Wednesday, December 10, 2008

Main Types of Gold-Bearing Ore Material (Part 2)

Here are items 6-10 of my two-post series on the main types of gold-bearing ore material:

6. Argentum-Rich Ores:
Argentum (Latin for silver)-rich (Ag) ores contain silver in greater quantities (70-95%) than gold, which is typically found in these ores as a secondary or "by-product" precious metal. Nevada's famous Comstock Lode is an excellent example of Ag-rich ore bodies that also contained substantial amounts of gold. Pure silver has a bright metallic white-gray color but oxidizes to a bluish-dark grey or black color when exposed to the atmosphere and elements.

7. Iron Sulphide Ores:
These ores are invariably associated with iron pyrites (FeS2, "fool's gold") or in pyritic host rock. They are easily identified by the presence of pyrite crystals in the ore and the reddish-orange iron oxidation stains that typically accompany them. FeS2 ores are usually found associated with other sulfides or oxides in quartz veins, and sedimentary and metamorphic rock formations, all well-known hosts for gold. Any pyritic rock or ore (with the exception of solid masses of FeS2 crystals) discovered in the field should be examined thoroughly for the presence of gold.

8. Oxidized Ores: Oxidized gold ores occur when the basic chemical structure of the ore-bearing material is weathered or impacted by the combined interaction of the elements and mineralization. The most common characteristic of oxide ore is the presence of hematite, magnetite, goethite, and limonite (all heavy "black sand" type minerals). Most of the time the presence or quantity of gold in oxidized ores is increased by the oxidation process but in isolated instances the ores can be so oxidized that the gold itself gets coated by layers of oxidation, making it more difficult to extract.

9. Placer Deposits: Though not ore bodies in and of themselves, gold placers are the most common type of gold-bearing material out there. Placers occur when gold erodes out of auriferous vein or ledge material and is deposited by gravity into low-lying areas such as washes and streambeds. These gold placers can be residual (near the host site), elluvial (downslope), alluvial (streambed), or of the bench (gravels left high and dry above stream courses) variety. Placers are the "bread and butter" of most small-scale gold mining activities.

10. Tellurides: The tellurides (Te) are a very common gold ore material in the American West, especially in areas of the Rocky Mountains such as Colorado. Chemically their structure varies (for example: [AuAg]Te2, [AgAu]2Te, etc.) with gold content running as low as 12% on up to 40% or even higher. Telluride gold ores are often found in conjunction with pyritic ores and can, at times, be misidentified as sulphide ores. Some of the oldtimers could spot tellurides in a heartbeat, but many miners and prospectors today have a hard time identifying them. So, if you're working in a telluride ore area or region, you may want to study up some on tellurides before heading out into the field on a prospecting trip.

(c)  Jim Rocha (J.R.)  2008


  1. Thanks so much for this - It was a great help!!

    Kalgoorlie WA

  2. You're most welcome Abi. Thanks for the comment and my best to all of you "Down Under." J.R.

  3. #8 Oxidized gold. How do you identify it and how do you seperate it from the black sand?


  4. If the gold is heavily oxidized it can be identified through specific gravity tests, cleaning (usually with acidic bases), or via high heat (jeweler's torch). More expensive but most accurate is a fire assay done by a reputable assayer. Seperation from heavy black sands is problematic at times, but can be accomplished with most of the concentrators/seperators sold on the market today ("Blue Bowl," Spiral Wheel, mini sluice, etc.). J.R. (Note here that I speaking in terms of small-scale or recreational mining, not commercial mining.)