Northeast Angola Diamonds - Catoca, Camatue, Camafuca-Camazambo, Camatchia-Camagico
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The provinces of Lunda Norte and Lunda Sul in the northeast of Angola hosts the bulk of the country's known kimberlites and diamonds reserves. The main diamondiferous kimberlites fields are at Catoca, Camutue, Camafuca-Camazambo and Camatchia-Camagico. These fields, which lie within the Archaean Kasai-Angola craton, contain a total of more than 70 kimberlite bodies distributed over a NNE trending interval of around 150 km and a width of up to 40 km, controlled by the NNE trending Lucapa regional fault zone.
The geology of Angola has been divided into three main stages (De Carvalho et aI., 2000; Guiraud et aI., 2005), namely:
• An Archaean orogenic stage, represented in the Central, Cuango and Lunda shields, most of which are composed of gabbro, norite and layered gneiss-migmatite and charnockitic complexes, which constitute the Angolan basement. The Central Shield is in the central part of the country, while the Cuango and Lunda shields are in the northeast.
• Three main Proterozoic cycles, i). the Eburnian orogeny during the Palaeoproterozoic, characterised by complex magmatic arc volcano-sedimentary groups, and reworked gneisses and migmatites, granites and syenites; ii). the Mesoproterozoic Kibaran cycle, which was related to extensional events that occurred on the border of Congo Craton and that later generated clastic-carbonatic sequences and local basic magmatism; and iii). the Neoproterozoic Pan-African event which was preceded by the extensional breakup of Rodinia and was associated with the development of Gondwana and led to compression to the generation of fold belts and granitic intrusions. The activation of zones of lithospheric weakness generated during these events, especially major fault zones, influenced the subsequent Phanerozoic break-up of Gondwana.
• The deposition of Phanerozoic sedimentary sequences resting unconformably on previously eroded surfaces (Pereira et aI., 2003). The break-up of Gondwana, from ~190 to 60 Ma, during the Jurassic to Cretaceous (e.g., Jelsma et aI., 2004), resulted in the development in Angola of extensional basins associated with deep fault systems, which, in turn, facilitated the emplacement of alkaline, carbonatitic, and kimberlitic magmas (Pereira et aI., 2003). The Lower Cretaceous regional extension controlled the development of deep faults and grabens with both trends NE-SW and NW-SE. One of these, the NE-SW trending Lucapa structure, has been a belt of recurring tectonic weakness since the Paleoproterozoic (Uelsma et aI., 2009), and exerted a strong control on the localisation magmatism.
The NE-SW trending Lucapa structural corridor is generally ~120 km wide in northeastern Angola, but broadens to the SW. Most of the diamondiferous kimberlites in Angola are located within the confines of the Lucapa structure in northeastern Angola, including those at Catoca, Camutue, Camafuca-Camazambo and Camatchia-Camagico, as well as the Luxinga cluster described below. Most of these have been variously dated between ~145 and 113 Ma (Lower Cretaceous; Eley et aI., 2008), although a separate population between ~252 and 216 Ma (median age of 235 Ma; Upper Triassic) has been inferred towards the SW (Jelsma et aI., 2012). The southwestern section of this structural corridor in central and southwestern Angola contains important occurrences of undersaturated alkaline rocks and carbonatites (Reis, 1972), as well as some minor kimberlite fields in southwestern Angola (Egorov et aI., 2007).
A large linear cluster of kimberlites, downstream from the Luxinga cluster along the northwestern Kwango River valley of north-central Angola, and along the adjacent DRC border appear to be related to the secondary cross-cutting NW-SE structural trend mentioned above.
Strike-slip and shear fault systems in northeastern Angola might have led to decompression (local extension) and compression, influencing the distribution of igneous activity within the Lucapa structure and may have had different expressions within the Angolan Shield and Kasai craton.
Continental sediments of the Karoo Supergroup unconformably overlie the Precambrian basement, deposited from the Late Carboniferous, commencing with the Lutoe Group tillites and clastic sedimentary rocks, followed by the Cassange Group sandstone-mudstone-shale sequence and Jurassic continental basalts. Following, and during an erosional hiatus, the Aptian to Albian (late Lower Cretaceous) Intercalar Group continental sandstones with interbedded mudstones and conglomerates, were deposited. These are overlain by the Cretaceous Kwango Group, including the 40 to 60 m thick Calonda Formation, the oldest sedimentary unit in the Lucapa structure. It comprises a fining upward lithostratigraphic unit that was formed by torrential deposits gradually changing to lagoonal facies and concluding with low-energy deposits associated with aeolian episodes. The Calonda Formation is the oldest sedimentary unit in Angola that contains detrital diamond and kimberlite clasts (Pereira et aI., 2003; and references therein). In most cases the basal conglomerate, which comprises a coarse fanglomerate/conglomerate, with a sandy-clay matrix, produces diamonds. It is reported to be Albian to Cenomanian (late Lower to early Upper Cretaceous; 113 to 93.9 Ma) in age, based on fish macrofossils, palynomorphs and tectonostratigraphic studies (Pereira et aI., 2003). The Kwango Group is unconformably overlain, after a hiatus from 93.6 to 55.8 Ma, by the Paleogene to Neogene Kalahari Group (Pereira et aI., 2003), which masks much of the older sequence whose exposure is largely restricted to the river valleys of the north-south to NNW-SSE aligned drainage system. Within the basal sections of the Kahalari Group, the Grés Polimorfos, there is occasionally a well-rounded gravel made up of clasts of quartz and quartzite with angular chalcedony
that contains scarce diamonds (Pereira et aI., 2003). The overlying Upper Series or Ochre Sands Formation of the Kalahari Group, largely comprise sands covering the former plateau gravels, sometimes several metres in thickness. These sands occur between altitudes of 800 to 900 masl, and are of aeolian origin, resulting from the redistribution of Kalahari sands in an arid climate.
Tectonism took place in the Holocene, producing a tilting to the west. The active slope erosion associated with this tilt, allowed drainage channels to cut into the easily friable Calonda and Kalahari deposits, and remobilised and deposited their materials directly over the sloping crystalline basement or in various terraces levels. Frequently, the ore grade diminishes, due to the dilution by large volumes of barren Kalahari sands and gravels. However, if the erosion cuts the basal conglomerate of the diamondiferous Calonda Formation, a fine deposit with rich diamond ore grades forms on top of the basement downslope, as eluvium, and is concentrated in alluvium on terraces in the associated river valley (Pereira et aI., 2003).
Catoca (#Location: 9° 24' 20"S, 20° 18' 8"E)
The Catoca kimberlite pipe and mine, is located 35 km NNE of the city of Saurimo in Lunda Norte Province, NE of Angola, and has been the main hardrock producer of diamonds in Angola.
The Catoca pipe is a 900 m diameter, weakly eroded diatreme which covers a surface area of 65.7 ha. The intruded country rocks comprise Archaean granite-gneisses and crystalline schists. The pipe is overlain by Palaeogene to Neogene sands of the Kalahari Formation, ranging in thickness from the several to 130 m. The Catoca pipe includes complete crater, diatreme and hypabyssal facies kimberlite rocks.
Crater facies volcaniclastic rocks comprise heterolithic breccias, psephitic and psammitic tuffs, alevrites and alevrolites with layered textures and variable amount of kimberlitic material.
Diatreme facies are represented by eruptive kimberlite breccias with brecciated, partly autolithic, with massive groundmass textures. The structure of the rocks is porphyritic and clastoporphyritic. The groundmass is composed by serpentine-carbonate with minor amounts of altered phlogophite, rare perovskite crystals and opaque oxide minerals.
The porphyritic kimberlite with massive textures and microporphyritic structures is described as a hypabyssal facies kimberlite. The porphyritic crystals are pseudomorphed olivine and phlogophite. The groundmass is composed by anhedral and euhedral olivine crystals, which are pseudomorphed to serpentine and a serpentine-carbonate mixture. Minor amounts of the phlogophite, spinel, ilmenite, perovskite and apatite are also present.
The chemical composition of the kimberlitic rocks from the Catoca pipe show significant compositional variation between the different facies.
The diamonds from the Catoca pipe are dominantly octahedral, although transitional and dodecahedral habits and flat faced octahedrons are also found. The primary kimberlite minerals are garnet, ilmenite, chrome-diopside and phlogophite.
The Catoca kimberlite pie has been dated at between 114.3±1.6 and 123.1±3.5 Ma by 206Pb-238U method (Robles-Cruz, 2012).
The estimated reserve of the Catoca kimberlite in 2002 is reported to be 271 Mt @ 70 carats per hundred tonnes for 189.3 million carats of diamonds.
The Catoca mine is operated by Sociedade Miniera de Catoca Ltda. (SMC), a joint venture between Empresa Diamantes de Angola - Endiama (32.8%), Russia's Almazy Rossii-Sakha Joint Stock Company - ALROSA (32.8%), Brazil's Odebrecht Mining Services Inc. (18.4%), and Israel's Lev Leviev (16%).
The Catoca pipe is the largest producer in Angola with an output of approximately 2.8 million carats in 2002. Total Angolan production for the same year was 5.02 million carats of diamond, much of the balance coming from the extensive placer diamond fields in the same province.
Camatue (#Location: 8° 25' 40"S, 20° 48' 40"E)
The Camatue diamondiferous kimberlite pipes are located ~10 km east of the town of Lucapa in northeastern Lunde Norte Province, and ~125 km NE of Catoca. A group of nine kimberlite pipes are known in the Camutue area. Five of these pipes have been shown to be diamondiferous and two have been mined historically to some extent, e.g., the Camatue-West kimberlite, which has an area of 19.8 ha, and was discovered in 1958 and mined between 1961 and 1974, resulting in the recovery of 33 673 carats at an average mined grade of 9 carats per hundred tonnes (cpht).
They occur in a tributary in the catchment of the Luachimo River which is parallel to and east of the Chicapa river. Placer diamond workings are developed downstream at Uari, Lucapa and Mafutu up to 30 km to the NNW.
Other Kimberlite and Placer Deposits - Chicapa River Valley (#Location: Camatchia - 8° 56' 47"S, 20° 28' 34"E; Camafuca - 8° 35' 16"S, 20° 33' 14"E)
Placer and palaeoplacer deposits include Chiri, Tchiuzo, Lapi, Canvuri, Luo-Camatchia-Camagico, Camafuca-Camazambo, Calonda and Yetwene, distributed in the following order, northward from Catoca, over a distance of 150 km within the valley and immediate tributaries of the Chicapa River. Other placer workings are located in neighbouring river systems progressively to the NE at Luarica and Luana, which are 25 and 45 km respectively from Camatue.
Clusters of kimberlites are also known along the Chicapa River valley, below and adjacent to the placer accumulations, including,
• Camatchia-Camagico, which is 55 km NNE of Catoca, where a mine was developed on two kimberlite pipes, the larger of which covers an area of 22.5 ha, with combined reserves of 80 Mct (Read and Janse, 2009);
• Camafuca-Camazombo [Kamazambo], which is ~100 km north of Catoca. Mining commenced in 2007 as a dredging operation to recover 13 Mct in five years, contained within fluvial mud and sand grading down into weathered kimberlite (Read and Janse, 2009). The kimberlite covered and area of 150 ha.
• Tchiuzo, which is 15 km north of the Catoca kimberlite.
Kimberlite and Placer Deposits - Kwango River Valley (#Location: Alto Cuilo - 9° 55' 46"S, 19° 22' 5"E)
Very extensive placer workings are distributed over an interval of 180 km along the SE-NW flowing Kwango (Cuango) river, at Alto Cuilo, Lulo, Tazua and Luzamba.
At Alto Cuilo, which is ~100 km WSW of Catoca, the large Luxinga cluster of kimberlites is developed in the headwaters of the Kwango River, and also lie within the Lucapa structure. These have been dated at 145 to 113 Ma and are covered by sandstone, litharenite and arkose of the Calonda Formation, Kalahari Group and Quaternary cover. These kimberlites exhibit crater and diatreme facies rocks and are composed of volcanic (mainly pyroclastic) rocks and re-sedimented volcaniclastic kimberlites.
A NNW-SSE trending clusters of kimberlites are found over a 300 km interval, bracketing this same NNW trending Kwango River valley towards and along the DRC border.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 12/2/2016
This description is a summary from published sources, the chief of which are listed below.
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