Hi
@gungadin,
Yes I think you are right. From the announcement
"Preliminary geological logging of RHD04 has confirmed four zones of significant
disseminated (+/- matrix style) pyrrhotite, pyrite and traces of chalcopyrite mineralisation in silica+/-sericite altered gneiss, each with downhole widths of between 8m and 18m."
Gold mineralisation in altered gneiss is a bit of an atypical style for the Yilgarn from my experience. I did find what might be an analogous deposit in India from the late Archean geological period (which is the right period). Western Australia and India were thought to be connected at one point in time so the example is pretty close to home in reality (see gif below). The gold deposit found in the Champion Gneiss in India has a reserve of 3.13 million tonnes averaging 4.7 g/t gold (~473,000oz). Besides this lode there are several gold loads occurring within mafic units as well as adjacent felsic units with different size reserves. See the article below for more description and the extract I've pulled out of it explaining the style of alteration and mineralisation. It is interesting to note from the article that where pyrrhotite occurs in massive form the deposit also contains Galena (a lead mineral) and this
is always associated with high gold values. It might be worth for the company to test for this Pb association should the Champion Gneiss deposit style fit the mineralisation, alteration and rock types they are finding at the Rebecca Project. Esh
View attachment 671271
http://www.vulkanisme.nl/images/begrippenlijst/platentektoniek.gif
Gold Mineralisation in Chigargunta Area of the Kolar Schist Belt, South India- A Part of the Archean Greenstone Belt
http://article.sciencepublishinggroup.com/html/10.11648.j.earth.s.2015040401.14.html
Extract from article describing mineralisation at the Champion Gneiss deposit which is shear controlled.
The Kolar schist belt, one of the auriferous Archean schist belt in the Dharwar craton, includes two types of gold deposit : 1) a stratiform sulphide type, associated with amphibolites and banded ferrugineous quartzite and 2) a gold quartz carbonate vein type associated with LREE- enriched komatiitic and tholeiite amphibolite on the eastrn part of the belt (Sivasiddaiah and Rajamani,[
12])
Gold mineralisation in the Chigargunta area is localised along zones of high deformation which are ductile to brittle in nature and occur over a strike length of 3 km in Chigargunta area (Fig 2b).
Though shear zones of different orientation are seen, only those which have a near parallel relationship to the foliation of the country rock are found to be mineralised. Those that transect the foliation at high angles are much younger, ductile in nature and are barren of mineralisation.
The mineralized shear zones are characterised by a strong mylonitic foliation and profuse hydrothermal alteration. Significant changes in grain size and texture are observed in the champion gneiss in proximity to shear zone. Major changes in silicate mineralogy and texture in and around the shear zone reflect the onset of extensive hydrothermal alteration that accompanied mineralization. Sericitization, biotitisation, silicification, tourmalization, flourtization and carbonatization are the noticeable wall rock alteration. Biotitization is the main alteration noticed in the mineralised zone. The biotite thus formed is honey brown colour. Fibrolite development is seen in the feldspar of Champion gneiss which in turn is sericitised. Tourmaline occurs as tiny prism, pale greenish brown to black in colour showing an orientation parallel to shear zones foliation. Presence of fluorite, tourmaline and scapolite in abundant quantites in the mineralised rock indicate that the hydrothermal solution were rich in volatiles like F, Cl and B. Presence of appreciable quantities of mica, some of them lilac brown coloured, coarse along with fluorite, tourmaline suggest a greisening effect.
Quartz occurs as veins, veinlets and pods parallel to the shear zone foliation and appears to be liberated from wall rock. Pegmatite and granitic materials are also seen as veins and pods within the mineralized zone. Those granitic veins frequently follow zones with high gold values. Hornfelsic texture is very much evident in the mineralized zone, because of the introduction of new materials probably at moderately high temperature.
Pyrite and pyrrhotite are the dominant sulphides present in the mineralized zones. They occur mainly in disseminated form in the Champion gneiss hosted mineralization. Frequently pyrite dissemination are seen for considerable width in the wall rock of the Champion gneiss.
In the mafic hosted mineralisation the proportion of sulphides are found to be more. Pyrrhotite is the dominant sulphide along with pyrite.
Pyrrhotite sometimes occur, in massive form. Aresenopyrite, chalcopyrite, galena, sphalerite and stibnite are noticed in minor quantities. Galena is always associated with high gold values. Gold occur in native form within quartz stringers, pods and veins.
From the available evidence it is apparent that gold emplacement took place in two stages in southern part of the Kolar schist belt. The sulphidic BIF are auriferous in many places showing high back ground gold values (about 100 ppb). It is likely that this interflow sediments got deposited in similar fashion to the ones which are forming around present day oceanic spreading centre (Seibold and Berger,[
13]). The association of pillowed tholeiite lavas with the auriferous BIF and the presence of minor subvolcanic basic intrusives with post-peak metamorphic mineral assemblage described earlier, are strong indication for such a process to have been operative. In the ore zone, native gold has also been found as finely disseminated grains within arsenopyrite. (Mukherjee,M.M.,[
9]). The second stage of gold mineralisation is epigenetic and shows an independent tectono-metamorphic evolution postdating the major folding and regional amphibolite facies metamorphic event. Auriferous zones are confined to discrete ductile shear zones where large scale retrogression of the silicate and quartz-sulphide-gold mineralisation occurred as a result of wide spread introduction of hydrous fluid. The spatial association of some acid porphyries within and close to the mineralised shear zones and the hydrothermal alteration of the host rocks point towards an acid magmatic source for the ore fluids. However, the tonalite-trondjhemite-granodiorite suit of Peninsular gneiss outside the schist belt was formed earlier than the major tectonic evolution of the Kolar schist belt. Immediately to the south of the Kolar schist belt, there is a transition zone where extensive migmatisation of high grade supracrustal rocks have taken place (Condie and Allen, [
14]. Further south of the transition zone, progressive metamorphism of gneisses, mafic and sedimentary enclaves to granulites by dehydration and CO
2 streaming along N-S trending shear zone at about 2500 Ma has been indicated (Gopalakrishnan et al,[
15] )
