One always expects ore bodies or mineral deposits more properly to be regular, tabular or lensoidal shapes of high school geometry. Figure a dip or plunge, plan a hole and hit the tail of the donkey. Practical reality looking the shape of the known bodies out there suggests strongly otherwise. They consist in most mines I have worked in as multiple, pipes lenses, tables, pods, flat lying, vertical, shallowly plunging, steep, changing in dip drastically, pinching and swelling, running out mysteriously and reappearing for no fathomable reason. Faulted, bent, imbricated (stacked) anastamosed (braided) cut off by dykes, thrown for 100's of feet to miles.
And all of this shape is also governed by economics itself. The shape of the economic sections is overprinted on the actual mineral deposition sometimes in an equally complex way, governed by semi random mineral variation due to precipitative pulses and remobilization(s), folding and faulting repetition, etc.. They can be complex.
The science of structural geology, the analysis of the formal laws of folding and faulting of beds, the sciences of geochemistry and petrology, orogenesis and economic geology are just a start at figuring out the many ways ore can occur subsurface. And the surrounding host rocks, which appear to be massive and regular often only appear so. Plastic rocks and rocks deposited later or earlier can appear to have no form except as blobs but may hide an occult structure that is massively complex. When you consider that as many as 12 phases of deformation may have taken place in a mineral area, and earlier phases may have been straightened out and reshaped again, it is no wonder that structure is hard to follow. Economic phases may even plunge or dip completely opposite to the major assumed host structures that control them.
The history of these phases is all important to understand, but is often far beyond the initial investigator's capabilities to divine. Often the real structure, which the initial holes have touched will await underground mining before it is finally realized what happened to the ore and its surrounding rocks billions of years ago. The Archean rocks were lain down flat, and are now obviously vertical or near vertical. In addition their straight edges have been crumpled into rough folds along strike, down dip and even down plunge. Axis of folds in turn become folded. Most often the mountains these orebodies were injected into, or laid down upon, were completely overturned, and then sheared off at their bases by 2 billion years of erosion. It is surprising how often geologists are unwilling to figure that the ore is upside down and the laws governing its drilling should be reversed.
I would say 60% of ore in the Archean camp is bottom up, so geochemistry that assumes higher metamorphic grade at depth does not apply at all. Kidd Creek, all 15,000 feet of its dip, is completely overturned. At the extreme depth, they are mining what was once the top of the ore body.
You don't know if you have only hit one of the peas in a larger pod, the top or bottom of a larger whacked out shape sort of like an octopus or hydra, or that is all there is, and all the rest of the ore that may have been around 2.8 billion years ago, is long eroded. The chances of that, geometrically can be roughly said it be only 30%. 2/3's of all the ore should still be under the surface, as as much rock got pushed down below the surface as got eroded, by the principle of isostasy. Only a certain weight of rock can be uplifted, the rest has to fall below to support the rising mass. In fact the probability that most of the rock and softer ore is eroded is slim. The ore was protected by 5000 feet of ocean, and uplifted gradually to surface becoming compressed in surrounding rock as the continental plates slid over each other. Some ore was compressed beneath the moving continent, reheated and remobilized into veins and masses that rose with volcanics, and are found in mountainous diapirically and manto intruded terranes, forming the Cordilleran and Andean porphyries.
But if you put the ore at more than 700 foot depth say, it is unlikely without a lot of luck that routine probing will reveal it, as geophysics cannot see that deep normally. Ore bodies can be 1000's of feet apart spread over dozens of miles. What you see in the initial stages, are the surface bodies, which consist of perhaps 15 to 20% of the possible bodies. And this is further complicated by the fact that many landholders cannot afford to or do not want to drill and kill of the possibility of an orebody on their promo properties. Despite the fact that drilling does not really prove a negative.
Gentlemen, idle your engines.
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