Heavy-Mineral Sand Evaluation Near Chavara
A heavy-mineral sand deposit near Chavara is being evaluated. The deposit is 2.5 km long, 400 m wide, and 4 m thick, with a bulk density of 1.7 t/m3. The heavy-mineral content is 7% by weight. The heavy assemblage is ilmenite 55%, garnet 20%, zircon 8%, rutile 6%, monazite 3%, and other minerals 8%. The water table is 1 m below the surface, and the beach is retreating at 1.5 m/year. Local fishermen report reduced catch near a previous dredge scar. Monazite contains thorium. The company proposes dry mining with a 200 m wide pit and no backfilling for two years. A village is 600 m inland, and the nearest house is 400 m from the deposit boundary.
1. Estimate the in-situ tonnage of heavy minerals, ilmenite, and monazite. Show each step and state the assumptions.
Read the worked answer
Step 1, volume: 2,500 m x 400 m x 4 m = 4,000,000 m3.
Step 2, total sand mass: 4,000,000 m3 x 1.7 t/m3 = 6,800,000 t.
Step 3, heavy minerals: 7% of 6,800,000 t = 0.07 x 6,800,000 = 476,000 t.
Step 4, ilmenite: 55% of 476,000 t = 0.55 x 476,000 = 261,800 t.
Step 5, monazite: 3% of 476,000 t = 0.03 x 476,000 = 14,280 t.
Assumptions: the grade and bulk density are uniform over the block; the figures are dry tonnage; there is no dilution by waste or moisture; and the deposit is fully accessible. In reality, grade varies laterally and with depth, so these are order-of-magnitude estimates, not reserves. A full evaluation would use borehole assays and a resource classification.
2. Evaluate the mining proposal from a resource and environmental standpoint. Recommend at least four conditions before mining.
Read the worked answer
The resource is substantial: about 476,000 t of heavy minerals, including about 262,000 t of ilmenite and 14,300 t of monazite. Ilmenite, rutile, zircon, and garnet have economic value, so the deposit is worth developing. But the proposal has serious problems. A 200 m wide pit in a 400 m wide deposit leaves little buffer, and no backfilling for two years means a large open scar on a retreating coast. The water table is only 1 m below the surface, so dry mining will require dewatering, which can affect wells and increase saltwater movement. The previous dredge scar and the fishermen's complaint indicate that coastal processes and livelihoods are sensitive.
Conditions before mining should include: an independent EIA and coastal-process study; a setback from the active shoreline and a plan for concurrent backfill and revegetation; groundwater monitoring and a dewatering plan that protects village wells; radiation and dust monitoring with sealed monazite handling and tailings containment; fisher consultation and compensation; and a financial bond for closure. Mining should not proceed on the current plan.
3. A local official says the monazite is only 3% of the heavies, so radiation is negligible. Assess this claim with reasoning.
Read the worked answer
The 3% figure is a grade, not a total quantity. Three percent of 476,000 t is about 14,280 t of monazite. If the monazite contains roughly 5-10% ThO2, that is about 700-1,400 t of ThO2, plus uranium and daughter products. That is a large inventory of radioactive material even though its concentration in the sand is low.
Radiation risk depends on dust, concentrate handling, tailings, and long-lived daughters, not only on the grade in the undisturbed deposit. Dry mining creates dust; mineral separation concentrates monazite; and tailings can remain radioactive for long periods. The correct approach is to assay the monazite, model the dose pathways, and prepare a radiation safety plan with monitoring, containment, and worker protection. The official's statement is not supported by the calculation.