Granite or migmatite? A quarry proposal in the Adamawa
A proposed quarry near Ngaoundéré in the Adamawa exposes a 25 m high face of coarse-grained leucocratic rock. Modal analysis gives quartz 35%, K-feldspar 30%, plagioclase 20% (An12), biotite 8%, muscovite 5% and garnet 2%, with accessory monazite, zircon and apatite. No hornblende is present. The rock contains 5–20 cm concordant leucosome bands and biotite-rich selvages; the bands are folded, with axial planes striking 060° and dipping 70° NW. About 10% of the face contains mafic microgranular enclaves. Specific gravity is 2.62, water absorption 0.4%, and uniaxial compressive strength is 95 MPa perpendicular to the banding but 60 MPa parallel to it. One company wants crushed aggregate for a bridge; another wants dimension stone for cladding. The quarry is 18 km from a paved road and 120 km from the railhead.
1. Use the modal mineralogy and field relationships to classify the rock and interpret its origin. Explain why calling it a 'granite' in a quarry brochure is incomplete.
Read the worked answer
Using QAPF, Q = 35, A = 30 and P = 20, so Q/(Q+A+P) = 35/85 ≈ 41% and A/(A+P) = 30/50 = 60%. That plots in the granite field. However, the rock is peraluminous: muscovite + garnet + monazite, no hornblende, and An12 plagioclase. This assemblage suggests a crustal, S-type melt rather than a metaluminous I-type granite. The concordant leucosome bands, biotite-rich selvages and folded axial-plane fabric show that the body is a migmatite or anatectic granite: part of it was mobilised, but much of it is in situ partial melt. The mafic enclaves could be restite or injected magma.
Calling it a 'granite' in a brochure is incomplete because it ignores the migmatitic structure and the peraluminous mineralogy, both of which control physical properties and commercial behaviour.
2. Assess the rock's suitability for the two proposed uses. Recommend which use, if either, should be pursued, and state the tests needed.
Read the worked answer
For aggregate, the rock is strong (95 MPa) and dense, but strength is anisotropic (60 MPa parallel to banding). Crushing will produce flaky fragments along foliation, and muscovite and garnet may lower impact and abrasion resistance. It could make acceptable base-course aggregate if tests pass; the quarry should be oriented to avoid weathered bands. For dimension stone, the 5–20 cm leucosome bands, folds and enclaves make it visually and mechanically anisotropic. It will not yield large uniform blocks, and water absorption 0.4% is not the main problem. It is poor for cladding unless a homogeneous facies is selected.
Recommendation: pursue aggregate after testing; do not market dimension stone without block-scale trial extraction. Tests needed: petrographic examination for alkali–silica reactivity, Los Angeles abrasion, sulphate soundness, water absorption, and orientation-specific uniaxial compressive strength.