Mount Pleasant Caldera
Encyclopedia
The Mount Pleasant Caldera is a large eroded Late Devonian volcanic caldera
complex, located in the northern Appalachian Mountains
of southwestern New Brunswick
, Canada
. It is one of few noticeable pre-Cenozoic
calderas, and its formation is associated to a period of crustal
thinning that followed the Acadian orogeny
in the northern Appalachian Mountains. It sits relatively near to the coastline.
. The volcano is north-south trending in its elliptical shape, with minimum dimensions of 13 by 34 km (8.1 by 21.1 mi) as outlined by regional gravitational and magnetic studies. The northern half of the volcano has since been covered by depositional rock strata. The caldera is bounded to the east and west by fused Ordovician
to Silurian
turbiditic metasedimentary rocks of the local Digdeguash and Flume Ridge geological formations.
Late Silurian to Devonian granitic rocks of the Saint George Batholith bound part of the southern margin of the caldera. Rocks within the summit itself date back to the Upper Devonian, and show multiple fill sequences late in its history.
The magma
produced by Pleasant is rich in silica, as indicated by a large amounts of ignimbrite
, tuff
, rhyolite
, and other igneous rock
s rich in the mineral. Silica-rich magma does have a high viscosity
, and therefore does not flow easily like basalt
. As a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth the rapid off-loading of overlying material causes the trapped gases to decompress rapidly triggering explosive destruction of the magma and spreading volcanic ash
over wide areas.
Intrusion-related gold
veins
have recently raised great interest among economic geologists. In southwestern New Brunswick, which is part of the Canadian Alleghenian orogeny
, several gold deposits have been recorded in the past. The positive early results have created great interest for gold-finding efforts, and Mount Meager
has proven to be an ideal candidate.
Granitic
intrusions within the caldera complex include the McDougall Brook Microgranite and the somewhat younger Mount Pleasant Granite. Gold quartz
breccia
s and veins cut the McDougall Brook Microgranite and its volcanic wall-rock, while molybdenum
-bismuth
-tungsten
and later polymetal
lic mineralization
are related to the multiphase Mount Pleasant Granite.
The numerous felsic
sections are associated with episodes of fractional crystallization
in a high-level, zoned magma chamber
. Fractionation was continually interrupted by eruption of material from the roof zone such that seven phases of caldera growth have been recognized.
Mount Pleasant lies along the southwestern margin of the caldera complex. Two mineralized zones, termed the Fire Tower Zone and the North Zone, occur within volcanic plug
s about 1 kilometre (0.621372736649807 mi) apart. The volcanic necks are defined by magmatic-hydrothermal breccia
s.
The Intercaldera Sequence comprises formations that crop out from overlying flows in triangularly shaped area, and includes thick volcanic ash
(tuff
), thick breccia
layers, and intermediate to felsic
igneous rocks that tend to intrude
the above layers and are typically located along caldera margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic
lavas, alluvial redbeds, and porphyritic
felsic lavas that are distributed across five different layers. The late Caldera-Fill Sequence contains rocks that are similar to those of the outflows of the other, older layers, and comprises two formations and two relatively minor intrusive lava flows. The volcanic flows are generally mafic
. The stratigraphic subdivision is supported by geochemical and mineralogical analyses, which indicate that the basaltic rocks are mantle
-derieved and have, unusually, relatively intraplate (or hotspot
) type chemical affinities. The multiple andesite
flows were probably derived from basaltic magma by the crystallization
of the magma material. The relatively rare, more felsic
flow units, are thought to have originated from high-end crystallization inside the magma chamber
. The various stages of fractionation are continually interrupted by eruptions, and have allowed scientists to recognize seven stages of caldera development. The genesis of the caldera is related to a period of lithospheric thinning that followed the Acadian Orogeny
in the northern Appalachians.
The relative position of the Exocaldera and Intracaldera sequences is based on several observations:
The first and oldest layer is the Hoyt Station basalt formation, which is composed of at least two flow units. There are two types of rock associated with the basalt, conglomerate
ranging in particle size from small pebbles to cobbles, and lithic
lapilli
-tuff
(well-worn, consolidated volcanic ash). It is only directly exposed at the surface in one area, as a tuff on an extended solidified multi-age lava flow that extends northeast from the northern section of the volcano, colored green in the diagram at right.
The next oldest layer is the Rothea formation. It can be divided into three major lava flow layers. The lower member consists mainly of unwelded (unjoined), but heavily compacted, pumice
ous lapilli-tuff and crystal tuff strata. The middle member consists of rock ranging from nearly aphyric tuff at the base to crystal tuff near the top. The top section of the middle unit contains pyroxene
. The top member consists of a fine-grained, typically reddish lava flow and another lithic tuff unit. It can be found exposed in a limited region, encompassing the flank of the Hoyt Station basalt, colored light yellow in the diagram at right; it also peeks out closer to the volcano twice.
The next layer, the South Oromocto Andesite formation, is composed of at least three flow units, with basalt flows being the most extensive. Only one exhibits a porphyritic
texture. Calcite veins
and hematite bands
near the top of the layer indicate that this was a period of degassing in the flow. It forms a thin band around the Rothea formation.
The Carrow formation is a predominantly fine-grained redbed (consisting of typically red sedimentary rocks) unit that has a grade ranging from pebbles and cobble in conglomerate
at the base to mudstone
laced with calcrete at the top. The flow is littered with abundant flows from the Seelys (in the intercaldera sequence) and Rothea (earlier) formation towards the southeast, but in the north metasedimentary rocks, which are sedimentary rocks exhibiting metamorphosis
, predominate. The lower part of the formation consists of unwelded and highly compacted lapilli tuff with abundant amounts of pumice fragments. A basalt and basalt-clad mudflow
occurs near the top if the formation in some places. A dated locality from the top of the formation has been dated to the Late Devonian, around 350 million years ago. The exposed part of the formation forms a long band around the South Oromocto Andesite formation, and is colored brown on the map.
The last formation in the later, the Bailey Rock Rhyolite formation, is composed of porphyritic lava
, and is characterized by an absence of angular crystals and pumice derivatives. In places its rhyolite
composition intrudes
older stratas. This layer is unique for it bridges the exocaldera and intracaldera sequences. A layer of saprolite
(chemically weathered rock) separates the Bailey Rock Rhyolite formation from the overlying Caldera-Fill Sequence-era rocks. The exposed part of the flow overlies part of the Carrow formation, and is colored pink in the diagram above-right.
and one mafic dyke that intrude the Scoullar Mountain and Little Mount Pleasant formations, respectively.
The first sequence in the order is the Scoullar Mountain formation. The layer is characterized by sedimentary breccia and interbedded andesitic lava. In addition felsic pyroclastic rocks are very common in places and one sandstone-conglomerate lava flow can be found. The sedimentary breccia is dominated by pebble to boulder size angular metasedimentary stratas, and a few crystal tuff layers that contain about 1% biotite, the only such high amount of a typically trace mineral outside of Rothea formation in the Extracaldera sequence. A pumice
-ridden lapilli-tuff section near the believed top section additionally has a 1% composition of the mineral amphibole
. The exposed portion of the flows loosely flank the slopes of the volcano on the left and right, and are represented by bright orange on the map.
The next layer is the Little Mount Pleasant formation, which is composed of crystal tuff and banded rhyolite
. The tuff is characterized by, recrystallized pumice with microscopic crystal
s and amphibole
. The crystal structure inside the pumice are larger than those outside, indicating that significant mechanical breakage during the eruption in which they were deposited. The exposed part of this layer is located around the southern base of the main caldera, and is colored ochre (light orange) on the diagram.
The Seelys formation, next in the order, consists of lithic tuff and pumice-littered lithic lapilli-tuffs, banded, pumiceous, crystal tuff, and densely welded crystal tuff. The basalt contains clasts of both the Scoullar Mountain andesite and Little Mount Pleasant formation. Quartz
and feldspar
increase in size and abundance yoward the top. Biotite is virtually absent, but zircon
is very common. The formation is colored the lightest orange on the map, and swathes across the main body of the caldera.
The McDougall Brook Granite formation consists mostly of porphyritic
granite
, a borderline feldspar (sometimes quartz) porphyry
, and minor amounts of fine-grained quartz monzonite
. The grain size of the porphyry and the size and abundance of feldspar increases inward. Amphibole with apatite
is the main mineral phase in all three of the flows. Parts of the feldspar porphyry are hydrothermally altered. The deposit is extensively exposed, and is colored red on the map.
The Mount Pleasant Porphyry formation occurs as dykes and small rock areas that have been associated with local breccia. The dyke structures seem to indicate multiple periods of intrusion
. Two types of breccias have been identified, and older and more common felsic series, and a younger neutral phase. The porphyries making up the rocks were emplaced at the pre-existing volcanic margin.
The Big Scott Mountain formation consists of rhyolite of variable composition, lapilli-tuff, and crystal tuff. Most of the rhyolites are characterized by large amounts of pyroxene
. One rhyolite unit appears to overlie the McDougall Brook Granite formation. The tuff is littered by clasts from the Seelys and McDougall Brook Granite formations. The tuff also appears to be layered. It is colored dark orange on the map, and the exposed part partially rings the north of the volcano.
The final formation, the Kleef formation, includes redbed, basalt and pumice-ridden tuff. Pebble to cobble sized conglomerate is also seen complimenting the volcanic rock, and parts seem to come from older formations. The basalt is characterized by large crystals (up to 2 cm (0.78740157480315 in)). The tuff are characterized by their reddish-brown color and abundant fossil-pumice. The formation is very rare exposed and of interest to economic geologists
, as it contains many of the possibly gold-bearing intrusive areas. It is colored ligh blue on the map.
Mount Pleasant has a long history of exploration and development in mining. Tin
was first discovered on the mountain in 1937. The focus of exploration over the years has shifted from tin-base metals to porphyry tungsten
-molybdenum
-bismuth
deposits, and then to porphyry tin deposits, and now indium
, a rare element that is important to new technologies such as LCD screens and solar cells, computers and smart phones.
The Mount Pleasant mine is located 80 km (50 mi) south of Fredericton, the provincial capital. The mining company Adex Mining Inc. holds 102 prospective claims covering approximately 1,600 hectares (4,000 acres) and 405 hectares (1,000 acres) of surface rights at Mount Pleasant, dominating the local extraction rights. It is situate close to infrastructure and potential employees. With a population of 50,000, Fredericton is important to the mine, as is Saint John
, with a population of 70,000, which lies about 80 km (50 mi) o the southeast. Mount Pleasant is also only 65 km (40 mi) from the Canada – United States border. When the mine was last producing - 1983 to 1985 - tungsten ore was regularly transported through Saint John by means of a provincial highway for shipment to Europe. The mine site is accessible via all-weather roads from Fredericton, Saint John and St. George. Electricity is provided by the New Brunswick transmission grid and water from a pump house located on the nearby Piskahegan River.
On September 25, 2005, the Society of Economic Geologists
hosted a small field trip to the caldera and the nearby Mount Pleasant Mine and Clarence stream deposit. The trip was divided into three stages: the Mount Pleasant Caldera, the Mount Pleasant Mine, and the Clarence Stream deposits. The field crew described two trenches that seemed to outline the relationship between the volcanic rock and the possibly gold-rich intrusive areas.
On June 25, 2008, Geodex Minerals Ltd., a gold venture firm
, announced the results of a 2007 - early 2008 diamond drilling program on the west side of Mount Pleasant. Along with the development of the Sisson Brook tungsten-molybdenum-copper project north of Fredericton, the area surrounding the former Mount Pleasant mine has been a major focus of the company. Research is ongoing, and focuses on the two most likely candidates: the McDougall Brook Granitic suite (MBG) and the Mount Pleasant Granitic suite (MPG).
In June 2011, the owners moved their corporate offices from Toronto to Fredericton with the intent of refocusing efforts onto getting the mine running again by 2012.
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
complex, located in the northern Appalachian Mountains
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
of southwestern New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. It is one of few noticeable pre-Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
calderas, and its formation is associated to a period of crustal
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
thinning that followed the Acadian orogeny
Acadian orogeny
The Taconic, Acadian and Alleghenian orogenies are the three tectonic phases responsible for the formation of the present Appalachian Mountains. The Acadian orogeny is a middle Paleozoic mountain building episode dating back 325-400 million years which should not be regarded as a single event but...
in the northern Appalachian Mountains. It sits relatively near to the coastline.
Geology
The large elliptical feature is dated back to the late Devonian Period, and is partially covered in the north by overlying Middle Mississippian and Pennsylvanian Period strataSTRATA
Strata Marketing Inc. is a Chicago, Illinois-based software company involved in connecting media buyers and sellers. It is owned by Comcast. As of 2010, it processes about $50 billion worth of media buys annually.-History:...
. The volcano is north-south trending in its elliptical shape, with minimum dimensions of 13 by 34 km (8.1 by 21.1 mi) as outlined by regional gravitational and magnetic studies. The northern half of the volcano has since been covered by depositional rock strata. The caldera is bounded to the east and west by fused Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
to Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
turbiditic metasedimentary rocks of the local Digdeguash and Flume Ridge geological formations.
Late Silurian to Devonian granitic rocks of the Saint George Batholith bound part of the southern margin of the caldera. Rocks within the summit itself date back to the Upper Devonian, and show multiple fill sequences late in its history.
The magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
produced by Pleasant is rich in silica, as indicated by a large amounts of ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
, tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
, rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
, and other igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s rich in the mineral. Silica-rich magma does have a high viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, and therefore does not flow easily like basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
. As a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth the rapid off-loading of overlying material causes the trapped gases to decompress rapidly triggering explosive destruction of the magma and spreading volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
over wide areas.
Intrusion-related gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
veins
Vein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
have recently raised great interest among economic geologists. In southwestern New Brunswick, which is part of the Canadian Alleghenian orogeny
Alleghenian orogeny
The Alleghenian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 1957....
, several gold deposits have been recorded in the past. The positive early results have created great interest for gold-finding efforts, and Mount Meager
Mount Meager
Mount Meager, originally known as Meager Mountain, is a complex volcano in the Sea-to-Sky Corridor of southwestern British Columbia, Canada. It is located north of Vancouver at the northern end of the Pemberton Valley. Part of the Cascade Volcanic Arc of western North America, its summit is above...
has proven to be an ideal candidate.
Granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
intrusions within the caldera complex include the McDougall Brook Microgranite and the somewhat younger Mount Pleasant Granite. Gold quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s and veins cut the McDougall Brook Microgranite and its volcanic wall-rock, while molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
-bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
-tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
and later polymetal
Polymetal
Primarily used in chemistry or mining, polymetal or polymetallic refers to a substance composed of a combination of different metals. When the substance contains only two metals the term bimetal is sometimes preferred .* A polymetallic ore is...
lic mineralization
Mineralization
Mineralization may refer to:* Mineralization , the process through which an organic substance becomes impregnated by inorganic substances...
are related to the multiphase Mount Pleasant Granite.
The numerous felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
sections are associated with episodes of fractional crystallization
Fractional crystallization
Fractional crystallization may refer to:* Fractional crystallization , a process to separate different solutes from a solution* Fractional crystallization , a natural process occurring in igneous rocks during which precipitation of minerals takes place...
in a high-level, zoned magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
. Fractionation was continually interrupted by eruption of material from the roof zone such that seven phases of caldera growth have been recognized.
Mount Pleasant lies along the southwestern margin of the caldera complex. Two mineralized zones, termed the Fire Tower Zone and the North Zone, occur within volcanic plug
Volcanic plug
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an...
s about 1 kilometre (0.621372736649807 mi) apart. The volcanic necks are defined by magmatic-hydrothermal breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s.
Eruptive history
The eruptive history of the Mount Pleasant Caldera can be divided into three stages of activity: the exocaldera sequence, the intracaldera sequence, and the late caldera-fill sequence. These can further be subdivided into strata based on their depth below the sourrounding rock.The Intercaldera Sequence comprises formations that crop out from overlying flows in triangularly shaped area, and includes thick volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
(tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
), thick breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
layers, and intermediate to felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
igneous rocks that tend to intrude
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
the above layers and are typically located along caldera margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
lavas, alluvial redbeds, and porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
felsic lavas that are distributed across five different layers. The late Caldera-Fill Sequence contains rocks that are similar to those of the outflows of the other, older layers, and comprises two formations and two relatively minor intrusive lava flows. The volcanic flows are generally mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
. The stratigraphic subdivision is supported by geochemical and mineralogical analyses, which indicate that the basaltic rocks are mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
-derieved and have, unusually, relatively intraplate (or hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
) type chemical affinities. The multiple andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
flows were probably derived from basaltic magma by the crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
of the magma material. The relatively rare, more felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
flow units, are thought to have originated from high-end crystallization inside the magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
. The various stages of fractionation are continually interrupted by eruptions, and have allowed scientists to recognize seven stages of caldera development. The genesis of the caldera is related to a period of lithospheric thinning that followed the Acadian Orogeny
Acadian orogeny
The Taconic, Acadian and Alleghenian orogenies are the three tectonic phases responsible for the formation of the present Appalachian Mountains. The Acadian orogeny is a middle Paleozoic mountain building episode dating back 325-400 million years which should not be regarded as a single event but...
in the northern Appalachians.
The relative position of the Exocaldera and Intracaldera sequences is based on several observations:
- The upper part of the Rothea formation (Exocaldera) contains about 1% biotiteBiotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
. The only intracaldera rocks with this much biotite is volcanic strata within sedimentary breccia of and a tuff unit near the Scoullar Mountain formation (Intracaldera). - Andesitic strataAndesiteAndesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
occurs only in two flow units: the South Oromocto Andesite of the Exocaldera sequence, and the Scoullar Mountain formation of the Intracaldera sequence. - The exocaldera Carrow formation contains clasts from the intracaldera Seelys formation.
- The exocaldera Bailey Rock Rhyolite intrudesIntrusionAn intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
and overlies the Carrow formation, but is intruded by the intracaldera McDougall Brook Granite formation.
Exocaldera sequence
The exocaldera sequence consists of, in ascending stratigraphic order, the Hoyt Station Basalt, Rothea Formation, South Oromocto Andesite, Carrow Formation and Bailey Rock Rhyolite. The first and last flow units have the least extent, evidence of erosional activity after their formation but before the next deposited layer.The first and oldest layer is the Hoyt Station basalt formation, which is composed of at least two flow units. There are two types of rock associated with the basalt, conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
ranging in particle size from small pebbles to cobbles, and lithic
Lithic flake
In archaeology, a lithic flake is a "portion of rock removed from an objective piece by percussion or pressure," and may also be referred to as a chip or spall, or collectively as debitage. The objective piece, or the rock being reduced by the removal of flakes, is known as a core. Once the proper...
lapilli
Lapilli
Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.By definition lapilli range...
-tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
(well-worn, consolidated volcanic ash). It is only directly exposed at the surface in one area, as a tuff on an extended solidified multi-age lava flow that extends northeast from the northern section of the volcano, colored green in the diagram at right.
The next oldest layer is the Rothea formation. It can be divided into three major lava flow layers. The lower member consists mainly of unwelded (unjoined), but heavily compacted, pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
ous lapilli-tuff and crystal tuff strata. The middle member consists of rock ranging from nearly aphyric tuff at the base to crystal tuff near the top. The top section of the middle unit contains pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
. The top member consists of a fine-grained, typically reddish lava flow and another lithic tuff unit. It can be found exposed in a limited region, encompassing the flank of the Hoyt Station basalt, colored light yellow in the diagram at right; it also peeks out closer to the volcano twice.
The next layer, the South Oromocto Andesite formation, is composed of at least three flow units, with basalt flows being the most extensive. Only one exhibits a porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
texture. Calcite veins
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
and hematite bands
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
near the top of the layer indicate that this was a period of degassing in the flow. It forms a thin band around the Rothea formation.
The Carrow formation is a predominantly fine-grained redbed (consisting of typically red sedimentary rocks) unit that has a grade ranging from pebbles and cobble in conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
at the base to mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
laced with calcrete at the top. The flow is littered with abundant flows from the Seelys (in the intercaldera sequence) and Rothea (earlier) formation towards the southeast, but in the north metasedimentary rocks, which are sedimentary rocks exhibiting metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
, predominate. The lower part of the formation consists of unwelded and highly compacted lapilli tuff with abundant amounts of pumice fragments. A basalt and basalt-clad mudflow
Mudflow
A mudslide is the most rapid and fluid type of downhill mass wasting. It is a rapid movement of a large mass of mud formed from loose soil and water. Similar terms are mudflow, mud stream, debris flow A mudslide is the most rapid (up to 80 km/h, or 50 mph) and fluid type of downhill mass...
occurs near the top if the formation in some places. A dated locality from the top of the formation has been dated to the Late Devonian, around 350 million years ago. The exposed part of the formation forms a long band around the South Oromocto Andesite formation, and is colored brown on the map.
The last formation in the later, the Bailey Rock Rhyolite formation, is composed of porphyritic lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
, and is characterized by an absence of angular crystals and pumice derivatives. In places its rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
composition intrudes
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
older stratas. This layer is unique for it bridges the exocaldera and intracaldera sequences. A layer of saprolite
Saprolite
Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds...
(chemically weathered rock) separates the Bailey Rock Rhyolite formation from the overlying Caldera-Fill Sequence-era rocks. The exposed part of the flow overlies part of the Carrow formation, and is colored pink in the diagram above-right.
Intracaldera sequence
The Intercaldera Sequence is divided into, in ascending order, the Scoullar Mountain formation, Little Mount Pleasant formation, Seelys formation, and McDougall Brook Granite formation. In addition, there are felsic dykesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
and one mafic dyke that intrude the Scoullar Mountain and Little Mount Pleasant formations, respectively.
The first sequence in the order is the Scoullar Mountain formation. The layer is characterized by sedimentary breccia and interbedded andesitic lava. In addition felsic pyroclastic rocks are very common in places and one sandstone-conglomerate lava flow can be found. The sedimentary breccia is dominated by pebble to boulder size angular metasedimentary stratas, and a few crystal tuff layers that contain about 1% biotite, the only such high amount of a typically trace mineral outside of Rothea formation in the Extracaldera sequence. A pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
-ridden lapilli-tuff section near the believed top section additionally has a 1% composition of the mineral amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
. The exposed portion of the flows loosely flank the slopes of the volcano on the left and right, and are represented by bright orange on the map.
The next layer is the Little Mount Pleasant formation, which is composed of crystal tuff and banded rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
. The tuff is characterized by, recrystallized pumice with microscopic crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s and amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
. The crystal structure inside the pumice are larger than those outside, indicating that significant mechanical breakage during the eruption in which they were deposited. The exposed part of this layer is located around the southern base of the main caldera, and is colored ochre (light orange) on the diagram.
The Seelys formation, next in the order, consists of lithic tuff and pumice-littered lithic lapilli-tuffs, banded, pumiceous, crystal tuff, and densely welded crystal tuff. The basalt contains clasts of both the Scoullar Mountain andesite and Little Mount Pleasant formation. Quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
increase in size and abundance yoward the top. Biotite is virtually absent, but zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
is very common. The formation is colored the lightest orange on the map, and swathes across the main body of the caldera.
The McDougall Brook Granite formation consists mostly of porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...
granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, a borderline feldspar (sometimes quartz) porphyry
Porphyry (geology)
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...
, and minor amounts of fine-grained quartz monzonite
Quartz monzonite
Quartz monzonite is an intrusive igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende...
. The grain size of the porphyry and the size and abundance of feldspar increases inward. Amphibole with apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
is the main mineral phase in all three of the flows. Parts of the feldspar porphyry are hydrothermally altered. The deposit is extensively exposed, and is colored red on the map.
Late Caldera-fill sequence
The Late Caldera-Fill sequence includes the Mount Pleasant Porphyry, the Big Scott Mountain formation, and the Kleef formation. The ages of the rocks are not well-established, but they are most likely Late Devonian and Mississippian. In addition the Intracaldera sequence is cut into multiple intrusions of varying origins. Many of the intrusions are thought to have formed without eruption, when mineral deposits "leaked."The Mount Pleasant Porphyry formation occurs as dykes and small rock areas that have been associated with local breccia. The dyke structures seem to indicate multiple periods of intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
. Two types of breccias have been identified, and older and more common felsic series, and a younger neutral phase. The porphyries making up the rocks were emplaced at the pre-existing volcanic margin.
The Big Scott Mountain formation consists of rhyolite of variable composition, lapilli-tuff, and crystal tuff. Most of the rhyolites are characterized by large amounts of pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
. One rhyolite unit appears to overlie the McDougall Brook Granite formation. The tuff is littered by clasts from the Seelys and McDougall Brook Granite formations. The tuff also appears to be layered. It is colored dark orange on the map, and the exposed part partially rings the north of the volcano.
The final formation, the Kleef formation, includes redbed, basalt and pumice-ridden tuff. Pebble to cobble sized conglomerate is also seen complimenting the volcanic rock, and parts seem to come from older formations. The basalt is characterized by large crystals (up to 2 cm (0.78740157480315 in)). The tuff are characterized by their reddish-brown color and abundant fossil-pumice. The formation is very rare exposed and of interest to economic geologists
Economic geology
Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals, construction-grade stone, petroleum minerals, coal, and water. The term commonly refers to metallic mineral deposits...
, as it contains many of the possibly gold-bearing intrusive areas. It is colored ligh blue on the map.
Mining
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
was first discovered on the mountain in 1937. The focus of exploration over the years has shifted from tin-base metals to porphyry tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
-molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
-bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
deposits, and then to porphyry tin deposits, and now indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
, a rare element that is important to new technologies such as LCD screens and solar cells, computers and smart phones.
The Mount Pleasant mine is located 80 km (50 mi) south of Fredericton, the provincial capital. The mining company Adex Mining Inc. holds 102 prospective claims covering approximately 1,600 hectares (4,000 acres) and 405 hectares (1,000 acres) of surface rights at Mount Pleasant, dominating the local extraction rights. It is situate close to infrastructure and potential employees. With a population of 50,000, Fredericton is important to the mine, as is Saint John
Saint John, New Brunswick
City of Saint John , or commonly Saint John, is the largest city in the province of New Brunswick, and the first incorporated city in Canada. The city is situated along the north shore of the Bay of Fundy at the mouth of the Saint John River. In 2006 the city proper had a population of 74,043...
, with a population of 70,000, which lies about 80 km (50 mi) o the southeast. Mount Pleasant is also only 65 km (40 mi) from the Canada – United States border. When the mine was last producing - 1983 to 1985 - tungsten ore was regularly transported through Saint John by means of a provincial highway for shipment to Europe. The mine site is accessible via all-weather roads from Fredericton, Saint John and St. George. Electricity is provided by the New Brunswick transmission grid and water from a pump house located on the nearby Piskahegan River.
On September 25, 2005, the Society of Economic Geologists
Society of Economic Geologists
The Society of Economic Geologists originated from a 1919 gathering of a group of Geological Society of America members who were especially interested in economic geology. The Society was established on December 28, 1920, during a constituting meeting of 60 distinguished professionals...
hosted a small field trip to the caldera and the nearby Mount Pleasant Mine and Clarence stream deposit. The trip was divided into three stages: the Mount Pleasant Caldera, the Mount Pleasant Mine, and the Clarence Stream deposits. The field crew described two trenches that seemed to outline the relationship between the volcanic rock and the possibly gold-rich intrusive areas.
On June 25, 2008, Geodex Minerals Ltd., a gold venture firm
Venture capital
Venture capital is financial capital provided to early-stage, high-potential, high risk, growth startup companies. The venture capital fund makes money by owning equity in the companies it invests in, which usually have a novel technology or business model in high technology industries, such as...
, announced the results of a 2007 - early 2008 diamond drilling program on the west side of Mount Pleasant. Along with the development of the Sisson Brook tungsten-molybdenum-copper project north of Fredericton, the area surrounding the former Mount Pleasant mine has been a major focus of the company. Research is ongoing, and focuses on the two most likely candidates: the McDougall Brook Granitic suite (MBG) and the Mount Pleasant Granitic suite (MPG).
In June 2011, the owners moved their corporate offices from Toronto to Fredericton with the intent of refocusing efforts onto getting the mine running again by 2012.
See also
- Volcanism of Eastern CanadaVolcanism of Eastern CanadaVolcanism of Eastern Canada has led to the formation of hundreds of volcanic areas and extensive lava formations, indicating volcanism played a major role in shaping its surface. The region's different volcano and lava types originate from different tectonic settings and types of volcanic...
- List of volcanoes in Canada