Abstract
The Mesoproterozoic Bushmanland Subprovince of northwestern South Africa
forms the western continuation of the transcontinental Namaqua-Natal Metamorphic
Province, a crustal domain affected by the 1020-1220 Ma Namaquan Orogeny.
Cross-cut by several large faults, the Bushmanland Subprovince can be subdivided
into a southern Garies Terrain and northern Aggeneys Terrain. The supracrustal rocks
of the Aggeneys Terrain (i.e. Bushmanland Group), comprise a thin (<1 km thick)
metavolcano-sedimentary succession composed of a very consistent, shallow marine
duplex of sandstone-shale to chemogenic metasedimentary and metavolcanic rocks
that have undergone multiple phases of deformation and metamorphism. Since the
discovery of the Broken Hill-type (BHT) mineralization in the Aggeneys-Gamsberg
district (~440 Mt, 5.2% Cu+Zn+Pb) in the early 1970’s, controversy has persisted
regarding the stratigraphy of the Bushmanland Group, its lateral correlation
throughout the Aggeneys Terrain, environment and age of deposition, as well as
classification and origin of its base-metal sulfide ± barite deposits. For these reasons,
the present study primarily focuses on two aims, namely: (1) regionally based
comprehensive lithostratigraphic, geochemical and geochronologic analysis of the
Bushmanland Group to be used in the construction of a basin model; and (2)
petrographic and geochemical analyses of Fe-Mn-rich rocks and barites to determine
if they are related to base-metal mineralization and if so, to what extent.
New lithostratigraphic data for the Bushmanland Group indicate that it can be
subdivided into two subgroups and thirteen formations that are directly correlatable
throughout the terrain as well as similar supracrustal successions in neighboring
portions of the Namaqua Metamorphic Province. The base of the Bushmanland
Group (Wortel Subgroup) comprises a thin (250-350 m thick) sequence of
interbedded upward-coarsening psammo-pelitic schists and mature quartzite (i.e.
meta-orthoquartzites) of the Namies Schist Fm., Pella Quartzite Fm., Bloemhoek Fm.
and laterally equivalent Kangnas Fm. In contrast, the metasedimentary rocks of the
unconformably overlying Kouboom Subgroup can be separated into facies terrains
divided by the Pofadder-Tantalite Valley Shear Zone (PTV Shear Zone). West of the
PTV Shear Zone the Kouboom Subgroup is characterized by a thin (205-225 m thick)
succession of interbedded mature quartzites and pelitic schists. East of PTV Shear
Zone the Kouboom Subgroup encompasses a thick (~1250 m thick) succession of
calc-silicate rocks hosted by biotite to calc-silicate-rich schists and metagreywackes.
The Koeris Fm., a variably thick (0-650 m) succession of psammitic schists,
metaconglomerates and ortho-amphibolites unconformably overlies the Kouboom
Subgroup.
Geochemical provenance and detrital zircon core populations of the Wortel
Subgroup suggest the metasedimentary rocks were derived from the Paleoproterozoic
continental island arc rocks of the Vioolsdrift Intrusive Suite and Gladkop Suite, as
well as an unidentified sedimentary/metasedimentary succession. Deposition took
place in a passive continental margin environment between 1140 to 1650 Ma. In
contrast, the unconformably overlying Kouboom Subgroup is characterized by larger
plutonic derived zircons of the basement rocks to the Orange River Group, suggesting
deposition in a tectonically active environment marked by repeated periods of tectonic
uplift. In addition, new age constraints reveal that deposition in the upper part of the
Kouboom Subgroup (possibly upper part of the Gams Fm.) was synchronous with
emplacement of the Little Namaqualand Suite (~1190 Ma) into the lower portions, i.e.
Wortel Subgroup, of the Bushmanland Group. The geochemical attributes and detrital
zircon populations of metagreywackes from the Driekop Fm. suggest they were
eroded from the newly exposed, i.e. fresh to poorly weathered, intrusions of the Little
Namaqualand Suite, indicating a renewed period of tectonic uplift. Lastly, unlike the
other lithologic units of the Bushmanland Group, the Koeris Fm. exhibits four detrital
zircon age populations at 1125-1325, 1605-1695, 1730-1910 and 1935-2075 Ma. The
older sub-populations indicate sediment derivation from various units of the
Richtersveld Subprovince and Steinkopf Domain, while the younger sub-populations
suggest derivation from various units in the Rehoboth Inlier of Namibia and the
Gordonia Terrain to the east. The provenance signature of the younger subpopulation
implies that deposition of the Koeris Fm. occurred after continental
collision between the Rehoboth Inlier-Kaapvaal Craton and the Namaqua
Metamorphic Province.
With regards to the base-metal deposits of the Aggeneys-Gamsberg district,
petrographic and geochemical analysis of the Bushmanland Group Fe-Mn-rich rocks
suggests that they can be subdivided into several types: (1) primary Fe-Mn-rich
metasedimentary rocks; (2) magnetite-amphibole-rich Fe-Mn-rich rocks; (3) coticules;
and (4) epigenetic Fe-Mn-rich rocks. Primary Fe-Mn-rich metasedimentary rocks
occur throughout the western and central portions of the study area and appear to have
been formed through the deposition of Fe-Mn-rich hydrogenous precipitates in areas
of localized sediment starvation. However, as illustrated by the primary Fe-Mn-rich
metasedimentary rocks of the Lemoenpoort prospect, a syn-diagenetic, hot (>250°C),
metalliferous hydrothermal fluids infiltrated and altered these hydrogenous Fe-Mnrich
metasedimentary rocks, resulting in the deposition of base-metal sulfides,
formation of magnetite-amphibolite-rich Fe-Mn-rich rocks, as well as hydrothermal
alteration of the siliciclastic wall rocks to form coticules. The spatial restriction of
epigenetic Fe-Mn-rich rocks to shear zones, high Fe2O3
T (ca. 65 wt %), low ΣREE
(ca. 13 ppm), presence of recrystallized quartz crystals, elevated concentration of Cu
in some occurrences and general similarities with some hydrothermal iron/iron-oxide
copper-gold (IOCG) deposits, suggests that the epigenetic Fe-Mn-rich rocks may have
formed during prograde metamorphism.
Low concentrations of SrO (0.5 ± 0.2 wt %), highly radiogenic Sr/ Sr ratios
(0.7164 ± 0.0028), elevated δ S (27.3 ± 4.9 ‰) and δ O (7.7 ± 3.1 ‰) values in the
barites, as compared to contemporaneous Mesoproterozoic seawater, suggests
precipitation of stratiform and stratabound barite layers in the Bushmanland Group
occurred through mixing of an evolved continental crustal source and
contemporaneous seawater sulfate,
87 86
34 18
modified by bacterial sulfate reduction. Most
importantly, δ O values suggest possible minimum temperatures of formation
ranging from
18
<150°C for the Gamsberg deposit to >250°C for occurrences in the
Aggeneys area. These obvious differences in temperature of formation are in good
agreement with the Cu-rich, Ba-poor nature of the sulfide mineralization characteristic
of the Aggeneys deposits versus the Cu-poor, Ba-rich character of the Gamsberg
deposit. In conjunction with this, the isotopic and petrographic arguments favor a
sub-seafloor replacement model for the stratabound barite occurrences of the
Aggeneys deposits, while at Gamsberg, deposition at the sediment-water interface as a
true sedimentary exhalite appears more acceptable.
Data obtained in the present study, combined with the results of previous
investigations can be used to develop a comprehensive model for the geological
evolution of the Aggeneys Terrain and Namaqua Metamorphic Province. The
tectono-sedimentary evolution of the Aggeneys Terrain and Namaqua Metamorphic
Province is marked by two important tectonic events separated by an episode of
tectonic quiescence. Extrusion and deposition of the metavolcano-sedimentary rocks
of the Orange River Group at 1908 Ma marks the start of the Orange River Orogeny.
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Prior to emplacement of the Vioolsdrift Intrusive Suite, the Orange River Group
appears to have undergone a period of folding and low-grade metamorphism [D1/M1]
that was subsequently followed by emplacement of the Main Phase Vioolsdrift
Intrusive Suite roughly dated at 1890 Ma. Rapidly following emplacement of these
intrusions, the lower crustal rocks of the Richtersveld Subprovince underwent a
second, higher, amphibolite-facies metamorphic event [M1B] from 1870-1840 Ma.
This event may have resulted in lower crustal melting and emplacement of the
Gladkop Suite into an unknown package of metasediments or metasedimentary rocks
south of the present day Orange River at roughly 1820 Ma. The Gladkop Suite was
subsequently subjected to high-grade metamorphism at 1800 Ma. The Orange River
Orogeny was terminated by emplacement of the Late Phase Vioolsdrift Intrusive Suite
at approximately 1765 Ma and later northward-directed thrusting.
Following termination of the Orange River Orogeny, deposition of the
Bushmanland Group began in a tectonically stable environment marked by punctuated
periods of tectonic activity that lasted until emplacement of the Little Namaqualand
Suite at 1190 Ma. The detrital zircon populations of the Pella Quartzite Fm. and
Koeris Fm. support (a) regional correlation of these stratigraphic units throughout the
study area, (b) confirms sediment derivation from various local source terrains and (c)
suggests a maximum depositional age of 1650 Ma. Furthermore, new age constraints
reveal initiation of the O’okiepian Episode (Namaquan Orogeny), characterized by
regional-scale mid- to high-grade contact metamorphism, was synchronous with
emplacement of the Little Namaqualand Suite and deposition of the upper Kouboom
Subgroup. Furthermore, the detrital zircon populations for the Driekop Fm. (upper
Kouboom Subgroup) contain a large population of 1190 Ma (i.e. O’okiepian-age)
detrital cores, suggesting a renewed period of tectonic uplift. Analogously, age
constraints for the Koeris Fm. indicate a maximum depositional age of 1130 Ma, as
well as derivation from a number of local and exotic source terrains indicating that
deposition of the Koeris Fm. must have occurred in response to continental collision
between the Rehoboth Inlier-Kaapvaal Craton and the Namaqua Metamorphic
Province. Furthermore, these new age constraints also constrain the timing of D2-D3
deformation to between 1130-1080 Ma and regional peak metamorphism to 1020-
1040 Ma.
Prof. N.J. Beukes
Prof. J. Gutzmer