|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.
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 |
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 So, the Carissa likely has a very large gold resource - but it was withdrawn from mining - not by the Federal government, but by its evil twin - the State government (and all done quietly - like a ninja in the night).
|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.|