Friday, October 28, 2016

Types of Gold Occurrences (Conclusion Part A)

 ("Sliced" section of gold ore from Australia.)

In this post I'll start wrapping this series up. I hope to clarify a few potentially confusing areas and leave you with a solid understanding of this entire gold and characteristic issue, so please read on.

The Greater Whole

First off, some of you may be wondering about a similarity between this series and some posts I wrote in the not-so-distant past about gold ores. The distinguishing factor in this series is that I'm talking about the main types of gold occurrences that dominate the gold generation in a given area, not just a piece of ore you may find out in the field. So, in a bismuth sulfide occurrence zone, the dominant ore bodies will reflect that sort of mineralization; in an iron sulfides zone the main gold ores will be Fe-based; and so on. Remember however, that Ma Nature doesn't always lay things out in a nice, neat way. Some occurrence zones can have more than one host carrier with one type being dominant over the others. Any pieces of ore or "float" found in an occurrence zone are simply small parts of a greater whole. The occurrence zone itself is the "father to the child." Make sense?

 (Ma Nature does her own thing when it comes to gold occurrences.)

Gold Size in Extraction and Processing

Perhaps it should go without saying, but the size of the gold contained in the ore from a gold occurrence zone is the main factor dictating it's economic value and the relative ease or difficulty in extracting and processing that gold. For you and I, the larger the gold contained in the ore the easier it is to recover. The smaller it is (microscopic, sub-microscopic) the greater the difficulty and cost in extracting it. Large mining companies will disagree with me on this point since they use cyanide leaching as the main way to extract gold in their operations and it's been proven out that larger, coarse gold contained in ores is resistant to cyanization and flotation due to its size and weight. Size then becomes problematic to those large operations. On the other hand, the big companies also struggle mightily with microscopic gold recovery. But remember, you and I are the small-scale guy or gal. We WANT that big, free-milling gold because we don't have the equipment or financial means to work on a massive scale. So the bigger the gold pieces contained in an occurrence zone's ore, the better off we are. What this should point out to you as well is the fact that certain gold occurrence zones aren't going to be worth our time or effort due to the small size of the gold they contain in their host rocks and general mineralization. Again, identifying the type of occurrence zone and the form of gold it carries becomes all important because of this.

 (Size does matter when it comes to gold occurrences.)

Finding Out What You Have

This section relates to those of you with the "What's in my rocks?" questions. It also provides a mini-guide for you veteran prospectors out there. First off, a good gold prospector is like an experienced gum-shoe detective. He or she knows how to research, what to look for in terms of leads and clues, and brings both experience and intuition into tracking down a suspect. The same is true for identifying gold occurrence zones and the ores they carry. A good detective uses proven investigative procedures to get the criminal. So it is for us.

 A good gold prospector is like an experienced gum-shoe detective.)

Here's your set of "investigative" procedures:
  • Determining if visible, free-milling gold is present in those rocks or ores you spot or pick up. This includes the gold's size, its distribution, quantity, the minerals its associated with, and how easy or hard it's going to be to remove it from its carrier or host.
  • Determining if the gold is associated with sulfides, silicates (i.e., quartz), or carbonaceous hosts (if carbons are present that is).
  • Determining if any microscopic or sub-microscopic gold is present and in what quantities.
  • Determining if other valuable metals are present (silver, gold, copper) and what sort of potentially "difficult" minerals or rocks are part of that gold occurrence you're investigating. Those hard-to-handle minerals or rocks make ore processing very tough from a gold extraction standpoint and include things like serpentine, cyanide-based ores, talc, graphite, and any mineral that's easily soluble in water.
  • Determining if your occurrence zone materials contain carbons or other forms of resistant gangue (host) materials.
And you thought it was simply about picking up gold-laden rocks, didn't you? Well, I hate to break the news to you, but it ain't!

 (There are any number of ways to process gold ore.)

I've told many of you via e-mail that your best, single approach for making these procedural determinations is via fire or chemical assay. Additionally, microscopy (electron or otherwise) or X-Ray diffraction may be needed as well to determine the constituents of your "rocks." Sure, it costs money but there's no easier, softer way if you really want to know EXACTLY what you have in your hot little hand. And usually what you're looking for in an assay is the presence of gold, what form it takes, how much your rocks contain, how the gold is distributed, how big it is, and whether minerals or metals such as silver, antimony, sulfur, tellurium, bismuth, or arsenic are present as well. This is the real science behind prospecting for gold and for knowing what you've it or not.

There's more to come so stay tuned.

(c) Jim Rocha 2016

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