Internationa l Journal of Geosciences, 2014, 5, 107-121
Published Online Januar y 2014 (http://www.scirp .org/journal/ijg)
http://dx.doi.org/10.4236/ijg.2014.51012
Carbonate Enrichment in Volcanic Debris and Its
Relationship with Carbonate Dissolution Signatures of
Springs in the Sabga-Bamessing, North West, Cameroon
Raymond Beri Verla1,2*, Germain M. M. Mboudou1, Olivier Njoh1,
Gilles Nyuyki Ngoran2,3, Aloysius Ngambu Afahnwie1
1Department of Geology, Facult y of science, Un iversity of Buea, Buea, Cameroon
2Explorers 33 Consultants, Yaounde, Cameroon
3Ministry of Min es , Yaounde, Cameroon
Email: *verla6@yahoo.com
Received October 25, 2013; revis ed November 27, 2013; accepted Dece mber 13, 2013
Copyright © 2014 Raymond Beri Verla et a l. This is an open access article d istributed under the Creative Co mmons Attrib utio n Li-
cense, which permits un restricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In
accordance of the Creative Commons Attribution License all Copyrights © 2014 are reserved for SCIR P and the o wner of the int el-
lectual property Raymond Beri Verla et al. All Copyright © 2014 are guarded by law and by SCIRP as a guardian.
ABSTRACT
Sabga-Bamessing is a part of the Bamenda Mountains, an extinct volcanic center of the West Cameroon High-
lands along the Cameroon Volcanic Line (CVL). The pristine volcanic rocks of the Sabga area are alkali mafic to
felsic (basanites, phonolites, trachytes and rhyolites). Some weathered sections of a heterolithologic debris flow
with a suppositious primary chemistry of the original volcanic rocks prior to weathering have shown significant
calcium carbonate enrichment. CaO and LOI values of up 61.31% and 41.72% respectively show correspo nding
enrichment of 16.54 and 10.88, when compared with average fresh volcanic rocks. Na+ normalized molar ratios
computed from the chemistry of springs and rivers show carbonate dissolution signature which is contrary to
silicate disso lutio n ex pect ed in a cid vo lca nic rocks. Sat ur at ion indi ces (SI) cal culat ed wit h PH REEQC reve al t hat
brackish to saline spri ngs are supersatura ted with C alcite (CaCO3) , Aragonite (CaCO3), Dolomite (CaMg(CO3)2
and Hydroxyl apatite (Ca5(PO 4)3OH). Recharging contributions to spring water chemistry deviate from those
produced by rock weathering, precipitation and evaporation/crystallization. An enrichment process is therefore
predicted as a recharging contributor to water chemistry.
KEYWORDS
Cameroon Volcanic Line (CVL); Carbonate Enrichment; Carbonate Dissolution; Weathering
1. Introduction
The Cameroon Volcanic Line (CVL) is one of the major
geological lineaments of the African plate and continent
[1]. This N30˚E mag m a t o-tectonic megastructure of
Central Africa is a paradigm of active hot lines on Earth
made up of continental and oceanic sectors; a unique
feature in Africa and even in the world [2].
The Sabga-Bamessing area is a small locality on the
west Cameroon highlands an extinct volcanic center of
the CVL.
Numerous researchers have worked on the CVL and
most attention i s directed to wards the genesis, petr ogene-
sis and petrology, geochronology, and emplacement me-
chanisms of the volcanic rocks [2-7] and other references
therein].
The Sabga-Bamessing area conforms to the general
geology of the CVL but has certain peculiarities. Explo-
sive volcanic activity evident by the presence of exten-
sive ignimbrite sheets characterizes the area [8]. Field
evidence has shown the brackish to saline nature of water
draining certain sections of Sabga. The geochemical as-
pects of these fluids have not been fully studied but am-
ple evidence shows that the fluids do not reflect the che-
mistry of the volcanic rocks. There is an evidence that
CaO and Carbonate enrichment has occurred across the
profile of a thick and extensive debris flow. The water
*
Corresponding author.
OPEN ACCESS IJG
R. B. VERLA ET AL.
108
chemistry of some springs has evolved; bearing a Car-
bonate dissolution signature. This paper presents new
data showing the peculiarities in ground and surface wa-
ter of Sabga due to enrichment and its non conformance
to the chemistry of acid volcanic rocks which constitute
the main li tholo gy in the are a.
2. Geographical and Geological Context of
the Sabga-Bamessing Area
The Sabga area is located in the Central African Region
in North West Cameroon. The area lies between latitude
6˚00'N - 6˚2'N and longitude 10˚20'E - 10˚22'E.
The North West Region of Cameroon falls within the
Equatorial climatic domain and precisely influenced by
the Cameroon mountain type climate with two seasons; a
rainy season from April to October and a dry season
from November to March. Annual precipitation and
temperature are about 1778 - 2286 mm and 20.5 - 22.3˚C
respectively. The vegetation of the study area is generally
made of Terminalia trees and shrub savanna derived
from moist evergreen forest [9]. The occurrence of bush
fires and other a nthro pogen ic activitie s have modifie d the
vegetation. The study area equally has a guinea savanna
type of vegetation with a mixture of stunted shrubs and
grassy undergrowth and has been beneficial for cattle
rearing for several decades. The Bamenda country-side
also presents a vegetation of raffia palm bush limited to
valleys and depressions [10]. Geologically, the Sabga
area (Figure 1) is p art of the Bamenda Mountains [11]. It
is an extinct volcanic center of the West Cameroon
Highlands. This center constitutes a very important part
of the continental sector of the Cameroon Volcanic Line
and lies mid-way between Bambouto in the south west
and the Oku massif in t he north eas t .
Figure 1 . Location of the study area w ithin t he CVL; the study are a is sho wn on the satellite image a s a rectangle.
OPEN ACCESS IJG
R. B. VERLA ET AL.
109
The Sabga area lies just a few miles (about 15 miles)
SE of Big Babanki, one of the stratovolcanoes on the
Oku massif. The volcanic rocks range from mafic to fel-
sic and include basalts, Mugearites, basanites, trachytes,
rhyolites and benmoreites [11-13]. The absolute ages
determined for the mafic volcanic rocks range from 10 to
22 Ma from earlier studies; 12 to 27 Ma following recent
geochronogical data [11]. The ages determined for the
felsic volcanic rocks range from 18.7 - 24.2 Ma for the
oldest rocks and 12.7 - 13.2 Ma for the youngest ones.
Geochemical studies had previously been done on nearby
volcanic centers (Oku and Ndu) and similar geochrono-
logic, petrogenetic and petrological conclusions had been
drawn [14,15]. The volcanic rocks are emplaced on a
granit ic b asement of P an A frican a ge wit h exte nsive out-
crops in the Bamessing, and Ndop plains. Field evidence
reveals abundant muscovite in the mode of these gra-
nites.
3. Sampling and Analytical Procedures
Five representative samples from 33 fresh volcanic sam-
ples were selected for analysis. Five samples collected
from the top and bottom of a weathered volcanic debris
flow were equally selected. The samples were oven-dried
at 105˚C. Each sample was disaggregated. The resulting
chips were put in a porcelain cup together with porcelain
balls, cocked tightly and milled for about 30 minutes.
The resultant powder measuring about 100 µ or more
was put into clean porcelain mortar and using the physi-
cal energy, pulverized with a porcelain pestle to finer
sizes. A clean 50 µ sieve was used to sieve the pulp ob-
taini ng a <50 µ powder. Glass beads and pressed powder
discs were made from the powder for major oxide and
trace element analyses respectively.
A Pioneer S4 Brucker XRF apparatus was used to
analyze for major and trace elements in ten samples at
MIPROMALO (Mission de Promotion des Matériaux
Locaux), Yaounde, Cameroon. Loss on ignition (LOI)
was determined by weight difference after ignition at
1000˚C.
Twelve water samples (10 springs and 2 river samples)
were collected using the conventional methods given in
FOREGS geochemical manual [16]; water samples were
collected with a syringe which had been rinsed twice
with the water sample and put in 500 ml ethylene bottle
passing through a filter, followed by the addition of 1.0
ml of conc. HNO3 acid. The samples were shaken tho-
roughly and placed in an ice box. Field parameters (pH,
EC, TDS, Temp,) were measured for the springs and
river samples using pH, EC, TDS meters (Hanna instru-
ments, H1 9811) and a digital thermometer. The water
samples were analyzed for major cations and anions at
the Institute of Agronomic Research and Development
(IRAD) in Ekona, SW Region of Cameroon.
The concentrations of Na+, K+, and Ca2+, were deter-
mined using the Gallemkamp flame photometer. Mg2+,
3
NO
,
2
4
SO
,
4
NH
+
,
3
4
PO
were analyzed by Colori-
metry and
3
HCO
and Cl b y titratio n.
The major oxides and few trace elements in volcanic
materials have been used to characterize the volcanic
rocks and compute the enrichment and depletion of ele-
ments. Major cations and anions in water are used to do
the aqueous speciation calculations to determine satura-
tion indic es (SI) using PH R EEQ C [17].
Field data and geochemical data are used together with
satellite images to produce geologic maps and models
using GLOBAL MAPPER and SURFER 9 software
which further highlig ht the spatia l distribution of volcan-
ic materials, springs and river s.
4. Resul ts
4.1. Geospatial Distribution of Rock Types
A 1:500 geologic map of the study area drawn using
combined field and sate llite imagery data with GLOBAL
MAPPER 11 and SURFER 9 software reveals that the
area has five main rock types, namely, Trachytic, Rhyo-
litic, Basaltic, Tuff breccia/pyroclastic breccia, and Vol-
canic debris flow (Figure 2). The crystalline rocks are
more or less fresh but the debris flow has suffered in-
tense chemical weathering. The debris flow has an esti-
mated perimeter of 14884.4 m and area of 7175.5 m2.
The bulk of the material is a mixture of fresh aphanetic
rock frag ments wit h very fine grain friable material in th e
ratio 1:3. The overall texture is volcaniclastic and many
radial cracks are present qualifying the outcrop as a lahar
(Figure 3(a)). Ro unded clayey a ggregat es also consti tute
large parts of these outcrops. These sections of the flow
are at relatively high topography at about 1650 - 1700 m
or more above sea level.
The river Nkambie has exposed steep walls about 50 -
60 m high of se ctions of the debris fl ow (Figure s 3(a)-(e)).
The wall shows a viscoplastic rheology and abundant
sub-rounded boulders at the bottom. The outcrop is pre-
dominantly made up of volcaniclastic material. The re-
mainder is made up of huge volumes of precipitates oc-
curring at the bottom of the exposed wall. A pure white
soft precipitate with acicular and feathery habit and a
taste peculiar to bicarbonate salts occupies the interstices
of the sub rounded boulders (Figure 3(b)). Another dull
yellow precipitate with an earthly to massive habit, a
sweetish astringent taste and a slight sulphurous odor is
also dominant (Figure 3(e)). Most of the samples effer-
vesce on the addition o f HCl. This botto m section is rela-
tively at lower topography (1450 - 1500 m above sea
level). SURFER generated DEM combined with the
geolo gic map sho ws that the mai n river ( Nkambie ) flo ws
through the base of the debris flow where appreciable
OPEN ACCESS IJG
R. B. VERLA ET AL.
110
Figure 2. Geospatial data added on the topographic design (geologic map) both in 2D and 3D for the study area. The main
rock types are classified using field classifications [18-22] and other identi fication parameters .
OPEN ACCESS IJG
R. B. VERLA ET AL.
111
Figure 3. (a)-(e): Bott om sect ions of a debris flow in Sabga. (a) Outcr ops of volcani c debris fl ow show ing fragments of ro cks
in a fine grained matrix that has been weathered. (b) Bottom section of a volcanic debris flow. The upper section above is
about 60 m on average. (c) Saline pre cipi t ates on th e bott o m se cti on of the lahar. (d) Basaltic boulders cemented by c arbon ate
material. (e) Yellow precipitates occurring with the white precipitates. (f) Evidence of plastic rheology which characterizes
debris flow.
secondary precipitation and salts have been observed
(Figure 2).
4.2. Characterization of Volcanic Rock and
Weathered Volcanic Debris
The volcanic rocks are geochemically classified on the
Harke r diagr am usin g the To tal Alkali ( Na2O + K2O) vs.
Silica (SiO2) classification (TAS) [22], as Basanite/
Trachy-basalt, Phonolite, Trachyte/Trachydacite and
Rhyolites (Figure 4). SiO2 concentrations range from
47% - 73.31%, while concentrations of Al2O3 ra nge fro m
11.93% - 20.49%. CaO concentrations range from
0.136% - 9.86% and MgO range from 0.103 - 4.55. Na2O
and K2O concentrations range from 2.227 - 6.534 and
2.22 - 6.776 re s pec t ively .
Incompatible trace elements (LIL, HFSEs), Rb, Nb, Zr,
Y, and Zn are pr esent with N b and Zr having the hi ghest
frequency of occurrence in the samples. The major o xide
chemistry is pr e sented in Table 1.
Comparing the major oxide chemistry of volcanic
Figure 4. Classification of Sabga volcanic rocks on TAS
diagram [22].
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R. B. VERLA ET AL.
112
Table 1. Major oxides, normative mineralogy and trace
element concentrations in v olcanic rocks.
Sample code B4 BR10 T5 RH1 T25
SiO2 47 73.21 56.98 72.66 67.28
TiO2 3.264 0.316 0.679 0.616 0.389
Al2O3 15.71 1 6.1 20.49 12.02 11.93
Fe2O3t 11.38 0.6869 7.014 2.833 5.811
MnO 0.19 0.0794 0.143 0.01 0.106
MgO 4.551 0.32 0.103 0.35 0.38
CaO 9.86 0.434 0.49 0.136 0.371
Na2O 4.311 2.27 6.534 4.485 6.106
K2O 2.22 4.633 6.075 5.061 6.776
P2O5 0.645
0.041 0.128
SO2 0.116 0.0693
0.0221 0.0334
LOI 0.76 1.89 1.51 1.8 0.69
SUM 100.007 100.0086 100.018 100.0341
100.0004
CIPW NORM
Q
40.868
29.058 21.104
C
6.562 2.274
Or 13.12 27.38 35.901 29.909 4 0.044
Ab 30.292 19.208 43.499 33.648 23.637
An 16.958 2.153 2.431
Ne 3.352
6.387
Ns
1.001 6.524
Di 13.326
0.051
Hy
0.797
0.872 0.923
Ol 3.614
0.18
Il 0.406 0.17 0.306 0.021 0.227
Tn
0.287 0.662
Pf 5.193
Ru
0.227 0.518 0.488
Ap 1.528
0.097 0.303
Sum 87.789 97.364 91.496 95.382 93.474
DI 46.764 87.456 85.787 92.618 84.606
Mg# 30.76455
34.1075 1.605462 12.07021
6.773739
Rb
81 190
251
Ba
794
Nb 69 86 424 226 425.9
Sr 969.4
Zr 319 555.6 2139 1017 2864
Y
52 36 88 77
Ga
50 40 49
V 198
Cr 267
Cu 70
Zn
66 363 124 183
Ce
530
rocks of Sabga with some rocks in other parts of the
world e.g. comparison of sample B4 and the alkali oli-
vine basalt sample KLPA-1, of Molokai volcano, Hawaii
[23] and RH1 with Circle Creek rhyolite, sample
60NC145, Nevada [24], has shown almost the same
chemistries. No peculiarities are presented by the Sabga
volcanic rocks.
The Mg number s (Mg# = MgO/MgO + FeO) with Fe O
calculated as 0.9Fe2O3t range from 1.605% - 30.765%
with an average of 17.064.
Norms are computed from the major oxides using
CIPW normative mineral calculations [25,26], giving
hypothetical mineral phases that might be present in the
volcanic rocks.
Differentiation index (DI) calculated from norms as
the sum of Q+ Or + Ab+ Ne [27] show that DI range
from 46.764 - 92.6 with an average of 79.44. Normative
minerals include quartz, corundum, orthoclase, albite,
anorthite, nepheline, nos ea n, di opside, hypes t hene, olivine,
ilmenite, titanite, perovskite, rutile and apatite (Table 1).
Normative quartz, orthoclase and albite dominate mean-
while normative anorthite is in lesser percentages. Other
mineral phases containing CaO such as Diopside, Hyper-
sthene, perovskite, and titanite occur in very small con-
centrations.
Weathered volcanic material and secondary precipi-
tates all parts of a single volcanic debris flow produce
major o xides essentially similar to the oxides in the fresh
volcanic rocks (Table 2). The concentrations of the
oxides vary especially in the precipitates but the wea-
thered materials at higher topography (CL1) have SiO2
and Al2O3 concentrations of 70.01% and 11.3% and these
are similar with those of the fresh rhyolite (RH1).
K2O concentrations are equally similar but there is a
marked reduction of about 92.7% in the concentration of
Na2O. Ash and precipitates at the base of the volcanic
debris have very high concentrations of CaO (32.4% av-
erage), SO2 (11.5 %) a nd ver y lo w conc entr atio ns o f S iO2
(17.91% average), though the base ash (ASH B) has a
SiO2 concentration of 60.3% similar to that of fresh in-
termediate trachytic rocks. CaO concentrations in some
of the precipitates (SIL, SAL1) are higher than those
commonly found in limestones.
Fe2O3t and SO2 are high (16.13 and 45.99 respective-
ly). These precipitates and bottom volcanic ash are also
characterized by high losses on ignition of average
25.9%.
4.3. Estimating Element Losses and Gains from
Fresh and Wea thered Volcanic Ma ter ial s
The lahar (Figure 3) has been studied as a zone of in-
tense weathering and leaching of material. To estimate
major losses and gains in elemental concentrations, an
enrichment and depletion diagram (Figure 5) is used.
The following assumptions are made in the calculation:
The volcanic debris is considered as heterolithologic
consisting of all the primary rock types present in the
area.
OPEN ACCESS IJG
R. B. VERLA ET AL.
113
Table 2. Major oxides and trace element concentrations in
weathered v ol canic materials and secondary preci pitates.
CL1 SIL SAL1 SAL 3Y ASH B
SiO2 70.01 1.51 3.68 6.11 60.34
TiO2 0.272 0.023 0.49 0.27 0.675
Al2O3 11.3 0.314 1.2 2.17 5.733
Fe2O3t 3.884 0.253 4.235 16.13 5.244
MnO 0.0557 0.165 0.121 0.0697 0.194
MgO
0.652
0.403
CaO 0.473 54.7 61.31 6.894 6.842
Na2O 0.328 0.472
3.28
K2O 5.971 0.136 0.775 2.076 3.675
P2O5
0.015 0.13 0.19 0.0928
SO2 0.042 0.05 0.128 45.99 0.013
LOI 7.7 41.72 27.94 20.11 13.54
SUM 100.0357 100.01 100.009 100.0097 100.0318
Rb 1055
70
Nb 202
30
Sr
835.7 267 40 130
Zr 1255 14 204 92 208
Y 235
Ga 50
Zn 349
120 70 99
La 560
0
Tb
55
Figure 5. Enrichment depletion diagram for average com-
positions of fresh and weathered volcanic rocks/precipi-
tates.
The average composition of all the fresh rock types is
considered as the original debris irrespective of the
grain size.
The material at the top of the lahar, though weathered
is considered together with the fresh rocks because of
its chemical similarity with rhyolites.
The average chemistry of all the material at the bot-
tom of the debris flow is calcu la te d.
The difference in magnitude when the two sets of
averages (major and trace elements) are considered as
ratios is calculated as the depletion (negative) or as
enrichment (positive).
CaO is seen to have been enriched up to 16.54 times
its original average value of about 1.96 while LOI has
been enriched 10.88 its original average value (2.37).
Fe2O3t, MnO, and Sr have also been enriched at the base
of the lahar but in le sser amounts (Figure 5).
High loss on ignition is related to high carbonate, bi-
carbonate and sulphate ions associated with secondary
precipita tion at the botto m of the lahar.
4.4. Hydrochemistry of Springs and Rivers
The hydrochemical data (Table 3) reveal that the domi-
nant major cation is Ca2+, followed by K+ for the brack-
ish-saline water springs and rivers. Mg2+ dominates as
second main cation for the fresh water springs. The
anions are in the order
3
HCO
>
4
SO
> Cl for the
brackish-saline water springs and
3
HCO
> Cl >
4
SO
for the fresh water springs and rivers. The total cations
(TZ+) and total anions (TZ) range from 20.09 - 917.72
mg/l and 20.83 - 12,292.32 mg/l respectively, resulting in
averages of 251.87 and 2971.9 mg/l. Ca2+ constitutes on
average 65.21%, of the total cations in meq/l while Mg2+
and Na+ + K+ contribute 21.33% and 13.45% respec-
tively.
3
HCO
constitutes averagely 76.75% of the total
anions in meq/l while Cl and SO4= contribute 19.26% and
3.98% respectively. Anomalous concentrations especial-
ly of Ca2+, K+,
3
HCO
and
4
SO
are associated with
springs labeled SP1, SP2A, SP7, W1 and W2. The per-
centage of Ca2+ increases further to an average of 71.55%
and
3
HCO
to an average of 92.16% in the highly con-
centrated springs. There is a great variation between the
cations and anions. Resulting in negative ion balances
(–65 to 92.2) in the highly bicarbonate concentrated
springs. TDS as high as 5188.8 - 11,570.2 mg/l classify
the highly concentrated spring water as brackish to saline
[28]. Piper diagram (Figure 6) places the water into Ca +
Mg-HCO3 + CO3 a nd HCO3 + CO 3 hydrochemical facies
[29]. The water from both the rivers and the springs
straddle the Ca- Mg apex of the major cation triangular
plot in Figure 6. This shows the presence of an important
Ca2+ and Mg2+ source in the area. This signature is dif-
ferent from t hose of spr ings al ong the CV L with Na+ and
K+ signatures interacting with almost the same rocks (in-
termediate-ac i d) l ike t ho se found in S ab ga . This mig ht be
signifying another origin other than the intense water-
silicate-rich rock interaction [30] we will expect in areas
with acid rich rocks; where the major cations generally
cluster around the Na+ and K+ apex of triangular plots.
Na+ normalized molar ratios [31] are calculated and
presented in Table 4 and summarized in Figure 7. It
should be noted that these values for rivers were not cor-
OPEN ACCESS IJG
R. B. VERLA ET AL.
114
Table 3 . Sa mple l ocation p oints, summary of the hydroche mistry and field parameters (EC, TDS, pH, Temp) of springs (1st
ten) and rivers (last three) .
Long. Lat
Elevation
Na+
(mg/l)
K+ Ca2+ Mg2+
Cl
HCO3
2
4
SO
NO3
NH4+
2
4
HPO
TZ+ TZ- IB EC
(µ s/ cm)
TDS
(mg/l) pH Temp
(˚C)
SP1 10.34118
6.03052 1518 6.9 2 60 .45
240.8
8.51 8 4782 82.13 0.67
0.1 565 .99
4954.93
69.3
7744.5 5188.8 7.1 20 .3
SP2A 10.3481 6.02177 1389 5 .91 44 .07
185.4
4.97 24
3050 64.64 0.1 3
0.06 430 .72
3203.41
65 5043.3 3379 7.4 22 .1
SP2 10.34824
6.02185 1389 0.4 1 5.46 18.6 6.54 8 43 1.79 0.02
0.79 56 .15 54 .6 2 5 .3 125 .1 83 .8 7.2 21
SP3 10.34981
6.01488 1322 0.2 1 2.73 11.2 6.75 9 20 0.48
0.97 0.0 8 38 .84 29.96 33.7 75 .2 50.4 8 24.2
SP4 10.33232
6.02016 1680 0.2 1 2.34 7.4 4.78 12
29
0.02
0.13 26 .91 41 .02 1.1 83 .1 55.7 6 .4 20 .6
SP6 10.32604
6.021451
1657 0.23 3.51 11.2 0.57 9 23 0.21
0.24 27 .28 32 .42 5.2 71 .2 47.7 6 .8 18 .9
SP7 10.33678
6.02327 1477 7.9 8 65 .52
27.8 8.3 2 12
6015 72.49 0.0 9
1.29 145 .74
6172.07
92.2
9267.3 6209.1 7.7 19 .7
SP8 10.33752
6.030219
1604 0.23 3.12 14.8 6.75 5 41 2.13 0.47
0.48
46.45 50.73 23 .5 108 .9 73 7.9 20 .1
W2 10 .33893
6.02003 1457 13.25
97.89
370.6
32.69
156
9248 1064.26
0.34 2.4 1 917 .72
11532.5
76 16392.1
10982.7
8.6 23 .3
W1 10 .33809
6.02029 1449 13.25
96.72
370.6
14.24
184
9675 1216.39
0.56
1.37 879 .65
12292.3
78.6
17269.0
11570.2
7.87
21.6
ST1 10.34108
6.01915 1459 0.8 5 7.8 33.4 8.1 5 9 201 6.2 0.71
1.26 0.1 2 91 .75 223.11 17.7
397.6 266.4 7 .6 18.9
RNK1
10.34111
6.02419 1446 0.2 5 2.34 11.2 1.07 6 12 0.9 1.03
27.13 20.83 30 .2 5 0 .4 33.8 7 .6 18 .1
RNK2
10.32582
6.021451
1663 0.12 1.95 7.4 1.61 5 22
0.98
0.24
20.09 27.98 5 .2 56 .8 38.1 8 .6 1 8 .4
Table 4. Na+ normalized molar ratios for Sabga spring and rive r water.
K+/Na Ca+/ Na Mg+/Na H CO3/Na SO4/Na Cl/Na
SP1 5.136324 19.96005 1.162994 259.9827 2.840482 0.749733
SP2A 4.384477 17. 99424 0.795285 194.1571 2.617642 2.63358
SP2 7.830167 26.02198 15.08509 39.45713 1.044878 12.65403
SP3 7.643734 30.59215 30.39754 35.8304 0.547039 27.79367
SP4 6.551772 20.21267 21.52596 51.95407
37.05823
SP6 8.973079 27.93196 2.343695 37.62192 0.218518 25.37683
SP7 4.827621 1.998265 0.985994 283.5787 2.17406 0.975217
SP8 7.97607 36.91009 27.75428 67.06515 2.2164 14.09824
W2 4.343948 16.04356 2.333206 262.5868 19.22331 7 .635394
W1 4.292029 16.04356 1.016361 274.711 21.97118 9.005849
ST1 5.395577 22.53922 9.067607 88.96476 1.745698 6.866672
RNK1 5.503488 25.69741 4.047602 18. 05852 0.861587 15.56446
RNK2 9.554668 35.37217 12.68816 68. 97351
27.02163
rected for atmospheric inputs because of the negligible
concentrations in rain water. Average values for Ca/Na,
Mg/Na, HCO3/Na, and Ca/Mg molar ratios are 22.87,
9.94, 129.5 and 7.5 respectively. These values are several
magnitudes higher than the high values [31] for riv-
ers/water draining basalts (Ca/Na: 0.2 - 3.15, HCO3/Na:
1 - 10, Mg/Na: 0.15 - 3.15), though the Mg/Na for the
HCO3-concentrated springs reflect those for water drain-
ing basalts (Mg2+/Na: 0.15 - 3.15).
The Na+ normalized plots show a trend which gener-
ally shifts towards and occupy the carbonate dissolution
end and a general shift away from Na+ (Figure 8). In
comparison with the diagrams in [31], the springs with
anomalously very high bicarbonates lie above the carbo-
nate dissolution zone and can be considered as zones of
bicarbonate concentration. It is also evident that these
springs are less concentrated in Mg2+ relative to the other
spri ngs a nd ri vers a nd have Mg+/Na+ mola r val ues withi n
the range for water draining basalts. Na+ normalized mo-
lar ratio trends for the brackish-saline springs (Figure 9)
sho w a gradual concentration of springs down slope rela-
tive to their sampling point elevation.
The brackish-saline springs are supersaturated with
certain mineral phases: SI of Aragonite (0.8 - 2.61), cal-
cite (0.96 - 2.86), dolomite (1.7 - 5.0), and hydroxyl
apatite (0.16 - 8.62). Mineral saturation is presented in
OPEN ACCESS IJG
R. B. VERLA ET AL.
115
Figure 6 . Piper diagra ms for the Sabga spring/ river water samples showing the major hydrochemical facies after [29].
Figure 7 . H istogram showing the Na normalized molar ratios for the spring and river w at er samples.
Figure 8 . Na normaliz ed molar rat io plots [31] f or th e spr ing and riv er w at er sa mple s s how ing the i mpo rta nt c ontr ibut io n of
carbonat e dissolution in modi fy ing the che mistry of the springs and river s.
Total Dissolved Solids
(Parts Per Million)
0.0
20,000.0
40,000.0
60,000.0
80,000.0
100,000.0
OPEN ACCESS IJG
R. B. VERLA ET AL.
116
Figure 9 . Conc e ntr a ti o n tr en d for the brac kis h/ sa line spr i ng
samples down gradient.
Table 5. The abundance of calcium oxide phases in the
minerals is indicative of the dissolution of calcium car-
bonate rocks.
Plot of TDS vs Na+/Na+ + Ca2+ (Figure 10) show a
deviation of spring water samples from the three re-
charging contributions postulated for spring water che-
mistry [32]. Such a deviation supports one of the major
outlined short comings [33] with the diagram, that there
is no place for waters which have been affected by do-
mestic contamination or other sources of hydrochemical
enrichment outside of the three proposed sources.
4.5. GIS/Hydrogeologic Models
The concentrations of major ions considered as third
coordinate axes are plotted in 3D, on the same longitude
and latitude space (first two coordinates) used in gene-
rating the 3D topographic model and stacked on the later
(Figure 11). The highest concentrations coincide with a
geomorphic structure; a shallow volcanic maar.
5. Discussion
The geologic field mapping presents a volcanic area with
volcanic domes and plugs characteristic of acid volcanism.
The satellite image (Figure 1) confirms that the area is
characterized by explosive volcanic activity due to the
presence of the Foleshelle cr ater. The rock types are gen-
erally acid rocks with the trachytic rocks comprising
about 34%, rhyolitic about 24% and tuff breccia and ba-
saltic r ocks making 15% and 7 % respec tively. The north-
ern slope of the area is made up of volcanic debris flow
(lahar) mater ial making up to 29% of the study area and is
evidence of the explosive and strongly aci di c nature of the
Sabga lithologies [8]. The rock types studied are consis-
tent with names mentioned [11,13] for r ocks occurring in
the Bamenda Mountains and also for those [14] in ne arby
Oku and Ndu. The trachytic plug, together with rhyolitic
and basaltic outcrops present very little weathering
(chemical decomposition) meanwhile the massive vol-
canic debris flow (lahar) has been weathered. Field stu-
Figure 10. TDS vs. Na+/Na+ + Ca2+ plot [32] of springs in the
Sabg a -Bamessing and environs.
dies also show that the debris is relatively weak as com-
pared to the other rock types from the deep river valleys
cut through by the Nkambie River (F igur es 2 and 10).
Where the river has been cut so deep, carbonates and
salts are seen to precipitate from fluids seeping at the
bottom of the debris. The saline springs (highly concen-
trated bicarbonate springs) are associated with very shal-
low circular marshy depressions and breccia (SP7, SP2A,
W1, and W2) or occur and have an intimate connection
with the bottom of the debris flow (SP1). Using spatial
distribution of rock types and normative mineralogy,
albite and orthoclase are dominant and by theory should
dominate in the water chemistry. The dominance will be
reflected in spring and river water draining this area as
Na+ and K+ when considering the acid nat ure of the vol-
canic rocks. The dominance of K+ and Na + has been stu-
died in many fluids draining acid and intermediate rocks
along the CVL [30]. The water chemistry has evolved
showing a chemistry dominated by Ca2+ and
3
HCO
.
This evolved chemistry closely is consistent with the
chemistry of the weathered materials and precipitates
(high CaO and LOI) at the bottom of the debris flow. The
carbonate character of some springs such as the Fossette
spring along the CVL has been linked to the decomposi-
tion of volca nic ash [3 4] . In S ab ga the re gio n enr iche d i n
secondary carbonates might be a result of prolonged
chemical weathering, resulting in element leaching and
concentration.
3
HCO
results exclusively from wea-
thering [31]. High concentrations of
3
HCO
in water
result in concentration or enrichment of Ca2+ [35]. Al-
though enriched, the same elements found in fresh vol-
canic rocks are represented in the enriched lithologies
and sa line pr ecip itates i n var ying amou nts sup por ting th e
idea of source rock isochemical dissolution [36] . The Na+
normalized molar ratio plots showing carbonate dissolu-
tion signature is a clear indication of a proximal calcium
carbonate mineral source. This is further supported by
the high SI values for Calcite, Aragonite and Dolomite
computed using aqueous speciation calculations [17]. An
enrichment process is therefore envisaged as a rechargin g
OPEN ACCESS IJG
R. B. VERLA ET AL.
117
Table 5. S a turat ion i ndices for mineral p hases in the Sabga spring a nd river water c omputed using PHREE QC.
Samples Phases SP1 SP2A SP2 SP3 SP4 SP6 SP7 SP8 W2 W1 ST1 RN K1 R N K2
Anhydrite
CaSO4
SI 1.81 1.93 3.93 4.68 / 5.01 2.83 3.94 1.03 0.84 3.23 4.38 /
log IAP 6.16 6.28 8.28 9.04 / 9.35 7.18 8.29 5.38 5.18 7.57 8.72 /
log KT 4.34 4.35 4.3 5 4.3 6 / 4.34 4.34 4.34 4.35 4.35 4.34 4.34 /
Aragonite
CaCO3
SI 1.09 1.21 1.51 1.1 5 3.08 2.48 0.8 0.9 2.68 2.1 0.24 1.86 0.76
log IAP 7.22 7.11 9.82 9.48 11.39 10.78 7.49 9.2 5 .6 5 6.21 8.54 10.16 9.06
log KT 8.31 8.32 8.3 1 8.3 3 8.31 8.3 8.3 8.3 1 8.33 8.32 8.45 8.3 8.3
Calci te CaCO3
SI 1.24 1.35 1.36 1.0 1 2.93 2.33 0.96 0.75 2.82 2.25 0.09 1.71 0.61
log IAP 7.22 7.11 9.82 9.48 11.39 10.78 7.49 9.2 5 .6 5 6.21 8.54 10.16 9.06
log KT 8.45 8.46 8.4 6 8.4 7 8.46 8.45 8.45 8.45 8.47 8.46 8.45 8.44 8.45
CO2 (g)
SI 0.56 1.01 2.64 3.7 2.23 2.6 1.02 3.32 1.75 1 2.36 3.57 4.29
log IAP 1.98 2.44 4.06 5.16 3.64 3.99 2.42 4.73 3.2 2 .4 2 3.75 4.95 5.68
log KT 1.41 1.43 1.4 2 1.4 6 1.41 1.39 1.4 1.14 1.45 1.43 1.39 1.38 1.39
Dolomite
CaMg(CO3)2
SI 1.32 1.46 2.89 1.9 5.77 5.69 1.7 1.55 5 3.41 0.53 4 .1 1.62
log IAP 15.66 15.56 19.8 18.97 22.75 22.63 15.27 18.53 12.05 13.6 17.48 21.12 18.55
log KT 16.98 17.02 17 17.07 16.99 16.95 16.96 16.97 17.05 17.01 16.95 16.93 16.93
Gypsum
CaSO4:2H2O
SI 1.58 1.7 3.7 4.46 / 4.77 2.6 3.71 0.81 0.61 2.99 4.14 /
log IAP 6.16 6.28 8.28 9.04 / 9.35 7.18 88.29 5.39 5.19 7.57 8.72 /
log KT 4.58 4.58 4.5 8 4.5 8 / 4.58 4.58 4.58 4.58 4.58 4.58 4.58 /
H2 (g)
SI 22.2 22.8 22.4 24 20.8 21.6 23.4 23.8 25.2 23.74 23.2 23.2 25.2
log IAP 25.33 25.94 25.5 27.15 23.93 24.72 26.53 26.93 28.34 26.88 26.32 26.32 28.32
log KT 3.13 3.14 3.1 3 3.1 5 3.13 3.12 3.13 3.13 3.14 3.14 3.12 3.12 3.12
H2O (g)
SI 1.63 1.59 1.61 1.53 1.63 1.67 1.65 1.64 1.56 1.6 1.67 1.69 1.68
log IAP 0 0 0 0 0 0 0 0 0 0 0 0 0
log KT 1.63 1.59 1.61 1.53 1.63 1.67 1.65 1.64 1.56 1.6 1.67 1.69 1.68
Halite NaCl
SI 8.91 8.49 10.0 10.25 10.11 10.19 8.69 10.46 7.44 7.35 9.66 10.33 10.73
log IAP 7.34 6.91 8.44 8.67 8.54 8.62 7.12 8.89 5.86 5.78 8.09 8.76 9.16
log KT 1.57 1.58 1.57 1.58 1.57 1.57 1.57 1.57 1.58 1.57 1.57 1.57 1.57
Hydroxylapatite
Ca5(PO4)3OH
SI 0.6 0.16 0.32 0 .2 8 9.24 5.15 0.71 / 8.62 5.65 0.11 / /
log IAP 3.6 3 3.38 3.6 3 12.27 8.02 2.23 / 5.35 2.53 2.98 / /
log KT 3 3.16 3.06 3 .34 3.02 2.87 2.94 / 3.27 3.12 2.87 / /
O2 (g)
SI 40.41 38.58 39.7 35.49 43.1 42.1 38.22 37.28 33.37 36.88 38.9 39.18 35.08
log IAP 43.26 41.45 42.6 38.38 45.96 44.94 41.07 40.13 36.25 39.74 41.74 42.02 37.92
log KT 2.86 2.87 2.8 6 2.8 9 2.86 2.85 2.85 2.86 2.88 3.87 2.85 2.84 2.84
NH3 (g)
SI / / / 7.26 / / / 7.87 7.32 / 7.81 / 7.59
log IAP / / / 5 .4 7 / / / 6 5.52 / 5.92 / 5.69
log KT / / / 1.79 / / / 1.87 1.8 / 1.9 / 1.91
OPEN ACCESS IJG
R. B. VERLA ET AL.
118
Figure 1 1. Major ion conc entration stacked on 3D topog raphic des ign revealing the region of ion anomaly .
OPEN ACCESS IJG
R. B. VERLA ET AL.
119
contributor to water chemistry due a deviation from
normal on the Gibbs diagram [32].
GIS and hydrogeologic models have contrib uted
enormously in the present study and these are quoted
amongst the best tools available used to characterize the
environment [37-39]. The regenerated Digital Terrain
Model (DTM) presents a real world object which is one
of the objectives of 3D GIS [37]. Geospatial data on both
2D and 3D presentation reveals a true picture of the
study area. The 3D representation brings a real picture
where friable, porous volcanic debris flow material is
weathered and the route of resultant chemical load can be
hypothesized as being vertical. Groundwater flow vectors
produced on the DTM show that the vectors are sub ver-
tical to vertical and the ions must have followed the same
pathway. The portion shown to be occupied by the vol-
canic debris flow extends on the slopes from about con-
tour 1380 m to > 1720 m above sea level. Sections of the
debris flow cut by the Nkambie River expose heights of
about 90 m on average. Thickness in synergy with spe-
cial lithology, weathering and removal of chemical load
in vertical- sub vertical pathway across the entire profile
of a volcanic debris flow might have led to a modified
chemistry which is capable of precipitating salts. Sec-
ondary carbonate deposits precipitated at about contour
1550 m are considered a point source for the high con-
centrations of Ca2+, K+, Na+,
3
HCO
, and
2
4
SO
in
some springs in the Sabga area.
Adjacent to the volcanic debris flow, the crystalline
volcanic rocks are more in an intact stage and have un-
dergone very little chemical weathering. It would rather
be erroneous visualizing such a high input of salt- for m-
ing ions to resul t from these rocks.
The topographic model with geospatial data added also
sho w how d eep the ri ver N kamb ie has dug i nto the co un-
try rock. The ease of vertical erosion and the resultant
steep and deep river valleys penetrate the zones of car-
bonate concentration and salt precipitation. The river
thus acquires the carbonate-bicarbonate signatures also
presented by the springs.
Hydrogeologic models defining the zone of anomaly
for groundwater concentration show highest concentra-
tion at low top ographic levels away fr om a hi gh elevation
source. The expected fate of the ions will be that ions
disperse and loose their salinity unless another process
causes them to concentrate.
The presence of a rim of tuff breccia surrounding a
shallow depression as shown in the topographic and
geologic maps both in 2D and 3D supports the presence
of a maar. The peak in groundwater ion concentration
coincides wit h this local explo sion structure.
6. Conclusions
As op p o sed t o the che mistr y of fluids in parts of the CVL
interacting with acid to intermediate volcanic rocks, the
Sabga fluids (springs and rivers) have adopted a carbo-
nate dissolution signature. Carbonate enrichment at the
base of a volcanic debris flow is in line with the carbo-
nate signature s adopted b y the fluids. H igh salinity (TD S)
in fluids of the Sabga is linke d to bicarbonate salts pr eci-
pitated from volcanic ash material at the base of volcanic
debris flow.
High concentrations in salinity south of this ion rich
point source (lahar) are associated with a local maar act-
ing as a sink for e lements.
This geologic spectacle whereby high salinities have
developed in a purely volcanic environment might be
linked to the following:
Explosive volcanic activity leading to the formation
of volcanic ash debris flows.
Massive thickness of the debris and associated struc-
tures such as cracks.
Intense weat hering a nd leachi ng result ing in Ca2+ and
3
HCO
enrichment.
Carbonate/bicarbonate-mineral dissolution by rivers
and springs.
Structural control which further concentrates the Ca2+,
3
HCO
2
4
SO
and other i ons.
Ackno wledgements
This article is a part of a M.Sc. thesis on water rock inte-
raction at the University of Buea by VRB. We acknowl-
edge the support and contributions of colleagues at the
University over the years. The editorial comments of the
editor are highly appreciated.
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