Pre- and post-Katangan granitoids of the Greater Lufilian Arc - geology, geochemistry, geochronology and metallogenic significance
Date
2008-06-27T10:42:37Z
Authors
Sanz, Alberto Lobo-Guerrero
Journal Title
Journal ISSN
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Abstract
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.