Summary:
The manganese and iron mineralization that forms the Plymouth Deposit is bedded and stratiform in nature and is recognized as being of primary sedimentary origin. Manganese is predominantly in the form of the carbonate mineral rhodochrosite and iron occurs in both oxide (hematite, magnetite, and ilmenite) and carbonate minerals (predominantly siderite). The deposit has been classified by some past workers as being of the Algoma Type banded iron-formation (BIF) group as defined by Gross (1965; 1996). More recent research reported by Way (2012) and Way et al. (2009) indicates that the Plymouth Deposit and its correlatives in this area that include the North Hartford Deposit, also held by CMC, are more appropriately classified as Clinton Type ironstones.
Government mapping in the Project area shows it to be underlain by a belt of Ordovician and Silurian siltstones and slates, collectively referred to as the Aroostook-Percé belt. This includes Late Ordovician to Early Silurian sediments of the Matapedia Group’s Whitehead Falls Formation that are overlain by Early Silurian sediments of the Perham Group’s Smyrna Mills Formation, which are laterally extensive over much of western and northwestern New Brunswick and Maine (Fyffe and Fricke, 1987; NBDNRE, 2000).
The Woodstock area manganese-iron deposits are interpreted to represent a series of Early Silurian manganiferous banded iron formations (BIFs). Six main deposits were identified by gravimetric survey results from the mid-1950s and defined as being large, lenticular-shaped bodies within the Silurian Smyrna Mills Formation. These deposits are interpreted to have formed in a shallow marine basin and are in sharp contact with units of red or green shale (Sidwell, 1957; Roberts and Prince, 1990; Force et al., 1991). Stratigraphic lensing and compositional variation of the manganiferous units has been interpreted to indicate that the deposits are stratigraphically separated and not one continuous unit. The current orientation of bedrock units is primarily a function of two folding generations (F1 and F2). F1 folds trend northeast and are slightly overturned south of the Plymouth Deposit and have axial planes ranging from nearly vertical, to 85° northwest. Fold axes plunge shallowly (< 5 degrees) to the northeast and southwest. F2 folds overprint F1 structures and have axial planes trending northwest, (approximately 320°) and dipping steeply north at approximately 80° (Roberts and Prince, 1990). Both sets of folds were generated during the mid-Devonian Acadian Orogeny and were affected by associated regional sub-greenschist metamorphism. Earlier Silurian deformation is not well represented in the Plymouth area.
The Plymouth Deposit has been described as an assemblage of iron and manganese oxide and carbonate-silicate-oxide facies rocks that formed within a shallow marine basin. Roberts and Prince (1990) described the Plymouth deposits as Banded Iron Formations (BIFs) within a series of sedimentary-volcanic units, but alternative hypotheses suggest the iron-manganese mineralization could have originated from a variety of sources including oceanic hydroxides and/orthe weathering of terrestrial bedrock, with deposition occurring in a continental margin marine setting (Way et al., 2009; Way, 2012). The lack of volcanic association in the associated stratigraphy supports the more recent assessment, which is favoured at present.
Historical interpretation of the mineralization of the Plymouth Deposit indicated that the iron-manganese mineralization can be subdivided into oxide,silicate- carbonate-oxide, and carbonate facies(Sidwell, 1957; Gilders, 1976; Roberts and Prince, 1990). These stratiform deposits are analogous to the Type IIA deposits of bedded manganese oxides and carbonates described by Macharmer (1987). The iron-manganese oxide facies present in the Project area is represented by red to maroon siltstone, and red chert characterized by the mineral assemblage magnetite, hematite, braunite (Mn+2Mn+36[O8SiO4]) and bixbyite ([Mn,Fe]2O3) and ranges between 30% and 80% iron-manganese oxides. Manganese mineralization is predominantly present in the form of rhodochrosite (MnCO3) and minorsursassite (Mn2Al3[(SiO4)(Si2O7)(OH)3]) crosscuts syngenetic iron-manganese mineralization (Sidwell, 1957). Greenish grey siltstone units interbedded with the red and maroon units also carry potentially economic levels of manganese. Bedded layers of both iron-manganese mineralization types are locally observed to be crosscut by veins of quartz, quartz-carbonate, chlorite and iron sulfides (Way et. al., 2009).
As a result of work completed by BMC-CMC and Thibault on the Plymouth Deposit since 2011, it has been recognized that the manganese mineralization in both the red and grey siltstone lithotypes that comprise the deposit is dominated by manganese carbonate in the form of rhodochrosite. The iron mineralization in red and grey siltstones was found to be different, with the dominant iron minerals in the red siltstones being oxides in the form of hematite, magnetite and ilmenite, while the dominant iron mineral in the grey siltstone is a carbonate, predominantly in the form of siderite.