Mineralogy and geochemistry of the impact breccias of the Bosumtwi impact structure, Ghana : genesis and secondary proccesses
Date
2009-05-28T10:56:53Z
Authors
Coney, Louise
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Abstract
Abstract
The 10.5 km diameter and 1.07 Ma old Bosumtwi structure in Ghana
is a complex impact structure that displays a variety of preserved
impactites from both within and outside of the crater and that is
associated with the Ivory Coast tektite strewn field. Consequently,
this crater structure provides a rare opportunity to examine a wide
range of impact-related deposits, including within-crater deposits and
proximal as well as distal ejecta. Access to the within-crater deposits
was provided by two drill cores obtained by a 2004 drilling project of
the International Continental Scientific Drilling Program (ICDP). The
focus of the present study has been to determine the petrographic
and geochemical characteristics of the different impactites, inside
and outside of the crater and how these relate to the regional target
rocks / drilled crater basement with the aim of understanding how the
impactites formed during the cratering process.
Two hard rock cores were recovered from the ICDP drilling project:
one through the thickest section of the crater fill (LB-07A) outside of
the central uplift and one into the flank of the central uplift (LB-08A).
The former has been the focus of this study. No coherent melt sheet
was intersected and the shock state of the impactites is lower than expectations from pre-drilling numerical modeling (2.6 – 3.9 km3
expected; <0.2 km3 melt encountered). Suevites from south and
north of the crater have also been examined in a similar manner to
those from within the crater.
The LB-07A core has been subdivided on the basis of different
impactite types into: An upper impactite section (333.28 - 415.67 m
depth), which consists of seemingly alternating polymict lithic and
suevitic breccias; this sequence is underlain by a lower impactite
section (415.67 - 470.55 m depth) consisting of monomict breccias
(after metagreywacke, phyllite or shale), which, in turn, is underlain
by basement metasediments (470.55 - 545.08 m depth). The basement metasediments are dominated by shale over
metagreywacke. Both the lower impactites and basement
metasediments contain cm-scale thin dykes (at 430.13 m, 445.22 m,
483.00 m and 513.90 m depths) with suevitic fill. The matrices of all
the suevites are clastic and contain discrete and tiny (50 – 100 μm)
melt fragments. A 30 cm thick granophyric-textured lithology,
interpreted to represent a hydrothermally altered granitioid intrusion,
occurs at 487.12 m (in the basement metasediments). The
subdivision of the core has provided a useful correlative tool for
geophysical and geochemical observations.
The target rock variety comprises metasediments (mostly
metagreywacke, shale and phyllite) that form the bulk (ca. 95%) of
the material from which the impactites were derived, and granite that
forms a minor component (< 0.5 %). The metagreywacke and shale
(together with phyllite) form near equal contributions (44 and 47 %
respectively) to the target stratigraphy. Additionally carbonate, in the
form of calcite bands, contributes to the target rock composition (ca.
2 %). Other contributors include schists and quartz derived from argillitic or greywacke metasedimentary precursors (6.5 % overall).
The impactites within the crater are chemically quite homogeneous.
Platinum group element (PGE) and osmium isotope analysis failed to
detect an unequivocal contribution from the meteoritic projectile in
the within-crater breccias. It is concluded that the meteoritic signature
is masked by the high abundances of siderophile and platinum group
metals in the target lithologies (associated with the regional, preimpact
gold mineralization).
The degree of shock of clasts in the impactites of the within-crater
deposits is lower than expected from pre-drilling numerical modeling
(in that no melt layer is in evidence). A maximum of 3 PDF sets in
quartz clasts are noted. On average, only 5.9 % of all quartz grains
display 1 or more PDF sets in the upper impactites, with this number decreasing to 4.4 % in the lower impactites, and 1.8 % in the
basement metasediments. No PDFs in feldspar have been noted.
Diaplectic quartz glass is present in the upper and lower impactites,
with a distinct absence of this shock feature in the basement
metasediments. Rare diaplectic feldspar glass has been observed. A
general decrease in shock pressure, in terms of proportions of melt
and shocked quartz, is noted along both cores. Melt particles reach a
maximum of 1 cm in size, and a maximum proportion of 36 vol% in a
non-representative sample, but are generally less than 5 vol% in
abundance. Melt particles of the different core sequences differ in
terms of colour and, to a lesser degree, chemical composition. The
particles are chemically heterogeneous at the single particle scale.
The melt particles consist of mixes of minerals derived from the
target lithologies (quartz, feldspars, phyllosilicates).
Small areas of suevites have been found to both the north and south
of the crater (1.5 km2). The suevites from outside of the crater have a
larger proportion of granite - and a distinct absence of carbonate –
clasts. Overall, the suevites from the south have clast populations
dominated by shale, phyllite and slate, followed by granitoid,
metagrewacke and schist; those from the north are dominated by
granitoid, schist, and relatively smaller proportions of phyllite, slate
and shale. The clast populations are notably different from those of
the within-crater suevites that are dominated by metagreywacke and
shale, though more similarities can be observed between those from
the south and the within-crater suevites. Additionally, the out-ofcrater
suevites are distinct from the within-crater suevites in terms of
their geochemical signature (which reflects the different clast
populations), and – particularly - shock degree of clasts (up to 4 PDF
sets in quartz, presence of ballen cristobalite and higher proportions
of diaplectic glass in the out-of-crater suevite). Melt particle sizes
range up to 40 cm and melt is volumetrically more significant (18 –
37 vol%) than in suevite within the crater. Melt particles outside the
crater are more vesiculated. Additionally, some melt particles from the north contain microlites. The particles also display internal
heterogeneity (particularly noticeable for those from south of the
crater), and those from the north are relatively enriched in Al2O3 and
depleted in FeO and MgO, in comparison to those from south of the
crater. The particle compositions from south of the crater are more
similar to the within-crater melt analyses than to those from north of
the crater. The melt compositions indicate that these melt particles
also formed from mineral mixes.
The impactite distribution (out-of-crater suevites occur to the north
and south of the crater, but are absent from the east and west) is
consistent with a low-angle oblique bolide impacting from the east.
Due to the differing petrographic and shock characteristics of the
suevites, it has been concluded that the within-crater and out-ofcrater
suevites formed by different mechanisms. A method of
formation involving relatively less shocked material and derivation
from metagreywacke and shale involves either slumping off the
crater walls or lateral movement of melted and displaced target rock
within the crater. Limited admixture of fallback material from the
ejecta plume is proposed to explain the accretionary lapilli layer
found above the impactite sequence. The out-of-crater suevites were
formed by fallout from the ejecta plume (thus resulting in higher
proportions of shocked material). However, the ejecta plume itself
was differentiated laterally, which resulted in the manifestation that
the clast populations of the suevites outside of the crater are different
to each other. A weak post-impact hydrothermal system affected the
crater fill, as testified by calcite veins and pods, quartz and chlorite
veins, and sulphide formation in the breccia matrices. This constrains
the hydrothermal system to lower greenschist facies conditions, as
no minerals from higher metamorphic assemblages have been
found.
The present study has indicated that the impactites from Lake
Bosumtwi are distinct from each other in terms of petrographic and geochemical characteristics. Furthermore it is proposed that these
impactites formed by different mechanisms. This study has provided
petrographic and geochemical data against which to correlate predrilling
expectations.