Evaluating a Phulbani Granite Quarry for Dimension Stone and Aggregate
At a granite quarry near Phulbani, a 35 m face is being evaluated for dimension stone and concrete aggregate. The rock is a coarse pink granite with 2–5 cm alkali feldspar phenocrysts, 1–3 m aplite dykes, and abundant 10–50 cm khondalite xenoliths near the western contact. A 20 cm chilled margin is exposed against khondalite. Sheeting joints are spaced 3–6 m, but two subvertical fracture sets are spaced 0.4–0.8 m and carry thin pyrite coatings. Laboratory tests on three blocks give water absorption 0.35%, specific gravity 2.64, dry compressive strength 112 MPa and wet compressive strength 86 MPa. Petrography notes 2% pyrite and 1% clay-altered feldspar. One block shows brown staining along a fracture after 14 days of wetting. The quarry has orders for 500 m3 of 20 mm aggregate and 80 m3 of polished slabs. The owner proposes to sell all rock as premium dimension stone and to use the pyrite-bearing blocks as unbound base course.
1. Interpret the western contact and the emplacement history of the granite.
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The contact is intrusive and shallow-level. A 20 cm chilled margin against khondalite means the granite cooled rapidly against cold country rock, and the 10–50 cm khondalite xenoliths show stoping of the wall rock. The 1–3 m aplite dykes are late differentiates, and the 2–5 cm feldspar phenocrysts indicate crystallization from a hydrous, relatively evolved melt. The 3–6 m sheeting joints are unloading joints formed as the pluton was exhumed, not primary bedding. This is not a lava flow: a flow would have a vesicular or glassy base and no granite xenoliths. The implication for quarrying is that the freshest, least fractured rock is likely in the interior, away from the western contact, while the contact zone is more heterogeneous and fractured.
2. Assess the durability of the rock and the owner’s proposal for the pyrite-bearing blocks.
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Not all rock should be sold as premium dimension stone. Water absorption of 0.35% is acceptable, but the 23% loss in compressive strength from dry to wet (112 to 86 MPa) and the 1% clay-altered feldspar indicate sensitivity to water. Pyrite coatings are the main disqualifier for polished slabs: oxidation produces brown staining, as the 14-day wetting test shows, and sulfuric acid can attack mortar. Blocks with visible pyrite or open fractures should be rejected for slabs. The pyrite-bearing rock is also unsuitable as unbound base course because oxidation and sulfate heave can cause staining and volume change; use it only as bound aggregate after testing, or reject it. Fresh, pyrite-free blocks from the interior can be sold as dimension stone; the rest should be directed to tested concrete aggregate.
3. Recommend a testing programme before the quarry ships either product.
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Testing should separate the two products. For dimension stone, run petrography, flexural strength, freeze–thaw, salt crystallization and water absorption on blocks from three locations, and inspect every block for pyrite. For concrete aggregate, test alkali–silica reactivity by mortar bar, sulfate soundness, Los Angeles abrasion and water absorption; test the pyrite-bearing stock separately for potential sulfate attack. Also test the clay-altered feldspar for durability under wetting and drying. Until those tests are complete, do not sell the pyrite-bearing rock as base course, and do not market all 500 m3 as premium stone.