Mount Pleasant Caldera Field Trip

Gently north-dipping redbeds in the road cut are typical of the lower part of the Shin Formation. Poorly sorted, coarse-grained sandstone beds (up to 0.5 m in thickness) have sharp basal contacts and are normally graded from pebbly granule-conglomerate to fine-grained sandstone. Pebbles are subangular to well rounded and mostly non-volcanic, i.e. not derived from the Piskahegan Group. The interbedded reddish brown mudstone locally contains calcareous nodules (calcrete), which are so abundant at one locality downstream from the bridge over Shin Creek. The Shin Formation, part of the Mabou Group, is post middle Viséan (middle Mississippian) and unconformably overlies the Piskahegan Group.

The reddish brown subcrop on the right side of the road is representative of the upper member of the Kleef Formation, part of the Late Caldera-Fill Sequence (Piskahegan Group). Pseudomorphed pumice fragments, which are darker in colour than the matrix, are up to 3 cm long and make up 10-15% of the individual blocks. Many preserve remnant tubular structure that is manifested as lineation on the flattened surfaces of the fragments. Some calcitized fragments are unflattened and consequently, better preserve this original structure. These calcitized fragments demonstrate that the eutaxitic foliation in the rock resulted from burial compaction rather than welding. This tuff also contains 5-10% lithic fragments, mostly felsic volcanic rocks, which rarely exceed 1 cm in size. Quartz and feldspar phenocrysts (< 1.0 mm) constitute less than 5% of the rock.

Flat outcrops and subcrops of olive gray to greenish gray basalt occur along side the trail. This basalt constitutes the middle member of the Kleef Formation and is characterized by plagioclase phenocrysts up to 2 cm long and locally glomerocrysts up to 4 cm across. This basalt also contains scattered dark reddish brown amygdules, up to 1 cm long. These amygdules are filled or rimmed by reddish brown hematite and are commonly cored by chalcedony. The Kleef Basalt is underlain by pebble to cobble conglomerate (lower member) that can be seen approximately 500 m to the northeast, where the power line crosses Shin Creek. This conglomerate contains abundant clasts of Bailey Rock Rhyolite, part of the Exocaldera Sequence. About 10 km to the west, Kleef conglomerate contains clasts of Seelys crystal tuff, part of the Intracaldera Sequence. Thus the age relationship of these two sequences with the Late Caldera-Fill Sequence is established.

Grayish red, feldspar phyric, flow-banded rhyolite on the east side of the road marks the base of the Late Caldera-Fill Sequence. The contact with the underlying McDougall Brook Granite (Intracaldera Sequence) at the south end of the outcrop is concealed but the flow banding near the base dips gently northward. The rhyolite contains abundant spherulites and examples of flow folds. Feldspar phenocrysts, mostly less than 1 mm, constitute 10-15% of the rock. Brecciated silica veins cut the outcrop in several places. Subcrop of porphyritic microgranite can be seen about 50 m south of the rhyolite.
      

The road cut shows the intrusive contact of the McDougall Brook Granite with crystal tuff typical of the upper part of the Seelys Formation (Intracaldera Sequence). Grayish red, crystal tuff in the southern part of the outcrop contains abundant (25-35%) quartz and feldspar phenocrysts/phenoclasts that are mostly < 2 mm in size. Locally, this tuff contains greenish gray metasedimentary clasts (generally < 1.5 cm); some clasts have reaction rims but others do not. The microgranite in the northern part of the outcrop has a mottled appearance and contains abundant (15-35%) feldspar phenocrysts (up to 10 mm but mostly < 3 mm) and lesser amounts (5-15%) of ferromagnesian minerals (mostly < 1 mm) in a microcrystalline groundmass. The grain size of the groundmass decreases from phaneritic to aphanitic in proximity to the contact with the tuff. Minor quartz phenocrysts occur near the contact.       

The outcrops on the hillside constitute a reference section for the lower part of the Seelys Formation. The rocks exhibit very little lithologic variation over the 29m of section from apparent base to apparent top of the section. They comprise grayish red to pale reddish brown crystal tuffs, containing highly flattened pumice pseudomorphs. Pink feldspar is slightly more abundant than clear quartz, and combined, these crystals (0.5-3.5 mm) constitute 15-25% of the rock. The largest crystals are inside the pumice pseudomorphs that make up 10-20% of the rock. Poorly developed columnar jointing, which is perpendicular to a well-developed, northerly dipping eutaxitic foliation, is visible on the vertical face of each plateau. At the top of the hill, flattened and stretched pumice pseudomorphs can be seen on the tops of the columnar joints. A supcrop of Little Mount Pleasant Formation occurs less than 100 m southeast of the base of the hill. The northerly dipping (about 25°) eutaxitic foliation and higher topographic elevation indicate that the Seelys Formation overlies the Little Mount Pleasant Formation.

At this location, flat lying outcrops of crystal tuff (Little Mount Pleasant Formation, Intracaldera Sequence) contain about twice as much feldspar as quartz; collectively, these crystals (0.5-3.0 mm) comprise 30-40% of the rock. Where unaltered, this tuff is generally grayish red in colour but here near the contact with the McDougall Brook Granite, the rock is greenish gray because of effects of hydrothermal alteration. Outcrops of pale red, feldspar porphyry that constitute a mappable border phase of the McDougall Brook Granite occur 150m to the northeast. The porphyry contains two populations of subhedral to euhedral feldspar phenocrysts (1-12 mm) with plagioclase generally slightly more abundant than K-feldspar. Combined, these phenocrysts make up 20-35% of the rock. The porphyry also contains minor (< 2%) clear quartz phenocrysts (1-4 mm) Note the aphanitic nature of the groundmass.

Heterolithic silicified breccia exposed on the north side of the road contain recognizable fragments that include Little Mount Pleasant Formation and feldspar porphyry. Some fragments exhibit evidence of rounding but most are angular. The breccia is part of a small enclave within the McDougall Brook Granite and is interpreted as a hydrothermal breccia, i.e. nonvolcanic but part of the Intravolcanic Sequence. The McDougall Brook Granite exposed about 100 m farther east on the north side of the road is a porphyritic microgranite. At this point it is reddish gray in colour but 150 m farther along, on the south side of the road, it is greenish gray and contains inclusions of older granitoid rocks. Farther along (150 m), on the north side of the road, the rocks are grayish red and transitional between microgranite and feldspar porphyry.
      

Quartz-feldspar crystal tuff with moderately well-developed eutaxitic foliation is exposed in the streambed of Hatch Brook on the east side of the road. The foliation is outlined by dark green fiamme or extremely flattened pumice pseudomorphs; this is one of few outcrops of this formation where eutaxitic foliation can be seen.
     

10A – Quartz-Feldspar Porphyry (Little Mount Pleasant Formation) is partly chloritized but can be recognized by the quartz and feldspar phenocrysts (typically 2 to 3 mm in size). Fractures and quartz veinlets in the quartz-feldspar porphyry locally contain sulfide minerals. Walk 15 m back toward the road. 10B – Feldspar Porphyry (McDougall Brook Granite) is distinguished from quartz-feldspar porphyry in having feldspar phenocrysts up to 1 cm in size and generally less than 5% quartz phenocrysts. The contact between the feldspar porphyry and the quartz-feldspar porphyry is marked by the local change in slope. 10C – On the south side of the road, breccia and Mount Pleasant Porphyry, fine-grained tin minerals (cassiterite and stannite) and base metal sulfides occur locally in fractured, brecciated and intensely silicified feldspar porphyry intruded by dykes of banded porphyry (Granite II).
    

11A – Fire Tower Breccia and Mount Pleasant Porphyry: early silicified breccia is intruded by banded porphyry dykes. The breccia consists of bluish gray to white, greisen-altered fragments that are angular to subrounded and generally are 1-10 cm in size. The fragment protoliths are mostly unknown because the original textures have been obscured by alteration, but locally quartz-feldspar porphyry can be recognized. The dyke rocks are greenish black, sparsely porphyritic (quartz + feldspar < 5% and generally < 1 mm), and have a cryptocrystalline groundmass. Near the contacts, the dykes contain xenoliths of country rock and exhibit well-developed flow banding. Crosscutting relationships between dykes indicate multiple stages of intrusion. 11B – Mount Pleasant Porphyry: a plug of greenish gray granite porphyry (Granite II) underlies most of the open area. Phenocrysts are more abundant in the microgranite than in the banded dykes but they generally constitute < 10% of the rock with subequal amounts of quartz and feldspar (up to 2.5 mm). The groundmass is microcrystalline rather than cryptocrystalline. Yellowish gray silicified zones that are localized along, and dissipate outward from, fractures cut the plug rocks. A few pebble dykes, up to 1 m wide but mostly < 30 cm, also cut the plug rocks. Most of the clasts in the pebble dykes are microgranite but bluish gray, greisenized feldspar porphyry clasts are also present. 11C – Mineralized Fire Tower Breccia: fluorite, molybdenite and wolframite occur in fractures and quartz veinlets in silicified felsic breccia exposed in the large trench east of the drill road. A dyke of banded porphyry that locally contains comb quartz layers intrudes this breccia. A pit in this breccia a few meters west of the drill road contains an indium-bearing sulfide-rich vein.

Mount Pleasant Porphyry is exposed in road on east side of the adit; fractured and altered quartz-feldspar porphyry occurs on the west side of the adit. The adit was developed to explore a small tin-bearing sulfide replacement deposit (the No. 7 lode); samples of massive sulfides including sphalerite, galena, chalcopyrite, cassiterite and stannite may be found on the dump on the north side of the road. Roquesite (CuInS2) has been identified in polished sections of samples from this area.
     

Cassiterite-bearing samples from the Endozone deposit may be found in the parking lot south of the main gate at the Mount Pleasant Mine. Metasedimentary rocks of the Silurian Sand Brook Formation, which formed the walls of the Mount Pleasant caldera, are exposed in several places on the mine site.