Background

Description

Roman Formation 1 (nArmn1): Felsic Volcaniclastic Rocks

Roman Formation 2 (nArmn2): Intermediate Tuff and Andesite

Intermediate tuff and andesite outcrops are very heterogeneous from a compositional perspective. Three main facies that cannot be individualized on the map have been described. In the Bohier Island area, intermediate tuffs are affected by at least two shistosity phases.

Amphibole and Biotite Cluster Intermediate Tuff

The amphibole and biotite cluster intermediate tuff consists of a pale beige-grey rock in altered surface and medium grey in fresh exposure. This is the most abundant facies of unit nArmn2. It is distinguished by the presence of amphibole and biotite clusters oriented to the main lineation. These clusters are millimetre in size and reach 1 cm. When present, bedding is marked by a change in size. The tuff matrix is composed of amphibole, saussuritized plagioclase, quartz with varying amounts of biotite, chlorite and epidote. Traces of zircons, sphene, rutile and opaque minerals are present. Two types of amphiboles (actinolite and hornblende) and traces to 3% garnet porprhyroblasts are present. Hornblende is coarse grained and poeciloblastic. Actinolite forms elongated rods. Plagioclase phencrystals make up between 0 and 10% of the rock. A few lapillis and blocks of garnet amphibolite can be seen in the tuff. It is generally non-magnetic and frequently altered in chlorite, epidote, hematite and silica.

Porphyritic Andesite

Porphyritic andesite is an intermediate rock that contains 3-15 % plagioclase phenocrystals. It has a pale green, sometimes greyish, altered surface and a medium grey fresh exposure. It is non-magnetic and is composed mostly of amphibole (30-60 % actinolite or hornblende) with sericitized quartz and plagioclase and varying amounts of biotite, epidote, garnet and chlorite. Feldspath, quartz and carbonate veins cut the unit.

Intermediate Lapilli Tuff

Intermediate lapilli tuff outcrops were observed. The matrix of these tuffs is composed of amphibole, plagioclase, quartz with varying amounts of chloritized biotite, chlorite, epidote, muscovite and garnet. The fragments are of felsic or mafic composition and represent between 5 and 40% of the rock.

Unit nArmn2 has a predominantly intermediate composition (andesitic and dacitic) and calc-alkaline affinity. It is characterized by 53-68 % SiO₂, 0.4-1.4 % TiO₂, 17-39 ppm Y, 100-182 ppm Zr, 1.7-3.8 ppm Yb and a Zr/Y ratio of 11.4-59. Unit nArmn2 rocks have fractionated rare earth profiles, characterized by light rare earth enrichment relative to heavy rare earths (mean [La/Yb]_CN = 5; mean [Gd/Yb]_CN = 1.5). In addition, only a few diagrams have very low negative Eu anomaly (Eu/Eu* = 0.5 to 1).

Roman Formation 3 (nArmn3): Gabbro

Gabbroic intrusions are difficult to differentiate from basalt outcrops (nArmn4). Gabbroic sills are cogenetic and have geochemistry similar to mafic effusive rocks. The gabbros are dark green in altered patina and slightly lighter in fresh exposure. They are medium grained and weakly foliated. They are mesocratic to melanocratic and have a subophitic to ophitic structure. They contain 60-85 % amphiboles (hornblende or actinolite), 10-30 % plagioclase with less quartz, epidote, clinozoisite, calcite, rutile, sphene and opaque minerals. Plagioclase is weakly to moderately altered in sericite or epidote. Some samples contain amphibolitized pyroxene pseudomorphs (20 to 50 mm). 

Roman Formation 4 (nArmn4): Amphibolitized Basalt

Mafic lava are blackish in altered surface and forest green in fresh exposure. These rocks have a wide variety of facies. The two main facies that have been documented are the pillow facies and the massive flows. Massive flow basalt is highly amphibolitized. Because of metamorphic overprinting, the more coarse-grained basalt can easily be confused with a gabbroic sill contemporary with the construction of the volcanic band (Hocq 1985; Roy 1988; Talla Takam et al., in preparation). In the Léran Lake area, an outcrop of coarse-grained (hornblende 0.5 cm) pillow basalt was observed. Pillows are well individualized and range from 0.2 to 1 m. Edges are millimetric to centimetric. Pillows are often flattened in the direction of regional foliation. Unlike basalts of the Érasme and Clément Formations, it is difficult to define precisely stratigraphic polarity in the basalts of the Roman Formation since the peduncles and quartz chambers are deformed and rarely preserved. In some places, mega-pillows are found up to 2 m in diameter and decametric micro-pillows. Porphyritic basalts with plagioclase phencrystals were observed in the Tiers-État Lake area (Roy 1988). Plagioclase phenocrystals, which account for 10 to 15% of the rock, are 2 to 3 cm in diameter (Roy 1988), and are altered in sericite and epidote. The paragenesis of amphibolitized basalt is relatively constant. They contain 50-70 % green hornblende (locally actinolite), damouritized plagioclase, epidote, sericite and lower amounts of chlorite, calcite, quartz, biotite, garnet, sphene, zircon and opaque minerals. The most magnetic basalts contain more actinolite than hornblende. Basalts are rarely magnetic and of variable grain size, ranging from very fine to medium. They have subophitic to ophitic structures. Biotite is locally present and partially chloritized. Altered basalts contain more biotite and calcite. Some basaltic facies contain elongated quartz plagioclase amygdules. Some pyroxene phenocrystals (10 to 20 mm) are present in the  massive flow facies and appear to be pseudomorphs of amphibolitized pyroxene grains.

Unit nArmn4 consists of subalkaline basalt, amphibolitized andesitic basalt, magnetic basalt and amphibolite. Mafic volcanics are subalkaline and of predominantly tholeitc to locally transitional affinity. Mafic rocks are characterized by relatively flat rare earth profiles, with contents 5 and 40 times higher than chondrite. They show moderate depletion of light rare earths ([La/sm]_CN = 0.6 to 1.9; [La/Yb]_CN = 0.6 to 2.6) and very small positive or negative anomalies in europium ([Eu/Eu*] = 0.7 to 1.2). These rocks are low-depleted in Th and U, highly depleted in Nb, Ta and Ti, and have small plateaus in Zr-Hf. As defined by tectonic environmental diagrams, mafic rocks of the eastern branch of the UEGB are oceanic ridge basalts that have undergone shallow melting. These are N-MORB-type rocks that appear to have been affected by crustal contamination (Pearce 2008).

Roman Formation 5 (nArmn5): Iron Formation and Garnet Amphibolite

Iron formations represent only a very small proportion within the Roman Formation. They are in thin, folded bands, discontinuous and boudinaged. Their extension is interpreted from the continuities on aeromagnetic charts. Iron formation outcrops are banded. These include partially silicified and partially oxidized mixed iron formations or silicate facies iron formations. They are usually composed of alternating millimetric to centimetric bands of greyish chert and greenish yellow to orange-brown bands containing amphiboles (grunerite, hornblende), garnet porphyroblasts and a few magnetite veinlets. The quartzite recrystallized chert bands are weakly boudinaged. Iron formations contain 2-10 % sulphide (pyrite and pyrrhotite).

Amphibolites with garnet porphyroblasts of tholeitic affinity highly enriched in iron (MgO between 4 and 5 wt. % and Fe2O3_T between 21 and 29%) are associated with iron formation bands. Amphibolites are dark greenish in altered surface and medium green in fresh exposure. They are medium grained and have nematoblastic structures. Their paragenesis consists mostly of green amphibole, moderately to highly sericitized plagioclase, ilmenite and smaller amounts of biotite, garnet, epidote and quartz. Amphibolites contain 10-30 % garnet porphyroblasts. Garnet is hypotomorphic to idiomorphic, poecilitic and is 2 to 10 mm in diameter. Edges of the grains are irregular and their growth is overprinted on the main foliation. Garnet is late-tectonic to post-tectonic.

Roman Formation 6 (nArmn6): Mudstone and Graphitic Chert

Some mudstone and graphitic chert outcrops were mapped in the northern portion of the UEGB north branch, specifically north and east of Harbour Lake (Talla Takam et al., in preparation). These outcrops are a minority within the Roman Formation. The thickness of this unit varies from 5 cm to a maximum of 5 m. The mudstones and graphitic cherts have a rusted blackish altered patina and are greyish to blackish in fresh exposure. These horizons are very fine grained. They are finaly laminated (stratified) and affected by the main deformation. Tight folds are common. They are made up of quartz, graphite, magnetite and 2-10 % sulphides, mainly pyrrhotite and pyrite with minor quantities of chalcopyrite.

Roman Formation 7 (nArmn7): Paraschist, Paragneiss and Wacke

In the upper part of the eastern branch of the Upper Eastmain Greenstone Belt, horizons of paraschist and wacke paragneiss sedimentary rock were emplaced between volcanic flows. These horizons were not observed in the latest mapping campaign conducted by the Ministère (Beauchamp et al., 2018). However, they were intercepted by diamond and reverse-flow drilling and described in exploration reports.

The majority of the paraschist is composed of quartz and plagioclase, which account for more than 80% of the rock, as well as biotite, garnet and hornblende (Chapdelaine 1995). A millimetric to centimetric banding consisting of alternating bands of light grey (70% of the rock), red brown (20% of the rock) and dark green (10% of the rock) is visible. The rock is fine grained and not magnetic. At drilling level 40349-95-06, the protolith is more difficult to identify. The rock is more schistose, more epidotized, and plagioclase has been replaced by sericite. The rock also contains up to 10% siliceous and stretched centimetric fragments.

Wacke paragneiss horizons were also described. They are grey, brown grey, green-grey and black grey, fine to medium grained, and rarely magnetic. Rarely sheared, the rock shows millimetric to centimetric bands that are distinguished by variations in grey shades. The main minerals are quartz, plagioclase, biotite, garnet and chlorite (Chapdelaine 1995). Hornblend is also present (Beland 2015). Ferromagnesian minerals generally account for 2-5% of the rock, but some horizons can be much richer (up to 50%). Almandine garnet shows shades of pink to orange. Primary bedding was observed in drill core LHRC-15-047 (Beland 2015). Finally, the rock contains minor amounts of calcite.

Roman Formation 8 (nArmn8): Ultramafic Sill

Ultramafic sill horizons are interbedded in the volcanic sequence of the eastern branch of the UEGB. It is possible that the Roman Formation contains horizons of ultramafic volcanics, but no volcanic structures have been observed on outcrops. Chapdelaine (1995), however, reported locally observing a spinifex structure in the northern portion of the branch.

In the southern portion of the branch (sheet 23D05), a discontinuous band could be interpreted based on magnetic anomaly maps and outcrop 17-MC-3172. It is a peridotite with a cumulate structure, blackish in fresh exposure and orange in altered surface. The rock is very magnetic. It contains mainly olivine or pyroxene, whose intense retrograde metamorphism into serpentine makes accurate mineralogical identification difficult. Olivine and pyroxenes are found in a matrix consisting of carbonates, acicular crystals of tremolite, talc and magnetite. The relatively low Cao content (3.6%) relative to the amount of carbonate and the orange patina of the rock indicate that the carbonate present is ankerite or even siderite. The rare earth profile of the sample from outcrop 17-MC-3172 shows a very clear enrichment of light rare earths relative to heavy rare earths, with a La/Yb ratio of 235. The mineral carrier of light rare earths could not be identified in thin section.

In the northern portion of the branch, the nArmn8 unit consists of a few ultramafic rock klippes and a more continuous horizon observed at outcrop 16-MP-2177, TR 13-04 stripping site (Béland 2013) and likely intercepted by drilling 4034-97-8, 4034-97-9 and 4034-97-10 (Tyson 1997) and 4034-95-03 (Chapdelaine 1995). The rock is green-grey to light-green, homogeneous, altered, massive to well foliated and fine to medium grained. It is generally non-magnetic, except when pyrrhotite is present. At drilling 4034-97-9, it is brecciated and contains fragments of basalt. Mostly made of tremolite, the rock also contains talc, actinolite and some biotite or phlogopite locally. Pyrite, pyrrhotite and chromite are also observed locally. The main alterations are chloritization, locally intense, and carbonation.

Thickness and Distribution

The Roman Formation outcrops in the south and east branches of the UEGB. Units nArmn1a and nArmn2 are located in the south branch of the belt. They form a continuous band whose true thickness varies between 250 m and 3.5 km. The eastern branch, which includes units nArmn1b and nArmn3 to nArmn8, is 75 km long and its true thickness varies between 3 and 4.5 km. 

Dating

Two dates made it possible to constrain the age at which the Roman Formation was emplaced. A felsic lapilli tuff (nArmn1b) sampled in the eastern branch of the UEGB yielded an age of 2770 ±6 Ma. A felsic porphyritic volcanic rock of rhyodacitic composition at the top of the Roman Formation pile (nArmn1a) was dated 2770 ±3 Ma. 

Stratigraphic Relationship(s)

The Roman Formation is part of the René Group. It corresponds to the second volcanic event of the Upper Eastmain Greenstone Belt. The Roman Formation is younger than the Érasme Formation (2800 ±6 Ma; Davis and Sutcliffe, 2018b) and older than the Clément Formation (<2770 Ma) and Dolent Formation (2751 ±5 Ma; Davis, personal communication, 2019).  This unit is in discordant and faulted contact with Bohier Group sedimentary rocks. Contacts between various formations of the René Group were not observed.

The Île Bohier Pluton (2771 ±6 Ma; Davis and Sutcliffe, 2018b) is synvolcanic and contemporary to the emplacement of the Roman Formation. Tonalitic and granodioritic rocks of the Bohier Island Pluton are geochemically very similar to felsic rocks of the Roman Formation. This observation suggests that the Bohier Island Pluton is the source of felsic and intermediate volcaniclastics and volcanics of the Roman Formation.

The Chiyaaskw Pluton, the Wahemen Suite and the Gaël Intrusive Suite distinctly cut the Roman Formation.

Paleontology

Does not apply.

References