RENDJOONTl Sod~t4 Jtlllfllnll dl Mln~'1I1oqI1l ~ Pefroloqla, 31 rI', JUt: pp. 3B1-406

BEN£DETI'O DE VIVO·. GlU5EPPE GruNTA ••• ANNAMARIA LIMA •••

VINCENZO MARIGLlANO RAMAGLlA ••••• GIOVANNI OR51···...VlNCENZO PUJLONE •••• AGOSTII"o ZUPPETTA •••

AN APPLICATION OF MOVING AVERAGE ANALYSIS ANDR-MODE FACTOR ANALYSIS TO A REGIONAL GEOCHEMlCALRECONNAISSANCE ON RESIDUAL SOILS OF SOUTHERN SUDAN

AaSTlACT. - In Southern Sudan a geoc:hanical reconnaissance survey has been carriedom on the residual soils of thl!: arta belWttll the Amadi.Bor·Kapotta aligntmem and theborders of Sudan with Uganda, Zaire, Kenya and the Ctnmtl African Republic.

aimatieally this area falls in the Tropical Savannah Beh. Physiographycally thrtt sectorscan be distingllished: the Southern Moumains, e~st of the White Nile, with soils of theHumid Montane Catenae; the Ironstone Plateau with soils of the LatOsols Catenae and theSouthern Clay Plain of the Upper Nile Basin with alkaline.day soils.

LithologicalJy thl': crystalline basemenl is maul': up of ml':dium to high grade melamorphitts,extensively migmat.izl':d and granitized. Wilhin this basement, mylonilic 10 psl':udOlachilitic bandsan: found along the Nimule·Rumbek lrend. Eas' of this belt, Palaeozoic basic dykc:s andCret8CC'OlJJ alkalinl': magmatites cut Ihl': basernl':nL In the eastern end of the studil':d .rea,there .n: volcanica repn:stnting an extension of the Ethiopian Trap Series. The .rea is coveredby continent.1 e1asties (Umm Ruw.ba Formation) Ttrtiary-Pleistoctne in age .nd residual soils.

1bt B horiton, at .n .verage depth of 20 cm, has bttn sampled, with • dauity of onesample per JOO.}~ km". 1bt minU5 80 mesh fllK1ion has bttn anaI~ by mtanS of X-rayfluo~ for AI, Fe, Mn, Ti, Cu, Mo, Pb, Zn, l\o and Sn.

The log·tranSformed data have been treated statistically by mean. of a moving averageanalysis and R·mode factor analyais.

The dtmt::lltal distribution of the rqional trends has been determined and maps rdniveto the moS! interesting distribution .n: n:portl':d.

Using the R-modc factor analysis foor metal associations (Sn.Mo-Fe; Ti·Nb.Zn-Al; Cu-Zn-AI;Pb-Mn-Zn-Fe) havc been recognized. Thi. fllClor model ac:counting for 74.4' 9& of the daUlvari.bility, has heeD chosen as the most meaningful solution in terms of the geological featuresand Kmndary trIvironmc:nt characterizing the studied area.

On the basis of the relationships between recognized metal association and geologicalfeatures, bedrock andlor rurncial, three target areas havc bttn identified n:pn:scnting .Ikali­-granites, nephelinc-sycnites and manc rocks (dykes, grttrlstones). One of them, the area inwhieh nephclinc-sycnites and alkali.granitcs OCC\lr, has been identified for imml':diale follow.upevaluation.

RIASSUNTO. - £ stata oondOIll, nel Sudan Meridionale, una prospezione grochimicarioonoscitiva sui suo!i residuali dell'area oompresa tra l'allineamento Amadi-Bor-Kapoeta ed iconfini imernuionali del Sudan oon Kenya, Ugand" Zain: e Repubblica Cc:mufricana.

Da un pumo di vista climatico I'area ricade nel «Tropical Savannah Belt., cal'lltterinato

• CantO di studio per la geocronologi. e la grochimica delle fonnazioni recenti, clo lstituto diGeochimica, Otd Univen:it.ria, Roma (Pn:stnt address: htituto di Gtologia e Gco6sica,Largo S. Marcdlino 10, Napoli). .. Servizio Geo1ogico e Geofisico dcl1a Regionc Sicilia,Palermo. .... Istituto di Gcelogia e Geo6sia, Largo S. M.rcdlino 10, Napoli. ..... lstituto-li Fl$iCll Teorka e NucI~, Mosu'l d'Oltrem.n:, N.poli. ........ IlIitutO di MineraIogia cr>e~rafia, Uni'lC:fl:iti di Bari.

388 B. DE VIVO, C. OIUNTA, A. LIMA, V. MARICLlANO RAMACLlA E ALTRI

dall'alternanza cli una stagione secta e cli una piovosa. Sill le piogge che la dU1'lItl!. delbstagione umida aumentano regolHmente da nord~st verso sud-ovest.

Per que! che riguarda morfologia e pedologia possono essere distinti tre sectad principali:le «Southern Mountains,., ad est del Nilo Bianco, con suoH della .. Humid Momane Catenae .. ;1'« Ironstone Plateau", ad ovesl del Niln Bianoo, COJl suoli della Latoso[s Calenae e la «SouthernOay Plain .. del Baeino dell'Upper Nile oon suoH alc:alino-argillosi.

Da un puma cli vista litologico e presence: un Basamento Cristallino con metamorfiti dimedio-aho grado, estesamentc migmatizzate e granitiuate, cl'cd Prccambriano-Paleozoico infe­riore. In questo basamento lungo j'allineamento Nimule-Rumbek si rinvengono bande damilonitiche a pseudolachilitiche. Ad est di questa fascia i1 basamento e lagliato da dicch;basid di etil paleozoica e da magmatiti akaline cretaciche. All'estremita orientale dell'area studiatasi ritrovano vukaniti riferibili alle Trap Series de1I'Etiopia. L'area, infine, e coperta da depositiclaslid conlinentali (Umm Ruwaba Formation) la cui ela va dal Terziario al Pleistocene e dasuoli residuali.

f: stato campionato I'orizzonle B dei suoli, ad una pmfondita media di 20 cm e con unadensid. di un campione ogni 300·350 km'. La frazicne inferiore ad 80 mesh e stata analizzatlla mezzo di fluorescenza a raggi X per AI, Fe, Mo, Ti, Cu, Mo, Pb, Zn, Nb e Sn.

I dad ouenuti sono stati log·trasformati e trattati statisticamente per mezzo dell'analisidelle medie mobili e dell'analisi fattoriale; e stata cosl determinata la disuibuzione degli elementie si sono costruite carte dei trends regionali, delle quali vengono qui riportate le piu interessanti.

Per mezzo dell'analisi fauoriale seno state riconosciute qualtro associazioni di metalli(Sn-Mo-Fe; Ti-Nb-Zn.AI; Cu-Zn.AI; Ph-Mo·Zn-Fe). II modello fattoriale 4, entm cui ricadonoqueste quauro associazioni, responsabile del 74,45 % della variabilita dei dati, e slato sceltocome il piu significativo in re1azione aUe caratterisdche geologiche cd all'ambiente superficialedella zona in studio.

SuUa base delle relazioni esistenti tra l'associazione dj metalli riconosciuta, le strulturegeologiche, la litologia e I'ambiente superfidale, son') state identificate ue aree caratterizzate dagraniti alcalini, sieniti a ndelina e rocce basich~, su cui orientate una futura prospezionegeochimica a carattere strategico.

Introduction

A reconnaissance geochemical survey of the Southern Provinces of Sudan(Bahr el Ghazal, eI Buheyrat and Equatoria), where no mineralization is known,was carried out. The area is bordered in the south and west by Kenya, Uganda,Zaire, and Central African Republic; in the east by the Kapoeta meridian; andin the north by the Bor-Amadi alignment (figs. 1 and 2).

The study was· divided into three stages: I) photogeological mapping; 2) fieldwork to check the photogeological map and to collect samples for petrographicand radiometric analyses; 3) preliminary geochemical prospecting on soils, alluviumand stream sediments. The principal results of the geochemical survey on theresidual soils are reported in this paper.

Sampliqg was carried out at nominal density of one sample per 300-350 sq./km.The samples were analyzed by X-ray Auorescence for AI, Fe, Mn, Ti, Nb, Cu,Pb, Zn, Sn, and Mo.

The data were processed by means of the moving average and R-mode factoranalyses. The geochemical dispersion patterns have been interpreted with respectto the know~ geology of the area.

AN APPLICATION OF MOVINC AVEUCE ANALVSIS ETC. 389

S UDIN

,,".",'" ",

"(.../.'

",eN

~St...di.d~""

",

"

,,'

",, ...... ....

ETHIOPIA

NAIR081•

KEHYA

"

",.........

... ;

....I(HAII:TOUM•

[G yP TN

t....

l_' _~\~\t' t '~\l~~\\" '" ,....

Z AI RE

,..'

,,'

",

rl&". I. Gcl>Cl"al kleation Imp cl l10dd am.

,

all1Ul .v1llC.Ul.0..

§, nmo, ~)""I~ •c::J< rn, Q. Ult(

~l O· 0·,. -, .'

Fig. 2. - Lilholosial u.etch Imp of the: Southern Sudan. I - Biol:ilC·garnet and .illimanilC gMiss;horncblc:nde anei$o; cban."ckile ,Miss; amphibolitu; garnet and pyrO:lCDC amphibolitu. 2 - Variou.typa of migmatilC gneW., au~n,~ ,raniloid ,nn". 3 • Migmatile graniw., frequently withK·fddapar mqa<ryata!s, miao-,ranodi.ocileli and miaogranito concordalll with the wall rocks... - Vat...., typer of hi,h grade mrnlloorphites. miglmtite ana" and migmalilC grarnlo withfrequent mylonitC$ and bl:ulomylonitn bands. 5 - Alkali-granito, nephcline-,ycnilo etC. 6 . LatmlKCOv"" on inditfcrenzialC baacmenl. 7 . Vo!cani«. 8 _ Umm Ruwaba Formation. 9 - Faults.

390 B. DE VIVO, C. CIUNTA, A. LIMA, V. MARICLlANO RAMACLI" E ALTRI

Ceology

a) BedrockGeological studies on the Southern Sudan are very scarse (WHlTEMA1'o', 1971;

Southern Sudan Geological Map, 1976; CIVElTA et al.. 1979). Our work was carriedout by means of a photogeological survey, followed in the field by geological­structural and geochemical regional surveies. The lithological map shown in figure 2has been prepared from the photogeological survey results, supplemented by limitedfield evaluations.

The following terrains, stratigraphically, from youngest to oldest, have beendistinguished (WHITEMAN, 1971; CIVETIA et al .. 1919):- Umm Ruwaba Formation (Tertiary-Pleistocc:ne);- Volcanics (Tertiary);- Laterites (Mc:sozoic?-Tertiary);- Alkali-granites and nephdine-syenites (Cretaceous);- Crystalline Basement (Precambrian and Precambrian-Early Palaeozoic).

The ctystalline Basemem is made up of medium-high grade metamorphicrocks, extensively migmatized and intruded by granitic bodies. In this lithologicalcomplex the following can be found: biotite-garnet gneiss, charnockire gneiss,garnet-sillimanite gneiss, amphibolite gneiss, amphibolites and garnet amphibolites.Due to a very intensive isoclinal folding, it is impossible to reconstruct thestratigraphy of this complex, which underwent migmatization until concordantgranitic bodies were formed,

In Eastern Equatoria, discordant basic dykes cut through the Basement rocks.An intensive post<rystalline deformation phase with the formation of bands rangingfrom mylonites to pseudotachylites, with northwest-southeast trends, occurs fromNimule almost reaching Rumbek.

In the same area and particularly near Juba, structures connected with an alkalinemagmatism have been found. The rock typo are alkali-granites and nephdinc:­syenites, Cretaceous in age, and discordant with the Basement. Around Naghishot,on the Sudan-Uganda international boundary, analogous phenomena have bet:nreported (VAIL, 1976).

Large areas underlain by volcanics, representing an extension of the EthiopianTrap Series, occur on the Basement complex in the eastern part of the studiedarea. Clastic (Umm Ruwaba Formation) and residual (laterites) continental depositshave been found as well. The latter, Mesozoic-Tertiary in age, outcrops especiallywest of the White Nile. Their thickness is variable' and never exceeds 15 m.

The Umpl Ruwaba Formation, Tertiary-Quaternary in age, is made up mostlyof sandy-argiflaceous Auvio-Iacustrine deposits with minor conglomerates. It outcropsnorth of the Kapoeta-Wau alignment; the thickness of this formation increasestoward the north, pinching out toward the south where e1uvial deposits overliethe Basement.

AN Al'PUCAT10N OF MOVING AVERAGE ANALYSIS ETC. 391

b) EconomicFrom me mining point of view me Soumern Sudan is completely uncxplorccl.

In fact exu:pt for the Hofrat en Nahas mines, which arc anyway situated in theDarfur, at the western extremity of the studied area, along the border with theCentral African Republic, explored and studied by various companies, other evidenceof mineralizations is fruit of sporadic and occasional informations.

The Hofrat en Nahas deposits are a polymetallic sulphide ones related topcgmatitic-pneumatolitic and hydrothermal processes associated with ignrous andmetamorphic rocks of the Basement complex. The mineralizations consist mostlyof coppe.r sulphides with uraninite, gold and molybdenum (AF1A and WIDAT/lLL/l,1961). According to U.N.M.S.P. (1971) these deposits could be attributed to ac Porphyry copper system ». The ore reserves amount to 8,741,000 lOns with anaverage copper content of 4.01 %. The mines are not exploited because of lackof transport and energy in the area. Other indications of mineralizations concernplacers of gold around Kapocta and Juba, and of cassiterite in the Ibba.sue basinat the border with Zaire (ANDREw. 1943), while columbite is reportccl in the areabctwttn Juba and Nimule (ANDREW. 1946).

It has to be stressed anyway the high minaal potential of the nepheline­syenites, because it is well known that in East Africa with such a type of magmaticrocks, singenetic and residual ore deposits of Nb, Mo, P, U'+Th, base metal sulphidesand aluminium. are associated (DEANS. 1966; BLOOMFIEU> et al., 1971; REEDMAN, 1974).

High potential for uranium epigenetic deposits, finally, has the Umm RuwabaFormation, because its belt, bordering the oxidizing environment of the lateriticcover, could h~t uranium rol1-type. deposiu (ClVf:ITA et al., 1980).

Physiography

Physiographycally three areas can be distinguished in the Southern Provincesof the Sudan (fig. 3).

I) The Southern Clay Plain. corresponding to the alluvial plain of the UpperNile Basin, is characterized by negligible relief, resulting in the formation ofcxt~nsive swamp areas during the rainy season. The surficial terrains of this a~arc dsentially clayey, with sandy gravelly lenses.

2) The lronftone Plateau, which extends northwards from the Nile-Congowatershed, is characterized by groups of subdues hills. In this area a very extensiveand usually thick lateritic cover occurs.

3) The South~n Mountainf. which fall to the East of the White Nile,are characterized by high relief (Mount Kinyet:i, 3,168 m).

Climatically the study area falls in the c Tropical Savannah Belt» (LUON,1965). Only in a narrow belt along the Zaire boundary, owing to rainfalls exceeding1,500 mm/years, does a tropical rain forest grow. Rainfall regularly increases south­west-wards from 800 mm/year to more than 1,600 mm/year (fig. 3). To the eastof the White Nile, in the Southern Mountains area, it exceeds 2,000 mm/year.

392 8. DE VIVO, C. CIUr-'TA, A. LIMA, V. MARICUANO RAMACLlA E ALTIU

. H, - I, B $<>lId>orw 0., "'"'",,

IQ ....C..., rr=o 1",....- PIo,-ou

, -- , ~S<>ut..... __

, ' )1% e-r"""",oI8"""''''- -~ ---- R..nIoll .........

(InUl RUMilt ~HIltu , . ,UPIILlC , -, --

~, , ""-, ,

.--.-.~.~ $h. ~~~ •,""~

, , (.....l:i~~~%'" , '-' ' ....., , , - ~.,;;w.,. -, , -\' .... :::• ,

nul' ~I"'ll.'" -Fig. 3. - Climatic ll=graphic mall.

(unuIflltllI(,nue

H

I

Fig. 4. - Soils map.

i:m~llm......................

In tha ludied area the (Xdology is largely inAuenc('d by the climate andmorphology (LEBON. 1965; 6g. 4). Alkaline Clay Plain Soils 3fC prest:nt in theSouthern Clay Plain. LE.IlON (1965) notes tha~ in this region extensive areas c 3rc

annually inundated, and include the great extension of "intermediate land". aswell as the lower lands nearer the perennial marshes, which is fooded or moistened{or much of each year. The soils are cracking montmorillonitic days, alkaline andcalcareous, but without gypsum. Locally, there is evidence of impeded drainage.

AN APPLICATiON OF MOVINO AVER-AOE ANALYSIS ETC. 393

The top soil can be acid, but further down is alkaline).Soils which can be attributed to the Alkaline Soils Catenae have also been

identified in the Southern Mountains, east of the White Nile. These areas arecharacterized by outcrops of felsic rocks and subhumid climate. In the SouthernMountains, covered by montane forest the presence of soil of the Humid MontaneCatenae is very probable, although no accoullt of this has so far been given.

In the Ironstone Plateau, finally, soils of the utosol Catenae are present.These soils are weakly acid, becoming more acid after cultivation. The clayfraction is mainly kaolinite and the silica mtio and content of exchangeable basesare low. Three belts (Red Loam, Ironstone and Toich) essentially related to therainfalls have been distinguished among the terms of this Catenae.

Sampling, analylical techniques and data processing

a) Field samplingThe Southern Sudan has a very extensive and, in places, thick residual soil

cover. The generally flat topography of this area lends itself to the formationof a thick residual cover since the flowing surface waters remain long enough toleach the soluble compounds of the soil, leaving in situ the insoluble ones.

X-ray analyses of samples from the lateritic cover have shown the presence ofquartz, goethite and kaolinite in some areas, and quartz, kaolinite, hematite andgibbsite in others.

The presence of quartz and gibbsite in the soils and their pisolitic texturesuggest that the material sampled from the residual soils in this study would beclassified as B horizon, according to BAYLlss (1972).

A reconnaissance sampling of the residual soils was carried out in the abovedescribed area (fig. 4). The samples were collected at an average depth of 20 cm.

About 200,000 sq./km were sampled at a nominal density of one sample per300-350 sq./km. Because of the already mentioned physiographical aspects of theregion under investigation, the sampling was carried out mostly along themotorable roads.

b) Sample preparation and analysisThe collected samples were dried, sieved to minus 80 mesh and then crushed,

powdered and analyzed for AI, Fe, Mn, Ti, Cu, Mo, Pb, Zn, Nb and Sn byX-ray fluorescence following a regression method based on full matrix correction(FRANZINI and LEONI, 1972; LWNI and SAlnA, 1976). The precision and the accuracyof the analyses for the different elements, at the 68 % confidence levd, is as follow:Ab03: 2.7 %; Fe203: 3110; MnO: 5.0-10; Ti02: 2,3 10; Cu: 3.5 %; Mo: 5.0%;Pb: 3.610; Zn: 3.5%; Nb: 4.00/0; Sn: 3.510.

c) Data processingStatistical methods were utilized in the identification of anomalous and

background population of the geochemical data for the residual soils. Generally

394 B. DE VIVO, C. ClUNTA, A. LIMA, V. MAll.ICLlANO Il.AMACLlA E ALTll.1

TABLE 1Rang~ and thresllOld /laltles

(lo... nt R:on~ ;: • la

Al 2 - 16 , 16 ,

fe 1-35 50

Mo 11· 8239 p.p... 4365 p.p .•.

TI 100 - 53200 10000

Nb 7-321 50

So 1-9 15

"" 1 - 23 11

20 ~_ 219 81

l'b 2- 25~ 114

tu 2- 122 111

the raw data distributions obtained In

this survey are markedly positivelyskewed, whereas when log-transformedtheir distribution more closely ap­proxImate normality (AHRENs, 19-54 a;1954 b; 1957). For this reason all datahave been log-transformed pnor tosubsequent statistical work.

For all elements the threshold values(Tab. 1) were selected at the mean plustwo standard deviations of the log­transformed data (LEPELTlER, 1969;ROSE, 1972).

Histograms showing the distributionof the log-transformed data are indicatedon their respective geochemical maps(fig,. 5 to 11).

To determine the regional trends ofthe elements a moving average analysi:;

...........

ml1l1"tlClIIll1lm

,

Lx:., • ,

•I

........­­~, .. "... "0,,%

0'·"[ill.H1

@".u.,11

F'II. 5. - Aluminium r<gional distribution (%) rn~p of r<sidua] soils.

was employed in this study. This technique has been used by different authorsin simiLar studies in Afriea (GARRET and N1CHOL et aI., 1969; ARMOUR-BROWN andNICHOL, 1970). f: The moving averages are computed by moving a search areaacross the data. The mean contents of a variable are computed for samples containedwithin the search area at various point as the search area moves progressivelyover the entire region of the surveY:t (HAll.BAUGH and MERRIAM, 1968).

In the area under investigation the sample density was not always constant;

AN APPLICATION OF MOVING AVERAGE ANALYSIS ETC.

TABLE 2Factor models derived from the R-mode factor analysis

1 32.6

o••• ~~ M.~;

2 51.39 ii " KEY

""~

"•• h

~O'D•• '0 •• ~ 0.6636<

M.P~ ..3 ~ 6 0.4. '" "'" •• ..... 0.2

'" " •• .. + -" ••.. ••

4 4.45 •• ~ ••ri. " o.•• " "~

~,.'; h .. ••

'" .. ••o.5 81£ ,

" ,,~. M. :l_ "•• 0

",..:.,

" ~ •• ,. M• "~ " ,...6 87.62,. .

,. •• o•; •• " ••" ~ h P~ M. '" '0.. o•••7 91.93 ~ ,.

" .. ~o. O•'0 ,. ••" .. .. .. '" " ,.

"h o.8 95.45 "o. f, h~ ,. ,. " .." •• ,. AI Z. .. ••

" ~ ~ .. ~ '" " .. ,.o.

9 "'.3o. ..o. ~ ~

,,. " o. h o. •• .. •• ••" " •• .. ~ " '0 ~ ..

10 100

" "o. o. ••.. ~ ,. " ..=+ - + - + + + + + + + - + -EL v~ 1 2 3 4 5 6 7 8 9 10

395

howe::ve::r, the average:: density was 1 sample per 300-350 sq./km. A search areaof 10,000 sq./km (100 X 100 km) was employed with an overlap of SO ro. Thisresulted in an average of 20-25 sample::s per search area and each sample wascomputed four times for the:: calculation of the average value to be: fixed in thecentre of the search area. For every search area the mean, the variance and thestandard deviation of the log-transformed data were computed.

396 B. DE VIVO. G. G1UNTA, A. LIMA. V. MAfllGUANO RAMAGl.1A E Al.TIll

Computt:r program uSt:d for tht: movlllg avt:ragt: analysis was writtt:n byDE. VIVO, LI"t/r, and Mu.IGLlAl"O (1980).

To portray tht: distinctivt: mt:ta) associations occurring in tht: art:a a R-mOOt:factor analysis has 1>«n carrit:d out. This tt:Chniqut: is t:xtt:nsivdy describt:d In

publisht:d tcxts (Rmmn. 1970; OAVIS. 1973) and it has 1>«n throughly usW bydifft:rt:nt authors In grocht:mical survt:lt:S (Anlooll.BJ.OWN and NICHOL, 1970;CLOss and NICHOl.. 1975; NICHOL t:t al., 1969; HAKLI, 1970; SMCElt and SIKCU"Il,

TABLE 3Communa/iti~s and dge"ntJalues for the R-mode faettw analysis

Flttor....1 "' c. '" " "

E1g.et1­ulues

•32.61

94.8 99.'

99.' 100.0

•,,

••

~.,

....

....87.2

".1

92.1

"..

65.5

15.3

16.9

11.3

81.2

92.8

91.0

97.4

35.0

~9.3

"'.,91.'7

....97.1

"..

40.0

82.0

8J.~

....

...,

...."..

~5 .1

51.8

8J.2

M.'".1

91.\

".,....

33.2

35.1

10.3

11.4

".."..".."..

11.3

15.9

16.1

n .•

....

60.2

....15 .~

15.8

n .•

97.5

48.9

52.8

~ ..81.1

91.6

92.1

".."..

15.2 51.39

85.5 63.60

86.1 7~.45

86.9 81.62

81.4 87.62

87.9 91.93

90.2 95.45

9\.5 98.35

10 100.0 100.0 100.0 100.0 \00.0 100.0 100.0 100.0 100.0 100.0 100.00

1974; TI.IPATHI, 1979). It can be: St:t:n that most of tht: works with factor analysishas 1>«n carrit:d out on strt:am 5t:dimt:nts, wht:rt:a5 Iittlt: uSt:. up to now, hasb«n madt: on soil grocht:mieal data.

Mt:tal associations havt: be:t:n obtaint:d using varimax critt:rion (KAISEIl, 1958).Computt:r programs uSt:d in this invt:stigation were from the GAS package

by WU.lINCTON (1973).Tht: factor analysis (ARMOUR·BROWN and NICHOL, 1970) is a sophisticatt:d

mt:thod by mt:ans of which tht: intt:rrdati~ns t:xisting be:twt:en diflert:nt variablescan bt: t:stablished on tht: basis of their corrdation coefficit:nts, that allow tot:xprt:ss tht: intt:rrelations in tt:rms of charactt:ristics mt:tal associations.

Among tht: differt:nt solution arising from this type: of analysis (Tab. 2·3)the prope:r factor mood, and therdort: tht: mt:tal associations portrayt:d in it, isSt:lt:Ctt:d in function of tht: geology and of tht: surficial t:nvironmt:nt charactt:rizingthe study art:a.

AN APPUCATION OF MOVING AVERAGE ANALYSIS ETC. 397

Plotting on maps the: factor scorc=s of e:ach metal associations for e:ach sample:,it is possible:, the:n, to de:te:rmine: the: stre:nght of that association for e:ach sp«:i6csample: site:. In this way the g«JChe:mical parame:te:rs rdat«l to the: main grologicalprocc:ssa charaete:rizing the: study are:a can b<: isolat«l with re:sp«:1. to the: totalvariability of the: data (CLOSS and NICHOL, 1975).

Resuhs and discU88ion of rl!:&uh.

tn the: studie:d area no mine:ralizations have: bttn re:port«l. The anomalousvaluc=s rc=sulting from this study may the:rdore: b<: rdate:d e:ithe:r to not ye:t knownmine:ralizations, or to a variation of the: grological fe:aturc=s or to changc=s in the:secondary e:nvironment.

11I1I1l

""1"11"'''I~

-----... t....-•

•0'<100 _

o <1OO·nOll

1m3 1500·15OQ

JZ::I 1600·7500..-------

Fig. 6.-TiwUum rrJK.tu.l diwibution (p.p.m.) moop of ru:dual ~\r..

The: reason for the: anomalic=s can be establishe:d with a fallow-up surve:y 10

the specific targe:t areas resulting from this study.The aamination of the element distributions of the: rc=sidual soils shows,

the pres«:nce: of significant variations in the: background concentrations. In ce:rtaincas«:s different ele:me:nts show the same: variation and distribution in the: backgroundconce:ntration, indicating the: e:xiste:nce: of me:tal associations. Diffe:re:nce:s in the:distribution of the: ele:me:nts are: indicate:d in the: other case:s. Noteworthy arethe anomalies shown by: the Nb (> 50 p.p.m.) north of Juba and around Rum!xk;the: Mo (> 17 p.p.m.) around Juba; the Ti (> 10,000 p.p.m.) around Rumbek,Juba and particularly at the: borde:r with Uganda; the: Pb (> 114 p.p.m.) in thewe:stern sector and particularly lKtwe:e:n Amadi and Rumb<:k; the: Zn (> 87 p.p.m.)e:ast of the: white: Nile.

Moving Au"ag~ AnalysisThe: most distinctive demental distribution of the: regional trends rc=sulting

398 B. DE VIVO, C. GIUNTA, A. LIMA, V. l.tAll.IGLlANO RAMACUA E ALTIl.I

from the moving average analysis are here discussed and illustrated. The iSlogramsshowing the approximate normal distribution of the log-transformed data are givenbelow the maps (figs. 5-11).

-- •I ---~...... ,.... "'D·ONI-

0 ....•DJ­E2:l ...........

F"lJr. 7. - Niobium r~ion:al d~ribulion (v.pm.) m:ap of rnidu:al toils.

~' .....~~~

•I

_._.- ~::-'

-----~... ,....

• I •• , •

~

0-1_0 1-3

DJ··~.·t

•••

Fig. 8. - Tin rrgion:al di.tribution (p.p.m.) m:ap of rtlidu:al soib.

The location of each high-low pattern and its relationships to known geologicalfeatures, bedrock and/or surficial environment, when possible, is discussed.

The principal results may be summarized as follows:Aluminium (fig. 5) shows three regional highs, northeast-southwest oriented

on the Yambio-Tonj-Rumhek alignment. These highs correspond to areas wher(:the lau=ritic cover is thicker and more extensive.

The titanium regional distribution (fig. 6) shows onc high around Rumbek.This high coincid~s with IWO highs of I.h~ aluminium.

...N APPLICATION OF MOVING ...V£....GE ...N",LYSIS ETC. 399

The niobium regional distribUlion (fig. 7) shows one distinct high in the areaaround Juba and a rdative one in the Rumbek area coinciding with the titaniumregional high. The high niobium concentrations may be rdated to the nephdine­syenite bodies outcropping in this area.

The tin distribution (fig. 8) give one high in the Rumbek area as for titaniumand aluminium.

•

<..............

(mu, "I'-._--~1II'W .....amm <....

",-~\-'_.~

, 0 •~-,- -, 4, ...../.,..-....J

1L ".~.._ ..... l

~

0'·-0>'ITa ..~ n.,11

.. 2)IO"m

Fig. 9. - Molybd"num r",ion~l diwibUlion (p.p.m.) m~p of .rlidu~l .oils.

cmu,''''UI~..

--------- ~ ,"-~

0'-­0-­D-­~ '100""'"..-

,. "i""'"

Fig. 10. - Mang~nnt: rC'gion~1 dillribution (p.p.m.) m~p of raidu~1 soils.

The molybdenum regional distribution (fig. 9) shows an high on the Tonj­Rumbek area and a rdative high in the Juba area coinciding with the niobiumhighest concentration!;. As for niobium these highs may be related either to thenepheline-syenites or to alkali-granites outcropping in the area.

The manganese (fig. 10) and the zinc (fig. ll) regional distributions show

400 B. DE VIVO, G. CIUt.'TA. A. LIMA, v. MARICLlANO ......MACLlA £ ALTlU

high concentr.nions in the eastern sector, whereas a different distribution is shownin the western sc=ctor. In fact the manganese: indicates high values in the Ragaarea and the zinc in the Wau area. The high concentrations in the eastern sectorshould be: related either to the basic dykes (dolerite, gabbro) or to the volcaniesthere extensively occurring. The high concentrations in the western sector aswell as (hI:' high values for cop~r, lead, and iron aTC related to me lateritic cover.

1.. , ...

''''10111"11I1

• • ' ..

----....-., ..-.-,

b0-,10-CJ~'

~ .,·so.'... ...

Fill· I L - Zin~ regional dinribUlion (p.p.m.) map of residual soils.

,,

_._.__.---------. ..,~-

jnllU

•

Fig. 12. - Distribution of Cu·Zn-AI utoauK.n factor sc.orn..

R·Mod~ Factor Analyn$The metal associations already qualitativdy established by means of single

element distribution can be defined through the determination of interdementcorrdation coefficients (Tab. 4). In fact it can be 5ten that corrdations

AN APPLICATION OF MOVINC AVERACE ANALYSIS ETC. 401

exist for example between niobium and titanium; aluminium, zinc andiron; tin, molybdenum and iron, whereas poor correlalions exist between otherelements as for niobium with tin, molybdenum with zinc and titanium. Negativepoor correlations are also evidenced as for niobium with iron, copper and lead,for copper with molybdenum and titanium.

•' ..i, 0

!-,

'''ClOlIICOtuCEITULumuU'UBtlC

i UUloa

Fig. 13. - Distribution of Ti-Nb·Zn-AI association faclor S£OfCS.

_._._,-""'"'~_____..... 0<

h.Id ,<0"

o.. U6Uoa

Fig. 14. - Distribution of Sn-Mo-Fc association factor s£orcs.

The series of models generated by means of R-mode factor analysis are shownIn tab. 2.

The similar distribution of titanium and niobium, of copper and zinc andof molybdenum and tin resulting from the moving average analyses indicate theexistence of metal associations related la the geological and/or surficial processes.

Elements with loading in the range 0.4-1.0 are considered significant members

402 B. DE VIVO, C. CIUNTl\. It. LIMit, v. MMlIGLlItNO RItMItGLIIt £ ItLTRl

of a particular association. Th~ four faclOrs model accounting for 74.45 % of thedata variability was chosen as the most meaningful solution. In fact th~ e1~m~nt

association appearing in this model can bt: considered rdat~d ~ilher to the geologicalphenom~na (~.g. Cu-Zn-AI related to mafic dykes; Ti.Nb-Zn.AI rdat~d lO nepheline.syenites) or lO the secondary cnvironm~nt of th~ studied area (~.g. Pb-Mn-Zn-F~

to lat~ritization processes). Th~ el~m~nt associations distributions resulting fromth~ four factors model ar~ shown on the re!ativ~ maps (fi81. 12-15). On th~ mapsis also drawn a fr~qu~ncy histogram of th~ factor scor~s charact~rizjng eachassociation.

_. _.-::.::::.:""----~:.:' f

o•

";

•1,uIU

Fig. 15. - Distribu'ion of Pb-Mn-Zn·F" a~fion belot" scorr..

C~p"-Zinc-A./uminjum

The Cu-Zn-AI association (fig. 12) accounts for 43.89 % of th~ data variability(tab. 5) with loadings of 0.8, 0.4 and 0.4 for Cu, Zn and Al respectively.

The distribution of high factor scores of this association is concentrat~d ID

the eastern part of the area where basic dykes and volcanics outcrop.

Titaniu m·NiOMu m-Zinc-A/umini11 mThis association (fig. 13) accounts for 25.13 % of the data variability (tab. 5)

with strong loadings of 0.8 for Ti and Nb and weaker lo.1dings of 05 for Zn and AI.This trac~ element suite is thought to bt: related to nepheline-syenite bodiesoutcropping in the eastern area and particularly north of Juba.

The high values present in the Rumb~k and Wau area cannot be, at this stage,related to any known geological features because of the thick lateritic cover.Consid~ring the extensive outcrops of alkaline rocks in Southern Sudan, theymay indicate in these areas the presence of such rock types under the lat~ritic cov~r.

Tin.Molyhd~num-lron

This assemblage (fig. 14), representing 16.41 % of the data variability (tab. 5),

AN APPLICATION OF MOVINC AVERACE ANALYSIS ETC. 403

is a strong association with loadings of 0.8 for Sn and Mo and 0.7 for Fe. Alsothe high values of this association must be related to the extensive alkali-granite andnephe1ine-syenite bodies. Noteworthy is the fact that this association in some areas(i.e. Wau and Rumbek) has an anthipathetic behaviour compared to the Ti-Nb·Zn.AIassociation.

TABI.E 4Corrt:/ation matrix

AI n • >0.5

NI 0 I @0.4-0.5

Ph 0 - I1 00.2-0.4

So 0 0 - 11o 0.0 -0.2- Ne9'!I,vely

0 ® ® 11corre~ted

Mo 0

Co 0 - 0 0 - I1ZO • 0 0 0 0 0 I1ITi 0 • - 0 0 - 0 I1Mo 0 0 0 0 0 0 ® 0 I1F. • - 0 ® • 0 ® 0 •1111

AI Nb Pb Sn Mo Cu Zn TI Mn Fe

TABLE 5Positive t:igenvalut:s fo, tIle "n and

four factor models

r~etor "_~. P~n."l £19~"..h...

het<), .,d~l 10 heto, _1 4

32.67 32.67 43.89 43.89

51.39 18.71 6'.02 2~. 13

63.60 12.21 85.43 16.41

14.45 10.85 100.00 14 .57

81.62 7.18

87.62 '.00

9\.93 4.31

• 9!i.(3 ,~

98.35 2.93

'" 100.00 1.65

Lt:ad-Manganese-Zinc-lronThis association (fig. 15), with loadings of 0.8, 0.6, 05 and 0.4 for Pb. Zn

and Fe respectively, accounts for 1457 '710 of the data variability.High levels of this association occur only on the western sector of the

investigated area, where a thick and widespread lateritic cover outcrops. In particularthe scavenging effect of Mo and subordinately of Fe oxides should have playeda decisive role in the determination of this association (CHAO and THEOBALO, 1976).In fan in the samples the concentrations of the other elements, and particularlythat one of the Pb, increast: with increasing concentrations of Mn and Fe oxides.This is also confirmed by the fact that the high levels of this associations aresimilar to the features portrayed by the single elements distribution noted in themoving average analysis of Pb, Zn, Mn, Fe and Cu.

Conclusions

The minor elements distribution and their associations, resulting from ageochemical. reconnaissance survey on the residual soils of the Southern Sudan.have been interpreted in the light either of the geological features or of thesecondary surface phenomena related to lateritiz:ltion processes.

404 B. DE VIVO, G. GIUNTA. A. LIMA, V. MARIGlIANa P,AMAGUA E ALTRI

The distribution of the minor elements resulting from the moving averageanalysis and particularly the use of the R-mode factor .malysis has led to anotable simplification of the data because it has been possible to identify the metalassociations of the dominant features contributing to the residual soils composition.Consequently it has been possible to describe the distribution of 10 minor elementsin terms of four metal associations representative of different bedrocks and secondaryenvironment.

In addition, because different rock-types are characterized by distinctivegeochemicaI associations, the use of factor analysis can give a notable support ininterpreting the geology of areas where outcrops are scarse, as it the case of mostof the surveyed area.

The principal results can be summarized as follow:a) the metal association Sn-Mo-Fe is related to the alkali-granites or to the

nepheline-syenite bodies extensively outcropping in the eastern sector;b) the metal association Ti-Nb-2n-AI showing similarity with the distribution

of the regional trends of Ti, Nb and AI, is clearly related to the nepheline­sycnites outcropping north-northwest of Juba. In the areas of Rumbek and Wau.similar rock types can be hypotized to exist under the thick lateritic cover,according to the metal associations there portrayed;

c) the metal association Cu-2n-Al, showing some similarities with the 2n andMn distribution, is very probable related to the basic rocks (dykes, greenstones)and/or to the volcanics outcropping east of the Nimule Belt;

d) the metal association Pb-Mn-2n-Fe shows high kvels only in the north-westernpart of the studied area. They are to be related to the thick and extensivelateritic cover there occurring.

On the basis of the above stated relationships between metal associationsand geological features, bedrocks and/or surficial, a), b) and c) have been identifiedas target areas for their mineral potential.

It is well known that all over Africa tin deposits are associated with rock~

«(. younger granites:.; OLADE, 1980) comparable to the ones occurring in the targetarea a). Particularly special attention has been paied to the search of primarysource of tin, as lode mineralizations and low grade disseminated ore bodies inthis type of rocks? (TAYLOR, 1969; SAINSBURY and HAMILTON, 1%7). In other partsof Sudan itself tin, molybdenum and wolfram mineralizations connected with alkali­granites are reported (WHITEMAN, 1971).

The target area b) is characterized by the presence of nepheline-syenites as aproduct of an alkaline magnetism.

It is well known that with such a type of magmatism syngenetic and residualore deposits of Nb, Mo, P, Rare Earths, U +Th, base metal sulphides and aluminiumare associated (DUNS, 1966).

This is consistent with the anomalies found in this area.In East Africa such a metal association has been found in connection with

AN APPLICATION OF MOVING AVERAGE ANALYSIS ETC. .05

carbonatite complexes to which undersaturated alkaline igneous rocks are associated(DEANs, 1966; BLOOMFIELD et aI., 1971; REED~L\N, 1974).

The metal association similarity and the presence of an alkaline magmatism bothin the surveyed area and in East Africa could suggest that the Southern Sudanesenephdine-syenites belong to a carbonatite complex. In addition, the data of aradiometric survey, carried out in Southern Sudan by the same authors, showthe existence of a radiometric anomaly In the areas where nepheline-syenites outcrop(CIVErrA et al., 1980).

The target area c), characterized by mafic rocks and volcanics, is interestingbecause the Cu-Zn·AI association identified could be related to the existence ofsulphide mineralizations. In fact sulphide ore deposits have now been found onmost of the continents in connection with mafic intrusions (Sudbury, Canada;Great dyke, Zimbabwe; etc.).

The area d) has not been considered as a target because the metal associationsidentified, clearly related to the widespread lateritic cover are too sparse and cannotbe, at this stage, related to any definite geological feature or bedrock.

Among the thr~ target areas individuated, following the above statedconsiderations, a priority for a follow·up survey has been suggested for the areasin which nepheline·syenites and alkali-granites occur, because its mineral potentialseems to be very high. This in consideration of the fact that its mineral associationand its geological features show close similarities with well known mineralizedadjacent areas in East Africa (Uganda, Tanzania, Kenya and Zambia).

Acknowldgtmmts. - The: authors ehank the: Miniscry of Industry, the: Geological Surveyof Southern Sudan, Dr. P. EyoBO and his seafl for ehe: useful and friendly collaboration givc:oduring the: fidd work.

The: authots also wish co thank deeply Dr. L. G. CLOSS (Colorado Sdlool of Mines)for aitia.lly~ the: manuscripe and for his helpful suggestions; and Mrs. KJ,YSTYNAUUANLU: for the: English review of lhe: manuscript.

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