Pre- and post-Katangan granitoids of the Greater Lufilian Arc - geology, geochemistry, geochronology and metallogenic significance
Sanz, Alberto Lobo-Guerrero
This document reports observations, findings and conclusions of the research project entitled “Pre- and Post-Katangan Granitoids of the Greater Lufilian Arc - Geology, Geochemistry, Geochronology and Metallogenic Significance”. The project, structured and supervised by Professor Laurence Robb, was designed to study granitoids that comprise the Greater Lufilian Arc. Its main aims were to define the various granitoids, and study their role in Katangan orogenesis and mineralization. Main fieldwork was concentrated in northwestern Zambia and northern Namibia. The Greater Lufilian Arc is a curvilinear belt of Neoproterozoic Katangan sediments that was deformed during the Pan African orogeny in Zambia and the Democratic Republic of Congo, and the westward extension of similar rock sequences into Botswana, Angola and Namibia. The mobile belt of the Greater Lufilian Arc also comprises a dominantly Paleoproterozoic basement of deformed granitoids, and a diverse suite of Pan-African granitoids that intrude the Katangan sequences. A total of 1500 samples were collected in the field; 351 plutonic rocks were analysed. 157 chemical analysis were compiled from various well-documented sources, to reach a total of 508 samples analysed in the database. 38 new zircon U-Pb SHRIMP II and laser ablation ICP-MS ages were produced. The majority of intrusive rocks from the Greater Lufilian Arc that were analysed (60%) had midalkaline characte. 33% were subalkaline and 7% were alkaline. Mafic rocks are closely associated to felsic rocks in most domains of the Arc. Two thirds of the gabbroids were midalkaline, 1/6 alkaline and 1/6 subalkaline. The average rock type distribution for the entire Lufilian Arc closely resembles that of the Hook Granite Batholith in Zambia. A frequent field observation is the persistent clustering of small bodies of red-altered granitoids, gabbroids, massive magnetite-hematite and quartz pods that are linked to ages around 550 and 750 Ma. The four-rock association is related to iron oxide-copper-gold (IOCG) mineralization, and seems to be a characteristic of continental extension anorogenic environments. Another recurrent feature observed in most outcrops of the study area is the presence of two or more contrasting types of plutonic rocks, including mafic, ultramafic and alkaline plugs and dikes. The multiplicity of rock types in a small area seems to be a characteristic of continental extension anorogenic environments. Quartz pods, hydrothermally-emplaced iron oxide bodies and round-pebble hydrothermal breccias are features that occur often in and around IOCG systems throughout the Greater Lufilian Arc. The main granitoid periods of emplacement present in the study area of the Arc are listed on Table 1. Several more restricted events occurred at 1700, 1600, 880 and 460 Ma. Table 1 Main Granitoid Terranes in the Greater Lufilian Arc Age (Ma) Rock types Location Environment of Emplacement Notes 550 ±50 Granite, alkali granite, quartzmonzonite, syenite, gabbroids Otjiwarongo, central Namibia, Kaokoland, Damaran intrusives (Namibia), Hook Granite, NW Zambia (Zambia) Continental epeirogenic uplift The period may be broken into 3 discrete events. 750 ±50 Granite, alkali granite, syenite and gabbroids with felsic and mafic volcanics Copperbelt, Kalengwa- Kasempa, NW Zambia (Zambia); Khorixas Inlier and Summas Mountains (Namibia) Rift-related and continental epeirogenic uplilft. Intrude Roan and Nguba Lithologies; overlain by Kundelungu and equivalent sediments. 1100 ±50 Granitoids and felsic to mafic volcanics South of the Copperbelt, West of Lusaka (Zambia); around Omitiomire, Kaokoland and the Witvlei area (Namibia) Continental riftrelated environments Surrounds Kapvaal Craton from Namaqualand to Irumide Belt in Zambia 1900 ±100 Foliated alkali granie, quartzmonzonite and granite Copperbelt basement, Mkushi- Serenje, NW Zambia, Domes region (Zambia); Kaokoland, central Namibia, Kamanjab Batholith, Grootfontein Inlier (Namibia) Not well defined; probably formed in an anorogenic continental extension environment Period can be broken into 4 discrete events The Zambian Lufilian Arc and Damara region of Namibia behaved as independent entities from 2200 to 2000 Ma. They also behaved significantly different from 1400 to 850 Ma. Geological history of the two main portions of the Greater Lufilian Arc is consistent from circa 800 Ma to the present, and especially during the last 600 million years. Most areas studied in the Arc show polycyclic geological histories. Repeated anorogenic intrusive events are a common denominator. Prolonged crustal histories have resulted in superimposition of events. Granitoid rock suites with closely matching chemistry and macroscopic features have been found to form two or three times in the same region, with up to a thousand million years of age difference. These features preclude lithological or detailed geochemical correlation of plutonic rocks. At least ten clusters of ring complexes were identified in the Arc. Clustering of multiple anorogenic ring complex intrusions can form batholithic size bodies. Clusters are made by amalgamation of multiple ring complexes of varying chemical composition and size. Most of their rocks are midalkaline. Volcanic and plutonic rocks of roughly the same composition occur together. Total duration of ring complex cluster cycles averages 110 Ma, and their plan view geometry is roughly that of an isosceles triangle. Information currently available on geophysics, geochronology, rock distribution and geochemistry from the Hook Granite Batholith (Zambia) fit quite well with an intracontinental, anorogenic, ring complex cluster origin. The Nchanga Granite (Zambia) has all the characteristics of an anorogenic granite ring complex, and might have contributed to the origin of copper in its environs. Several sources of evidence indicate that the Kamanjab Batholith (Namibia) is an anorogenic cluster of ring complexes. Volcanic and plutonic rocks of similar composition make the batholith. Geological history for the Khorixas Inlier and the Kamanjab Batholith are significantly different. Complete Wilson cycles were not identified in the study areas of the Greater Lufilian Arc. The dominant magmatic process, as evidenced by the volume of extruded rock, is anorogenic continental epeirogenic uplift, closely-followed in time by a rift-related granitoid emplacement. Coalescing and overprinting aulacogens seem to be the main geological event in the Arc. Incipient migmatitization and alteration of Paleoproterozoic rocks modified their chemistry to a point where their environment of emplacement cannot be identified by traditional geochemical means. The anomalous thorium content in some granitoids of the Greater Lufilian Arc induced and maintained long-lived, large convective cells of hydrothermal fluid flow. E-W-trending regional fracture systems, that run parallel to the elongation of the Arc, play an important role in the emplacement of magmatism and IOCG mineralization. Those structures are generally parallel to the main Lufilian Arc trend, and could have been normal syn-rift faults reactivated multiple times during geological history. At least eight discrete periods of mineralization were identified in the Greater Lufilian Arc. There is a wide-spread series of midalkaline intrusions emplaced around 750 Ma that produces a variety of mineral deposits. Another event took place around 540±40 Ma. Five less well defined events occurred at ~1970, ~1930, ~1866, 1097-1059 and ~460 Ma. The dominant deposit type is iron oxide-copper-gold mineralization, but other types of mineral deposits are present in the Arc. At least two distinct events of disseminated copper mineralization associated to midalkaline granitoid intrusives were identified in the Kamanjab Batholith; the first took place around 1975 Ma and the second around 1928 Ma. The main IOCG events that have been identified in the Greater Lufilian Arc took place during eight time periods. The rocks of many IOCG deposits and prospects in the Arc are pristine. There is no significant deformaton involved. Hydrothermal brecciation and other mineralization features are un-deformed. Three discrete time periods show IOCG mineralization in close temporal spatial association with sedimentary-hosted copper deposits. The first took place around Witvlei (Namibia) from 1108 to 1059 Ma. The second and third ocurred in the basement to the Zambian Copperbelt from 882 to 725 Ma and from 607 to 500 Ma. This idea may generate a new concept for the origin of sedimentary-hosted copper and cobalt deposits.