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\DOI{10.5802/crgeos.230}
\datereceived{2022-11-11}
\daterevised{2023-03-26}
\datererevised{2023-04-27}
\dateaccepted{2023-07-25}
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\dateposted{2023-09-28}
\begin{document}

\begin{noXML}

\TopicEF{Geochemistry, cosmochemistry}{G\'eochimie, cosmochimie}
%\TopicEF{Research article}{Article de recherche}

\title{First evidence of sinter and travertine in Cameroon: fault
reactivation and geothermal implications}

\author{\firstname{Milan Stafford} \lastname{Tchouatcha}\CDRorcid{0000-0003-2693-1084}\IsCorresp}
\address{Department of Earth Sciences, Faculty of Science, University
of Dschang, P.O. Box 67, Dschang, Cameroon}
\email[M. S. Tchouatcha]{inter\_milanac@yahoo.fr}

\author{\firstname{Arnaud Patrice} \lastname{Kouske}}
\address{University Institute of Technology, University of Douala, 
P.O. Box 8698, Douala, Cameroon}
\email[A. P. Kouske]{arnaudpatricek@gmail.com}

\author{\firstname{Primus Azinwi} \lastname{Tamfuh}}
\address{Department of Soil Science, Faculty of Agronomy and
Agricultural Sciences, University of Dschang, P.O. Box 222, Dschang,
Cameroon}
\address{Department of Mining and Mineral Engineering, National Higher
Polytechnic Institute, University of Bamenda, P.O. Box 39, Bambili,
Cameroon}
\email[P. A. Tamfuh]{aprimus20@yahoo.co.uk}

\author{\firstname{Alain} \lastname{Pr\'{e}at}}
\address{Res. Grp. - Biogeochemistry and Modeling of the Earth System
-~Sedimentology and Basin Analysis,  University of Brussels, Av. F-D
Roosevelt 50, B-1050, Brussels, Belgium}
\email[A. Pr\'{e}at]{alainpreat@icloud.com}

\author{\firstname{Ren\'{e}} \lastname{Toyama}}
\address{Department of Earth Sciences, Faculty of Science, University
of Bangui, P.O. Box 908, Bangui, Central African Republic}
\email[R. Toyama]{toyamarene@gmail.com}

\author{\firstname{Roger} \lastname{Feumba}}
\address{Department of Earth Sciences, Faculty of Science, University
of Yaound\'{e} I, P.O. Box 812, Yaound\'{e}, Cameroon}
\email[R. Feumba]{rfeumba2002@gmail.com}

\author{\firstname{Vannelle} \lastname{Tiokeng Ngounfack}}
\addressSameAs{1}{Department of Earth Sciences, Faculty of Science, University
of Dschang, P.O. Box 67, Dschang, Cameroon}
\email[V. T. Ngounfack]{ngounfackvanelle@gmail.com}

\author{\firstname{Vivant} \lastname{Madjingain}}
\address{Department of Earth Sciences, Faculty of Science,  University
of Ngaound\'{e}r\'{e}, P.O. Box 454, Ngaound\'{e}r\'{e}, Cameroun}
\email[V. Madjingain]{madjingainvivant@gmail.com}

\author{\firstname{Yaya Berinyuy} \lastname{Konglim}}
\addressSameAs{1}{Department of Earth Sciences, Faculty of Science,
University of Dschang, P.O. Box 67, Dschang, Cameroon}
\email[Y. B. Konglim]{berinyuyyaya@gmail.com}

\author{\firstname{Rigobert} \lastname{Tchameni}}
\addressSameAs{8}{Department of Earth Sciences, Faculty of Science,  University
of Ngaound\'{e}r\'{e}, P.O. Box 454, Ngaound\'{e}r\'{e}, Cameroun}
\email[R. Tchameni]{rigotchameni@yahoo.fr}

\shortrunauthors

\keywords{\kwd{Plio-pleistocene to actual}
\kwd{Cameroon volcanic line}
\kwd{Laopanga basin}
\kwd{Central Cameroon shear zone}
\kwd{Precambrian fault}}

\begin{abstract}
The Laopanga hot spring deposits along the ``Cameroon Volcanic Line''
(CVL) are distinctive in being both siliceous sinter and 
travertine, made up of immature amorphous silica and mainly calcite, and associated
with detrital deposits such as claystone, sandstone and conglomerate.
Their age range from Plio-Pleistocene to Actual. Sr concentrations (17
to 2304~ppm) suggest an enrichment by epithermal outflows.
$\updelta^{13}\mathrm{C}$ and ${\updelta}^{18}\mathrm{O}$ values,
ranging respectively from 1.5{\textperthousand} to
2.9{\textperthousand} V-PDB and ${-}$10.1{\textperthousand} to
${-}$6{\textperthousand} V-PDB, reflect a high temperature of the parent
solution (40~\textdegree C) related to an elevated geothermal gradient.
The europium anomalies values (Eu/Eu*: 0.54--1.78) indicate the
temperature variation of precipitated deposits related to distance of
the deep hot water flow. The~chemical compositions of the lithofacies
show the diversity of the spring deposits related to complex phenomena
of internal migration of mineralized hot water and the tectonic
controls during the Precambrian fault reactivations. 
\end{abstract}

\maketitle

{\vspace*{3pt}}

\twocolumngrid

\end{noXML}

\section{Introduction} \label{sec1}



Numerous hot springs are located along the ``Cameroon Volcanic Line''
(CVL) which is a mega-structure with oceanic and continental volcanic
fabrics from Pagalu Island to Lake Tchad displaying a ``swell and
basin'' structure \citep{Tchoua1974,TchuimegnieNgongangetal2016}.  Some
of these hot springs have travertine deposits near or far from the
water source as for the Bongongo (near Mount Cameroon) and the Ngol
(near Mount Manengouba) springs 
\citep{LeMarechal1976,Tchoua1977,Bisseetal2018,Tchouatchaetal2018}. The
study area (Figure~\ref{fig1}) concerns a small-sized ({${<}$}0.15~Km
length) sedimentary basin with thermogenic depositions. The deposits
are atypical and made up of an alternation of detrital (conglomerates,
sandstones and claystones) and chemical (travertines and siliceous
sinters) facies (Figure~\ref{fig2}). The case of alternation of
travertine and detrital deposits is also reported in southern Tunisia
\citep{Henchirietal2017}.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Geological context; (A)~Location of the
Cameroon map in Africa; (B)~Location of the studied area in the
Cameroon Volcanic Line. (C)~Geological map of the studied area
\citep[modified from][]{Lasserre1961,Lasserre1962,Temdjimetal2004,Temdjimetal2010}.}
\end{figure*}

\begin{figure}
{\vspace*{8pt}}
\includegraphics{fig02}
{\vspace*{12pt}}
\caption{\label{fig2}Synthetic litho-stratigraphic log in the study
area with location of samples. Abbreviations: Cl $=$ Clay; S $=$ Sand; Cg $=$
Conglomerate. Collected sample names: TLT02, TLT0, TLT01, TLT04, TLT1,
TLT2, TLT3, TLALI, TLAS, TLAS1, TLTSB, TLTSV (Chemical samples) and
TLAN, TLANb, TLMGF, TLMGFb, TLMGA, TLMGAb (Detrital 
samples).}
\end{figure}
 
Travertines and sinters are typical hydrothermal deposits in geothermal
fields, and like ``CVL'', they are usually associated with recent
volcanic eruptions \citep{Suhetal2003}. Two origins have been suggested
for the travertine formation 
\citep{Magninetal1991,PentecostViles1994,Pentecost2005,Ollivieretal2009}: 
(i) hydrothermal and (ii) fresh water or meteoric. Moreover,
\citet{Pentecost2005,GandinCapezzuoli2008} and
\citet{Capezzuolietal2014} distinguish the travertine and tufa deposit,
based on the following characteristics such as the precipitation water
temperatures (high in for the travertine deposits and ambient for the
tufa deposit), the fabric (mainly regularly bedded to fine laminated in
the travertine deposits and mainly poorly bedded in the tufa deposits),
the $\updelta^{13}\mathrm{C}$ values (${-}$1 to ${+}$10 V-PDB  for the
travertine deposits and  ${<}$0 V-PDB for the tufa deposits) or the
primary porosity (generally low in the travertine deposits and high in
the tufa deposits). Meanwhile, according to \citet{DellaPortaetal2017a}
travertine can be very porous indicating the\unskip\break complexity of the hot
spring carbonate deposits nomenclature.

The CVL travertines have been subject of many studies that led to their
deep origin \citep[e.g.][]{Bisseetal2018,Tchouatchaetal2018} from hot
springs, [31--49~\textdegree C, \citealp{LeMarechal1976}]. Chemical
elements such as Li, Ba, Sr and Sr in the host spring waters are
commonly associated with CO$_{2}$ {ranging from 500 to 
1050~mg/l \citep{LeMarechal1976}. The high temperature parent
solution with} ${\updelta}^{18}\mathrm{O}$ (${-}$8.4{\textperthousand} to
${-}$6.4{\textperthousand} V-PDB) and $\updelta^{13}\mathrm{C}$
(0.4{\textperthousand} to 0.5{\textperthousand} V-PDB) values at Ngol,
and ${\updelta}^{18}\mathrm{O}$ (${-}$5.8{\textperthousand} to
${-}$5.9{\textperthousand} V-PDB) and $\updelta^{13}\mathrm{C}$
(1.1{\textperthousand} to 2{\textperthousand} V-PDB) values at
Bongongo, belonging to the CVL, reflect a mixture of hydrothermal and
meteoric fluids for the genesis of travertines \citep{Bisseetal2018}.
This suggests a mixture between inorganic CO$_{2}$  and soil
CO$_{2}$ from plant respiration 
\citep[e.g.,][]{Giustinietal2017}. The heavy mineral content of these deposits
reveals that the deep fluids crossed varied types of rocks such as
plutonic and metamorphic rocks \citep{Tchouatchaetal2018}. Gastropods
and leaves occurring in the travertines suggest present-day climate
conditions \citep{Tchouatchaetal2018}. Despite the genetic link
between travertine deposits and climate is well established
\citep{Kronfeldetal1988,OsmondDabous2004,Weisrocketal2008,Tokeretal2015,Berardietal2016,Vignarolietal2016,Mancinietal2021}, 
tectonics can also be determinant 
\citep{Faccennaetal2008,DeFilippisetal2013a}. 
The link between travertines and active
tectonics has been established \citep{Tchouatchaetal2016,Tchouatchaetal2018} 
and
well known elsewhere
\citep{AltunelHancock1993,AltunelKarabacak2005,Mescietal2008,Brogietal2012,Brogietal2020,Brogietal2021a,Brogietal2021b,DeFilippisetal2013b,Kanellopoulosetal2017,Materaetal2021}.

Sinter or siliceous hot spring deposits are surface chemical
precipitates related to geothermal systems, they are largely produced
in near-neutral pH alkali chloride fluids oversaturated with silica
\citep{Hamiltonetal2019}. Fluid composition may evolve to form
acid-sulfate (--chloride), or bicarbonate springs related to alkali
chloride parent fluids that migrated\unskip\break laterally before exiting in distal
outflow zones
\citep{Giggenbachetal1994,Handleyetal2005,Schinteieetal2007,Hamiltonetal2018,Hamiltonetal2019}.
Thermogenic travertines may therefore have been deposited from the hot
spring surface distal from epithermal up-flows \citep{RenautJones2011}.

The microtextures of hot spring precipitated deposits are much
diversified and record the conditions of precipitation 
\citep[e.g.,][]{DellaPortaetal2017a,Tchouatchaetal2018,Hamiltonetal2019}.
They are widely described in the literature: for the travertine
deposits; e.g., \citet{Keleetal2011} (branching or layered textures),
\citet{Kanellopoulos2012} (shrub-like, laminated, botryoidal or
spicular textures), \citet{Polaetal2014} (porous, laminated or
feather-like laminated textures), \citet{GandinCapezzuoli2008}
(dendritic, fibrous or plume-like textures), 
\citet{DellaPorta2015,Crocietal2016,DellaPortaetal2017a,DellaPortaetal2017b}
(tabular, clotted peloidal, prismatic, dendritic, fibrous, porous or
columnar textures) and sinter deposits, e.g. \citet{Hamiltonetal2019}
(acicular needle-like growths, laminated, crenulated concentric,
layered feathering or varved textures). 

The textural characteristics (e.g., massive crystalline, vugular or
coated textures) of hydrothermal deposits depend on fluid composition
and CO$_{2}$  concentration 
\citep[e.g.,][]{Folketaletal1985,Pentecost2005,Tchouatchaetal2018}. For example,
they display a diagenetic continuum of the silica phase from amorphous
silica to hexagonal microcrystalline quartz
\citep{Herdianitaetal2000,Rodgersetal2004}.

According to \citet{Sillitoe1993} and \citet{GuidoCampbell2011}, sinter
deposits occur at the intersection of water table and Earth's surface,
in subaerial volcanic fields where heat input is sufficient to move
hydrothermal flow of ground water from deep-seated reservoirs. 

The world geography of mineralized hot springs is largely dominated by
travertine facies. Although travertine-siliceous sinter coexistence is
rare, it is however reported in the Yellowstone National Park, USA
\citep{Foukeetal2000,BraunsteinLowe2001,ChafetzGuidry2003,LoweandBraunstein2003,HinmanWalter2005} 
and at southern Lake Bogoria, Kenya \citep{Renautetal2017}.

In this study, the geochemical (major and trace elements, stable carbon
and oxygen isotopes) and mineralogical (XRD and heavy minerals)
analyses of varied hydrothermal deposits are presented, with the aim to
constrain the origin of fluids and dissolved elements for the
hydrothermal deposits in the Laopanga area, and their conditions of
deposition and chemical variations. A correlation with other CVL hot
spring deposits will be established.

\section{Geological setting}\label{sec2}

The CVL (Figure~\ref{fig1}A), on which is located the study area, is a
volcanic megastructure \citep{Benkhelil1982,Fitton1983} comparable to
the Benue trough \citep{Fitton1983,Dunlop1983}. Its origin is
attributed to the rejuvenation of the Central Cameroon Shear Zone
(CCSZ) and Precambrian faults
\citep{Burkeetal1971,Ngangom1983,BrowneFairhead1983,Tchouatcha2011}.
From Niger-Nigeria to Cameroon, the interaction between the Precambrian
faults and hot spots controlled the magmatic expression during the
Phanerozoic times \citep{Ngakoetal2006}. Precambrian tectonic
rejuvenation during Phanerozoic times are well documented 
\citep{LeMarechalVincent1971,Ngangom1983,Dumont1984,Dumont1987,NjikeNgaha1984,Tchouatchaetal2010,Tchouatchaetal2016,Tchouatchaetal2018}. 
They induced the Cenozoic thermo-metamorphism \citep{Ngangom1983} that
affected the basal Cretaceous conglomerates along the CCSZ in
Djerem-Mbere \citep{Tchouatcha2011,Tchouatchaetal2016} and are also
related to the South Atlantic Ocean opening. In this particular
context, the CVL represents a N30E megasplit stress related to the Pan
African N70E leap reactivation
\citep{CornacchiaDars1983,Moreauetal1987,Moreauetal1994,Deruelleetal1991,Montignyetal2004}.

The study area is located on the Adamawa Plateau near the Ngaoundere
region (Figure~\ref{fig1}B), and varied basement and volcanic rocks,
consisting of pre-Pan African granitoids and metamorphic formations are
exposed in this area \citep{Tchamenietal2006}, Figure~\ref{fig1}D). The
granitoids are intensively deformed and metamorphosed under the
amphibolite-facies. Monazite dating is not conclusive, but strongly
indicates pre-magmatic monazite inheritance at ca. 926~Ma
\citep{Tchamenietal2006}. The volcanic rocks composed of mafic lava
flows (basanites, alkali basalts and hawaiites) dated at  $7.8 \pm
1.4$~Ma to $6.5 \pm 0.2$~Ma are associated with abundant phonolite and
trachyte domes and plugs dated from  $10.9 \pm 0.4$~Ma to
$6.2\pm0.2$~Ma \citep{Marzolietal1999,Temdjimetal2004}.

Our field work at Laopanga shows that the sedimentary basin is crossed
by hot spring (40~\textdegree C) which lead to the formation
chemical precipitates (travertines and siliceous sinters) alternating
with detrital deposits containing varied volcanic debris
(conglomerates, sandstones and claystones) from continental erosion,
indicating the post-volcanic deposits, probably the Plio-Pleistocene to
Actual age, according to the geochronological data of volcanic rocks
\citep{Marzolietal1999,Temdjimetal2004}.  Intense fracturing and
shearing affecting the granitic basement overlain in unconformity by
these deposits, indicate the important role of tectonics control on
the deep fluids and their circulation.  The other hot springs,  with
only travertine deposits, are known  along the CVL such as at Ngol
(29~\textdegree C) near Mount Manengouba with a cascade morphology
and  Bongongo (49~\textdegree C) near Mount Cameroon in a swampy
area \citep{Bisseetal2018,Tchouatchaetal2018}.

\section{Methods}\label{sec3}

The study area is located in a mall depression ({${<}$}0.15~Km length)
near Laopanga village. Twenty-five representative samples of various
rock types (detrital and chemical facies) were collected on an 8 to 10
m stratigraphic spacing, were subjected to petrography, geochemistry
(major and trace elements, REE and carbon and oxygen isotopes) and
mineralogy (XRD and heavy minerals). 

Twenty polished thin sections (sandstone, claystone, travertine,
calcareo-siliceous sinter and sinter) were prepared at Langfang Rock
Detection Technology Services Ltd in Hebei (China). Their microscopic
study was carried out using a polarized microscope at the Laboratory of
Petrology and Structural Geology of the University of Yaound\'{e} 1 in
Cameroon. 

X-ray diffraction patterns were obtained from a Bruker D8-Avance Eco
1~Kw diffractometer (Copper $K{\upalpha}$ radiance,
${\uplambda}=1.5418~\AA$,  $V= 40,125$~KV, $I = 25$~Ma) with Lynxeye Xe
energy dispersive detector in the laboratory of ``Argiles,
G\'{e}ochimie et Environnements S\'{e}dimentaires (AGES)'' at the
University of Li\`{e}ge in Belgium. The analyses were carried out on
the non-oriented powder with grinded particles ${<}50~\upmu\mathrm{m}$
of bulk material of eleven (11) representative chemical samples
(travertine, calcareo-siliceous sinter and sinter).\looseness=-1

Geochemical analyses of eighteen whole rock samples were carried out at
Bureau Veritas Commodities, Vancouver, Canada. Prepared samples
(homogenized powder) were mixed with LiBO2/Li2B4O7 flux. Crucibles were
fused in a furnace at 1000~\textdegree C. The cooled bead was dissolved
in ACS grade nitric acid. Trace elements (including rare earth elements
$=$ REE) were determined by the inductively coupled plasma mass
spectrometry (ICP-MS). Major elements oxides were obtained by
inductively coupled plasma-atomic emission spectrometry (ICP-AES). Loss
on ignition (LOI) was determined by igniting a sample split then
measuring the weight loss. The assays uncertainties varied from 0.1\%
to 0.04\% for major elements, 0.1 to 0.5\% for trace elements and 0.01
to 0.5~ppm for rare earth elements. Accuracy for REE is estimated at
5\% for concentrations ${>}$10~ppm and 10\% when lower. The analyzed
samples concern twelve (12) pure chemical samples (travertine,
calcareo-siliceous sinter and sinter) and six (06) detrital samples
with chemical binder (claystone and sandstone).\looseness=-1

Chemical deposits from study area are highly dominated by siliceous
facies. The carbonate beds differ from sinter beds in colour and
structure, and contained enough carbonates to be analyzed for stable
isotopes. Nine samples (09) from carbonate beds (${<}$1~m-thick for
the total) were used for stable isotope analyses. They were
micro-drilled from cut surfaces corresponding to areas subjected to
thin section study. Analyses were performed at the University of
Erlangen (Germany). Samples were reacted with 100\% phosphoric acid
[density $>$ 1.9; \citealp{WachterHayes1985}] at 75~\textdegree C~using a
Kiel III online carbonate preparation line connected to a
Thermo-Finnigan 252 mass spectrometer. All values are reported in per
thousand ({\textperthousand}) relative to V-PDB. Reproducibility was
checked by replicate analysis of laboratory standards and was better
than ${\pm}0.04${\textperthousand} (for $\updelta^{13}\mathrm{C}$)
and 0.07{\textperthousand} ($\updelta^{18}\mathrm{O}$).

The method for heavy minerals analysis is that of
\citet{Parfenoffetal1970}. After crushing (detrital deposits:
sandstone and conglomerate matrix) and decalcification by HCl
(travertine deposits), the heavy minerals ($d > 2.89$) of six (06)
samples were extracted using ``bromoform'' (heavy liquid).  Their
identification and proportions were carried out using a polarized
microscope at the Laboratory of Petrology and Structural Geology of the
University of Yaound\'{e} 1 in Cameroon.

\section{Results}\label{sec4}

\subsection{Field results and facies description}\label{sec41}

The studied outcrops, located in a vegetated river valley
(Figure~\ref{fig1}C), extend over 50 m and has a height varying from 8
to 10 m. From bottom to top, the succession consists of an alternation
of precipitated (travertine and sinter) and detrital (claystone,
sandstone and conglomerate) facies that overlain the sheared and
fractured granitic basement (Figure~\ref{fig2}), the contacts between
these lithologies are well visibles but sometimes buried. Their
thicknesses vary from 50~cm to 4~m and 20~cm to 1.5~m respectively. 

\subsubsection{Precipitated deposits}

They are represented by three lithofacies based on CaCO$_{3}$ and
SiO$_{2}$ concentrations. They are as follows: 
\begin{itemize}
\item  \textbf{Siliceous sinter}  \textbf{lithofacies}  with 
$\mathrm{SiO}_{2}> 90\%$ (between 90.2 and 94.8\%), $\mathrm{CaCO}_3<
3\%$ (between 0.14 and 2.7\%); 
\item  \textbf{Calcareous--siliceous sinter lithofacies}  with 
$\mathrm{SiO}_2> 50\%$ (between 63--67\%), $\mathrm{CaCO}_3 < 50\%$ 
(between 28.59 and 29.91\%); 

\item \textbf{Travertine lithofacies}  with $\mathrm{CaCO}_3 > 50\%$
(between 54 and 83\%), $\mathrm{SiO}_2< 40\%$ (between 9.1 and 39.1\%). 
\end{itemize}
 
The main characteristics of the deposits of each of the three groups
are as follows: 

\paragraph{Calcareo-siliceous sinter deposits}

The deposit is located near the actual hot spring with thickness
${<}$25~cm and stacked leafs  (Supplementary Figure~1). They are whitish
grey to yellowish grey, laminated with abundant bioclasts (plant debris
fossils), compact and less porous than the previous deposits, sometimes
showing varied and vertical composition or lithology (calcareous
siliceous sandstones and mudstones). They are very rich in well
preserved gastropods and leafs of varied shapes. Under the microscope,
they show various microstructures such as clastic (Supplementary
Figure~1) or bioclastic (Supplementary Figure~1) microstructures, with
rare angular and infra-millimetric quartz grains and pores. Bioclasts
are mainly represented by plant debris (cuticles, tissues, tiny
leaves). The laminae are thin (millimeter to infra-millimeter), sigmoid
or horizontal.\pagebreak~The pores have variable sizes and shapes (millimeter to
pluri-millimeter, elongated and sub-spherical). Ooids are very rare.

\paragraph{Travertine deposits}

The travertine thickness range from about 1 to 120~cm. Their surface
generally bears dissolution marks (Supplementary Figure~1), with
sometimes botryoidal fabric made of calcite crystals. Their color is
grey-yellowish and they are either compact or wavy laminated and
porous. Under the microscope, the facies is laminar with rare oolites
{(${<}$0.2~mm}
in diameter) and abundant bioclasts (cuticles, tissues and
small-sized leaves, sometimes ${>}$2~mm size) in a porous matrix. The
pores are plentiful, sometimes ${>}$50\%, and vary in size and shape. The
laminae are horizontal or wavy. Quartz grains are rare. The laminar
fabric is highlighted by oriented calcite grains. The main diagnostic
features of the travertine microfacies are the branching sparitic
(Supplementary Figure~1), crystalline sparitic  
(Supplementary Figure~1) and sometimes fibrous sparitic crystals.

\paragraph{Siliceous sinter deposits}

The siliceous sinter deposits is the dominant precipitated deposits
with 80 to 150 cm thick. Their colors vary from greyish to black. They
are distorted laminated with elongate voids  (Supplementary Figure~1),
displaying erosional horizontal or vertical contacts with the detrital
facies (conglomerates), thin and irregular laminated, rough bedded
affected by vertical thin mud cracks with calcite filling
(Supplementary Figure~1), showing sometimes variation between compact
and porous aspects and vertical color variation linked to varied
chemical composition. Under the microscope, they are laminar with ooids
(${<}$2~mm in size), bioclasts mixed with detrital grains (quartz
grains mainly). Siliceous laminae are infra-millimetric and of variable
shapes with wavy fabrics, anastomosed and folded, horizontal and
sometimes exhibit radial structure with syntaxial needles
(Supplementary Figure~1). Spherical to sub-spherical ooids or oncoids
are sometimes abundant and most often agglutinated. Bioclasts are
numerous and diversified with millimetric to pluri-millimetric plant
debris (cuticles, tissues) and small-sized (${<}$2~mm) gastropods and
ostracods (Supplementary Figure~1). The pores can be very abundant,
with variable shapes and sizes (${<}$1~cm). Detrital grains are
essentially angular quartz grains associated with highly weathered
minerals.

\subsubsection{Detrital deposits}

Detrital deposits consist of three main facies (claystone, sandstone
and conglomerate) enclosing\unskip\break terrigenous debris (metamorphic, plutonic,
volcanic detritus and sometimes sinter fragments in the upper part of
the sequence) and sillicicalcareous binder. As the chemical deposits,
they outcrop on both sides of Mambere stream that crosses the basin
(Supplementary Figure~2). The main sedimentary structures affecting
these facies are the bedding (Supplementary Figure~2) and lamination
(Supplementary Figure~2). They are as follows:

\begin{itemize}

\item \textbf{Black claystone}  with about 50~cm thick, is rich in
organic matter (Supplementary Figure~2) with abundant angular quartz
and feldspar grains, in a very fine to fine-grained fabric
(Supplementary Figure~2);


\item \textbf{Pink sandstone rich}  in feldspars (microcline
essentially associated with plagioclase and orthoclase) and quartz
associated with ferricrusts, basalts, granites, gneiss debris,
siliceous/calcareous binders and sometimes clayey matrix in a fine to
coarse-grained fabric (Supplementary Figure~2). The thickness range
between 25 to 100~cm;

\item \textbf{Pink conglomerates} with same mineralogical
composition as the previous sandstones associated with pebbles of same
lithology (ferricrusts, basalts, granites, gneiss) but in a medium to
very coarse-grained fabric, and the thickness varying from 60 to
200~cm.
\end{itemize}

\subsection{Geochemistry} \label{sec42}

The precipitated materials (Supplementary Tables~1, 2, and 3) were
divided into two groups: (i)~travertine/carbonate group (T:
$\mathrm{CaCO}_{3}>50\%$), and (ii)~siliceous sinter group (ST:
$\mathrm{SiO}_{2}>50\%$). An additional detrital sedimentary rocks
(DSR) is also recognized and will allow comparisons with purely
chemical facies.

\subsubsection{Major elements} 

Normalized to PAAS [Post Archean Australian Shale,
\citealp{McLennan2001}] (Figure~\ref{fig3}), ST samples are, as
expected, more enriched in SiO$_{2}$ which is depleted in the T group.
ST and T samples are enriched in CaO, mainly in the T group, and
depleted in the DSR group. This latter is enriched in TiO$_{2}$ which
is depleted in the two other groups. The three groups (T, ST, and DSR)
are depleted in all the remaining elements (Al$_{2}$O$_{3}$,
Fe$_{2}$O$_{3}$, MgO, Na$_{2}$O, K$_{2}$O, P$_{2}$O$_{5}$ and MnO). 


\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Stick diagram of average major elements normalized
to PAAS \citep{McLennan2001} Abreviations: ST $=$ siliceous sinter and
calcareo-siliceous sinter; T $=$ travertine; DSR $=$ detrital sedimentary
rock.}
\end{figure}

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Stick diagram of average trace elements normalized
to PAAS \citep{McLennan2001}. Abreviations: ST $=$ siliceous sinter and
calcareo-siliceous sinter; T $=$ travertine; DSR $=$ detrital sedimentary
rock.}
\end{figure}


\subsubsection{Trace elements}

Normalized to the PAAS \citep{McLennan2001} the T samples are more
enriched in Sr, and  ST and DSR samples are enriched in U 
(Figure~\ref{fig4}). The main remaining elements (Ba, Ni, Cs, Hf, Nb,
Ta, Th and Zr) are depleted in the precipitated facies and enriched in
the detrital ones, except for Sc, Co, Ga, Rb, V and Y depleted. Th
contents vary from 3.2 to {${<}$}0.2~ppm in the precipitated deposits
and from 9.4 and 31.1~ppm in the detrital facies, with U concentrations
ranging from 1.1 to 13.3~ppm and 3 to 12.1~ppm respectively. Sr varies
from 17.5 to 2304.1~ppm (17.5 to 2304.1~ppm and 184.1 to 455.9~ppm
respectively in precipitated and detrital facies). The dominant
elements in the studied facies are Sr, Ba and Zr. 

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Distribution patterns of rare elements normalized
to PAAS (A) and Chondrite (B) after \citet{McLennan2001} and
\citet{McDonoughSun1995} respectively.\vspace*{-5pt}}
\end{figure}

\begin{figure*}
\includegraphics{fig06}
\vspace*{-2pt}
\caption{\label{fig6}Vertical X-ray diffraction from the chemical
deposits (see Figure~\ref{fig2} for stratigraphic position of the studied
samples).}
\vspace*{-2pt}
\end{figure*}

\subsubsection{Rare earth elements}

PAAS and chrondrite-normalized [\citealp{McLennan2001} and
\citealp{McDonoughSun1995} respectively] REE pattern show that the
precipitated deposits are more depleted in REE than the detrital
deposits grouped upwards (Figure~\ref{fig5}A and B). The precipitated
sample TLALI from ST group is more enriched in high REE probably linked
to a contamination from the detrital deposits. The precipitated sample
TLTO from ST group shows a negative Pr anomaly. The fine-grained
sediments (TLAN and TLANb) from DSR facies are more enriched than those
of coarse-grained. 


\subsection{Mineralogy}\label{sec43}
\vspace*{2pt}
\subsubsection{X-ray diffraction (XRD)}
\vspace*{3pt}

Amorphous silica is characterized by a diffuse peak in all samples from
10 to 30\textdegree~angular limits (XRD diagrams) typical of
non-crystallized silica \citep{Biswasetal2018}. The strong intensity
of the peaks indicates a high proportion of amorphous silica,
cristobalite  \citep{LeMarechal1976} in the siliceous samples from  ST
group) except for samples rich in calcite  samples from T group
(Figure~\ref{fig6}). 

In the ST group, the remaining samples (TLTO, TLTSV and TLTSB) contain
calcite. The composition is strongly dominated at the base of the
profile by silica (sample TLO2), then occurs a slight 
\mbox{silica} \mbox{depletion}
(sample TLT0), followed by an important depletion of silica and an
enrichment of calcite (samples TLT01, TLT1 and TLT3). Finally, an
enrichment in silica (samples TLALI and TLAS) with a 
slight\unskip\break depletion
and enrichment in silica and calcite respectively (samples TLTSB and
TLTSV) occur at the top of the profile. 

\subsubsection{Heavy minerals}

Heavy minerals from chemical and detrital samples reveal a great
diversification with kyanite, epidote, tourmaline, zircon, hornblende
and oxides  (Supplementary Figure~3). However, the mineral assemblages
present a strong heterogeneity despite, as a general rule, oxides
dominate. Meanwhile, tourmaline is rare to absent in the detrital
samples and basaltic hornblende is present in the detrital samples and
absent in the chemical deposits. Their grain shapes are very angular to
angular. 

\subsection{Stable carbon and oxygen isotopes}\label{sec44}

Carbon and oxygen isotope compositions of nine samples are given in
Supplementary Table~4. The carbon and oxygen isotope values are
perceptibly similar or very close, with $\updelta^{13}\mathrm{C}$
varying from 1.54 to 2.90{\textperthousand} V-PDB and
$\updelta^{18}\mathrm{O}$ from ${-}$10.1 to ${-}$6{\textperthousand} V-PDB or
20.50 to 24.34{\textperthousand} SMOW. These values are comparable with
the average carbon and oxygen isotopic compositions of hot spring
deposits out and from Cameroon (Figure~\ref{fig7}) \citep{DellaPortaetal2017a}. 

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig07}
\vspace*{-3pt}
\caption{\label{fig7}Combined plot of ${\updelta}^{18}\mathrm{O}${\textperthousand} 
(PDB) ${\updelta}^{13}\mathrm{C}${\textperthousand}
(PDB) values from travertines presently forming in terrestrial
environments \citep[slightly modified][]{DellaPortaetal2017a}. The
${\updelta}^{18}\mathrm{O}$ and ${\updelta}^{13}\mathrm{C}$ of our
studied samples fall in the upper part of the diagram confirming a
geothermal provenance of the depositing parental water.}
\vspace*{-4pt}
\end{figure*}

\section{Discussion}\label{sec5}

\subsection{Origin of deposition}\label{sec51}

The water origin is given by stable isotopes 
\citep{Craig1953,Minissaleetal2002,Keleetal2011,DellaPortaetal2017a,
Bisseetal2018,Materaetal2021}
and the solute give idea on the fluid rock  \citep{LeMarechal1976}. 

\subsubsection{Stable isotope and hot spring composition interpretations} 

Although only nine oxygen and carbon isotopic compositions have been
recorded from the studied stratigraphic column, they are representative
of the series because they cluster around values that are very close to
each other. Carbon isotopes are widely used to characterize the origin
of travertine deposits
\citep{Craig1953,Turi1986,Cerlingetal1991,Keleetal2011,Bisseetal2018},
as their isotopic compositions are essentially controlled by
$\updelta^{13}\mathrm{C}$ values of parental CO$_{2}$ sources.
These values can be superficial (atmospheric and soil gas) or of deep
origin; crustal metamorphism,
magma/mantle degassing, ancient deep
crystallized igneous melt or hydrocarbon
\citep{OhmotoRye1979,Hoefs1987,PentecostViles1994}.
\citet{PentecostViles1994} have, for example, differentiated thermogenic
or hydrothermal travertine from meteoric deposits or ambient
temperature deposits (tufa) according to the carbon origin. These
travertines are characterized by $\updelta^{13}\mathrm{C}$ values
that vary from 0{\textperthousand} to ${-}$11{\textperthousand} while the
thermogenic ones range from ${-}$4{\textperthousand} to
${+}$8{\textperthousand} \citep{Turi1986,Pentecost2005,GuoChafetz2014}.
According to \citet{JonesRenaut2010,Rodriguez-Berrigueteetal2012}
and \citet{Pentecost2005}, the positive carbon isotopic signatures are
characteristic of thermogenic travertines. The
$\updelta^{13}\mathrm{C}$ values from our studied samples are positive
and vary from 1.54{\textperthousand} to 2.90{\textperthousand}, very
low than other values such as those from the travertine in the Central
Italy \citep[e.g.,][]{DellaPorta2015,DellaPortaetal2017b}
indicating likely the downstream CO$_2$ degassing while thermal water is
flowing and  the low effect of dissolved substrate carbonate 
\citep[e.g.,][]{Gonfiantinietal1968,Minissaleetal2002,DellaPorta2015,DellaPortaetal2017a}
that could be mainly the Precambrian magmatic and metamorphic rocks as
indicate the hot water source composition  \citep{LeMarechal1976}.
Also, the combined plot of carbon and oxygen isotopic compositions
indicates a thermogenic origin, in agreement with data of
\citet{GandinCapezzuoli2008,Polaetal2014,DellaPortaetal2017a}. 

The hot spring compositions from which precipitated deposits have been
used to characterize the origin of various travertines
\citep{Bisseetal2018,Tchouatchaetal2018}. Spring of the study area is
warm (40~\textdegree C), highly mineralized (1680~mg/l), with slight
acidity (pH $=$ 6) and rich in dissolved CO$_{2}$ (560~mg/l) 
\citep{LeMarechal1976}, characteristic of deep-derived endogenic water
as suggested by \citet{Crosseyetal2006}. The hot nature of this spring
suggests therefore a thermogenic origin the travertines
\citep{Casanova1981,Magninetal1991}. This quite weak temperature
(40~\textdegree C) is lower than the one expected for water heated by a
magma chamber and could be explained by the consequent heat loss during
the water rising toward the subsurface. $\updelta^{13}\mathrm{C}$
values (1.54{\textperthousand} to 2.90{\textperthousand}) point
effectively to a  mixture of deep and fresh waters
\citep{Crosseyetal2006}.

The presence of a thin layer of travertine in the siliceous sinter
deposit can be related to late-stage fluid deposits of different
composition \citep{Drakeetal2014,GuidoCampbell2017,Campbelletal2019},
on the one hand, and on the other hand,  \mbox{deposition} of both silica and
calcite is likely controlled by microchemical conditions and local
temperature gradient \citep{Campbelletal2002}, moreover, the
evidence of the  Cyanobacteria and other bacteria role in calcite and
aragonite nucleation in hot springs (40~\textdegree C  for Laopanga
hot spring) as the thermophilic microbes have long been implicated in
the formation of travertine and siliceous sinter 
\citep[e.g.,][]{RenautJones2000,RenautJones2011,Giggenbachetal1994,Schinteieetal2007,DellaPortaetal2022}. 
The vertical variation of some characteristic chemical minerals is
given in Supplementary  Figure~4 showing the same evolution between CaO
and Sr curves from the bottom to the top, contrary to the SiO$_2$ 
evolution, and the same evolution between $\updelta^{13}\mathrm{C}$
and ${\updelta}^{18}\mathrm{O}$ curves from the bottom to the top.

\subsubsection{Geological hypothesis}

Main mineral/water rock genetic types in the Earth's crust related to
recent volcanic and thermo-metamorphic activities have been described
by \citet{Ivanovetal1968}. Their data have been applied in the Mbere
sub-basin exposing travertines \citep{Tchouatchaetal2016} along the
active CVL \citep{Suhetal2003}. Relationships between CVL and CCSZ
(Central Cameroon Shear Zone) \citep{Tchouatchaetal2016}, and several
reactivations of the Precambrian faults indicate that an intense
fracturation was associated with exsurgence and resurgence phenomena in
CVL related to a deep endogenic spring \citep{Tchouatchaetal2018}. The
weak thermos-metamorphism that affected the Cretaceous conglomerates
along CCSZ indicates a Cenozoic reactivation of these faults
\citep{Ngangom1983,Dumont1987,Tchouatchaetal2010,Tchouatchaetal2016,Tchouatcha2011}. 
Many authors have established a link between
travertines (hot spring deposits) and active tectonics
\citep{Pentecost1995,Tchouatchaetal2016,Tchouatchaetal2018}.  In Cameroon, some
compounds appear to be specific to the geological environment; in the
hot spring, chlorine (Cl) element is generally linked to volcanic
phenomena and sulphur (SO$_{4}$) to tectonic phenomena, while carbon
dioxide (CO$_{2}$) appears to be ubiquitous, although its dominant
activity in a volcanic environment  \citep{LeMarechal1976}. These
values are 1054.72~mg/l (Cl), 40.30~mg/l (SO$_{4}$) and 500~mg/l
(CO$_{2}$) at Bongongo, 5.67~mg/l (Cl), 7.20~mg/l (SO$_{4}$) and 1050
mg/l (CO$_{2}$) at Ngol, and 12.40~mg/l (Cl), 41.79~mg/l (SO$_{4}$) and
560 mg/L (CO$_{2}$) at Laopanga respectively  \citep{LeMarechal1976};
at Laopanga, the SO$_{4}$ value (41.79~mg/L) is high than that of Cl
(12.40~mg/L) and those of Bongongo (40.30~mg/L) and Ngol (7.20~mg/L)
hot springs, indicating the important role of tectonics control on
Laopanga hot spring deposits, as the cases in  Turkey 
\citep[e.g.,][]{AltunelHancock1993,AltunelKarabacak2005,Mescietal2008}, Italy
[e.g., \citealp{Brogietal2012,DeFilippisetal2013a}; and Greece
\citealp{Kanellopoulosetal2017}]. Moreover,  the variety of
hydrothermal deposits grow in the cooling geothermal system when and
where outflow from distal silica-rich chloride parent fluids or deep
alkali chloride parent fluids upflow are replaced by CO$_{2}$-rich
fluids leads to the deposits of travertine \citep{RenautJones2011}.

Heavy mineral distribution in the chemical deposits indicates that the
deep water flowed across metamorphic (such as gneisses) and plutonic
rocks (such as granites). The reported epidote is probably derived from
hydrothermal alteration of plutonic rocks through which flowed the hot
water. Kyanite indicates metamorphic sources while zircon, tourmaline,
garnet and hornblende are from both plutonic and metamorphic sources.
Meanwhile, the presence of basaltic hornblende in the detrital deposits
indicates the erosion of volcanic source rocks.

Non-crystalline or amorphous silica in the chemical deposits is related
to diagenetic immaturity according to \citet{Campbelletal2001} and
\citet{Rodgersetal2004} and can be explained by the lack of lattice
reordering and structure water preservation, with conservation of low
density and abundant porosity.

\subsection{Evidence of deep hydrothermal migration and tectonic control}\label{sec52}

The morphological types of the studied deposits are not easy to
recognize. Field observation indicates a concatenation of chemical and
detrital deposits. The main preserved outcrops of chemical deposits are
related to intense erosion giving sometimes galleries and one main
stream crossing the deposits. 

The Sr, ${\updelta}^{18}\mathrm{O}$ and $\updelta^{13}\mathrm{C}$
values have used to depict the origin of chemical deposits 
\citep[e.g.,][]{Keleetal2011,Bisseetal2018}. In this study, the Sr
concentrations values range from 17.5 to 2304.1~ppm suggest a
deposition far (low values of Sr) and close (high values of Sr) to
paleosprings \citep{Keleetal2011}. The Sr values indicate that the
travertine deposits for Bongongo and Ngol areas are respectively close
and far from the paleosprings \citep{Bisseetal2018}. Meanwhile, Sr
values generally increase or decrease following SiO$_2$ values  (See
Supplementary Figure~4), low in high siliceous deposits and high in low
siliceous deposits, suggesting that the silica enrichment could be
derived from chemical alteration of saturated bedrock with silica
during distal hot water flows. 


Moreover, the $\updelta^{13}\mathrm{C}$ values (varying from 1.5 to
2.9{\textperthousand}~V-PDB), the fabrics (bedding, lamination,\,{\ldots})
or some characteristic microstructures (shrubs,\,{\ldots}) indicate that
the study chemical deposit were mainly precipitated directly from hot
water. 

Meanwhile, some deposits with high porosity, compact structure and with
snail's preservation could indicate the periodic precipitations from
cooled thermal water \citep{Capezzuolietal2014}.

Positive Eu anomalies are the signatures of the hydrothermal fluids
\citep{Bauetal1996}. The PAAS-normalized patterns of the studied
precipitated deposits display negative and positive Eu anomalies, but
mainly ${>}$1 (Eu/Eu*: 0.54--1.78). These values range from 0.54 to 1.78
in the travertine facies and 0.67 to 1.66 in the siliceous to
calcareous siliceous facies indicating the temperature variation of
precipitation related to the distance of the deep hot water
circulation. Moreover, these values are ${>}$1 and range from 1.21 to
1.27 in the detrital deposits suggesting likely the thermogenic effect
from precipitated deposits as they constitute the matrix of these
facies.

Furthermore, the black claystones associated with the laminations and
bedding that affected the basal conglomerates point to more reduced
environment created by tectonics. The presence towards the upper part
of the sequence of a distorted bed of siliceous sinter suggests a
filling up period. So, sedimentation and precipitation took place in an
active paleo-tectonic basin (Figure~\ref{fig8}). Evidence of the
reactivation of old faults is reported both on the basement and ancient
sedimentary deposits of the Cretaceous age
\citep{Ngangom1983,Dumont1987,Tchouatcha2011}. 

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig8}(A)~Location of the Laopanga Basin in a
faulted corridor \citep[modified from][]{LeMarechal1976} and evidence of
relationship with the Precambrian faults (CCSZ) reactivation.
(B)~Schematic geological sketch and interpretative depositional
model between detrital and chemical deposits and showing intense fault
networks affecting the Precambrian basement and recent deposits
indicating the Post-Precambrian or Recent faults reactivation.}
\end{figure}

\subsection{Age of deposition}\label{sec53}

The Laopanga Basin is located in the volcanic zone of the Ngaoundere
area. The basal detrital deposits of this basin contains the volcanic
rock debris as indicated above (heavy minerals data). The presence of
siliceous and carbonated cements in this basal deposits, on the one
hand, and on the other hand, the vertical contact between the basal
detrital and chemical deposits, suggests approximately the same age of
deposition for these deposits. Moreover, the basal deposit of the
Laopanga Basin is discordant on granite affected by a
N60\textdegree E trending fault similar to the CCSZ and these
two faults are believed to be contemporaneous.\looseness=-1 

In the Ngaoundere area, mafic lava flows (basanites, alkaline basalts
and hawaiites) are dated at $7.8 \pm 1.4$ to $6.5 \pm 0.2$~Ma, with
abundant trachyte and phonolites domes and plugs emplaced at 
$10.9 \pm 0.4$ to $6.2 \pm 0.2$~Ma
\citep{Marzolietal1999,Temdjimetal2004} indicating a
Mio--Pliocene volcanic activities in this area. The age
inferred from the palynological data as well as on the detrital
deposits [Plio-Pleistocene, \citealp{Tchouatchaetal2010}] and that of
travertine deposits [Pleistocene Holocene, \citealp{Tchouatchaetal2016}]
in the Mbere Basin located along the CCSZ, and the ages of volcanic
rocks in the studied area \citep{Marzolietal1999,Temdjimetal2004} 
suggest a Plio-Pleistocene to Actual age for
the Laopanga hot spring deposits.

\section{Conclusion}\label{sec6}

\begin{itemize}
\item According to the major elements chemical composition, and
the Sr concentrations, the hot spring deposits of Laopanga vary from
travertines to siliceous sinters, and are likely related to the upflow
and outflow of parental fluids, and The  Eu/Eu*  values  indicate the
temperature variation of precipitated deposits related to distance of
the deep hot water flow;

\item $\updelta^{13}\mathrm{C}$ and ${\updelta}^{18}\mathrm{O}$
isotope values of carbonates fall in the values typical of a
thermogenic source for the deposits, although they are very far from
the isotopic composition of magmatic carbon, because there is isotopic
fractionation during water rising. The carbon isotope value 
(${>}$0{\textperthousand}) 
suggests that the deposits are from deep and hot
waters rich in CO$_{2}$;

\item The location of the study area in a volcanic zone belonging to
the Cameroon volcanic line, the relationship between the Cameroon
volcanic line and the Central Cameroon Shear Zone, and the intense
fracturing and shearing affecting the granitic basement  indicate the
important role of tectonics control on the Laopanga deposits;

\item The variation between the siliceous sinter and travertine,
indicate likely intense geothermal up-flow and outflow along the CVL,
and the age of Laopanga deposits ranges from Plio-Pleistocene to Actual
according to the volcanic rocks age in the studied area and the
relationship between these volcanic rocks and deposits.
\end{itemize}

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgements}
We thank Professors Giovanna Della Porta and Stanislas Sizaret, and the
Editor for their for their constructive comments to improve the quality
of this work.

\back{}

\section*{Supplementary data}
Supporting information for this article is available on the journal's
website under \printDOI\ or from the author. 

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-230-suppl.pdf}}

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