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Friday, June 29, 2012

Time and Stratabound Ore Deposits:

Native copper
Photo by Rob Lavinsky

Time and strata bound ore deposits are formed because of a specific geological event that causes the deposition of ore.  This ore is not necessarily deposited at the same time as the rocks, but can be the result of a later infusion of metal bearing liquids with a chemical reaction depositing the metals from solution.  A typical reaction may occur between the solution and organic components in the host rock.

Another type of time and Stratabound deposit is volcanogenic massive sulfides.  These deposits are the result of metal sulfides being the first minerals to form in a magma, and sinking to the bottom of the magma chamber due to their higher density over the other components of the magma.

A common form of this kind of ore deposit is one that forms where there is a fossilized log jam in the bed of a paleostream deposit where the logs provide a large bed of organic matter that can be replaced by later mineral bearing fluids. 

These fluids are generally the result of volcanism, and are coming from both groundwater sources and juvenile waters that are expelled from the magma as it cools.  These waters can also dissolve metals from the surrounding country rock as they are passing through then.  The majority of ore deposits are of this type.

Another type of Stratabound deposit occurs when you have a paleo-karst terrene where the mineral laden waters follow the overlaying sediments that are usually sandstone that has also filled the nooks and crannies in the karst terrene.  An example of this is the lead/silver deposits that were mined in Leadville, Colorado.

The presence of petroleum can also supply a source of organic materials to trap metal bearing solutions and make them give up their riches.  This usually happens in volcanic intrusions into sedimentary rocks that produce enough heat to turn some of the petroleum into an analog of amber called “Pyrobitumen.”  This often happens in back arc basins like the Sea of Japan.

The metals are usually deposited anywhere they meet with a reducing environment.  A back arc basin is one of these places.  A marine basin is another of these places, and a good modern example is the Mediterranean and Black Seas.  Other then the mane made Suez Canal this large area of seawater only has one natural entrance and exit the Straits of Gibraltar.  The waterflow is restricted through these straits due to their narrowness, and shallowness.  The waters from the Atlantic flow inwards along the bottom, and the water from these seas flows outwards from the top.

Most of the world’s ore deposits are found in Time and Stratabound deposits. 


Cisternas, M. Eugenia, et al. The role of bitumen in Stratabound copper deposits in the Copiapo region of Northern Chile, Springerlink,  

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