Wednesday, October 30, 2013

A Short Course on Hard Rock Gold (Part 2)

 (Arsenic sulfide ore.)

In this post of the series I'll be talking a bit more about gold ores as they relate to hard rock mining. I'll beg your indulgence and ask that you hard rock old timers bear with me as I try to pass along some useful info to the newcomers and hard-core placer miners out there.

Arsenic Sulfide Ores: Despite their sometimes colorful appearance, these types of gold ores can be difficult to identify for the "average Joe" prospector out there. One reason for this is the gold that arsenic sulfides contain is often sub-microscopic in size, although larger potentially visible gold particles do show up in certain instances. One clue to finding arsenic sulfides is that they're frequently associated with minor amounts of pyrite, traces of chalcopyrite, galena, sphalerite, and arsenopyrite. Aside from pyrites (which most of you are familiar with) some of these terms can be daunting for the uninitiated, so let me give you a quick rundown:

Chalcopyrite is a mineral composed of copper and iron that forms in crystals with seven lattice points. Like regular pyrite, it streaks greenish-black.

Galena is essentially lead sulfide and the most commonly found sulfide material out there. Galena ores will typically contain at least some silver or silver sulfides, so don't throw samples out with the baby's bathwater.

Sphalerite is the mineral most directly linked to zinc ores. It's identified by it's cubic gray-black crystals and streaks yellow or very light brown.

 (Sphalerite sample containing visible gold.)

Arsenopyrite is essentially an iron and arsenic sulfide combination. It is very hard and dense and gives off an unpleasant odor (arsenic isn't nice stuff even in sulfide form). Arsenopyrite crystals are somewhat brassy in appearance and frequently form in striated column type formations.

Gold Concentrates Gold Pans Gold Concentrators
 
Here's the good news about arsenic sulfide ores. They're often  associated with large, significant gold deposits. Now for the bad news. Arsenic sulfide ore bodies are invariably refractory in nature which means it's very difficult to extract the gold from them without resorting to chemical treatment on a very large scale. The Getchell-Carlin open pit mine in Nevada is a good example of a "hit-it-big" arsenic sulfide deposit.

Antimony Sulfide Ores: Antimony is known as Sb on the chemical element chart and has a silvery, metallic appearance. In pure metallic form, it's often used as an alloy with other metals. Antimony sulfide ores can contain free gold in small particles or they can be refractory with the gold still in chemical (sulfide) form, They're often associated with a mineral called aurostibite that's actually a form of pyrite and is frequently found in hydrothermal-spawned gold quartz veins. In this latter context, antimony sulfide ores can provide a good amount of free-milling gold but in sulfide form they need processing and treatment approaches far beyond what most of us small-scale miners can handle. The Big Bell Mine in Australia provides an excellent example of a rich antimony sulfide ore body.

(Antimony sulfide ore.)

Bismuth Sulfide Ores: I would hazard a decent bet that many of you out there have never heard of this one. Bizmuth gold ores are often associated with a mineral called maldonite, which is a type of copper sulfide typically found in mineralized zones containing copper, gold, and silver deposits. Bismuth (Bi on the Periodic Chart) has a whitish-silverish or even metallic appearance and is nearly as dense as lead. Bismuth is often found in areas containing antimony and arsenic ores...now you can see the connection to gold. Bismuth gold ores can also contain visible, free milling gold or sub-microscopic gold particles trapped in sulfide form. In some locations iron staining may appear nearby or on the bismuth ore itself, but your best bet for a visual identification key is the whitish-silverish-metallic appearance. The Pogo Mine in the United States provides a good idea of what a large bismuth sulfide deposit is all about.

That should keep you busy for now.

(c) Jim Rocha (J.R.) 2013

Questions? E-mail me at jr872vt90@yahoo.com

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