Gore Mountain Garnet, found in the

Adirondack Mountains The Adirondack Mountains (; a-də-RÄN-dak) form a massif in northeastern New York with boundaries that correspond roughly to those of Adirondack Park. They cover about 5,000 square miles (13,000 km2). The mountains form a roughly circular ...

in New York, contains the world's largest

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

s. The rock that holds these garnets, garnet

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

, is sometimes referred to as 'black ore' or 'dark ore.' This rock formation formed during

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

during the Ottawan phase of the

Grenvillian orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...

, and extremely high temperatures combined with introduction of fluids is what most likely contributed to the unusual size of the megacrystic garnets.

Geography

The Gore Mountain Garnet outcrop is located in the south central Adirondack mountains in northern New York. The mountains themselves, which are part of the Canadian Grenville Province, are composed of three main parts, the first of which is a

dome A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...

of anorthosite. This dome underlies a mantle of

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

, which in turn underlies a metasedimentary sequence of

marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

, amphibolite, and other types of gneiss. The rock formation that Gore Mountain is famous for, the garnet amphibolite, is located 2600 ft up its northern slope. Said rock outcrop measures about 50 m by 600 m and trends east-west, grading into garnet-bearing meta-gabbro to the east. To the south of the garnet amphibolite is a fault that runs parallel to the contact line of a meta-syenite formation. The garnet amphibolite also borders the southern margin of an olivine meta-gabbro formation.

Geological Overview

The garnet amphibolite in Gore Mountain formed during the Ottawan Orogeny via metamorphism. Past studies suggested that the growth of garnets was aided by magma intrusion and partial melting, but that hypothesis has since been disproven by experiments. The abnormal size of the garnets is attributed to high temperatures and fluid flow introduced by faults.

Metamorphism

The protolith is a

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

-clouded

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

metamorphosed

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...

(metagabbro). Metamorphism took place during the Ottawan phase of the Grenville orogeny, when crustal thickening was occurring. The peak temperature of metamorphism was notably high, being over 900 °C. Peak pressure was also quite high, ranging from 9-10 kbar. The metamorphosis of the protolith into garnet amphibolite is marked by orogenic folding and plastic flow, as well as a hydration phase. Garnet growth, introduction of fluid, retrograde metamorphism, and ductile deformation all occurred simultaneously.{{Cite journal, last1=Goldblum, first1=Deborah R., last2=Hill, first2=Mary Louise, date=1992, title=Enhanced Fluid Flow Resulting from Competency Contrast within a Shear Zone: The Garnet Ore Zone at Gore Mountain, NY, url=https://www.jstor.org/stable/30065046, journal=The Journal of Geology, volume=100, issue=6, pages=776–782, doi=10.1086/629628, jstor=30065046, bibcode=1992JG....100..776G, s2cid=129087018, issn=0022-1376 The fluid flow was likely channelized, as evidence from the large variability in trace elements.Morgan, Elizabeth R., "Geochemical and Petrographic Analysis of Gore Mountain Garnets, Adirondacks NY" (2011). ''Honors Theses''. 1036. https://digitalworks.union.edu/theses/1036 Fluid flow induced ion mobility and plasticity, as well as enriching the whole rock chemistry with

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

and

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high me ...

and depleting cesium. Aside from the introduction of fluids, metamorphic transformation was isochemical as evidenced by the homogeneity of the garnets. Having all chemical constituents continuously available and constant pressure and temperature conditions during formation is most likely what allowed isochemical transformation and homogeneity in the garnets. The garnets underwent two growth phases and one reabsorption phase. The first growth phase was homogenous, while the second phase was marked by the enrichment of yttrium and

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

in the garnets. In the reabsorption phase, garnets partitioned yttrium but were depleted in titanium. Two periods of cooling occurred, a fast one (91 °C/Ma) and a slow one (26 °C/Ma). Advection dominated cooling happened during topography driven lower crustal flow, while conduction dominated cooling happened one flow stopped.

Petrological description

The rock formation that bears the large garnets in Gore Mountain is an

that has been enriched in yttrium and lithium.Denny, Alden. "Determination of the Duration of Retrograde Metamorphism at Gore Mountain and Ruby Mountain, NY."Hollocher, Kurt, et al. "Petrology of Big Garnet Amphibolites, North Creek–Warrensburg Area, Adirondacks, NY." ''Geological Society of America Abstracts with Programs,''. Vol. 40. No. 2. 2008.

Mineralogy

Minerals in the garnet amphibolite include: * Hornblende *

Plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

Garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

Biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

Orthopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

There are some minor cases of

sulfides Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...

, and there are no

olivines The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

. The garnets in Gore Mountain are abnormally large, with the largest being 1 m in diameter. They have a hardness of 8-9 on the Mohs scale and an average density of 3.95 gm/cm³. The composition of the garnets are as follows: * 43%

pyrope The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for ''fire'' and ''e ...

* 40%

almandine Almandine (), also known as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia M ...

* 14% grossular * 2%

andradite Andradite is a mineral species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12. Andradite includes three varieties: * ''Melanite'': Black in color, referred to as "titanian andradite".spessartine Spessartine is a nesosilicate, manganese aluminium garnet species, Mn2+3Al2(SiO4)3.Gemological Institute of America, ''GIA Gem Reference Guide'' 1995, This mineral is sometimes mistakenly referred to as ''spessartite''. Spessartine's name is a ...

Trace element geochemistry

The garnet amphibolite is strongly enriched in

and depleted in

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

, cesium,

, and

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

. Trends in the garnet amphibolite include: * Hornblende and

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

growing bigger with the garnets *

Manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

concentrations increasing towards the outer rim of garnets due to reabsorption * Enrichment of yttrium and titanium during garnet growth * Depletion of titanium and enrichment of yttrium during garnet reabsorption * Overall enrichment of

in the whole rock and overall depletion of

, cesium, thorium, and uranium * Green-

included

becoming white plagioclase with no inclusions.

Structures

The garnets bear the inclusions in this rock unit, which range from minerals such as acicular rutile,

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

, hornblende, ilmenite,

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

, and

. The most common of these is the acicular rutile. Also shown in the garnets is well-developed tectonic parting. Viewing the garnet amphibolite as a whole, there is a fault to the south that runs parallel to the contact with a meta-syenite formation. Lineation in the garnet amphibolite is formed by the parallel alignment of hornblende, elongate mafic and felsic minerals, plagioclase pressure shadows, and few instances of elongate garnet.

References

Geology Petrology