1000s of hours sorting, picking, analysing indicators

Diamond Indicator Minerals
Peace River District, British Columbia & Alberta

Peace Diamond Project

concentrating
sediments for
heavy minerals

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KIMBERLITE INDICATOR GEOCHEMISTRY (KIMs)

olivine garnet eclogite pyroxene zircon phlogopite bentonite ilmenite chromite diamond

kimberlite, olivine, garnet, eclogitic garnet, pyroxene-clinopyroxene, ilmenite
ilmenite rind, ashes-clays, Whitestone, diamond

DIAMOND INDICATOR MINERALS

Kimberlite Pipes and Diamond Inclusion Field Minerals (DIFs) see: AGS KIMBERLITE info Summary

DIAMOND INDICATORS (DIMs)

picking diamond indicators from heavies grains

(Photo left: A concentrated sample of heavies. Like panning for gold. These are ready for picking and microprobe.)

DIMs are minerals formed in the upper mantle as companion crystals with diamond. They are millions of times more numerous in the kimberlite host rock than diamonds, so we search for these first. Common indicators:

olivine - serpentine garnet - pyrope, chrome pyrope eclogite - eclogitic garnet pyroxene - clinopyroxene, diopside and chrome diopside ilmenite - picro ilmenite oxides - chromite zircon kyanite mica - phlogopyte, sericite clays - bentonite, ashes and derivatives of kimberlite

OLIVINE-SERPENTINE

(Olivine - soft waxy white, green or lemon coloured mineral. Often alters to serpentine.) olivine The prime exploration mineral. Igneous rocks are formed by successive melt and solidification. The reaction sequence from silicate melts to formation of Olivine Group minerals depends upon the reaction being discontinuous. The Discontinuous Reaction Sequence results in olivine, pyroxene, amphibole, biotite, quartz. The degree of polymerisation, starting with the orthosilicate forming; rings, chains, sheets and networks; proceeds on the reaction sequence. The Continuous Reaction Sequence results in plagioclase (Ca-rich), plagioclase (Na-rich), alkali feldspar, quartz. Olivine and pyroxene solidify first on the discontinuous reaction sequence, so these (kimberlites) are the silica-poor ultrabasic rocks with magnesium being predominant.

GARNET GROUP

(This photo shows how exquisitely beautiful some of the mineral grains are. This is a ballas, possibly a diamond, partially encased in eclogitic garnet minerals.)

Garnet is a resistant silicate most easily identified in the field and recoverable by panning. Best noted with heavies on beaches and bars. Continuous variations in compositions, except between the; pyrope, almandine, spessartine; and the uvarovite, grossular, andradite, series. Divided into twelve groups, G1 to G12, on a statistical basis by Dawson & Stephens. Presence in diamondiferous kimberlites or as grains found as inclusions in diamonds were the criteria.

(Electron scanning microprove provides the chemical composition analysis of the grains. Probability of formation with diamonds can then be determined on a statistical basis.)

Pyrope with 3-8% Cr2O3 are common constituents of kimberlite. These tend to have a greenish violet or purple hue. Almandine species kimberlite indicators, G3-G6, may also contain Pyrope and Grossular molecules; Ti, Ca and Mg setting them apart as indicators. Of the G-7 and G8 Grossulars, the G7 uvarovite is rare and does not travel well. Chrome content gives it the dark green to emerald green colour. Microprobe is used to establish composition of garnet.

PYROXENE GROUP

Pyroxene is a component of most kimberlites, lamproites and lamprophyres. Olivine is in the Pyroxene Group. Dawson and Stephens established a kimberlite-lamproite diamond related pyroxene group classification system for the orthopyroxenes and clinopyroxenes. Clinopyroxene is a common groundmass of kimberlite. Orthopyroxenes are rare in kimberlites. (Mitchell, 1986) In hypabyssal facies, Low Titanium Low Chrome Diopside occurs (TiO2,AlO2,Na2O<1%;CrO3<.5%). This is a characteristic groundmass phase and in micaceous kimberlite (lamprophyre-Type II K) Kimberlite I crystallize monticellite and serpentine but not diopside, potassic amphibole, and potassium feldspar." (Mitchell,1995,p247).

(Photo: concentrated heavies from gravel sometimes contain garnet (orange), pyroxene (green), ilmenites (black),magnification X 200.)

Picking kimberlitic indicators and diamond indicators is based on colour. Colour results from Fe, Mn, Cr content. Pyrope - pinkish red, crimson to purple; Chrome-Pyrope greenish violet to purplish. Almandine; deep red to brown-black. Grossular; colourless, pink, yellow-green depending on Fe and Mn present.(Deer,1993) Picking based on colour takes experience and a good eye for variations in colour. It is a difficult, subjective and inexact process. DIMs are .25 to 2 or 3 millimeters, so you need good eyes and a good microscope.

In river, creek and soil samples, we look for Peridotic and Eclogitic garnets - G1 to G10, clinopyroxines in the pyroxene group, such as chrome diopside -CP2 to CP5. High-pressure garnet grains indicate if geochemistry is consistent with diamond formation. The pyroxenes are soft, so they indicate distance of travel from their volcanic pipe sources. DIMs Trends: http://www.ags.gov.ab.ca/publications/ABSTRACTS/MIN_19990025.html

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