Greyish-Black Rutile Megaclasts from the Nsanaragati Gem Placer , SW Cameroon : Geochemical Features and Genesis

Greyish-black rutile megaclasts from the Nsanaragati gem placer in south western Cameroon display a wide range of lithophile and siderophile elements in LA-ICP-MS analyses. TiO2 abundances exceed 94 wt.%, with FeO (up to 4.2 wt.%), SiO2 (up to 1.5 wt.%) and Al2O3 (up to 1.8 wt.%) forming noticeable contents. Minor and trace elements with significant to moderate values (ppm) include Nb (965-4814), V (729-1846), Cr (495-756), Ta (44-180), and Zr (43-210). Nb/Ta ratios range between 10.0-44.9 and place the Nsanaragati rutile grains within the Niobium rutile. The measured contents for other elements including total REE are < 150 ppm, mostly falling below detection limits. Al2O3-MgO plots (wt.%) indicate that most rutile grains fall within a crust-derived rutile field, with rare plots in the mantle-derived field. Cr-Nb plots suggest the grains are related to rutile from metapelitic rocks, rather than metamafic rocks. Temperatures calculated from Zr in rutile thermometry range from 470 to 675°C, compatible with a likely crustal metapelitic source.

Mineralogical studies of the Nsanaragati gem corundum placer in the south western region of Cameroon, show rutile, zircon and ilmenite enrichment within the heavy minerals (Kanouo et al., 2012).No detailed study has been performed in the rutile grains.Their origin is still uncertain.Given the important role played by rutile, we present in this paper, the geochemical feature of greyish-black rutile megaclasts from this ore minerals deposition in other to contribute to its characterization and determine its origin.

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Study area

Cameroon volcanic line
The Mamfe sedimentary basin in the south western region of Cameroon is one of the south eastern branches of the Benue Trough largely found in the Nigerian territory (Figure 1).The western part of this basin (Figure 2) is essentially filled by assumed Cretaceous age lithified clastic rocks of the Cross River Formation of Le Fur (1964-1965).These rocks are locally covered by the Nsanaragati and the Munaya gem placers (Laplaine & Soba, 1967, Kanouo, 2008), and underlain by gneisses, schists, and granites of the Precambrian age basement (Wilson, 1928;Dumort, 1968).The sedimentary rocks in this area include immature sandstones and conglomerates, mudstones (Le Fur, 1964-1965;Eyong, 2003) of assumed Albian to Cenomanian age (Wilson, 1928;Le Fur, 1964-1965;Dumort, 1968).Recent works on the Nsanaragati gem-bearing revealed an important concentration of megaclastic rutile, sapphires and zircons within the heavy mineral suite (Kanouo, 2008;Kanouo et al., 2012).U-Pb dating of reddish zircon grains provided an average age of 12.39±0.55Ma relating their sources to magmatic Serravalian age rocks (Kanouo et al., 2012).

Materials and Methods
A total of nineteen greyish-black rutile megaclasts from the Nsanaragati gem placer deposit were morphologically examined at the laboratory of Applied Geology-Metallogeny, Department of Earth Sciences, University of Yaoundé I, Cameroon.They were characterized with a binocular microscope after separation of the coarse to very coarse grained opaque and non magnetic heavy minerals.The characterization was based on their physical properties including habit, shape, color and degree of roundness as presented by Duplaix (1958), Parfenoff et al. (1970), and Mange & Maurer (1992).
The rutile megaclasts (NSG 1 -19) were analyzed for their major, minor and trace element contents, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analytic techniques at the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences in Wuhan (China).Each clast was mounted with epoxy resin on thin section and observed under a microscope directly linked to the Laser ablation system (LA).This system was used as the sampling tool for the Agilent 7500a quadrupole ICP-MS.The analytical protocol and data acquisition techniques are the same as in Liu et al. (2008) with GeoLas 2005 and Agilent 7500a ICP-MS instruments used for Laser sampling and acquisition ion-signal intensities.The carrier (He) and make-up (Ar) gas were mixed via a T-connector before entering the ICP.This mixture forms the central gas flow of the Ar plasma, in which N was added to decrease the detection limit and improve precision (Hu et al., 2008).Individual analysis was performed, using the Agilent Chemstation.

Rutile Morphology and Geochemistry
The grain size ranges from 3 to 6 mm.They are tabular, rounded or slightly elongated.The tabular grains are angular, whereas elongated and rounded grains are sub-blunt or blunt.The rutile megaclasts are all opaque with sub-metallic luster.
Over 45 chemical elements were analyzed in the Nsanaragati rutile megaclasts covering a wide range of lithophile and siderophile, with few chalcophile elements (Table 1).The concentration of these elements varies from one sample to another.Ti is the only quantified major element (TiO 2 content in Table 2, exceed 94.0 wt.% with a peak at 98.9 wt% (Figure 3).The other elements form the minor and trace elements suites.The trace element suites in Table 3 and 4 The rare earth element (REE) abundances (up to 98 ppm) in Table 4 are low falling within the range of trace elements.The relative low contents for rare earth elements and most of the trace elements show that they do not readily enter rutile structure during crystallization (Miller et al., 2007).The total REE (TREE) abundance ranges from 0.3 to 221.2 ppm, with the highest value been recorded in sample NSG2; differentiating this sample from the others.

Clastic Rutile Morphology
The grain size of the Nsanaragati grayish rutile megaclasts is up to 6 mm, and fall within the range of gravels or pebbles clasts in Wentworth (1922) clastic particles size classification table.The size of the rutile grains suggests that they were probably sorted from course to very coarse grained rocks.This interpretation is based on James (1851) and Roux and Rojax (2007) characterization of other clastic minerals.For these authors, the grain size of hard and weathering resistant displaced and deposited minerals such as rutile, quartz, tourmaline, kyanite, and zircon highly depends on the textural feature of their source rocks.
The studied rutile are mostly angular or blunt in feature and therefore show some morphological differences that help for the discrimination of their source parameters; as Cailleux (1964), Berthois (1970) and Selley (1982) for other clastic minerals.The angular nature is a conserved primary relic feature during transportation.These angular grains were probably sorted not too far from their source area, and may be product from local sedimentation (Cailleux, 1964;Berthois, 1975).As suggested by Kanouo et al. (2012), the blunt nature of the rutile grains may be due to constant reworking or long distance hydro-mechanical transportation in water from distal sources.

Geochemical Characteristic and Correlation
The Nsanaragati rutile megaclasts appear as an important reservoir of minor and trace elements (up to 45 elements were identified).The element contents vary from one sample to another.TiO 2 content exceed 94 wt.%, and some values are similar to those of Garon Lake (Cu-Zn) ore (Clack & William-Jones, 2004) and Yaoundé rutile (Stendal et al., 2005); although major correlations do not exist for other chemical elements.
The FeO values (0.20-4.20 wt.%) contents are significant.The FeO enrichment in sample NSG5 (> 4.0 wt.%) differentiate this sample from the others.The highest MgO (0.07 wt.%), Na 2 O (0.018 wt.%),K 2 O (0.017 wt.%), and MnO (0.027 wt.%), and lowest TiO 2 (94.4 wt.%) contents were also measured in this sample.That shows a possible enrichment of Fe, K, Na, and Mn in the source environment.Correlation between these elements may be suggested.This geochemical anomaly differentiates NSG5 from the others, and may relate their source to Fe enriched and Ti fairly depleted environment.In general, Fe contents of the studied rutile grains exceed 0.2 wt.%, and such values have been attributed to the presence of ilmenite lamellar in inclusion (Meinhold, 2010).Another feature of NSG5, is the relative high content in LREE (13.88 ppm).It is the second highest values after that of sample NSG2 (153.5 ppm).This sample also has the highest SiO 2 (1.48 wt.%) and P 2 O 5 (0.03 wt.%).The high P 2 O 5 and LREE content in this grain can be attributed to apatite inclusion.This has been reported in some gabbroic and eclogitic source rutile from eastern Alps (Miller et al., 2007).Sample NSG15 is rather characterized by the high Al 2 O 3 contents (1.77 wt.%) with P 2 O 5 and SiO 2 similar to those of NSG5.That may relate its crystallization to an aluminum rich environment.Cr, Nb, Zr, Ta and Hf are known as good source constrainers for rutile (Zack et al., 2004a(Zack et al., , 2004b;;Luvizotto and Zack, 2009;Meinhold et al., 2008;Meinhold, 2010).For example the Nb ( 964-4817 ppm) and Cr (495-756 ppm) contents (Table 2 and 3) are within the range values of metapelitic rutile ( Triebold et al., 2007;Meinhold et al., 2008) consistent with their main crystallization environment.However some of the studied rutile grains display moderate Cr and V correlation that may indicate mafic igneous contributions (Rosendaal et al., 2009) whereas the two highest Nb contents 3454 and 4814 in ppm are within the range of metasomatic rutile (3000-5000 ppm) of Rudnick et al. (2000).Silica fluid contamination from surrounding rocks, can be suggested here to explain the high Si content in some of rutile grains ( Zack et al., 2004a;2004b;Luvizotto and Zack, 2009).The fact that the studied rutile grains have high Nb and Ta contents and display Nb, Sc and Sn correlations, is mostly consistent with their crystallization in felsic environment, following the result obtained by Rosendaal et al. (2009) for other studied rutile crystals.

Zr Thermometer
Zr abundance in rutile has been used for thermometric measuring and source constraining (Zack et al., 2004a(Zack et al., , 2004b;;Luvizotto & Zack, 2009;Meinhold, 2010).Zr versus T plot diagram (Figure 8) confirms the existing correlation between these two parameters in rutile.The calculated crystallization temperature using Zack et al. (2004b) equation ranges from 470 to 675 o C (Table 2).They show that rutile with temperature ranging from 470 to 550 o C were crystallized in greenschist and bleuschists while those whose temperature ranges from 550 to 675 o C has amphibolitic to eclogitic affinities as presented in Morton & Chenery (2009) and Meinhold (2010) rutile thermometric table (Table 5).

Conclusion
The Nsanaragati greyish black rutile megaclasts are eroded product from local sedimentation and long distance hydrodynamic transportation that were probably sorted from coarse to very coarse grained rocks.They are chemically enriched rutile grains mostly grown in metapelitic source rocks.

Figure 1 .
Figure 1.Location of the Mamfe sedimentary basin in the SW region of Cameroon and SE region of Nigeria byBenkhelil (1989)

Figure 2 .
Figure 2. Geology map showing the distribution of lithology in the Mamfe sedimentary basin modified from Eyong (2003)

The
Cr and Nb values in the rutile megaclasts range from 495 to 756 ppm, and, 964 to 4814 ppm respectively, fall within the range values ofTriebold et al. (2007) andMeinhold et al. (2008)  metapelitic rutiles.ForMeinhold  et al. (2008)  rutile from metapelitic source rock are characterized by Cr<Nb and Nb> 800 ppm (eg micaschists, felsic granulites), while, those from metamafic rocks (eg eclogites and mafic granulites) have low Nb (Nb<800 ppm).If based on this hypothesis, all the Nsanaragati rutile grains are within the range for metapelitic rutile.The plotted data in Nb versus Cr (Figure6) confirm their relation with metapelitic source rutiles.They were probably crystallized in metapelitic rocks.Some of the studied rutile megaclasts reflect moderate Cr and V correlations that may indicate mafic igneous contribution for the crystallization of some the rutile(Rosendaal et al., 2009).

Figure 8 .
Figure 8. Temperature versus Zr correlation plot diagram for the Nsanaragati greyish-black rutile megaclasts

Table 3 .
, include elements whose abundance is below 1000 ppm.With more than 37 elements in each rutile, trace elements are highly represented.Within these suites, Na, Mg, P, Ca, Mn, Zn, K, Cr, W, Zr, Ta, and Sn contents do not exceed 700 ppm, while, Pb, Hf, Ba, Sb, Mo, Sr, Cu, Ga, Co, U, Th, and Sc contents are below 50 ppm.The Li, Be, B, Co, Cd, Ni, Ag, Ge, Rb, Bi, Cs, and Y contents are less than 10 ppm.Trace elements abundance in the Nsanaragati greyish-black rutile megaclasts

Table 4 .
Rare Earth elements abundances in the Nsanaragati greyish-black rutile megaclasts