PARTICLE SIZE ANALYSIS AND YIELD EFFICIENCY OF ZIRCON AND APATITE SEPARATION METHODS

Mineral separations concentrate the minerals zircon and apatite necessary for geochronology; three processing methods before magnetic and density (LST, MEI) separation were tested: a control hand-washed sample, samples processed with a spiral panning table, and a sample processed with a Gemini Table. Laser diffractometry determined the grain size distribution of processed samples. The test sample was a hbl-bt tonalite from Mount Stuart, WA; expected yields per kg, from a CIPW-norm: 300 mg zircon; 3200 mg apatite. After sieving to <500 μm, the raw crushed sample contained 2% of grains <20 μm, 13% <125 μm, and 30% <250 μm. The hand-washed control sample recovered the most zircon and apatite (72 mg zircon and 333 mg apatite, considered the maximum yield); LST lights contained 0.02% of grains <20 μm, 12% <125 μm. Panner #1 (58 g/min feed-rate, 9 rpm rotation) recovered 3.7 mg zircon and 1.0 mg apatite from 5.2 g of panner-heavies; panner lights contained 0.5% of grains <20 μm, 15% <125 μm. Panner #5 (36 g/min, 25 rpm) recovered 13.2 mg zircon and 13.0 mg apatite from 34 g of panner heavies; panner lights contained 0.2% of grains <20 μm, 12% <125 μm grains. LST lights from a set of panner heavies contained 0% of grains <20 μm, 0.2% <125 μm. Faster panner rotation increased yield by concentrating more heavies. The Gemini table recovered 330 mg of zircon + apatite (zircon not yet separated from apatite) from 45 g of heavies; Gemini lights contained 0% of grains <20 μm, 4% <125 μm. Yield differences of the techniques are mainly explained by grain size. In thin section, most zircon and apatite are <125 μm, and ~30% of zircons are <20 μm; the control process most efficiently recovered small grains explaining its higher yield. Using magnetic minerals as a proxy for heavies (mostly hornblende) shows that the Gemini heavies (61% <250 μm, of which 72% were magnetic) more efficiently concentrated small grains than the panner (#5 heavies had 16% <250 μm, of which 59% were magnetic). However, the panner concentrated zircon relative to apatite 4-5X more than the control. Even the best yield (control) recovered only 12% of the zircon + apatite expected from the CIPW norm, probably due to loss of zircon + apatite as inclusions in other minerals. In thin section, ~25% of zircons were inclusions in magnetic minerals, and therefore would be lost during magnetic separation.