|Gold in the San Juan Mountains of Colorado. As a gold prospector you should immediately notice the mine dumps, lack of trees growing on the yellow, brown, reddish brown and tawny colored rock and soil known as gossan. It's obvious that these gossans were mined in the past, but look at all of the exposed gossan in the high peaks that have not been mined. It is likely there are several rich gold and silver deposits still to be found in this area. Just search Google Earth for 'Red Mountain No. 1, Colorado', or 'Silverton, Colorado' to find this and other gossans scattered over a very large region near the 'Million Dollar Highway (550) in southwestern Colorado.|
Gold Prospectors like to generalize gold deposits into two groups - lode gold (gold which is found in place in outcrop) and placer gold (detrital gold eroded from a nearby lode and deposited in a stream). Both deposits are important, but for the gold prospector, placer gold deposits are most important because many can be worked with smaller resources and less cost that most lode gold deposits. But still, you need to consider scientific prospecting methods to insure that you have a commercial gold deposit and that you do not make mistakes like many of us have seen on the TV program Gold Rush.
This morning, I'm going to talk a little about gossans - something every gold prospector needs to know about as these are often guides to lode gold, and they are constantly overlooked by gold prospectors. They are also guides to placer gold simply because if they are gold bearing, erosion will tend to release the gold over time and transport the precious metal down slope to a nearby stream. As an example, while watching Gold Rush, I keep seeing a distinct gossan each week located at one of the featured deposits, and it is never mentioned on the program by anyone, nor is it ever explored. So after you learn about gossans, start looking for these on Gold Rush, but also look for gossans on sites like Google Earth.
|Pyrite cubes in chlorite groundmass. Note not only the pyrite, but also the|
cubic pits where other crystals of 'fool's gold' fell out of the rock.
|One, large, massive piece of pyrite (fools gold) showing it's cubic crystal |
habit. This specimen has relatively high heft.
|Gold from Douglas Creek, Wyoming. Note the distinct warm yellow color of|
gold as compared to the brassy color of pyrite. Also the gold is
characteristically rounded and much heavier than pyrite.
But as a gold prospector, you need to also realize that pyrite can fool you again - particularly if you throw it away without assaying it. In my 2011 gold book co-authored my son, we describe some pyrite around the world that has considerable hidden gold! That's right, as much as 2000 ppm (parts per million) gold has been noted hidden inside the pyrite crystals. That means as much as 60 ounces of gold per tonne might be hidden right under your nose and you may never see it if you don't have it assayed, crush the pyrite to a very fine powder and pan it for gold, examine many specimens under a microscope, or look closely at the gossan produced by the pyrite for visible gold. But one more way it can actually fool you is that you may have a good gold deposit in what is known as noseeum gold (invisible gold). Well, the gold is not really invisible except to our eyes. It is in stealth mode with individual gold atoms replacing some of the iron atoms in the pyrite atomic structure. So how do you know its there and how do you get it?
In this case, it has to be assayed. Then to recover it requires some serious chemistry mentioned in our gold book. If you have a lot of invisible gold, it might be in your best interest to try to sell it to a mining company as it is not going to be cheap to mine and recover.
|United Verde mine, Jerome, Arizona. Note the well-defined gossan exposed in the highwall. This mine was so rich in pyrite, that the pyrite actually caught on fire deep underground and burned for many years.|
|Sample of massive pyrite collected from the Tin Cup district. Note that the brassy, metallic pyrite appears to be partially replaced by brownish mud. The brown material is actually limonite, goethite and hematite (rust) or the classical gossan!|
|Gold in rusty gossan from the Mary Ellen Mine, South Pass district. All of |
the brown material was originally pyrite that rusted to limonite and goethite
while exposiing the gold hidden inside the pyrite.
A few months ago, someone contacted me and sent an excellent photo of a piece of limonite with quartz and lots of visible gold. This photo is perfect to illustrate what I'm writing about, but I need to dig through my email and get this person's permission and name to give them credit. Hopefully, I can find that email - if not, please send me your gold photos (with limonite), name and permission to use. Until I get permission from that person, the photo above also works quite well. This is a specimen found in the Mary Ellen mine by Steve Gyorvary who donated it to the Wyoming Geological Survey (before it became a den of scum from 2004-2008) and I photographed it using a binocular microscope (the scale is not really in meters as I like to tell some people, it is a millimeter scale).
|Chalcopyrite (also known as copper pyrite) also will rust to produce gossan with some copper minerals such as tenorite, cuprite, malachite, etc.|
|Limonite boxworks after pyrite. Note the distinct porous appearance of this |
sample - the pores are often good places to look for visible gold. Such samples
from Arizona, Alaska, California, Montana, Nevada, Colorado, Utah,
Idaho, Oregon, Washington and Wyoming often have visible gold.
Pyrite and chalcopyrite not only produce gossans, but other valuable minerals also produce gossans. Often the gossans will have slighting different colors due to trace metals.
|Beautiful fluorite producing a distinct yellow limonite. Fluroite is sometimes found with gold. This specimen from the Bear Lodge Mountains district where some gold and rare earth deposits are found.|
|Arsenopyrite with blood reddish brown limonite gossan. Arsenopyrite smells like garlic (its the arsenic) and sometimes yields a greenish-yellow limonite. Often arsenopyrite also contains hidden gold as we discovered in Alaska.|
|Iolite (water sapphire) altering to limonite. This very high-quality gemstone is part of a world-class iolite gemstone deposit that I discovered in the central Laramie Mountains.|