Mount Pleasant Caldera

Simplified geology map of the Mount Pleasant Caldera, southwestern New Brunswick. Exocaldera, intracaldera and late-caldera fill sequences are recognized; older and younger rocks are respectively coloured pale blue and grey. Click the blue Group and Formation names in the map legend for detailed descriptions. Numbered blue dots are links to Field Trip Stop descriptions.
      

Representative stratigraphic columns of the Mount Pleasant Caldera. Columns 7 to 9 are representative of the exocaldera sequence. Columns 2 to 6 are representative of the intracaldera sequence. Columns 1, 4, 5 & 7 are representative of the late caldera-fill sequence. Click the blue names in the legend to view photographs of these units.
     

The Exocaldera Sequence, in ascending stratigraphic order, consists of the Hoyt Station Basalt, Rothea Formation, South Oromocto Andesite, Carrow Formation and Bailey Rock Rhyolite. The first and last units have the least areal extent.

The Hoyt Station Basalt comprises at least two flow units. Minor pebble- to cobble-conglomerate and lithic lapilli-tuff is associated with this basalt.

The Rothea Formation includes a lower member consisting of unwelded, but compacted, pumiceous lapilli-tuff and crystal tuff units, a middle member which grades from nearly aphyric tuff at the base to crystal tuff at the top (pyroxene(?) pseudomorphs typify this unit) and an upper member consisting of a lower fine-grained redbed unit and an upper lithic tuff unit.

The South Oromocto Andesite is composed of at least three flow units with the basal flow being the most areally extensive and the only one exhibiting porphyritic textures. Calcite veins and hematite bands near the top reflect degassing of the flow interior.

The Carrow Formation is predominantly a fining-upward redbed unit that grades from pebble- to cobble-conglomerate at the base to mudstone with intercalated calcrete at the top. Toward the southwest, the conglomerate contains abundant clasts of the Seelys (intracaldera sequence) and Rothea formations, but to the northeast, metasedimentary clasts predominate. In the lower part of this formation, an unwelded, but highly compacted, pumiceous lapilli-tuff contains abundant pumice fragments. Locally, a basalt and basalt-clast mudflow occurs near the top of the formation. A spore locality from the upper part of the Carrow Formation has yielded a precise Late Famennian age (McGregor and McCutcheon, 1988).

The Bailey Rock Rhyolite is a porphyritic lava and, like other lavas, is characterized by an absence of angular crystal fragments and pumice pseudomorphs. In places this rhyolite is intrusive into older rock units. It is unique because it crosses the boundary between the Exocaldera and Intracaldera sequences. A saprolite separates the Bailey Rock Rhyolite from the overlying Late Caldera-Fill Sequence.

The Intracaldera Sequence comprises, in ascending order, the Scoullar Mountain Formation, Little Mount Pleasant Formation, Seelys Formation and McDougall Brook Granite. In addition, there are felsic dykes and one mafic dyke that intrude the Scoullar Mountain and Little Mount Pleasant formations, respectively.

The Scoullar Mountain Formation is characterized by sedimentary breccia and interbedded andesitic lavas; felsic pyroclastic rocks are voluminous in places and one sandstone-conglomerate unit is present. The sedimentary breccia is dominated by pebble- to boulder-size angular metasedimentary clasts, and a few undeformed crystal tuff clasts that contain about 1% altered biotite. A pumiceous lapilli-tuff near the apparent top has about 1% amphibole and accessory apatite.

The Little Mount Pleasant Formation is composed of crystal tuff and flow-banded rhyolite. The crystal tuff is characterized by unflattened to weakly flattened, microcrystalline, recrystallized pumice fragments and chloritized amphibole with associated apatite. Phenocrysts inside these recrystallized pumice fragments are an order of magnitude larger than those outside, indicating that significant mechanical breakage of phenocrysts occurred during eruption.

The Seelys Formation, consists of lithic tuffs and pumice-bearing lithic lapilli-tuffs; banded, pumiceous, crystal tuff; and densely welded crystal tuff. The basal part contains clasts of Scoullar Mountain andesite and Little Mount Pleasant Formation. Quartz and feldspar phenocrysts increase in size and abundance from base to top in the upper part of the sequence. Platy biotite is virtually absent but metamict zircon is a common accessory in all units.

The McDougall Brook Granite consists mostly of porphyritic monzogranite, a border phase feldspar (± quartz) porphyry, and minor equigranular to subporphyritic, fine-grained quartz monzonite. The groundmass grain-size of the porphyry, the size and abundance of feldspar phenocrysts increase inward, away from the contact with country rocks. Chloritized amphibole with associated apatite is the main ferromagnesian mineral phase in all three units. Parts of the feldspar porphyry are hydrothermally altered, and a small hydrothermal breccia or diatreme cuts the microgranite.

The relative stratigraphic position of units in the Exocaldera and Intracaldera sequences is based on the following observations:

• The upper part of the Rothea Formation consistently contains about 1% platy biotite pseudomorphs. The only intracaldera rocks with this much biotite are volcanic clasts within sedimentary breccia of, and a tuff unit near the apparent base of, the Scoullar Mountain Formation.
• Andesitic rocks occur only in two units: the South Oromocto Andesite of the Exocaldera Sequence and the Scoullar Mountain Formation of the Intracaldera Sequence.
• The Carrow Formation contains clasts from the Seelys Formation.
• The Bailey Rock Rhyolite, which occurs in both sequences, intrudes and/or overlies the Carrow Formation but is intruded by, or grades into, the McDougall Brook Granite.

The Late Caldera-Fill Sequence includes the Mount Pleasant Porphyry and its associated breccias, and felsic dykes, the Big Scott Mountain Formation and the Kleef Formation. The ages of the Late Caldera-Fill rocks are not firmly established; they are most likely Late Devonian but could range into the Mississippian.

The Mount Pleasant Porphyry is restricted to the Mount Pleasant area where it occurs as dykes and small, plug-like bodies associated with magmatic-hydrothermal breccias, as defined by Sillitoe (1985). The dykes commonly exhibit flow-banding, and crosscutting relationships between the dykes indicate multiple stages of intrusion. Two types of hydrothermal breccia are present: an older and more voluminous felsic phase, and a younger, chloritic phase (Kooiman and others, 1986). The porphyry, which grades into granitic rocks at depth, and associated breccias were emplaced at the pre-existing caldera margin.

The Big Scott Mountain Formation consists of, porphyritic to nearly aphyric rhyolite, lithic to lithic lapilli-tuff and crystal tuff. Most of the rhyolites are characterized by pyroxene(?) pseudomorphs. One of the rhyolite units appears to disconformably overlie the McDougall Brook Granite. The lithic tuffs contain clasts that were derived from the Seelys Formation, McDougall Brook Granite, and aphyric rhyolite of uncertain correlation. Primary layering is discernible in the crystal tuff and is defined by slight differences in crystal size and abundance.

The Kleef Formation includes redbeds, porphyritic to glomeroporphyritic basalt and pumiceous, lithic tuff to lithic lapilli-tuff. Pebble- to cobble-conglomerate contains clasts of the Scoullar Mountain and Seelys formations, plus the Bailey Rock Rhyolite and Big Scott Mountain Formation. The basalt is characterized by large plagioclase phenocrysts (up to 2 cm) and, near the top of the unit, some plagioclase glomerocrysts (up to several centimetres). The lithic tuffs are characterized by their reddish brown colour and abundant fossil-pumice.

The Mount Pleasant deposits are associated with hydrothermal breccias and intrusive rocks that cut the Intracaldera Sequence. The various granite phases and the fluids that produced the mineral deposits were probably derived by in situ (i.e. no eruption) cooling of peraluminous anorogenic magma by convective fractionation (diagram), a term coined by Rice. (1981). Mass balance calculations show that under the above conditions, small volumes (10-20 km3) of magma with an initial composition like the Little Mount Pleasant Formation could yield quantities of metal and fluid capable of producing the Mount Pleasant deposits.

Host Rocks
The intracaldera host rocks have been highly brecciated, altered and mineralized in two areas at Mount Pleasant designated as the North Zone and the Fire Tower Zone (diagram). In both areas, breccias and associated intrusive rocks form irregular, roughly vertical, pipe-like complexes that were centers of subvolcanic intrusive and related hydrothermal activity. The breccias range from matrix-supported with rounded fragments to clast-supported with mainly angular fragments. Both fragments and matrix material have been altered extensively and in many places the fragment protoliths are difficult to identify.

The contact relationships among and distribution of various granitic phases at Mount Pleasant (diagram) have been determined from drill cores and exposures in underground workings. These units, from oldest to youngest, have been designated Granite I, Granite II and Granite III, referring to fine-grained granite, granite porphyry and porphyritic granite respectively. All are considered to be part of the Mount Pleasant Porphyry.

Granites I, II and III are considered to represent successive cooling stages of one magma body. Granite I occurs as irregular bodies closely associated with the hydrothermal breccias. Its contacts with the breccias are commonly gradational and fragments of Granite I are abundant locally within the breccias. Granite I is typically fine-grained and equigranular in relatively unaltered specimens. However, in most areas textural features of Granite I have been obscured by pervasive chloritic and/or silicic alteration.

Granite II gradationally underlies Granite I, although in places dyke-like bodies of Granite II have intruded Granite I and the overlying breccias. Banded porphyry dykes that crop out at surface are probably derived from Granite II. Granite II varies from aplitic to porphyritic in texture. Parts of Granite II contain abundant miarolitic cavities and comb quartz layers. The comb quartz layers consist of parallel to subparallel layers in which quartz crystals are oriented approximately perpendicular to the planes of layering. They are one of a family of unidirectional solidification textures (USTs) that are associated with fluid saturated and/or undercooled magmas.

Granite III forms a large body that gradationally underlies Granite II and locally intrudes both Granites I and II. The contacts are commonly sharp and in many places are marked by thin (0.5 to 2 cm wide) layers of USTs, mainly K-feldspar, in Granite III. Granite III varies from fine- to medium-grained and equigranular porphyritic and pegmatitic. Miarolitic cavities filled with very fine-grained sericite are locally abundant.

The absolute age of the granitic rocks at Mount Pleasant, collectively referred to as Mount Pleasant Porphyry, is uncertain. It has to be younger than the Exocaldera Sequence of the Piskahegan Group, which is constrained by a U-Pb radiometric age of 363.4 ± 1.8 Ma on the Bailey Rock Rhyolite. K-Ar and Rb-Sr studies indicate a Late Mississippian age of 340 to 330 Ma. However, a K-Ar date of 361 ± 9 Ma from biotite hornfels in sedimentary breccia that is underlain by Granite III appears to confirm a Late Devonian-Early Mississippian age.

Mineral Deposits
The mineralization at Mount Pleasant is granite-related. Tungsten-molybdenum deposits appear to be related to Granite I; tin deposits are associated mainly with Granite II and associated with porphyry dykes. Only a few isolated tin zones have been found within Granite III.

Tungsten-molybdenum deposits: The resource in the Fire Tower Zone prior to mining totalled 22.5 million tonnes grading 0.21% W, 0.10% Mo and 0.08% Bi (Parish and Tully, 1978); approximately 11 million tonnes of similar grade material are present in the North Zone. Included in this resource was a higher-grade deposit in the Fire Tower Zone containing 9.4 million tonnes grading 0.39% WO3 and 0.20% MoS2 (Kooiman and others, 1986). During the two years of mining this deposit from 1983 to 1985, the Mount Pleasant Mine produced more than 2000 tonnes of concentrate grading 70% WO3 from about one million tonnes of ore.

The tungsten-molybdenum deposits are hosted mainly by breccia, Granite I and, to a lesser extent, by associated country rocks. The deposits consist of mineralized fractures, quartz veinlets and disseminations in breccia matrix. Wolframite and molybdenite are the principal ore minerals; minor amounts of bismuth and bismuthinite are also present. Quartz, topaz, fluorite, arsenopyrite and loellingite are the principal gangue minerals.

Alteration associated with the tungsten-molybdenum deposits includes several different types. Intense and pervasive silicic or greisen-type alteration occurs within and above the high-grade tungsten-molybdenum zones. This type of alteration is characterized by the complete or nearly complete replacement of host rocks by quartz, topaz and fluorite. This alteration grades outward to a less intense silicic alteration that is limited mainly to narrow selvages on mineralized fractures and quartz veinlets. Quartz, biotite, chlorite and minor amounts of topaz are the principal minerals of this alteration stage that extends laterally up to 100 m beyond the high-grade tungsten- molybdenum zones. Propylitic alteration consisting of chlorite and sericite surrounds the silicic alteration and extends for more than 1000 m before grading into relatively unaltered rock.

Tin-Indium deposits: Tin-base metal deposits occur as sulphide-rich polymetallic veins and replacement bodies, which are superimposed on the tungsten-molybdenum mineralization. Sphalerite, chalcopyrite, arsenopyrite and cassiterite are the dominant ore minerals and are associated with chlorite, fluorite and a complex assemblage of sulfides and sulpharsenides, including loellingite, galena, pyrite, marcasite, molybdenite, tennantite, bornite, bismuthinite, wittichenite and roquesite.

Most of the potentially economic tin deposits occur in the North Zone at depth of 200 to 400 m below surface. They include the Deep Tin Zone, Contact Crest, Contact Flank and Endogranitic Zone deposits (diagram).The Deep Tin Zone is a relatively large, irregular deposit that consists of fracture-controlled and disseminated cassiterite in silicified and chloritized breccia and Granite I. Other minerals associated with cassiterite include arsenopyrite, sphalerite, chalcopyrite and galena. The Contact Crest and Contact Flank deposits occur mainly in breccia or other associated host rocks at the upper contact or along the sides of Granite II. The Endogranitic Zone deposit, on the other hand, occurs mainly within Granite II. In these deposits, cassiterite occurs as finely disseminated grains and as fine- to medium-sized grains in veins or veinlets and along fractures. Associated minerals include arsenopyrite, sphalerite, chalcopyrite, pyrite and pyrrhotite. Chlorite, fluorite, quartz, topaz and sericite are the main alteration minerals. Crosscutting relationships indicate that as many as 6 stages of alteration and mineralization may be present. The total inferred and indicated resources in these North Zone deposits are 4.8 million tonnes grading 0.82% Sn and 129 g/t In (Sinclair and others, 2006, their Table 1).

Some of the tin-bearing polymetallic deposits in the Fire Tower Zone contain significant amounts of indium, with grades ranging from 50 to 300 g/t In. The indium occurs mainly as solid solution in sphalerite and, to a lesser extent, in chalcopyrite and stannite. The total inferred and indicates resources in the Fire Tower Zone are 0.28 million tonnes grading 0.30% Sn and 207 g/t In (Sinclair and others, 2006, their Table 1).