Theo: I'll give my explanation another try ...
1) Glad to hear you no longer think that I'm calling Tom Dodge "a liar" <VBG>
2) If you're on the beach or in the desert, and if you pick up a stick and you draw a box in the sand, you have just drawn a boundary on a 2D surface; if a stream runs over this surface and deposits a little extra sand in this box, it has just deposited a thin layer of sediment on a 2D plane (the third dimension, its thickness, is relatively insignificant until several layers are accumulated i.e. a stacked series of 2D planes)
3) You can use the word "placer" if you want - I didn't invent any of these terms, nor do I claim to have, I only work them
4) In typical hardrock PM deposits, processes are three-dimensional - veins, dikes, sills, etc take various paths thru the host rock by moving upwards, laterally, criss-crossing each other, etc; these things also vary in thickness substantially ...
5) We still seem to be having problems in this discussion with my usage of the term 'homogeneity' - I would ask you to compare the following examples, and would ask you to consider which is more homogeneous:
i) a 200m(wide)x200m(long)x200m(deep) 'cube' of old, hard (metamorphic) rock that has been folded, sheared, twisted, baked, etc (just like BRN); inside this 'cube' you have drilled 5 holes down to 200m, one hole in the center and 4 holes on the corners of your property; only the one hole in the center intersected anything, but that was three randomly oriented quartz veins, each 1m wide and containing 4.0 oz/ton of PMs; you can make money mining these, but only if you can figure out how they are oriented in the subsurface; so you have to drill-up most of your 200x200x200m property to figure out where they are in 3D ...
ii) now consider a 200m(wide)x200m(long)x200m(deep) 'cube' of desert dirt (just like BRB); again you drilled 5 holes on this property down to 200m, one hole in the center and 4 holes on the corners of your property; this time you found average grades of 0.05-0.45 oz/ton over 200m depth in each of the 5 holes drilled; grades are lower than the quartz veins and they are 'variable' from hole to hole, BUT YOU DON'T CARE BECAUSE:
- you can make money at an average of 0.20 oz/ton
- even though grades vary from 0.10-0.45 in the different holes, if you consider the ENTIRE 200x200x200m 'cube', you have an average GREATER than 0.20 ...
- and better still, your company sedimentologist has told you that, because of the LARGE scale nature of the original depositional system (ephemeral streams on a 10x10 KILOmeter braidplain) relative to the SMALL scale nature of the property (200x200x200), he is absolutely convinced that the 5 holes are a statistically valid representation of a bulk average grade of 0.20 oz/ton for the entire 'cube' of dirt
- SO, ALL YOU HAVE TO DO IS DIG UP THE ENTIRE 200x200x200m PILE OF DIRT AND FEED IT THRU YOUR VIBRAMILL/PROCESS; YOU DON'T CARE IF YOU RECOVER A LITTLE LESS PMs TODAY AND A LITTLE MORE TOMORROW, BECAUSE IT ALL AVERAGES OUT IN THE END - THE ENTIRE 'CUBE', WITH IT'S 'VARIABILITY', IS STILL ESSENTIALLY HOMOGENEOUS AS LONG AS YOU MINE THE ENTIRE THING ...
- ... and best of all, YOU DIDN'T HAVE TO DRILL-UP YOUR ENTIRE PROPERTY AS YOU WOULD HAVE IF YOU WERE HUNTING FOR THOSE 3 QUARTZ VEINS.
6) Finally, you said: "All the sedimentologists in the world won't take the place of one driller, ever, in any placer mine"
I would never say this either, nor did I! Dirt-mine development will require BOTH sedimentologists AND drillers. My point was (and still is) simple, and that is ONE sedimentologist who gets a grip on the PM depositional process can probably save the company the cost of drilling several un-necessary infill holes.
Hope this helps,
Rock
|
