At most large mines today, the gold is invisible It is so finely disseminated in the rock that geologists are heard to say, "I found a lot of gold, but I haven't seen it yet." In any case, different environments produce different gold exploration conditions. The following is a comparison of some of those environments.
Portugal. Of three properties investigated, one is now producing. That is a very good batting average! The properties were mined in Roman times and this was a re-evaluation of them. The Romans were very thorough; there is no gold left in the old workings. The deposits are in brecciated quartz veins (angular rock fragments in a quartz matrix). The gold here is associated with pyrite. The Portuguese government did not mind that historical areas were slightly disturbed in the search for more gold. Many countries, however, will not permit this.
Mexico. Better known as a silver producer, Mexico still mines a lot of gold. The Guanajuato District alone has produced more than 20 million ounces. Current gold mining here is very modern and obviously well financed! In quartz veins, amethyst is a good indicator of silver, which is often in black sulphide minerals. Red "ruby silver" minerals are also common. Some tailing ponds are high on hillsides above a human habitation. This is very dangerous as tailings are unstable.
Costa Rica. Although not known as a prosperous country, Costa Rica nevertheless has a strong environmental awareness and mining permits are difficult to obtain. The country is mainly thick rain forest and one can explore for many days and never even see a rock! Also, the soil is so deep that geochemistry doesn't work well. Contast this with arid regions where rocks are everywhere. Troops of monkeys in the trees follow explorers everywhere.
Belize. All available mineral land in this 4,000 square-mile country was leased by the Anschutz Corporation. Like Costa Rica, it is mainly thick jungle and it is hard to find a rock, much less a gold mine. Project strategy was to follow streams and hope for waterfalls or other places where rock might be exposed. There were snakes venemous enough to cause nearly instant death! Camp fare included large amounts of a native rodent, notably tough and gristly. Although there was potential for other minerals, there were no significant gold finds.
Nevada. This is one of the great gold-producing regions of the world. Most of this is in the northeast part of the state in the "Carlin Trend," which was discovered in the early 1960's and has been expanding since. Total reserves are now about 150 million ounces. Nearly all is invisible gold, not seen until poured into bars for shipment. Arsenic minerals are commonly associated with the gold. Another good indicator is jasperoid, a massive multi-colored quartz rock. In some mines, black carbon material can be seen and is believed to be former petroleum steamed out by gold-bearing fluids.
Colorado. All the old veins in Cripple Creek were completely mined out. The only gold production, and the only producing gold mine in the state, is in low grade areas mined by open pit methods. The typical grade is one gram per ton, which means one part per million, or, put another way, 33 tons of rock might contain one ounce of gold. But of course, not all of the gold will be recovered.