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Gold/Mining/Energy : Precious and Base Metal Investing -- Ignore unavailable to you. Want to Upgrade?

To: The Barracuda™ who wrote (34172)	2/4/2005 11:49:27 AM
From: E. Charters	Respond to of 39344

Well, you don't know if the origin in some cases is not completely eroded as we don't know the depth of the intermontane deposit in the first place. Many characteristics are unknown. It could have been shallow, or it could have been high sulphide.. or low grade. In lower B.C. in the Penne D'Oreille there are two types of sources. One is HG quartz vein gold and another is gold associated with sulphide Pb-Zn-Ag-Au veins, which are plentiful. Pb-Zn veins in BC are ubiquitous with 1000's of small mines and showing in the Pac. NW. Placers are fair and medium grade, but not extensive. Some are associated with nearby volcanics. (the author did extensive placer exploration in BC for a major company) In the Yukon old time miners observed quartz grains with gold attached, and rounded pebbles of quartz which contained gold. Assays of quartzose gravel without free gold, often had returns. Despite this indication of a point source, that was never looked for immediately. Since then, from the 1930's on in Atlin and the Cariboo, and Yukon they have identified many lode type deposits that in part could have been the source of the placers. Atlin was probably formed from volcanic sources. In Californication, there were some identification of sub-economic sources. The Motherlode district of deep mines in granite were mined many years after the placers were found. The high Sierras yielded the odd lode gold mine. Many sources were never found, cut off from exploration these days by environmental regs and movements. Oddly the initial geological surveys of the area prior to the rush of 1849 identified the rocks in situ and the quarts and sericite micas that indicated that gold was formed there. These ideas were never followed up on en-masse as the placer riches took predominance being the far cheaper form of mining. This has been the case since China 4000 BC. If one type of gold is there for the taking, why look to where it comes from? The Ross Placer, a great placer in CA, was found to be made by many medium to low grade veins cutting gabbro in the valley. The grade of the veins fit the grade of the Ross, which was marginal. Despite this, the volume methods of the Ross produced economically. Mile long sluice boxes running at 25 miles an hour were fed by multiple hydraulic monitors or giants. The spray from these water engines that cut banks would kill a man or a horse at 1/4 mile. Horses drowned in the sluice boxes. I believe in general that the grade of the placer fits the grade of the source to a degree. The mass wasting of the mountains and the carry away of the rivers is never that efficient. You cannot get ten to one reduction of mass. The larger boulders and gravel will remain. No amount of river flow will carry them far. The just roll into the valleys and make the level the same, but even a 100 million years leaves the majority of the mass intact. Where the placer is formed is a valley channel. The amount of concentration can only be surmised. Perhaps the sands are displaced two to one, to 4 to one to heavy sands. Much of the lights can be washed away but not the cobble. Placers are formed not from transport of gold, but washing away of lights in rotary wave systems. They river constantly lifts lights, re-deposits gold and transports quartz. The swiftest river and most manifest mass wasting cannot send its heavy product more than 5 or 6 miles in geologic time. The idea that gold is concentrated 100's of miles from source does not bear up. All placers since early times have been shown from China to Switzerland to have nearby point sources. It is now the predominant sedimentary theory. That is why there were mounted exploration programs to examine the source of the river placers of AB. The assertion that they were Rocky Mountain derived from a distance of 100 miles is absurd, if one knows anything about the mechanism of placer formation. Dispersal would be the operative condition if that were the case. One can see that clearly in overburden exploration down ice of major gold systems in Ontario. Gold grains a mile from the veins are rare. Due to the mass of sericitized quartz in the Yukon White Channel one can surmise that the type of gold emplacement is similar to Ontario Gold mines, but later in time. The fuchsite and sericite of the remaining rock is greenshcist facies. This points to hypothermal, extreme temperature veins. Never mind what is said about epithermal giant deposits. Epi-thermal veins systems are usually small, less than a million tons. On the average high heat vein deposits are much larger. (The Timmins vein systems go to 100 million tons of rich ore. (McIntyre-Hollinger) Want to look for a Timmins gold mine? It would appear to be a ripping idea, but people are shy. Geological ignorance is the rule not the exception.) If a volcano-sedimentary environment was associated, the veins may have precipitated to a depth of several thousand feet. Our Archean gold systems are the deepest in the world. (Russian Archean systems bottom out at 1500 feet) The reason for that is the extreme uplift of our systems. The precipitation of the system may have been shallow, but it is no longer. In places where the rocks themselves are hotted and deeper, though the veins themselves are retrograde in temperature. They are greenschist. (Hemlo). I am encouraged however despite the young systems and low uplift, that the veins could extend to perhaps 1000 to 3000 feet. Their temperature tells me that they could. Their lateral extent may be generous. One thing worries me is that the ubiquity of placer gold in that area indicated a massive and widespread sourcing system. That itself does not augur well for anything but multiple smallish rich and nuggety veins or limited extent. Over the coming decades of lode exploration in the Yukon, we shall definitely see. EC<:-}

To: The Barracuda™ who wrote (34172)	2/4/2005 11:55:53 AM
From: zebra4o1	Read Replies (2) \| Respond to of 39344

How do we know if the source of placer gold had not already been totally eroded and that there is no "source" any longer? That's true in many cases. As I understand it, most of the California gold rush placers were 'twice removed' from the original bedrock source. The modern placers were created by erosion of much older placers. But in the case of Klondike Star, they tell a fairly plausible geologic story. They follow the placer creeks upstream until the reach a point where there the placer gold dies out. They do this for several of the richest placer creeks and find that these points all fall along a line, and this line corresponds with a long shear zone. And in one of the few locations where there is an extensive bedrock outcrop of this shear zone, there is an old hardrock mine (active 1912 to 1914). Thus they infer that the shear zone is the source of the gold. But I guess its still possible that the richest parts of this shear zone have already been eroded. And maybe the grade of the shear zone was always low. But 1000's of meters of erosion concentrated the gold. Of course it's always a bit suspicious when a geologic interpretation is this simple and understandable - could be a 'just so story'.