Monday, February 8, 2016

Gold-Bearing Veins (Part 2)

 (A beautiful specimen of calcite containing iron pyrite. Remember, calcite can be a crystallization "carrier" of gold.)

In this post I'll be continuing with my discussion of gold-bearing veins with an emphasis on carbonates. There's always something new to learn when it comes to gold mining (small-scale or otherwise), so please read on.

More on Carbonates

In my previous post I briefly mentioned carbonates and their relationship to gold veins and large-scale gold deposits so I thought it might be best to discuss them a bit more. Without knowing for certain, I suspect that fewer than 50% of you out there are familiar with carbonates. So the need is there, I believe.

 (One form of calcite gold ore from Nevada.)

The term carbonate (CO3-2) essentially refers to rocks and minerals containing carbonate ions. Carbonates vary widely from a chemical constituency standpoint but the most common carbonates associated with gold deposition are calcite and calcium carbonate. If you read the previous post in this series carefully, you already learned that calcite represents a major crystallization carrier of gold (the popularity of quartz in this regard notwithstanding). Iron carbonates like FeCO3 are a major form of iron ores and some copper and silver ores are also associated with carbonates. Carbonate host rock ("country" rock) typically contains induced sulfides or other forms of oxidizing agents. Crystalline gold carriers in carbonates include quartz, barite, and calcite. These carriers are often associated with pyrites (FeS2) which are, of course, a source of oxidation to host rock.

(A chunk of barite, another carrier for gold in carbonaceous rock.)

More on the Carlin

I've spoken about Nevada's Carlin Mines in the past and the Carlin Trend deserves mention again here since the Carlin gold deposits are carbonaceous in origin. The Carlin Mines (large-scale, open pit operations) have been steadily producing large amounts of micron-sized refractory gold since 1965, when gold was still pegged at $35.00 a troy ounce. Forty years later in 2005, the Carlin Mines produced 2,500,000 troy ounces with spot gold prices averaging around $450.00 per troy ounce. That's some heavy duty folding green brothers and sisters, and with gold spot prices over $1,000.00 an ounce these days, one can only imagine the monetary value of the gold Carlin continues to produce.

(One type of carbonaceous gold ore. Note the presence of pyrites and iron sulfides.)

 (Carlin's main open-pit operation in Nevada.)

The trouble (if that's the right word to use here) with carbonate veins is that they are quite often refractory in nature. I've discussed this term in the past but let me run it by you again since there are undoubtedly some newbies or greenhorns reading this. Refractory ores contain gold either in the form of chemical constituents or locked into those very same constituents. These latter include chemical solutions in the host rock containing gold chloride, gold cyanide, or various organic gold compounds that offer quite a challenge to the small-scale prospector or miner in terms of identification or most importantly, ore processing and gold recovery. For example, a major source of the Carlin gold deposits was discovered by a solitary prospector who obviously knew his stuff when it came to identifying carbonaceous ore bodies. This same old timer also knew he was up shit creek when it came to getting the gold out of the veins he'd discovered because the ore was so frustatingly refractory. So what'd he do? He sold off his claims to the first company that began mining the Carlin on a large scale. I hope this sage individual ended up richer than Old King Midas, but you know how that can go...

(An open-pit mining operation containing a large body of rich, gold-bearing carbonaceous ore.)

This doesn't mean that carbonate formations don't contain free-milling gold too, because they do. However, in the case of the Carlin Geologic Trend, much of that gold is widely dispersed and very tiny in size. That makes gold recovery only suitable for large-scale, open pit operations where the average amount of gold per ton of material processed runs about 7-8 grams. On the other hand, there have been instances where prospectors or geologists have located some very rich carbonate veins containing lots of concentrated free-milling gold. Whatever the case, you should add some knowledge of carbonates and carbonate veins and ores to your mining lexicon because you never know what you might run into out there. You could stroll right by a rich carbonaceous gold vein and not have a clue as to what what you just passed by. But beyond that, every little bit of knowledge you acquire regarding small-scale gold prospecting and mining gives you that much more of an edge out in the field.

In my next post I'll be talking about how gold ends up in crystalline carriers in country rock. Until then, be safe, work smart, and be good to one another.

(c) Jim Rocha (J.R.) 2016

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