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Mineral Zones in the Herkimer District
It is common knowledge that within Herkimer diamond deposits throughout the Herkimer diamond mining district there are major pocket bearing zones producing Hermimer diamonds in a layer, fairly horizontal. This layer is often capped and underlain by a hard resistant rock which is here called the cap rock and the table rock. The major Herkimer diamond producing zone is here called the "pocket zone". The pocket zone is but one of three distinctly recognizable Herkimer diamond producing zones. Each zone has its own unique set of features. The second zone is called the "druze zone". The third zone is known by all Herkimer diamond enthusiasts but has not been officially recognized as a seperate zone. This zone is here called the "vug zone". Below is a discussion with photos, or links to photos.
A Very Generalixed Description of the Zonation in the Herkimer District
In the future we will have more information about the behavior of these zones, and their occurence at each of the mines in the district. What is presented here are only preliminary findings.
The relationship between each of these zones, their timing in relation to each other, and the processes that caused their unique features are all questions that have yet to be researched in detail. The zones also vary in behavior from deposit to deposit across the district. But below is a generalized summary of the layering ( preliminary findings ).
Overburden - dirt, plants, large gaps in the rock, weathered out holes may contain a few loose crystals. These are called "float".
Barren Rock - dolostone that generally is absent of any crystals. This layer can by inches thick (old days at DA) to 10s of feet thick (AD). It is interbeded with sandy layers. Breaks into large blocks.
Cap Rock - This is a very hard dolostone that is "sparkling" in the sunlight. Seldom contains any crystals. It is usually a thin layer, less than a foot in thickness. When weathered and near the surface this is sometimes called "shatter rock".
Pocket Zone - Contains pockets ranging in size from 1/2 a foot in diameter up to several feet. The shape of the pockets is not consistant across the district, but the "dome and dougnut" model has been proposed for the Middleville deposits.
This author also believes that this zone may repeat as a zone of smaller pockets above the larger pocket zone. This is seen at the TCR location. And it may repeat below the druze zone (seen at DA). There is also a small pocket zone and a large pocket zone at HH.
Table Rock - This is shown in some of the pocket zone pictures and videos. It is the floor that is followed under the pocket zone. It is of a similar nature to the cap rock (it is quite dense), except it may have abundant signs of black carbon material.
Vug Zone - dolostone that generally heavily dotted with small holes ranging from less than an inch to six inches in diameter, but average around 2 to 3 inches. Herkimer diamonds are scattered and do not occur in every vug. Black carbon material is present when a Herkimer diamond is in a vug. The rock is "sparkling" hard, but easier to work than the table rock, because of the vugs. The thickness of the vug zone varies, but it seems to always be more than a few feet thick but not more than 10.
It is a layer of many small holes, dolomite lined, and some with the black carbon material and no druze! Occassionally there will be a small Herkimer diamond, or a couple, within one of these holes, or vugs. This is the common collecting rock for tourists at the Herkimer Diamond Mine in Middleville, NY. A photo of the vug zone at TCR is provided on right.
Barren Rock - dolostone that generally is absent of any crystals. This layer ranges in thickness. It is dolostone interbeded with sandy layers. Breaks into large blocks.
Druze Zone - this zone is like the vug zone in that it is dotted with many small holes, but in this zone the holes are lined with pyramidal druze (first phase) quartz. There are veinlets of quartz running throughout the host rock. As with the vug zone, an occassional vug will have a Herkimer diamond on the druze. Sometimes a cluster can be found on the druze. In general the druze vugs tend to be a bit larger than the vugs in the vug zone. The thickness of the zone varies across the district, as do it features. Thickness ranges from a few feet (at DA) to 20 feet thick (at TCR, photo right).
Barren Rock - dolostone that generally is absent of any crystals. It is interbeded with sandy layers. Breaks into large blocks.
Please note that the above sequence is a very generalized sequence and is subject to updated information as more field evidence is gathered. It is presented here for public enlightenment, but should not be considered the "final word" on the subject. The fact that such zonation is observed, that it changes in character across the district and that some features remain fairly consistant across the district are remarkable points.
We do not know very much about the nature of the zones across large distances, anything more than 10 to 50 feet. The nature and behavior of the zones and layers can be observed across a long horizontal surface (100s of feet) at the Benchmark Hanson Quarry. Unfortuantely no researcher is allowed in there to study them in detail.
The pocket zone has been mined in Middleville along 100s of feet, in two mines. Because of this we have some idea of how that one zone behaves. But the mining is following a flat intersection with the zone and thus it presents as "thin slices" through the zone giving us only that type of view at any moment in time. In addition little attention has been paid to the details, while most of the attention has been on "getting the sparkle". A more detailed gathering of information, with documentation, would be needed during the mining in order to understand the zone behavior.
In a district that covers more than 600 square km you can imagine that examining 50 feet of exposed rock is only a tiny sample. It is quite likely that the zones and layers have more variability than has been documented thus far.
This page will be updated when more information is made available.
Photo from Bill and Anne, 2008, two pockets at the base of the wall.
Photo from Mike H., 2008
W. David Hoisington, Ph.D.
More pocket photos are located on each mine's web page - click Directions at the top.
photo to be added