Pre- and post-Katangan granitoids of the Greater Lufilian Arc - geology, geochemistry, geochronology and metallogenic significance

摘要

This document reports observations, findings and conclusions of the research project entitled “Pre- andudPost-Katangan Granitoids of the Greater Lufilian Arc - Geology, Geochemistry, Geochronology andudMetallogenic Significance”. The project, structured and supervised by Professor Laurence Robb, wasuddesigned to study granitoids that comprise the Greater Lufilian Arc. Its main aims were to define theudvarious granitoids, and study their role in Katangan orogenesis and mineralization. Main fieldwork wasudconcentrated in northwestern Zambia and northern Namibia.udThe Greater Lufilian Arc is a curvilinear belt of Neoproterozoic Katangan sediments that was deformedudduring the Pan African orogeny in Zambia and the Democratic Republic of Congo, and the westwardudextension of similar rock sequences into Botswana, Angola and Namibia. The mobile belt of the GreaterudLufilian Arc also comprises a dominantly Paleoproterozoic basement of deformed granitoids, and auddiverse suite of Pan-African granitoids that intrude the Katangan sequences.udA total of 1500 samples were collected in the field; 351 plutonic rocks were analysed. 157 chemicaludanalysis were compiled from various well-documented sources, to reach a total of 508 samples analysedudin the database. 38 new zircon U-Pb SHRIMP II and laser ablation ICP-MS ages were produced.udThe majority of intrusive rocks from the Greater Lufilian Arc that were analysed (60%) had midalkalineudcharacte. 33% were subalkaline and 7% were alkaline. Mafic rocks are closely associated to felsic rocksudin most domains of the Arc. Two thirds of the gabbroids were midalkaline, 1/6 alkaline and 1/6udsubalkaline. The average rock type distribution for the entire Lufilian Arc closely resembles that of theudHook Granite Batholith in Zambia.udA frequent field observation is the persistent clustering of small bodies of red-altered granitoids,udgabbroids, massive magnetite-hematite and quartz pods that are linked to ages around 550 and 750 Ma.udThe four-rock association is related to iron oxide-copper-gold (IOCG) mineralization, and seems to be audcharacteristic of continental extension anorogenic environments.udAnother recurrent feature observed in most outcrops of the study area is the presence of two or moreudcontrasting types of plutonic rocks, including mafic, ultramafic and alkaline plugs and dikes. Theudmultiplicity of rock types in a small area seems to be a characteristic of continental extension anorogenicudenvironments. Quartz pods, hydrothermally-emplaced iron oxide bodies and round-pebble hydrothermaludbreccias are features that occur often in and around IOCG systems throughout the Greater Lufilian Arc.udThe main granitoid periods of emplacement present in the study area of the Arc are listed on Table 1.udSeveral more restricted events occurred at 1700, 1600, 880 and 460 Ma.udTable 1 Main Granitoid Terranes in the Greater Lufilian ArcudAgeud(Ma)udRock types Location Environment ofudEmplacementudNotesud550ud±50udGranite, alkali granite,udquartzmonzonite,udsyenite, gabbroidsudOtjiwarongo, central Namibia,udKaokoland, Damaran intrusivesud(Namibia), Hook Granite, NWudZambia (Zambia)udContinentaludepeirogenic upliftudThe period may beudbroken into 3 discreteudevents.ud750ud±50udGranite, alkali granite,udsyenite and gabbroidsudwith felsic and maficudvolcanicsudCopperbelt, Kalengwa-udKasempa, NW Zambiaud(Zambia); Khorixas Inlier andudSummas Mountains (Namibia)udRift-related andudcontinentaludepeirogenic uplilft.udIntrude Roan andudNguba Lithologies;udoverlain byudKundelungu andudequivalent sediments.ud1100ud±50udGranitoids and felsic toudmafic volcanicsudSouth of the Copperbelt, West ofudLusaka (Zambia); aroundudOmitiomire, Kaokoland and theudWitvlei area (Namibia)udContinental riftrelatedudenvironmentsudSurrounds KapvaaludCraton fromudNamaqualand toudIrumide Belt in Zambiaud1900ud±100udFoliated alkali granie,udquartzmonzonite andudgraniteudCopperbelt basement, Mkushi-udSerenje, NW Zambia, Domesudregion (Zambia); Kaokoland,udcentral Namibia, KamanjabudBatholith, Grootfontein Inlierud(Namibia)udNot well defined;udprobably formed inudan anorogenicudcontinentaludextensionudenvironmentudPeriod can be brokenudinto 4 discrete eventsududThe Zambian Lufilian Arc and Damara region of Namibia behaved as independent entities from 2200 toud2000 Ma. They also behaved significantly different from 1400 to 850 Ma. Geological history of the twoudmain portions of the Greater Lufilian Arc is consistent from circa 800 Ma to the present, and especiallyudduring the last 600 million years.udMost areas studied in the Arc show polycyclic geological histories. Repeated anorogenic intrusive eventsudare a common denominator. Prolonged crustal histories have resulted in superimposition of events.udGranitoid rock suites with closely matching chemistry and macroscopic features have been found to formudtwo or three times in the same region, with up to a thousand million years of age difference. Theseudfeatures preclude lithological or detailed geochemical correlation of plutonic rocks.udAt least ten clusters of ring complexes were identified in the Arc. Clustering of multiple anorogenic ringudcomplex intrusions can form batholithic size bodies. Clusters are made by amalgamation of multiple ringudcomplexes of varying chemical composition and size. Most of their rocks are midalkaline. Volcanic andudplutonic rocks of roughly the same composition occur together. Total duration of ring complex clusterudcycles averages 110 Ma, and their plan view geometry is roughly that of an isosceles triangle.udInformation currently available on geophysics, geochronology, rock distribution and geochemistry fromudthe Hook Granite Batholith (Zambia) fit quite well with an intracontinental, anorogenic, ring complexudcluster origin. The Nchanga Granite (Zambia) has all the characteristics of an anorogenic granite ringudcomplex, and might have contributed to the origin of copper in its environs. Several sources of evidenceudindicate that the Kamanjab Batholith (Namibia) is an anorogenic cluster of ring complexes. Volcanic andudplutonic rocks of similar composition make the batholith. Geological history for the Khorixas Inlier and theudKamanjab Batholith are significantly different.udComplete Wilson cycles were not identified in the study areas of the Greater Lufilian Arc. The dominantudmagmatic process, as evidenced by the volume of extruded rock, is anorogenic continental epeirogenicuduplift, closely-followed in time by a rift-related granitoid emplacement. Coalescing and overprintingudaulacogens seem to be the main geological event in the Arc.udIncipient migmatitization and alteration of Paleoproterozoic rocks modified their chemistry to a pointudwhere their environment of emplacement cannot be identified by traditional geochemical means.udThe anomalous thorium content in some granitoids of the Greater Lufilian Arc induced and maintainedudlong-lived, large convective cells of hydrothermal fluid flow.udE-W-trending regional fracture systems, that run parallel to the elongation of the Arc, play an importantudrole in the emplacement of magmatism and IOCG mineralization. Those structures are generally paralleludto the main Lufilian Arc trend, and could have been normal syn-rift faults reactivated multiple timesudduring geological history.udAt least eight discrete periods of mineralization were identified in the Greater Lufilian Arc. There is audwide-spread series of midalkaline intrusions emplaced around 750 Ma that produces a variety of mineraluddeposits. Another event took place around 540±40 Ma. Five less well defined events occurred at ~1970,ud~1930, ~1866, 1097-1059 and ~460 Ma. The dominant deposit type is iron oxide-copper-goldudmineralization, but other types of mineral deposits are present in the Arc. At least two distinct events ofuddisseminated copper mineralization associated to midalkaline granitoid intrusives were identified in theudKamanjab Batholith; the first took place around 1975 Ma and the second around 1928 Ma.udThe main IOCG events that have been identified in the Greater Lufilian Arc took place during eight timeudperiods. The rocks of many IOCG deposits and prospects in the Arc are pristine. There is no significantuddeformaton involved. Hydrothermal brecciation and other mineralization features are un-deformed.udThree discrete time periods show IOCG mineralization in close temporal spatial association withudsedimentary-hosted copper deposits. The first took place around Witvlei (Namibia) from 1108 to 1059udMa. The second and third ocurred in the basement to the Zambian Copperbelt from 882 to 725 Ma andudfrom 607 to 500 Ma. This idea may generate a new concept for the origin of sedimentary-hosted copperudand cobalt deposits.