Geochemical investigation of stream sediments from the nlonako area; littoral, Cameroon: implications for Au, Ag, Cu, Pb and Zn mineralization potentials

Nlonako area belongs to Cameroon Volcanic Line. To determine the mineralization potentials of the area, stream sediment survey and geochemistry were carried out. The results show weak anomalies in Gold and Silver with threshold values of 0.001 and 0.073 respectively. Copper, Lead and Zinc have some relatively high values which are 690ppm, 182ppm and 378ppm with mean values statistically calculated to be 73.90, 46.51 and 198.58 respectively. The data analyzed using multivariate statistical methods yielded 5 factors from Principal Component Analysis (PCA). These five factors are; Factor 1 (Ce, La, Nb, Y), Factor 2 (W, Mo, Sr, As, Cd, Ni), Factor 3 (Pb, Sn, Zn, Sb), Factor 4 (Au, Ag, Cu) and Factor 5 (Cr, Co, Bi). These factors point mostly to a possible sulphide mineralization. Gold shows high correlation with Ag and Cu while Y correlates more highly and positively with Ce and La. There is a relatively high input of the light rare-earth elements which form the highest factor (34.363%) of the total PCA variance and possibly points to a granitic source rock. The existence of mafic–ultramafic igneous rocks in the study area underlies high correlations between Cr and Co and other factors gotten from PCA.


Introduction
Sampling and analysis of stream sediments is a well-known method of mineral exploration especially at the reconnaissance and detailed scales in many areas (e.g. Ottesen & Theobald 1994). Active sediments in the channels of streams and rivers can contain high concentrations of metals derived from the weathering of mineralized rocks within the upstream catchment. When geochemical data of stream sediments are spatially displayed as well as statistically treated using appropriate procedures, they can unravel element associations that are relevant to primary exploration in a region (Atsuyuki et al, 2005). These associations can also be useful in deciphering the source region lithology and the nature of the primary mineralization, (e.g. Levinson, 1974, Plant and Hale, 1994, Key et al., 2004. Generally, the chemical composition of stream sediments can be dependent on many factors. Some of these factors are the rock type, morphology and structural setting of the catchment, and the effect of climate.  Omang et al, (2014). These works have investigated the concentrations of gold and associated elements in stream sediment in some areas of Cameroon with some giving clues on future exploration guides (e.g. Embui et al., 2013). More so, the areas have had a history of at least some legal artisanal mining and somehow explored.
However, the history of exploitation in the Nlonako area and environs is not quite clear because it is not published, although some indications of such an activity exist in the area but information on exploration here is virtually absent. The available outcrops in this area consist petrographically of some volcanic and plutonic rocks. To search for mineral potentials in Nlonako, a preliminary stream sediment survey was designed targeting some of the accessible stream beds in the area. The objective of this paper is to present and analyse preliminary baseline geochemical data which brings some information on the geochemical distribution of some elements within the heavy mineral fraction of the stream sediments of Nlonako and environs which will provide a useful guide for future exploration strategies.

Geological setting
The study area is located along the Cameroon Volcanic Line (CVL) which is an extensive range of volcanoes (Kamgang et al, 2008). The volcanoes of the CVL (Fig.1 B) are Tertiary to Recent in age and extend within oceanic and continental domains measuring about 1700km in length. It is made up of prominent features like the the Mounts Fako, Bamboutos, Bamenda, Oku, Manengouba, Nlonako, Koupe etc. Nlonako area (Fig. 1C) is made up of both volcanic and plutonic rocks both related to the CVL (Abolo et al, 2014). Most of the volcanic rocks are basaltic while the plutonic rocks include gabbros and granites mostly related to the Nlonako anorogenic complex. Gneisses and other rocks types are also found in the area (Fig. 2).

Field methods
Heavy mineral fractions of stream sediments were collected along stream beds of the study area by panning. They were sampled mostly at depths of 30-45 cm and panned on site in order to concentrate the heavy fraction that was stored in clean polyethylene bags and transported to the laboratory. During the sampling process, simultaneous site surveys were carried out in order to provide specific information in relation to the geology of the catchment.

Laboratory methods
Laboratory works were carried out by the commercial analytical laboratory (www.alsglobal.com) Australia Laboratory Services (ALS). There, the samples were first dried at 60 0 C, crushed and milled to fine powder. Five grams of each sample were then decomposed by aqua regia digestion in a graphite heating block. After cooling, the resulting solution was diluted with deionizer water, mixed and analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) followed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analytical results were corrected for interelement spectral interferences.

Results
The concentrations of various elements in the stream sediment samples are presented on Table 1. For base metals; concentrations are characterized by high values and wide ranges e.g. Cu (4.28-690ppm), Pb (3.84-182ppm), Zn (53.6-373ppm) with mean concentrations of 73.902ppm, 46.508ppm and198.58ppm respectively ( Fig. 3 and 4). Precious metals like Silver (Ag) values are all above detection limit with the maximum being 1.15ppm. Gold (Au) was not detected in all samples but the highest values of concentration were found in sample ARSS12 (0.102ppm). However, the ranges of concentration of precious metals are 0.0001 -0.102ppm for gold and 0.004-1.15ppm for silver (Fig. 3) and the means of Ag and Au are 0.11 and 0.08 respectively ( Table 1). Some of the transition metals include Cr, Ni and Co with concentration ranges between 39.1-311ppm, 14.15-349ppm and 16.3-81.6ppm respectively with their concentrations in all stream sediment samples above detection limit. Some Platinum Group Elements (PGE) were also found in the stream sediment samples like Palladium and platinum but Pd was found only in sample ARSS08 and ARSS13 while platinum was found in all other samples except in samples ARSS03, ARSS04, ARSS05, ARS06 and ARSS13. Values of PGEs found range from 0.06-0.001ppm and0.004-0.002ppm respectively for Pd and Pt respectively. From Table1 Cerium (Ce) is the most abundant light REE with mean concentration values of 143.96, with maximum and minimum values in ARSS12 and ARSS09 respectively, followed by La and Y. Thorium (Th) is the most abundant Radioactive Element with mean concentration values of 74.30ppm, with maximum and minimum values in ARSS01 and ARSS09 respectively followed by Pb, Sr, U and Rb.     The highest concentration of Cu (690ppm) was found in ARSS13 sampled around locality 12 which yields the highest gold concentration. The same location (locality 13) which is made up of granitic and gabbroic rocks yields the highest concentration of Cr (389ppm) and the values of Pb, Co, Zn, Ni are amongst the highest values in the whole study area. The highest Zn concentration (373ppm) amongst all samples is found in ARSS10 and the lowest concentration of Zn (53.6ppm) was found in ARSS12. La and Ce have concentration ranges of 9.56-186ppm and 31.3-389ppm respectively. For the presentation and interpretation of the data to be enhanced, graduated point symbol maps for some elements that make up the factors 3 and 4 were generated (Fig. 5). These plots show that most of the gold greater than 0.0009ppm occurs to the North eastern part of the study area as well as one sample in the south western part (Fig. 5). Cu and Pb (Fig. 5) concentrations above 52ppm are found both in the south western and north eastern parts of the study area while concentrations of Zn greater than 250ppm are found both in the north eastern and south eastern parts of the region (Fig. 5). Some high values of Zn (>250ppm) and Au (0.0009ppm) are found in the same sample found to the north eastern part of the study area. More so, it can be observed that some of the very high concentrations of Au, Ag, Cu, Pb and Zn are found in the same samples except for a sample around the central area which is not very high in Zn concentration compared relatively.

Discussion and interpretation
Threshold values from which background and anomalous concentrations can be gotten are given by 2standard deviation × mean (2S*M, Table 1). However, from the data in this work it is evident that only two elements Au and Ag show very weak anomalies compared to geological standards. The threshold value of 0.001 for discriminating background from anomaly is lower than one Au concentration (0.102ppm in ARSS12) in the stream sediments of the studied area suggesting a weak Au anomaly.
It is also shown that the threshold value of 0.07343 for discriminating background from anomaly is lower than the Ag value in one sample (1.15ppm in ARSS13). Worth mentioning is the fact that the average concentration of Cu (73.902) is very far lower than certain Cu concentrations (690ppm in ARSS13 and 100.5 in ARSS08) in the stream sediments of the studied area. This occurs in Essel village in the North eastern part of the study area. Furthermore there was comparison of the values gotten in this work with stream sediments geochemical data from where world stream sediment geochemical baselines for certain elements including Cu, Pb, Zn As and Cd are gotten (Callender, 2004) and have equally been used to establish geochemical anomalies elsewhere gotten from both lognormalised and raw geochemical data (e.g. Carpenter 1971, Adiotomre 2014, Yilmaz, 2007, Cocker, 1996, Stanislav, 2008. Most of these anomalies are accounted for in stream sediments from streams draining over areas of indicated mines and known mineralization at points in various areas clearly described in their literature. As such, a value of Cu (50.1ppm) below average in this study is a high anomaly elsewhere, where threshold was calculated by 2×standard deviation+mean (6.22ppm as anomaly for Adiotomre, 2014). Also, the average value of Zn (198ppm) for Zn is lower than the maximum Zn content of the analyzed stream sediments. This value is found in sample ARSS8 and compared to world baseline values of Oyarzun et al (2006) relatively, it is a very strong anomaly (e.g. in Adiotomre, 2014). This is the same case with some Pb concentration which (maximum is 144ppm) is higher than the average of 46.51ppm. Worth mentioning equally is the fact that the concentration of Cu, Zn and Pb in these studied stream sediments are far higher than those studied elsewhere in Cameroon e.g. in Gouap Nkollo ( From table 3, it can be seen that Au has the highest correlation coefficient (r) of 0.566 with Cu and 0.561 with Ag. The strongest positive correlations occur between, La-Ce, Ag-Cu, La-Y, Zn-Cr, Sr-Mo. Other element pairs with positive r values include As-Sr, As-Mo, Bi-Sb, Cd-Co, Ce-Y, Co-Zn, Co-Cr, W-Ni, Pb-Y, Pb-Sb, Pb-Sn, Sn-Sb. Many elements have equally very low positive r values. These are for example Ni-Co, Pb-Cu, and Zn-Cu amongst others. To further investigate these element associations, the data were reduced by being subjected to principal component analysis (PCA), (Grunsky, 2002(Grunsky, , 2010 using the five factor model by rotation using quartimax method with Kaiser Normalization. Five factors or components were generated ac-counting for 90.975% (Table 3) of the total variance with each representing a cluster of interrelated elements within the data set. These five factors are Factor 1 (Ce, La, Nb, Y) from a REE lithology, Factor 2 (W, Mo, Sr, As, Cd, Ni) which is from sulphideultramafic inputs , Factor 3 (Pb, Sn, Zn, Sb) and factor 4 (Au, Ag,Cu), related probably to a sulpide mineralization and Factor 5 (Cr, Co, Bi) indicating an ultramafic lithology. Factor 1 accounts for 36.477% of the total variance which is made up of four of the Light Rare Earth Elements; Ce, La, Nb, Y which mostly point to a felsic rock origin especially intrusive granites. More so, the strong correlation between La and Ce (

Conclusions
1) The element anomalies found in this study are the results of geological influence and not as a result of pollution. 2) The element associations defined by multivariate statistics between Au, Cu and Ag suggests that the sulphides of Ag and Cu depositions and gold mineralization are contemporaneous, thus rocks with these sulphides are the most prospective during litho geochemical surveys in the region. 3) Since there are visible sulphide mineralizations in the field hosted by granites and factor 1 of PCA points to that effect, it can be thought that most of the indications of the sulfide mineralization can be sought in the granites in this region. More so since the gold is found in the factor with Cu we might think that searching for this sulphide mineralization might possibly lead to finding anomalous gold mineralization. This equally does not preclude the idea of the search for the presence of mafic -ultramafic rocks which can as well host such mineralization. 4) It can be suggested that input of the various elements results from weathering of mineralized rocks from within the catchment of the study area evident from field observations of such mineralization. The study equally points to the fact that there is a good mineralization potential for plutonic rocks of the Nlonako area and environs. 5) Heavy mineral geochemistry has been shown to be a suitable starting point for exploration for gold, silver, copper, zinc and lead amongst other elements within the Nlonako area and environs as points have been identified for their high concentrations within the study area. All these point to the fact that there is a good mineralization potential within the Nlonako area and environs needing further research on.