Hi aubin, UFX, roblun, MB49 and other posters who like rocks,
Apologies for the technical nature of this post, but I thought it may be interesting to some to share.
I have added some of GAL's latest holes to your graphic aubin (
Post #:63496702) on the cross section under the map. The map below shows the depth in metres (yellow numbers) that the heavily disseminated sulphides in the black shale were intersected in GAL and GSR-CNJ's holes to date. Aubin's modified cross section under the map has been flatten at the top of the black shale. As seen in GAL’s recent holesNRC278,279, 280and NRC292-293,aswell as GSR-CNJ’s newholes, ifthey are drilled deep enough, they encounter what appears to be this ubiquitous blackshale /shale horizon (sediment)whichis sandwiched between intrusive ultramafic sills of pyroxene and/orperidotite. As we know, this blackshale / shale contains heavily disseminated sulphides with onsite visualsidentifying pentlandite(Ni), chalcopyrite(Cu) and sphalerite (Zn) as well is the gangue sulphides pyrrhotite (Fe(1-x)S and pyrite (FeS2).
Variousstudies suggest that the formation of magmatic Ni-Cu-platinum group element(PGE) sulphide deposits requires addition of sulphur (S) from the country rocks(
in this case the sulphidic, black shales at Mount Thirtsy) into the magma. If indeedthisiscase,theblackshale/shalesof Mount Thirsty may play a key roleasasourceofsulphur(S anion)needed in the formation of metal sulphides such as pentlanditeand chalcopyrite. Some of the principal observations thatrelate to many, but importantly not all, major Ni sulphide deposits includesuch features as available source of metal ions (e.g. Ni, Cu) in mafic and ultramafic magmas,a source of sulphur (e.g., sulphidic black shales) to saturate the magma,gravitational segregation of dense immiscible sulphide liquid, andconcentration of the sulphides into physical traps at the base of intrusions and in the case of Mount Thirsty within the black shale as well.Studies suggest that blackshale assimilation is the most plausible trigger mechanism for the formation ofthe magmatic Ni-Cu-PGE deposits in the Duluth Complex, Minnesota. The DuluthComplex magmas assimilated a large proportion of sulphur from footwall blackshales of the Paleoproterozoic Virginia Formation, which enhanced sulphidesaturation in the magmas. Consequently, the sulphide deposits occursystematically close to the footwall rocks. The details of the mode of sulphurtransport during the assimilation is are not fully understood. Both fluid andmelt phases are likely to mobilise sulphur during contact metamorphism (
e.g between the Mount Thirsty black shale and the intruding ultramafic sills above and below the black shale) and partial melting of the black shale.ThePartridge River Intrusion of the Duluth Complex represents an ideal andwell-documented intrusion for studying contamination processes, because Ni-Cusulphide deposits are found in the basal part of the intrusion close to thecontact with sulphur-rich black shales of the Virginia Formation. This has similarities to Mount Thirsty where pyroxene andperidotite sills surrounded the black shales and may have partially melted them; certainly there would be contact metamorphism.
Anyway, just some thinking around the possible significance of the black shales we are intersecting in the GAL and GSR-CNJ holes.
Also, about 20 kms NE of Callistro, black shale crops out. I have included the photo below to give posters some idea what it looks like at the surface. It very similar to what we are seeing the the diamond cores around Callistro.
Kind regards OldGeo
