We are excited about the Gold Rush. To get your share of the gold, be sure to learn what you can about gold deposits, geochemistry, geophysics and geology. The more you know, the better your chances of striking it rich.
GOLD RUSH
The search for lode and placer gold, how to find gold and where to find the precious metal.
Monday, January 6, 2020
Monday, November 11, 2013
Ore Shoots
Many chemically formed ore shoots yield good ore deposits. During vein formation, hot silica (quartz-rich), hydrothermal acidic fluids react with rocks with favorable chemistry; particularly rocks with calcium carbonate (limestone, dolomite). Thus, following a vein that cuts through limestone could lead prospectors to find skarns, replacement deposits, etc for gold and other valuable minerals. Replacement deposits and skarns are often formed when such mineralizing fluids come in contact with limestone or even rocks such as limy shales, or limy sandstones. Replacement deposits can be very rich.
Cupriferous gossan - note the tawny limonite (hydrated iron oxide) with fissures filled with azurite (copper carbonate) typically found in many mining districts in Arizona and Utah. |
The tan matrix in this specimen invaded fractures in a quartz vein and partially replaced the quartz. |
Thursday, March 28, 2013
Ore Shoots and How to Trace Lode Veins
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The Duncan mine and mill as they appeared in 1977 (photo by Dan Hausel). |
So, why did these ore shoots occur and how was someone going to be able to identify them without crawling across the 16+ miles of vein-shear structures and assaying every inch? Unfortunately, its not so easy. And then there are large areas that are hidden under soil and aluvium that could easily have cover a very good ore shoot.
While mapping around the Duncan and Carissa gold mines, I found some ore shoots where the gold values increased dramatically. But what was causing this was not quite clear at first. These ore shoots were difficult to recognized because of considerable soil cover and also the rocks that hosted the ore shoots were all black amphibolites and it was very difficult to see entire structures. But at the Duncan mine, I was able to collect enough samples across the structure and was able to actually plot the gold value increases leading into an ore shoot. The values increased about 100 times the normal gold values.
It turns out that the high gold values I detected (a 2-foot long channels sample that assayed 1.06 opt Au) were part of an ore shoot that was structurally controlled by folding. While mapping the Carissa mine, I found this structure as also intensely folded. Gold values at the Carissa were enriched more than a hundred times the average of the shear structures in the district.
Copper (chalcopyrite) breccia from Wyoming. Note the angular rock fragments |
So, the Carssa ore shoot plunges down into the earth for at least 970 feet (that was the deepest anyone ever drilled in the district), but there is no reason to believe this ore shoot does not continue to the base of the greenstone belt (2000 feet, 3000 feet, 5,000 feet, 10,000 feet?). So, the Carissa likely has a very large gold resource - but it was withdrawn from mining - not by the Federal government, but by its evil twin - the State government (and all done quietly - like a ninja in the night).
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The rocks at the Carissa mine are like an accordian - folded and squeezed |
Other ore shoots may form where there is considerable breccia, or an increase in open space by faulting or by a rising gaseous vent. So, breccias should be examined for enriched amounts of ore. There are many rich breccias that have been found in Arizona that are the result of high-pressure mineralized gases erupting from depth. Many such breccias have been found about giganic porphyry copper deposits.
Ore shoots are areas in veins where valuable mineralization increases, often dramatically. It has always been a subject of economic geology to try to understand where these rich shoots form and why, so miners can predict where the greatest amount of ore will be found in a vein, fault or shear zone.
Ore shoots can form chemically. During vein formation, hot quartz-rich, acidic fluids will react with rocks of favorable chemistry, particularly rocks with calcium carbonate (limestone). Thus following a vein in any area that may have limestone should lead any prospector to search the limestones for gold and other valuable minerals. Replacement deposits and skarns are often formed when such mineralizing fluids come in contact with limestone or even rocks such as limy shales, or limy sandstones. In some cases, the replacement deposits can be very rich.
Copper (green) breccia with common quartz prisms in vugs surrounding angular rock fragments. |
Thursday, February 21, 2013
Lode Deposits
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Classical quartz vein or lode emplaced in a fracture in tonalite (granitic) host rock. This quartz vein contains patches (shoots) of gold that are surrounded by milky quartz gangue. Photo taken in the lower level of the Mary Ellen Mine at South Pass. Note that the vein has been faulted and offset. |
According to one the glossary of geology, “a lode is a mineral deposit consisting of a
zone of veins, veinlets, disseminations, or planar breccias; a mineral deposit
in consolidated rock”. This is
straight forward to most geologists, but in may not be for many prospectors.
Most prospectors think of lodes as veins. This is partially correct, but a vein
is only one type of lode.
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Visible gold in limonite and hematite from a shoot at the Mary Ellen mine. |
The dictionary definition of a vein is more complex and I
will only include those summaries that apply to our discussion as a vein can be
much more encompassing. Some of the definitions include:
“(1). An epigenetic mineral filling of a fault or fracture
in a host rock, in tabular or sheet-like form, often with associated
replacement of the host rock; a mineral deposit of this form and origin.
(2). A zone or belt of mineralized rock lying within boundaries clearly separating it from neighboring rock. It includes all deposits of mineral matter found through a mineralized zone or belt coming from the same source, and appearing to have been created by the same processes.
Gold-bearing quartz vein exposed at the Vulture Mine, Arizona. The flat surface on the vein on the left is the dip plane of the vein (the angle that the vein projects into the earth). This is a very distinctive vein. |
(4). The term lode is commonly used synonymously for vein”.
Most prospectors visualize a
lode to be a distinct vein of quartz. But lodes include more than just veins.
But let’s briefly look at veins first. Veins can be formed primarily of quartz, but
another common gangue mineral is calcite. Gangue minerals are worthless
material that is found with valuable minerals, and when combined, the two
produce ore. So ore is just a mixture of the good and bad stuff.
The formation of ore shoot in veins is not entirely understood, but they appear in many cases to be the result of changes in chemistry sometimes due to changes in the rock type in the wall rocks. They have been identified where veins are folded or where veins intersect other veins or fractures.
Often prospectors and geologists get so focused on the vein, they miss all of the other good stuff nearby. At the Vulture mine, adjacent to the vein is a very attractive looking gossan. Apparently, only a minor amount of this gossan was mined by open pit - it may have been because it was low grade, but at any rate, it should be sampled in detail. In Arizona, there are numerous deposits of detrital gold found in fanglomerates that contain enough gold for placer miners. This may also be true at the Vulture mine where a distinct fanglomerate was dug into in the past.
At the Vulture mine, the gold-quartz vein has a prominent gossan adjacent to the vein and this gossan also has a fanglomerate (a conglomerate deposited as an alluvial fan and derived from a nearby gold deposit).
Adit (tunnel entrance) in a conglomerate adjacent to the Vulture gold mine, Arizona. |
This fanglomerate appears to have had some values because the miners drifted into the eluvium.
At the Commonwealth mine in the Pearce Hills of the southern Dragoon Mountains of Arizona, in some places the lode is very district forming a prominent veins. But other veins in this area are referred to as stockworks and may not be so well-defined to prospectors even though most geologists would get very excited over these stockworks simply because the stockworks (veinlets) are often mineralized as are the rocks fragments in between. So these deposits often represent large tonnage deposits of low grade ore.
At the Commonwealth mine in the Pearce Hills of the southern Dragoon Mountains of Arizona, in some places the lode is very district forming a prominent veins. But other veins in this area are referred to as stockworks and may not be so well-defined to prospectors even though most geologists would get very excited over these stockworks simply because the stockworks (veinlets) are often mineralized as are the rocks fragments in between. So these deposits often represent large tonnage deposits of low grade ore.
This vein at Huddy Hill in the Pearce Hills of Arizona crops out as a well-defined quartz vein containing silver and gold. |
Stockworks in the Pearce Hills of Arizona. Many narrow silver-gold veinlets with breccia fragments inbetween that are also mineralized in silver and gold. |
Stockworks at the Copper King mine in Wyoming. This deposit may not look like much, but it has a 2 million ounce gold equivalent ore deposit that has excited me since I first looked at it back in 1981. The ore deposit is open in several directions and potentially could be even larger. |
Most quartz vein gold deposits in Wyoming are mesothermal, meaning that they were formed at great depth. These are usually boring veins that show little textural or color difference and are difficult to predict if they contain commercial mineralization or just 'bull' quartz. One of the principal guides for gold in these types of veins are tawny to light yellow-green limonite associated with the veins.
Upper Gold Road vein (lode) in Arizona with open vugs and banded quartz. |
Ribbon vein (mixture of bands of quartz and rock) in the back of the Giant King mine, Mother Lode, California. |
The San Ignaceo vein in Arizona showing breccia, open vugs and banded quartz typical of an epithermal vein. |
One of my favorite lodes is that of the Carissa mine in the South Pass greenstone belt of Wyoming. This lode is incredible. Everytime I led a field trip to this mine (possibly a dozen times or more), at least one person (or several) found samples of quartz with visible gold. Drilling established that the deposit was commercial grade and actually very high grade for a vein deposit. It averaged between 0.15 to 0.3 opt Au!
Being curious, while mapping this greenstone belt, I checked the wall rock at the Carissa lode and discovered that the wall rock was actually part of the lode! I found gold in the wall rock over a few hundred feet and found evidence that the shear zone (lode) was as much as 1,000 feet wide!!! Drilling by past companies also established that the gold mineralization continued to depths of just under 1,000 feet (no one had drilled any deeper). Based on the geology and ore deposit type, I would expect this lode to continue to much greater depth- 2,000, 3,000, 5,000 or more feet! So here is was, the mother lode sitting in Wyoming. The gold deposit likely contains a few million (or more) ounces.
So, state government again proved they were filled with geniuses and purchased the mine, withdrew it from mining, and opened up a tourist attraction that brings as many as 200 to 300 people a year to the South Pass City historical site at no cost.
Central Vein in the Mother Lode district, California. Numerous white quartz veins enclose black schist. |
No visible quartz but lots of tawny limonite - is this a lode? Could be. One would need to take samples in the limonite initially, to see if it has any gold. |
Saturday, February 16, 2013
Gold and Gossan
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. |
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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! |
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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.
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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. |
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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. |
Saturday, December 15, 2012
GOLD PROSPECTING
Follow us on Facebook and Gem Hunter and learn more about gemstones and gold.
A search for gold, gold placers, gold lodes, veins, shear zones, and old mining districts. About prospectors and other interesting people. How to find gold, where to find gold, how to pan for gold, gemstones found with gold in your pan, and more.
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Finding the Mother Lode in Alaska |
Lost Dutchman gold preserved in matchbox |
The Gem and Gold Hunter searching for gold in Wyoming, 2012 |
Visible gold in milky quartz vein |
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Placer gold from Dickie Springs, Wyoming (photo courtesy of Dr. J.D. Love) |
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Searching for gold at 350 feet deep in the Carissa Mine. |
Searching for gold in the Giant King Mine, California |
Seaching for gold at Gleeson, Arizona |
Gold at the Resolution mine, Arizona |
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Discovery of a new gold district in Wyoming similar to Cripple Creek, Colorado |
Gold at Oatman Arizona |
Superstition Mountains, Home of the Lost Dutchman gold mine, Arizona. |
The Lost Dutchman mine? |
San Juan Mountains gold, Colorado |
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The Duncan gold mine, Wyoming |
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Carissa mine, Wyoming |
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