Friday, May 11, 2012

Deposit Models

Beginning shortly after its founding in 1879, the USGS pioneered sophisticated studies of mining districts - and all the mining company geologists pored over these elegant "Professional Papers" to learn everything they could... so they could find more billion-dollar mining districts. 

These Professional Papers pioneered a new idea - the first rudimentary mineral deposit models. That is, if we are looking for gold, or copper, or platinum, or diamonds... well, how do they form or get concentrated in the first place? Do you find diamonds on mountain tops? Do you find gold in pastures?  If not, then, why not?  If you can codify this mineral formation, concentration, and deposition process, then you create a model of these deposits. If the model is good enough - correct enough - then you can use it to find other similar deposits.

Deposit models are based on a few basic assumptions, the first assumption being that you can even describe them as a class in the first place. Among other things, can you carefully work up a description of:
  • what the host rocks must be like,
  • what the source of the mineral of interest might be,
  • is it close enough to where it will be deposited?
  • then what is the transportation and concentration process, and
  • what "clue" minerals are associated with the one you're specifically looking for?
The USGS has actually produced a whole BOOK (called a "Bulletin") of mineral deposit models:  (by Dennis Cox and Don Singer). It keeps evolving and expanding as more examples and data are added to the huge mineral resource databases.

Behind this dry, tabular format is an incredible amount of raw effort to gather information on just about every single ore deposit known to man (at least at the time this Bulletin was initially published). LOTS of information about each one. 

Here are a few simple rules (just part of their model descriptions) for some different types of minerals:

Tungsten is always found in granite bodies, that is, a slowly-solidified magma mush with a lot of silica in it.

Platinum (and the related platinum-group elements like palladium and rhodium) form in layered ultramafic rocks, that is a slowly-solidified magma mush withOUT much (if any) silica in it.

Bottom line: don't look for platinum when you're looking for Tungsten. They won't be in the same place.

Diamonds are formed by extreme heat and compression applied to carbon. The "normal" square sheet-like lattice of carbon graphite (that's why pencil lead smears on paper) must be made much more compact. Making it much more compact makes it (a) denser, and (b) harder.

The kind of monstrous pressure necessary to form diamonds means that they had to form at least 50 - and more like 100 - kilometers deep inside the Earth - in the upper Mantle. Well, OK. But that means that they must somehow get to where we can lay our hands on them. There must be some sort of mechanism that transports them to the surface. Experience says that these are narrow pipes - filled with material that shoots up from the mantle in an extremely violent fashion. The minerals dragged up along with the diamonds are not what you would usually find on the Earth's surface (things like chrome spinel), and in fact these rocks have a distinctively bluish color. They don't look like anything you will see anywhere else (well, almost anywhere else) on the planet. These rocks even have a name, derived from where they were first found: Kimberly, South Africa: Kimberlites. It turns out that this blue stuff is highly conductive - so an airborne electromagnetic survey can theoretically find a covered-over Kimberlite by simply looking for small, bulls-eye circular conductors. I'm not making this stuff up. There are geophysicists in Canada who make a living out of this.

Potash is a salt - actually many different kinds of salt, sometimes mixed with manganese, found in diluted form in the sea. Potash is one of the three critical elements necessary for the Green Revolution: you know, why 8 billion people on this planet have enough food to eat. Rice requires fertilizer to grow well, and there is just so much human feces that you can throw into a rice paddy. To be effective, a fertilizer must have nitrogen, phosphorus... and potassium. Get the German language involved here - most of Germany is underlain by potassium deposits - and you get the word potash. Actually, some of the first potash was obtained by burning certain kinds of vegetation into an ash. Potassium + ash = potash.

It's not economic to mine potash from the sea - it's not concentrated enough. So geologists look for where a natural process has already concentrated it for them. It turns out that an excellent way to do this is when an smaller sea gets closed off from the rest of the world ocean... and then evaporates. Near the end of the drying-out process (a process that results in thick, layered salt beds), the potash salts will be nearly the last to precipitate out, so they are higher in the stratigraphic column. Potash salts are typically distinctive - pink or even red - which makes them easy to see and pick out in a salt mine. There is a dried-up ancient sea that now extends from west-central Canada all the way down into North Dakota. This is called the Elk Point "deposit", but that word is used loosely here, because this "deposit" is HUGE, and has many different evaporite layers in it - many different seawater incursions followed by evaporation. Seawater first got into a great continental depression near modern central Alberta in the middle Devonian epoch. The entry was then blocked, the sea dried up, and formed a layer of salt with some reddish stuff at or near the top. This happened over and over again: the ocean breaches again, fills the salt-flat basin, is blocked, and the cut off arm of the new sea dries up yet again.

You'd think it would make nice layers that you could mine from Alberta to North Dakota. In many cases, the potash salts ARE rather nicely layered. But salt has a habit of easily deforming. Under pressure, it behaves like plastic. Salt can thus slowly pour itself into places where the overlying rock pressure is slightly lower - that's why we have all those salt domes in the southeastern US and the Gulf of Mexico.

This up-welling process is very much like making a batch of bread dough, letting it rise in the pan, then putting your open-fingered hand down in the center: the dough oozes up like little walls and plugs between your fingers. In the case of salt, this is called "diapirism" or "halokinetic" (salt + moving) behavior. When salt pushes up through a weakened zone, past sediments laid down later that are now lying above it, it bends them. Then oil found in one layer will start oozing up along those sediments gets blocked by the salt plug (or salt dome, or salt diapir). Salt has very low density, so a gravity survey will "see" these salt diapirs and salt walls as gravity lows: they typically look like bulls eyes on a county map in Texas or Sascatchewan. The trick is to not drill the center, but to drill around the edges, because that's where the oil has been trapped. Finding the potash is even harder - you need to figure out where the layers have flowed.

Copper: there are at least two ways copper gets concentrated into an economic deposit: (1) As fluids squeezed out of sedimentary rocks, leaching the small amounts of copper out of them and concentrating them in layers above them. The Central Africa Republic and the eastern Congo have an enormous resource of copper that formed and was concentrated this way. (2) As fluids associated with hydrothermal cells that form when granite and similar monzonite bodies extrude up from the Earth's mantle. The intrusive body heats up the groundwater (which is pretty much everywhere if you recall earlier chapters). This water starts slowly circulating, and it leaches out copper from surrounding rock, and concentrates it in fractures created by the crystal mush (still-fluid granite or monzonite magma) punching upward into the Earth's crust. These are called "porphyry coppers" because of the way some crystals form as it slowly cools... and then the thing breaches the Earth's surface and what's not yet formed into crystals quenches quickly, too fast for the rest of the crystals to form. These rocks are cool-looking: typically white crystals of feldspar in a gray groundmass of quenched magma. One of them in Arizona has the unusually descriptive name of "Turkey-Track Porphyry".

Well. That may be the shortest single paragraph summary of copper deposits ever written.

Gold: But I know you're just waiting for the gold deposit model, right? Unfortunately there are a number of very different gold deposit models. There is the Carlin Trend "no-see-um" gold deposits in central Nevada, and the Comstock placer gold deposits that led to the California Gold Rush of the late 1840's, and the low-sulfide quartz vein gold deposits... In fact, I have a thick book on my shelf titled simply "GOLD" that tries mightily in ~300 pages to make some sense of all the different gold deposits in the world. For a long time, some of the most brilliant minds in economic geology actually despaired at ever getting any gold models articulated. This is because every gold deposit is different - like bears. Or like humans: each one has a different "personality" if you will.

But if you apply brilliant human minds at a problem long enough - especially to a problem that could pay handsomely in a brilliant yellow-orange metal - then you will eventually succeed. Several of these gold deposit models are described in that USGS Bulletin 1693. The "low sulfide quartz vein" gold model was developed from cataloging hundreds and hundreds of small gold deposits in the Canada, Western Australia, and California. This model turned out to apply almost perfectly to the gold deposits we were finding in southern Venezuela. They are found in ancient rocks. They are found in association with somewhat younger intrusives like granites or quartz monzonites. They tend to concentrate the gold in ancient volcanic rocks... because those rocks rather nicely fracture. The gold then precipitates out as the fluids move through those fractures to the cool, low pressure surface of the Earth.

There was one exception to the LSQV gold in Venezuela: there was no ARSENIC found in concentric halos around these deposits like you found elsewhere in the world. The reason for this dearth of grandma's favorite rat-poison was because the particular ancient continental crust  - the Guayana Craton that popped up out of the primordial Earth under what is now southern Venezuela - just happened to be an arsenic-poor crust. No one knows why; it's like the Fine Structure constant of physics: it just is that way. Oddly, the Guayana Craton WAS loaded with beryllium, however, so we learned to look for little "wire stars" of dark-green tourmaline crustals in the quartz veins.

You've heard probably that gold is a "noble" element - that it doesn't dissolve in acids or oxidize. Don't believe it. While it won't indeed oxidize like iron, if you can get the ground water just a bit acidic with carbonation, it will slowly collect and concentrate the gold from many kilometers away into the fractures around the intrusive body. Carbonation! Think of soaking the rock with Pepsi - and ever so slightly it WILL move and collect the gold in the rock it seeps through. The water will reach the intrusive, get hot, and rise. As it approaches the surface, the pressure and temperature will quickly drop... and all sorts of things will precipitate out into the fractures. Eventually all those precipitating the minerals - mostly quartz but also a cream-colored carbonate called anchorite - clogs up the fractures until they are plugged. The fractures then  become the light colored "veins" you see in many beat-up old rocks. But there is a much higher concentration of gold these veins than the surrounding rock.

I wrote in considerably more detail about Venezuelan gold and diamond deposits in a book I authored with my wife called "2 Worlds, The Real Venezuela: Living on the Edge of the Jungle and the Rise of Hugo Chavez". That cute blondie on the cover is our 16-yr-old daughter holding a monkey that likes to eat flower petals from your hand.

Something I haven't really addressed here are secondary deposits. The diamonds found in Venezuela are not generally found in Kimberlite pipes, but are found in placers: where they have been weathered out and washed down to some secondary place where they are concentrated with other heavy minerals like titanium oxides. In fact, the diamonds in southern Venezuela  have been washed yet again out of paleoplacers into a third generation of deposit. The ancient placer concentrations have themselves been broken up, washed down to somewhere else in modern rivers like the Caroni where they are again being concentrated or trapped by clay from weathered-out diabase dikes. Most of the gold mined in Alaska and the California Mother Lode is found in placer deposits.

Too many words. This is supposed to be the easy way to learn geology, so I'll leave it here. This is a whole lot shorter than that book on "Gold" however.


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