RE: Tweet21 Aug 2022 04:46
Those with time to spare and a large flask of strong coffee might care to look up Algoma-type BIFs, which tend to be more constrained than the better-known vast economic BIFs of the Hammersley Range in Australia, or those in Brazil. They are associated with volcanic vents or crustal fissures, and are often coincident with VMSs (Volcanic Massive Sulphides).
Re. the speculation about Carlin-type (finely disseminated) gold in the Ditau core, here's a good starter:
https://www.sciencedirect.com/science/article/abs/pii/S0895981118304255
"Important host rocks for orogenic gold mineralization, BIFs have a composition that favours the replacement of iron bands by sulphides, accompanied by the precipitation of gold. They host gold deposits in many granite-greenstone terrains such as the Slave Craton, Churchill and Superior Provinces of Canada, and in the Amalia-Kraaipan greenstone-granitoid terrane of the Kaapvaal craton, South Africa. In BIF-hosted orogenic gold deposits, hydrothermal magnetite is commonly an important phase, as is the case at the giant Golden Mile deposit, Kalgoorlie." (slightly edited for brevity & clarity).
On 18th May here, I said about the Ditau core "The stripy red stuff is hæmatite, which is an iron ore normally formed from sedimentary deposits - commonly as in Australia's huge banded iron ore deposits, but more rarely from volcanic activity. From its banded appearance, I suspect this is of sedimentary origin, but has been metamorphosed (heated & squashed), which is why the bands are wavy. Next it has been fractured - hæmatite is very hard, but very brittle. This brecciation may have been caused by pressure from a nearby intrusion, such as a carbonatite - for example, Olympic Dam mineralisation is hosted by brecciated granite. Next it has been subjected to probably two injections of hot aqueous fluids - one rich in silicates, giving the milky quartz depositions in the cracks, one rich in iron sulphides, giving the pyrite." and
"this looks a bit like hæmatitic jasperoid. That's a very unusual rock which I think is only found (so far) in the Proterozoic near Lightning Ridge, NSW. It contains finely disseminated gold (Carlin type), mostly within pyrite".
I don't want to be accused of rampant speculation, so perhaps I shouldn't mention that the Algoma-type BIF Cuiabá deposit in Brazil is estimated to contain 160 tonnes if Au. Enjoy the rest of your weekend.