Argyle Diamond Pipe, Western Australia

Location: 128.3E, 16.6S

The Argyle diamond pipe is in a small valley at the southend of the Matsu Range. The pipe is about 2 km long by 150-500 m wide and steep-sided. The shape of the pipe has been modified by faulting, regional tilting, and the coalescence of two or more vents (Boxer and others, 1989). In this photo, the dirt road runs on the pipe along the valley floor. All photos courtesy of Grant Boxer.

The most common rocks are quartz-rich lapilli ash tuffs (shown is this photo) and tuffs devoid of quartz. The quartz in the tuffs was derived from disaggregation of sediments as the diatreme was emplaced. It did not crystallize from the magma. The tuffs are made of juvenile clasts, accretionary lapilli, armored lapilli and ash, and accidental clasts from country and basement rocks. Bedding within the deposits suggests deposition by base surge (Boxer and Jaques, 1990).

Intrusive olivine lamproite dikes and fine-grained quartz-rich sediments are present in minor amounts. The dikes, such as the one in this photo, serveas samples of the lamproite magma that were not contaminated by the quartz-rich surface sediments.

By definition, lamproite is a potash and magnesia-rich volcanic or hypabyssal rock with one or more of the following major minerals: olivine, diopside, phlogopite, leucite, amphibole, or thopyroxene,sanidine, and glass. Accessory minerals may include priderite, apatite, nepheline, spinel, perovskite, wadeite, and ilmenite. Xenoliths and xenocrysts of uppermantle origin may be present. Diamond is a rare accessory mineral (Jacques and others, 1984).

Mean major-element chemical composition of the lamproite dikes at Argyle:
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 H2O+ CO2
44.0 3.40 5.29 8.02 0.14 19.7 5.27 0.16 3.92 1.25 4.83 2.69
FeO = FeO total. Data from Jacques and others (1989) and calculated free of H2O-.

Based on their chemistry, lamproite is either basic or ultrabasic (MgO, 16-23%), with high K2O/Na2O (>3), high nickel (560-1080ppm) and high chromium (880-1340 ppm) with high concentrations of Rb, Sr, Ba, Ti, Zr, Nb, Pb, Th, U and light-rare-earth elements. These chemical characteristics are derived by melting in the mantle to form ultra basic magma. Enrichment of incompatible elements coupled with strongly radiogenic strontium (and non-radiogenic neodymium) isotopic compositions (see Jacques and others, 1989) suggest that the magma originated by small degrees of partial melting of subcontinental lithosphere that had undergone long term geochemical enrichment (Boxer and Jaques, 1990).

Isotopic dating of rocks and minerals from the pipe give an age of 1.178 billion years. This is the age of the eruption. Isotopic dating of garnets and clinopyroxene in the diamonds indicated that the diamonds formed about 1.580 billion years ago. Thus, the diamonds formed in mantle eclogite and mantle peridotite almost 400 million years before the magma picked them up and carried them to the surface.

Most of the diamonds at Argyle are resorbed dodecaheda.

A small percent of the diamonds from Argyle are sharp-edged octahedra with hexagonal surface pits.

Most diamonds are found in kimberlite. The rocks at Argyle are lamproite. Since the recognition of diamond-bearing lamproite in Western Australia, lamproite pipes in other areas have become the target of exploration teams.

The morphology of juvenile clasts and the presence of bedded tuffs, accretionary lapilli, and water escape structures indicated that the Argyle diatreme formed from several phreatomagmatic eruptions. The explosive eruptions were caused by the interaction of lamproite magmaand shallow groundwater. Cross-section of the Argyle diatreme from Boxer and others (1989).

Diamond exploration in the Kimberley and the difficulty in determining the grade of ore in diamond pipes are described by Grant Boxer.

Most diatremes, like those in the Navajo volcanic field, do not contain diamonds. Most young diatremes have maars or tuff cones at the surface like Coliseum Maar and Menan Buttes respectively. The surface features of many diatremes in Australia have been eroded away.

Diamonds in Crater of Diamonds, Arkansas are hosted in lamproite but the diamond content (0.5 carat per 100 tons) is not economical for mining.


Sources of Information:

Boxer, G.L., and Jaques, A.L., 1990, Argyle (AK1) diamond deposit, in Geology and Mineral Deposits of Australia and Papua New Guinea, Hughes, F.E., ed., The Australian Institute of Mining and Metallurgy, Melbourne, p.697-706.

Boxer, G.L., Lorenz, V., and Smith, C.B., 1989, The geology and volcanology of the Argyle (AK1) lamproite diatreme, Western Australia, in Kimberlite sand Related Rocks, Vol. 1, Their Composition, Occurrence, Origin and Emplacement (Eds J. Ross and others), geol. Soc., Aust. Spec. Publ. No. 14, p. 140-152.

Jaques, A.L., Haggerty, S.E., Lucas, H., and Boxer, G.L., 1989, The mineralogy and petrology of the Argyle (AK1) lamproite pipe, Western Australia, in Kimberlites and Related Rocks, Vol. 1, Their Composition, Occurrence, Origin, and Emplacement (Eds. J. Ross and others), Geol. Soc. Aust. Spec. Publ. No. 14, p. 153-169.

Jaques, A.L., Lewis, J.D., and Smith, C.B., 1986, The kimberlites and lamproites of Western Australia: Geological Survey of Western Australia Bulletin 132, 268 p.

Jaques, A.L., Lewis, J.D., Smith, C.B., Gregory, G.P., Ferguson, J., Chappell, B.W., and McCulloch, M.T., 1984, The diamond bearing ultrapotassic (lamproitic) rocks of the west Kimberley region, Western Australia, in Kimberlites I: Kimberlites and related rocks, edited by J. Kornprobst: Amsterdam, Elsevier, p. 225-254.


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