re: Ann: Drilling Commences Curnamona SA Iro...
I like our chances of hitting a sepergene copper segment maybe even high grade gold!!!
ALKALIC MAGMATISM IN THE OLARY DOMAIN: GENESIS AND IMPLICATIONS FOR Cu-Au MINERALISATION Lachlan Rutherford1, Andy Burtt2, Karin Barovich1 and Martin Hand1 1Continental Evolution Research Group, Geology and Geophysics, School of Earth & Environmental Sciences, University of Adelaide, 5005, Adelaide SA. 2Primary Industries & Resources of South Australia, Mineral Resources Group, PO Box 1671, Adelaide, SA 5001 INTRODUCTION Both silica-saturated and silica-undersaturated alkaline magmatism have been associated with mineralisation styles including porphyry Cu-Au, Fe-oxide Cu-Au and carbonatite-hosted Cu- Au-rare earth element (REE). Alkaline magmatism often occurs in broad-scale magmatic provinces, some of which are temporally associated with more intermediate compositions. Silica-undersaturated alkaline magmatism at Billeroo, north of Plumbago in the northwestern margin of the Olary Domain has affinities to other alkaline magmatic complexes that host Cu- Au mineralisation. This abstract will briefly address the petrogenetic evolution of the Billeroo magmatic complex. In view of these petrogenetic interpretations, the prospectivity of alkaline magmatism will be assessed with regard to its potential for Cu-Au mineralisation in the Curnamona Province, particularly in the Olary Domain. PETROGENESIS The Billeroo alkaline magmatic complex is a highly heterogeneous intrusive body outcropping over <1km2. However geophysical data indicates that the complex has a two dimensional extent in excess of 5km2. The complex has ijolite, syenite, alkaline mafic dyke and breccia phases that are characterised by variable textural and petrological features. A primitive and depleted, mantle-derived source fractionated to first produce feldspathic-bearing ijolite phases. Typical ijolite mineralogy includes albite, phlogopite, garnet, Na-pyroxene, nepheline, cancrinite and accessory titanite, Fe-(Ti)-oxides, epidote, sodalite, analcime, Kfeldspar, apatite and calcite. Continued fractionation of the primitive source produced porphyritic and equigranular alkali syenite phases. Syenites are composed of K-feldspar phenocrysts within a matrix of albite, muscovite, biotite, K-feldspar, sericite, titanite, Feoxides, sulphides and calcite. Syenite intrusion was contemporaneous with the intrusion of an orthomagmatic or diatreme breccia. The breccia contains clasts of metasediment and syenite annealed by syentic magma, massive carbonate and pulverised rock flour. The final products of fractionation produced alkaline mafic dykes that are composed of albite, epidote, Fe-oxide, phlogopite, biotite, muscovite, garnet, cancrinite and accessory fluorite, titanite and apatite. Late-stage carbonate veining was ubiquitous in all the phases. Fractionation and hydrothermal alteration of a primitive source is supported by trends in major- and trace-elements with respect to SiO2 and Zr. During the fractionation process CaO, MgO, Na2O and P2O5 decreased as SiO2 increased, as would be expected through crystallization of Na-Ca-pyroxenes, nepheline, cancrinite and apatite. A dramatic increase in F, Sr and Ba in the final stages of differentiation is interpreted due to a high volatile content during the final stages of fractionation. Chondrite-normalised REE patterns are LEE enriched and become progressively more enriched as would be expected during fractionation processes. Reducing magmatic conditions are supported through the mobility of sulphides in the system. The introduction of H2O during emplacement of the syenite resulted in changes in redox conditions such that Cu precipitated, leading to anomalous Cu-Au values. The fractionation of a mantle-derived, primitive source is reflected in the åNd signature that 153 ranges between 0.3-3.5. These values are slightly more depleted that those obtained for the mantle-derived, rift-related 1700Ma A-type granitoids (Ashley et al., 1995). A pervasive northeast trending fabric has developed in the ijolites, breccia and around the margins of the syenite. This produced a schitose to mylonitic texture to the complex, particularly the ijolitic phases, coincident with pervasive dynamic recrystallisation. The general trend of the fabric is similar in orientation to the Olarian D3 fabric of Berry et al. (1984). Thus based on current tectonic evolution models for the Olary Domain and field relations, magmatism is interpreted to have occurred after the initiation of the Olarian Orogeny (~1600 Ma), and before 1580 Ma. MINERALISATION POTENTIAL Effects of Na-K-Fe-Ca hydrothermal alteration within the Billeroo alkaline intrusives have had a profound effect on the mineral assemblages. The hydrothermal alteration is interpreted to have occurred synmagmatically, although the majority of alteration is interpreted to have occurred during the final stages of crystallisation. Alkalic alteration is ubiquitous in all the different magmatic phases. However evaluating its full extent is difficult due to the primary feldspathic magmatic nature. Andradite, epidote, titanite, biotite, albite, Fe-oxides and calcite are characteristic minerals associated with Na-K-Fe-Ca alteration. The hydrothermal assemblages at Billeroo are similar to those observed in world-wide alkalic porphyry Cu-Au systems and mesothermal Fe-oxide Cu-Au systems. Syenitic intrusions and such alteration assemblages are common in documented alkalic porphyry Cu-Au deposits such as Rayfield River, Galore Creek and Copper Canyon in the Canadian Cordillera, British Columbia (Lang et al., 1995). Anomalous Cu-Au values in the syenite may indicate a metalbearing magmatic-hydrothermal system operated at Billeroo. At the current level of exposure, Cu-rich, Au-poor upper levels of such a system would have been eroded away. This has implications for supergene enrichment of Cu at the current level of exposure, and placer Au deposits in adjacent palaeo-drainage channels. Au-rich, Cu-poor mineralisation could still plausibly be at depth in the Billeroo system. Such a system would be of the high-grade, lowtonnage type. Skarn replacement-style mineralised systems may also be associated with the Billeroo intrusion if carbonate metasedimentary lithologies are nearby, similar to mineralisation in the Cadia system. The style of alteration observed in the Billeroo magmatic complex is also similar to those observed in Fe-oxide Cu-Au provinces. However this type of mineralisation is rarely associated within magmatic bodies. However large hydrothermal systems are known to have operated in the Olary Domain, making the Billeroo alkaline magmatism one of the potential reservoirs for Fe-oxide Cu-Au mineralisation in the Olary region.
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