The fabulous Golconda. Chlorine and bromine. 1899. In 1895 they introduced roasting and cyanide in Colorado (Kokomo, also the birth place of the long-axis turning continuous throughput ball mill.) which improved recovery and increased mine life.
By the 50's roasting and custom milling was out of favour because of air pollution, high grade mine depletion, high labour and low gold price and this method of treating telluride and refactory ores of the SW faded. Most of the ores of the SW US are refactory without roasting and the chlorine gas method which was popular pre 1895 gets low recovery as it will not break down sulphides. The Russians have gas technique they used for aluminum ores, that is similar and might work. They key to cyanide was roasting though, and being exothermic it was cheap.
Slate at the granite contact sounds like the Motherlode ores which were basically granite associated. I wonder if the granite is really granite and not synenite or granodiorite. And the slate, is not slate but a proto metased tuff or wacke of some type. All kinds of Archean ore in Canada is in metaseds, wacke, conglomerate or vocanic tuff, cut by qtz feldsapr porphry and contacting subaqueous volcanics. Volcanics or seeming intrusives are always around near the contact but are not talked about in the States. Since ore does not occur in the volcanics they US geologists ignore them. I find that in different regimes or areas, different mythologies of geology develop.
In Canada we always set up on the pillow lavas, drill through the volcanic tuff ore zone, (sediments) and into the porphyry. The common phrase is "drill the shit out of the shear zone." You get fancy terms for it, like mylonite, but it's a shear. Rock associations of the archean gold ore are iron formation, quartz-horneblende diorite, granodiorite, aplite, syenite, andesite, pillow lava, quartz-feldpar porphyry, mafic tuffs, potassic granite, basalt, and diorite. You always find the vein in the contacting quartz-flooded iron rich sediment (wacke, conglomerate, or tuffaceous rock. I tend to differ but dating seems to put all ore formation at 2.8 billion to 2.6 billion years ago.
If however I am right and the age of the vast Matachewan dyke swarms is proterozoic at 1.8 billion, then in fact the ore formation which is always last in gold, must post date the folding event which shifted the dykes. The age of the dyke swarm then predates the emplacemrnt of gold, which hydrothermally-structurally must post date the folding. (The host quartz had to have dilation zones created by the folding to precipitate into. If the folding displaced proterozoic dyes, then quartz and gold is not archean, unless remobilized. The latter, I doubt) This would mean our fabled gold of Geraldton is proterozoic or one billion years younger than the Archean. This would explain why it is in oxide iron formation which was formed when the planet had an oxyen atmosphere. Otherwise the iron formation has to be sulphidic as it could get no oxygen otherwise. It is magnetite implying formed at the time of oxygen. The ore being structurally emplaced in this it must be later. It is quite evident that the east west faulting that moved the last great orogeny of the shield must be a late event. How late? Are we to place three orogenies and folding events prior to 2.8 billion and then cratonize the area for all time? I favour, since Sudbury is a mere 500 miles east, and 30,000 foot Grenville mountains loomed east of it, shoving into the basin and falling over 1.8 billion years, ago that some of that orogeny must have influenced Geraldton at that time. So we have contemporaneous nickel and gold deposition in Ontario just prior to life arising on the planet. Structure seems to favour this theory. (These 6 mile high mountains were the highest ever on the planet, they are gone today and only the laurentians remain. A tad of erosion must have gone on since then. I always wondered where the great placers of that time must be since much of the mountain arc area contained gold. These placers have to have formed like the ancient SA reefs in the residual sediments of the post Archean and Proterozoic age.
In looking for them they ran across a sandstone-conglomerate near Georgian Bay in the 70's that ran 1/10 of an ounce across 15 feet. There were numerous gold veins in the conglomerate east and west of Subury that contained gold. Were they veins or residual gold placers? The Cobalt Conglomerate is identical to the conglomerates of Ghana that carry placer reefs. No assay of note has got gold out these rocks. But in the Vermillion River east of Sudbury there is a famous unconsoldiated placer that runs for 100 miles. In ever occurence of placer gold there is broken down, much conglomeritic rock and quartz pebbles. Hydrothermal or fossil placer source? I am sure there is gold in our sediments like South Africa. I have been sure of it for 27 years. No one has looked with enough money or guts. Like the Whiteshell gravels of the Yukon, which were a chrome mica carbonate and pebble quartz, identical to the Kerr Addison mine, no one has looked for the source. It is there. (All placers have point source economic or near economic point sources, it is the nature of concentration that his must be. The motherlode concept of the early prospectors is really finally true.)
Basically you can stake any subaqueous mafic volcanic which has tension fractures, shear with iron-quartz, carbonate, and porphyry. You will generally find gold. The old timers used to say, greenstone from the colour of the lavas and altered sediments.
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