b'Don Emersons best of Exploration GeophysicsFeatureTable 2.Magnetic petrophysical classes of intrusive rocks and alteration zones associated with mineralisation: TYPE OF MINERALISATION MAGNETIC PETROPHYSICAL CLASSIFICATIONAu-rich ( 0.4 g/t) porphyry Cu within MFM mafic intermediate igneous hostSFM granitoid (M- or I-type)+ SFM potassic alteration zone PM phyllic zone + rocks WFM to MFM propylitic zone Porphyry Cu within MFM mafic-intermediate igneous host rocks MFM to SFM granitoidSFM potassic alteration zone + PM phyllic zone + PM argillic zone + WFM to MFM propylitic zonePorphyry Mo WFM granitoid directly associated with mineralisation, zoned to MFM less fractionated granitoid phaseAu-scheelite-quartz exogranitic plutonic veinWFM to MFM granitoid(scheelite contains Mo)W-Mo-Cu skarn MFM to SFM granitoid + SFM skarn W-Cu-Sn veins. Tungsten mineral is wolframite orPM granitoidMo-free scheeliteW-Sn-F skarnPM granitoid + WFM to SFM skarn Sn-W greisen PM granitoidCr, PGEs, Ni-Cu in lower levels of layered mafic complex PM to WFM gabbros overlying PM unserpentinised ultramafics or, more commonly, MFM to SFM serpentinised ultramaficsTi, V in upper levels of layered mafic complexSFM gabbros Sn-W, Be, Li and U associated with peraluminousPM granitoidtwo-mica granitesNb-Ta, REE mineralisation associated with peralkalinePM granitoidMFM to SFM carbonatiteanorogenic ring complexesPM = paramagnetic; WFM = weakly ferromagnetic; MFM = moderately ferromagnetic; SFM = strongly ferromagnetic; VSFM = very strongly ferromagnetic (see text)and lower pyrite contents in the altered host rocks allowcountry rocks can produce substantial quantities of fine-some of their magnetite to survive alteration. The dioritegrained magnetite that is capable of carrying intense and stable model is applicable usually to syenite monzonite porphyriesremanence.associated with diorites and often contain gold, as well as copper, but no economic molybdenum mineralisation. The magnetic anomaly at Mount Leyshon is a pronounced low Phyllic alteration, argillic alteration and intense propyliticthat arises from reversed remanent magnetisation (Q 1) of alteration associated with porphyry intrusions tend tothe biotite-magnetite altered metasediments and dolerites. destroy magnetite within the intrusion and in surroundingMagnetite-bearing skarns with reversed remanence are also rocks. Weak to moderate, but pervasive, propylitic alterationresponsible for negative anomalies at the Red Dome Au deposit may leave most of the magnetite in host rocks relativelyin NE Queensland (Collins 1987). Monoclinic pyrrhotite may also unaffected. On the other hand, the potassic alteration zonecarry intense remanence and produce large magnetic anomalies. associated with oxidised, magnetic felsic intrusions is oftenTherefore, pyrrhotite skarns may produce strong magnetic magnetite-rich. This is commonly observed for Au-richanomalies that are dominated by remanent magnetisation.porphyry copper systems (Sillitoe 1979). It is evident fromMagnetite-rich alteration zones around calc-alkaline porphyry the above descriptions of the Lowell-Guilbert and Hollisterscopper deposits have been extensively discussed by Clark and diorite models that the magnetic signatures of the two typesArancibia (1996). These authors argue that magnetite-rich vein of system should differ substantially. Clark et al. (1992a; 1992b)systems in and around some porphyry systems are often early presented a theoretical magnetic signature of an idealised(pre-mineralisation) and are distinct from magnetite-biotite gold-rich porphyry copper deposit, based on the Sillitoepotassic alteration that is associated with sulphides and Cu-(1979) model and magnetic petrological concepts. EarlyAu mineralisation. The magnetiteamphibole plagioclase potassic (biotite-rich) alteration around the gold-mineralisedalteration, with very little sulphide, represents the initial stage Mount Leyshon Complex (Queensland), which is comprisedin the evolution of a subclan of porphyry copper deposits. of intrusive breccias and trachytic to rhyolitic porphyry plugsDeposition of this assemblage is favoured by host rocks of and dykes, produced abundant magnetite in metasedimentarymafic-intermediate composition. Host rocks influence deposition and doleritic host rocks that adjoin the southern half of theof magnetite around these systems, but iron metasomatism Mount Leyshon Complex. That alteration is therefore largelyeffected by magmatic conditions is also demonstrably responsible for the Mount Leyshon magnetic anomaly (Sextonimportant. The early strongly magnetic alteration appears to et al. 1995). However, the equivalent alteration within felsic,be associated with strongly oxidised intrusions that contain iron-poor, granitic host rocks, around the northern portionmagnetite + sphene rather than the less oxidised assemblage of the complex, produces K-feldspar alteration with little ormagnetite + ilmenite.no secondary magnetite. Thus, the Mount Leyshon magneticWall and Gow (1996) recognise a magnetite-rich Cu-Au class anomaly is centred over the southern portion of the complexand a haematite-rich Cu-Au (U, REE) class of deposits associated and its adjoining metasomatised aureole, rather than beingwith Proterozoic felsic plutons. Magnetite precipitation may symmetrically distributed around the complex. be an important chemical control on sulphide precipitation In most porphyry systems, both primary magmatic magnetitein granitoid roof zones. The haematite association overprints and hydrothermal magnetite are generally in the multidomainthe magnetite-rich bodies and results from highly oxidised size range. Multidomain magnetite boosts susceptibility butlower temperature fluids with major meteoric component. La is not an efficient or stable carrier of remanent magnetisation.Candelaria-type magnetite mineralisation in Chile and Peru has Thus, most of the magnetic signature of such porphyry systemssome similarities and may be related to Mesozoic granitoids. is attributable to induced magnetisation, perhaps slightlyThe granitoids associated with these types of mineralisation are enhanced by viscous remanence. However, alteration of certainoxidised, high temperature, magnetite-series plutons.65 PREVIEW APRIL 2020'