«Arkhangelskgeoldobycha» JSC (Arkhangelsk, Russia):

Pendelyak R. N., Leading Geologist, e-mail: RPendelyak@agd.lukoil.com

Verichev E. M., Deputy Head of Geological Department, Candidate of Geological and Mineralogical Sciences, e-mail: EVerichev@agd.lukoil.com

Golovin N. N., Head of Department — Chief Geologist, Candidate of Geological and Mineralogical Sciences, e-mail: NGolovin@agd.lukoil.com

The Grib Diamond Deposit is comprised of two kimberlite pipes: Grib Pipe and North Pipe. Host rocks (1150 m) are composed of argillite, siltstone and Riphean and Wend sandstone. Overburden rocks are limestone, dolomite, sandstone, mid-Carboniferous siltstone and loose quaternary deposits 53 to 83 m thick. The Grib Pipe size is 575500 m. The crater portion (to 147 m) is composed of sheet-like bodies and lens of sand, tuffsandstone, tuffite, tuff and Wend breccias. Diatreme zone has the two-phase intrusive history. The first phase is tuff breccias and xeno-tuff breccias; the second phase is kimberlite. The rocks feature a pyrope-picro-ilmenite assemblage if sinking fraction at the yield up to 50 kg/t. The distinguishing characteristic is joints of megacrysts of garnet, chrome-diopside, phlogopite and picro-ilmenite and diverse unweathered xenolyth of mantle rocks. Magmatic component of kimberlite is serpentine in the top layers and olivine in the bottom layer. The rocks of the crater and first intrusive phase contain saponite. Kimberlite is characterized with high magnesia content, low alumina content and alkalinity. The average diamond content is 0.39 carat/t in the crater, 0.47 carat/t in tuff–xeno tuff breccias and 1.63 carat/t in kimberlite. By weight, the prevailing size grades of diamonds are –4+2 mm and –2+1 mm. In the diamond size grade +0.5 m, octahedral and dodecahedron crystal habits are upper most; in microcrystals octahedron shape dominates. The North Pipe occurs 200 m northward of the V. Grip Pipe, is 50100 m in dimension and is composed of sandstone, tuffsandstone and tuffite; the diamond content is poor.

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