GORE MOUNTAIN GARNET

    Famous for its garnet ore, Gore Mountain lies within a syenite mantle on the northeast side of an anorthosite dome (Bartolome, 1960). The ore is located in an amphibolite unit which contacts a olivine meta-gabbro unit to the north, a meta-anorthosite unit to the east, and is in fault contact with a meta-syenite unit to the south (Brady, 1996). A garnet hornblendite with little to no feldspar is located at the west end of the garnet ore (Brady, 1996). The amphibolite is a mafic body while the olivine meta-gabbro, meta-anorthosite and units are feldspar rich (more than 80 percent).

    There is a gradational contact between the olivine meta-gabbro and garnet amphibolite of approximately 2 - 3 meters wide (Brady, 1996). Ranging from 5 to 20 percent, the average modal percent of garnet in both the olivine meta-gabbro and the amphibolite is 13 percent. Across this transition zone there is a sharp increase in garnet size from 1 mm in the olivine meta-gabbro to 3 mm in the transition zone to 50 - 350 mm in the amphibolite (Brady, 1996). This increase in garnet size is matched by a ten fold increase in amphibole and biotite (Goldblum, 1992). In addition, in the transition zone olivine is lost, clinopyroxene decreases, hornblende increases, green-spinel included plagioclase feldspar change to white inclusion free plagioclase and hornblende almost completely replaces pyroxene (Goldblum, 1992). Such petrologic evidence indicates that the amphibolite was derived by retrograde metamorphism of the olivine meta-gabbro (Goldblum, 1992). The garnet amphibolite formed when gabbroic magma intruded at the edge of the meta-anorthosite unit. This magma cooled under tectonically quiet conditions and, after crystallization, corona structures formed in the interior of the gabbro at 800^oC (Luther, 1977). Water was absorbed by this intrusion and created a nearly isochemical transformation to the garnet amphibolite (Luther, 1977). The few garnets that did nucleate during this transformation grew to large sizes under static temperature and pressure (Luther, 1977).

    The olivine meta-gabbro unit to the north of the garnet ore is rich in clinopryoxene, orthopyroxene, garnet and olivine. These minerals all contain parallel microfractures; indicating that they were deformed in brittle fashion. This unit has poorly developed foliation and weak to moderate lineation. Reactions during the metamorphism of this unit happened so slowly that not all of the reactions were completed. These incomplete reactions are represented by coronas of minerals around the metamorphic mineral reactions.

    The garnet amphibolite is defined by elongated hornblende and biotite, plagioclase shadows around garnet and ellipsoidal garnet (Goldblum, 1992). Large hornblende almost totally replaced pyroxene; indicating that there was a large fluid influx during the deformation of this unit (Goldblum, 1992). This unit was composed of plagioclase feldspar, olivine, clinopyroxene and ilmenite before metamorphism (Brady, 1996).

    A meta-anorthosite body lies in the Central Highlands of the Greenville basin. This large rock unit is composed mainly of plagioclase feldspar. The meta-anorthosite unit originated as anorthosite magma from the earth^,s mantle and crust and rose to the surface; breaking off pieces of surrounding rocks asthe magma moved towards the surface. This unit is to the east of the meta-syenite, amphibolite and olivine meta-gabbro units. The meta-anorthosite is often intruded by olivine meta-gabbro with nearlyvertical contacts (Bartolome, 1960).

    Located to the south of the garnet amphibolite is a meta-syenite unit. This unit is composed of approximately 50 percent perthite, plagioclase feldspar, less then ten percent biotite, opaque minerals, and garnet and traces of apatite (Goldblum, 1992). This unit strikes NNW and dips shallowly to the west (Goldblum, 1992). A weak subhorizontal foliation is present and lineation is absent in this unit (Goldblum, 1992).

    The garnets at Gore Mountain are very a high quality abrasive, first used in the sandpaper industry by Mr. Henry Hudson Barton in the mid 1850^,s. The garnets, still mined by the same mining company started by Mr. Barton in 1878, are now also used for glass grinding, metal and glass polishing, coated abrasives, creating non-skid surfaces, for color picture tubes in televisions, and in removing the red hulls from peanuts (Brady, 1996). Although garnets are known as gemstones, no gems come from the Barton mine. However, gems cut from the Gore Mountain garnets are usually between one and five carats and are dark red in color (Kelly, 1992).

    Garnets at Gore Mountain range in size from 2.5 cm to 90 cm in diameter. Some crystals have even grown to one meter in diameter. They are composed of approximately 37 - 43 percent pyrope, 40 - 49 percent almandine, 13 - 16 percent grossular and 1 percent spessartine (Brady, 1996). Mineralogically, the garnets at Gore Mountain are not unusual. Similar garnets exist throughout the world; however, garnets of this size are unusual. The garnets have a hardness between 8 and 9 and they have an average specific gravity of 3.95 gm/cm-3 . Garnets found at Gore Mountain have a pseudo planar cleavage which is well developed tectonic parting.

    The garnet ore at Gore Mountain is prophyroblastic with rims of pure plagioclase feldspar around the garnets (Bartolome, 1960). The growth of these large garnets consumed the plagioclase feldspar which brokedown to form garnet and hornblende shells around each garnet (Luther, 1977).

    The garnet ore displays strong consistent lineation of shear zone trends WNW (Goldblum, 1992). This lineation is defined by the parallel alignment of prismatic hornblende grains, elongate segregation of felsic and mafic minerals, plagioclase shadows and occasional elongated garnets (Goldblum, 1992). The plagioclase shadows formed by a later deformation of the rock.

    Chemical analysis of the garnets in the garnet amphibolite and the olivine meta-gabbro show that the garnets are all chemically homogeneous. Such homogeny indicates that the garnets grew under conditions in which all chemical components were continuously available and that temperature and pressure conditions must have been uniform during the period of garnet formation (Brady, 1996). The growth of such obviously larger garnets in the garnet amphibolite is due to a influx of water during the time of garnet formation. The water penetrated the rock throughout the narrow transition zone between the olivine meta-gabbro and the garnet amphibolite. Evidence of such an influx of water is the abundance of hydrated minerals, such as hornblende. In addition, ion diffusion and increase in ductility can be seen in textural changes that occur from the olivine meta-gabbro through that transition zone and into the garnet amphibolite (Goldblum, 1992).

    Inclusions of cristobalite, a low density polymorph of SiO2 , is present in the garnets of Gore Mountain (Darling, 1997). The high temperature form of cristobalite is cubic and it is stable only between 1470^oC and 1728^oC at a pressure of 1 atm (Darling, 1997). In the case of the inclusions in the Gore Mountain garnets, the cristobalite formed meta-stably, at temperatures below its stability field (Darling, 1997). Although some of the inclusions are irregular or circular, most of the cristobalite inclusions are elongated or blade-like. The largest inclusions range from 3 -10 mm in width, 20 - 80 mm in length and 2 -4 mm thick (Darling, 1997). The cristobalite inclusions are believed to have originated as hydrous Na-Al siliceous melt that was trapped in the garnets during prophyroblastic growth (Darling, 1997). As a result of high tensile strength and low thermal expansivity and compressibility of garnet, this melt would crystallize under virtually constant volume conditions (Darling, 1997).

    To study the Gore Mountain garnets, samples were collected from the middle and the edge of both a large (15 cm in diameter) sample and a small (2.5 cm in diameter) sample collected at the Barton mine at Gore Mountain. The samples were placed into separate compartments in a metal cap, coated with epoxy and ground with a series of diamond laps to a smooth surface. Next, the sample was coated with a thin layer of carbon using a vacuum evaporator. The sample was placed into a Scanning Electron Microscope (SEM) with an accelerating voltage of 20 Kv. By using x-ray microanalysis and the Kevex system, the chemical composition of each garnet was taken in three places; the left edge, the right edge and the middle of each sample. For reference, the Kevex system used a sample of pyrope for Mg, Al, Si, and Ca ions found in the samples. A sample of rhodonite was used for reference of the Mn ions found in the samples and fayalite was used as reference of the Fe ions. All of the calculated equations are based upon twelve oxygens

The outer edge of the small garnet (left edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5487	6.75	MgO	11.19	1.2090
Al KA	0.9861	12.53	Al2O3	23.68	2.0235
Si KA	0.9771	18.99	SiO2	40.61	2.9452
Ca KA	1.0723	3.90	CaO	5.45	0.4237
Mn KA	0.0087	0.29	MnO	0.38	0.0233
Fe KA	0.3318	18.18	FeO	23.39	1.4183

Mg_1.2Ca_0.4Mn_0.02Fe_1.4Al_2.02Si_2.9O₁₂

The outer edge of the small garnet (middle of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5610	6.89	MgO	11.42	1.2395
Al KA	0.9711	12.35	Al2O3	23.34	2.0022
Si KA	0.9745	18.92	SiO2	40.47	2.9460
Ca KA	1.1025	4.01	CaO	5.61	0.4374
Mn KA	0.0074	0.25	MnO	0.33	0.0201
Fe KA	0.3280	17.97	FeO	23.12	1.4078

Mg_1.2Ca_0.4Mn_0.02Fe_1.4Al_2.0Si_2.9O₁₂

The outer edge of the small garnet (right edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5722	7.02	MgO	11.64	1.2476
Al KA	0.9857	12.54	Al2O3	23.70	2.0089
Si KA	0.9847	19.15	SiO2	40.97	2.9465
Ca KA	1.0689	3.89	CaO	5.44	0.4190
Mn KA	0.0062	0.21	MnO	0.27	0.0165
Fe KA	0.3328	18.23	FeO	23.45	1.4106

Mg_1.2Ca_0.4Mn_0.02Fe_1.4Al_2.0Si_2.9O₁₂

The inner portion of the small garnet (left edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5561	6.88	MgO	11.40	1.2315
Al KA	0.9743	12.44	Al2O3	23.51	2.0082
Si KA	0.9708	18.90	SiO2	40.44	2.9311
Ca KA	1.0020	3.64	CaO	5.09	0.3953
Mn KA	0.0090	0.31	MnO	0.40	0.0243
Fe KA	0.3455	18.91	FeO	24.33	1.4745

Mg_1.2Ca_0.39Mn_0.02Fe_1.4Al_2.0Si_2.9O₁₂

The inner portion of the small garnet (middle of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5523	6.83	MgO	11.32	1.2239
Al KA	0.9752	12.45	Al2O3	23.52	2.0107
Si KA	0.9717	18.92	SiO2	40.47	2.9356
Ca KA	0.9937	3.61	CaO	5.05	0.3925
Mn KA	0.0089	0.30	MnO	0.39	0.0241
Fe KA	0.3446	18.86	FeO	24.27	1.4722

Mg_1.2Ca_0.39Mn_0.02Fe_1.4Al_2.01Si_2.9O₁₂

The inner portion of the small garnet (right edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5536	6.83	MgO	11.32	1.2257
Al KA	0.9766	12.45	Al2O3	23.52	2.0133
Si KA	0.9731	18.93	SiO2	40.50	2.9408
Ca KA	1.0060	3.66	CaO	5.11	0.3979
Mn KA	0.0062	0.21	MnO	0.27	0.0167
Fe KA	0.3408	18.66	FeO	24.01	1.4581

Mg_1.2Ca_0.39Mn_0.016Fe_1.4Al_2.01Si_2.9O₁₂

The outer edge of the large garnet (left edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5317	6.56	MgO	10.88	1.1899
Al KA	0.9758	12.41	Al2O3	23.44	2.0275
Si KA	0.9652	18.74	SiO2	40.09	2.9420
Ca KA	1.0488	3.81	CaO	5.33	0.4192
Mn KA	0.0056	0.19	MnO	0.25	0.0153
Fe KA	0.3353	18.37	FeO	23.63	1.4504

Mg_1.18Ca_0.4Mn_0.01Fe_1.4Al_2.02Si_2.9O₁₂

The outer edge of the large garnet (middle of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5490	6.80	MgO	11.27	1.2235
Al KA	0.9751	12.46	Al2O3	23.54	2.0202
Si KA	0.9639	18.78	SiO2	40.17	2.9253
Ca KA	0.9582	3.48	CaO	4.87	0.3798
Mn KA	0.0093	0.31	MnO	0.41	0.0251
Fe KA	0.3477	19.03	FeO	24.48	1.4906

Mg_1.2Ca_0.37Mn_0.02Fe_1.4Al_2.02Si_2.9O₁₂

The outer edge of the large garnet (right edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.5456	6.77	MgO	11.22	1.2180
Al KA	0.9633	12.32	Al2O3	23.28	1.9973
Si KA	0.9717	18.92	SiO2	40.47	2.9462
Ca KA	0.9124	3.31	CaO	4.63	0.3614
Mn KA	0.0081	0.27	MnO	0.35	0.0217
Fe KA	0.3525	19.28	FeO	24.81	1.5105

Mg_1.2Ca_0.36Mn_0.02Fe_1.5Al_1.99Si_2.9O₁₂

The inner portion of the large garnet (left edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.4667	5.92	MgO	9.81	1.0630
Al KA	0.9706	12.48	Al2O3	23.58	2.0191
Si KA	0.9608	18.77	SiO2	40.16	2.9183
Ca KA	0.8446	3.05	CaO	4.27	0.3327
Mn KA	0.0141	0.48	MnO	0.62	0.0379
Fe KA	0.3992	21.76	FeO	27.99	1.7012

Mg_1.06Ca_0.33Mn_0.03Fe_1.7Al_2.01Si_2.9O₁₂

The inner edge of the large garnet (middle of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.4651	5.88	MgO	9.75	1.0625
Al KA	0.9628	12.35	Al2O3	23.33	2.0101
Si KA	0.9646	18.80	SiO2	40.22	2.9401
Ca KA	0.8478	3.07	CaO	4.29	0.3361
Mn KA	0.0165	0.56	MnO	0.72	0.0444
Fe KA	0.3872	21.12	FeO	27.18	1.6615

Mg_1.06Ca_0.33Mn_0.04Fe_1.6Al_2.01Si_2.9O₁₂

The inner portion of the large garnet (right edge of the sample)

Element &Line	K-Ratio	Weight Percent	Formula	Oxide Percent	No. of Cations in Formula
Mg KA	0.4647	5.88	MgO	9.75	1.0636
Al KA	0.9590	12.31	Al2O3	23.26	2.0055
Si KA	0.9614	18.74	SiO2	40.10	2.9329
Ca KA	0.8661	3.13	CaO	4.38	0.3435
Mn KA	0.0154	0.52	MnO	0.67	0.0416
Fe KA	0.3907	21.31	FeO	27.42	1.6773

Mg_1.06Ca_0.34Mn_0.04Fe_1.6Al_2.00Si_2.9O₁₂

    After researching the Gore Mountain garnet ore, it was not surprising to find that the garnets were chemically homogeneous. The inclusions of cristobalite described by Darling, Chou and Bodnar were not observed in either secondary electron mode or backscatter electron mode on the SEM. The samples of garnet used in this analysis were small and rather select pieces. Therefore, the samples may simply have not contained cristobalite inclusions, while other parts of the mineral may have such inclusions.

    In collaboration with John Brady, the influx of fluid theory was brought into question. The rocks at Gore Mountain were buried at depths up to 30 km and subjected to temperatures up to 800^oC. At such temperature and pressure, some hydrous minerals become unstable and begin to break down into their components. Some of the water involved in the fluid influx most likely came from the breakdown of such hydrous minerals present in the rock. In addition, there is the question as to where the remnents of these hydrous minerals are today. Some of the Al ions and Na ions probably contributed to the formation of the cristobalite inclusions described by Darling, Chou and Bodnar. Other ions, such as Fe, Mg, Si, Ca, Mn,and Al, contributed to the formation of the garnets present in the rock. The quantites of these elements; however, are not equivalent. For example, by looking at the equations K(Mg,Fe)₃(AlSi₃O₁₀)(OH) for biotite (one of the minerals in the formation of garnet) and an average of Mg_1.18Ca_0.38Fe_1.51Mn_0.03Al_2.01Si_2.93O₁₀ for the garnets at Gore Mountain, there are several ions missing from the reaction. It has been hypothesized that a slight amount of magma was another catalyst in the formation of the large garnets. The presence of magma would allow the ions in the minerals to communicate easily. Good communication between the ions in the rock would create larger mineral assemblages. The magma would also remove the excess ions left behind after the metamorphic reactions had created the new assemblages. If magma was a factor in the formation of the large garnets, it is evident tht there was not a large amount. There is no evidence of large mass flows in the garnet amphibolite. The magma would have formed from the water present in the rock during garnet formation. At temperatures up to 800^oC and the pressures of burial at 30 km a slight amount of water, such as that created from the breakdown of hydrous minerals, would have created a small amount of magma in the rock. It is this magma that is hypothesized to have been the second catalyst in the formation of the large garnets at Gore Mountain.

1.) Bartolome, P. Genesis of the Gore Mountain Garnet Deposit, New York. in Economic Geology and the Bulletin of the Society of Economic Geologists. edited by Alan Mara Bateman. volume 55. The Economic Geology Publishing Company. New Haven, CT. 1960. pg. 255 - 277.

2.) Brady, John and Jack Cheney. Guidebook to Selected Mineral Localities in the Northeast and Their Geological Context. Teching Mineralogy Workshop. Smith College, Northampton, MA. June 1996. pg. 14 - 21.

3.) Darling, Robert and I-Ming Chou and Robert J. Bodnar. An Occurence of Metastable Cristobalite in High Pressure Garnet Granulite. in Science. volume 276. American Association for the Advancement of Science. Washington D.C.. 4 April 1997. pg. 91 - 93.