KIMBERLEY-TYPE PYROCLASTIC KIMBERLITES (KPK) (formerly Tuffisitic Kimberlites) Botswana Lethlakane Mine, LM1 Kimberlite Sample BOT/LET/1 – 1 slab and 2 thin sections; see description South Africa Premier Mine, Grey Kimberlite Sample RSA/PRE/1 – 2 slabs and 3 thin sections Sample RSA/PRE/5 – 2 slabs and 2 thin sections; see description Lesotho Mothae Sample LES/MOT/1 – 1 slab and 2 thin sections Canada (see POSTER for comparable textures ) Renard 2 Pipe, Quebec Sample CAN/QE/REN2/1 - 1 slab, 2 thin sections; see mini-description Sample CAN/QE/REN2/4 - 1 slab, 1 thin section ; see mini-description 5034 Pipe, Gahcho Kue, NWT Sample CAN/NT/5034/47 – 2 slabs and 2 thin sections Tuzo Pipe, Gahcho Kue, NWT Hole Depth MPV-02-111C 163.8m – 1 slab and 2 thin sections MPV-02-111C 202m – 1 slab and 2 thin sections Hearne North, Gahcho Kue, NWT - HK to KPK Textural Transition. Hole Depth Texture – 1 slab and 2 thin sections MPV-99-16C 146.5m KPK-HK – 1 slab and 2 thin sections MPV-99-16C 163m HK-KPK – 1 slab and 2 thin sections MPV-99-16C 203.6m HK 1
KIMBERLEY-TYPE PYROCLASTIC KIMBERLITES (KPK) - CONSTITUENTS TO LOOK FOR – List their features (see POSTER for comparable textures ) Xenoliths – type, alteration of xenolith, alteration of adjacent kimberlite. Macrocrysts/mantle-derived xenocrysts Olivine Phenocrysts Olivine Groundmass Perovskite Spinel Carbonate – primary Serpentine – (and deuteric replacement) Textures Magmaclasts comprise pre-existing minerals/xenoliths with thin selvages Interclast (inter-magmaclast) matrix: serpentine and microlitic minerals 2
DRAW A PICTURE OF KPK 3
KIMBERLEY-TYPE PYROCLASTIC KIMBERLITE ILLUSTRATIONS FROM MITCHELL 1997 A Petrographic Atlas Xenoliths: Figs. 43, Macrocrysts Olivine: Fig. 43, Phenocrysts Olivine: Fig. 43, 51-54, Texture: Magmaclasts: Figs. 44, Interclast Matrix: serpentine and microlitic minerals: Figs. 59-57 USEFUL REFERENCES – KIMBERLEY-TYPE PYROCLASTIC KIMBERLITES (formerly TUFFISITIC KIMBERLITES) CLEMENT C.R. and REID A.M. 1989: The origin of kimberlite pipes: an interpretation based on a synthesis of geological features displayed by southern African occurrences. Geol. Soc. Australia Spec. Pub. 14, 1, pp. 632-646. HETMAN, C.M., 2008. Tuffisitic kimberlite: a Canadian perspective on a distinctive textural variety of kimberlite. Proceedings of the Kimberlite Emplacement Workshop, Saskatoon, Canada, September 2006. Journal of Volcanology and Geothermal Research, 174, 57- 67. HETMAN C.M., SCOTT SMITH B.H., PAUL J.L. and WINTER F.W. 2003. Geology of the Gahcho Kué kimberlite pipes, NWT, Canada: Root to diatreme transition zones. Eighth International Kimberlite Conference, Extended Abstract 352, 5 pages. HETMAN C.M., SCOTT SMITH B.H., PAUL J.L. and WINTER F.W. 2004. Geology of the Gahcho Kué kimberlite pipes, NWT, Canada: Root to diatreme magmatic transition zones. Proceedings of the Eighth International Kimberlite Conference. Lithos 76, 51-74. Reviewed. MASUN K.M. and SCOTT SMITH B.H. 2008. The Pimenta Bueno kimberlite field, Rondonia, Brazil: Tuffisitic kimberlite and transitional textures. Proceedings of the 2006 Kimberlite Emplacement Workshop, Saskatoon, Canada. Journal of Volcanology and Geothermal Research, 174, p. 81-89. Reviewed. SKINNER, E.M.W. 2008. Proceedings of the Kimberlite Emplacement Workshop, Saskatoon, Canada, September 2006. Journal of Volcanology and Geothermal Research, 174, 40- 48. 4
Sample #: BOT/LET/1 Locality: Lethlakane Mine, Botswana Rock Type: Xenolith-rich Kimberley-type Pyroclastic Kimberlite (xKPK) Macroscopic characteristics: The sample is dark green-brown in colour, competent and massive. It displays a magmaclastic texture and is composed of abundant magmaclasts and altered country rock xenoliths set in a dark green-brown interclast matrix. The numerous magmaclasts consist of dark brown totally altered, fine- to medium-grained (f-m) olivine macrocrysts and phenocrysts with thin ‘frosty’ magmatic selvages. The abundant (> 15 % vol) pale coloured, highly altered basaltic country rock xenoliths are subangular to irregular in shape. Some have alteration halos, some have thin magmatic selvages. Ilmenite and kelyphitised garnet xenocrysts are common. Microscopic characteristics: The magmaclastic texture of the rock is confirmed in thin section. The magmaclasts comprise abundant totally altered (serpentine) olivine macrocrysts and phenocrysts with thin partial or complete magmatic selvages that contain ubiquitous microlitic clinopyroxene. The texture varies from clast- to matrix-supported. Most of the altered basaltic xenoliths also have magmatic selvages. The primary groundmass mineralogy is difficult to determine, however spinel and coarse perovskite are present in some magmatic selvages and also in the interclast matrix. Some mica xenocrysts (derived from disaggregated xenoliths), and mantle-derived ilmenite, spinel and garnet xenocrysts are also present. The interclast matrix consists of serpentine and clusters of microlitic clinopyroxene. Discussion The sample contains common xenolithic material. The xenoliths are partly altered but less so than xenoliths in typical hypabyssal kimberlites. The thin selvages of very fine grained kimberlite on most of the olivine and xenoliths are typical of KPK-type magmaclasts. The selvages contain microlitic clinopyroxene. The interclast matrix is composed of cryptocrystalline serpentine which contains microlitic clinopyroxene. The magmaclasts, the nature of the interclast matrix with microlitic textures and the abundance of partly altered xenoliths are all characteristic features of the xenolith-rich Kimberley-type pyroclastic kimberlites (formerly termed ‘tuffisitic kimberlite breccias’). Other characteristic features include the absence of carbonate and the presence of the full olivine population and lack of sorting resulting a mega-and macroscopic massive appearance. Classification: The sample is a xenolith-rich olivine macrocrystic, spinel- and perovskite-bearing Kimberley-type pyroclastic kimberlite. 5
Sample: RSA/PRE/5 Locality: Premier Mine, so-called Grey Kimberlite Rock Type: Xenolith-rich Kimberley-type Pyroclastic Kimberlite (xKPK) Macroscopic characteristics: The sample is dark greenish-grey in colour, hard and competent. The rock appears to be massive but displays a magmaclastic texture. Abundant dark brown totally altered, fine- to medium- grained (f-m, few c) olivine macrocrysts and phenocrysts, The olivines have thin ‘frosty’ magmatic selvages and a few have thick-selvages showing that they are magmaclasts. A high proportion (> 15 % vol) of subangular to angular, varied altered granitic and mafic basement xenoliths is present. The majority of the xenoliths have either clearly magmatic or altered ‘frosty’ selvages. Ilmenite xenocrysts are common throughout the sample. Microscopic characteristics: The sample displays a distinctly magmaclastic texture in thin section and consists of abundant close-packed magmaclasts and country rock xenoliths set in a serpentine-dominated interclast matrix. The majority of magmaclasts are made up of totally altered (serpentine) olivine macrocrysts or phenocrysts that are surrounded by a thin partial or complete magmatic selvage, which in most cases contains microlitic clinopyroxene. The latter produces the frosty appearance in the polished slab. The magmaclasts with thicker magmatic selvages indicate comprise primary groundmass spinel, altered perovskite and phlogopite in a serpentine base. The diagnostic microlitic clinopyroxene is ubiquitous, occurring not only in the magmaclasts, but also as clusters within the serpentine interclast matrix. The country rock xenoliths are altered and most have at least a partial magmatic selvage which also contains microlitic clinopyroxene. Ilmenite and rare altered kelyphitised garnet xenocrysts are also observed. Discussion The sample contains common xenolithic material. The xenoliths are partly altered but less so than xenoliths in a typical hypabyssal kimberlites. The thin selvages of very fine grained kimberlite on most of the olivine and xenoliths are typical KPK-type magmaclasts. The selvages contain microlitic clinopyroxene. The interclast matrix is composed of cryptocrystalline serpentine which also contains microlitic clinopyroxene. The magmaclasts, the nature of the interclast matrix with microlitic textures and the abundance of partly altered xenoliths are all characteristic features of the so-called xenolith-rich Kimberley-type pyroclastic kimberlites (formerly tuffisitic kimberlite breccias). Other characteristic features include the absence of carbonate and the presence of the full olivine population and lack of sorting resulting a mega-and macroscopic massive appearance. Classification: The sample is a xenolith-rich olivine macrocrystic Kimberley-type pyroclastic kimberlite. 6
Recommend
More recommend
