A reassessment of the diamondiferous Pamali Breccia, southeast Kalimantan, Indonesia: Intrusive kimberlite breccia or sedimentary conglomerate?

The “Pamali Breccia” is a small (<100-m diameter, 10- to 35-m thick), semicircular peridotite body that rests on the Bobaris ophiolite in southeast Kalimantan, Indonesia. Generally it has been regarded as a primary source rock for Borneo diamonds, which represents a tectonic breccia that is possibly related to a kimberlite. New geologic, geochemical (major and trace elements), and petrographic data indicate that the Pamali Breccia is, in fact, a sedimentary conglomerate with local Bobaris ophiolite provenance. The Pamali Breccia consists of well-rounded to subangular clasts (0.1 to 20 cm; mean, 0.7 cm), mostly serpentinites and pyroxenites, with clasts of gabbros and greenstones forming minor constituents. Abundant sedimentary features characterize the Pamali Breccia and include well-sorted, matrix-supported, normally graded beds (coarse sand to cobble clast-size), and planar stratification with imbricated pebbles and cobbles. When corrected for near-surface, secondary calcite alteration and normalized to an anhydrous basis, whole-rock samples of the breccia and its component clasts and matrix display geochemical traits typical of ophiolites and other ultramafic rocks (SiO 2= 42–45 wt. %, 9–12% FeO T, 25–35% MgO, 3–12% CaO, 6–9% Al 2O 3, 0.1–1.3% Na 2O, <0.2% K 2O). Compatible trace element contents of the Pamali Breccia are more typical of ophiolites than kimberlites or lamproites and range from 600 to 2,000 ppm Ni, 1,400 to 4,000 ppm Cr, and 60 to 100 ppm Co. Large ion-lithophile (incompatible) element contents of the Pamali Breccia display the same features and are in the range of Rb = 10 to 30 ppm, Sr = <10 to 80 ppm, Y <10 ppm, Ce = 5 to 8 ppm, and Th <0.2 ppm. Various Pamali Breccia elemental ratios are also more characteristic of ophiolites than those of kimberlites or lamproites. Thus the Pamali Breccia is not a kimberlite or lamproite. The primary source of the diamonds in the breccia may be the host ophiolite, or, alternatively, a distant kimberlite or lamproite, the unambiguous identification of which requires further study.