Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India

Gondwaria Research, V 8, No. 2, p p . 213-222. 02005 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Gondwana Res eaTCh

Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India M. Nakagawal, M. Santoshl, C.G. Nambiar2and C. Matsubaral I

Department of Natural Environmental Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520,Japan, E-mail: rnnakagawOcc.kochi-1i.ac.jp Department of Marine Geology and Geophysics, Cochin University of Science and Technology, Kochi, India

(Manuscript received July 12,2004; accepted November 27,2004)

Abstract We report the morphological, textural and chemical characteristics of gold grains in stream gravels from the Siruvani River in Attappadi Valley, southern India. The placer gold deposits contain both primary grains with jagged grain contours and secondary grains with smooth grain margins. The primary and secondary gold grains are also distinguished by marked contrast in microtextures with the latter displaying a range of corrosion textures including striations, etch pits and chemical corrosion cavities that coalesce to form honey-comb patterns. Some of these cavities are filled with fine clay derived from lateritic weathering front. While the primary grains are characterized by high silver content (up to 35.77 wt.%) with marginal overgrowths of high purity gold, the secondary grains show exceedingly high fineness (1000 Au/Au+Ag) levels (up to 984) with no marked compositional variation indicating selective extraction of Ag and/or reprecipitation of Au. From morphological and chemical characteristics, we propose that the high purity gold grains were not derived directly from primary sources, but underwent chemical refinement in the weathering front before they were transferred to the fluvial systems. Our findings have important implications for gold exploration in the Attappadi Valley. Key words: Placer gold, morphology, chemistry, lateritic weathering front, Attappadi Valley.

Introduction The major gold deposits of the globe occur within quartz lodes associated with magmatic or metamorphic processes. The origin of the hot hydrothermal fluids which concentrated such vein gold mineralization has been variously ascribed to metamorphic devolatilization reactions, felsic magmatism, granulite formation or equilibration with a compositionallyvariable crust (Groves et al., 1988; Burrows et al., 1986; Cameron, 1988; Kerrich, 1989; Santosh et al., 1995). Gold in economic grade has also been reported from laterites in some terrains, where dust, coarse grains and even large nuggets occur in easily amenable form (Wilson, 1984; Freyssinet et al., 1989; Santosh et al., 1992). While natural gold is often an alloy of gold, silver and copper, among other impurities, intense weathering and gold mobility in lateritic profiles can sometimes result in the formation of very high purity gold grains, such as those reported from Nilambur region in southern India (Santosh and Omana, 1991). The differential solubilities of gold and silver in low temperature fluids lead to Au-Ag decoupling in the

weathering front. Selective extraction of silver from the primary ores, or differential precipitation of gold at the fluid front can generate supergene gold deposits with enhanced grade (Santosh, 1994). Mechanical translocation of dispersed gold grains in the primary and supergene source regions, physical transport in fluvial systems, and accumulation in the form of placers constitute another potential means of generating commercially exploitable reserves of gold. Such fluvial transport of gold grains often lead to an enhancement in their purity by leaching out silver (Desborough, 1970; Grant et al., 1991; Santosh et al., 1992). Thus, gold deposits form through primary, supergene and secondary processes, with the latter two generating high purity gold. In this study, we report the morphological characteristics of gold grains recovered from stream placers along the Siruvani River in Attappadi Valley of Palakkad district in the northern Kerala region, southern India. We investigate the microtextural parameters and geochemicalcharacteristics of the gold grains and evaluate their mechanism of formation.

214

M. NAKAGAWA ET. AL.

Geological Background The geologic framework of southern India is broadly defined by a granite-greenstone terrain in the north and a granulite facies terrain in the south. Both terrains contain gold deposits of economic significance (Radhakrishna and Curtis, 1991). While the Kolar, Hutti and Ramagiri gold fields are located within the Archaean low-grade terrain, the Wynad gold field occurs in the Proterozoic high-grade terrain. The divide between the Archean and Proterozoic terrains in southern India is defined by the PalghatCauvery Shear Zone System which comprises a complex system of shears including the southern Palghat-Cauvery arm and the northern Moyar-Bhavani arm (Fig. 1). Gold in laterite was previously reported from the Nilambur Valley in Wynad district (Nair et al., 1987; Santosh and Omana, 1991). Gold grains were also reported from stream placers along the Chaliyar River and its tributaries draining the Nilambur Valley (Santosh et al., 1992). The Attappadi Valley where gold occurrence has been discovered recently (Nair and Varma, 1993a, b; Nair et al., 2005) is located along the Bhavani Shear Zone where amphibolite facies gneisses formed through the retrogression of granulite facies rocks characterize the major lithounits. Enclaves of supracrustal rocks like metamorphosed mafic-ultramafic and metasedimentary rocks occur within charnockite, hornblende gneiss, migmatitic amphibolite and granitoid of the gneissic complex. The manifestation of the shear zone in the Attappadi Valley is represented by a series of parallel/sub-parallel NE-SW trending 100 m wide mylonitic zones which define a cumulative width of nearly 8 km (Nair et al., 2005). Deformed pillow and ocelli structures are seen in metapyroxenite and talc-tremolite/actinolite schist. Nair and Varma (1993b) carried out preliminary bulk geochemical investigations for placer gold along the Siruvani River in the Attappadi Valley, where placer gold recovery has been going on since long by local miners. They correlated the gold occurrence in the placers to primary vein gold mineralization in the amphibolite/ metaultramafic enclaves within the gneisses and migmatites of the Attappadi Valley. The small bands of banded iron formations seen in the area are also reported to be auriferous (Nair and Varma, 1993b). Nair et al. (2005) reported detailed studies at Kottathara, a gold prospect towards the northern part of the present study area, where gold-quartz veins traverse chlorite schists or biotite-quartz schists developed proximal to the contact zone of metapyroxenite/amphibolite occurring within hornblende gneiss. Both massive (up to 2 m width) and non-massive (1m width) quartz veins occur here. A major quartz lode trending NE-SW discontinuously over a strike

length of 500 m has also been traced (Nair et al., 2005). The quartz vein formation post-dates penetrative deformation of the host rocks. Visible specks of gold occur in the veins, particularly where the associated sulphides have been leached out. Pyrite is the major sulphide mineral with minor chalcopyrite, chalcocite, galena and covellite. The chemistry of gold grains from various zones show that those from the weathered zone are more enriched in gold relative to silver as compared to samples of primary gold recovered from drill cores (Nair et al., 2005). The Attappadi Valley is drained by rwo third-order streams, namely the Bhavani River and the Siruvani River. Owing to the highly weathered nature of the rocks of the catchment area and the heavy rainfall in the region, the streams transport large quantities of sediments. The sandy and gravelly sediments of the low order streams, especially the main channel of the Siruvani River of the area are being panned by local miners for gold.

Sampling and Analytical Techniques Samples for the present study were collected from recent sandy/gravelly sediments deposited in the channel/point bars mostly adjacent to the bends and meanders along the course of the Siruvani River from Tumbappara in the south to Kottathara in the north (Fig. 1). Placer gold along with heavies was obtained by panning from ten locations along the stream course. Gold grains were isolated by heavy liquid separation and hand-picking under a binocular microscope and their shape and size were documented. Grain morphology was examined under the microscope and detailed investigation of the surface textures was carried out using JEOL-JSM6100 Scanning Electron Microscope (SEM) housed at the Kochi University. Selected gold grains were analysed by Scanning Electron Microscope with Energy Dispersive X-ray Spectrometry System (SEM-EDS, JEOL JSM-6500-F with EX-23000 BU) at the Center for Advanced Marine Core Research of Kochi University. Grains mounts on glass plate using petropoxy and polished well with diamond paste were analysed by electron microprobe (EPMA) at the Hiroshima University to obtain quantitative estimates on grain chemistry. Traverses were made in representative grains to check chemical gradients within the domain of individual grains. EPMA analyses were performed with a JEOL JXA-8200 Superprobe at an accelerating voltage of 20 kV and a beam current of 15 nA. The quantitative calculation of Au and Ag was done following Bence and Albee method using a-factors given in Taguchi and Hirowatari (1976). ZAF correction was applied to Cu and Fe which occur as trace components. Gondzuana Research, V . 8, NO.2,2005

PLACER GOLD FROM ATTAPPADI, SOUTHERN INDIA

Charnockite Amphibolite Metadolerite Biotite granite gneiss Hornblende gneiss Migmatitic amphibolite Metapyroxenite/ Talc-tremolite actinolite

n

OL

Pegmatite Quartz biotite gneiss/ Schist SiIIimanite quartzite/ Fuchsite quartzite Quartzo-feldspathic gneiss River Gold prospect Sample point

Fig. 1. Geological map of Attappadi Valley area showing sampling locations along the Siruvani River (modified after Nair et al., 2005).

Gondwana Research, V . 8, No. 2,2005

215

216

M.NAKAGAWA ET. AL.

Results Grain morphology Gold grains from the placer deposits along Siruvani River display a variety of shapes and size. Primary gold is distinguished by the platy, elongated or filamental nature with sharp contours and jagged edges. They generally range in size from 200-500 pm, although some large grains measuring 0.5-lmm or more are also found. Secondary gold grains are characterized by sub-rounded to rounded shapes with ovoid, botryoid or spherical outlines. Grains transitional between primary and secondary types exhibit slight rounding of edges and smoothening of the protrusions. Grain sizes of these categories vary widely from less than 200 pm up to 800 pm. Exceptionally large grains of gold up to few millimeters in size also occur rarely. SEM photographs of representative grains belonging to the different categories are shown in figure 2. The large primary grain in figure 2a shows euhederal crystal outlines and jagged edges. Gold grain in figure 2b displays the transition stage between primary and secondary, with the crystal outline still preserved although the grain contours

have been smoothened. Grains showing sub-rounded nature (Fig. 2c) and clear botryoidal shapes (Fig. 2d) characterize the secondary type. High power images under the SEM clearly bring out the textural features and growth and/or dissolution patterns of the gold grains. The primary gold grains are characterized by smooth and regular grain texture (Fig. 3a). Often the growth zones resulting from layeritic growth during precipitation from hydrothermal fluids are also visible (Fig. 3b). In secondary gold grains, a range of textures denotes various stages of corrosion and/or dissolution. In the initial stage, the grain margins become corroded generating fine grooves and etch pits, sometimes appearing as a dendritic network (Fig. 3c). The grooves and etch pits soon enlarge and coalesce to produce textures representing extreme corrosion (Fig. 3d). In some cases, textures resembling honey-comb (Fig. 4a) or etch pits enlarging to cavities (Fig. 4b) are seen. Some grains preserve the relict textures of the primary grains adjacent to corrosion cavities and honey-combs (Fig. 4c). Interestingly, many of the cavities and honey-combs are filled with a bright material (Fig. 4d) that was identified to be lateritic clay (see below).

Fig. 2. SEM photographs of gold grains from Siruvani River placers showing grain morphology. See text for discussion.

Gondwana Research, V . 8, No. 2,2005

PLACER GOLD FROM ATTAPPADI, SOUTHERN INDIA

217

Fig. 3. SEM photographs of gold grains from Siruvani River placers showing microtextures. See text for discussion.

Grain chemistry

EPAU data

EDS data

The EPMA data on gold grains are given in table 1. Backscattered electron images (BSE) of representative grains with analytical spots are shown in figure 7. The contrast in brightness distinguish the relatively dull primary gold grains (Fig. 7 a, b which are the polished grain mounts of those shown in Fig. 2 a, b) from the bright secondary grain (Fig. 7 d, which is the polished grain shown in Fig. 2d). The primary grain in figure 7a shows a thin bright rim of secondary overgrowth. Relatively darker zones also occur within various zones in the primary grains (Fig. 7c). The contrast in brightness of BSE images is a reflection of their compositional variation (see below). The electron microprobe data classify the gold grains into the following three categories. (1)Grains which are pure gold with Au content >95 wt. % and exceedingly high fineness (1000 Au/Au+Ag) of up to 983. The BSE images of these grains show high brightness (Fig. 7d). There is no marked core to rim Au/Ag variation in these grains. (2) Grains that have high silver content (up to 35.77 wt. YO)and low fineness (642) that show relatively dull appearance in BSE images (Fig. 7a, b). These grains correspond to electrum. In some cases, Ag-rich domains (31.97-35.49 wt. YOAg) can be clearly demarcated by

Representative spectra from EDS analyses of gold grains are shown in figures 5 and 6. Figure 5a shows a primary gold grain and figure 5b displays a transition between the primary and secondary types. Both grains yield sharp peaks in EDS for Au. While the spectra for the grain in figure 5a show only minor peaks for Ag, those in figure 5b include prominent peaks for Ag, in addition to the major Au peak. These gold grains thus contain both Au and Ag. On the other hand, the EDS spectra shown in figure 6a for a secondary gold grain lack any significant Ag peak. Many of the secondary gold grains carry bright zones of fine powdery material that are clearly distinguished from their brightness contrast under SEM (cf. Figs. 2c, d, 4d). These bright areas are often seen filling corrosion cavities, etch pits and honey-combs within the grains. Notably, such zones are absent in primary grains. Figure 6b shows the EDS analysis of the white band filling the depressions within a botryoidal grain. The spectra show that these bright patches are lateritic clays consisting of Al, Si and Fe. The presence of such fine clayey material within pits and cavities of secondary gold grains provides important clues on their origin, and will be discussed in a later section.

218

M. NAKAGAWA ET. AL

Fig. 4. SEM photographs of gold grains from Siruvani River placers showing microtextures. See text for discussion.

their contrast in brightness (Fig. 7 c). ( 3 ) Primary grains cores with high silver content (Au- 64.10 wt.%; Ag- 35.77 wt.%) are sometimes mantled by exceedingly fine margins (Au-98.56wt.%; Ag-1.75 wt.940) which show up as bright thin mantles in BSE images (Fig. 7a). Almost all the grains are alloys of gold and silver, with Cu occurring as trace amounts in some grains. Fe is negligible or absent in most of the grains. Compositional verification using qualitative analyses of representative gold grains did not reveal the spectra for any elements other than Au, Ag and Cu.

Discussion The primary vein gold mineralization in the study area occurs as flakes, stringers and veins or as tiny inclusions within sulphide minerals. In contrast, the secondary gold grains recovered from stream placers show sub-rounded nature and smoothening of grain contours. While the microtextures of primary gold grains are characterized by smooth surface and layeritic growth patterns, the secondary gold grains show a variety of corrosion textures starting from minor isolated etch pits to systems of regular corrosion cavities that link to form honey-comb like patters, and closely spaced elongate corrosive channels that coalesce to form a dendritic network. While some of

these textures have developed by mechanical transport of the gold grains in the stream sediments, many of the corrosion cavities and etch pits indicate chemical action, rather than simple mechanical transportation. Santosh and Omana (1991) reported textures of gold grains formed under supergene environments within lateritic weathering zones in the Nilambur Valley in southern India. The gold grains released from the primary mineralization during weathering underwent chemical dissolution, migration and reprecipitation in lowtemperature fluids of the supergene environment. The irregular grain contours of the primary grains were chemically dissolved, resulting in rounded grains found towards the upper zones of the weathering profiles, while those occurring towards the lower zones preserved jagged grain contours. Santosh and Omana (1991) proposed that the chemical rounding is distinctly different from that developed through extensive physical transport, such as in alluvial sediments. The varying intensities of etched pits developed along the grain surface and the incipient growth patterns displayed by these gold grains support the conclusion that they were developed by reprecipitation during lateritization. Santosh and Omana (1991) also reported the chemistry of these gold grains which are characterized by extremely high fineness levels (991-997) and near absence of silver as against the relatively Gondwana Reseavch, V . 8, No.2,2005

PLACER GOLD FROM ATTAPPADI, SOUTHERN INDIA

219

-

Table 1. Electron microprobe analyses of gold grains from Siruvani River, Attappadi Valley. Grain No.

Spot No.

Position

SR- 1 1

1 2

SR- 2 2 2 2 SR- 3 3 3 3 3 SR- 4 4 4 4 SR- 5 5 5 SR- 6 6 6 6

4 5 6 7 8 9 10 11 13 14 15 16 17 18 19 20 21 22 23 24

Core Intermediate Core Margin Margin Core Core Margin Core Core Margin Core Margin Core Core Core Core Core Core Core Margin Margin

0.00

100

2.00

Au

97.04 96.58 96.13 95.58 96.61 96.93 63.85 96.97 64.10 64.49 98.56 97.84 96.10 97.00 97.36 94.70 95.65 95.23 80.75 80.80 67.16 64.87

3.00

Ag

Total (wt.%)

Fe

Cu

Fineness (1000 Au/Au+Ag)

1.67 0.76 0.00 99.47 1.64 2.90 2.87 3.00 2.99 35.36 2.49 35.77 35.20 1.75 1.76 1.77 1.71 1.78 4.03 4.20 4.11 19.68 20.01 31.97 35.49

400

0.62 0.14 0.11 0.10 0.18 0.03 0.00 0.00 0.00 0.02 0.06 0.02 0.13 0.06 0.07 0.13 0.06 0.00 0.04 0.61 0.46

5.00 keV

0.03 0.00 0.04 0.00 0.00 0.00 0.02 0.00 0.01 0.08 0.02 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.03 0.02 0.00

6.00

98.87 99.17 99.00 99.71 100.10 99.24 99.48 99.87 99.70 100.23 99.68 97.89 98.84 99.20 98.80 99.99 99.40 100.43 100.88 99.76 100.82

7.00

8.00

983 983 971 971 970 970 644 975 642 647 984 982 982 983 982 959 958 959 804 802 677 646

9.00

10.00

high silver content of primary gold grains from this area (Ag-13.13wt.%). This study indicated that the gold grains underwent chemical refinement by low-temperature fluids and natural purification. Similar process of chemical purification of gold in the weathering front have also been reported from some other regions (e.g., Freyssinet et al., 1989). The microtextures of gold grains observed in the present study compare closely with those reported by Santosh and Omana (1991). Aithough simple striations and abrasions can result by high energy fluvial transport, the intense corrosion cavities and etch pits displayed by these grains indicate that all of the grains were not derived directly from primary source rocks, but underwent chemical corrosion in the weathering front before they were transferred to the fluvial systems. Importantly, the occurrence of fine lateritic clay within the etch pits and corrosion cavities as brought out in this study indicate the residence of the gold grains in the lateritic weathering front, where they underwent chemical modification. An analogous occurrence of fine-grained gold crystals associated with kaolinite is reported as one of the various parageneses in which gold occurs in the supergene zone

000

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

keV

Fig. 5. EDS data on gold grains from Siruvani River placers. The white squares inside the grains represent the analytical spots.

Gondwana Research, V . 8, No. 2, 2005

9.00

10.00

M. NAKAG AWA BT. AL.

220

(a)

3000 Oo8 2700 2400

2100 1800 $1500

1200 900

300

krV

000

100

200

300

400

500

‘‘’

Kfc K

600

700

Au I

Au L

800

900

1000

keV

Fig 6 EDS data on gold grain from Siruvani River placers The white and black squares represent the analytical spots in (a) and (b) respectivcly

overlying the primary gold in the Fazenda Brasileiro gold deposit, Brazil (Vasconcelosand Kyle, 1989).Further proof for weathering-related chemical refinement and formation of high purity gold is provided by the gold grain chemistry. While the primary gold grains have high silver content, the secondary gold grains, including the bright secondary overgrowths on primary gold grains, are characterized by exceedingly high fineness. Although high fineness rims can sometimes develop by leaching of silver during fluvial transport (e.g., Desborough, 1970; Grant et al., 1991), the fact that the cores of most of the secondary gold grains also possess very high fineness and lack any marked

compositional gradients from core to rim suggests that these grains underwent chemical reprecipitation in the laterites before they were transferred to the fluvial system. An alternate process of formation of high-purity gold in the fluvial channel itself is as hypothesized by Seeley and Senden (1994), who provided evidence for the origin of high-purity gold by the mechanism of selective dissolution/aggregation near clay zones within paleochannel sands and gravels beneath the present river sediments where colloidal aggregates form small grains (usually >Au (Boyle, 1979; Mann, 1984). An increase in the purity of gold and enhancement in the grade of ore can be achieved by either or two processes, namely, (1) selective extraction of Ag from the primary gold grains, or (2) differential precipitation of Au from mixed Au+Ag fluids (Santosh, 1994). Where silver is selectively dissolved, the gold grains develop a high fineness margin with a concomitant decrease in the grain size. Where selective gold precipitation occurs over alloyed primary grains, there is Gondwam I
PLACER GOLD FROM RTTAPPADI, SOUTHERN INDIA

221

Fig. 7. BSE iinagcs of polishcd grain mounts of gold from Siruvani River placers. The numbers within the grains represent the analytical spots. In figure 7a, the analytical spots 9 and 13 are along the fiiic bright margin of thc grain

a progressive rimward increase in the fineness, as well as an enhancement in grain size. In the present case, the development of high fineness rims around silver-enriched cores of some of the gold grains may indicate selective extraction of silver. However, the pure gold grains with high fineness cores and margins attest to chemical dissolution, migration and selective reprecipitation of gold in the weathering front.

Acknowledgments We thank G.N. Hariharan and K.R. Baiju of the Cochin University of Science and Technology (India) for help with sample collection. Thanks are due also to Prof. M. Fukuoka and Mr. Y. Shibata of Hiroshima University (Japan) for their kind help with EPMA analyses. Dr. M. Murayama and Dr. H. Asahi of the Center for Advanced Marine Core Research of Kochi University extended help with the SEMEDS analyses. We also thank Dr. R.V.G. Nair for useful review comments. Gondzvaria Research, V . 8, No. 2,2005

References Boyle, R.W. (1979) The geochemistry of gold and its deposits. Geol. Surv. Canada Bull., v. 280, pp. 431-445. Burrows, D.R., Wood, PC. and Spooner, E.T.C. (1986) Carbon isotopic evidence for a magmatic origin for Archean goldquartz vein ore deposits. Nature, v. 321, pp. 851-854. Cameron, E.M. (1988) Archean gold: relation to granulite formation and redox zoning in the crust. Geology, v. 16, pp. 109-112. Desborough, G.A. (1970) Silver depletion indicated by microanalysis of gold from placer occurrences, western United States. Econ. Geol., v. 65, pp. 304-311. Fisher, N.H. (1945) The fineness of gold, with special reference to the Morobe gold field, New Guinea. Econ. Geol., v. 40, pp. 449-495. Freyssinet, P., Zeegers, H. and Tardy, Y. (1989) Morphology and geochemistry of gold grains in lateritic profiles of southern Mali. J. Geochem. Expl., v. 32, pp. 17-31. Grant, A.H., Lavin, 0.P and Nichol, L. (1991) The morphology and chemistry of transported gold grains as an exploration tool. J. Geochem. Expl., v. 40, pp. 73-94. Groves, D.I., Golding, S.P., Rock, N.M.S., Bailey, M.E. and McNaughton, N.J. (1988) Archean carbon reservoirs and

222

M. NAKAGAWA ET. AL

their relation to the fluid source for gold deposits. Nature, 331, pp. 254-257. Kerrich, R. (1989) Archean gold: relation to granulite formation or felsic intrusion? Geology, v. 17, pp. 1011-1015. Mann, A.W. (1984) Mobility of gold and silver in lateritic profiles: some observations from Western Australia. Econ. Geol., V. 79, pp. 38-49. Nair, N.G.K., Santosh, M. a n d Mahadevan, R. (1987) Lateritisation as a possible contributor to gold placers in Nilambur Valley, southwest India. Chem. Geol., v. GO, pp. 309-315. Nair, R.V.G. and Varma, A.D.K. (1993a) Preliminary investigation for gold in parts of Attappady Valley, Palakkad district, Kerala. Records Geol. Surv. India, v. 126, Part 5, pp. 177-179. Nair, R.V.G. and Varma, A.D.K. (1993b) Preliminary investigation for placer gold along Siruvani river, Palakkad district, Kerala. Records Geol. Surv. India, v. 126, Part 5, pp. 173-177. Nair, R.V.G., Nair, M.M. a n d Maji, A.K. (2005) Gold mineralization in Kottathara prospect, Attappadi Valley, Kerala, India. Gondwana Res., v. 8, pp. 203-212. Radhakrishna, B.P. and Curtis, L.C. (1991) Gold: the Indian scene. Geol. SOC.India Pub., Min. Res. India, No. 3, 163p. Santosh, M. (1994) Gold-silver decoupling in weathering front: implications for gold exploration in lateritic terrains. J. Geol. SOC.India, v. 43, pp. 51-65. V.

Santosh, M. and Omana, P.K. (1991) Very high purity gold from lateritic weathering profiles of Nilambur, southern India. Geology, v. 19, pp. 746-749. Santosh, M., Philip, R., Jacob, M.K. and Omana, P.K. (1992) Highly pure placer gold formation in the Nilambur Valley, Wynad Gold Field, southern India. Mineral. Deposita, v. 27, pp. 336-339. Santosh, M., Nadeau, S. and Javoy, M. (1995) Stable isotopic evidence for the involvement of mantle-derived fluids in Wynad gold mineralization, South India. J. Geol., v. 103, pp. 718-728. Seeley, J.B. and Senden, TJ. (1994) Alluvial gold in Kalimantan, Indonesia: a colloidal origin ? J. Geochem. Expl., v. 50, pp. 457-478 Taguchi, S. and Hirowatari, E (1976) Quantitative electron microanalysis of electrum by Bence and Albee method. J. Mineral. SOC.Japan, Spec. Issue v. 12, pp. 82-85 (in Japanese). Vasconcelos, P. and Kyle, J.R. (1989) Supergene geochemistry and crystal morphology of gold in a semi-arid weathering environment: application to gold exploration. J. Geochem. Expl.,v. 40, pp. 115-132. Wilson, A.E (1984) Origin of quartz free gold nuggets and supergene gold found in the laterites and soils: a review of some new observations. Australian J. Earth Sci., v. 31, pp. 303-316.

Gondwana Research, V . 8, No. 2,2005