Thursday, November 20, 2008

3 Theories on the Geology of Gold Deposits

As miners, every bit of information we can glean about gold and its formation can help us out in the field. This information includes the geology of gold deposits. Currently, there are three main scientific theories on the geology of gold deposits and how gold is formed. Here they are:

1. Geothermal Action

These deposits are formed in areas of volcanic or sedimentary rocks when circulating ground water is driven out of molten rock or magma (more commonly known as lava) by heat and then is forced into the top 2-5 miles of the earth's surface. If you want a clearer idea of what I am talking about here, the best analogy is a underground geothermal system that can be tapped for energy or even, in some instances, as a surface mineral hot spring. The key point to remember here is the term "mineral," for without some sort of mineralization occurring gold will not be formed.

Rainfall accounts for most of the water contained in geothermal geological systems. The rainfall waters seep downward primarily through paths of least resistance such as fissures, cracks, and permeable layers in the cooler portions of the earth's crust. Eventually, the water is drawn sideways into pockets heated by magma and then forced upward through fractured layers. Metals such as gold are then dissolved from the surrounding rock. More importantly, when superheated water interacts with cooler rock layers at shallow depths various metallic minerals form gold veins or ore bodies.

2. Precipitation

No, this ore formation theory is not about rain, snow, or sleet. Geologic precipitation occurs when gold-bearing or or other mineralized chemical solutions are ejected from magma as it cools. As these natural chemical solutions are ejected or otherwise forced out, they "precipitate" into the cooler "host" or country rock surrounding them. In many instances this type of gold ore precipitation can be found in areas where large granitic fields or "batholiths" occur, such as California's Motherlode Region, where large granitic formations represent the cooled magma (albeit in a much altered state).

3. Metamorphism

The third theory is relates to auriferous veins in metamorphic rocks occurring in mountainous regions at the margin of various geologic "contact" zones. During the mountain uplift process, sedimentary or volcanic rocks can either be buried more deeply beneath the earth's crust or "pushed" laterally under the edge of other geologic zones or masses, including continents.

When this happens the two types of rock are subjected to extremely high temperatures under terrific pressure, resulting in a complete alteration or metamorphosis where the essential chemical and structural bonds of the rock are transformed into something entirely new. Highly mineralized, superheated water is expelled from the rocks and begins migrating toward the surface creating gold ores as the deeper earth pressures and temperatures decrease.

My Familiarity

I am most familiar myself with the geologic processes described in the latter 2 theories. I've spent many years working in California's Motherlode Region where the underlying granitic batholith is the primary source for most of the regions' lode and placer gold. Additionally, I have worked (at one time or another) any number of metamorphic gold zones in Arizona, New Mexico, and southeastern California. Perhaps this is true of you as well.

(c)  Jim Rocha (J.R.)  2008

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