Gold is often found associated with milky hydrothermal quartz that is called “bull quartz. Although gold has many other associations with base metals this is its association with iron pyrite (fool’s gold). Gold is found with many other metals and ores. One common association is with iron ore where it is often found in association with granular magnetite. This is a common occurrence in places like Nova Scotia and the Cornwall, Pennsylvania iron mines.
The common way gold in quartz is found is when hydrothermal water with both gold and quartz percolate up through the fissures in bedrock that is hot enough to hold both minerals dissolved in solution. Both minerals drop out of solution when the water cools. Quartz falls out of the solution first. The speed of the dissolution can be judged by the size of the quartz crystals. Basically, the larger the crystals the slower the hot fluids cooled. Gold is one of the last minerals that to come out solution. Gold is one of the last minerals the last minerals to be deposited out of solution.
There are several other minerals that are likely to fall out of the solution first, quartz is only the first of many. Other common minerals that are apt to be deposited ahead of gold are galena lead sulfide, sphalarite zinc sulfide and bornite a sulfide of copper and iron that is called peacock ore. Gold is usually found as an encrustation on other minerals that form in the spaces in the quartz matrix. Quartz containing gold is called by miner’s “bull quartz” a milky quartz having the crystal faces obscured in the body of the rock.
They are there none-the-less you just can’t see them with the naked eye however, they can be seen by using an X-ray Diffraction Spectrometer. These are usually called “Powder Cameras” by the geologists that use them. They work by mixing the powdered mineral with caladium, a mixture of acetone and nitrocellulose. They are then formed into small rods containing the powdered mineral that are placed in the X-ray beam. By photographing these rods with a special X-ray camera using a photographic plate that shows the resulting diffraction patterns of each mineral. Every mineral has a distinctive diffraction pattern that is used to identify it just like fingerprints.
Gold can be found in just about any rock, sometimes in specks that are too small to be seen with a conventional microscope. They are only seen with an electron microscope. These deposits are diffused into the rock and weren’t discovered until the 1970s. These deposits hold enough gold as tiny specks to be economically viable.
One such mine is the Carlin mine operated by Newmont Mining in northern Nevada. Another mine is one the author tried to get control of in Mexico years ago where the gold was diffused through marble as equally tiny specks. We discovered the deposit had the potential of producing several billion dollars worth of gold, except the owners were not willing to sell the property.
Gold Reef Mining, Wikipedia, http://en.wikipedia.org/wiki/Quartz_reef_mining
Igneous Geology of the Carlin Trend, Michael W. Ressel, et al, Geoscience World, http://econgeol.geoscienceworld.org/cgi/content/abstract/101/2/347