Stratiform copper and zinc mineralization in the Cretaceous of Angola
Stratiform
Copper and Zinc mineralization
in the Cretaceous of Angola
Introduction
The coppcr
occurrcnces in the Cretaceous of Angola have long been
known. Copper was mincd near Bengucla during the late
19th century, and the deposit of Cachociras de Binga
still awaits development. A recently discovered zinc
occurrence some 60 km south of Novo Redondo may prove to
be of economic interest. The Angolan occurrences form
part of an extensivc metallogenic province that contains
mostly small, hut occasionally economically feasible
deposits, of which the best known are Merija (Cu-Pb) and
Rou-Setlam (Pb-Zn) in Morocco, Kroussou (Pb) in Gabon,
and Aïn-Sefra (Cu) in Algeria. Pb (and Cu)
minerali/ation has also been found in the Lowcr
Cretaceous of the Araripe basin in Brazil, (Farina,
1974).
As pointed out
by Caïa (1976), the setting of all these Cretaceous
mineral occurrenees is very sitnilar. The mineralization
is associated with the transitionl continental-marine
phase at the start of a transgrcssive marine sequcnce
which developed in subsiding epicontinental basins. The
ubiquitous presence of copper in all Cretaceous basins
bordering the South Atlantic is attributed to their
common sedimentary-tectonic development related to
Continental drift. Though a terrigenous source is
inferrcd for the heavy mctals, these metals were dumped
in the sedimentary basin with the interstitial waters of
the detritic scquence and concentrated only after a
complex migrational history. Diagenetic proccsses in
peculiar local sedimcntary environments are the
principal factors in the final concentration of the
metals to ore-grade deposits.
The Cretaceous
deposits have many charactcristics in common with an
important class of mainly cupriferous deposits in other
shales and sandstones of marine association, among which
the Zambian Copperbelt and the European Kupferschiefer
are the outstanding examples.
Stratigraphy
The
Stratigraphy of the Cretaceous is well known as a result
of extensive driüing in the search for petroleum (see
Brognon and Verrier, 1966). The Cuanza basin, which
stretches along the coast from Luanda to just north of
Novo Redondo (Fig. 1), has the most complete
stratigraphic succession (Fig. 2). South of Novo Redondo
a string of smaller but stratigraphically similar basins
is found. Three broad stratigraphic units can be
distinguished. a basal terrigenous unit of Lower Aptian
age, unconformably overlying a Precambrian Basement, a
middle evaporitic unit of Aptian-Albian age, and the
overlying predominantly marine succession, which extends
up to the Miocene.
Lower Cuvo
The reddish
continental Iwds of Lower Aptian age which lic at the
base of the Cretaceous were named the Lower Cuvo
Formation by Brognon and Verrier (1966). The formation
varies from O to over 100 m in thickness as it fills
depressions of the underlying basement. Deeply incised
paleovalleys, which indicate a pre-Aptian runoff toward
the west, contain boulders of more than l m in diameter,
directlv overlying the basement gneisses, followed by
conglomerates and cross-bedded sandstones in an
upward-fining secuence. These immature terrigenous
sediments were laid down as alluvial fans in a rapidly
sinking basin that opened up to the west at the start of
continental rifting.
The Lowcr Cuvo
sediments were derived from a deeply weathered crust. on
which continental sediments had accumulated since the
Paleozoic, and Cretaceous beds with a pure continental
affiliation cover vast areas of northeastern Angola. The
continental Cretaceous is part of a
several-thousand-meter-thick sedimentary pile that was
laid down in the intra-
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