The Upside Down Mine

Sterling Hill Mine in Ogdensburg, New Jersey
Photo by Dmadeo

No, this isn't about a gold mine it's about a zinc mine specifically the one at Sterling Hill, New Jersey that for many years produced zinc and was one of the largest mines and the East Coast of the United States. The ore produced from this mine is noted worldwide for its many fluorescent minerals, but like all mines in eventually came to it in and was closed down although it still operates as a museum. At this mine and the ones in nearby Franklin, New Jersey form a canoe shaped orebody hosted in marble that is been the subject of much controversy over the years as to its origins.

During the 1960s Brian Skinner of Yale University was working for the USGS and with a colleague started speculating on how this ore body was emplaced. Under normal circumstances it is assumed that the ore came from deeper down in the bowels of the earth. However in this case under the marble was a layer of impervious rock that apparently did not allow the ore bearing hydrothermal fluids to rise up from below the marble.

Willemite, Franklinite and Rhodocrosite under Ultra-violet light from Sterling Hill
Photo by Rob Lavinsky

Skinner and his colleague were working in the mine one night when the question of the deposit’s origin came up.  The layer below the marble was impervious, but the marble itself wasn’t.  Many layers of sediments were deposited above the marble including some that were volcano-clastic in nature.  These rocks were also pervious allowing ground water to perk down from higher up that were capable of bearing ore bearing fluids.  They came to the conclusion that the ore bearing fluids came down from above and perked through the marble until they hit an impervious layer below the marble.  Here the ore bearing fluids stopped and gave up their load of dissolved minerals creating the deposit. However there is zinc minerals dispersed throughout the marble above the actual deposits.

This is all theoretical, but there are other examples of this same mechanism at work.  One glaring example can be found in the iron deposits found in northwestern Connecticut.  For almost two centuries the iron mines in Salisbury yielded some of the richest iron ore on earth that had its origin in the overlaying Walloomsac formation of schist interbedded   with marble.

The same specimen as above under normal light
Photo by Rob Lavinsky

The origin of this formation is volcano-clastics that came from an island arc that formed off the then east coast of North America that collided with the continent during the late Ordovician and caused the Taconic Orogeny.  This volcano-clastic material was the source of the iron that formed the famous Salisbury iron deposits, and supplied the magnesium that turned the limestone into dolomite.  The Walloomsac formation extends south from Lake Champlain to Manhattan where it underlies among other things the New York Stock Exchange.  Throughout its length at its base and in the underlying marble iron deposits are sprinkled like plums in a plum pudding.

It is theoretically possible that some gold deposits that are hosted by marble were deposited by the same mechanism i.e. leached from the overlying deposits.  A possible example of this mechanism in action are the gold deposits of the Carlin Trend in Nevada.  Another suspect is the San Pedro mine in San Luis Potasi, Mexico.  Both of these deposits are hosted in marble.  Keeping this mechanism in mind might lead to the discovery of other deposits of valuable metals.

The World of Volcanogenic Massive Sulfides (VMS)

As a source of gold volcanogenic massive sulfides are not the primary source, but they do contain a substantial amount of gold as a byproduct. These deposits also hold significant amounts of copper, lead, zinc as well as gold and silver. Throughout history the VMS deposits are generally not the first choice of miners, as they are usually small and can be difficult to process. In today's world they have become a more significant source of metals than they were in the past because of advances in technology as well as the recent high metal prices that have caught the attention of prospectors.

This is Rouyn-Noranda the home of a VMS deposit. The Horne Smelter is in the background.

Deposits of VMS minerals can take in a variety of shapes occurring typically as lenses of polymetallic massive sulfide subform in patches on the seafloor near black smokers. Deposits of this nature can be found in extensional tectonic settings that include both spreading from the tectonic ridge, or can also be found in back arc basins such as the modern Sea of Japan. They range in age from some of the oldest rocks on Earth to the modern black smokers that are found in oceanic spreading centers.

Many deposits of this type that were once on the ocean floor have been incorporated in the continental crust by tectonic earth movements, so they are now on land. Even with the location of many of these deposits on dry land there are still many other exploration and mining companies that are interested in recovering minerals from the seafloor VMS deposits.

A black smoker where VMSs are formed in the Atlantic Ocean

Deposits of the VMS type are grouped according to the type of rock they are hosted in or on the basis of ore composition. This type of deposits can be classified in three different ways.

1. These are felsic volcanic hosted making up the most popular category that accounts for at least 50% of the deposits. Examples of this type of deposits can be found in the Abitibi region of Canada in Québec and Ontario.

2. These are mafic volcanic hosted accounting for about 30% of the deposits,

3. These are interbedded with a mixed bag of volcanic and sedimentary rocks that account for an additional 20% of VMS deposits.

The ore found in these deposits is broken into two groups: copper, zinc and zinc, lead and copper they also contain significant amounts of gold and silver.