Finding Gold Ore in Old Mines
Most people think of finding gold nuggets. The truth is that very little comes from nuggets – nearly all newly mined gold comes from ores mined from the natural hard rocks that contain gold in tiny, even microscopic particles. The following are some examples of gold ore.
When searching for or finding gold ore it is good to remember that, gold deposits in the western United States are frequently found mixed within iron ore deposits so look for dark and rust colored iron ore on the rock walls of the mine and in the tailings piles (rock waste from the mines). However the appearance of gold ore can vary quite a bit. Any good assayer will be able to tell you that gold ore mined in Ghana Africa is going to look different from gold ore mined in Nevada’s Carlin Trend and even from mine to mine the gold ore is going to vary quite a bit. Recognizing gold ore will take some experience.
The good news is that it is often easy to distinguish between veins and the host-rock. Once you find the veins you will want to chip out the ore. In the picture with the white quartz vein you can see the dark red/brown iron ore, just above the quartz. When quartz and iron were formed they cooled at a different temperature and separated. You can think of it like cow’s milk. The cream floats to the top and separates from the milk, but both are found right next to each other. If you were to mine this vein you would chip out the white quartz and the iron ore. Because, as noted in the picture, not all of the iron ore has completely separated from the quartz and small pieces of iron ore are found encased in the quartz. Also note the dull, grey, thin, line below and above the quartz. This is where silver will be found. When finding gold ore remember that concentration of gold and silver will vary so it’s a good idea to gather samples from different sections of the mine so that they can be assayed. Fill a five gallon bucket from different section of your mine. Make sure to label the bucket and the section of the mine and either crush the ore yourself or get it assayed. In this photo you can easily see the vein of iron ore. You would want to chip out the whole vein for processing. The highest concentration of gold will be found on the right (the darker brown section). The red in the middle of the photo indicates a higher concentration of iron. Iron will separate from the gold while panning or sluicing. The density of gold is over twice that of Iron. A gallon of water weighs a little under 8.4 pounds. The same volume of Iron would weigh just under 65.7 pounds, and the same volume of gold would weigh just over 161.2 pounds. Sluicing gold is effective because gold is over 19 times denser than water and over twice the weight of Iron. The table below can give you a good idea of the comparison of different common metals.
Densities of materials
In this photo the dull black or dark gray area (large circled blue area on the left of the picture) is a combination of silver lead and zinc. Again the red rock gets it’s color from oxidized iron. Gold can be found mixed with the iron, but the higher concentration of gold will be found in the brown rock, which is mixed in with the black rock. Also next to the yellow sulfur you can see some brown rock (circled in blue) and that rock also shows evidence of gold.
The photo with the black glove is another good example of iron ore that cooled and separated frome the quartz. The iron ore separated from the quartz when it cooled, but is found all around it. You would want to pull out all of the rock including the quartz for processing. Sometimes the gold is in a finely divided state, sometimes it’s found in larger sizes, such as nuggets, grains, scales, plates, threads and wires in quartz rock.
Sometimes gold is found in tiny specs scattered through slate and some sedimentary rocks like limestone. The gold was placed there by the flow of heated and mineralized waters. Sulfur is often a transporter of gold. So it’s not surprising that native gold is also very commonly found within sulfide minerals such as pyrite. Pyrite can contain up to 30% of gold content. Iron pyrite acts as a reducing agent. This means that the pyrite won’t bond with the gold. So, whenever gold is found in pyrite, it is always present as free milling gold. This last picture shows a yellow layer of gold between quartz. Also note a dark gray vein of lead and silver. You would want to chip out the whole vein from the rock.
Density information courtesy of Hoadley, Rick. http://www.coolmagnetman.com1998-2012