Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold mine, Guiana Shield

Applied Geochemistry 17 (2002) 321–336 www.elsevier.com/locate/apgeochem

Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold mine, Guiana Shield Gabriel Voicua,b,*, Marc Bardouxa a

De´partament des Sciences de la Terre et de l’Atmosphe`re, Universite´ du Que´bec a` Montre´al (UQA`M), C.P. 8888, Succ. Centre-Ville, Montre´al (Qc), H3C 3P8, Canada b Omai Gold Mines, 176-D Middle Street, Cummingsburg, P.O. Box 12249, Georgetown, Guyana Received 21 August 2000; accepted 31 May 2001 Editorial handling by R. Fuge

Abstract Mineralogical, petrographical, and geochemical studies of the weathering profile have been carried out at Omai Au mine, Guyana. The area is underlain by felsic to mafic volcanic and sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleoproterozoic greenstone belts of the Guiana Shield. Tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith. The top of the weathering profile consists of lateritic gravel or is masked by the Pleistocene continental-deltaic Berbice Formation. Mineralogical composition of regolith consists mainly of kaolinite, goethite and quartz, and subordinately sericite, feldspar, hematite, pyrite, smectite, heavy minerals, and uncommon mineral phases (nacrite, ephesite, corrensite, guyanaite). A specific feature of the weathering profile at Omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon. Chemical changes during the weathering processes depend on various physicochemical and structural parameters. Consequently, the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile, but rather an index that measures the degree of supergene alteration that has affected each analyzed sample, independently of the depth of sampling. Thus, the mineralogical index of alteration (MIA) can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock. It can also aid in establishing a quantitative relationship between intensity of weathering and mobility (leaching or accumulation) of each element in each analyzed sample. At Omai, some major and trace elements that are commonly considered as immobile (ex: TiO2, Zr, etc.) during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance. In addition, due to higher ‘‘immobile element’’ ratios, the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of
20 units is necessary for correct classification. In contrast, these elements could aid in defining the material source in sedimentary rocks affected by weathering. Generally, the rare-earth element (REE) patterns of the bedrock are preserved in the saprolith horizon. This can represent a potentially useful tool for geochemical exploration in tropical terrains. Strong negative Ce and Tb anomalies are displayed by weathered pillowed andesites, which are explained by the influence of the water/rock ratio. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Over the past several decades large databases have been collected to document variations of major and

* Corresponding author. E-mail address: [email protected] (G. Voicu).

trace elements related to chemical changes during intense weathering in various climatic environments (Duddy, 1980; Topp et al., 1984; Davies et al., 1989; Ange´lica and da Costa, 1993; Braun et al., 1993; Boulange´ and Colin, 1994; Walter et al., 1995; Valeton et al., 1997; Hill et al., 2000; Sharma and Rajamani, 2000). Although the chemical changes that affect the bedrock during weathering are complex, the geochemistry probably

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represents the most promising tool for discriminating weathered rock types, and, consequenctly an exploration guide for mineral deposits (Davy and El-Ansary, 1986). The degree of weathering has been commonly considered as related to the depth of sampling, i.e. weathering index gradually decreases with depth. Fundamentally this is true, but, as observed at Omai, there is not a linear relationship between weathering intensity and depth, due to specific physicochemical and structural characteristics of the bedrock. These characteristics include the original chemical composition of the rock, mineralogy, fabric, hydrothermal and metamorphic alteration, joints, faults, veins and geological contacts between rock types, pillow structures, etc. In addition, special attention must be paid to the structural characteristics of the weathering profiles formed on steeply dipping alternating successions of felsic to mafic volcanic and sedimentary sequences, as in the underlying greenstones at Omai. Their petrographical and tectonic contacts can act as migration paths for elements that might result in a mixed behavior of the chemical patterns. Meteoric water circulation along the migration paths can create local zones of more intense weathering closer to the bottom of the weathered profile and, by contrast, zones less weathered in the upper parts of the regolith. Therefore, depth should not be the principal criterion for comparison purposes of geochemical behavior within the weathering profile, but rather this compatison should be an index that quantitatively measures the replacement of primary mineral phases by secondary minerals for each analyzed sample. In this study, discussion of the geochemical pattern focuses on the chemical changes of each sample collected from several weathered rocks types as a function of intensity of weathering rather than depth of sampling. The degree of weathering, which yields distinct values for different weathered materials at a similar depth, can be evaluated by quantitative measures, using the whole-rock chemical analyses. These values, representing the average weathering index for each analyzed system (sample), can also be applied to the determination of the weathering index of each separate mineralogical component of the system. The main assumptions are that the index of alteration of a sample is the same for all its mineralogical pairs used for the partition of a chemical element between a primary and its equivalent secondary mineral and that the system is closed, without mass transfer (loss or gain). (Voicu et al., 1996, 1997b). The first step is represented by the calculation of the chemical index of alteration (CIA: Nesbitt and Young, 1982; Fedo et al., 1995) for each analyzed sample, using the following equation: CIA ¼ ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 O þ K2 OÞ  100 ð1Þ

where oxides are in molecular proportions. Because CIA values range between 50 and 100 and cannot be directly applied for the normative calculations, the second step is represented by the calculation of the mineralogical index of alteration (MIA), using the following equation (Voicu et al., 1996, 1997b): MIA ¼ 2  ðCIA- 50Þ

ð2Þ

The mineralogical index of alteration evaluates the degree of mineralogical weathering, i.e. the transformation ratio of a primary mineral into its equivalent alteration mineral. It has the advantage of indicating the degree of weathering for each analyzed sample, independently of the depth of sampling. The MIA value indicates incipient (0–20%), weak (20–40%), moderate (40–60%), and intense to extreme (60–100%) weathering. The value of 100% means complete transformation of a primary mineral into its equivalent weathered product and, by extrapolation, complete weathering of the parent rock. The main objective of the present paper is to discuss the nature of weathering undergone by the greenstone belt of the Omai area, Guyana, mineralogy and geochemistry of parent rocks and weathering profile. In particular, the chemical behavior of major, trace and rare-earth elements for each rock type and its equivalent weathered product as a function of a mineralogical index of alteration (MIA), rather than depth of sampling, were examined. Understanding the geochemical behavior within the weathered profile helps to detect the presence of specific chemical elements that are usually associated with ore deposits, which has an obvious impact on exploration in tropical regions.

2. Regional geology The north-central part of Guyana represents a typical greenstone belt sequence consisting of Paleoproterozoic volcano-sedimentary rocks of the Barama–Mazaruni Supergroup (Gibbs and Barron, 1993) (Fig. 1). The volcano-sedimentary sequence was metamorphosed generally to greenschist facies during the Trans-Amazonian orogeny (
2100 Ma, Norcross et al., 2000). The sedimentary rocks have been dated by U-Pb on zircons at 2250 106 Ma (Gibbs and Olzseswki, 1982) and the subvolcanic porphyry dikes at 2120 2 Ma (Norcross et al., 2000). The low-grade greenstone-granite terranes are spatially associated with granulites, gneisses and micaschists (Bartica Formation). Although the stratigraphic relationships are not fully understood, the Bartica Formation seems to be the higher metamorphosed equivalent of the overlying greenstone belt. The greenstone sequence was intruded by syn- to post-tectonic, intermediate to felsic batholiths and stocks of the Granitoid Complex, which yielded U-Pb zircon ages between

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323

Fig. 1. Simplified map of north-central Guyana showing the Paleoproterozoic bedrock geology (modified from Walrond, 1987; Elliott, 1992; Voicu et al., 1997a).

2094 6 and 2096 11 Ma at Omai (Norcross et al., 2000). The younger terranes, which post-date the greenstone-granite belt and associated mineral deposits, comprise suprajacent anorogenic sedimentary sequences of the Paleoproterozoic Roraima Formation and associated mafic dikes of the Avanavero Suite, as well as Triassic diabase dikes related to the opening of the Atlantic Ocean referred to as the Apatoe Suite. The combined effects of prolonged uplift and weathering under warm, humid conditions have led to the development of a thick weathered profile, partially recovered by the Pleistocene continental-deltaic Berbice Formation (Wong, 1984).

3. Local geology The Omai Au mine is a large-scale open pit mining operation in the Potaro district of Guyana, located about 200 km SW of Georgetown. Small-scale mining targeted the placer Au, while in situ mineralization was

sporadically explored and exploited by several mining companies. Since 1992, two distinct orebodies (Fennell and Wenot) hosted by both bedrock and weathered profile are in production, with mineable reserves (including past production) of 80 Mt grading 1.5 g/t Au. The Omai mine area is underlain by various volcanic, plutonic and sedimentary rocks. Extensive petrographical description and bedrock geochemistry are provided by Bertoni et al. (1991), Elliott (1992) and Voicu et al. (1997a, 1999a,b, 2000). As the weathered profile in this study refers to the Wenot orebody, a brief bedrock description for this part only of the Omai deposit follows (Fig. 2). The Wenot zone consists of subvolcanic rhyolite and quartz-feldspar porphyry dikes that intrude mafic volcanic and sedimentary rocks. This entire sequence is part of the Barama–Mazaruni greenstone belt. The mafic volcanic sequence comprises calc-alkaline andesitic (and subordinately basaltic) flows with fluidal or amygdaloidal textures and pillow-lava structures. Due to the regional low metamorphic grade, the pillows are

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Fig. 2. Bedrock geological map of the Wenot orebody, Omai deposit (modified after Voicu et al., 1999b, 2000).

slightly deformed, except the central part of the Wenot zone, affected by an east–west striking shear corridor, where the pillows are strongly deformed. Thin beds of andesitic tuffs occur within the volcanic flows. Sedimentary rocks consist of fine-grained siltstones and sandstones (phyllitic tuffs in mine terminology). The geochemical patterns suggest that sedimentary rocks contain a dominantly calc-alkaline volcanoclastic component. The contact between sedimentary and volcanic rocks is tectonic, along the E–W striking Wenot shear zone, discontinuously marked by a 2-m thick, strongly deformed cataclasic breccia. A quartz-feldspar porphyry dike, which trends 100–110 and steeply dips in either direction, intruded the contact between the mafic volcanic and sedimentary sequences along the Wenot shear zone. The thickness of the dike averages 10 m and it can be traced for more than 2.5 km along trend and at least 300-m downdip. The quartz-feldspar porphyry has rhyodacitic composition and consists of albite, Kfeldspar, quartz, biotite (replaced by chlorite), calcite, and traces of apatite, rutile, and pyrite. The rhyolite dikes (4 major dikes) are highly irregular in shape and frequently pinch-and-swell along trend and dip.

Rhyolites are strongly silicified, massive, aphanitic to very fine grained. The southern and eastern parts of the Wenot orebody are recovered by the Pleistocene Berbice Formation, which consists of a clayey facies at the base and extensive sand levels interstratified with clay lenses in the upper part. The basal clays have been deposited in extensive swamps characterized by reducing conditions that favored preservation of organic matter. The contact between Berbice Formation and underlying in situ lithology is marked by a thin conglomerate level (
40 cm), which consists of angular quartz fragments derived from the mineralized quartz veins, in a partially indurated, sandy matrix. The predominantly sandy upper part formed under regressive conditions in a continental-fluviatile and deltaic environment. The Berbice Formation has an average thickness of 15 m and dips between 10 and 30 S. The Wenot orebody consists of stockwork quartz veins hosted mainly by the felsic dikes, and subordinately by adjacent andesites and sediments in both bedrock and weathered rock (Bertoni et al., 1991; Elliott, 1992; Voicu et al., 1997a,b, 1999b). The stockwork veins show highly

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variable thickness, ranging between a few mm and 0.6 m. Vein-forming minerals in the bedrock are quartz, ankerite, chlorite, fuchsite, muscovite, scheelite, pyrite, native Au, and minor rutile, chalcopyrite, molybdenite and magnetite. Phyllitization, carbonation, silicification, propylitization, and pyritization characterize the hydrothermal alteration haloes surrounding the mineralized veins. Vein mineral assemblage in the weathered profile consists mainly of quartz, pyrite, goethite native Au. Pyrite, generally considered as one of the first minerals that oxidizes during weathering, occurs at Omai as fresh grains in quartz veins and wallrocks in the lower weathered profile (saprolith) and it is transformed in Fe oxihydroxides in the upper part (pedolith) only. Refractory Au that occurs in pyrite as globular inclusions is preserved in saprolith, while Au that occurs in bedrock as fracture filling in pyrite is leached and replaced by secondary minerals (kaolinite). Wallrock pyrite in the weathered profile carries up to 600 g/t Au.

4. Climate and geomorphology The Omai mine is located near the Omai River, a tributary of the Essequibo River, and it is covered by tropical rainforest. The general topography is gentle, with a maximum relief of about 100 m above sea level. The climate is warm and rainy, with annual average minimum and maximum temperatures of 21 and 32 C, respectively. The site receives an average of 2.6 m of rainfall annually. Although significant amounts of precipitation fall in all months, there are two rainy seasons during the year (May–July and December–February).

5. Description and mineralogy of the weathering profile Intensive weathering of fresh rock results in major chemical, mineralogical and fabric changes. Although the supergene processes are integrated and complex, specific supergene alteration zones (horizons) are produced within the regolith (Tardy, 1992, 1997; Lawrance, 1994; Lecomte and Zeegers, 1992). The horizon characterized by secondary mineral formation associated with important isovolumetric chemical changes, but with preservation of primary rock fabric by weathering products is referred to as saprolith, which is divided in saprock (or coarse saprolite) and lithomarge (or fine saprolite). Lithomarge very often contains no primary mineral but the parent rock fabric is conserved. The upper part of the weathered profile, characterized by total extinction of the parent rock fabric is referred to as pedolith. It consists of mottled zone, ferricrete and latosol (Lecomte, 1988; Nahon and Tardy, 1992). Typical sections through the weathering profile at Omai are shown in Fig. 3.

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The saprolith represents more than two-thirds of the whole weathering profile. The transition zone between bedrock and saprolite, characterized by the preservation of primary rock fabric and consisting of less than 20% of alteration products of weatherable minerals, forms the saprock. At Omai, the thickness of the saprock zone is highly variable, generally ranging between 5 m (in the felsic dikes) and 15 m (in sediments), depending mainly on bedrock lithology, frequency of quartz veining, and structural discontinuities. Above the saprock, continued fluid-rock interaction and leaching associated to further loss of mobile elements results in lithomarge formation. The weathered rock becomes softer, of lower density, more porous and friable than the saprock from which it was formed (Lawrance, 1994). The initial fabric and volume of the rock are preserved because the expansion that accompanies the transformation of some primary minerals into ‘‘weathering plasma’’ is offset by the reduction of volume resulting from the weathering of other components (Trescases, 1992). The pedolith profile is well developed over the andesites in the northern part of the Wenot orebody. Its thickness depends on the chemical composition of the bedrock and landscape topography. Mafic volcanic rocks, associated with topographically high parts or stream divides are characterized by thick (up to 15 m) laterite caps, rich in hardened Fe-rich zones, whereas the Fe-poor felsic rhyolite and porphyry dikes in more mature topographic areas are overlain by a thinner pedolith profile (several meters). The typical pedolith profile is generally composed of two horizons, mottled zone and ferricrete. Mottled zone represents the lowermost laterite horizon affected by surface oxidation. The transition between the upper saprolite horizon and the mottled zone is gradual within tens of cm to a few meters and it is always horizontal. Oxidation, associated with a fluctuating watertable under wet climate, causes Fe3+ precipitation as localized spots and patches of hematite and goethite and the almost complete breakdown of the secondary minerals of the lithomarge into clay minerals. With the increase of weathering intensity, only the most stable clay types are produced, especially kaolinite. At Omai, the mottled zone consists of up to 80% normative kaolinite and up to 10–15% normative goethite (Voicu et al., 1997b). Generally, the mottled zone overlying mafic rocks is goethite-rich and kaolinite-poor. Conversely, over felsic rocks the Fe spot proportion diminishes and the mottled zone is characterized by significant kaolinite enrichment. Towards the upper part of the mottled zone, increasing weathering results in further Fe remobilization and reconcentration as hematite, forming partially to completely indurated nodules and pisoliths, referred to as ferricrete. Surface physical and chemical reworking of this horizon may separate pisoliths from their matrix, resulting in lateritic gravel. In

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Fig. 3. Typical section through the weathering profile in the Wenot orebody, Omai deposit, showing the relative thickness of the weathering profile.

the eastern part of the Wenot zone (below the Gilt creek), in situ pedolith is covered by several meter thick Aubearing, transported overburden, mainly represented by nodules and pisoliths in a partially indurated matrix. In the southern part of Wenot pit, the weathered profile covered by the Pleistocene Berbice Formation is characterized by saprolith only, the pedolith being completely absent. The Berbice Formation itself is slightly lateritized in the upper part. The mineralogy of the weathering profile was defined by microscope observations, XRD analyses and by normative calculations using the MINNOR software (Voicu et al., 1996, 1997b). The saprolith horizon at Omai is characterized by high normative kaolinite and goethite contents. However, the presence of fresh pyrite grains in saprolith suggests that most of the Fe incorporated into the goethite structure is leached from ferromagnesian silicate minerals rather than sulfides. Depending on the bedrock primary mineralogy and on the weathering intensity, other important minerals are quartz, chlorite, sericite and smectite, while albite, anatase, apatite, corundum, orthoclase, titanite and pyrolusite are minor mineral phases. XRD analyses evidenced the presence,

in minor amounts, of several minerals rarely described in weathering profiles: (nacrite [Al2Si2O5(OH)4], ephesite [Na2Al2(Al2Si2)O10(OH)2], corrensite [(Mg,Fe)9 (Si,Al)8O20OH10*H2O], and guyanaite [Cr2O3*1.5H2O]).

6. Sampling and analytical techniques Geochemical study at Omai was carried out on 29 samples from the weathering profile and the unweathered protolith (parent rock). More geochemical data for the unweathered protolith are provided by Voicu et al. (1997a). For this study, only one representative fresh (unweathered) sample of each rock type has been used for comparison purposes with the weathered profile. Samples were collected either from diamond drillholes or from the Wenot open pit. Depending on the degree of weathering, the samples were dried for 1–5 days at 30 C. After crushing and pulverizing in an agate mill, samples were analyzed for major oxides and most of the trace elements by X-ray fluorescence spectrometry at the Geochemical Laboratories, McGill University in Montreal. Rare-earth elements (REE), U, Th, Ta, Hf and Cs

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were analyzed by INAA at UQA`M and E´cole Polytechnique, Montreal. Precious metals (Au, Ag, Pt, and Pd) were analyzed by FA at McGill University.

7. Geochemistry of the weathering profile Representative major oxide, trace and rare-earth element compositions of the weathering profile and underlying bedrock are shown in Table 1. 7.1. Major and trace element redistribution 7.1.1. Saprolith The chemical signature is preserved for several oxides, while other oxides show either enrichment or leaching

327

(Fig. 4). Their behavior is related to the weathering intensity (MIA) and the rock type. The SiO2 contents from all rock types are generally not affected by weathering. However, some higher SiO2 values in saprolith could represent local hydrothermal silicification zones, frequently observed in bedrock. Negative correlation is observed between MIA and Na2O, CaO and MgO contents for all rock types. Complete Na2O leaching occurs with intense weathering (MIA>85) in andesites and sedimentary rocks and between MIA=70–80 in felsic rocks (porphyry and rhyolite), while CaO is almost totally leached in all rock types at MIA
40, i.e. in saprock. The MgO content in weathered andesites is variable, although it has significantly lower values compared to fresh andesite. No direct relationship is observed between the Fe and Mg contents, which suggests that Fe

Fig. 4. Behavior of major oxides in bedrock, saprolith and pedolith horizons as a function of MIA (mineralogical index of alteration) values. See text for discussion.

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Table 1 Whole-rock chemical analyses for the weathered profile and underlying bedrock at Omai MIAd SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI

Total

Ba

Rb

Sr

Ga

Cs

Nb

Hf

Zr

Y

Th

349 2s ss5 ss4 3432 10w 19AW ss6 18aw

21.60 62.58 75.85 76.10 79.00 87.80 93.64 97.70 99.14

60.01 66.20 58.96 63.24 56.67 62.08 58.53 51.36 58.77

0.41 0.71 1.09 1.25 1.16 1.19 1.60 1.27 1.33

16.21 16.16 17.35 19.01 18.59 18.78 22.52 19.71 18.63

6.89 10.84 15.69 8.43 13.31 10.37 6.36 18.08 14.19

0.12 0.01 0.03 0.01 0.02 0.02 0.01 0.06 0.01

4.29 0.07 0.01 0.00 1.45 0.19 2.47 0.13 0.00

7.53 0.05 0.07 0.07 0.13 0.20 0.01 0.15 0.01

2.84 0.80 1.96 2.12 1.42 0.75 0.21 0.07 0.06

0.08 2.87 0.35 0.39 0.63 0.27 0.52 0.01 0.02

0.09 0.06 0.04 0.04 0.08 0.06 0.05 0.03 0.09

1.67 3.10 5.32 5.68 6.53 6.26 8.10 9.80 7.85

100.44 101.01 100.99 100.44 100.15 100.32 100.59 10.79 101.06

76 450 148 159 122 87 525 84 29

1.1 82.0 6.1 6.8 15.0 5.6 15.4 0.0 0.0

105.6 109.1 138.8 169.0 100.4 51.4 50.7 13.1 2.9

16.2 19.7 16.6 21.4 19.8 18.3 26.2 20.4 19.1

0.23 1.70 0.60 0.93 0.91 0.38 0.68 0.50 0.40

6.8 5.7 5.4 6.2 5.4 5.3 11.2 6.5 6.5

5.66 1.40 3.32 2.95 1.95 2.20 6.41 2.73 2.48

94.3 51.7 67.4 80.3 69.8 67.4 229.33 77.8 77.7

10.2 10.6 29.0 38.2 32.0 33.4 44.4 38.6 14.3

2.12 0.02 0.40 0.47 0.35 0.14 0.11 0.42 0.52

622/123 Quartz porphyry 664/80 +Quartz porphyrya 6w *Quartz porphyry 738 *Quartz porphyry

15.76 40.52 66.48 73.36

75.94 70.50 75.84 69.30

0.18 0.44 0.42 0.62

9.29 16.51 15.99 19.92

2.40 2.73 1.57 1.85

0.06 0.08 0.00 0.00

2.67 0.23 0.18 0.14

2.67 0.53 0.02 0.01

3.34 4.76 0.63 0.25

0.75 1.70 2.57 2.81

0.05 0.14 0.02 0.02

4.00 2.43 3.28 5.25

99.83 100.17 100.69 100.27

180 670 1108 871

26.7 58.7 76.6 80.4

128.4 412.8 104.7 47.8

11.6 19.5 16.6 24.0

2.19 3.27 2.49 3.23

6.1 6.5 7.0 8.0

4.45 6.47 4.18 6.86

59.2 120.3 147.7 154.6

4.1 7.5 21.5 15.0

6.82 11.21 0.04 14.21

5428 739

Rhyolite *Ryolite

32.14 77.33 0.10 12.05 2.63 73.82 79.60 0.73 13.81 0.53

0.04 0.01

0.01 0.03

0.10 5.52 0.03 0.64

0.70 0.01 1.41 0.02

0.78 3.16

99.70 489 100.07 553

18.9 44.1

32.3 22.7 0.64 32.7 16.70 489.0 101.5 17.6 1.97 12.8 7.43 206.4

139.0 7.91 35.2 5.74

23423 15423 13423 10423 1513 11424 18bw

Phyllitic tuff * Phyllitic tuff * Phyllitic tuff * Phyllitic tuff * Phyllitic tuff * Phyllitic tuff * Phyllitic tuff

38.06 49.40 58.72 64.22 64.68 78.10 99.52

69.85 68.15 70.96 62.49 57.16 58.53 58.73

0.66 0.72 0.69 0.82 0.99 1.02 1.44

12.86 15.44 13.65 17.81 21.77 19.78 20.74

6.62 6.73 6.45 9.44 9.68 10.70 11.06

0.10 0.02 0.03 0.13 0.06 0.09 0.01

1.81 1.59 1.56 1.62 1.68 3.03 0.01

1.27 0.30 0.37 0.18 0.24 0.31 0.01

3.27 2.91 1.62 0.66 1.43 0.17 0.04

1.23 2.02 1.56 3.04 3.00 1.52 0.01

0.11 0.12 0.12 0.14 0.17 0.18 0.03

2.91 2.47 3.37 4.41 4.34 6.21 8.42

101.00 100.66 100.50 100.87 100.71 101.54 100.61

267 797 434 574 739 619 4

47.7 80.1 61.1 118.2 112.9 50.5 0.0

126.5 108.0 66.4 111.2 204.1 28.5 2.9

13.8 17.1 14.9 21.6 25.8 20.5 21.7

1.27 2.44 nae na 3.79 1.50 0.13

9.1 10.8 10.1 11.3 11.1 11.3 6.8

4.33 4.16 5.40 5.50 5.34 4.01 2.55

158.7 157.6 167.8 173.2 209.3 153.4 85.7

17.8 19.7 18.5 26.5 27.9 22.1 10.8

5.47 6.06 na 0.80 7.47 5.96 0.04

1cw 1s 1aw 1423 1432 7s 2423

**Mottled zonec **Mottled zone **Mottled zone **Mottled zone **Ferricrete **Ferricrete **Ferricrete

96.28 97.14 97.32 97.68 67.78 69.28 98.26

56.10 45.91 65.88 48.86 41.93 43.93 48.16

1.72 2.31 1.38 1.89 1.66 1.07 1.63

27.2 28.24 20.63 33.12 23.01 24.86 27.43

2.06 7.91 4.56 2.81 21.12 16.14 10.77

0.01 0.05 0.01 0.01 0.20 1.26 0.01

0.10 0.61 0.01 0.07 0.26 0.47 0.06

0.01 0.30 0.02 0.01 0.03 0.01 0.01

0.02 0.07 0.02 0.05 1.15 0.15 0.05

0.51 0.04 0.24 0.34 3.24 4.36 0.19

0.06 0.14 0.05 0.05 0.16 0.15 0.05

11.25 14.93 7.80 13.03 7.34 8.27 11.94

99.84 100.77 100.77 100.33 100.42 101.02 100.36

199 39 69 133 1061 11.78 90

29.9 9.3 6.6 15.3 89.0 153.2 11.7

40.4 12.6 23.3 38.0 104.0 12.6 31.1

31.8 31.7 27.5 38.3 35.0 19.5 33.8

2.52 4.45 0.49 1.29 0.38 2.36 0.81

35.6 15.1 11.9 34.8 9.5 5.6 31.3

15.08 5.75 5.33 16.73 3.84 1.68 16.10

605.8 213.9 199.5 596.5 140.6 76.8 583.2

22.0 22.4 10.1 20.1 38.2 28.4 18.3

1.76 4.52 0.15 25.45 0.20 0.14 30.32

Andesite *Andesiteb *Pillowed andesite *Pillowed andesite *Andesite *Pillowed andesite *Andesite *Pillowed andesite *Pillowed andesite

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

Sample Lithology

MIA U

Ta

Ni

Co Sc

V

Cu

Pb

Zn Bi Sn W

Mo Au

Pt Pd Ag As

Se Sb Te La

Ce

Sm

Eu

Tb

Yb

Lu

349 2s ss5 ss4 3432 10w 19AW ss6 18aw

21.60 62.58 75.85 76.10 79.00 87.80 93.64 97.70 99.14

0.76 0.19 0.03 0.39 0.23 0.04 1.91 0.36 0.16

0.59 0.18 0.25 0.21 0.12 0.21 0.41 0.21 0.38

117 19 176 234 155 96 69 152 58

40 10 14 14 42 10 35 18 7

39.4 24.0 84.5 103.6 35.0 31.0 41.0 67.8 40.0

115 302 320 373 314 308 291 342 296

68 188 149 432 186 305 263 87 192

0.0 0.0 0.0 0.5 0.0 0.0 8.1 0.0 0.0

92 73 102 164 229 103 147 73 84

0.0 0.1 0.0 0.0 0.0 0.0 0.2 0.0 0.1

na 2.3 na na 2.4 1.3 0.0 na 2.2

1.3 4.6 7.8 5.1 1.9 1.4 4.2 2.9 2.0

4.1 0.0 0.0 0.0 0.1 0.3 0.0 0.0 0.1

6 19 9 6 39 21 2 3 2

0 0 0 0 0 0 0 0 0

0 0 0 4 0 9 2 3 0

na 0.8 na na 1.6 1.0 1.2 na 1.2

5.8 3.1 10.0 2.6 1.9 12.2 0.0 7.7 2.4

na 0.2 na na 0.8 0.2 0.2 na 0.3

na 0.0 na na 0.0 0.0 1.8 na 0.0

2.2 4.6 2.3 1.5 6.6 6.1 4.3 2.2 6.2

14.91 1.78 9.35 6.45 10.64 10.15 45.91 20.23 5.65

28 0 1 1 11 29 96 1 4

3.30 0.98 10.32 12.63 4.70 5.34 12.72 8.41 2.96

1.24 0.32 2.91 3.72 1.23 1.58 3.01 2.18 0.82

0.56 0.02 1.74 2.21 0.85 0.06 1.98 1.38 0.33

2.13 1.13 6.60 7.61 2.73 3.86 4.05 4.08 2.27

0.28 0.17 0.96 1.16 0.41 0.52 0.52 0.62 0.31

622/123 Quartz porphyry 15.76 2.80 664/80 +quartz porphyry 40.52 4.10 6w *quartz porphyry 66.48 0.93 738 *quartz porphyry 73.36 3.97

0.19 0.40 0.36 0.43

16 11 15 4

12 0 4 0

7.6 10.4 12.0 18.1

50 40 67 79

56 20 214 58

0.0 7.0 2.9 9.3

66 66 74 54

0.0 0.0 0.0 0.0

na na 0.0 na

4.8 0.0 0.0 1.9

0.0 0.0 0.3 0.0

79 5 32 5

0 0 0 0

0 3 0 0

na na 0.2 na

0.0 4.8 0.9 6.0

na na 0.2 na

na na 2.1 na

0.0 0.0 2.4 0.0

14.58 40.36 11.01 42.21

22 43 40 44

2.03 5.63 2.08 5.88

0.77 1.43 0.56 1.55

0.23 0.54 0.21 0.63

0.46 0.99 2.38 1.65

0.06 0.12 0.37 0.26

Andesite *andesite *pillowed *pillowed *andesite *pillowed *andesite *pillowed *pillowed

andesite andesite andesite andesite andesite

5428 739

Rhyolite *rhyolite

32.14 2.36 2.22 29 73.82 2.02 0.99 4

13 0

2.0 36.2

50 157 122 9

2.9 1.2

232 0.0 3.4 23.5 5.9 24 37 0.0 na 3.3 0.0 2

0 0

0 3

0.4 3.5 na 1.5

0.1 0.0 2.0 45.47 123 15.18 1.52 3.10 15.00 2.05 na na 0.0 42.66 62 9.89 1.75 1.31 5.32 0.79

23423 15423 13423 10423 1513 11424 18bw

Phyllitic tuff *phyllitic tuff *phyllitic tuff *phyllitic tuff *phyllitic tuff *phyllitic tuff *phyllitic tuff

38.06 49.40 58.72 64.22 64.68 78.10 99.52

1.98 1.77 na 0.00 2.46 1.65 0.11

0.53 0.65 na na 0.67 0.69 0.25

53 47 40 66 92 111 95

27 30 22 24 35 33 1

14.0 11.0 11.0 21.0 15.0 22.0 95.0

109 102 80 118 137 151 359

521 191 175 201 250 214 226

4.8 3.9 3.6 7.5 12.7 7.6 0.0

186 123 124 148 170 175 90

0.0 0.0 0.5 0.4 0.0 0.0 0.2

1.6 2.7 0.2 2.3 2.8 1.8 0.5

0.0 2.6 7.0 5.7 8.8 3.3 0.7

3.1 1.0 0.0 0.0 0.0 5.2 0.0

2 8 2 23 26 621 2

0 0 0 11 0 0 0

0 0 0 15 0 0 0

1.0 1.0 0.8 1.0 1.0 1.7 1.0

2.7 0.3 1.5 1.3 1.0 2.1 0.0

0.2 0.5 0.0 0.3 0.1 0.0 0.7

0.0 0.0 0.0 0.9 3.1 1.0 0.0

3.5 4.7 5.3 5.5 6.2 5.3 3.9

21.21 27.98 na na 28.46 21.32 4.14

42 50 na na 61 39 8

3.46 4.31 na na 5.37 4.29 2.12

0.76 1.30 na na 1.14 1.01 0.54

0.45 0.53 na na 0.66 0.76 0.03

2.03 1.57 na na 2.62 2.33 1.64

0.29 0.21 na na 0.34 0.33 0.28

1cw 1s 1aw 1423 1432 7s 2423

**mottled zone **mottled zone **mottled zone **mottled zone **ferricrete **ferricrete **ferricrete

96.28 97.14 97.32 97.68 67.78 69.28 98.26

4.41 1.01 0.90 3.28 0.81 0.79 3.51

2.24 0.81 0.58 2.42 0.45 0.39 1.95

20 110 43 26 99 229 128

10 61 3 4 71 647 2

19.0 56.0 23.0 17.0 59.0 44.0 19.0

122 349 364 193 430 304 161

132 1367 138 126 354 531 128

55.2 1.7 12.0 28.9 10.8 0.0 25.5

76 437 69 80 221 235 79

0.0 0.0 0.0 0.0 2.3 0.5 0.0

0.0 2.0 0.0 0.0 5.2 3.8 3.3

4.7 5.2 19.7 2.3 55.3 0.0 2.4

1.7 0.4 0.7 0.7 7.5 0.0 1.8

7 306 198 54 4261 29 2

0 16 0 0 0 0 0

0 19 7 0 0 0 0

0.4 1.0 0.6 0.6 3.6 1.4 1.0

1.1 2.1 7.8 0.7 13.9 0.7 1.5

0.2 0.1 0.8 0.6 0.9 0.5 1.5

2.2 0.0 0.1 0.5 0.0 0.0 0.0

1.6 5.1 2.7 0.3 8.2 8.3 4.3

69.01 9.47 16.96 59.76 19.55 9.40 48.39

106 26 44 89 24 59 58

4.34 2.80 2.09 3.98 5.80 4.36 2.88

0.83 0.77 0.59 0.79 1.46 1.43 0.62

2.29 0.59 0.25 0.02 0.10 0.07 0.48

3.68 2.26 1.32 3.31 3.70 2.80 3.22

0.35 0.31 0.06 0.49 0.47 0.43 0.54

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

Sample Lithology

a + b c d e

—Saprock. *—Lithomarge. **—Pedolith (mottled zone and Ferricrete). MIA—mineralogical index of alteration (Voicu et al., 1997b); major oxides are in %, trace elemetns in ppm, except Au, Ag, Pt, and Pd, which are in ppb. na = not analyzed.

329

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G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

is accumulated mainly outside the ferromagnesian minerals. In porphyry, the MgO content decreases sharply at MIA values
40 (saprock) and it is completely leached with intense weathering (MIA
75). In rhyolite, the almost complete lack of ferromagnesian minerals in the fresh rock precludes any interpretation. The K2O shows leaching in mafic volcanic and sedimentary rocks, while in felsic rocks there is a gradual enrichment with increasing weathering. The K-bearing minerals (muscovite, fuchsite and minor feldspar) at Omai generally persist throughout saprolith. In addition, the Au mineralization is associated with strong phyllic alteration, which is widespread in felsic rocks (Voicu et al., 1999b). Hence, it can be supposed that weathering overlapped the hydrothermal alteration, which caused a residual increase of K in felsic rock by remobilization from mafic volcanic and sedimentary rocks. The Fe2O3 contents show both gain and loss in mafic volcanic rocks, gradual gain in sedimentary rocks and loss in felsic rocks. The highly variable Fe contents in the mafic volcanics are mainly due to the weathering of less stable minerals (amphibole, biotite, pyroxene) and formation of secondary Fe oxides. Some leaching of Fe is probably related to a lower watertable that characterizes the post-weathering periods. On the other hand, the sedimentary rocks are characterized by a more uniform Fe oxide redistribution. This is due to more limited weathering because of the overlying Berbice Formation that acted as a blanket and to a more uniform fabric and chemical composition of sediments when compared to mafic volcanics. The Fe distribution in sediments suggests a gradual accumulation towards the upper part of the saprolith, undisturbed by frequent water-table variations. Al2O3 is the only major oxide

that shows a positive correlation with MIA in all rock types due to its retention in secondary mineral phases. TiO2 is generally immobile in mafic volcanics, but shows gradual enrichment in weathered felsic volcanics and sedimentary rocks. Therefore, the weathered felsic volcanic rocks have Ti/Zr ratios similar to andesites in the Hallberg (1984) diagram (Fig. 5). Meanwhile, the weathered sediments plot mainly within the andesitie field. This fact confirms their andesitic component and, on the other hand, suggests that the Ti/Zr plot can be used to define the source of weathered, relatively homogeneous sediments. P2O5 is completely leached in saprolite formed on volcanic rocks, while in sediments it is mostly immobile, regardless of the degree of weathering. Trace elements have various behavior patterns during weathering (Fig. 6). Some of them are not affected by weathering, suggesting relative immobility (Nb, Hf, Ta and partially Y), some show moderate to strong enrichment (V, W, Ga, Sc) or leaching (Sr, Mo) and others have a mixed behavior (Cs, Ba, U, Th, Ni, Cu, Zn, Pb, Te). Rubidium is generally completely leached during extreme weathering (MIA
95). 7.1.2. Pedolith The pedolith horizon is formed by extreme weathering of the andesitic-basaltic rocks only. The mottled zone and ferricrete are SiO2 depleted. Leaching of K2O and Fe2O3 in the mottled zone and high K2O and Fe2O3 contents in ferricrete compared to saprolith indicate remobilization of primary and secondary micas and Fe oxihydroxides in the lower part of the pedolith and subsequent accumulation as sericite ‘pockets’ preserved in ferricrete pisoliths and nodules. Sericite ‘‘pockets’’

Fig. 5. Ti–Zr plot (Hallberg, 1984).

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

331

Fig. 6. Behavior of selected trace elements in bedrock, saprolith and pedolith horizons as a function of MIA (mineralogical index of alteration) values. See text for discussion.

and, implicitly, high K content directly affect the MIA values, which are anomalously low in two ferricrete samples. The Al2O3 contents show general increase in the mottled zone and ferricrete when compared to the underlying saprolith and bedrock. TiO2 has commonly been considered to be immobile and it has been used as an index element to calculate mass balance during the weathering processes (Oliveira and Campos, 1991; Boulange´ and Colin, 1994; Porto and Hale, 1995; Freyssinet and Itard, 1997). Although pedolith formation is not isovolumetric, the variance of TiO2 content when compared to that of lithomarge, saprock and parent rock suggests that this element could become mobile in the upper parts of the weathering profile. Similar behavior is observed for some trace elements (Zr, Nb, Hf, and less Y). The Zr/TiO2 vs Nb/Y classification diagram (Winchester and Floyd, 1977) shows that these element

ratios in pedolith horizons are generally higher than those in the underlying lithomarge formed on andesites (Fig. 7). Similarly, the Ti–Zr plot (Hallberg, 1984; Fig. 5) shows a large dispersion limiting the reliability of discrimination. These observations indicate that these chemical elements cannot be used to calculate the mass and volume balance in the upper horizons of the weathering profile at Omai. Hill et al. (2000) reach a similar conclusion based on strong Y depletion in paleolaterite from Northern Ireland. As Ti, Zr, and Nb-bearing minerals are not (or only slightly) affected by weathering, their enrichment in pedolith is considered as mechanical rather than chemical. In the overlying Berbice Formation, partially affected by lateritization, Zr content is up to 700 ppm, Nb 35 ppm, Hf 15 ppm and TiO2 2 wt.%, which suggests a general enrichment of these elements from the mottled zone towards the present-day surface. This observation

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Tb anomaly. Hence, it is suggested that weathered homogeneous sediments with MIA values <90 preserve the REE fractionation patterns of the unweathered protolith.

8. Discussion and conclusion 8.1. Behavior of major and trace elements in weathered profiles and its impact on exploration

Fig. 7. Zr/TiO2 vs Nb/Y classification diagram (Winchester and Floyd, 1977) for the Wenot bedrock and weathered profile. Symbols: open lozange: andesite; filled lozange: lithomargeandesite; open square: quartz porphyry; half-filled square: saprock-quartz porphyry; filled square: lithomarge–quartz porphyry; open triangle: rhyolite; filled triangle: lithomargerhyolite; open circle: sediment; filled circle: lithomarge–sediment; rotated cross: mottled zone; cross: ferricrete.

agrees with frequently described translocated zircon-rich zones at the top of the morphological sequence (Brimhall et al., 1985; Colin et al., 1993). 7.2. Rare-earth element (REE) redistribution The REE chondrite-normalized patterns for the parent rocks and the weathered equivalents are shown in Fig. 8. Generally, the REE fractionation depends on the petrographical nature of the bedrock and degree of weathering. Lithomarge formed on pillowed andesites is characterized by slight to strong negative or positive Ce anomalies (between 0.01 and 1.18), where the Ce anomaly has been calculated relative to straight line interpolation between the flanking element. Several samples have negative Tb anomalies also. Except for La and Ce, the other REEs in weathered andesites are generally more enriched and less fractionated than in their parent rock. Lithomarge formed on quartz-feldspar porphyry is characterized by HREE enrichment and less REE fractionation compared to the unweathered rock (average (La/Yb)ch is 8 and 26 for lithomarge and parent rock, respectively). The LREE behavior is similar for both weathered and unweathered porphyry. The Eu anomaly that characterizes unweathered rhyolites is not observed in the weathered equivalent. The REEs are more fractionated in the rhyolitic lithomarge, which is also characterized by a prominent HREE depletion. Sedimentary rocks (phyllitic tuffs) show similar REE behavior between the lithomarge and the parent rock, except the most weathered sample (MIA=99.5), which shows LREE depletion and has a strong negative

Use of a weathering index (MIA or other similar indexes) allows one to quantitatively measure the supergene alteration of each individual sample, in contrast with qualitative estimation of weathering intensity either by costly methods (XRD, mineralogical studies, etc.) or, frequently, by subjectively visual observations only. It can provide more accurate information about the trends of major and trace element in regolith as opposed to unweathered bedrock. In particular, the weathering index can aid in relating the intensity of supergene alteration in the degree of mobility (accumulation or leaching) of chemical elements and in better understanding of the influence of specific physicochemical and structural features of bedrock upon weathering processes. Plotting major oxides vs the weathering index shows that some elements are highly mobile in all analyzed rock types, while chemical behavior of other oxides depends on physicochemical characteristics of the unweathered rock type. Some oxides (CaO, MgO, Na2O) are completely leached in the saprolith horizon, while others (Al2O3, Fe2O3, K2O) persist up to the top horizons of the weathering profile. Some major and trace elements that are considered generally immobile (ex: TiO2, Zr, etc.) during weathering could become mobile in certain lithologies. This implies that they cannot be used to calculate the mass and volume changes due to weathering. Furthermore, they show higher ratios that affect rock discrimination diagrams. At Omai, higher ratios of ‘‘immobile elements’’ in felsic volcanic rocks displace the rock identification towards more mafic rock types and a negative adjustment of
20 units are necessary for correct classification. In contrast, these elements could aid in defining the material source of sedimentary rocks affected by weathering. At a weathering index >80, the chemical weathering and random mechanical concentration of ‘‘immobile elements’’ at the top of the weathering profile preclude any use of these elements for rock discrimination or quantitative mass calculations. The most obvious impact of weathering on exploration is the masking of ore deposits and bedrock features by secondary products and element dispersion. Understanding the geochemical behavior of major and trace elements within regolith helps in defining bedrock petrography and in detecting the presence of specific chemical

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

333

Fig. 8. Chondrite-normalized REE patterns for Wenot bedrock and weathered profile. Normalizing values from Sun and McDonough (1989). Open symbols represent bedrock, filled symbols represent equivalent weathered products. For pedolith horizon (mottled zone and ferricrete), which formed on andesite only, the REE patterns of unweathered andesite (symbol: lozenge) are also shown for comparison purposes.

elements that are usually associated with ore deposits. For many Au deposits, as at Omai, the presence of felsic rocks can represent a major indicator for Au, especially when associated with specific element anomalies. The Wenot orebody was discovered due to a K radiometric anomaly superposed over a Au geochemical anomaly. The K anomaly can now be explained by gradual enrichment in K2O during weathering due to remobilization from mafic volcanic/sedimentary rocks coupled with the presence of K-rich hydrothermal minerals that were preserved in the upper part of the weathering profile. 8.2. Behavior of rare-earth elements The REE behavior is different in lithomarge formed on volcanic rocks compared to that formed on sedi-

mentary rocks. Lithomarge formed on mafic volcanic rocks shows HREE enrichment and variable Ce and Tb anomalies, lithomarge formed on felsic rocks is HREEenriched or depleted, but it lacks REE anomalies, while lithomarge formed on sediments has the same REE patterns as the unweathered protolith. This different REE behavior could be useful in geochemical exploration in the tropical regions for determining the nature of the bedrock by analyzing the saprolith horizon for REE. Anomalous Ce behavior in weathering profiles has been reported in several studies. Boulange´ and Colin (1994) and Valeton et al. (1997) described positive and negative Ce anomalies in bauxite formed on alkaline rocks and carbonatites. Braun et al. (1990, 1993) and Ange´lica and da Costa (1993) noted negative and positive anomalies in the Fe duricrust formed on alkaline/ ultramafic rocks. Several minerals from the weathering

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profiles also have Ce anomalies, either positive (zircon, phlogopite—Walter et al., 1995) or negative (barite, dolomite—Walter et al., 1995). At Omai, negative Ce anomalies are associated mainly with pillowed basaltic andesites. The Ce behavior does not depend on the weathering intensity, but on the textural characteristics of the protolith. Ange´lica and da Costa (1993) argued that negative Ce anomalies are related to low anatase content, due to substitution of Ti and Nb in Nb-rich anatase by Ce4+. However, at Omai no correlation is observed between Ce content and usually immobile elements (TiO2, Zr, Nb, P2O5). The results presented above suggest that a relationship might exist between the content and distribution of Ce and the water/rock ratio, i.e. the openness of the porosity system. Interpillow spaces (e.g. sample 10w), characterized by intense fluid circulation and high water/rock ratio, are moderately Ce-enriched, while the inner pillow parts are strongly Cedepleted, suggesting Ce leaching from the inner parts of the pillows and its subsequent precipitation by the supergene fluids in the interpillow channels. More intense weathering in the pedolith horizon (mottled zone and ferricrete) results in Ce redistribution and homogenization, which lead to total lack of positive or negative Ce anomalies. A possible explanation for Tb anomalies could be analytical error or the ’tetrad effect’ (Hidaka et al., 1994; McLennan, 1994). In addition, Walter et al. (1995) found Dy and Er anomalies (Tb has not been analyzed) in several secondary minerals, which were explained by complex physical and chemical conditions during weathering. The persistence of Tb anomalies in the pedolith horizon at Omai might, therefore, be explained by the presence of secondary minerals with Tb anomalies in the whole weathering profile. 8.3. Pyrite preservation within saprolith Wallrock hydrothermal pyrite is preserved as fresh grains in saprolith at Omai, a particular situation since sulfide oxidation is generally described as occurring at the weathering front, and, by consequence, it is accompanied by a rapid loss of S during the earliest stages of weathering. It is known that initial penetration by surface water of a fresh mineral depends on factors such as mineral cleavage and strain (Robertson and Eggleton, 1991). A possible explanation for the pyrite preservation at Omai could be related to the deformation history of the Wenot orebody. Geochronology data show that the mineralizing event post-dates the regional metamorphism and deformation. In consequence, the sulfide mineral phases (and generally, all hydrothermal minerals) have not been affected by significant post-depositional strain or stress. The fracture-filling minerals in pyrite have been removed and replaced by secondary Al-rich minerals (mainly kaolinite), although no Fe oxihydroxides

have been noted as replacements on pyrite fractures, an indication that Fe2+ from pyrite was not oxidized. As a result, one can suppose that Au deposits that post-date regional and local deformation could still preserve the hydrothermal Au-bearing sulfides unaffected by supergene alteration, which would represent a potential tool for Au exploration in tropical terrains. 8.4. Stages of weathering The distribution of saprolith and pedolith horizons within the weathering profile and their spatial relationship with the Pleistocene Berbice Formation at Omai could provide information about the relative timing of the supergene alteration stages. Lack of pedolith beneath Berbice Formation and its presence in zones not recovered by Berbice have two possible explanations. First, lack of pedolith means that the latest stage of weathering post-dates the Berbice deposition, i.e. it has a post-Pleistocene age. This stage weathered the saprolith into pedolith in zones not recovered by Berbice as well as the upper part of Berbice. Second, the pedolith was eroded before Berbice deposition started. As described earlier, a mineralized quartz-pebble conglomerate level marks the basal part of Berbice. Meanwhile, the eastern part of the Wenot zone is recovered by mineralized transported overburden. It is suggested that post-lateritic modification of the regolith resulted in removal and transport of the pedolith from south Wenot and its redeposition as overburden in east Wenot. This instability was probably due to drainage rejuvenation (Gilt creek formation) or climatic change. The pedolith removal has resulted in the lower horizons (saprolith) being exposed at the surface. Chemical weathering and mechanical desegregation of the hydrothermal veins formed angular quartz fragments that were incorporated within sand and clay during the earliest stage of Berbice deposition. In conclusion, the main stages of the supergene alteration are considered as pre-dating Pleistocene, while minor weathering continued until the present-day.

Acknowledgements Supporting funds from Omai Gold Mines Ltd./Cambior Inc./Golden Star Resources and a Natural Sciences and Engineering Research Council of Canada scholarship to GV are gratefully acknowledged. The X-ray fluorescence spectrometry analyses were performed at the Geochemical Laboratories, McGill University, under the supervision of T. Ahmedali. We thank L. Harnois (UQAM) for his advice during the INAA analytical procedures and M. Preda (UQAM) for X-ray diffraction analyses. We wish to thank Dr. Y. Tardy and an anonymous reviewer for constructive comments and suggestions.

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References Ange´lica, R.S., da Costa, M.L., 1993. Geochemistry of rareearth elements in surface lateritic rocks and soils from the Maicuru complex, Para, Brazil. J. Geochem. Explor. 47, 165–182. Bertoni, C.H., Shaw, R.P., Singh, R., Minamoto, J., Richards, J.M., Belzile, E., 1991. Geology and gold mineralisation of the Omai property, Guyana. In: Ladeira, E.A. (Ed.), Brazil Gold ’91; The Economics Geology Geochemistry and Genesis of Gold Deposits. Balkema, Rotterdam, pp. 767–773. Boulange´, B., Colin, F., 1994. Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro, Minais Gerais, Brazil. Appl. Geochem. 9, 701–711. Braun, J.J., Pagel, M., Muller, J.P., Bilong, P., Michard, A., Guillet, B., 1990. Cerium anomalies in lateritic profiles. Geochim. Cosmochim. Acta 54, 781–795. Braun, J.J., Pagel, A., Herbillon, A., Rosin, C., 1993. Mobilization and redistribution of REEs and thorium in a syenitic laterite profile: a mass balance study. Geochim. Cosmochim. Acta 57, 4419–4434. Brimhall, G.H., Alpers, C., Cunningham, A.B., 1985. Analysis of supergene ore-forming processes using mass-balance principles. Econ. Geol. 80, 1227–1254. Colin, F., Veillard, P., Ambrosi, J.P., 1993. Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment. Earth Planet. Sci. Lett. 114, 269–285. Davies, T.C., Friedrich, G., Wiechowski, A., 1989. Geochemistry and mineralogy of laterites in the Sula Mountains greenstone belt, Lake Sonfon gold district, Sierra Leone. J. of Geochem. Explor. 32, 75–98. Davy, R., El-Ansary, M., 1986. Elemental behaviour in the Saddleback, Western Australia, laterite profile as a guide to mineralization. J. Geochem. Explor. 26, 119–144. Duddy, I.R., 1980. Redistribution and fractionation of rareearth and other elements in a weathering profile. Chem. Geol. 30, 363–381. Elliott, R.G. 1992. The Geology and the Geochemistry of the Omai Goldfield, Guyana. PhD thesis, Oxford Brookes Univ., UK. Fedo, C.M., Nesbitt, H.W., Young, G.M., 1995. Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance. Geol. 23, 921–924. Freyssinet, P., Itard, Y., 1997. Geochemical mass balance of gold under various weathering conditions: application to exploration. In: A.G. Gubins (Ed.) Proc. Exploration 97: 4th Decennial Int. Conf. Mineral Exploration. 14–18 September, Toronto, Canada, pp. 347–354. Gibbs, A.K., Olzsewski, W.J., 1982. Zircon U-Pb ages of Guiana greenstone-gneiss terrane. Precam. Res. 17, 199–214. Gibbs, A.K., Barron, C.N., 1993. Geology of the Guiana Shield. Oxford Monographs on Geology and Geophysics No. 22. Clarendon Press, Oxford. Hallberg, J.A., 1984. A geochemical aid to igneous rocks identification in deeply weathered terrain. J. of Geochem. Explor. 20, 1–8. Hidaka, H., Holliger, P., Shimizu, H., Masuda, A., 1994. Lanthanide tetrad effect observed in the Oklo and ordinary

335

uraninites and its implication for their formation processes. Geochem J. 26, 337–346. Hill, I.G., Worden, R.H., Meighan, I.G., 2000. Geochemical evolution of a paleolaterite: the Interbasaltic Formation, Northern Ireland. Chem. Geol. 166, 65–84. Lawrance, L., 1994. Supergene ore deposit geochemistry. One Day Short Course, Geol. Soc. Zimbabwe, Harare, 70 pp. Lecomte, P., 1988. Stone line profiles: importance in geochemical exploration. J. Geochem. Explor. 30, 35–61. Lecomte, P., Zeegers, H., 1992. Humid tropical terrains. In: Butt, C.R.M., Zeegers, H. (Eds.), Handbook of Exploration Geochemistry, vol. 4, Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Elsevier, Rotterdam, pp. 241–294. McLennan, S.M., 1994. Rare earth element geochemistry and the ‘tetrad effect’. Geochim. Cosmochim. Acta 58, 2025– 2033. Nahon, D., Tardy, Y., 1992. The ferruginous laterites. In: Butt, C.R.M., Zeegers, H. (Eds.), Handbook of Exploration Geochemistry, Vol. 4, Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Elsevier, Rotterdam, pp. 41–56. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climate and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. Norcross, C.E., Davis, D.W., Spooner, E.T.C., Rust, A., 2000. U-Pb and Pb-Pb age contraints on Paleoproterozoic magmatism, deformation and gold mineralization in the Omai area, Guyana Shield. Precamb. Res. 102, 69–86. Oliveira, S.M.B., Campos, E.G., 1991. Gold-bearing iron duricrust in Central Brazil. J. of Geochem. Explor. 41, 309–323. Porto, C.G., Hale, M., 1995. Gold redristibution in the stone line lateritic profile of the Posse deposit, Central Brazil. Econ. Geol. 90, 308–321. Robertson, I.D.M., Eggleton, R.A., 1991. Weathering of granitic muscovite to kaolinite and halloysite and of plagioclase-derived kaolinite and halloysite. Clay. Clay Min. 39, 113–126. Sharma, A., Rajamani, V., 2000. Weathering of gneissic rocks in the upper reaches of Cauvery river, south India: implications to neotectonics of the region. Chem. Geol. 166, 203– 233. Sun, S-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implication for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins. Geol. Soc. Spec. Publ., 42. Blackwell, Oxford, pp. 313–345. Tardy, Y., 1992. Diversity and terminology of lateritic profiles. In: Martini, I.P., Chesworth, W. (Eds.), Developments in Earth Surface Processes, Vol. 2. Elsevier, Amsterdam, pp. 379–406. Tardy, Y., 1997. Petrology of Laterites and Tropical Soils. Balkema, Rotterdam. Topp, S.E., Salbu, B., Roaldset, E., Jørgensen, P., 1984. Vertical distribution of trace elements in lateritic soil (Suriname). Chem. Geol. 47. pp. 159–174. Trescases, J.-J., 1992. Chemical weathering. In: Butt, C.R.M., Zeegers, H. (Eds.), Handbook of Exploration Geochemistry, Vol. 4, Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Elsevier, Rotterdam, pp. 25–40. Valeton, I., Schumann, A., Vinx, R., Wieneke, M., 1997.

336

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336

Supergene alteration since upper cretaceous on alkaline igneous and metasomatic rocks of the Pocos de Caldas ring complex, Minas Gerais, Brazil. Appl. Geochem. 12, 133–154. Voicu, G., Bardoux, M., Harnois, L., Cre´peau, R., 1997b. Lithological and geochemical environment of igneous and sedimentary rocks at the Omai gold mine, Guyana, South America. Explor. Mining Geol. 6, 153–170. Voicu, G., Bardoux, M., Je´brak, M., 1999a. Tellurides from the Paleoproterozoic Omai gold deposit, Guiana Shield. Can. Mineral. 37 (39), 559–573. Voicu, G., Bardoux, M., Je´brak, M., Cre´peau, R., 1999b. Structural, mineralogical, and geochemical studies of the Paleoproterozoic Omai gold deposit, Guyana. Econ. Geol. 94 (8), 1277–1304. Voicu, G., Bardoux, M., Je´brak, M., Voicu, D., 1996. Normative mineralogical calculations for tropical weathering profiles. Winnipeg ’96, Ann. Meet., Geol. Asso. Can. Mineral. Assoc. Can. Prog. with Abstr., 21, A-69. Voicu, G., Bardoux, M., Stevenson, R., Je´brak, M., 2000. Nd and Sr isotope study of hydrothermal scheelite and host

rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits. Mineral. Depos. 35 (4), 302–314. Voicu, G., Bardoux, M., Voicu, D., 1997a. Mineralogical norm calculations applied to tropical weathering profiles. Mineralogical Magazine 61, 185–196. Walter, A.V., Nahon, D., Flicoteaux, R., Girard, J.P., Melfi, A., 1995. Behaviour of major and trace elements and fractionation of REE under tropical weathering of a typical apatite-rich carbonatite from Brazil. Earth Planet. Sci. Lett. 136, 591–602. Walrond, G.W., 1987. Geological Map of Guyana, Scale 1: 1 Million. Guyana Geology and Mines Commission, Georgetown, Guyana. Wincester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chem. Geol. 20, 325–343. Wong, T.E., 1984. Stratigraphy and sedimentary history of the Guiana Basin. Geol. Mijnb. Dienst van Suriname, Med. 27, 83–90.