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Application of mineral liberation analysis to determine the spatial association of accessory minerals with biotite in granitic rocks

Bachmann, K.; Gilbricht, S.; Renno, A.; Hamilton, D. C.; Clarke, D. B.


We apply mineral liberation analysis (MLA) to resolve and automate highly specific petrological questions. In this study, we quantify the spatial association of more than 15,000 grains of accessory apatite, monazite, xenotime, and zircon with essential biotite, and clustered with themselves, in a peraluminous biotite granodiorite from the South Mountain Batholith in Nova Scotia (Canada). A random distribution of accessory minerals demands that the proportion of accessory minerals in contact with biotite is identical to the proportion of biotite in the rock, and the binary touching factor (percentage of accessory mineral touching biotite divided by modal proportion of biotite) would be ~1.00. Instead, the mean binary touching factors for the four accessory minerals in relation to biotite are: apatite (5.06 for 11168 grains), monazite (4.68 for 857 grains), xenotime (4.36 for 217 grains), and zircon (5.05 for 2876 grains). Shared perimeter factors give similar values. Monazite and zircon have approximately log-normal grain-size distributions, but apatite is strongly skewed toward larger grain sizes, and xenotime is skewed toward smaller grain sizes. Accessory mineral grains that straddle biotite grain boundaries are larger than completely locked, or completely liberated, accessory grains. Only apatite-monazite clusters are significantly more abundant than expected for random distribution. The high, and statistically significant, binary touching factors and shared perimeter factors suggest a strong physical or chemical control on their spatial association. We discuss several petrogenetic processes that may lead to this spatial association. This study is an example of how modern methods of automated mineralogy, combined with powerful statistical methods, allow petrographic observations defined as "well known" and "given" to be transformed into viable scientific statements that are verifiable and falsifiable.

  • Poster
    Geoanalysis 2022, 06.-12.08.2022, Freiberg, Deutschland