Trace elements in indicator minerals: area selection and target evaluation in diamond exploration

Abstract

Early recognition and rejection of uneconomic prospects is essential to an economically rational diamond exploration program. Some powerful new techniques for prospect evaluation have been developed by the CSIRO, based on the trace-element capabilities of the proton microprobe. On the larger scale, these techniques also can contribute to the process of area selection. The nickel content of chrome-pyrope garnet equilibrated with mantle olivine increases with temperature. This “Ni thermometer” can be used to measure the distribution of equilibration temperatures ( T n i ) in garnet concentrates from exploration targets such as kimberlites and lamproites. A “Cr barometer”, based on the partitioning of Cr between garnet and orthopyroxene in equilibrium with chromite, gives a minimum estimate of pressure ( P cr )for each grain. By combining T ni and P cr , the position of the local paleogeotherm can be derived from garnet concentrates, and the depth of origin of each garnet grain is determined by referral of its T ni to the derived geotherm. The depth to the base of the lithosphere can be derived from plots of Y content vs. T ni , and the “diamond window” is defined as the range of T ni between the intersection of the geotherm with the diamondgraphite equilibrium curve and the base of the lithosphere. Areas with elevated geotherms are inherently less prospective for diamonds, since the geotherm enters the diamond stability field only within the deepest part of the lithosphere, or not at all. Diamond-rich pipes contain a large proportion of garnets with T ni in the diamond window, while diamond-poor pipes typically contain a high proportion of garnets with lower T ni , reflecting greater sampling of mantle within the graphite field. Many poorly diamondiferous and barren pipes also contain abundant garnets with high Zr, Ti and Y contents, reflecting metasomatic processes in the mantle. A combined measure (Γ) of T ni distribution, rock type proportions and metasomatism in garnet concentrates shows a strong correlation with diamond grade. Combined major- and trace-element data for chromite macrocrysts define distinct spinel populations typical of kimberlites (Group I vs. Group II) and lamproites. These populations can be used to recognize the source rocks of chromites found in exploration, and to distinguish xenocryst and phenocryst spinels. The Zn content of chromites equilibrated with olivine is strongly temperature-dependent, and this “Zn thermometer” divides xenocryst spinels from kimberlites and lamproites into those derived from the diamond stability field and those from shallower, barren levels of the mantle. The Cr- T zn distribution in chromite concentrates is a useful adjunct to the garnet geotherm, because it shows the T range over which garnet and chromite can coexist; it also provides an independent, if rough, guide to the position of the geotherm. Trace levels ( > 6 ppm) of Zr and Nb occur in many chromites from kimberlites, lamproites and ultramafic lamprophyres, but are essentially absent in chromites of similar major-element chemistry from greenstone terranes and ophiolites. Combined major- and trace-element data on ilmenite macrocrysts in kimberlites and lamproites show smooth trends that can be explained by fractional crystallization from single magma chambers. Suites from different kimberlites in the same area commonly are distinct from one another, and this feature can be used to inventory drainages.