Vents with fumaroles on the 1999 eruptive fissures (Figure 1(c)) were mapped diligently and representative sample collected. Basaltic rocks bearing precipitates were sampled around the eruptive vents and a preliminary mineral identification done using the hand lens. The structural configuration of various fissures and splays were measured. Both the fresh and hydrothermally altered basaltic rocks sampled were split into two parts. While a section of the sample was used for thin section preparation the remaining samples were pulverized for X-ray diffraction (XRD) and X-ray Fluorescence (XRF) analysis.

Thin sections were prepared following standard procedures at the Institut de Recherche Geologigue et Miniere (IRGM), Nkolbison Yaoundé (Cameroon) and studied using transmitted and reflected light microscopy at the Department of Geology, University of Buea. The presence of major minerals where determined semi-quantitatively on the bulk fraction using PW1710X-ray diffractometer with automatic divergence slit and monochromatic CoKα radiation at the University of Botswana, Gaborone. Powder from each sample was mounted with a random orientation in an aluminum sample holder. Instrumental conditions were as follows: 40 kv, 35 nA, goniometerscan from 5˚ to 70˚ 2θ for bulk samples, step size = 0.02˚, scan speed = 1.2˚/min and count time = 0.5 s. An interactive software package (MacDiff, freeware from http://www.geolpal.uni-franfurt.de/Staff/Homepages/Petschick/classisoftware.html) was used to identify the minerals. Identifications are based on multiple peak matches using the mineral data base provided with MacDiff.

A portion of each sample was crushed in an alumina plate jaw crusher and then pulverized in a shatter box with an alumina container and puck. These powders were analyzed for major elements by X-ray Fluorescence (XRF) at Actlabs in Canada. Loss on ignition was determined by weight difference after ignition at 1000˚C. The element dispersion between the fresh basalts and the hydrothermally altered varieties were then computed using the mass balance method of [20] with TiO₂ as the least mobile element of choice. The mass change for a given element in an altered rock is calculated using the following equation:

Δ C TiO 2 . J = 100 ∗ [ ( C J . A / C J . F ) / ( C TiO 2 . A / C TiO 2 . F ) ] − 1

where C_J.A is the concentration of the element in the altered rock, C_J.F is the concentration of the element in the fresh/parent rock. The composition of the parent rock (fresh basalt) was used as a reference for the mass balance calculation.

Vent- and fissure-controlled active fumaroles sites observed along the 1999 fissures at Mount Cameron volcano are semi-circular to circular in shape (Figure 2(a), Figure 2(f), Figure 2(h)). They lie along NE-SW-trending deformational fissures that define the general trend of the Mount Cameroon rift. The fissures range in width from 0.5 m - 1 m (Figure 2(e), Figure 2(g)). Some of the fissures occur as splays characterized by low rate degassing. The walls of the vents are covered with coatings of various thicknesses and colors (Figures 2(b)-(d)). The altered rocks vary from reddish-whitish to yellowish in color (Figure 2(d)). Some are covered with thin silica films with typical lava drain features (Figure 2(d)) reminiscent of repeated lava flux in the conduit and prolonged fumarolic alteration.

The main petrographic features of the fresh basalts and their altered counterparts are presented in Figure 3. The rocks are coarse-grained and porphyritic with dominant phenocryst phases being olivine and clinopyroxene (Figure 3(a), Figure 3(c), Figure 3(e), Figure 3(h)) while plagioclase is common in the microlite-laden groundmass (Figure 3(b), Figure 3(d), Figure 3(f), Figure 3(g)). Some of the olivine phenocrysts show fracture-controlled alteration (Figure 3(a), Figure 3(c), Figure 3(e), Figure 3(h)). The phenocrysts are usually euhedral to anhedral, amoeboidal and skeletal in shape. Intergrowth textures are common (Figure 3(b)) with recognizable simple and polysynthetic twins in augite and anorthite, respectively (Figure 3(b), Figure 3(d), Figure 3(f), Figure 3(g)). Reaction textures are also common and include sieve-textured cores in olivine, serrated core-rim boundaries in clinopyroxene and cleavage-controlled coloration and alteration seams. The alteration phases are resolvable only by XRD as the material is often amorphous.

The primary and secondary minerals identified by XRD are presented in Figure 4. The dominant mineral coating on the altered rocks is sulfur (octa-sulfur, native sulfur and s-brimstone). The other alteration products include silica, feldspar (albite), gypsum, magnetite, jarosite and alunite while relicts of primary minerals phase such as augite and olivine are still noticeable (Figure 4).

The geochemical composition of both fresh and altered basaltic rocks is presented in Table 1. Compared to the fresh basalt, the altered products have elevated concentrations of SiO₂ but show significant depletion in Al₂O₃ (avg. = 4.8), Fe₂O₃^T (avg. = 4.6), MgO (avg. = 2.4), CaO (avg. = 3.6) and Na₂O (avg. = 0.8). It is remarkable that the altered samples have high loss on ignition values since the precipitates are essentially water-bearing or hydrous phases. On the gain and loss plot (Figure 5), the altered basalts show a significant gain in SiO₂ while most of the other major elements are depleted. Exceptionally, Al₂O₃, Fe 2 O 3 T , MnO, MgO, CaO and K₂O also show an increase (Figure 5) in some samples.

At active volcanoes, magmatic gases tend to interact with surrounding wall rock materials and pre-existing hydrothermal systems thereby greatly modifying the composition of the fluids, the residual gas and the wall rock [23] . Basaltic rocks

from the 1999 eruptive vents at Mount Cameroon volcano are hydrothermally altered and coated with oxidized siliceous material as well as sulfur-rich amorphous phases (Figures 2-4). This coating developed as a result of the chemical interaction between the rock substrate (basaltic glass and primary minerals) and fluids in its environment [24] [25] . Although this chemical

DL = Detection Limit.

interaction may involve external deposition from silica-saturated fluids [18] [26] , in this study microtextural evidence such as reaction rims of mineral grains suggests that the silica coating formed by in situ dissolution and re-precipitation of silica mobilized in solution as noted previously by [27] . The textural characteristics of coatings formed in this study reveal an interaction with acidic fluids and vapor. A potential source of acidity for alteration of the basaltic surfaces at active volcanoes is volatile degassing of the magma before, during and after an eruption. The high sulfur content of the altered basalts at Mount Cameroon (Figure 4) indicates that SO₂ is the principal acid source involved in coating formation at the 1999 eruptive vents. According to [5] , the most primitive glass inclusions from this volcanic edifice are volatile-rich with 1.7 wt.% H₂O, 967 ppm CO₂, 1530 ppm F, 2400 ppm S, and 1270 ppm Cl; with the F content being the highest known for basaltic glasses. These gases, once dissolved in shallow ground water produce acidic solutions capable of altering the primary minerals in the basaltic rocks. The melts upon experiencing fractional crystallization became S-saturated resulting in extensive S outgassing and the precipitation of native sulfur. This is supported by a continuous degassing of sulfur from the eruptive vents. High sulfur contents and tiny spherical sulphide grains in glass inclusions from the Mount Cameroon volcano is reported in [5] ; further evidence of S-rich melts at this edifice.

Replacement reactions in the basaltic rocks resulted in the formation of various secondary minerals. The breakdown of pyroxene and calcic plagioclase resulted in the liberation of silica and calcium. While the siliceous material is

progressively reddened by iron possibly derived from the breakdown of fayalitic olivine, the calcium is consumed in the formation of gypsum. Indeed ancient hydrothermal ore deposits that are rich in silica and iron also form by replacement reactions within silicious rocks at low temperatures [28] similar to the temperature expected at these 1999 vents. The occurrence of silica in the altered samples reflects cations depletion by acid leaching. Reddish to brownish Fe-oxyhydroxide coatings observed on the Mount Cameroon vent wall samples have been reported from mid ocean ridge basalts that have undergone acidic fluids-driven hydrothermal alteration due to the replacement of plagioclase, olivine and basaltic glass [29] . We show in this study that these processes although better documented at oceanic settings also occur on land. At oceanic settings, diffuse hydrothermal fluid flow is observed from cracks and crevices in the basaltic seafloor. At Mt Cameroon the fluids follow fissures and vents pointing to the similarity of both marine and continental volcanic degassing.

Alunite and jarosite as alteration minerals in numerous ore deposits represent advanced argillic alteration [30] . The presence of these phases in altered rocks at Mount Cameroon emphasizes the role of acidic hydrothermal fluids in the formation of such deposits. Alunite typifies S-saturated systems which upon oxidation develop sulfate-bearing phases. The presence of jarosite also supports the fact that a primary volcanically-derived source of acid is involved in the formation of the precipitates which occur as coatings [31] . Sodium mobilized from the breakdown of primary phases is used up in the formation of albite detected by the XRD technique. The occurrence of albite within this volcanic edifice points to a sodic alteration and generally, albite commonly replaces plagioclase in pervasively altered igneous rocks (e.g. [32] ).

The changes in the mineralogy of the altered basaltic rocks as described above are also reflected in their whole rock geochemistry. In fact the mass balance calculations have identified high contents of SiO₂, Fe 2 O 3 T , Al₂O₃, MnO, CaO and K₂O in some samples when compared to the fresh unaltered basalt. According to [18] , thin films of water acidified by contact with volcanic vapor can dissolve near surface basalts then precipitate amorphous silica. This results in high SiO₂ content in such samples. The sulfur dioxide emanating from the vents and fissures react with fluids such as rain water to form weak sulfuric acid which then attacks the surface of the basaltic rocks reacting with the cations and leaching them out from the system. Such remobilized cations constitute the main elements in the new mineral phases in the altered rocks, thus they have enrichments in these cations over the fresh basalt. The high loss on ignition values of the altered basalts as compared to the fresh basalt concords with the development of hydrated mineral phases through various cation-driven reactions.