, because there are many have been mined for magnetite. Such deposits are evidentlylinks between magnetic properties of igneous intrusions and associated with very large magnetic anomalies. their geological, chemical and mineralogical characteristics. Cu skarns (mostly associated with epizonal quartz monzoniteThese patterns arise directly in some cases, e.g. the correlation and granodiorite stocks in continental settings) are associatedbetween oxidation ratio and magnetite content, but in many with oxidised assemblages, including magnetite haematite,cases they are indirect.with the less common magnesian skarns exhibiting higherOne example of an indirect relationship is the strong association magnetite and lower sulphide contents than calcic skarns.between paramagnetic ilmenite-series granitoids and S-type Tungsten-bearing skarns (associated with mesozonal calc- granitoids. The magnetite-poor nature of most S type granitoids alkaline quartz monzonite to granodiorite intrusions) have aarises from their reduced character, which reflects incorporation more reduced calc-silicate and opaque mineralogy than Cu- of crustal carbon. Carbon content is an incidental, rather than a skarns, but typically contain minor magnetite and/or pyrrhotitedefining, characteristic of an S type granitoid.and would therefore be expected to exhibit a relatively weak, but nevertheless detectable, magnetic signature in most cases.Data on geological, geochemical and mineralogical associations Calcic Zn-Pb skarn deposits associated with granodioriticwith magnetite in granitoids are summarised in Table 1.to granitic magmatism and Mo skarns associated with felsicOf particular importance to exploration is the clear, albeit granites appear to contain relatively little magnetite. Sn skarnsindirect, relationships between a number of important types of are associated with reduced ilmenite-series granites andintrusive-related mineralisation and the magnetic properties have relatively low sulphide contents. The skarns themselvesof the associated intrusions and their alteration systems contain magnetitepyrrhotite and exhibit a substantially larger(Table2). There is now sufficient knowledge to develop and test susceptibility than the paramagnetic granite and unalteredimproved magnetic exploration models for intrusive-related host rocks. Massive sulphide replacement tin orebodies inmineralisation. Key elements that need to be incorporated into dolomite (e.g. Renison and Cleveland deposits, Tasmania) aremagnetic exploration models include:rich in monoclinic pyrrhotite and have high susceptibilities, with substantial remanent magnetisation. This type of orebody may represent the low temperature distal analogue of magnesian Sn1. Regional-scale links, summarised in Table 2, between skarns. intrusive-related mineralisation of a given type and the magnetic petrology of the associated intrusions.Webster (1984) analysed magnetic patterns over a number of2. The magnetic expression, at regional to deposit scale, of granitoids associated with tin mineralisation in the Lachlan Foldstructural controls on location of intrusions or fluid pathways, Belt and contrasted these with unmineralised and Cu-Mo-Wand of lithological controls on deposition of mineralisation.mineralised granitoids. The characteristic magnetic signature3. The magnetic expression of fractional crystallisation (zoned of granitoid-associated tin mineralisation is: a granitoid withmagnetic properties or complex patterns that suggest low magnetic relief, surrounded by a more magnetic aureole,multiple comagmatic intrusions of varying fractionation) at a with significant magnetic anomalies associated with thedistrict to deposit scale.mineralisation. 4. Use of magnetic petrological principles and magnetic Wyborn and Heinrich (1993) and Wyborn and Stuart-Smithpetrophysical data to predict the magnetic properties of (1993) have suggested that particular host rocks favourintrusive phases, host rocks and alteration zones for each deposition of Au mineralisation from oxidised fluids thatdeposit type.emanate from felsic granitoids and move up to 5 km from the5. Incorporating predicted magnetic properties into granitoid contact. Graphite-, sulphide- and magnetite-bearingconventional ore deposit models to enable calculation of lithologies are capable of reducing the fluids and depositingtheoretical magnetic signatures of deposits for a range of Au and Cu, whereas Pb and Zn are preferentially depositedgeological settings.in carbonate rocks. Au-only mineralisation will preferentiallyClark et al. (1992b) showed some examples of simple magnetic be deposited within graphite-bearing but magnetite- andexploration models. Although such models are inevitably sulphide-poor rocks, whereas magnetite and or iron sulphide- simplistic, if they are based on petrophysical data and magnetic rich rocks tend to precipitate Cu and Au together. Thesepetrological principles they should improve the utility of relationships appear to have been observed in the easternmagnetic surveys in exploration.APRIL 2020 PREVIEW 66'