Evidence for Vertical Flow and Hydrothermal Alteration in the Herkimer Diamond Mining District

In the science of geology processes have a tendency to duplicate themselves.  If gas is thought to be related to fracture systems just south of the Herkimer Mining District, then it is likely the oil (and the minerals) in the Herkimer Mining District are also tied to fracture systems.  We also know from observation in the Herkimer Mining district that the oil and minerals followed along rock layers that had lot of holes (also true for the gas deposits).  In the gas deposit exploration they recovered core samples of the rock and through their analysis they developed the model in slide 1.  In the Herkimer Mining District we do not have access to such extensive core samples, and we have very few places where there is fresh rock exposed over a large vertical distance - large enough for us to actually see the influence of vertical mineralization cutting across flat rock layers.  Most of the mines/deposits in the district have very limited vertical depth.  This makes it quite hard to see the vertical structures, and most likely explains its absence from discussion in connection to the birth of Herkimer diamonds.

Gathering the Evidence

The largest fresh vertical surface in the Herkimer Diamond Mining district is at HBQ. If this analog theory is to be applicable to the district then vertical mineralization should be observable at HBQ.  PROBLEM! The site is off limits.  Fortunately some photos were donated and one provides excellent evidence for these large vertical mineralized structures. This is shown in slides 2 through 4. An unexpected discovery, but one that makes sense, is that when these vertical structures cut across the dominant pocket producing layer then larger pockets are found at, or very near, that intersection. In the summer of 2015 this author (along with David Beck) was fortunate to find veins in breccia (broken, usually displaced, rock that is filled in with minerals)  from Middleville. Photos of this breccia are shown in slides 5 through 7. In May of 2016 (with Frank M.and Adam S.) samples of mineralized veins were found at the Little Falls mine - photos 8 and 9 (more micro-vein photos - slides 19 to 23 and special photos 23a and 23b). Slide 10 shows what is likely to be a fracture filled breccia* in connection with the veins and vertical pocket at HBQ - slide 11. DA also has large vertical pockets - see slide 12. Slides 13 and 14 relate fluid flow variations to variations in crystal groupings seen in crystal pocket reconstructions - Middleville versus DA.  Slide 15 adds the laboratory evidence for hydrothermal fluid flow in the Herkimer Mining district.

Alteration and Zoning Evidence - Locating the Hot Spots

Two features that are common to hydrothermal - fracture systems are 1) alteration of the host rock, and 2) zoning of the deposited minerals as you move away from the hottest parts of the system.  Slides 16 through 31 show examples of the wall rock alteration and also some examples of changes in the intensity of this alteration (possible zoning examples).  These 30 slides offer many different pieces of evidence for hydrothermal fluid movement throughout the Herkimer mining district. This is an important piece of the new story about how Herkimer diamonds were formed.

*Breccias in the Herkimer mining district need their own research in order to better understand their origins.