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The Fluids Make Dolomite: The limestone “birth place” rock is converted to a dolomite rock.
The Herkimer host limestone is changed into a dolomite rock, and in doing so it adds more “holes” (increased porosity)
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Throughout the Herkimer Mining District there are faults (large cracks in the Earth where the land mass on one side of the crack shifted relative to the land mass on the other side).  These faults pass through the birth place rocks of Herkimer diamonds and it is likely some were active while those rocks were being laid down. Geologists have found zones of dolomite around these faults, where the original calcium carbonate limestone has been altered, changed to a magnesium-calcium carbonate = dolomite. The process of doing this results in a change in volume within the rock and thus an increase in the porosity (amount of holes). The first dolomite event may have been wide spread around the faults (both here and in the gas well geology of the Finger Lakes) but the crystals are very small.  See the dolomite page:
The nature of mining for Herkimer diamonds is mostly a near surface process, and usually at fairly shallow depths (10-15 ft). The two mines in Middleville, which supply a good majority of market place Herkimer diamonds, have a little more rock exposure (20 to 50 ft). It seems that the mineral bearing zones in these mines are horizontally controlled.  But is this true across the entire district? Are ALL the deposits in the Herkimer district are strictly horizontally controlled? It is a reasonable belief given the sampling bias created by the surface mining conditions that limit our vision of vertical structures. But this view changes once you have more vertical surface to examine - as in the HBQ site - where we have large vertical rock exposure (hundreds of feet) and can see a clear relationship between vertical structures and the larger pockets (some big enough for a man to sit in comfortably).  Then factor in that we know that dolomitic solutions followed fault features and then moved out horizontally into the surrounding rock. The fluid system that helped to make Herkimer diamonds is a system that has both vertical and horizontal flow - and this is a critical part of understanding the diversity of mines across the entire Herkimer Mining District.  For more on the vertical and horizontal flow features visit the page:
Dolomite Page Dolomite Page Vertical and Horizonal Flow Features Vertical and Horizonal Flow Features
After the first dolomite event, dolomite was part of the crystal forming process that made Herkimer diamonds, and crystals of dolomite may have been forming along with them.
Note! The fluids in this mineral system flow both vertically and horizontally.
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