Tuesday, May 15, 2012

Exploration Geochemistry, Part 2

The US Geological Survey at one time carried out a mineral resource inventory of most of the United States and Alaska. This was done by organizing teams of geologists, geochemists, and geophysicists to gather data and evaluate a quadrangle. In the Lower-48 a quadrangle was a one-degree-by-two-degree, 1:250,000-scale topographic quadrangle, typically 100 km x 160 km (60 miles x 100 miles) in size. This was called the Contiguous United States Mineral Appraisal Program, or "CUSMAP" for short. In Alaska it was done slightly differently: it was called "AMRAP", and the quadrangles were one degree by three degrees in size - pretty much the same surface area, but the 3-degree size was necessitated by the convergence of the lines of longitude as one got farther and farther north.

My first introduction to Alaska involved the usual training in handguns and "long guns", the purchase of rubberized rain suits and X-traTuff boots - "cane cutter boots" with an Alaskan attitude. At first I wondered about all the gear, but I soon learned why "Southeast" is famous for two things: bears and rain. The average rainfall in Craig, Alaska, where we first motored to for our work, is 12 feet of rain per year. That's 365 cm of rainfall. It seems like rain every day, all the time. I've seen bumper-stickers in southeast Alaska that say "The Bright Yellow Ball is the Sun."

We started work that first year at the docks of Ketchikan, a small city in the southeastern Alaska Panhandle, where we boarded the R/V Don J Miller, a 35-meter (116-ft), 80-year-old vessel that had been refurbished - outfitted with a map-room and a helipad above it, in lieu of a back deck. We were taught how to drive a skiff - a 5-meter (17-foot) aluminum boat, or in later years this became an inflatable "Zodiac" of about the same size. We were also taught how to make emergency repairs, and how to anchor such a vessel on a coast line. This is much trickier than you might think - this is a region where the tide could shift the sea-level up or down by as much as 10 meters (33 feet or more) in just 6 hours. You do NOT want to tie off your boat, climb up a Devil's-Club-infested creek to collect your stream-sediment sample, and then come back to find your 200-kilo (440-lb) transportation is:
(a) floating out in the fjord a stone's throw away, or
(b) stuck on rocks a stone's throw from the water's edge...
...and thus unusable in either case for up to 12 hours.

Doing an "overnighter" in these circumstances is actually worse than the discomfort of trying to sleep hungry in the cold rain all night. You had to face your buddies sheepishly the next day when they stopped all their own work to come looking for you.  Or for your gnawed bones.

I should mention that the R/V Don J Miller was named after a USGS geologist who died while working in Alaska. By an amazing coincidence this man's daughter, also a geologist, was actually working on that ship with us that summer.

Here were my working parameters: If the cloud cover on any given day was high enough, I would fly out in the helicopter and collect gravity stations. I used a $25,000 gravimeter that could detect changes in the pull of the Earth's gravity field down to  0.0000000001 - that incredible sensitivity is why the thing cost so much. There will be more on geophysics in subsequent chapters.

However, if the cloud cover was below 125 meters (500 feet), our airship was grounded - the pilot sat and read novels and drank coffee all day. Since we couldn't fly, I would instead go out and help the geochemists (I read slowly and I don't drink coffee). As I mentioned, we would motor to an area that we had no data for, then climb up through Alaska's nastiest weed (the thorn-infested, and aptly-named Devil's Club) to a point where we were well above the highest ocean tide. There we would collect several shovels of sediment, crudely sieve out the larger rocks and pebbles to reduce what we had to transport, write field notes on the location, and return to the skiff.

Repeat this 15 or so times a day, with a brief stop at an unforgettable viewpoint somewhere for lunch. Any single stream-sediment sample we acquired was then representative of the entire drainage area to the highest peaks above. We didn't have to crawl through the whole thing to know what might be hidden there. Southeast Alaska's rain brought it right to us.

This process might sound straightforward, but in practice it is truly arduous work. Getting just a single shovel of sediment in a stream bed made up almost exclusively of rocks the size of your head is one issue. Bucking through the Devil's Club is another. Watching the Tide Tables closely enough to ensure that you are tying off your skiff at the right place is another... and timing your climbing and sample-collecting so that you get back when you planned to is yet another. Humping around a .45-70 carbine or rifled-slug-loaded shotgun - and keeping alert for mother bears - is yet another.

Why did we do this?

The geochemists explained to me that when gold, or sulfide minerals like copper, lead, silver, or molybdenum, are deposited somewhere, there are a lot of other minerals frequently associated with the process. For instance, you may not see "puntos" of gold a mile away from the core of a gold deposit, but you could very well detect a higher-than-normal level of arsenic a mile away. Hydrothermal mineral deposits are concentrations of something you want, something you value. Nature has gathered these minerals from a vast volume of surrounding rock and concentrated them in one place for you. This gathering process is caused by water being circulated through an immense volume of older rock that was already there. The engine driving the wateris driven by a heat source - say a granite or monzonite intrusive punching up from the Earth's Mantle. (A monzonite is like granite, but with less quartz and more calcium in the rock). This hot, usually acid water circulation system picks up gold and sulfide minerals in distant rock and concentrates it in the vicinity of the hot intruding body - seemingly like how moths are drawn to a light.

Early on, miners noticed a haloing effect. As you moved outward from the center of a primary mineral deposit, you would see roughly concentric rings of other minerals. You could see different sulfide minerals as you progressed outward. You would also see "alteration": clay minerals like aluminum-bearing feldspars that had been cooked to a powdery, grungy form by hot, acidic water - close in to the deposit. You may also see greenish-looking rock - less-strongly altered or less "cooked" rocks, but still with some extra chlorine in them - farther out from the center.

The geochemists would collect as many samples as they could during the summers, driving themselves to do 14-hour days while they had the chance. The wild beauty of wilderness Alaska was a nontrivial reward, I might add. They would then spent the cold winters in Denver doing the tedious sample grinding, sieving, and chemical analyses. By the time they were ready to come back for another, perhaps final field season, they would already have preliminary contoured the results. THIS island has copper on this side but not on the other side, while THIS peninsula is just glowing with lead sulfides. So we can look at a map of the stream drainages and get a sense of the area we are talking about in each case - each area being represented by one or two stream-sediment samples.

The economic geologists, thinking through their various deposit models, would then begin to make resource estimations of yet-undiscovered resources. It seems counter-intuitive, but this is surprisingly easy. With time they had gathered enough examples of grade and tonnage that they could even begin to make some realistic predictions of undiscovered resources. Example: You will generally find primary (as opposed to placer) gold in regions with "Greenstone Belt rocks" - chlorine-tainted, "cooked" volcanic rocks. These are usually ancient volcanic island arcs that continental drift has slammed into the edges of the early continental crusts as they were forming. There are areas in Colorado, in California, in Canada, and in Australia where they have really, really, searched and mapped thoroughly. It turns out that there is a consistent pattern: you statistically have a 0.06 chance per square kilometer of finding a gold deposit in Greenstone Belt rocks. Any Greenstone Belt rocks. That means you can - more or less - count on one deposit per every 17 square kilometers.


However, when we got to Venezuela the first thing we noticed was that everything was covered with jungle. Where are the Greenstone Belt rocks in all that jungle? Well, Greenstone Belt rocks are usually pretty magnetic. Along with the metal sulfides that formed with the ancient volcanoes, there is a significant amount of iron. Some of it is in the form of pyrite - "fools gold" - but some of it is also in a form called magnetite. We therefore searched until we could find the old aeromagnetic data collected in the 1950's when US Steel was working in Venezuela (hint: they were looking for banded-iron deposits). We converted those data into a form where the magnetic anomaly showed as a high right over the magnetic source rocks, and used that magnetic data to then figure exactly where the Greenstone Belt rocks were. We could then calculate area. We multiplied that area by 0.06 and got how many deposits must be there. We subtracted out the few known gold deposits, and were left with what must still be there waiting to be found. Using the grade and tonnage curves from North America and Australia, we could also then calculate, through something called Monte Carlo simulation, exactly how many tons of gold were hiding there.

I did this calculation for a quadrangle in Venezuela called NB-20-4; this was the Venezuelan name for a 1-degree-by-1-1/2-degree quadrangle that happened to include a known mining district in one corner called Bochinche. Our Venezuelan counterparts thought they had mapped the quadrangle and found no sniffs of gold, so were preparing to move on and start looking farther west. I showed them that a bit more than seven tons of gold were not accounted for by the known mining district... and our host agency reprogrammed their efforts to continue searching in the NB-20-4 quadrangle.

So science (and an enormous amount of very tedious data-gathering) again actually paid dividends here.


No comments:

Post a Comment