"Along these lines, the S-source was suggested by Naldrett to be upper crustal because once the sulfide fluid was created (Ni from olivine+S from host) is could not travel far (relatively speaking via fluid mechanics). Sulfur isotopic studies at Voisey were inconclusive as at many other sites. Off the top of my head I can't think of any mantle-derived S in Canadian Ni-deposits????"
With metastability being operative and possible catalytic forming of minerals it is bard to say if certain minerals actually prove the geobarythermometry inferred. However much of the Gabbro and Serpentine appears to mantle derived as in the TBay area you can see omphacites in them. (very high pressure pyroxene, next to perovskite as an indicator of depth). The sulfur is different. But also the nickel itself may be crustal and not mantle derived, at least in the last cycle, the proterozoic. The Tasisuak gneiss is geochemically anomalous in Ni and domimantly so of that suite in the Churchill, for 300 miles. I would think the Sudbury and Thompson gniesses are similarly so.
I agree with Naldrett in that the sulphide fluid is too turgid to get anywhere. That and the fact that you do no see any pyrite cinder cones or massive pyrite mountains sort of tears the fluid magma theory. Also it would demand that all nickel bodies be at the base or near the base of gabbros in some dilation zone. They appear to be at the base of what appear to be intrusives frequently, but all too frequently they are also are formed late in the cooling cycle and are seemingly separated from the parent magma. So there is another explanation for their frequent migration from the parent. The remobilization during outgassing could cause vent deposition far from the host transport/remobe of the metal. This favours the sulfidization more than intimate contact with a fractionating melt. The co-incidence of the contact nature of the deposits and the relative lack of non- stuctural transgressive radial localized deposits leads one to believe that water was the more effective transport medium than fluid channels of molten melt. The ubiquitous formation temperatures at around 350 degrees, which corresponds to hot vent precipitation on cold seawater contact, being explained by the magmatists as "metastable late formation after passing the eutectic" shows the weakness of their theories. Not to mention that in the Sudbury belt, to support magmatism one would have to conveniently overturnm the entire basin. A neat trick that is not observed in the actual age succession of the sediments. The other old saw about isotopic rations being preserved in nickel deposits at the mantle sulfur ratios as proof of closed systems is also bit stretched. They have not shown the mechanism that would change the ratios in a precipitative environment. After all, what you dissolve you also precipitate en masse. The other problem with magmatism is that it serves to deny all other operative forms of orebodies that may take place and would take place with all that magma being around. No secondary hydrothermal veining with all that hot sulphur and iron contacting all that water? So we have 3 types of deposits, layered exactly like the many copper zinc deposits, sitting on sedimentary rocks like many SEDEX deposits and within 1000's of feet of volcanic and other extrusive structures at later date and similar levels of erosion and stratigraphy, and they ask in Sudbury for one type of deposition for the nickel. The Strathcona, a trellis of dendritic branching veins in a dilation structure looking all the world like a hydrothermal vein and the fractionating disseminated pipe -like Creighton and they call for one type of deposition. Seems a shame.
I have no trouble with it but I could not reconcile the explanation or the rationalization to grade school children.
Finally we have the astonishing, lame rationalization that all the nickel is at embayments corresponding to basins in the sedimentary sublayer and at the fine grained flow like contact all around a sedimetary basin, because it is the preferred plane of weakness My word! Ain't it the truth. 350 miles of basin edge and there aint one other nice little offshoot fault or trap to be found! (Ok the nickel offsets.)And if the nickel is so much later than the basin, then of course you find it all through the center and coming up everywhere at crazy angles in great volcanic blobs. Along with the Norite. But lo! this is not so. So little rug, so much loose dirt.
So we have Gneiss, then sedimentary sublayer, then volanics, then sediments, then Norite, then Nickel ore. Ok. And the Nickel and Norite inset themselves between the masses of sediment and the sublayer and the Norite right at the contact. Transgressively and neatly, let's not miss that contact as their is so much to intrude. The nickel was so careful. The Norite was fantastically polite! How considerate!
They would be far ruder and more careless if you made them flows and sediments. Then they could do what they liked and they would end up at exactly the same place. Occam needs to shave. Pass the razor.
I like nice rocks. They obey all my theories.
EC<:-} |
