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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,  

Prospecting in Gabbro

Gabbro a dark colored mafic rock
Photo by Tanno4595

Gabbro deposits are noted for their polymetallic sulfides that are often associated with the base of the intrusion in the form of Volcanogenic Massive Sulfides caused by settling of the sulfides to the bottom of the intrusion.  They are usually found on the footwall of the intrusion where it meets with a different kind of rock that was intruded. This is assuming that the intrusion was injected in one shot as many of them are.

If the intrusion underwent more then one injection of magma you are more likely to have a series of bedded ore bodies that are at the bottom of each individual injection as a result of segregation of the components of the magma.  These are primary ore deposits derived from the magma itself.

The ores most apt to be found associated with a Gabbro intrusion aside from iron pyrite and phyrottite are the sulfides of nickel and copper with accessory mineralization of chromium, silver, gold and platinum group metals (PGM).  More often then not these are segregated into the volcanogenic massive sulfides (VMS) at the bottom of the intrusion, or if the intrusion cooled quickly dispersed throughout the body of the intrusion. 

Some good examples of Gabbro intrusions exist on opposite ends of Connecticut.  The first intrusions of interest are located in Torrington and Litchfield they are the Hodges Complex in Torrington, and Prospect Hill in Litchfield.  Both of these prospects have had producing copper-nickel mines located on them during the early 1800s.  They were also explored for nickel after WW II.  Hodges Complex by Falconbridge mining from Canada and Prospect Hill by Dino Testone a local prospector who owned a diamond drilling rig.

In the case of the Hodges Complex not enough ore was found to make a modern mine.  The Prospect Hill Complex was another story, the diamond drill turned up a large quantity of nickel ore, but Mr. Testone passed on before he could develop a mine as told to the author by Mr. Testone.

At a later time with another geologist the author did a bedrock mapping project on the Hodges Complex and found it to be divided into three lobes that they were norite interbeded with an untramafic rock called pyroxenite.  We found several minerals in the intrusion including: ilmanite, sperrylite, erytherite, chalcopyrite, cordierite, pyroxene, labradorite, plagioclase, serpentine, and pyrite.

Both of these intrusions were diatremes that from other like diatremes were injected as wine glass shaped intrusions.  Most of the separated minerals were out of sight, and probably at the bottom of the intrusions, or in their footwalls.

The other two intrusions are located in Eastern Connecticut.  The first one is the Preston Gabbro located in the towns of Preston and Ledyard.  Most of these intrusions are found on the lands of the Mashentucket Pequot’s and on the grounds of their casino “Foxwoods.”   According to their former chief Richard Hayward in a telephone conversation several years he told the author that they found platinum in the drill cuttings when they were drilling wells for Foxwoods.  There is a platinum nugget on display in the Wesleyan University Science Museum that came from the river draining this intrusion.

There is another intrusion of Gabbro to the Northwest just below Willimantic that may be a larger part of the Preston Gabbro that could be connected below the younger rocks that form its roof.  It will take some drilling however to prove this however.

To prospect in either of these intrusions probably the best place to look for mineralization is in their footwalls where any sulfide minerals might have collected.  This may be done looking for magnetic anomalies or by core drilling.

The State of Connecticut maintains records of wells that have been drilled in the state at DEP Headquarters, 79 Elm St., Hartford, CT.  This is a good place to look because this is the office of the State Geologist as well as the Reading Room for State Environmental Records.  It also houses the DEP Store that sells State Publications.

Another nearby source of information is the State Library on Capitol Ave. across the street from the State Capitol.  They keep aerial photos reaching back to 1934 as well act as a government repository for documents published by the US Government.


General Types of Auriferous Deposits, Alluvial Exploration and Mining,

Bedrock Geological Map of Connecticut, Compiled by John Rodgers, 1985,