The Grain Size Tin Problem: why identical assays are not identical deposits
Updated: 1 day ago
Part 1 mapped a register of tin and tungsten deposits that have been known for a century. This note explains the variable that decided which of them became mines: the size of the ore mineral grains. Grain size sets recovery, recovery multiplies grade, and no drill intersection reports it. Eastern Australia hosts all four answers to the problem, side by side.
Samso Insights | Research Insights | The Eastern Australian Tin - Tungsten Register - part 2 of 3 | Samso Geology Series |
WHAT THIS PIECE COVERS Why every deposit has two grades, and why only one of them is assayed. Where the four archetypes sit, redrawn from Part 1. The gravity window, in plain English. Taronga against Windermere, and what that pairing demonstrates. Coarse veins, the ore that built the district. Locked skarns, tin you can assay but cannot recover. Watershed, the fine-grained skarn with a 2026 answer. Heemskirk, and what clean ore looks like. Ore sorting, fuming, and the strongest case against this note. Effective recovered grade, and how it re-sorts the whole register. |
1.00 — THE PREMISE
Every deposit has two grades
Part 1 closed with three exhibits. Taronga, at 0.10% tin, has a completed definitive feasibility study. Windermere, at 0.27% tin, nearly three times the grade, has never been mined through any price cycle since its discovery. Gilmore's ore ran 7.6% tin and paid for itself with 1930s technology, while Gillian's 0.65% defeated a 2013 study advised by the world's largest tin producer. If grade decided development, every one of those outcomes would be reversed.[1][6][8][9][11]
The variable that reconciles them is grain size. A tin deposit really has two grades: the assay grade a laboratory reports, and that grade multiplied by the recovery a processing plant can actually achieve. Grain size sets the second one, because every recovery machine ever built has a particle-size floor, and mineral grains finer than the floor leave the plant in the tailings no matter what the drill intersection said. The result is an invisible cut-off grade that never appears in a resource table.[2]
This matters more in 2026 than it has in forty years, because the tin price is at records and the temptation is to read every old intercept at today's prices. Price is a multiplier on recovered metal, not on assayed metal. A higher tin price can pay for more grinding, more flotation and more furnace time; it cannot make a cassiterite grain bigger. This note works through the physics in plain English, then walks the four metallurgical archetypes that Eastern Australia hosts within two belts.
