Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold...

Geochemical behavior under tropical weatheringof the Barama–Mazaruni greenstone belt at

Omai gold mine, Guiana Shield

Gabriel Voicua,b,*, Marc Bardouxa

aDepartament des Sciences de la Terre et de l’Atmosphere, Universite du Quebec a Montreal (UQAM), C.P. 8888,

Succ. Centre-Ville, Montreal (Qc), H3C 3P8, CanadabOmai 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 lateritizationprocesses and formation of a deeply weathered regolith. The top of the weathering profile consists of lateritic gravel or ismasked by the Pleistocene continental-deltaic Berbice Formation. Mineralogical composition of regolith consists mainly ofkaolinite, goethite and quartz, and subordinately sericite, feldspar, hematite, pyrite, smectite, heavy minerals, and uncom-

mon mineral phases (nacrite, ephesite, corrensite, guyanaite). A specific feature of the weathering profile at Omai is the pre-servation of fresh hydrothermal pyrite in the saprolith horizon. Chemical changes during the weathering processes dependon 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 degreeof supergene alteration that has affected each analyzed sample, independently of the depth of sampling. Thus, the miner-alogical 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 ofweathering and mobility (leaching or accumulation) of each element in each analyzed sample. At Omai, some major andtrace 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 ele-ment’’ ratios, the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards moremafic rock typesand a negative adjustment of �20 units is necessary for correct classification. In contrast, these elements could aid in definingthe 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 intropical terrains. Strong negative Ce and Tb anomalies are displayed by weathered pillowed andesites, which areexplained by the influence of the water/rock ratio. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Over the past several decades large databases havebeen collected to document variations of major and

trace elements related to chemical changes during

intense weathering in various climatic environments(Duddy, 1980; Topp et al., 1984; Davies et al., 1989;Angelica and da Costa, 1993; Braun et al., 1993; Bou-lange 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 bedrockduring weathering are complex, the geochemistry probably

0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

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Applied Geochemistry 17 (2002) 321–336

www.elsevier.com/locate/apgeochem

* Corresponding author.

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

represents the most promising tool for discriminatingweathered rock types, and, consequenctly an explora-tion guide for mineral deposits (Davy and El-Ansary,1986).

The degree of weathering has been commonly con-sidered as related to the depth of sampling, i.e. weatheringindex gradually decreases with depth. Fundamentally this

is true, but, as observed at Omai, there is not a linearrelationship between weathering intensity and depth, dueto specific physicochemical and structural characteristics

of the bedrock. These characteristics include the originalchemical 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 mustbe paid to the structural characteristics of the weatheringprofiles formed on steeply dipping alternating succes-

sions of felsic to mafic volcanic and sedimentary sequen-ces, as in the underlying greenstones at Omai. Theirpetrographical and tectonic contacts can act as migration

paths for elements that might result in a mixed behaviorof the chemical patterns. Meteoric water circulationalong the migration paths can create local zones of more

intense weathering closer to the bottom of the weath-ered profile and, by contrast, zones less weathered in theupper parts of the regolith. Therefore, depth should not

be the principal criterion for comparison purposes ofgeochemical behavior within the weathering profile, butrather this compatison should be an index that quanti-tatively measures the replacement of primary mineral

phases by secondary minerals for each analyzed sample.In this study, discussion of the geochemical patternfocuses on the chemical changes of each sample col-

lected from several weathered rocks types as a functionof intensity of weathering rather than depth of sam-pling. The degree of weathering, which yields distinct

values for different weathered materials at a similardepth, can be evaluated by quantitative measures, usingthe whole-rock chemical analyses. These values, repre-senting the average weathering index for each analyzed

system (sample), can also be applied to the determinationof the weathering index of each separate mineralogicalcomponent of the system. The main assumptions are that

the index of alteration of a sample is the same for all itsmineralogical pairs used for the partition of a chemicalelement between a primary and its equivalent secondary

mineral and that the system is closed, without masstransfer (loss or gain). (Voicu et al., 1996, 1997b). Thefirst step is represented by the calculation of the chemi-

cal index of alteration (CIA: Nesbitt and Young, 1982;Fedo et al., 1995) for each analyzed sample, using thefollowing equation:

CIA ¼ ½Al2O3=ðAl2O3 þ CaOþNa2OþK2OÞ� � 100

ð1Þ

where oxides are in molecular proportions. Because CIAvalues range between 50 and 100 and cannot be directlyapplied for the normative calculations, the second stepis 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 transforma-tion ratio of a primary mineral into its equivalent altera-tion mineral. It has the advantage of indicating the degree

of weathering for each analyzed sample, independently ofthe depth of sampling. The MIA value indicates incipient(0–20%), weak (20–40%), moderate (40–60%), andintense to extreme (60–100%) weathering. The value of

100% means complete transformation of a primarymineral into its equivalent weathered product and, byextrapolation, complete weathering of the parent rock.

The main objective of the present paper is to discussthe nature of weathering undergone by the greenstonebelt of the Omai area, Guyana, mineralogy and geo-

chemistry of parent rocks and weathering profile. Inparticular, the chemical behavior of major, trace andrare-earth elements for each rock type and its equivalent

weathered product as a function of a mineralogicalindex of alteration (MIA), rather than depth of sam-pling, were examined. Understanding the geochemicalbehavior within the weathered profile helps to detect the

presence of specific chemical elements that are usuallyassociated with ore deposits, which has an obviousimpact on exploration in tropical regions.

2. Regional geology

The north-central part of Guyana represents a typicalgreenstone belt sequence consisting of Paleoproterozoicvolcano-sedimentary rocks of the Barama–Mazaruni

Supergroup (Gibbs and Barron, 1993) (Fig. 1). The vol-cano-sedimentary sequence was metamorphosed gen-erally to greenschist facies during the Trans-Amazonian

orogeny (�2100 Ma, Norcross et al., 2000). The sedi-mentary rocks have been dated by U-Pb on zircons at2250106 Ma (Gibbs and Olzseswki, 1982) and the

subvolcanic porphyry dikes at 21202 Ma (Norcross etal., 2000). The low-grade greenstone-granite terranes arespatially associated with granulites, gneisses and micas-

chists (Bartica Formation). Although the stratigraphicrelationships are not fully understood, the Bartica For-mation seems to be the higher metamorphosed equiva-lent of the overlying greenstone belt. The greenstone

sequence was intruded by syn- to post-tectonic, inter-mediate to felsic batholiths and stocks of the GranitoidComplex, which yielded U-Pb zircon ages between

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

20946 and 209611 Ma at Omai (Norcross et al.,

2000). The younger terranes, which post-date thegreenstone-granite belt and associated mineral deposits,comprise suprajacent anorogenic sedimentary sequencesof the Paleoproterozoic Roraima Formation and asso-

ciated mafic dikes of the Avanavero Suite, as well asTriassic diabase dikes related to the opening of theAtlantic Ocean referred to as the Apatoe Suite. The

combined effects of prolonged uplift and weatheringunder warm, humid conditions have led to the develop-ment 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, locatedabout 200 km SW of Georgetown. Small-scale miningtargeted the placer Au, while in situ mineralization was

sporadically explored and exploited by several mining

companies. Since 1992, two distinct orebodies (Fennelland Wenot) hosted by both bedrock and weatheredprofile 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 petrographicaldescription 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 thisstudy 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 vol-canic and sedimentary rocks. This entire sequence ispart of the Barama–Mazaruni greenstone belt. Themafic volcanic sequence comprises calc-alkaline andesi-

tic (and subordinately basaltic) flows with fluidal oramygdaloidal textures and pillow-lava structures. Dueto the regional low metamorphic grade, the pillows are

Fig. 1. Simplified map of north-central Guyana showing the Paleoproterozoic bedrock geology (modified from Walrond, 1987;

Elliott, 1992; Voicu et al., 1997a).

G. Voicu, M. Bardoux / Applied Geochemistry 17 (2002) 321–336 323

slightly deformed, except the central part of the Wenotzone, affected by an east–west striking shear corridor,where the pillows are strongly deformed. Thin beds of

andesitic tuffs occur within the volcanic flows. Sedi-mentary rocks consist of fine-grained siltstones andsandstones (phyllitic tuffs in mine terminology). Thegeochemical patterns suggest that sedimentary rocks

contain a dominantly calc-alkaline volcanoclastic com-ponent. The contact between sedimentary and volcanicrocks is tectonic, along the E–W striking Wenot shear

zone, discontinuously marked by a 2-m thick, stronglydeformed cataclasic breccia. A quartz-feldspar porphyrydike, which trends 100–110 and steeply dips in either

direction, intruded the contact between the mafic volca-nic and sedimentary sequences along the Wenot shearzone. The thickness of the dike averages 10 m and it can

be traced for more than 2.5 km along trend and atleast 300-m downdip. The quartz-feldspar porphyryhas rhyodacitic composition and consists of albite, K-feldspar, quartz, biotite (replaced by chlorite), calcite,

and traces of apatite, rutile, and pyrite. The rhyolitedikes (4 major dikes) are highly irregular in shapeand frequently pinch-and-swell along trend and dip.

Rhyolites are strongly silicified, massive, aphanitic tovery 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 exten-sive sand levels interstratified with clay lenses in theupper part. The basal clays have been deposited in

extensive swamps characterized by reducing conditionsthat favored preservation of organic matter. The contactbetween Berbice Formation and underlying in situ

lithology is marked by a thin conglomerate level (�40cm), which consists of angular quartz fragments derivedfrom the mineralized quartz veins, in a partially indu-

rated, sandy matrix. The predominantly sandy upperpart formed under regressive conditions in a con-tinental-fluviatile and deltaic environment. The Berbice

Formation has an average thickness of 15 m and dipsbetween 10 and 30S.The Wenot orebody consists of stockwork quartz

veins hosted mainly by the felsic dikes, and subordinately

by adjacent andesites and sediments in both bedrock andweathered rock (Bertoni et al., 1991; Elliott, 1992; Voicuet al., 1997a,b, 1999b). The stockwork veins show highly

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


variable thickness, ranging between a few mm and 0.6m. 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 hydro-thermal alteration haloes surrounding the mineralized

veins. Vein mineral assemblage in the weathered profileconsists mainly of quartz, pyrite, goethitenative Au.Pyrite, generally considered as one of the first minerals

that oxidizes during weathering, occurs at Omai as freshgrains in quartz veins and wallrocks in the lowerweathered 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 occursin bedrock as fracture filling in pyrite is leached and

replaced by secondary minerals (kaolinite). Wallrockpyrite 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 tri-butary of the Essequibo River, and it is covered by tro-pical rainforest. The general topography is gentle, with a

maximum relief of about 100 m above sea level. Theclimate is warm and rainy, with annual average mini-mum 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 pre-cipitation fall in all months, there are two rainy seasonsduring the year (May–July and December–February).

5. Description and mineralogy of the weathering profile

Intensive weathering of fresh rock results in majorchemical, mineralogical and fabric changes. Althoughthe supergene processes are integrated and complex,

specific supergene alteration zones (horizons) are pro-duced within the regolith (Tardy, 1992, 1997; Lawrance,1994; Lecomte and Zeegers, 1992). The horizon char-

acterized by secondary mineral formation associatedwith important isovolumetric chemical changes, butwith preservation of primary rock fabric by weathering

products is referred to as saprolith, which is divided insaprock (or coarse saprolite) and lithomarge (or finesaprolite). Lithomarge very often contains no primary

mineral but the parent rock fabric is conserved. Theupper part of the weathered profile, characterized bytotal extinction of the parent rock fabric is referred to aspedolith. It consists of mottled zone, ferricrete and

latosol (Lecomte, 1988; Nahon and Tardy, 1992). Typi-cal sections through the weathering profile at Omai areshown in Fig. 3.

The saprolith represents more than two-thirds of thewhole weathering profile. The transition zone betweenbedrock and saprolite, characterized by the preservationof primary rock fabric and consisting of less than 20%

of alteration products of weatherable minerals, formsthe saprock. At Omai, the thickness of the saprock zoneis highly variable, generally ranging between 5 m (in the

felsic dikes) and 15 m (in sediments), depending mainlyon bedrock lithology, frequency of quartz veining, andstructural discontinuities.

Above the saprock, continued fluid-rock interactionand leaching associated to further loss of mobile ele-ments results in lithomarge formation. The weathered

rock becomes softer, of lower density, more porous andfriable than the saprock from which it was formed(Lawrance, 1994). The initial fabric and volume of therock are preserved because the expansion that accom-

panies the transformation of some primary minerals into‘‘weathering plasma’’ is offset by the reduction ofvolume resulting from the weathering of other compo-

nents (Trescases, 1992).The pedolith profile is well developed over the ande-

sites in the northern part of the Wenot orebody. Its

thickness depends on the chemical composition of thebedrock and landscape topography. Mafic volcanicrocks, 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 theFe-poor felsic rhyolite and porphyry dikes in moremature topographic areas are overlain by a thinner ped-

olith profile (several meters). The typical pedolith profileis generally composed of two horizons, mottled zone andferricrete. Mottled zone represents the lowermost laterite

horizon affected by surface oxidation. The transitionbetween the upper saprolite horizon and the mottledzone is gradual within tens of cm to a few meters and it

is always horizontal. Oxidation, associated with a fluc-tuating watertable under wet climate, causes Fe3+ pre-cipitation as localized spots and patches of hematite andgoethite and the almost complete breakdown of the

secondary minerals of the lithomarge into clay minerals.With the increase of weathering intensity, only the moststable clay types are produced, especially kaolinite. At

Omai, the mottled zone consists of up to 80% norma-tive kaolinite and up to 10–15% normative goethite(Voicu et al., 1997b). Generally, the mottled zone over-

lying mafic rocks is goethite-rich and kaolinite-poor.Conversely, over felsic rocks the Fe spot proportiondiminishes and the mottled zone is characterized by sig-

nificant kaolinite enrichment. Towards the upper part ofthe mottled zone, increasing weathering results in fur-ther Fe remobilization and reconcentration as hematite,forming partially to completely indurated nodules and

pisoliths, referred to as ferricrete. Surface physical andchemical reworking of this horizon may separate piso-liths from their matrix, resulting in lateritic gravel. In


the eastern part of the Wenot zone (below the Gilt creek),in situ pedolith is covered by several meter thick Au-bearing, 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 ischaracterized by saprolith only, the pedolith being

completely absent. The Berbice Formation itself isslightly lateritized in the upper part.The mineralogy of the weathering profile was defined

by microscope observations, XRD analyses and by nor-mative calculations using the MINNOR software (Voicuet al., 1996, 1997b). The saprolith horizon at Omai is

characterized by high normative kaolinite and goethitecontents. However, the presence of fresh pyrite grains insaprolith suggests that most of the Fe incorporated into

the goethite structure is leached from ferromagnesiansilicate minerals rather than sulfides. Depending on thebedrock primary mineralogy and on the weatheringintensity, other important minerals are quartz, chlorite,

sericite and smectite, while albite, anatase, apatite, cor-undum, orthoclase, titanite and pyrolusite are minormineral phases. XRD analyses evidenced the presence,

in minor amounts, of several minerals rarely describedin weathering profiles: (nacrite [Al2Si2O5(OH)4], ephe-site [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 29samples from the weathering profile and the unweath-

ered protolith (parent rock). More geochemical data forthe unweathered protolith are provided by Voicu et al.(1997a). For this study, only one representative fresh

(unweathered) sample of each rock type has been usedfor comparison purposes with the weathered profile.Samples were collected either from diamond drillholes

or from the Wenot open pit. Depending on the degree ofweathering, the samples were dried for 1–5 days at 30 C.After crushing and pulverizing in an agate mill, sampleswere analyzed for major oxides and most of the trace

elements by X-ray fluorescence spectrometry at theGeochemical Laboratories, McGill University in Mon-treal. Rare-earth elements (REE), U, Th, Ta, Hf and Cs

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

weathering profile.


were analyzed by INAA at UQAM and Ecole Poly-technique, Montreal. Precious metals (Au, Ag, Pt, andPd) were analyzed by FA at McGill University.

7. Geochemistry of the weathering profile

Representative major oxide, trace and rare-earth ele-ment compositions of the weathering profile and under-lying bedrock are shown in Table 1.

7.1. Major and trace element redistribution

7.1.1. SaprolithThe chemical signature is preserved for several oxides,

while other oxides show either enrichment or leaching

(Fig. 4). Their behavior is related to the weatheringintensity (MIA) and the rock type. The SiO2 contentsfrom all rock types are generally not affected by weath-ering. However, some higher SiO2 values in saprolith

could represent local hydrothermal silicification zones,frequently observed in bedrock. Negative correlation isobserved between MIA and Na2O, CaO and MgO con-

tents for all rock types. Complete Na2O leaching occurswith intense weathering (MIA>85) in andesites and sedi-mentary rocks and between MIA=70–80 in felsic rocks

(porphyry and rhyolite), while CaO is almost totallyleached 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 tofresh andesite. No direct relationship is observedbetween 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.


Table 1

Whole-rock chemical analyses for the weathered profile and underlying bedrock at Omai

Sample Lithology MIAd SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Ba Rb Sr Ga Cs Nb Hf Zr Y Th

349 Andesite 21.60 60.01 0.41 16.21 6.89 0.12 4.29 7.53 2.84 0.08 0.09 1.67 100.44 76 1.1 105.6 16.2 0.23 6.8 5.66 94.3 10.2 2.12

2s *Andesiteb 62.58 66.20 0.71 16.16 10.84 0.01 0.07 0.05 0.80 2.87 0.06 3.10 101.01 450 82.0 109.1 19.7 1.70 5.7 1.40 51.7 10.6 0.02

ss5 *Pillowed andesite 75.85 58.96 1.09 17.35 15.69 0.03 0.01 0.07 1.96 0.35 0.04 5.32 100.99 148 6.1 138.8 16.6 0.60 5.4 3.32 67.4 29.0 0.40


3432 *Andesite 79.00 56.67 1.16 18.59 13.31 0.02 1.45 0.13 1.42 0.63 0.08 6.53 100.15 122 15.0 100.4 19.8 0.91 5.4 1.95 69.8 32.0 0.35

10w *Pillowed andesite 87.80 62.08 1.19 18.78 10.37 0.02 0.19 0.20 0.75 0.27 0.06 6.26 100.32 87 5.6 51.4 18.3 0.38 5.3 2.20 67.4 33.4 0.14

19AW *Andesite 93.64 58.53 1.60 22.52 6.36 0.01 2.47 0.01 0.21 0.52 0.05 8.10 100.59 525 15.4 50.7 26.2 0.68 11.2 6.41 229.33 44.4 0.11


18aw *Pillowed andesite 99.14 58.77 1.33 18.63 14.19 0.01 0.00 0.01 0.06 0.02 0.09 7.85 101.06 29 0.0 2.9 19.1 0.40 6.5 2.48 77.7 14.3 0.52

622/123 Quartz porphyry 15.76 75.94 0.18 9.29 2.40 0.06 2.67 2.67 3.34 0.75 0.05 4.00 99.83 180 26.7 128.4 11.6 2.19 6.1 4.45 59.2 4.1 6.82

664/80 +Quartz porphyrya 40.52 70.50 0.44 16.51 2.73 0.08 0.23 0.53 4.76 1.70 0.14 2.43 100.17 670 58.7 412.8 19.5 3.27 6.5 6.47 120.3 7.5 11.21

6w *Quartz porphyry 66.48 75.84 0.42 15.99 1.57 0.00 0.18 0.02 0.63 2.57 0.02 3.28 100.69 1108 76.6 104.7 16.6 2.49 7.0 4.18 147.7 21.5 0.04

738 *Quartz porphyry 73.36 69.30 0.62 19.92 1.85 0.00 0.14 0.01 0.25 2.81 0.02 5.25 100.27 871 80.4 47.8 24.0 3.23 8.0 6.86 154.6 15.0 14.21

5428 Rhyolite 32.14 77.33 0.10 12.05 2.63 0.04 0.01 0.10 5.52 0.70 0.01 0.78 99.70 489 18.9 32.3 22.7 0.64 32.7 16.70 489.0 139.0 7.91

739 *Ryolite 73.82 79.60 0.73 13.81 0.53 0.01 0.03 0.03 0.64 1.41 0.02 3.16 100.07 553 44.1 101.5 17.6 1.97 12.8 7.43 206.4 35.2 5.74

23423 Phyllitic tuff 38.06 69.85 0.66 12.86 6.62 0.10 1.81 1.27 3.27 1.23 0.11 2.91 101.00 267 47.7 126.5 13.8 1.27 9.1 4.33 158.7 17.8 5.47

15423 * Phyllitic tuff 49.40 68.15 0.72 15.44 6.73 0.02 1.59 0.30 2.91 2.02 0.12 2.47 100.66 797 80.1 108.0 17.1 2.44 10.8 4.16 157.6 19.7 6.06

13423 * Phyllitic tuff 58.72 70.96 0.69 13.65 6.45 0.03 1.56 0.37 1.62 1.56 0.12 3.37 100.50 434 61.1 66.4 14.9 nae 10.1 5.40 167.8 18.5 na

10423 * Phyllitic tuff 64.22 62.49 0.82 17.81 9.44 0.13 1.62 0.18 0.66 3.04 0.14 4.41 100.87 574 118.2 111.2 21.6 na 11.3 5.50 173.2 26.5 0.80

1513 * Phyllitic tuff 64.68 57.16 0.99 21.77 9.68 0.06 1.68 0.24 1.43 3.00 0.17 4.34 100.71 739 112.9 204.1 25.8 3.79 11.1 5.34 209.3 27.9 7.47

11424 * Phyllitic tuff 78.10 58.53 1.02 19.78 10.70 0.09 3.03 0.31 0.17 1.52 0.18 6.21 101.54 619 50.5 28.5 20.5 1.50 11.3 4.01 153.4 22.1 5.96

18bw * Phyllitic tuff 99.52 58.73 1.44 20.74 11.06 0.01 0.01 0.01 0.04 0.01 0.03 8.42 100.61 4 0.0 2.9 21.7 0.13 6.8 2.55 85.7 10.8 0.04

1cw **Mottled zonec 96.28 56.10 1.72 27.2 2.06 0.01 0.10 0.01 0.02 0.51 0.06 11.25 99.84 199 29.9 40.4 31.8 2.52 35.6 15.08 605.8 22.0 1.76

1s **Mottled zone 97.14 45.91 2.31 28.24 7.91 0.05 0.61 0.30 0.07 0.04 0.14 14.93 100.77 39 9.3 12.6 31.7 4.45 15.1 5.75 213.9 22.4 4.52

1aw **Mottled zone 97.32 65.88 1.38 20.63 4.56 0.01 0.01 0.02 0.02 0.24 0.05 7.80 100.77 69 6.6 23.3 27.5 0.49 11.9 5.33 199.5 10.1 0.15

1423 **Mottled zone 97.68 48.86 1.89 33.12 2.81 0.01 0.07 0.01 0.05 0.34 0.05 13.03 100.33 133 15.3 38.0 38.3 1.29 34.8 16.73 596.5 20.1 25.45

1432 **Ferricrete 67.78 41.93 1.66 23.01 21.12 0.20 0.26 0.03 1.15 3.24 0.16 7.34 100.42 1061 89.0 104.0 35.0 0.38 9.5 3.84 140.6 38.2 0.20

7s **Ferricrete 69.28 43.93 1.07 24.86 16.14 1.26 0.47 0.01 0.15 4.36 0.15 8.27 101.02 11.78 153.2 12.6 19.5 2.36 5.6 1.68 76.8 28.4 0.14

2423 **Ferricrete 98.26 48.16 1.63 27.43 10.77 0.01 0.06 0.01 0.05 0.19 0.05 11.94 100.36 90 11.7 31.1 33.8 0.81 31.3 16.10 583.2 18.3 30.32

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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 Andesite 21.60 0.76 0.59 117 40 39.4 115 68 0.0 92 0.0 na 1.3 4.1 6 0 0 na 5.8 na na 2.2 14.91 28 3.30 1.24 0.56 2.13 0.28

2s *andesite 62.58 0.19 0.18 19 10 24.0 302 188 0.0 73 0.1 2.3 4.6 0.0 19 0 0 0.8 3.1 0.2 0.0 4.6 1.78 0 0.98 0.32 0.02 1.13 0.17

ss5 *pillowed andesite 75.85 0.03 0.25 176 14 84.5 320 149 0.0 102 0.0 na 7.8 0.0 9 0 0 na 10.0 na na 2.3 9.35 1 10.32 2.91 1.74 6.60 0.96


3432 *andesite 79.00 0.23 0.12 155 42 35.0 314 186 0.0 229 0.0 2.4 1.9 0.1 39 0 0 1.6 1.9 0.8 0.0 6.6 10.64 11 4.70 1.23 0.85 2.73 0.41

10w *pillowed andesite 87.80 0.04 0.21 96 10 31.0 308 305 0.0 103 0.0 1.3 1.4 0.3 21 0 9 1.0 12.2 0.2 0.0 6.1 10.15 29 5.34 1.58 0.06 3.86 0.52

19AW *andesite 93.64 1.91 0.41 69 35 41.0 291 263 8.1 147 0.2 0.0 4.2 0.0 2 0 2 1.2 0.0 0.2 1.8 4.3 45.91 96 12.72 3.01 1.98 4.05 0.52


18aw *pillowed andesite 99.14 0.16 0.38 58 7 40.0 296 192 0.0 84 0.1 2.2 2.0 0.1 2 0 0 1.2 2.4 0.3 0.0 6.2 5.65 4 2.96 0.82 0.33 2.27 0.31

622/123 Quartz porphyry 15.76 2.80 0.19 16 12 7.6 50 56 0.0 66 0.0 na 4.8 0.0 79 0 0 na 0.0 na na 0.0 14.58 22 2.03 0.77 0.23 0.46 0.06

664/80 +quartz porphyry 40.52 4.10 0.40 11 0 10.4 40 20 7.0 66 0.0 na 0.0 0.0 5 0 3 na 4.8 na na 0.0 40.36 43 5.63 1.43 0.54 0.99 0.12

6w *quartz porphyry 66.48 0.93 0.36 15 4 12.0 67 214 2.9 74 0.0 0.0 0.0 0.3 32 0 0 0.2 0.9 0.2 2.1 2.4 11.01 40 2.08 0.56 0.21 2.38 0.37

738 *quartz porphyry 73.36 3.97 0.43 4 0 18.1 79 58 9.3 54 0.0 na 1.9 0.0 5 0 0 na 6.0 na na 0.0 42.21 44 5.88 1.55 0.63 1.65 0.26

5428 Rhyolite 32.14 2.36 2.22 29 13 2.0 50 157 2.9 232 0.0 3.4 23.5 5.9 24 0 0 0.4 3.5 0.1 0.0 2.0 45.47 123 15.18 1.52 3.10 15.00 2.05

739 *rhyolite 73.82 2.02 0.99 4 0 36.2 122 9 1.2 37 0.0 na 3.3 0.0 2 0 3 na 1.5 na na 0.0 42.66 62 9.89 1.75 1.31 5.32 0.79

23423 Phyllitic tuff 38.06 1.98 0.53 53 27 14.0 109 521 4.8 186 0.0 1.6 0.0 3.1 2 0 0 1.0 2.7 0.2 0.0 3.5 21.21 42 3.46 0.76 0.45 2.03 0.29

15423 *phyllitic tuff 49.40 1.77 0.65 47 30 11.0 102 191 3.9 123 0.0 2.7 2.6 1.0 8 0 0 1.0 0.3 0.5 0.0 4.7 27.98 50 4.31 1.30 0.53 1.57 0.21

13423 *phyllitic tuff 58.72 na na 40 22 11.0 80 175 3.6 124 0.5 0.2 7.0 0.0 2 0 0 0.8 1.5 0.0 0.0 5.3 na na na na na na na

10423 *phyllitic tuff 64.22 0.00 na 66 24 21.0 118 201 7.5 148 0.4 2.3 5.7 0.0 23 11 15 1.0 1.3 0.3 0.9 5.5 na na na na na na na

1513 *phyllitic tuff 64.68 2.46 0.67 92 35 15.0 137 250 12.7 170 0.0 2.8 8.8 0.0 26 0 0 1.0 1.0 0.1 3.1 6.2 28.46 61 5.37 1.14 0.66 2.62 0.34

11424 *phyllitic tuff 78.10 1.65 0.69 111 33 22.0 151 214 7.6 175 0.0 1.8 3.3 5.2 621 0 0 1.7 2.1 0.0 1.0 5.3 21.32 39 4.29 1.01 0.76 2.33 0.33

18bw *phyllitic tuff 99.52 0.11 0.25 95 1 95.0 359 226 0.0 90 0.2 0.5 0.7 0.0 2 0 0 1.0 0.0 0.7 0.0 3.9 4.14 8 2.12 0.54 0.03 1.64 0.28

1cw **mottled zone 96.28 4.41 2.24 20 10 19.0 122 132 55.2 76 0.0 0.0 4.7 1.7 7 0 0 0.4 1.1 0.2 2.2 1.6 69.01 106 4.34 0.83 2.29 3.68 0.35

1s **mottled zone 97.14 1.01 0.81 110 61 56.0 349 1367 1.7 437 0.0 2.0 5.2 0.4 306 16 19 1.0 2.1 0.1 0.0 5.1 9.47 26 2.80 0.77 0.59 2.26 0.31

1aw **mottled zone 97.32 0.90 0.58 43 3 23.0 364 138 12.0 69 0.0 0.0 19.7 0.7 198 0 7 0.6 7.8 0.8 0.1 2.7 16.96 44 2.09 0.59 0.25 1.32 0.06

1423 **mottled zone 97.68 3.28 2.42 26 4 17.0 193 126 28.9 80 0.0 0.0 2.3 0.7 54 0 0 0.6 0.7 0.6 0.5 0.3 59.76 89 3.98 0.79 0.02 3.31 0.49

1432 **ferricrete 67.78 0.81 0.45 99 71 59.0 430 354 10.8 221 2.3 5.2 55.3 7.5 4261 0 0 3.6 13.9 0.9 0.0 8.2 19.55 24 5.80 1.46 0.10 3.70 0.47

7s **ferricrete 69.28 0.79 0.39 229 647 44.0 304 531 0.0 235 0.5 3.8 0.0 0.0 29 0 0 1.4 0.7 0.5 0.0 8.3 9.40 59 4.36 1.43 0.07 2.80 0.43

2423 **ferricrete 98.26 3.51 1.95 128 2 19.0 161 128 25.5 79 0.0 3.3 2.4 1.8 2 0 0 1.0 1.5 1.5 0.0 4.3 48.39 58 2.88 0.62 0.48 3.22 0.54

a +—Saprock.b *—Lithomarge.c **—Pedolith (mottled zone and Ferricrete).d 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.e na = not analyzed.

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is accumulated mainly outside the ferromagnesianminerals. In porphyry, the MgO content decreases shar-ply at MIA values �40 (saprock) and it is completelyleached with intense weathering (MIA �75). In rhyolite,

the almost complete lack of ferromagnesian minerals inthe fresh rock precludes any interpretation. The K2Oshows leaching in mafic volcanic and sedimentary rocks,

while in felsic rocks there is a gradual enrichment withincreasing weathering. The K-bearing minerals (musco-vite, fuchsite and minor feldspar) at Omai generally

persist throughout saprolith. In addition, the Au miner-alization is associated with strong phyllic alteration, whichis widespread in felsic rocks (Voicu et al., 1999b). Hence, it

can be supposed that weathering overlapped the hydro-thermal alteration, which caused a residual increase of Kin felsic rock by remobilization from mafic volcanic andsedimentary rocks. The Fe2O3 contents show both gain

and loss in mafic volcanic rocks, gradual gain in sedimen-tary rocks and loss in felsic rocks. The highly variable Fecontents in the mafic volcanics are mainly due to the

weathering of less stable minerals (amphibole, biotite,pyroxene) and formation of secondary Fe oxides. Someleaching of Fe is probably related to a lower watertable

that characterizes the post-weathering periods. On theother hand, the sedimentary rocks are characterized bya more uniform Fe oxide redistribution. This is due to

more limited weathering because of the overlying Ber-bice Formation that acted as a blanket and to a moreuniform fabric and chemical composition of sedimentswhen compared to mafic volcanics. The Fe distribution

in sediments suggests a gradual accumulation towardsthe upper part of the saprolith, undisturbed by frequentwater-table variations. Al2O3 is the only major oxide

that shows a positive correlation with MIA in all rocktypes due to its retention in secondary mineral phases.TiO2 is generally immobile in mafic volcanics, but showsgradual enrichment in weathered felsic volcanics and

sedimentary rocks. Therefore, the weathered felsic vol-canic rocks have Ti/Zr ratios similar to andesites in theHallberg (1984) diagram (Fig. 5). Meanwhile, the

weathered sediments plot mainly within the andesitiefield. 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 homo-geneous sediments. P2O5 is completely leached in sapro-lite 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 byweathering, suggesting relative immobility (Nb, Hf, Ta

and partially Y), some show moderate to strong enrich-ment (V, W, Ga, Sc) or leaching (Sr, Mo) and othershave a mixed behavior (Cs, Ba, U, Th, Ni, Cu, Zn, Pb,

Te). Rubidium is generally completely leached duringextreme weathering (MIA �95).

7.1.2. PedolithThe pedolith horizon is formed by extreme weathering

of the andesitic-basaltic rocks only. The mottled zone

and ferricrete are SiO2 depleted. Leaching of K2O andFe2O3 in the mottled zone and high K2O and Fe2O3contents in ferricrete compared to saprolith indicateremobilization of primary and secondary micas and Fe

oxihydroxides in the lower part of the pedolith andsubsequent accumulation as sericite ‘pockets’ preservedin ferricrete pisoliths and nodules. Sericite ‘‘pockets’’

Fig. 5. Ti–Zr plot (Hallberg, 1984).


and, implicitly, high K content directly affect the MIA

values, which are anomalously low in two ferricretesamples. The Al2O3 contents show general increase inthe mottled zone and ferricrete when compared to the

underlying saprolith and bedrock. TiO2 has commonlybeen considered to be immobile and it has been used asan index element to calculate mass balance during the

weathering processes (Oliveira and Campos, 1991; Bou-lange and Colin, 1994; Porto and Hale, 1995; Freyssinetand Itard, 1997). Although pedolith formation is not

isovolumetric, the variance of TiO2 content when com-pared to that of lithomarge, saprock and parent rocksuggests that this element could become mobile in theupper parts of the weathering profile. Similar behavior is

observed for some trace elements (Zr, Nb, Hf, and lessY). 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 dis-

crimination. These observations indicate that these che-mical elements cannot be used to calculate the mass andvolume balance in the upper horizons of the weathering

profile at Omai. Hill et al. (2000) reach a similar conclu-sion based on strong Y depletion in paleolaterite fromNorthern Ireland. As Ti, Zr, and Nb-bearing minerals are

not (or only slightly) affected by weathering, their enrich-ment in pedolith is considered as mechanical rather thanchemical. In the overlying Berbice Formation, partiallyaffected by lateritization, Zr content is up to 700 ppm, Nb

35 ppm, Hf 15 ppm and TiO2 2 wt.%, which suggests ageneral enrichment of these elements from the mottledzone towards the present-day surface. This observation

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.


agrees with frequently described translocated zircon-richzones at the top of the morphological sequence (Brim-

hall et al., 1985; Colin et al., 1993).

7.2. Rare-earth element (REE) redistribution

The REE chondrite-normalized patterns for the parentrocks and the weathered equivalents are shown in Fig. 8.Generally, the REE fractionation depends on the petro-

graphical nature of the bedrock and degree of weathering.Lithomarge formed on pillowed andesites is char-

acterized by slight to strong negative or positive Ce

anomalies (between 0.01 and 1.18), where the Ce anomalyhas been calculated relative to straight line interpolationbetween the flanking element. Several samples havenegative Tb anomalies also. Except for La and Ce, the

other REEs in weathered andesites are generally moreenriched and less fractionated than in their parent rock.Lithomarge formed on quartz-feldspar porphyry is

characterized by HREE enrichment and less REE frac-tionation 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 bothweathered and unweathered porphyry. The Eu anomalythat characterizes unweathered rhyolites is not observed

in the weathered equivalent. The REEs are more frac-tionated in the rhyolitic lithomarge, which is also char-acterized 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

Tb anomaly. Hence, it is suggested that weatheredhomogeneous sediments with MIA values <90 preservethe REE fractionation patterns of the unweatheredprotolith.

8. Discussion and conclusion

8.1. Behavior of major and trace elements in weatheredprofiles and its impact on exploration

Use of a weathering index (MIA or other similarindexes) allows one to quantitatively measure the

supergene alteration of each individual sample, in con-trast with qualitative estimation of weathering intensityeither by costly methods (XRD, mineralogical studies,etc.) or, frequently, by subjectively visual observations

only. It can provide more accurate information about thetrends of major and trace element in regolith as opposedto unweathered bedrock. In particular, the weathering

index can aid in relating the intensity of supergenealteration in the degree of mobility (accumulation orleaching) of chemical elements and in better under-

standing of the influence of specific physicochemical andstructural features of bedrock upon weathering pro-cesses. Plotting major oxides vs the weathering index

shows that some elements are highly mobile in all ana-lyzed rock types, while chemical behavior of other oxi-des depends on physicochemical characteristics of theunweathered rock type. Some oxides (CaO, MgO,

Na2O) are completely leached in the saprolith horizon,while others (Al2O3, Fe2O3, K2O) persist up to the tophorizons of the weathering profile. Some major and

trace elements that are considered generally immobile(ex: TiO2, Zr, etc.) during weathering could becomemobile in certain lithologies. This implies that they can-

not be used to calculate the mass and volume changesdue to weathering. Furthermore, they show higher ratiosthat affect rock discrimination diagrams. At Omai,higher ratios of ‘‘immobile elements’’ in felsic volcanic

rocks displace the rock identification towards more maficrock types and a negative adjustment of �20 units arenecessary for correct classification. In contrast, these ele-

ments could aid in defining the material source of sedi-mentary rocks affected by weathering. At a weatheringindex >80, the chemical weathering and random

mechanical concentration of ‘‘immobile elements’’ at thetop of the weathering profile preclude any use of theseelements for rock discrimination or quantitative mass

calculations.The most obvious impact of weathering on explora-

tion is the masking of ore deposits and bedrock featuresby secondary products and element dispersion. Under-

standing the geochemical behavior of major and traceelements within regolith helps in defining bedrock petro-graphy and in detecting the presence of specific chemical

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: lithomarge-

andesite; open square: quartz porphyry; half-filled square:

saprock-quartz porphyry; filled square: lithomarge–quartz

porphyry; open triangle: rhyolite; filled triangle: lithomarge-

rhyolite; open circle: sediment; filled circle: lithomarge–sedi-

ment; rotated cross: mottled zone; cross: ferricrete.


elements that are usually associated with ore deposits.

For many Au deposits, as at Omai, the presence of felsicrocks can represent a major indicator for Au, especiallywhen associated with specific element anomalies. The

Wenot orebody was discovered due to a K radiometricanomaly superposed over a Au geochemical anomaly.The K anomaly can now be explained by gradual

enrichment in K2O during weathering due to remobili-zation from mafic volcanic/sedimentary rocks coupledwith the presence of K-rich hydrothermal minerals that

were preserved in the upper part of the weathering pro-file.

8.2. Behavior of rare-earth elements

The REE behavior is different in lithomarge formedon volcanic rocks compared to that formed on sedi-

mentary rocks. Lithomarge formed on mafic volcanic

rocks shows HREE enrichment and variable Ce and Tbanomalies, lithomarge formed on felsic rocks is HREE-enriched or depleted, but it lacks REE anomalies, while

lithomarge formed on sediments has the same REEpatterns as the unweathered protolith. This differentREE behavior could be useful in geochemical explora-

tion in the tropical regions for determining the nature ofthe 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 andnegative Ce anomalies in bauxite formed on alkalinerocks and carbonatites. Braun et al. (1990, 1993) and

Angelica and da Costa (1993) noted negative and posi-tive anomalies in the Fe duricrust formed on alkaline/ultramafic rocks. Several minerals from the weathering

Fig. 8. Chondrite-normalized REE patterns for Wenot bedrock and weathered profile. Normalizing values from Sun and McDo-

nough (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.


profiles also have Ce anomalies, either positive (zircon,phlogopite—Walter et al., 1995) or negative (barite,dolomite—Walter et al., 1995). At Omai, negative Ceanomalies are associated mainly with pillowed basaltic

andesites. The Ce behavior does not depend on theweathering intensity, but on the textural characteristicsof the protolith. Angelica and da Costa (1993) argued

that negative Ce anomalies are related to low anatasecontent, due to substitution of Ti and Nb in Nb-richanatase by Ce4+. However, at Omai no correlation is

observed between Ce content and usually immobile ele-ments (TiO2, Zr, Nb, P2O5). The results presented abovesuggest that a relationship might exist between the con-

tent 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 circula-tion and high water/rock ratio, are moderately Ce-enri-

ched, while the inner pillow parts are strongly Ce-depleted, suggesting Ce leaching from the inner parts ofthe pillows and its subsequent precipitation by the

supergene fluids in the interpillow channels. More intenseweathering in the pedolith horizon (mottled zone andferricrete) results in Ce redistribution and homogeniza-

tion, which lead to total lack of positive or negative Ceanomalies.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 beenanalyzed) in several secondary minerals, which were

explained by complex physical and chemical conditionsduring weathering. The persistence of Tb anomalies inthe pedolith horizon at Omai might, therefore, be

explained by the presence of secondary minerals with Tbanomalies in the whole weathering profile.

8.3. Pyrite preservation within saprolith

Wallrock hydrothermal pyrite is preserved as freshgrains in saprolith at Omai, a particular situation since

sulfide oxidation is generally described as occurring atthe weathering front, and, by consequence, it is accom-panied by a rapid loss of S during the earliest stages of

weathering. It is known that initial penetration by sur-face water of a fresh mineral depends on factors such asmineral cleavage and strain (Robertson and Eggleton,

1991). A possible explanation for the pyrite preservationat Omai could be related to the deformation history ofthe Wenot orebody. Geochronology data show that the

mineralizing event post-dates the regional metamorph-ism and deformation. In consequence, the sulfide mineralphases (and generally, all hydrothermal minerals) havenot been affected by significant post-depositional strain

or stress. The fracture-filling minerals in pyrite havebeen removed and replaced by secondary Al-rich miner-als (mainly kaolinite), although no Fe oxihydroxides

have been noted as replacements on pyrite fractures, anindication that Fe2+ from pyrite was not oxidized. As aresult, one can suppose that Au deposits that post-dateregional and local deformation could still preserve the

hydrothermal Au-bearing sulfides unaffected by super-gene alteration, which would represent a potential toolfor Au exploration in tropical terrains.

8.4. Stages of weathering

The distribution of saprolith and pedolith horizonswithin the weathering profile and their spatial relation-ship with the Pleistocene Berbice Formation at Omai

could provide information about the relative timing ofthe supergene alteration stages. Lack of pedolith beneathBerbice Formation and its presence in zones not recov-ered by Berbice have two possible explanations. First,

lack of pedolith means that the latest stage of weatheringpost-dates the Berbice deposition, i.e. it has a post-Pleis-tocene age. This stage weathered the saprolith into ped-

olith in zones not recovered by Berbice as well as theupper part of Berbice. Second, the pedolith was erodedbefore Berbice deposition started. As described earlier, a

mineralized quartz-pebble conglomerate level marks thebasal part of Berbice. Meanwhile, the eastern part of theWenot zone is recovered by mineralized transported

overburden. It is suggested that post-lateritic modifica-tion of the regolith resulted in removal and transport ofthe pedolith from south Wenot and its redeposition asoverburden in east Wenot. This instability was probably

due to drainage rejuvenation (Gilt creek formation) orclimatic change. The pedolith removal has resulted inthe lower horizons (saprolith) being exposed at the sur-

face. Chemical weathering and mechanical desegrega-tion of the hydrothermal veins formed angular quartzfragments that were incorporated within sand and clay

during the earliest stage of Berbice deposition. In con-clusion, the main stages of the supergene alteration areconsidered as pre-dating Pleistocene, while minorweathering continued until the present-day.

Acknowledgements

Supporting funds from Omai Gold Mines Ltd./Cam-bior Inc./Golden Star Resources and a Natural Sciences

and Engineering Research Council of Canada scholar-ship to GV are gratefully acknowledged. The X-rayfluorescence 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 ana-lytical procedures and M. Preda (UQAM) for X-ray

diffraction analyses. We wish to thank Dr. Y. Tardy andan anonymous reviewer for constructive comments andsuggestions.


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Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold...

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