My historical tours in Butte always include geological background, including what, where, who, when, how, and how much. Among the most frequent questions from visitors is why—why did Butte (or any other big mining district) end up with so much copper and silver in a tight, 5-square-mile zone, and otherwise similar areas just a few miles away did not?
My answer always is, “If I knew that, I could win the Nobel Prize in Economic Geology.” Not that there is such a prize, but you get the idea. Geologists understand mineral deposits pretty thoroughly, up, down, sideways, and through time. But that ultimate question of “Why?” typically remains unclear.
The easy cop-out answer is something like, “Well, more copper (or whatever) than usual was scattered in the rocks, and when magmas rose (or some other kind of geologic event happened) the metals were mobilized and concentrated into veins because there were more fractures (or some other rock feature that focused the mineralizing fluids).”
That’s fine, but it’s almost entirely “how,” not “why.”
Why was there more tourmaline in western Turkey to put boron in hot water there, making Turkey the world leader in boron production? Why did a 45-million-year-old lake in southwestern Wyoming end up with enough highly concentrated sodium carbonate (a compound critical in glass making) to make that deposit the largest in the world? A good answer is challenging to find.
We’re not totally clueless, of course. Researchers study primary mechanisms like black smokers on the sea floor, possible fundamental sources of deep-earth elements like copper, lead, and zinc. Such processes may represent the earth’s first version of mineral concentration. Later events, reasonably well understood, may take those primordial deposits and convert them into exploitable economic deposits in the earth’s crust, near enough to the surface for miners to work today.
But “Why?” can be the hardest question to answer in geology, just as it may be in human affairs.
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