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20/11/17
01:18
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Originally posted by Blue swims
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Yes definitely needs the mafic conglomerate and the MRB capping, but my own take on the rest is a bit different. Take it as you will.
Regionally- 2.8 Ga was about the start of period of intense earthquake / volcanic activity probably tectonic or rising magma derived.
Location- if we're looking at Northern Pilbara evidence is that theyre generally a shallowish lacustrine( lake) environments with an ecosystem.( stromatolites, methotrophs, methogens)
Timing -2.77 Ga is a date where several events/ processes can simultaneously occur . MRB flows , mafic conglomerate,hydrothermal vent alteration, dolerite dykes ( all zircon dated).
Phrenomagmatic eruptions from regional fissure eruptions creating large amounts of material -basalts (milling) , hydroclastites and large pyroclastic debris flows in a subaqueous ( lake)setting- Rhyodacitic formations.( volcanic conglomerates).
My take is the same events are driving a large regional metasomatic system ie hydrothermal fluids ( probably sulphides)from deep down dissolving gold ( probably from the greenstones) and pushing it upwards towards surface under pressure and temperature.
Fluids using the vocanistic conglomerates due to high permeability and other pathways.
As hot fluids reach cooler zone further chemical changes take place, the fluid slows,condenses and releases the condensed gold out of solution . Higher less permeable conglomerate layers means less gold.(?)
Lava can typically reach over the melting point of gold so possibly the subsequent MRB flows results in baking the conglomerate and further concentration of some deposits into nuggets(?),then the capping and slowing of the erosional process.
Metasomatic systems in the Fortescue group have been studied and indicated regionally over a 450km x 200km area. Possibly more than twice.
I've relied on work done by Hickman, Blake , Thorne and Trendall, Huston, White, Van Kranendonk
and other teams who've studied the Pilbara over the decades. Plus a splash of tomato sauce.
Apologies for a few terms, just trying to be brief and concise. Good luck to all gold heads
Hundreds of cubic kilometres of rock sucked dry of gold and concentrated near surface sounds alright to me.
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Thanks for this post Blue swims, the manner of emplacement or concentration appears complex and has been debated on these and other threads. Metasomatic activity certainly seems a valid option. I would like to follow up on some of your references if you could post more detail.
Cheers
BS