2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 64-2
Presentation Time: 1:50 PM


First order objections in predicting macro-diamond grades from micro-diamond [<0.5 mm] determinations (exploration and investment norms) are: 1. All kimberlites are multiple intrusions with highly variable diamond contents as nuggets from P and E sources. 2. Nucleation, growth and dissolution assure heterogeneity. 3. Modelled as oxidation of CH4 or reduction of CO2 should yield equal contents of diamond, yet macros and micros are unequal in size and abundance. 4. Conduit sampling is chaotic not coherent. 5. Micros are mineral inclusion- and surface etch-free, Type Ib octahedra or Type II. Macros are Type Ia octahedra commonly corroded to dodecahedra or tetrahexahedra. 6. In size vs abundance, micros are linear, macros are quadratic and the inferred log-normal trends are separated by a marked discontinuity “Chapman & Boxer discrepancy zone.” 7. The broad-based continuum from micro- to macro-diamonds ignores the ultra-ultra-fine population at one extreme that is never recovered, and mega-stones at the other that are rarely encountered. 8. With a huge disparity in the surface to volume ratio, micros and macros cannot be of the same age, or have resided over the same period of time in an environment of constant C, or in a setting affected by the same corrosive mantle fluids. 9. C-isotopes and H-contents are equivocal (frags are not micros).10. Having micros “protected in xenoliths” from kimberlite attack lacks credulity: It’s slow corrosion not rapid etching or combustion, and the mantle is solid not disruptively xenolithic.

Permissively, young micros are from D” carbon in plume-bearing proto-kimberlites. Large numbers of nucleation centers crystallized in short-order. Ancient macros of primordial C, variable fO2, and with small numbers of nucleation sites, were initiated by thermo-chemical cracking of gas-dominant species. Subsequent, hyperactive metasomatic fluids induced corrosion and dissolution over time. From the many differences noted, how can micros and macros possibly be related? And if unrelated, how can one possibly predict a macro-population from a micro-determination? Any inferred relationship is, therefore, mythical and the widespread log-normal association, so common in nature, is myopic for diamonds: “hyper-focused that fails to include a larger context beyond the focus!” viz. Genesis!