Appendix A – Mud mineralogy and geochemistry in equatorial Africa
This Appendix provides the extended detailed description of the mineralogy, geochemistry and
isotope signatures of the studied silt-sized fluvial sediments produced along the western branch of
the East African Rift in Uganda, Rwanda, Burundi and Tanzania, with the complete mineralogical
and geochemical datasets (Tables 2 to 5). Provided are also the synthetic description of each
identified sedimentary province (Table 1) and the element concentrations used for normalization and
calculation of weathering indices (Table 6).
Chemical composition is discussed with reference to the estimated element concentration in the
upper continental crust (UCC) [Taylor and McLennan, 1995; McLennan, 2001; Hu and Gao, 2008]
and standard shale (PAAS) [Taylor and McLennan, 1985]. REE data were normalized to CI
carbonaceous chondrites [McDonough and Sun, 1995]. Among the numerous indices proposed to
estimate chemical weathering [Duzgoren-Aydin et al., 2002], we considered specifically the
Chemical Index of Alteration (CIA) [Nesbitt and Young, 1982] and the Weathering Index (WIP)
[Parker, 1970]. Other indices including CIW [Harnois, 1988] and PIA [Fedo et al., 1995] were also
calculated. The CIA is 47 for UCC and 70 for PAAS, the WIP 81 for UCC and 51 for PAAS, and
the Eu anomaly 0.65 for both UCC and PAAS.
A1. River mud from the Virunga and South Kivu lava fields
Mud derived from the Virunga volcanoes is dominated by only slightly weathered bedrock
minerals, including feldspars (plagioclase, sanidine), pyroxene (mostly augite), Fe-oxides,
amorphous matter, little mica-illite and very little quartz. Secondary clay minerals are negligible
even in rill sediment. Magnetite is the dominant Fe-oxide in river mud, whereas hematite dominates
in the oxidised rill sample. Chemical composition reflects limited weathering of basaltic source
rocks. Muds are enriched in Cu, Au, Cd, Hg, Tl, Pb, As, Bi, Se, LOI, C, and depleted in alkali and
alkaline-earth metals relative to sand (CIAmud = 60±10, WIPmud
=
59±22 versus CIAsand = 44±4,
WIPsand = 87±13). Muds appear to be richer in Pb and poorer in Fe, Mg, Ca, Ti, Sr, Sc, V, Co than
parent lavas (CIALAVA = 37±6, WIPLAVA = 94±3) [Rogers et al., 1992; 1998]. Chondrite-normalized
patterns are quite steep for LREE and nearly flat for HREE, with moderately negative Eu anomaly.
The Ce anomaly is mildly positive (Ce/Ce* = 1.13±0.03), and the MREE bulge generally negligible
and marked only in the rill sample (MREE/MREE* = 1.16). Virunga muds yielded the lowest
Sr/86Sr ratios and among the least negative Nd(0) of our whole sample set, and middle to late
87
Mesoproterozoic model ages.
In the South Kivu Province, mud is dominated by amorphous matter, clay minerals and Feoxides/hydroxides, with little quartz and feldspar. Kaolinite (low-crystallinity metahalloysite)
dominates over smectite and illite. Concentrations of alkali and alkaline-earth metals (CIAmud = 99,
WIPmud = 5 versus CIAsand = 81, WIPsand = 18) are an order of magnitude lower than in parent lavas
for Mg, Ca, Na and Sr (CIALAVA = 37±3, WIPLAVA = 89±7) [Furman and Graham, 1999]. Relative
to parent lavas, Shangazi mud is depleted also in Si, K, Rb, Ba, and enriched in Al, Fe, Ti, Sc,
HREE, Zr, Hf, V, Cr, Cu, Pb. Relative to sand, mud is poorer in Si, Mg, Ca, Na, Sr, Ba, Cr, Co, Ni,
Cu, and richer in Al, Ti, Th, Zr, Hf, V, Nb, Ta, Au, Hg, Ga, with a higher LOI. Relative to Virunga
muds, Shangazi mud is poorer not only in all alkali and alkaline-earth metals but also in W, Au, Tl,
Pb, As, Bi, and richer in Al, Fe, Ti, Mn, Sc, Y, MREE, HREE, V, Cr, Co, Ni, Cu, Ga. The REE
pattern is less steep and displays small Eu anomaly, negligible Ce anomaly, and conspicuous
MREE bulge (1.42). Shangazi mud yielded slightly higher
87
Sr/86Sr ratio than Virunga mud, the
only positive Nd(0) of our sample set, and the youngest mid-Neoproterozoic model age.
A2. River mud from Paleoproterozoic basement in Rwanda and Burundi
Muds produced along the inner flank of the Kivu-Tanganyika rift shoulder include micas, clay
minerals, quartz, significant feldspars, some amphibole, Fe-oxides/hydroxides, and locally
amorphous matter. The plagioclase/total feldspar ratio seems to decrease from bulk samples to the <
63 m fractions. If true, this would suggest selective dissolution of unstable plagioclase in finer size
classes. Kaolinite prevails over illite and locally smectite in tributaries of Lake Kivu or minor
chlorite in tributaries of Lake Tanganyika, whereas the intervening Rusizi basin is characterized by
illite-kaolinite-smectite assemblages with minor chlorite and trace sepiolite or talc. Although
displaying higher concentrations than in sands, alkali and alkaline-earth metals are depleted by over
an order of magnitude relative to UCC in all mud samples (CIAmud = 804, WIPmud = 335 versus
CIAsand = 656, WIPsand = 237). Ca, Na and Sr are significanty depleted also relative to PAAS,
suggesting breakdown of plagioclase. Basement-derived muds of the Kivu-Tanganyika Province
yielded high 87Sr/86Sr ratios, strongly negative Nd(0) and early Paleoproterozoic model ages.
Along the eastern outer flank of the Kivu-Tanganyika rift shoulder (Gishwati, Nyungwe, and
Muramvya Provinces), muds include less feldspars (K-feldspar mostly more abundant than
plagioclase), more clay minerals, micas, quartz, Fe-oxides/hydroxides, and locally amorphous
matter and amphibole. Kaolinite dominates over illite and chlorite, and it is the virtually exclusive
clay mineral in the Nyungwe rill sample. Smectite is absent, and sepiolite and talc locally detected.
Alkali and alkaline-earth metals are most strongly depleted in the Nyungwe and Muramvya
Provinces, and display similar concentration as in sands (CIAmud = 913, WIPmud = 155 versus
CIAsand = 804, WIPsand = 127). Depletion is less pronounced in the Gishwati Province (CIAmud =
821, WIPmud = 404 versus CIAsand = 715, WIPsand = 175).
Anomalous concentrations in Y, REE, Th, U, Zr and Hf, reaching 6 times UCC in Mugere mud
(Tanganyika Province) or even 11 times UCC in Agatobwe mud (Nyungwe Province), and Eu
anomalies respectively as negative as 0.38 and 0.26, suggest either hydraulic enrichment of
ultradense minerals including zircon and monazite or inadvertent contamination by the heavymineral-rich fine tail of the sand mode during sampling [Garzanti et al., 2008]. Eu anomalies are
markedly negative in the Nyungwe Province (0.440.11) reflecting felsic protoliths and extensive
feldspar dissolution, and less negative than UCC in the Muramvya Province (0.730.06) again
suggesting more mafic protoliths. REE patterns are otherwise similar for all basement-derived
muds; the Ce anomaly reaches up to 1.34, and the MREE bulge up to 1.26.
Muds in the Gishwati and Nyungwe Provinces yielded very high 87Sr/86Sr ratios, strongly negative
Nd(0) and early Paleoproterozoic model ages. Upper Ruvubu mud yielded a lower 87Sr/86Sr ratio, a
slightly less negative Nd(0) and a notably younger middle Paleoproterozoic model age, reflecting
supply from the Neoproterozoic Upper Ruvubu alkaline plutonic suite. This is also revealed by
relatively high Na and Nb in both mud and sand.
A3. River mud from Kibaran and older granitoids
Mud derived from the Gitarama granite in central Rwanda includes feldspars (K-feldspar prevailing
over plagioclase), quartz, subordinate micas, clay minerals, Fe-oxides/hydroxides and amorphous
matter. In mud derived from the Mutara granite in NE Rwanda lowlands, quartz prevails over
feldspars (dominantly K-feldspar), clay minerals, micas and Fe-oxides/hydroxides. Clay minerals
include kaolinite, illite and smectite in both provinces.
Alkali and alkaline-earth metals are only slightly depleted in Victoria Nile sediments at Itanda falls
(CIAmud = 56, WIPmud = 64 versus CIAsand = 53, WIPsand = 66). Instead, depletion particularly in Na,
Ca, and Sr is significant in Gitarama mud (CIAmud = 68, WIPmud = 47 versus CIAsand = 562,
WIPsand = 288) and strong in Mutara mud (CIAmud = 832, WIPmud 206 versus CIAsand = 712,
WIPsand = 104), compared to literature values from parent rocks (CIAGRANITE = 592, WIPGRANITE
= 676) [Hildebrand, 1984; Fernandez-Alonso et al., 1986; Fernandez-Alonso and Theunissen,
1998]. Locally high concentrations in REE, Zr, Hf, Th, and U reveal occurrence of ultradense
minerals including zircon and monazite. REE patterns locally display more negative Eu anomaly
than in Kibaran granites (Eu/Eu* as low as 0.40 versus Eu/Eu*GRANITE 0.55±0.03 [FernandezAlonso and Theunissen, 1998] reflecting feldspar dissolution, and MREE bulge up to 1.35.
A4. River mud from Mesoproterozoic metasedimentary and sedimentary rocks
Mud derived from Western Domain metasediments consists of quartz, amorphous matter, clay
minerals (subequal illite and kaolinite in Rwanda, kaolinite prevailing over illite in Burundi), micas,
Fe-oxides/hydroxides and few feldspars (K-feldspar invariably prevailing over plagioclase).
Gibbsite was recorded locally. Depletion of mobile alkali and alkaline-earth metals reflect both
intense weathering and dilution by quartz, as reflected by low WIP particularly in sand (CIAmud =
866, WIPmud = 147 versus CIAsand = 856, WIPsand = 66). The rill sample is strongly depleted in
Mg and Ca, but otherwise similar to fluvial mud. High concentrations in Au (Cizanye) or in W and
As (Kora) may reflect local occurrence of ore bodies. REE, Ti, Zr and Hf concentrations may also
be relatively high locally, likely due to recycled ultradense minerals such as zircon, rutile and
monazite. REE patterns show Eu anomalies down to 0.36, Ce anomalies up to 1.16, and MREE
bulge up to 1.11.
Mud derived from siliciclastic units of the Eastern Domain has similar geochemistry, with higher
Ni, Cu, and locally U, V, Co and Tl. The CIA reaches up to 93 due to extreme depletion in Na and
Ca, and the Ce anomaly up to 1.25.
A5. River mud in the Malagarasi basin
Muds in the Malagarasi basin are invariably dominated by quartz, with amorphous matter, clay
minerals, Fe-oxides/hydroxides, and few feldspars (mostly K-feldspar) and micas. Smectite prevails
over illite and kaolinite. Anatase was locally detected. Low concentrations in alkali and alkalineearth metals are largely ascribed to dilution by recycled quartz (CIAmud = 825, WIPmud = 195
versus CIAsand = 61, WIPsand = 13), whereas concentrations in Fe, Mg, Mn, V, Co, Ni, Cu are
relatively high. Locally high concentrations in Zr and Hf reflect occurrence of zircon. REE patterns
are not steep, with Eu anomaly slightly less negative than UCC and commonly marked MREE
bulge reaching up to 1.36.
Mud of the Malagarasi River just upstream Lake Tanganyika yielded high
87
Sr/86Sr ratio, strongly
negative Nd(0) and a latest Neoarchean model age. Its Rumpungwe tributary draining the Eastern
Domain yielded the highest 87Sr/86Sr ratio of our dataset, and its Ruchugi tributary a model age as
old as early Mesoarchean. Mud from the Rugera River, a small river draining gneissic basement of
the Tanzania Craton in SW Burundi, yielded relatively low
87
Sr/86Sr ratio, very strongly negative
Nd(0) and late Mesoarchean model ages.
A6. River mud in the Kagera basin
Muds in the upper catchment include micas and clay minerals (predominant in Nyabarongo and
Akanyaru muds), amorphous matter and quartz (most abundant in Ruvubu mud), Feoxides/hydroxides and feldspars (equally abundant K-feldspar and plagioclase in Nyabarongo mud;
little and mostly potassic feldspar in Ruvubu mud). Feldspars (particularly K-feldspar) are abundant
in Upper Ruvubu mud. Kaolinite invariably dominates over illite. Mineralogy is intermediate
between Nyabarongo and Ruvubu muds downstream of their confluence. Kaolinite decreases and
smectite becomes significant along the lower Kagera.
CIA and αAl values are highest in Nyabarongo headwaters (Nyungwe Province) and decrease
downstream, where they become similar to those of muds in the Gishwati Province. Alkali and
alkaline-earth metals (particularly Sr), Ti, Nb, Ta and Au are higher in Mukungwa mud than in the
Gishwati Province, and the same elements increase in Nyabarongo mud downstream of the
Mukungwa confluence. Ruvyironza and Ruvubu muds are strongly depleted in mobile elements. In
their lower course, muds become enriched in Cr and Ni. Lower Kagera muds are enriched in Si.
87
Sr/86Sr ratios are highest for Nyabarongo mud, high for lower Ruvyironza mud, and similar for
lower Akanyaru and lower Ruvubu muds. They decrease regularly along the lower Kagera down to
the mouth in Lake Victoria; Nd(0) remains strongly negative throughout the catchment. Model ages
range from Neoarchean for Ruvyironza mud to Paleoproterozoic for Ruvubu and Akanyaru muds.
Model ages tend to decrease downstream in Kagera mud, where they are consistently 2.2 Ga except
for one sample yielding a younger Paleoproterozoic tDM age. Mud of the Muvumba tributary yielded
similar Sr-Nd isotopic ratios and model age as Kagera mud.
A7. River mud in Uganda and along the White Nile
Mud carried by the Ruizi River is highly quartzose, with amorphous matter, some feldspars (almost
only K-feldspar), clay minerals (dominant kaolinite with subordinate illite and significant smectite)
and micas. Concentrations of Si, Zr and Hf are high; concentrations of alkali and alkaline-earth
metals are low. Depletion of alkali and alkaline-earth metals is extreme for Si-poor Katonga mud
(CIA = 98). Muds of the northern Lake Albert graben and Albert Nile outlet are Si-rich and
depleted in most elements (CIA = 655, WIP = 126). Ratios of most chemical elements to Si are
higher in Albert Nile mud downstream.
87
Sr/86Sr ratios reach up to 0.800 for Nkusi muds and are lowest for the Victoria Nile (0.714-0.715)
and Albert Nile-Bahr el Jebel (0.716-0.720); Nd(0) is invariably strongly negative, and very
strongly negative (≤ -30) for Victoria and Albert Nile, Ora, and Nkusi muds. Model ages range
from 3.4 Ga for the Victoria Nile upstream Lake Albert to 2.6-2.4 Ga for the Kafu, Ruizi, and
Mubuku Rivers. Younger model ages were obtained for upper Sezibwa and particularly Rwimi
mud, which yielded the least negative Nd(0).
TABLE CAPTIONS
Table A1. Synthetic description of sedimentary provinces identified in equatorial Africa.
Provenance classification after Garzanti and Andò [2007]. Annual rainfall ranges from < 1 m (dry)
to ~ 2 m (hyperhumid).
Table A2. Bulk mineralogy of the studied mud samples. X-ray powder diffraction analyses by
Massimo Setti and Luigi Marinoni (University of Pavia). GSZ= grain size.
Table A3. Clay mineralogy. X-ray powder diffraction analyses of the < 2 m class by Massimo Setti
and Luigi Marinoni (University of Pavia).
Table A4. Geochemistry of the studied mud samples. Elements analysed by aqua regia digestion
(a.r.d.) may be underestimated because of only partial leaching of refractory minerals. Chemical
weathering indices CIA, CIW, PIA, WIP are after Nesbitt and Young [1982], Harnois [1988], Fedo
et al. [1995], Parker [1970], respectively. In order to avoid bias caused by hydraulic concentration
of heavy minerals hosting Ti, REE and Th, αAl values were normalized to non-mobile Al; αAl values
were calculated with reference to average Virunga [Rogers et al., 1992; 1998] and Kivu lavas
[Furman and Graham, 1999] for purely volcaniclastic muds, to Kibaran and Singo granitoids
[Fernandez-Alonso et al., 1986; Fernandez-Alonso and Theunissen, 1998; Nagudi et al., 2003] for
purely plutoniclastic muds, and to UCC for all other samples. The Eu anomaly is the measured
chondrite-normalized Eu value over the value that Eu would have in a linear extrapolation between
chondrite-normalized values of Sm and Gd. The Ce anomaly Ce/Ce*, indicative of redox state, is the
measured PAAS-normalized Ce value over the value that Ce would have in a linear extrapolation
between PAAS-normalized values of La and Pr. MREE is the average of Eu, Gd, Tb and Dy
normalized to PAAS, MREE* the average of LREE (La, Ce, Pr, Nd) and HREE (Er, Tm, Yb, Lu)
values [Haley et al., 2004 and references therein]. The chondrite-normalized LaN/YbN, LaN/SmN,
GdN/HoN, and HoN/YbN ratios are also given.
Table A5. Geochemistry of the studied sand samples [for full petrological description and
discussion of river sands in the same area see Garzanti et al., in review].
Table A6. Element concentrations used for normalization and calculation of weathering indices.
Data sources: 1) Rogers et al. [1992; 1998]; 2) Furman and Graham [1999]; 3) Hildebrand [1984],
Fernandez-Alonso et al. [1986], Fernandez-Alonso and Theunissen [1998]; 4) Taylor and McLennan
[1985; 1995], McLennan [2001], Hu and Gao [2008];
McDonough and Sun [1995].
5) Taylor and McLennan [1985]; 6)
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