Summary:
Graphite deposits occur in three forms: flake graphite, vein graphite, and amorphous graphite. These are described by Mitchell (1993):
- Graphite generally occurs as a result of metamorphism (regional or contact) of organic matter in sediments. Flake graphite is assumed to be derived from fine grained sediments rich in organic matter. As metamorphic grade increases, carbonaceous material converts to “amorphous” graphite. Flake graphite forms from its amorphous precursor at or beyond amphibolite grade metamorphism (Landis, 1971). Vein graphite is assumed to form by partial volatilization of graphite and subsequent recrystallization during regional granulite and/or charnockite facies metamorphism. Amorphous graphite is generally considered to have originated by thermal or regional metamorphism of coal or carbonaceous sediments.
Landis (1971) tentatively concluded that graphite formation is primarily dependent on metamorphic temperature and forms above 750°F (400°C), with pressure and variation in starting materia constituting secondary controls.
The Coosa graphite deposits are flake graphite deposits in high grade metamorphic rocks. They are associated with anomalous vanadium, including the vanadium-mica roscoellite, and nickel, as well as other elements.
Geology
The host of the Project is the Higgins Ferry Formation in the Wedowee Group of the Coosa Block. The Higgins Ferry Formation is defined as an interbedded sequence of three major lithologic units, from top to bottom:
1. Quartz-graphite schist (QGS);
2. Mixed QGS-quartz-muscovite-biotite-graphite schist (QMBGS) unit called INT or intermediate unit;
3. Quart muscovite biotite graphite schist (QMBGS);
4. Quartz-biotite-garnet schist (QBGS).
QGS grades downward into INT, then into QMBGS, which in turn grades downward into QBGS. QMBGS is more highly metamorphosed than QGS and less metamorphosed than QBGS. In most places, the mixed QGS- QMBGS unit (INT) consists of interlayers of finer grained QGS with QMBGS layers containing medium to large muscovites and biotites, reflecting a gradual increase in metamorphic grade from the QGS to the QMBGS (Greenan, 2022).
In some places, QGS grades downward into QMBGS without interlayering of the two units. The contact between these two units is somewhat arbitrary and is taken as where the muscovites and biotites become much larger and more common than in the QGS and where the rock has larger, coarser foliations. This change is reflected in the Cg assays, with the QMBGS generally having much lower Cg grades than the QGS.
Overall, metamorphism increases with depth. This may be caused by the depth of a lithological unit and/or regional metamorphism, or by a buried heat source below the QBGS (contact metamorphism). Near the bottom of some of the holes, the QBGS has gneissic intervals. These are suggested to be fingers/sills of a gneissic intrusive that may have caused the different metamorphic grades.
Metamorphic grade can also increase laterally, with a transition from QGS to INT to QMBGS. Changes such as these appear to be evidence of a contact metamorphic aureole, possibly from a buried amphibolite or other intrusive body. These types of changes, along with thrust faulting, help explain the apparent lack of horizontal and/or vertical continuity of the various lithologies in the cross sections. Increasing metamorphic grade from QGS to QMBGS also generally correlates with lower Cg values, indicating that the Cg was driven off by the metamorphism. In general, metamorphic grades increase from east to west in the Main Grid area. Kyanite and sillimanite increase closer to the large amphibolite intrusive that occurs to the east of the Main Grid area. These also increase closer to a postulated buried granitic intrusive that occurs on the southeast end of Line 05 in the Main Grid area.
There is no definitive correlation between the different iron oxides (jarosite, goethite, and hematite) and Cg mineralization. However, areas with more jarosite ± goethite generally occur above more pyritic unoxidized QGS. Quartz veins/veinlets that cut the foliations usually have more hematite than jarosite or goethite. Hematite is most common in and close to amphibolite sills and intrusives.
Mineralization
Graphite flakes occur as part of the rock forming minerals in the schists. They are often associated with disseminated pyrrhotite and minor pyrite. In places, the green vanadium bearing muscovite, roscoelite, also occurs. Minor late stage, straight-sided veinlets of cubic pyrite up to 10 mm wide with smectite clay cross cut the schistosity and pegmatites.
Graphite ores mined historically were almost entirely from the weathered zone (60 ft to 100 ft), partly because weathering is deep in this area and partly because the weathered rock could be gently crushed without blasting, liberating the graphite without significantly reducing the size of the larger flakes.
The oxide and transition zones were logged in core and modelled for resource estimation. The oxide zone is defined as the zone of total oxidation of sulfides to give an orange-redbrown color to core, and a much softer rock. The base of the oxide zone is often sharp and occurs over an interval of less than one foot. The transition zone is defined as the zone of partial oxidation of sulfides and the rock is significantly crumblier than the underlying reduced or sulfide zone. The top of the transition zone is marked by the first appearance of sulfides downhole, and the base is marked by the disappearance of iron oxides, and a marked increase in hardness. Generally, there is more oxidation on fractures and veins in this zone.
The main schist compositions and logging units defined by Westwater geologists in drill core are as follows:
- Quartz-graphite schist (QGS): This schist is both finer grained and better laminated than the QMBGS unit. The color varies from dark gray to black. Contorted foliation and pegmatites are less common. Pyrite and pyrrhotite are both finer grained and more abundant than in the QMBGS unit and form laminae parallel to foliation. Graphite is more abundant in this unit, with grades higher than 1% Cg.
- Mixed QMBGS/QGS (INT): The QMBGS and QGS are commonly interbedded at a centimeter scale forming a mixed unit with graphite grades higher than 1% Cg.
- Quartz-muscovite-biotite-graphite schist (QMBGS): This schist is medium to coarse grained in texture and is characterized by large porphyroblasts of muscovite. The color varies from medium gray to dark gray-green. It is moderately foliated and commonly contorted, with lenticular pegmatites parallel to foliation. Pyrite and pyrrhotite are common accessory minerals and occur as large, disseminated grains. Sillimanite fibers have been observed. Graphite flakes are generally coarse in this unit, although the average carbon content is generally less than 1% Cg.
- Quartz-biotite-garnet schist (QBGS): This unit is subordinate to both the QMBGS and QGS units. It is medium grained and medium to dark gray-green in color. The garnets are usually fine grained with a diameter of about one millimeter, but in places are up to 5 mm to 10 mm. The garnets are light pink in color, suggesting a high manganese (spessartine) content. Foliation is irregular with abundant pegmatites. Pyrite is sparse. Graphite is sparse and grades are less than 1% Cg.