Geology of the Gore Mountain Garnet Mine

Garnet Mine Tours

The following story of the geological history of the Gore Mountain Garnet Mine of the Baron Mines Corporation follows closely the generally accepted theories of Adirondack geologists; however, there are many divergent opinions as to the geology and ore genesis. My main objective is to supply a literary treatise on this fascinating natural phenomenon prepared in such a way that it is interesting, informative, and readable for the layman. The use of technical terms is minimized and there is no intention that this is to be a technical paper. I might add that today’s geologic picture of this unique mineral deposit leaves many questions unanswered and, needless to say, I have not attempted to compress into this article the wealth of knowledge which geologists and others have made available on this topic.

The mineral deposit I am writing about is widely recognized throughout the world as the most famous of its kind. It has been actively mined by Barton Mines since 1878 and its main product, garnet, is widely used as an industrial abrasive. It is visited regularly by eminent geologists and students from throughout the world. Its location near North Creek, New York, is 2,600 feet above sea level on the north side of Gore Mountain in the heart of the Adirondacks, the oldest known range of mountains in North America. Here, garnet crystals over two feet in diameter are often seen and crystals over three feet have been reported.

The story begins at the earliest known geological period of this deposit, a billion or more years ago in the Archeozoic Era which occurred a billion years or so after the beginning of known earth time. The story ends with formation of the garnet-bearing ore in the Precambrian Period, approximately five hundred million years or more from the present time.

During the Archeozoic Era the Grenville Sea, one of the great prehistoric seas, covered this area and extended northward into Canada approximately one hundred miles. This great sea carried in solution sediments that were deposited many feet deep on the existing rocks of the time. During the existence of the sea and after drainage, these sediments formed into the sedimentary rock. Following the depositing of the sediments and formation of sedimentary rock, three principal igneous rock intrusions occurred. These intrusions as we know them today may be compared to the eruption of a volcano except that they were not explosive and the hot molten liquid rock did not penetrate through the earth’s crust. These intrusions with their tremendous power were a part of the mountain building forces of the Adirondack Range.

The three igneous rock intrusions at the garnet deposit were, in order of their occurrence, anorthosite, gabbro, and syenite.

The anorthosite penetrated upwards from the interior of the earth into sediments forming a dome many miles wide. Whiteface anorthosite is the rock at the eastern contact of the mine and Marcy anorthosite forms the western contact.

The gabbro penetrated as a pipe upwards through the anorthosite and into the sedimentary rock. after alteration, a thin section of the rock along the contact of the latter intruding syenite formed the garnet-bearing ore. The country rock of this ore was altered to a diorite rock. Gabbro forms the sidewall on the north side of the mine.

The syenite was an intense intrusion that penetrated upward into the anorthosite and forced its path vertically and horizontally forming sheets of rock between the anorthosite and sedimentary rock lying on top. It is assumed that this rock may lie under garnet deposit. The near vertical south wall is composed of syenitic rock type.

Each of the three intrusions entered into the outer earth’s crust as a viscous liquid or plastic material. As each rock penetrated into the other, it absorbed a portion of the rock through which it passed. It is quite possible that the gabbro may have been contaminated with the sediments, anorthosite and syenite; however, the extent of the contamination is unknown.

Each of the intrusive rocks were subjected to severe periods of metamorphism which caused complete change in their crystal line form. We are little concerned with the changes in the sediments, syenite or anorthosite rocks, bordering the garnet deposit. However, the primary constituents of the garnet deposits appear to be from the gabbroic rock. as the gabbro was undergoing its period or periods of metamorphism from tremendous pressures exerted from the interior of the earth (five miles or more below the mass) and lateral pressure from the contacting rocks, it returned to a plastic stage at a temperature of approximately 1,200 degrees Fahrenheit (see Note 1). It absorbed some hydrothermal vapors and gave off others during this change allowing some elements to escape and others to be injected into the plastic phase. The intense hot liquid on slow cooling caused the necessary elements to collect together and form garnet pockets in the now altered rock. The rock remaining is a diorite that contains the minerals of hornblende, plasioclase, feldspar, and biotite mica. Both the garnet with the hornblende and the diorite were formed during these periods of alteration.

After the formation of the garnet deposit, the overlying rock forming the earth’s crust was attached by erosion. The work of the rain, wind, and frost caused the rock to decay and be washed and blown away. Later the great glacier, while moving southward, carried away some of the rock. after the ice cap receded, the surface rock was again subjected to severe erosion which worked downward into the igneous rocks leaving them in the form in which we see them today.

The mine at the present time is approximately three-quarters of a mile long. It varies in width from fifty feet to five hundred feet and its widest section can be seen at the western end of the mine. The strike of the ore body lies in an east-west direction and the dip of the deposits is seven to nine degrees to the west.

Diorite rock forms the matrix of the ore body. Each garnet crystal or pocket in the diorite rock is encased in a rim of hornblende with a thin layer of feldspar between the garnet and the hornblende.

The deposit lies as a narrow band between gabbro on the north and syenite on the south. Whiteface anorthosite forms the east contact and Marcy anorthosite the west contact. (See Note 2.)

Note 1:

Dr. R. H. Wentroff, Jr., of the General Electric Company, armed with hornblende form this deposit created the first artificially made garnet as a forerunner to a later achievement by him and his company of producing the first synthetic diamond. He accomplished the former by mechanically creating a metamorphic period, that is, he placed the hornblende under a pressure of 375,000 pounds per square inch and applied heat at a temperature of 2,200 degrees Fahrenheit. The slow cooling of this little mass formed the manmade garnet. from his experiment he concludes that the temperature of the plastic floe was much higher than 1,200 degrees Fahrenheit. Wentroff’s creation of a garnet by pressure, heat, and dehydration applied to hornblende indicates that the garnet deposit was formed during a metamorphic period.

Note 2:

Dr. P. Bartholome takes the position that the rock identified as Marcy anorthosite is not a result of the anorthosite intrusion but as a derivative of a magnetic rock of the olivine gabbro series and classifies it as anorthosite troctolite.