Guide horizons for gold mineralisation in lateritic crusts

Chemical Geology, 60 (1987) 293-298

293

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

01-9)

GUIDE HORIZONS FOR GOLD MINERALISATION IN LATERITIC CRUSTS A. BHASKARA RAO Department of Geosciences, University of Brasilia, 70910 Brasilia (Brazil) (Accepted for publication July 4, 1986)

Abstract Bhaskara Rao, A., 1987. Guide horizons for gold mineralisation in lateritic crusts. In: Y. Ogura (Guest-Editor), Proceedings of An International Seminar on Laterite, October 14-17, 1985, Tokyo, Japan. Chem. Geol., 60: 293-298. The structural classification previously proposed for laterite crusts in field exploration by the author has the objective of: (a) serving as a tool to geological mapping of the bed rock types; (b) recognition of greenstone belt units of economic interest; and (c) demarcation of target areas for mineral exploration. In localities of several small placer gold operations, outcrops of nodular and pisolitic laterite crusts are seen overlain occasionally by breccioid laterite, nodular laterite breccia, mottled cavernous laterite or banded concretionary laterite.Such nodular or pisolitic varieties, rich in goethite, characterise mafic to ultramafic parent rocks in/and greenstone belt units. This type of crust is thus the guide horizon in the lateriteprofile indicative of Au. Its surface expression accompanying the source rocks isoccasionally significant and thus can be a promoter of small-scale mining and a metallotect and guide for exploration programmes.

TABLE I

1. I n t r o d u c t i o n

1.1. Domains of laterite Laterite is a generic term frequently used in the countries limited by latitudes 30°N and 30 ° S. Ever since Francis Buchanan, in the early 18th century, discovered them in India, they have been characteristic of bad land topographies, non-fertile regions, and with only one application as a raw material for bricks. The process of lateritisation, in fact, has produced in the tropical and sub-tropical regions of the world useful mineral concentrations. The domains of tropical soils followed by those of desert/semi-desert soils of significance, especially in Latin America, Africa, 0009-2541/87/$03.50

Third World soil type coverage Soil type

% share of area

Vegetation type

Tropical soils (ferralite) Desert/ semi-desert soils Other

55

rain forest-savanna

35

desert thorn-bush steppe

10 montane flora, etc.

Source: P~ischl (1985).

Southeast Asia and Oceania is resumed in Table I. This proves the extensive coverage of such soil and the necessity to "emphasize" their

© 1987 Elsevier Science Publishers B.V.

294 nature and characteristics for better use by mankind. 1.2. Importance in ore deposits The process of lateritisation (bauxitisation) characteristic to tropical regions, is that of chemical decomposition acting on the rock types. The gradual transformation releases minerals, kaolinises them, conserving the original structures and non-destroyable minerals as residual ores. The further vertical movement of the solutions result in enrichment of chemical constituents, succeeded by the lateral movement of solutions with lateral migration of metallic ions producing the concentration of metalliferous minerals or metals in laterites. Such a process has resulted in extensive bauxite deposits from varied lithologies; Ni ores from ultramafic complexes; Mn ores from distinct protores; phosphates from sedimentary formations, and apatite-pyrochlore-Ti ores of carbonatite complexes; Zr ores from alkaline plugs besides Cr, Zn, Fe, Sn, Au and precious stones. 1.3. Importance for the Third World Thus, the process of lateritisation through the decomposition of the host rock and later enrichment of elements, has been of special interest from the economic point of view in the Third World countries. However, the degree of lateritisation varies and this is also an important factor to be considered in each context. The Third World countries with vast domains of such tropical soils, need to procure the understanding of this process in detail to further enhance the discoveries of metallic ore deposits which are basic for their industrial growth. 1.4. Lateritic crust concept It is important to state here the concept with which the term lateritic crust is used. Lateritic

crusts are indurated compact coverings on any bed rock type, with a deep-brown colouration indicative of abundance of goethite, either dominating all over or as intergranular locking material between rock fragments, or bands or nodules constituted of varying mineralogy. Such crusts are usually autochthonous but could be allochthonous, either way indicative of the nature of bed rock below or nearby. Studies are normally concentrated on in situ crusts.

2. Scope of study Small-scale mining ventures, often dominant in the Third World countries, are real vanguards for regional exploration strategies and programmes (Bhaskara Rao, 1982 ). Except Al, Mn, Ni and P ores derived through lateritisation processes, all other deposits are generally of small extension and limited occurrence as of' Au. Au, in some fluvial placers, is being considered as of tateritic derivation ( Evans, 1982 ). In Brazil, especially in the central-western region and in the vast Amazon area, many Au occurrences are recorded in lateritic crusts (Bhaskara Rao and Barros, 1982; Bhaskara Rao et al., 1982, 1983). Thus their studies are considered useful, especially when their structures are easily discernible in the field campaigns for exploration, with correlation to the bed rock. Fig. 1 and Tables II and III are the fundamental results of such a classification proposed.

3. N o d u l a r lateritic crusts The nodular types are of importance in the studies made on profiles of lateritic crusts. They have basically the following characteristics: (a) Spherical, semi-spherical, oblong, ovoidal, pisolitic or oolitic. (b) Concretionary, with a nucleus and banding, with or without repetition, with or without uniformity in thickness. (c) Constituted of goethite, with or without

295

3.2. Mineralogy

BRECCIA FRAGMENl~3

The mineralogical constitution of the nodules found in these lateritic crusts is essentially of: /~ "~"

~"

Bo

~

LATERrTE

LATE RITE

~

LAT~RT,E N: I::L ES

CONCRETIONARY

20

~~

B:If:IN G

PISOLITIC OR COLIT~C NODULES

C*'

(a) (b) (c) (d) (e) (f)

goethite gibbsite or clay (kaolin) hematite quartz as fragments, or silica as veinlets sulphides (pyrite, pyrrhotite, arsenopyrite, chalcopyrite - - as grains) Au (dust and fines; occasional plates, nuggets)

80-100% 5-10% < 10% < 5% 1-2% variable

CONCRETIONARY BANDED

50

/

80

BANDING

Fig. 1. Classification of laterites after their structures (Bhaskara Rao, 1983).

gibbsite as the growth material, directly reflecting the nature of the parent rock. 3.1. Nodular forms The form nodule includes pisolites, oolites, spheroidal and oval concretionary types. They are usually deep brown, reddish, brownish red and red when they are rich in goethite, hematite and lepidocrocite. The metallic luster and high hardness normally indicate the presence of specular hematite. The size of the nodules vary and can reach upto 20 mm in diameter. Often the nodules are indurated also by goethite, and occasionally other minerals like silica participate. These crusts may present cavities, upto 10 mm; and their origin may be similar to the pores in bauxite beds. Concretionary growth over a nucleus is considered as due to constant input of Fe as hydroxides, resultant of the decomposition of Fe-rich primary rock, usually ultramafic to mafic in nature. The growth of these crusts is conditioned to the extension of the bed rock, and the supply of Fe which if abundantly present can favour the development of pisolitic and oolitic forms.

Goethite is the essential mineral which forms part of practically all the nodular, pisolitic, oolitic and concretionary crusts. In some cases, spherical banding of goethite has intercalated material composed of clay or hematite. Gibbsite appears as veinlet material, fillings, or interbanding constituent. Quartz and silica are not infrequent though they are present as angular fragments (quartz) or as fine veinlets, stringers or lenses (silica). Sulphide minerals are interesting in their distribution. They are not intralayer, but are mixed within the spherical bands of goethite. They are angular, small fragments, do not show alteration, and are normally suggestive of Au association. 3.3. Gold distribution Usually Au is not visible in the lateritic crusts covering greenstone belt units. But at wellknown occurrences, it is represented by small nuggets or plates, discernible with weights of : upto 800 g known in Barro Alto, 15 g in Fazenda Nova, 1-2 g in many places, all in Goias State, Brazil. Au distribution can thus be classified as follows, irrespective of the nature of the bed rock, in the laterite crusts: (1) As small nuggets, with a fine band of goethite cover, normally showing growth forms. It is possible that this Au is conserved in the original form from the parent rock; or mobilised through chemical dissolution and redepo-

296 TABLE II Mineralogical constitution of some proposed structural lateritic crusts (Bhaskara Rao, 1985 ) Laterite type

Constituents

Mineralogy, lithology

Laterite breccia

fragments

quartzites, vein quartz, chert, sandstones

matrix

kaolinite, gibbsite, (goethite

bands

hematite, goethite, silica

fragments

ferruginous quartzite, chert, silica

Banded laterite

bands

hematite, goethite, quartz/silica

Concretionary pisolitic or oolitic laterite

pisolites

goethite, (gibbsite), ( hematite )

matrix

goethite, (kaolinite), (gibbsite), (quartz fragments)

nodules

goethite

matrix

goethite, (silica), ( hematite )

nodules

goethite, (gibbsite), ( kaolinite ), jasper

matrix

gibbsite, kaolinite, (goethite), (silica/jasper)

matrix

kaolinite, gibbsite, goethite

fragments

quartz

fine bands

( hematite, goethite ), (kaolinite ), (silica)

fragments

quartzites, (vein quartz )

nodules

goethite

matrix

goethite, gibbsite, etc.

Breccioid banded laterite

Nodular laterite

Laterite with nodules

Compact laterite

Nodular laterite breccia

sition to form nuggets; or a chemical growth resulting in the accretion a n d forms. T h e origin of such a nugget in the p a r e n t rock needs to be u n d e r s t o o d in each case a n d in the c o n t e x t of' the data. (2) As small plates, often discontinuous, indicating a filling in a relict structure of a fracture or a crack conserved from the p a r e n t rock. It is possible t h a t this Au might have been mobilised a n d has filled the fracture p a t t e r n in the p a r e n t rock. L a t e r laterit~sation neither mutilated the structures nor the Au, t h u s conserving t h e m in the r e s u l t a n t ambience of laterite crusts. (3) As fines a n d dusty Au in the goethite nodules or pisolites, due to dissolution, transport in colloidal form and deposition during lateritisation.

As extremely fine invisible Au inducing an i n t e r n a l reflection of goethite with sprinkled bright golden yellow shine in the polished section.

4. C o n c l u s i o n s

T h r o u g h the present study a n d with an analysis of earlier works ( B h a s k a r a Rao a n d Barros, 1982; B h a s k a r a Rao et al., 1983; M a n n , 1984; B h a s k a r a Rao, 1985, 1987 in this special issue ) some conclusions could be arrived at, to indicate a guide horizon for Au mineralisation in lateritic crusts. (a) Laterites with pisolitic, oolitic or nodular forms are recorded in tropical weathered

297 TABLE III Lateritic types and greenstone belt units (Bhaskara Rao, 1985 ) Laterite type

Possible parent rock

Laterite breccia

quartzites, sediments

Banded lateritic breccia

banded ferruginous quartzites, sediments

Breccioid laterite

heterogeneous sedimentary or metamorphic suite

Breccioid banded laterite

BICF's

Banded laterite

BIF's

Laterite with banding

gneisses with BICF's

Concretionary banded laterite

mafic and/or ultramafics dominated by BIF's

Concretionary pisolitic or oolitic laterite

mafics to ultramafics, basalts to peridotites: volcanics, metamorphic equivalents

Nodular laterite

ultramafic: dunite or peridotite, komatiites, metamorphosed equivalents

Laterite with nodules

andesites, diorites

Brsccioid nodular laterite

basic Mg and Fe schists, chlorite-talc schists, talc schists, etc., with veins of quartz, or quartzite lenses

Compact laterite

granite-gneisses and homogeneous metamorphic or sedimentary suite

Nodular laterite breccia

quartzites and basic schists

BIF = banded iron formation; BICF = banded iron cherty formation. TABLE IV Profiles of laterites with gold occurrences Fazenda Nova Goias State, Brazil (Bhaskara Rao et al., 1983)

Mara Rosa Goias State, Brazil (Bhaskara Rao, 1987 in this special issue)

Yilgarn Block, (idealised), Western Australia (Mann, 1984)

Breccioid laterite

lateritised gravel bed

pisoliticlayer of coarse ironstone pebbles cemented by ferruginous material

Banded concretionary laterite

nodular lateritic crust with pebbles and cavities

mottled zone uniformly stained (yellow,brown, purple, green) kaoliniticclays

mottled zone pale to off-white kaolinite with brown or orange ferruginous staining and angular Fe-rich fragments

Coarse-grained nodular laterite

nodular lateritic crust with mottles of gibbsite and clays

Pisolitic laterite Oolitic laterite

nodular to pisolitic laterite

pallid zone pale to off-white kaolinite

pallid zone

Transition zone with ferriferous infiltrations

altered bed rock

relict structures

relict structures

Bed rocks (greenstone belt units: mafic to ultramafic sequences)

bed rocks ultramafic to mafic volcanogenic (? ) suite

saprolite basic and mafic sequences (greenstone)

saprolite granite

zones of mafic to ultramafic bed rocks in/and greenstone belts (Table IV) . (b) This system is thus a potential indicator of Au mineralisation. (c) The frequent occurrences of Au in the fluvial placers in the viscinities suggest the derivation of Au from these crusts. (d) Old workings in several parts of Goias

State and new discoveries in the Amazon region in Brazil, have recorded lateritic crusts with visible gold. (e) The process of lateritisation in tropical and sub-tropical regions of the world should thus be viewed as a metallotect and a true guide for mineral exploration projects. (f) Field diagnosis of lateritic crusts is

298

indicative of the nature of the bed rock from which they are derived (Bhaskara Rao, 1985). (g) Breccioid lateritic or mottled cavernous laterite or banded concretionary laterite may be present in the lateritic profiles overlying the nodular (concretionary) lateritic crusts. In one case a "lateritised gravel bed" is now recorded and reported (Bhaskara Rao, 1987 in this special issue ). (h) Nodular (pisolitic, oolitic), goethiterich, in situ, laterite crust is a guide horizon that characterises a system with potential Au mineralisation. Au in laterite crusts, once considered as a curiosity and of no economic feasibility, should now be viewed as a potential indicator of smallto medium-scale enterprise, and as a guide to the location of primary mineralisation.

Acknowledgements Thanks are due to C N P q ( National Research Council in Brazil) for support to these investigations; Professor Maria S. Adusumilli for fruitful discussions; and Dr. Jorge G.C. Barros for collaboration.

References Bhaskara Rao, A., 1982. Small deposits as guides to regional exploration projects. 6th I.A.G.O.D. (Int. Assoc. Gen-

esis Ore Deposits) Symp., Tibilisi, Collect. Abstr., 1982, pp. 259-260. Bhaskara Rao, A., 1985. Laterite classification for exploration. In: W.C. Park, D.M. Hauser and R.D. Hagni (Editors), Applied Mineralogy. Metallurgical Society of AIME, Warrendale, Oreg., pp. 951-964. Bhaskara Rao, A., 1987. Lateritised gravel bed - - A new guide horizon for lateritic gold? In: Y. Ogura (GuestEditor), Proceedings of An International Seminar on Laterite, October 14-17, 1985, Tokyo, Japan. Chem. Geol., 60:287-291 (this special issue). Bhaskara Rao, A. and Barros, J.G.C., 1982. Perspectives of small scale mining in developing countries - - Brazilian example. AGID (Assoc. Geosci. Int. Dev.) News, Spec. Iss. on Small Scale Mining, No. 30, pp. 19-25. Bhaskara Rao, A., Barros, J.G.C. and Borges, M.R., 1982. Gold in laterites: perspectives. Gold-82 Symp., Harare. Abstr., 32 pp. Bhaskara Rao, A., Barros, J.G.C. and Adusumilli, M.S., 1983. Lateritic gold project. In: A.J. Melfi and A. Carvalho (Editors), Lateritisation Processes. Proc. 2nd Int. Semin. of Lateritisation Processes, S~o Paulo, pp. 159-176. Evans, D.L.C.. 1982. Lateritisation as a possible contributor to gold placers. Eng. Min. J., Aug. 1982, pp. 86-91. Mann, A.W., 1984. Mobility of gold and silver in lateritie weathering profiles: some observations from Western Australia. Econ. Geol., 79: 38-49. PSschl, A.G., 1985. The search for ore deposits in the Third World - - Chances of success. Nat. Resour. Dev., T~ibingen, 21: 35-43.