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Murowa, Sese |
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Zimbabwe |
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Diamonds
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Super Porphyry Cu and Au
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The Murowa and Sese diamond pipes are located 35 km south and 60 km ESE respectively from the township of Zvishavane in South Central Zimbabwe, 350 km SSW of Harare, 190 km ESE of Bulawayo and 200 km from the South African and Botswana borders with Zimbabwe.
The Cambrian-aged Murowa and Sese (50 km to the east) kimberlites (dated at 538±11 Ma), are hosted by a granite-greenstone terrane of the Archaean Zimbabwe Craton, within 10 to 50 km of the transition to the ENE trending Limpopo Mobile Belt (LMB) to the south that separates the Zimbabwe and Kaapvaal cratons. The Limpopo Mobile Belt contains two major zones of shearing and transcurrent dislocation that separate marginal granulite zones on the Archaean granite-greenstone cratons from a central zone which consists of complexly infolded Palaeoproterozoic cover rocks and reworked basement.
Other kimberlites within the region include Chingwisi, 70 km to the south within the LMB, River Ranch immediately to the north of the South Africa-Zimbabwe belt on the southern margin of the LMB, and Venetia 200 km SSW of Murowa in South Africa, also within the transition from the LMB to the Kaapvaal craton. A belt of small kimberlites are found over a 70 km interval from the Chiadzwa district of Marange to the Chimanimani Mountains on the Mozambique border, some 320 km to the NE of Murowa. These kimberlites are responsible for a series of high grade alluvial diamond accumulations at Chiadzwa, Marange and Mbada.
The Zimbabwe craton in this area is represented by the Chibi and Zimbabwe granite batholiths, which together form the Chilimanzi Suite of granites dated at ~2.6 Ga. These intrusives were emplaced into the older 3.5 Ga Archaean tonalitic gneiss complex and the 2.9 to 2.5 Ga Busha Greenstone Belt composed of schists, banded iron formations and ultramafics. Late dykes of the 1.8 Ga Mashonaland dolerites intrudes the basement to the west of Murowa. Although banded iron formation and micas schist are seen near Sese, all conacts with both Murowa and Sese kimberlites are of granite, mostly coarse- to medium-grained granitoid, leucocratic, hypidiomorphic inequigranular, slighty porphyritic in places and of adamellite to granite in composition.
Both the Murowa and Sese clusters contain a number of small complex pipes (currently exposed in their root zones) with multi-lobed outlines, near vertical dykes and shallowly dipping sills. Textural varieties of kimberlite at both Murowa and Sese include hypabyssal macrocrystic kimberlite (HMK), HMK breccia (HKMB), with minor tuffisitic kimberlite breccia and lithic tuffisitic kimberlite breccia. The dykes and sills are composed of HMK only. The pipes, which are elongated are composed of both HMK and the breccia types, with mica-rich HMK forming the matrix to HKMB. To types of HMK dyke are recognised, an early mica-rich and carbonate-poor which only occurs within the pipes and cuts the breccias, and a late mica-poor and carbonate-rich variety which cuts all kimberlite types and the country rock. HKMB occurs as irregular central masses within several of the Murowa and Sese pipes, containing irregular angular to sub-rounded xenoliths of fenitised granite (syenite) country-rock, autoliths of early-formed mica-rich HMK and occasional well-rounded mantle peridotites.
Adjacent to the pipes and dykes, the country rock granites are intensely fenitised with the development of green metasomatite vein networks which cut and disrupt the granite, imparting an in situ a brecciated appearance. The metasomatite chemistry is intermediate between granite and kimberlite. Fenitisation resulted in partial or complete replacement of quartz by calcite and chlorite and replacement of biotite by pyroxenes and amphiboles. The degree of fenitisation decreases outwards.
The diamonds from these pipes are mostly octahedral, moderately rich in nitrogen with moderate to high aggregation, and contain mainly dunite-harzburgite mineral inclusions. Dunite xenoliths predominate over harzburgite within the kimberlites, while eclogitic xenoliths are extremely rare.
The Murowa diamond mine produced 178 000 carats during 2010 (Rio Tinto, 2011).
The mining reserve within three kimberlite pipes as of December 2010 (Rio Tinto, 2011) were 16.5 Mt @ 0.9 carats per tonne plus
a mineral resource of 10 Mt @ 1 carat per tonne.
This summary was largely based on Smith (2004) and Smith et al. (2009).
The most recent source geological information used to prepare this decription was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Smith C B, Pearson D G, Bulanova G P, Beard A D, Carlson R W, Wittig N, Sims K, Chimuka L and Muchemwa E, 2009 - Extremely depleted lithospheric mantle and diamonds beneath the southern Zimbabwe Craton: in Lithos v.112 pp. 1120–1132
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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