Metasomatic Gold Deposits

Gold Nuggets

Geologists consider Metasomatism as the chemical alteration of rock by hydrothermal or other fluids that can come from either an igneous or metamorphic source.  

If it was caused by the action of magma intrusions or volcanic activity Metasomatism makes skarns, greisens and may also affect hornefels if there is a contact metamorphic aureole next to the intrusive magma.  In a metamorphic environment it takes the mass transfer of liquid from a mass of hot rock that has water driven from it by the action of metamorphism with the water or other fluids acting as a solvent.

This occurs whenever rocks buried in the deep crust are losing fluids (dewatered) as well as dissolved mineral components because of the breakup of hydrous minerals.  The fluid with its dissolved minerals percolate to the near surface and in some circumstances reach the surface as springs of juvenile water.  The action of this hot water can chemically alter and change the crustal rocks.

It is this mechanism that tells us that metasomatism is an open system that is different from the usual metamorphic process causing an in situ mineralogical change in a rock, but does not alter the chemistry of the rock.  In effect metamorphism and metasomatism go to gather like hand and glove.

In practice metasomatism is in reality a mass transfer process that is best shown by gold ore deposits that are the result of the focused concentration of gold bearing metasomatic fluids.  These fluids drop their load of dissolved gold in shear zones and lodes.  Often the first indicators of gold are altered rock.  It is this mechanism that removes gold from greenstone and deposits it in adjoining rock.

This fluid is extracted from many cubic kilometers of rock where the overall content of gold can be quite sparse yet it is concentrated in the host rock.  One common example of this is the so-called saddle reef where gold is concentrated at the nose of a vertical fold where sometimes a large quantity of gold occurs in a small deposit.  Another type of deposit is illustrated by the Carlin Trend in Nevada where the action of metasomatism has caused the depositing of gold throughout a massive formation of limestone.

In order to be affected by metasomatism it is necessary for the rocks to be permeable to water.  Some of the other minerals formed by this mechanism beside gold are: copper, iron, molybdenum lead, zinc and tin.  Some of the industrial minerals formed are: graphite, mica, talc and wollastonite.

Because the fluids are moving through permeable rock one of the best places to look for gold is where one rock type contacts another.  That area acts as a dam for holding the mineral laden water in place where it deposits its mineral load.