Ann: Outstanding Drill Results from Bell 50 Target at Renison, page-13

Sn has a higher (solid) solubility in the acidic granite than other more basic rocks. As the granite cools its exudes the Sn into carbonates, magnesite (Ca & MgCO3) or forms skarns for us (pegmaties for others). The concentration of the Sn occurs into the smaller areas of basic carbonate/ magnesite affected by heat and pressure some time called metamorphose. I presume as we get close to the granite we are vectoring close to the more heat (and pressure) affected area so the excellent grades. It's the carbonate & magnesite members at Rension we're mining not the granite itself.
My simplified summary.