Geochemical Prospecting 

Investigations in the 

Nyeba Lead-Zinc District 

Nigeria

GEOLOGICAL SURVEY BULLETIN 1000-B

A CONTRIBUTION TO GEOCHEMICAL PROSPECTING

FOR MINERALS

GEOGHEMICAL PROSPECTING IN THE NYEBA LEAD 

ZING DISTRICT, NIGERIA

By H. E. HAWKES 

ABSTRACT

A field study of the distribution of traces of lead and zinc in weathering 

products, particularly residual and alluvial soils, was made in the Nyeba lead-zinc 

district, southern Nigeria. Similar geochemical work in temperate climates has 

shown that systematic sampling and trace analysis of residual soils and other 

weathering products hold considerable promise as a method of mineral exploration. The purpose of the present investigation was to learn whether geochemical 

methods of prospecting would apply equally well in a tropical environment.

The program consisted principally of detailed studies of the distribution of 

lead and zinc in residual and alluvial cover over known lead-zinc lodes, followed 

by experimental reconnaissance sampling surveys to determine the feasibility 

of large-scale geochemical prospecting work. A chemical kit was shipped to the 

field area and used for obtaining prompt analytical data to assist in guiding the 

sampling program.

As a result of these experiments, it is concluded that the metal content of small, 

near-surface samples of residual soil collected at a spacing of 100 feet would be 

adequate to show the location of the important lead-zinc lodes of the Nyeba 

district, and that the metal content of deeper samples of alluvial soil collected 

at a spacing not greater than the width of the lode would indicate the presence 

of sulfides beneath alluvial cover.

An all-African crew was trained to do all but a minor part of the routine work 

of sample collection and analysis.

INTRODUCTION

Even a casual glance at a map of the World's major mineral 

producing areas shows a conspicuously uneven geographic distribution 

of the sources of our various mineral commodities. It may be, however, 

unsafe to assume that significant mineral resources are absent in a 

given region simply because they have not been found there. Caution 

is desirable when the distribution of metal production apparently 

coincides with climatic zones. For example, a study of the statistical 

summaries of metal mining (Minerals Yearbook, 1949) shows that in 

1949, 94 percent of the world's tin and 38 percent of the copper, but 

only 9 percent of the zinc came from mines located in the tropics.

51
52 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

Possibly the climate has had some critical effect on either the formation 

or the discovery of economic deposits of certain metals. Tin may be as 

abundant in temperate latitudes as in the tropics, but may not have 

concentrated there in rich alluvial deposits. It may be equally true 

that lode deposits of zinc sulfides are just as abundant in the tropics 

as elsewhere, but may have escaped discovery because of the greater 

depth of tropical weathering. It is, perhaps, significant that, with 

minor exceptions, the great mines of the tropics are confined to 

deposits of those metals whose primary minerals are either highly 

resistant to weathering, such as gold and cassiterite, or oxidize to 

conspicuous or easily recognized secondary minerals, such as the 

brightly colored copper carbonates. Very few important deposits of 

minerals such as sphalerite that are neither resistant to weathering 

nor conspicuous by their oxidation products are known in the tropics. 

If the full potentialities of the lead and zinc resources of equatorial 

regions are to be realized, better methods of locating deposits of those 

metals must be found.

Recently, new methods of prospecting based on chemical analysis 

of rock, soil, vegetation, and natural water for traces of metals have 

been developed and applied successfully to a variety of mineral exploration problems in the United States and Canada. Such techniques, collectively known as geochemical methods of prospecting, 

may have applications to problems in mineral exploration peculiar 

tb the tropics, if conditions are favorable and the operations are not 

too expensive.

The principal aim of the present investigation was to find out 

whether soil analysis could be used to locate those lead-zinc deposits 

that lie beneath a tropical weathering profile, and whether the cost 

of such a survey, if successful, could be reduced to a point where large 

reconnaissance exploration 010^1113 WOUld be practical.

The Nyeba (Ni-e'bti) lead-zinc district in Ogoja (Og5'jd) Province, southeastern Nigeria (fig. 1) was selected for these experimental

StlldieS because a private mining organization was actively developing the district, and because the Mutual Security AgGHCJ Of 

the United States Government was currently engaged there in a

geological research project which could include w. experimental geochemical prospecting program, The project was financed jointly by

the Mutual Security A.gency, and the Nigerian Geological Survey

Department, The writer served as a member of the staff of the

United States Geological Survey on special loan for M. S. A. WOfK 

in Nigeria.

Field Work was carried out from November 1951 to January 1952.

Mr. G. Jefford, chemist of the Nigerian Geological Suryej Department, collaborated with the writer during the month of January 

1952. R. G. Bogue, geologist of the U. S. Geological Survey in
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 53

^Geology from Provisionol Geological Mop of Nigeria, 1943

EXPLANATION

Upper Cretaceous Lower Cretaceous

FIGURE 1. Geologic Index map of southeastern Nigeria.

charge of the M. S. A. geological program in the area, took care of 

all arrangements for housing, transportation, and the management 

of the crew of African assistants. Harold Bloom of the U. S. Geological Survey assembled and packed the chemical equipment used 

in the .field laboratory. Except as otherwise noted, the chemical analyses quoted in this report were made by H. E. Crowe,' A. P. Marranzino, and J. H. McCarthy, of the U. S. Geological Survey. The technical staff of the Nigerian Lead-Zinc Mining Company, particularly J. H. Courtright and L. M. Wilson, provided advice, services, 

facilities and chemical supplies without which the work would have 

been impossible.

PRINCIPLES OF GEOCHEMICAL, PROSPECTING

Geochemical methods of prospecting for valuable minerals are 

based on the premise that the dispersion of ore metals in the country rock yields a characteristic distribution pattern which can be 

used as a guide in locating hidden ore bodies. Such distribution patterns, often referred to as geochemical "halos" or "anomalies", may 

encompass a very much larger volume or area of surface than the
54 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

deposit of ore itself. Systematic surveying of unexplored ground 

in search of geochemical anomalies may provide a step intermediate 

between geologic reconnaissance and diamond drilling, which under 

some conditions will materially reduce the total cost of exploration.

The principles on which geochemical methods of prospecting are 

based are as old as prospecting itself. Since prehistoric times the systematic panning of stream sediment and residual soil for native gold 

or other resistant minerals has been used to find bedrock lodes. Variations in the color of soils, or peculiarities in the vegetation have led 

prospectors to the discovery of many other important ore deposits. 

The application of chemical methods to the problem, however, is a 

relatively new feature. The first large-scale investigation of geochemical methods was done in the mid-1930's, when Soviet geologists 

developed methods of systematic sampling and spectrographic analysis 

of alluvium and soil (Sergeyev, 1941; Fersman, 1939). About the 

same time, workers in Sweden began large-scale programs of plant 

sampling and spectrochemical analysis for the purpose of mineral 

exploration (Rankama, 1947). Shortly after the last war, experimental work was started in the United States and Canada on a wide 

variety of geochemical methods of prospecting, all supported by chemical methods of trace analysis. (See Hawkes, 1948, 1950 and Harbaugh, 1953 for lists of published references.)

Recently, suitable chemical methods of trace analysis have become 

available, marking the turning point in the development of geochemical methods of mineral exploration. If a prospecting program based 

on chemical analysis of a great number of samples is to be practical, 

the analytical tests must be inexpensive and rapid. It is an added 

advantage, in that the equipment is portable and the procedures are so

simple that the analytical work can be done at the base of field operations, allowing the field man to get analytical data promptly. The 

analytical tests developed by the U. S. Geological Survey, and now in

current use in the United States and Canada (Lakin, Almond and

Ward, 1952) meet most or all of these requirements.

PRIMARY ANOMALIES 

Experience has shown that there are two important types of geochemical anomalies primary and secondary. Primary anomalies inelude all the effects of dispersicm of metals into the rocks surrounding

the mineral deposit by the ore solutions. Several kinds of primary

anomalies have been recognized, in the course of recent -work in the

United States and Canada. In some districts, traces of metals are 

dispersed in the wall rock near the contact of the minable ore (Morris, 

1952). Dispersion patterns of this kind might conveniently be referred to as "wall-rock" halos. Another type of primary halo 

consists of a pattern of hydrothermally dispersed metals in the rock
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 55

overlying deposits that do not crop out at the surface (Lovering, 

Sokoloff and Morris, 1948; Riddell, 1950; Cooper and Huff, 1951). 

Primary halos are basically three dimensional features, even though 

they appear as two dimensional patterns on surface plans.

Little is known about the mode of occurrence of the metals in primary halos. It is generally assumed that primary dispersions of 

metals take the form of intricate systems of very small stringers of 

the common vein-forming minerals. Dispersed mineralization of this 

kind undoubtedly occurs in many districts, but has been overlooked 

because of the inconspicuous character of the introduced minerals. 

Such patterns can, however, be brought out by systematic chemical 

analysis of channel samples of rock or of the derived soil.

SECONDARY ANOMALIES

Secondary anomalies include a wide variety of patterns of metal 

dispersed in surficial material, that have resulted either directly or 

indirectly from the weathering of primary deposits. Metal that has 

been leached or otherwise removed from an oxidizing ore may pass 

through many different phases from the time it first comes under the 

influence of weathering agencies until it ultimately reaches the sea. 

Some metals tend to be held within the crystal lattice of relict primary minerals, such as cassiterite, which may be brought into the 

drainage-ways as heavy alluvial minerals. Other metals may be dissolved, and either reprecipitated locally in gossans and in residual 

soils, or carried away in solution in ground and surface waters. Some 

of the metal in solution may be taken up by plants and trees where 

it is temporarily concentrated in the living vegetable tissue. Erosion 

of metal-rich residual soil and gossan will carry the dispersion pattern 

of the metal into the stream sediments.

Secondary anomalies may be conveniently classified according to the 

material in which the distribution pattern is developed. A residual 

soil anomaly is a geochemical pattern in residual soil that shows a 

higher-than-normal content of metal in the rocks from which the soil 

was derived. Plant anomalies, sometimes called "biogeochemical" 

anomalies, are patterns showing a high-than-normal metal content in 

plant material.

When dealing with secondary dispersion, it is important to bear in 

mind that each element behaves differently in the course of weathering. Some elements, of which zinc is the most conspicuous example, 

tend to be dissolved easily and be readily transported in solution. 

Others, such as lead, are relatively insoluble and tend to remain in the 

residual soil and in the solid fraction of the stream load. Copper is 

intermediate between zinc and lead in its tendency to move in the 

weathering cycle. From the contrasting behavior of different ele293432 55 2
56

ments in weathering has grown the concept of "mobility" of elements; 

we say, for instance, that zinc is a highly mobile element whereas 

lead is relatively immobile. Mobility, however, is strictly a qualitative concept, representing the resultant of all the factors that tend to 

dissolve the element, balanced against the factors that tend to keep it 

insoluble or to remove it from solution. Zinc, although relatively mobile, can apparently be rendered immobile by a variety of processes 

that are not at all well understood. Colloidal or very fine-grained 

material, organic matter, biological processes, and changes in pH may 

each, under certain conditions, be effective in removing zinc from 

solution. '

Under some conditions solution and local redeposition cause a rather 

special type of secondary halo, and high concentrations of metal may 

develop in transported cover directy above suboutcrops of sulfide ore. 

Presumably the metals are dissolved in the course of oxidation of the 

primary deposit, and migrate in solution up into the mantle of transported material where they are reprecipitated. According to Russian 

terminology, geochemical anomalies of this kind are "superimposed" 

halos, so named because the secondary geochemical pattern has been 

superimposed on otherwise barren material (Fersman, 1939, p. 3 of 

translation). In North American work, most of the known examples 

of superimposed halos have been observed in glacially transported 

material.

GEOCHEMICAL THRESHOLD

Geochemical prospecting is a process of finding areas of higher than 

normal metal content that indicate nearby ore deposits. This definition carries a double implication. First, the lowest content of metal 

that is economically significant in the material sampled must be known. 

This limiting value, or "threshold", should be sought by preliminary 

experimental work in the vicinity of known ore bodies. However, 

areas of higher than normal metal content can be identified and interpreted only when the normal or "background" metal content of material in unmineralized areas is known. The metal background depends 

on the content of metal in the various kinds of unmineralized parent 

rock, together with all the factors in weathering that tend to modify 

the content of metal in soil, vegetation, and water.

The effectiveness of a given geochemical method of prospecting

depends to a large extent on the contrast between the background and

the threshold value for the particular metal and the particular host

material in the area under consideration. For example, 1,000 ppm 

(parts per million) may be the local economic threshold for lead in

residual soil 5 in other words, the 1,000 ppm lead contour encloses areas 

deserving detailed exploration. The common range of lead background is from 20 ppm to 50 ppm, with an average near 30 ppm. Here
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 57

the contrast is 1,000 ppm against 30 ppm, or 30 to 1, which is a very 

satisfactory ratio for a geochemical reconnaissance method. For zinc, 

the threshold may be 200 ppm against 50 ppm background, giving a 

contrast of 4 to 1. Here the less satisfactory contrast for zinc may 

be offset by other factors, such as the greater simplicity of the analytical test for zinc and possibly the broader areal extent of the zinc 

anomalies.

CONTAMINATION

Contamination of surficial material as a result of human activity 

presents an ever-present hazard when variations in minute amounts 

of metal may be significant. The most common sources of contamination are old surface workings, where fresh or oxidized ore has been 

thrown out and scattered over the surface. A very small increment 

of oxidized zinc ore may raise the zinc content of residual soil well 

above the economic threshold.

When deposits are opened by either opencast or underground work, 

the primary minerals are subject to vastly accelerated oxidation and 

solution. This may be disastrous to geochemical studies of water, as 

the metal content of ground and surface water in the vicinity of old 

mines is probably tens or even hundreds of times greater than before 

the deposit was developed. Thus the small increment from an undiscovered deposit may be completely masked by the excessive contribution of metal to the drainage water by old prospects.

Stream sediment may become useless as a result of contamination 

from mine dumps and mill tailings. Smelter fumes and wind-blown 

dust from mill tailings have been known to contaminate surface soils 

within two or more miles of the smelter stack. Needless to say, the 

geochemical prospector must be constantly on guard against the 

possibility of contamination.

NEED FOR PRELIMINARY ORIENTATION STUDIES

Not enough geochemical prospecting work has been done to justify 

a forecast in a new area as to whether primary anomalies may be 

expected, which soil horizon will yield the most significant soil samples, what the optimum spacing will be for soil or vegetation samples, 

which types of vegetation will provide the most significant samples, 

whether soil or vegetation will give the best results, whether water 

or sediment sampling will be useful, and what will be the disturbing 

effect of contamination from human activity. It is virtually mandatory that at the beginning of any new field project, experiments be 

carried out to determine the answers to those questions pertinent to 

the local geochemical problem: Once these questions have been 

answered, then and only then can a routine geochemical exploration 

work program be intelligently planned.
58 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

GEOLOGY

The Nyeba district lies near the southern end of the Nigerian leadzinc belt, about 10 miles south of Abakaliki (A'ba.ka'li.ke1 ), and 50 

miles east of Enugu (E.noo'oo) (fig. 1). In some published reports, 

the mines of the Nyeba district have been included as part of the 

Abakaliki district, which comprises a considerably larger area covering the small prospects near Abakaliki as well as the principal deposits centering about the settlement of Nyeba.

GENERAL

The geological setting of the Nigerian lead-zinc belt has been summarized recently by Mackay (1950), Webb and Millman (1950,1951), 

Bogue and Reynolds1 , and Farrington (1952). The following summary is quoted from Webb and Millman (1951) :

(a) Sedimentary rocks. A belt of dominantly sedimentary rocks, probably 

of Lower Cretaceous age, trends northeast for about 400 miles from near Enugu 

in the Eastern Provinces of Nigeria. The stratigraphy and structure of the belt 

are imperfectly known (Tattam, 1943). Shales and siltstones are the principal 

sedimentary rocks, and sandstones are common in the north of the region. Thin 

intercalated limestones and calcareous horizons in the argillaceous rocks are not 

unusual. The sediments are folded into gently pitching anticlines or elongated 

domes, generally along predominantly east-west lines; dips rarely exceed 30°. 

In the Ameka-Ameri (A'mg.kd, A.me/re) area (herein referred to as the Nyeba 

district) NE-SW anticlinical folding constitutes the important structural feature (Tattam, 1930; fig. 4). Cross (ac) joints are well developed and oblique 

shear joints less so.

(b) Igneous rocks. The sediments are intruded by relatively small dykes, 

necks and stocks of syenitic to doleritic igneous rocks (Tattam, 1943, p. 41). 

Volcanic rocks Of more recent age occur in various parts of the Cretaceous belt. 

Occasionally lead-zinc veins cut basic igneous intrusions and at Abakaliki the 

mineralization occurs in volcanic rocks of andesitic type.

MINERAL DEPOSITS

Tke following description of the ore deposits of the Nyeba district 

is quoted from Bogue and Reynolds.3 The location of tke various 

deposits is shown on figure 2.

STRUCTURE

The known ore deposits in me district, those at Abakaliki and in the vicinity 

Of Nyeba, are on the north-south and northwest-southeast fault zones. Movement on these faults has not occurred on a Single fault plane but along a number 

of parallel Closely spaced fractures, forming a fault zone of considerable width. 

The mineralization is not confined to any single fracture in the fault zone. Tne 

deposits are composed of a nxam*>e* Of inaividual ore bodies within the fault zones 

and have an echelon pattern both Vertically and Horizontally.

1 Bogue, R. G., and Reynolds, R. R., 1951. Preliminary report on the lead-zinc deposits of 

the Abakaliki district, Nigeria: U. S. Geol. Survey unpublished report, 19 p. 

8 Bogue, R. G., and Reynolds, R. R., op. cit.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 59

EXPLANATION

Dolo or/ 

fig II

Bose mop compiled from surveys by R.G.Bogue, 

and Nigerian Lead-Zinc Mining Company

5000 0 10,000 Feet 

__I

FIGURE 2. Index map of Nyeba lead-zinc district.

Quartz veins which have been emplaced in the fault zones at Ameka and, to 

a lesser extent at Nyeba, have been resistant to erosion and have given local 

topographic expression to the outcrops. However, where there is no quartz in 

the veins, there is no topographic expression of the orebodies.

The individual ore bodies are apparently lenticular in form. The exact shape 

of most are not known but may extend several hundred feet horizontally and 

vertically and up to 40 feet in width. There is not enough evidence at the 

present time to determine the pitch of the ore lenses, but there is a suggestion 

of a general pitch of the longitudinal axes to the south at about 30° to 50°.

The mineralization is largely confined to the fault zones and there is almost 

no leakage of the ore minerals into the unbrecciated shales on either side of the 

faults. However, small veins of dolomite and siderite occur in the wall rocks. 

The introduced minerals have replaced the brecciated shales within the fault 

zones. There is no evidence of fissure filling in the deposits except marcasite, 

which was introduced late in the mineralization sequence.

Wall rock alteration is not apparent in the shales more than a foot or two 

from the ore bodies. In no place were the shales observed to be intensely altered, 

but small amounts of fine-grained mica, probably hydro-muscovite,. and clay
60 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

have been found. Hydromuscovite, clay and chlorite are often observed in the 

brecciated shales in the fault zones, especially in the vicinity of the mineralization. There has been some silicification of the wall rocks; however, it is not 

widespread and is erratically distributed throughout the deposits. It is believed 

that this silicification was caused by quartz which was introduced late in the 

sequence of mineralization and caused some silicification of the wall rock and 

formed veins in the surrounding area. These quartz veins are widespread 

throughout the district and usually occupy northwest-southeast fractures, 

though a small percentage are in fractures of random orientation.

MINERALOGY

The mineralogy of the ore deposits is relatively simple and consists of but few 

primary minerals. The minerals arranged in the order of their paragenesis 

are as follows:

Dolomite. Cream or pale pink in colour and occurs in small veins throughout, 

and adjacent to, the mineralized area. It is most abundant in the brecciated 

shale of the fault zones and small veinlets are often present in the wall rock.

Quartz. This is an early generation of quartz, which was introduced during 

and shortly after the dolomite. It is usually dense, finely crystalline, milky 

white, and occurs usually in small veins but in places has been found in veins 

several feet in width. There is evidence that some of this quartz may have been 

introduced about the same time as the siderite. However, in most places it is 

definitely later than the siderite.

Siderite. Usually light brown or grayish buff in colour on freshly broken 

surfaces but changes to a reddish brown when exposed. Most of the siderite is 

crystalline with crystal faces up to several inches in length, but some is dense 

and has a granular appearance. There is a late generation of siderite which is* 

usually a darker shade of brown.

Chalcopyrite. Widely distributed throughout the deposits in minor quantities. 

It usually occurs as small rounded grains in the siderite but occasional masses 

several inches in diameter have been found.

Sphalerite, The colour varies from light to dark brown and is generally well 

crystallized. Crystal faces up to 6 inches in diameter are not uncommon, but 

well-formed, euhedral crystals are quite rare.

Galena. There are two types of galena present, the early, which has a cubic 

form and a very minor amount of the late which has an octahedral crystal form. 

The early galena is usually well crystallized but occasional masses of .fine textured, granular galena are found. Late quartz, calcite, and barlte were introduced after the cubic galena. These minerals are usually in small veinlets and

erratically distributed throughout the deposits. Small amounts of late chalcopyrite associated with late siderite and some pyrite have been found. There

is not enough evidence to determine the exact sequence of the introduction of

these minerals except that it is known that all are later than the first galena 

and earlier than the marcasite.

Marcasite. This mineral is locally abundant in certain of the Ore lengeg at

Ameka and in lesser amounts in the other deposits. It usually is in botryoidal 

crusts but sometimes is found in masses several feet thick.

Galena. Small crystals of octahedral galena, have been found coating- the

marcasite. This type is probably the result of solution and redeposition of the

earlier galena.

There are a number of secondary minerals in the oxidized portions of the 

ore outcrops. The outcrops were mined down to the water table at Ameka and 

Nyeba about 20 years ago so that most of the secondary minerals are found
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 61

on the old dumps although occasionally they may be seen in place. The secondary minerals are listed below:

Limonite and related iron oxides

Cerussite

Calamine

Smithsonite .

Clay mostly kaolinite, some allophane( ?)

Chalcedony

Malachite

Azurite

Pyromorphite

Anglesite

Calcite

SURFICIAL FEATURES

In southern Nigeria, as in most tropical areas, geologic mapping 

and mineral exploration are severely handicapped by lack of natural 

exposures of the unweathered bedrock. Conclusions as to lithology, 

structure, and primary mineralization must be made by piecing 

together the fragmentary evidence obtained from studies either of 

weathered rock or of residual soil. Therefore, the effectiveness of 

geologic work depends in no small measure on the competence of the 

observer to interpret the effects of the many natural and human 

agencies that have contributed their share to the surficial history of the 

region.

In view of the importance of surficial factors in mineral exploration 

in the tropics, it is regrettable that their effects are so little understood, either generally or locally. The following remarks, therefore, 

must be kept largely on an empirical and descriptive level. What 

tentative conclusions are drawn are based on very incomplete evidence.

LAND FORMS

The topography in the general vicinity of the Nyeba district is 

nearly level or only slightly undulating. The general elevation is 

slightly less than 300 feet above sea level, although scattered hills of 

relatively resistant rocks stand a hundred feet or more above the level 

of the plain. Near the watersheds, the stream drainage follows broad 

swales in the gently rolling surface. Larger streams and rivers flow 

in sharply incised channels as much as 30 feet deep cutting the 

original, almost featureless surface. The present active downcutting 

of the larger streams undoubtedly results from a relatively recent 

regional uplift.3 Perennial springs are common throughout the 

district. Many of these springs have been enlarged and excavated 

by the natives as reservoirs or "juju (joo'joo) pools" to provide a source 

of water throughout the dry season.

3 Bogue, R. G., and Reynolds, R. R., op. cit.
62 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

CLIMATE

The average annual rainfall in the Nyeba district is 60 to 80 inches. 

Virtually all falls during the rainy season, which lasts from May to 

October. Much of the precipitation conies in torrential showers, with 

resulting high runoff and erosive action. From November through 

February the tropical continental air from the Sahara desert periodically moves southward, displaces the tropical maritime air of the 

Gulf of Guinea, and brings with it high winds, dust, and extremely 

arid conditions. The dry winds of the winter months are locally 

known as the "harmattan" (har'ma-tan'). During the harmattan 

period, the exposed soil dries out, and water disappears from all the 

smaller springs and drainage channels. Water remains throughout 

the year in many of the juju pools, although at the end of the dry season it may consist only of a stagnant and putrid residue. The temperature is uniformly warm except during the harmattan period, when 

night temperatures may fall as low as 50° F. Freezing weather is unknown in southern Nigeria.

WEATHERING

Two sharply contrasting weathering profiles were observed in the 

Nyeba district. One type, occurring in relatively well-drained areas, 

is characterized by the accumulation of oxides of iron, a process commonly referred to as "lateritic" weathering. The other type is confined to poorly drained and swampy areas and leads to the formation 

of a heavy clay.

WELIXDRAINED AREAS

The Weathering profile in the relatively well-drained areas of the 

Nyeba district is characterized by a tendency for silica to be leached 

and for.iron to be relatively enriched (see table 5). The content of 

organic matter is apparently very low, except for surface litter and 

liVG rOOtletS, Following is the typical profile, reading downward from 

the surface:

a. Surf ace organic litter.

b. Fragmental zone. This is a zone, rarely more than 18 inches 

thick, consisting of fragments Of weathered Shale mixed with varying 

proportions of bead-sized nodules or pellets of aluminous and ferrUginOUS material. This zone constitutes the "soil" that is periodically 

turned over and mixed during the course of native cultivation. TllllS 

it is in reality hardly a part of the natural weathering profile, as much

Of the original layer undoubtedly has been removed by erOSlOll, aild

also has been admixed with fragments broken off from the underlying

weathering shale or laterite. Virtually all the drained terrain in tlie 

district is now being or has been subjected to this treatment, and no 

natural, undisturbed profile could be seen. In its present state, the
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 63

fragniental zone is pervious and notably low in its content of claysized particles.

c. Laterite. In perhaps 10 to 20 percent of the drained parts of the 

district, the fragmental zone is underlain by a two- to four-foot layer 

of consolidated buff to red-brown material. When freshly exposed, 

this material can be chopped and shaped, much like stiff clay. Upon 

exposure to the air, it becomes hard and stony. It typically contains 

through-going, usually subvertical pencil-sized channels, which resemble worm-holes. These channels may be either empty or filled with 

earthy, claylike material. Many of the channels are now occupied by 

live rootlets. The term "laterite" has been locally applied to this 

zone, but because of the confusion regarding the exact definition of 

a laterite (see du Preez, 1948), any such classification should be applied with caution.

d. Weathered bedrock. The fragmental zone, or the laterite if present, is directly underlain by weathered bedrock, either shale or siltstone. The boundary is commonly very sharp. The weathered bedrock is essentially in place, except for a certain amount of slumping 

due to leaching or slope creep. It is commonly buff colored, and relatively soft and crumbly. Through a zone extending from 5 to perhaps 

50 feet below the surface of bedrock, the buff coloration grades irregularly into gray, and the consistency becomes harder and more compact.

POORLY DRAINED AREAS

During the rainy months, areas where the topographic gradient is 

sufficiently low are covered by standing water for long periods of time. 

Such areas occur in many of the broad-upland stream channels, and 

in level areas both near watersheds and in the larger valley bottoms. 

During the dry season, the standing water drains away, but the surface soil immediately below the vegetative cover remains moist. In 

such areas, the weathering processes apparently favor formation of 

clay rather than of iron and aluminum oxides. The poorly drained 

areas of the district are in general not arable and undisturbed weathering profiles remain that could be studied. The boundaries between 

well-drained areas of lateritic weathering and poorly drained areas 

where weathering leads to the formation of clay are relatively sharp, 

with a transitional zone of rarely more than a few tens of feet. Following is a typical profile, as observed in a trench 11 feet deep and 

100 feet long excavated in a swamp area near the Crocodile prospect

(fig- 2). 

a. Organic zone. As much as 12 inches. The roots of the swamp

flora, mainly grasses and reeds, are embedded in a dark-gray organic 

zone.

293432 55 3
64 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

b. Gray clay zone. 12 to 36 inches. The organic zone grades downward with decreasing amounts of organic matter into the underlying 

clay zone. The color grades downward from dark gray to light blue 

gray.

c. Mottled clay zone, below 36 inches. The zone below the gray 

clay is a mottled brick-red and blue-gray clay. The proportion of red 

coloration to gray increases somewhat with depth. Where live rootlets 

penetrate the mottled zone, they are surrounded by 2-inch zones of 

even-hued blue-gray clay, which suggests that in the vicinity of the 

rootlets the iron oxide of any red clay present had been reduced. Rootlets were observed at depths as much as 11 feet below the surface. 

The clay, although moist throughout, is very impervious to moving 

water. In a profile of residual clay exposed on the walls of a deeply 

incised modern drainage channel near the Crocodile prospect, the indistinct traces of the original bedding of the parent rock could be 

seen.

d. Weathered bedrock. The mottled clay . grades downward 

through a transitional zone of a foot or two into weathered bedrock, 

where the original sedimentary structures of the shale or siltstone are 

easily seen. Bedrock was found at a depth of 22 feet in a shaft in 

the bottom of the trench. In poorly drained terrains, weathering 

processes acting on parent materials of different origins lead to end 

products that are extremely difficult to distinguish. Thus from a 

cursory examination of near-surface material, it is virtually impossible to tell whether the parent material was indigenous shale bedrock 

or a deposit of transported alluvium. This problem will be discussed 

later in connection with the experiments in the Ameri swamp.

NATURAL VEGETATION 

Webb and Millman (1951) state that the Nyeba district is

well within the northern climatic limits of the forest zone, but owing to degradation by native agriculture and bush fires the vegetation is now mostly derived 

savannah, the main forest boundary lying a few miles to the south.

According to the most reliable local information, the original Jporest 

was cleared several hundred years ago by natives who were driven 

out of their original farmlands in the north by hostile tribes. At the

present time, remnants of the original hardwood forest may be seen

only in scattered patches, principally along the larger streams and 

in the bottomlands Which were unfavorable for agriculture.

AGRICULTURE AITO SOIL EROSION

practised by the local natives is a yam

culture. Yams are root vegetables that will rot before maturing if 

the enclosing soil is saturated with water over any substantial part 

of the growing season. It has become the local practice, therefore,
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 65

to plant the yams in conical hills of soil commonly 2 to 3 feet high. 

It is difficult to imagine an agricultural practice that would do more 

to promote soil erosion than the building of conical piles of this 

kind. Not only is fresh soil periodically exposed to the torrential 

tropical rains, but unobstructed channelways are opened up for the 

removal of eroded material by the runoff water. As a result, any true 

soil in the agricultural sense has long since been removed, and the 

fragmental surf ace material remaining is hardly more than the coarest fraction of the original soil that was not as readily washed away, 

plus fragments of weathered shale dislodged from the immediately 

underlying bedrock. It is conceivable that most of the soil erosion 

took place as an almost catastrophic event immediately after the clearing of the forest, some four hundred years ago. The thickness of 

material that has been eroded is probably more than 5 feet. Further 

field investigations aimed specifically at unraveling the recent history 

of the Nyeba district would undoubtedly provide answers for many 

of these questions.

The lack of a true soil, and the present negligible thickness of the 

layer of fragmental material overlying weathered bedrock, are of 

extreme importance in mineral exploration. Diagnostic fragments 

derived from the oxidation zone of sulfide lodes are broken off and 

mixed with the surface fragmental horizon in the course of building 

yam heaps. Native prospectors, sent out by the different previous 

European managements of the mines, have systematically searched 

the cultivated fields for fragments of vein quartz, limonitic gossan, 

and "lead float" or crystals of galena coated with gray lead sulfate. 

It is very unlikely that any large lode deposit in the well-draine'd 

areas of the district is undiscovered. This conclusion applies, however, only to the well-drained terrain, where the overburden is shallow ; prospecting the clay-covered swamps is quite another problem. 

In the poorly drained areas covered by clay where no surface expression of lode sulfides could be expected, ore bodies of substantial size 

could easily have been overlooked.

NATIVE MINING

. Although apparently no written records or native folklore survive 

of any iron smelting activity in southern Nigeria, iron making must 

have been an active industry there at one time in the past. This is 

attested by the widespread distribution of small fragments of slag in 

the surface material, and by large mounds consisting of many tons 

of slag in areas twenty to forty miles north of the Nyeba district. 

The remaining evidence for this activity is extremely fragmentary, 

although careful studies would almost certainly provide the answers. 

Samples of slag from widely separated localities, both within the
66 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

Nyeba district and elsewhere, were collected by the writer and analyzed for lead and zinc, with the following results:

TABLE 1. Lead and zinc content of slag samples

Locality

Do.... 

Do. 

14 miles north of Abakaliki ______ . __ . ____ . _____ . ____ . _ .

Parts per million

Lead

300 

4,000 

75 

130 '50 

25 

400 

50 

25

Zinc

160 

600 

300 

200 

100 

100 

350 

100 

100

Guillemaih (1910) reports that in the Cameroons, several hundred 

miles east of the Nyeba district, the natives used iron-rich laterite for 

iron ore. However, the high lead and zinc content of some of the slag 

samples reported in table 1 suggest that in part, at least, the raw material for the iron industry within 30 miles of Nyeba was metal-rich 

limonitic gossan.

The native tribes of southern Nigeria have also mined many of the 

oxidized outcrops of the lead-zinc lodes for crystals of galena, which 

were crushed and used for cosmetics. Only the larger cubic crystals 

were saved, as the fine-grained galena, for some reason, had no commercial value in the native markets. This activity has continued up 

to relatively recent times. T. G. H. Cotsworth, senior inspector of 

mines at Enugu, has learned from discussion with local natives that 

it was common practice for members of each tribe to forage outside 

of their own tribal boundaries for oxidized ore containing relict galena 

crystals. Blocks of unsorted gossan were gathered wherever they 

could be found and brought in baskets as much as 6 miles back to 

the home village for sorting. Spillage in transit and undesirable 

galena discarded at the sorting pile could leave a pattern of galena

float that would be most distracting to the exploration geologist.

A problem of this kind apparently was encountered at the Crocodile

prospect in the Nyeba district. One of the exploration trenches in

this area uncovered remains of a pit about three feet deep, nOW filled

With rubble Consisting mainly of somewhat discolored fragments of 

weathered shale together with a few fragments of charcoal and pottery.

A composited sample of the surface fragmental material collected

within 30 feet of this pit contained 1,000 ppm lead. The average lead 

content of nine soil samples taken at 50-foot intervals along a traverse 

across the area with 200 ppm, or about four times the lead content of 

normal soil. Within the same area, African scouts employed by the 

mining company had found scattered small fragments of galena in the
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 67

surface material. Thorough trenching of the area failed to find any 

evidence whatever of mineralization. The pit may indicate the existence here of a native village at some time in the past, where, conceivably, the inhabitants sorted gossan brought from the Nyeba deposit 

one mile to the west.

A similar problem may exist at the Ameka West prospect, also in 

the Nyeba district, where abundant fragments of galena had been 

found at the surface, but where extensive trenching also failed to 

find any bedrock mineralization.

For the purposes of the modern prospector, the significant conclusions to be drawn from these observations are that the presence of 

lead float, and geochemical soil anomalies in lead are not necessarily 

indicative of bedrock sulfides in the immediate vicinity.

PREVIOUS GEOCHEMICAL PROSPECTING WORK

The first geochemical prospecting work in the Abakaliki district was 

carried out by R. O. Roberts (1953) in 1948 and 1949 in conjunction 

with an experimental geophysical survey. Roberts made determinations of lead and zinc on a total of 102 samples of soil collected at 

intervals varying from 20 to 500 feet along three transverse lines. 

Although the resulting data, particularly for the distribution of lead, 

showed considerable promise, the project was discontinued before any 

very definite conclusions could be drawn.

At about the same time, J. S. Webb and A. P. Millman of the Royal 

School of Mines conducted experimental geochemical water and vegetation surveys in the Nyeba district. This work has been described 

in full in their published reports (Webb and Millman, 1950,1951), and 

will be reviewed and discussed later in connection with the results of 

the present investigation.

R. J. Lacy and L. M. Wilson of the geophysical department of the 

American Smelting & Refining Co., one of the parent companies of 

the Nigerian Lead-Zinc Mining Co., set up a geochemical prospecting 

laboratory at their field headquarters in Nyeba. They assembled the 

necessary reagents and equipment, and did some preliminary soil and 

water surveys. Officials of the mining company kindly made available 

for the writer's use the supplies and equipment from this laboratory.

ANALYTICAL PROCEDURES

For maximum economy and general effectiveness, the analytical 

methods used in geochemical prospecting work should be not only 

sensitive, but also both rapid and simple. Because of the usual wide 

range of metal content encountered in most problems in geochemical 

prospecting, high accuracy is not required. To meet this need the 

Geological Survey has developed a series of rapid and simple colorimetric tests suited to the practical requirements of geochemical pros-
68

pecting work. Tests for lead and zinc in soils were evolved from 

the standard colorimetric methods (Sandell, 1950) by elimination of 

many of the precautionary but tedious steps needed for highly 

accurate work.

The procedures used for the Nigerian project embody some modifications over those previously described in the literature (Lakin, 

Stevens and Almond, 1949; Almond and Morris, 1951; Lakin, 

Almond and Ward, 1952). The following pages give the analytical 

procedures as they were actually used on the samples from Nigeria 

and are essentially the same as those subsequently described by Bloom 

and Crowe.6

A selected group of samples were analyzed in the Denver laboratory 

for copper, cobalt, nickel, silver, arsenic, and antimony in addition to 

lead and zinc. The cobalt and arsenic procedures used for this work 

have been described by Almond (1953),4 the antimony procedure by 

Ward and Lakin,5 and the others by Lakin, Almond, and Ward 

(1952).

EQUIPMENT FOB FIELD LABOBATOBY

In the early stages of any experimental geochemical prospecting 

project, the analytical data should be available within a day or two 

after the samples are collected, to assist in the daily decisions needed 

in feeling out the most effective geochemical approach to the local 

exploration problem. Once the general approach has been, selected 

on the basis of such preliminary experimental work, then fairly large 

sampling programs may be laid out without the necessity for prompt 

analytical reports.

Reagents shipped by air to the project area included one bottle 

thymol blue solution, 150 g purified potassium cyanide, 300 g ammonium citrate, 200 g sodium thiosulfate, 400 g sodium acetate, 113 

g hydroxylamine hydrochloride, and 24 vials each containing 0.01 g

dithlZOne. Inasmuch as international air freight regulations prohibit the shipment of any corrosive liquids, itwas necessary io D01TOW 

nitric add, aCetiC acid, ammonium hydroxide, and carbon tetrachlori.de from the local laboratory at NyeDR.

« Also, in the same year, Almond, Hy, 1953, A field method for the determination flf 

arsenic to soils, in U. S. Geological Survey open-file report, No. 2O8, Additional field methods used In geochemical methods usedi in geochemical prospecting by the u. s. Geological

Survey, p. 8 12.

6 Ward, F. N.| find Lafcln, H. W., ISOS, Determination of microgram quantities of antimony in sons and rocks in u. g. Geological Survey open-file report, No., 20&, Additional

field methods used In seochemlcal prospectlns by the U. S. Geological Survey p 3-7 6 Bloom Harold, and Crowe, H. E., 1953, Determination Of readny sombie copper,' zinc,

and lead In soils and rocks : nitric acid digestion, in D. S. Gfifll. SllTO OPfifl-file TCpOrt, NO. 

208, Additional field methods used In geochemical prospecting by the U. S. Geological 

Survey, p. 16^24.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 69

The.analytical field kit included the following apparatus:

1 Stainless-steel sieve, 80-mesh

1 Stainless-steel spatula

1 Camel's-hair brush

1 li^-inch paintbrush

1 Lucite spoon, consisting of a lucite bar drilled with cavity sufficient to hold 0.1 g of soil

100 Pyrex culture tubes, 16 x 150 mm, marked at 10 ml

2 Glass-marking pencils

50 Boiling-tubes, for prevention of bumping during digestion of 

sample. These are 9-inch sections of glass tubing, inside diameter 

2 mm, on which the bore has been sealed off about one inch from bottom. This may be done by softening the glass over a flame, and 

crimping with a strong pair of crucible tongs.

2 Steel fusion racks for test tubes

2 Coleman petrol stoves

2 Test tube racks for 40 test tubes each

2 Test tube brushes

2 Test tube clamps

1 100-ml Glass-stoppered pyrex graduate

2 50-ml Glass-stoppered pyrex graduates

6 25-ml Glass-stoppered pyrex graduates

3 125-ml Pyrex separatory-funnels

1 Separatory-funnel rack

10 Culture tubes with plastic screw caps for standards

2 Culture tube racks for comparison of standards

3 500-ml Pyrex reagent-bottles

1 Polyethylene wash-bottle

6 Volumetric pipettes: 2, 2-ml; '2, 3-ml; 2, 5-ml

10 Graduated pipettes: 2,0.5-ml; 3,1-ml; 2,5-ml; 3,10-ml

1 Porcelain pipette-holder

4 Dropping-bottles

2 Sets of pH papers: 1-7 and 7-14

2 Demineralizer cartridges

Miscellaneous: stirring-rods, litmus paper, gummed labels, files, 

ear syringes, corks, stopcock grease, paper tissue, towels.

The following equipment needed for the field laboratory was borrowed from the mine laboratory at Nyeba: 

1 Pyrex still, for distillation of carbon tetrachloride. 

1 Analytical balance for weighing reagents. This is not necessary

if adequate quantities of prepared regents in previously weighed

batches are prepared in the supporting laboratory. 

1 Dispenser, for dithizone solution, consisting of a 1-pint thermos jug

fitted with a two-hole rubber stopper, an atomizer bulb, and glass

nipple and tubing. This device, developed by L. M. Wilson of the
70 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

American Smelting and Kefining Co., keeps the solution both dark 

and cool and provides an easily controlled discharge. Direct pipetting of dithizone solution should be avoided because of the toxic 

properties of carbon tetrachloride. 

1 Kerosene refrigerator for storing dithizone stock solution.

DETERMINATION OF LEAD AND ZINC IN SOILS

The following rapid procedures for estimating traces of lead and 

zinc in soils were used both in the field and in the laboratory of the 

U. S. Geological Survey. For a full discussion of colorimetric methods of analysis, the reader is referred to the publications of Lakin, 

Almond, and Ward (1952) and Sandell (1950).

PREPARATION OF REAGENTS

Water. Pass distilled water from a standard laboratory still 

through a resin demineralizer. All water used for the following 

reagent solutions were prepared in this way.

Nitric acid, 1:3. Dilute one part analytical-grade nitric acid with 

three parts water; the nitric acid borrowed from the mine laboratory 

was tested and found to be essentially free of lead and zinc.

Ammonium citrate, 10%. Dissolve 100 g commercial ammonium 

citrate in 900 ml water; if contaminated, this solution may be scrubbed 

by shaking with dithizone solution. To scrub, add approximately 15 

ml of 0.1 per cent dithizone solution to 1 liter of the ammonium 

citrate solution, and shake for about 1 minute. If the final color of the 

dithizone solution is not green, separate the CCU layer, and repeat 

until the final dithizone product is green. Then remove dithizone 

from the aqueous phase by shaking for a half minute with successive 15 ml portions of CC14 until the final CC14 solution is colorless.

Ammonium hydroxide, IN. Dilute 70 ml concentrated ammonia 

to 1 liter. Commercial ammonia, although it may be seriously contaminated with zinc, is commonly sufficiently free of lead to be suitable for use in the lead procedure,

Potassium cyanide. For 30 percent solution, dissolve 300 g metalfree KCN in TOO ml water. For 0.1 percent solution, dilute one part

30 percent solution with 300 parts water,

Thymol blue indicator, 0.4 percent. Dissolve 0.04 g of the sodium

salt in 100 ml of water.

Buffer for zinc. Dissolve 300 g sodium acetate (NaCaHaOs.SHzO)

and 125 g sodium thiosulfate (Na2S2Oa.5ILO) in water; add 60 ml of

glacial acetic acid; dilute to 2 liters; if contaminated, this solution

may be scrubbed by shaking with dithizone solution.

Dithizone solution. For 0.01 percent stock solution, dissolve 0.01 g 

in 100 ml carbon tetrachloride, For 0.001 percent working solution, 

dissolve 1 part stock solution in 9 parts carbon tetrachloride.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 71 

SAMPLE DIGESTION

Measure 0.1 g minus 80-mesh soil sample in lucite spoon and place 

in dry test tube. Add 2 ml nitric acid. Insert boiling-tube and boil 

for one-half hour, adding water from time to time to maintain level of 

liquid. Remove boiling-tube, dilute to 10 ml with water, mix, and 

allow to settle.

ESTIMATION OF LEAD

With pipette, transfer aliquot of solution from test tube to separatory funnel containing 4 ml ammonium citrate solution. Add 2 

drops of thymol blue and sufficient NH4OH to turn the solution distinctly yellow. Add about 0.1 g. of hydroxylamine hydrochloride if 

difficulties with oxidation of dithizone are anticipated. Add 10 percent KCN solution until thymol blue indicator turns tbward blue. 

Add 5 ml of 0.001 percent dithizone solution and shake gently for 

10 seconds. Drain CCU layer into cylinder containing 10 ml of 0.1 

percent KCN solution and shake. Compare color of CC14 layer with 

1-, 2-, and 3-microgram lead standards similarly prepared.

ESTIMATION OF ZINC

With pipette, transfer aliquot of solution from test tube to a 25 ml. 

stoppered graduate containing 8 ml. of zinc buffer solution. Add 5 

ml of 0.001 percent dithizone solution and shake vigorously for 30 

seconds. Compare color of CC1* layer with 1-, 2-, 3-, and 4-microgram 

zinc standards similarly prepared.

DIFFICULTIES AND PRECAUTIONS

Some of the reagents, particularly carbon tetrachloride and potassium cyanide, are toxic and should be handled with care. No worker 

should attempt to use these reagents without becoming thoroughly 

familiar with the dangers involved and the precautions necessary in 

their use by reference to any competent work on laboratory toxicology, 

as for example Elkins (1950).

Dithizone in carbon tetrachloride solution is easily oxidized and 

rendered useless by light, heat, and excesses of ferric iron in the aqueous phase. The oxidation product is yellow, and its presence when 

mixed with unaltered-green dithizone can be determined by observing 

the yellow-green or dirty-green hue of the resulting solution. The 

effects of light arid heat can be minimized by the use of the thermos 

dispenser described above. The effect of oxidation by ferric iron can 

under favorable conditions be offset by adding hydroxylamine hydrochloride, which acts as a reducing agent.

The most serious analytical difficulty encountered in the course of 

the field program in Nigeria was with tne lead test, where reproducible 

results could not be obtained with either standard or sample solutions. 

At the beginning of the project reasonably good repeat analyses

293432 55 4 v;j
72 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

could be obtained on samples previously run. The reproducibility 

became progressively worse until, in the latter part of the project, 

analysis for lead was abandoned because of inability to get consistent 

results. Later investigation showed that the difficulty apparently 

arose from cumulative impurities in the carbon tetrachloride after 

repeated redistillation. It was concluded that the impurity responsible for the difficulty was probably phosgene, which is known to be 

generated by distillation of impure carbon tetrachloride. Phosgene 

has the combined effect of acidifying the solution and destroying the 

lead-dithizone complex, and its presence could adequately account 

for all the difficulties encountered. Carbon tetrachloride contaminated with phosgene may be made suitable for use by a preliminary 

shaking with a strong aqueous solution of ammonia, or by distillation 

in the presence of CaO.

CHARACTERISTICS OF TESTS

During the progress of the field work, 735 determinations for zinc 

and 698 determinations for lead were made in the field laboratory 

on 685 samples. Subequently, 533 of these samples and 485 additional 

samples were analyzed for lead and zinc in the laboratory in Denver, 

by the same procedure as was used in the field. Also 150 replicate 

determinations by the rapid procedures and 15 analyses by quantitative colorimetric methods were run in Denver. These data are the 

basis for the following appraisal of the characteristics of the rapid 

tests as used in Nigeria.

PRODUCTIVITY

Two African laboratory assistants were trained to do the analytical 

procedure through the digestion of samples. This work included 

measuring the sample into the test tube with a scoop, labeling the

test tube, pipetting nitric acid into the tube, and boiling over the 

petrol burner for half an hour. The African help required some supervision for this, as they had difficulty in manipulating some of the 

equipment, particularly the petrol stove. Under ordinary operating

conditions, these two men could process one hundred samples in a sixhour working day.

In 18 days spent on laboratory work at the field headquarters, a 

total of 1,433 determinations were made on 685 samples. This is 

equivalent to an over-all productivity of about 80 determinations per

day, and about 35 samples per day, for the crew of one European with

two African assitants.

REPBODUCIBILITY

The data from the replicate determinations of zinc on 504 samples,

first in the field laboratory in Nigeria, and then in the Denver laboratory, are summarized in table 2. Bulk comparison of the two sets 

of data showed an unexplained systematic error equivalent to about a
' INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 73.

20-ppm deficiency in the field determinations. To make the data more 

nearly comparable, therefore, 20 ppm has been added to the field data 

before tabulation in column 2 of table 2. Furthermore, the figures 

obtained in the field have been grouped for the sake of simplifying 

the computations. By way of explanation, the fourth horizontal line 

of table 2 is computed on the basis of 32 samples (column 1), all 

of which gave 100 or 110 ppm Zn according to the original determination in the field laboratory; to this is added 20 ppm to compensate 

for systematic error, giving values that may be grouped as 125 ppm 

(column 2); repeat determinations on each of these 32 samples in the 

Denver laboratory averaged 123 ppm Zn (column 3); 9 of these 

replicates, or 36 percent (column 4) were reported 125 ppm Zn; 27, 

or 84 percent (column 5) were reported between 83 ppm and 187 ppm; 

and 31, or 97 percent (column 6) were reported between 62 ppm and 

250 ppm. In summary, the data of table 2 show that, with allowance 

for systematic error, 4 out of 5 of the pairs of replicate determinations 

differ by a factor of 1.5 or less, and that better than 9 out of 10 differ 

by a factor of 2.0 or less. Because of the difficulties encountered in 

the field with the lead procedure, and the resulting lack of reliability 

of the data, a similar study of lead determinations made in the field 

would be of no significance.

TABLE 2. Comparison of replicate rapid determination of zino m Denver 

Laboratory with original determinations in field laboratory

Number of samples

1

44-. 

16..... .............................. ...

7. ....................................... ..

32-. .................................

67. ..................................... ...

74.. .......................... .............

226. 

38.- - . -.

Original 

determination 

In field i 

(ppm)

2

>250

200 

150 

125 

100 

75 

50 

25

Mean of 

repeat 

determinations 

In laboratory (ppm)

3

187 

103 

123 

93 

72 

56 

41

Percent of repeat determinationsIdentical 

to original 

determination

4

55 

25 

28 

36 

40 

31 

55 

61

Within 

+50 pet 

and -33 

pet of 

original 

determination

5

75 

100 

71 

84 

81 

92 

74 

61

Within 

+100 pet 

and 50 

pet of 

original 

determination

6

100 

100 

71 

97 

99 

 92 

99 

82

i Figures In this column include 20 ppm added to original determinations to compensate for an apparent 

systematic error. For simplicity, the values have been grouped; thus 50 ppm includes original determinations of 40,50, and 60 ppm.

Sets each consisting of ten samples for which the original determinations were the same were selected from the total of 1018 samples 

analyzed for lead and zinc in the Denver laboratory. Table 3, column 

2, gives the original determinations for lead and zinc. Each one of 

these samples was reranalyzed by the same procedure. Columns 3 to 6
74 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

of table 3 are similar to the same columns in table 2. The conclusions 

to be drawn from these data are essentially the same as those enumerated for table 2.

ACCURACY

Column 7 of table 3 gives the results of quantitative colorimetric 

determinations on composite samples representing each set. These 

values are probably accurate to within ± 10 percent, and constitute a 

standard reference with which to compare the rapid determinations. 

It may be observed that the correspondence between rapid and quantitative determinations of lead is fairly good. The rapid determinations 

of zinc are substantially lower than the quantitative values, possibly 

as the result of incomplete extraction of zinc by the relatively weak 

half-hour digestion in nitric acid called for by the rapid procedure. It 

is difficult to explain the discrepancies in quantitative values between 

the sets containing high lead and low lead where the rapid zinc determinations were the same; it is likely that some lead is being reported as 

zinc in these samples.

TABLE 3. Comparison of replicate rapid determination of lead and zinc in

Denver laboratory

Number of samples

1

Original 

rapid 

determinations 

(ppm)

2

Mean of 

repeat 

determinations 

(ppm)

3

Number of repeat determinations 

Identical 

with original determination

4

Within 

+50 pet 

and 33 

pet of 

original 

determination

5

Within 

+100 pet 

and 50 

pet of 

original 

determination

6

Quantitative 

determinations of 

composite 

(ppm)

7

Lead

10 . .........................

10.. .-... ................

10....... .-. ... ......

10....... ............ ..........

10. ...

10.. .......... ........... 

10 

10 ...... .. .. .. -

5,000

2,000

1,000

500

200

100

50

3,600

1,950

715

370

190

109

52

1

5

3

1

6

4

5

9

5

9

5

4

8

6

5

10

9

10

9

10

10

10

10

8,000

4,700

2,125

900

430

220

98

45

10....... ......................

10...... ................

10.............. ............

10 

10...... ...... .................

200

1100

150

JSO

149

91

61

54

i

1

2

3

5

8

8

4

6

7

9

8

9

10

10

660

300

200

160

70

1 Lead by original rapid determination ?500 ppm. 

s Lead by original rapid determination 5100 ppm.

SHIPMENT OF SAMPLES

At the end of the field project, all the soil and rock samples were 

shipped to the Geological Survey laboratory in Denver, Colo. All
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 75

the data used in this report are from the subsequent analytical work 

by this laboratory.

Restrictions on international shipments of materials from tropical 

countries posed serious problems at the start of the investigations in 

Nigeria and should be studied carefully whenever geochemical work 

is contemplated in the tropics. Where soil or plant samples are imported from tropical countries into the United States, special prior 

.arrangements must be made with the Bureau of Entomology and 

Plant Quarantine of the U. S. Department of Agriculture in Washington, D. C., or the shipment is liable for confiscation and destruction. 

For example, when the soils were imported from Nigeria, an arrangement was made whereby the shipment was entered without inspection 

 on the condition that all the samples would be sterilized at 105° C 

for one hour before they were discarded. No similar regulations apply 

to imports into the United Kingdom.

DISTRIBUTION OF LEAD AND ZINC

A large part of the geochemical prospecting work in the Nyeba 

district was devoted to fairly detailed studies of the distribution of 

lead and zinc in surficial materials, both in mineralized and in unmineralized localities. Most of this experimental work was in the 

vicinity of the Palm Wine, Ameri and Ameka lodes, and the Crocodile 

prospect (fig. 2).

The data from each of these localities has contributed to the solution 

of several problems in the distribution of lead and zinc. Therefore, 

in the following discussion, the results of the experiments will be 

arranged according to the various topics under consideration rather 

than according to the localities where the work was conducted.

The Ameka and Nyeba lodes are characterized by strongly silicified 

wall rocks that are relatively resistant to erosion. As a result, these 

lodes crop out along low ridge tops, where the limonitic oxidation 

products of the sulfide minerals have been preserved by the protecting 

sheath of silicified shale. Work by J. H. Courtright, resident geologist 

at the mines, showed that it was possible to identify the primary minerals of these lodes, particularly marcasite, siderite, and sphalerite, by 

the texture of the secondary limonitic pseudomorphs in the gossan. 

With much of the limonitic material, even from these lodes, however, 

identification of the parent minerals was uncertain or impossible.

None of the other lodes of the Nyeba district showed the same degree 

of wall-rock silicification or topographic relief. Where the oxidized 

outcrops of the lodes are not protected from erosion, gossan material 

is seen on the surface only as small fragments of limonite scattered 

through the surface rubble, and shows no diagnostic relict textures.
76 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

In an attempt to find a more generally useful method of appraising 

the base-metal content of the parent lode, a collection of gossan samples was analyzed for lead and zinc (table 4). One group of samples 

was collected from the outcrop of the Ameka lode, which is known to 

be fairly rich in both lead and zinc; another group was taken from 

the Igbaja prospect, where the geochemical soil traverse directly over 

the ancient pit showed a well-developed zinc anomaly, but little or no 

pattern in lead. Characteristic pseudomorphic textures in the limonite 

could not be identified in any of these samples.

The results clearly show a high lead content of the Ameka samples, 

and a high zinc content of the Igbaja samples. Considerably more 

work of a similar nature over other lodes of known tenor will be necessary, however, before the metal content of limonite can be considered 

a reliable guide to the mineralogy of the parent lode. Nevertheless, 

the presence of relatively high concentrations of lead and zinc in 

limonite may provide some evidence for the presence of base metals 

in a deposit where no primary sulfides can be observed.

TABLE 4. Lead, zinc, and iron content of gossan samples

Locality

Ameka lode.. .......

Igbaja prospect.. ....

Sampl6

no.

1627

1628

1629

1630

1631

1632 

579

580

581

682

583

584

585

Percent 

Fe

45.0

5.6

46.0

4.9

37.0

50.0 

22.5

22.0

24.0

48.0

22.0

28.0

51.0

PP

Pb

5,000

3,000

3,000

4.000

3,000

150 

100

700

50

50

70

50

50

m

Zn

1,500

500

300

900

300

80 

80, 000

10, 000

6,000

180

8,000

200

10,000

Remarks

Llmonltic boxwork; yellow-brown, siliceous.

Siliceous core; spongy, red llmonitic coating.

Limonitic boxwork; yellow-brown, siliceous.

Siliceous skeleton filled with spongy red limonite.

Limonite, hard, banded, reddish-black with

siliceous stringers.

Limonite; hard, massive, reddish -black. 

Limonite, amorphous, red.

Limonite, amorphous, yellow; with quartz.

Limonite, yellow.

Limonite, hard, black and brown.

Siderite with limonitic coating.

Limonitic boxwork, siliceous.

Do.

Kelict galena crystals encased in a shell of white lead sulfate occur

commonly in surficial materials near sulfide yeins, Sulfate-coated

galena fragments, rather than limonitic gossan, appear to be the normal product of the weathering of primary galena in the Nyeba district.

Apparently galena is much more resistant to oxidation than the other

ore-forming sulfides, as similar relict crystals of sphalerite or marcasite are never encountered in undisturbed surface soils.

VERTICAL DISTRIBUTION IN RESIDUUM 

WEATHERING PROFILE OVER UNMINERALIZED ROCKS

Six suites of samples representing weathering profiles over five

types of normal, unmineralized rocks, were collected to determine the 

behavior of lead, zinc, silica, iron, and alumina in the weathering cycle. 

The results are given in table 5. Shale, diorite, and biotite gneiss pro-
5*-

INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 77

files were sampled in well-drained areas of relatively low relief that 

are characterized by lateritic weathering. The micaceous and siliceous 

schist profiles were sampled in an area of fairly rugged topography 

where no signs of lateritic-type weathering were seen.

TABLE 5. Distribution of certain elements in weathering profiles over 

unmineralized rocTcs

Parent rock

Shale (Nyeba district).

Diorite (Abakaliki).

Biotite gneiss (4 

miles N of 

Obudu»). 

Micaceous schist 

(20 miles E of 

Obudu).

Siliceous schist (20 

miles E of 

Obudu). 

Shale (Crocodile 

prospect).

Type of weathering

Lateritic, welldrained.

.... .do.. 

 do. 

Nonlateritic, welldrained.

 do.. 

Clay-type, poorly 

drained.

Zone

Laterite ...........

Partly weathered 

rock.

Partly weathered 

rock.

Soil. .... . 

Partly weathered 

rock. 

Fresh rock. .......

SoU...............

Partly weathered 

rock. 

Organic clay, 3- 

inch depth. 

Gray clay, 24-inch 

depth. 

Mottled clay, 

120-inch depth.

Percent

SiOa 1

36.4 

55.2 

51.6

57.2 

17.0 

51.4 

45.9

44.4 

66.8

64.6 

66.5

64.6 

73.6 

74.1

61.8 

71.5 

57.2

FeaOj i

41.1 

12.1 

11.2

7.2 

53.5 

12.4 

9.9

33.2 

6.8

8.2 

8.1

8.6 

1.3 

2.4

3.6

6.0 

7.3 

7.2

AhOa i

14.6 

17.4 

22.2

20.2 

14.2 

14.0 

13.8

14.6 

15.3'

15.8 

15.0

15.4 

15.6 

14.8

19.1 

12.9 

20.2

Parts per million

Pb

114 

75 

67

280 

210 

35

64 

19

64 

56

23 

50 

65

55 

23 

20

Zn

81 

155 

160

226 

176 

40

48 

85

110 

130

130 

32 

41

84 

161 

93

Sb

4 

4 

4

i Analyses by K. E. White, J. M. Dowd, and H. F. Phillips, U. S. Geological Survey, Beltsville, Md. 

sO'boo-doo').

The samples representing the lateritic profile over shale were taken 

from different parts of the Nyeba district, as the entire profile was 

not exposed at any one locality. Therefore, much of the chemical 

variation in the weathering products may be a reflection only of the 

variations in the composition of the parent rock. The diorite and 

biotite gneiss suites were collected from single localities, although, 

here again, local, undetermined variations in the composition of the 

parent rocks will be responsible for corresponding variations in the 

weathering products. Each of the schist suites came from the walls 

of an excavation where samples could be collected from a single layer 

extending from the surface down into relatively unweathered rock.

The samples from the lateritic profiles all show an enrichment of 

iron and lead and an impoverishment of silica in the course of weathering and laterite formation. Zinc is impoverished in the samples of 

laterite derived from shale and gneiss and is enriched in the laterite 

over the.diorite. The alumina content shows no significant trend.

In the nonlateritic profiles, over schists, the data show no systematic 

variation in any of the elements reported.
78 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

The clay-type weathering profile was sampled in the trench in the 

swamp near the Crocodile prospect (fig. 2). The figures in table 5 

for the clay-type profile over shale represent analyses of composite 

samples of each zone collected along the 100-foot length of the trench. 

This profile is described in the discussion on Weathering in poorly 

drained areas. The sample of "fresh rock" used in both shale profiles was taken from Palm Wine bore hole 1. These data indicate 

that, at least in the trench near the Crocodile prospect, lead is enriched and iron impoverished in the organic surface horizon, whereas 

zinc is highest at a two-foot depth.

WEATHERING PROFII^E OVER KNOWN LODES

To determine the distribution of lead and zinc with depth in the 

vicinity of a known sulfide vein, the sides of a small trench at the 

south end of the Ameri lode were sampled (figs. 3 and 4). In this 

area, the original, natural surface had apparently not been disturbed 

by prehistoric mining activity, so that the soil was probably relatively 

uncontaminated. Sets of two samples, one of fragmental material 

at a depth of 6 inches and the other of weathered shale at a depth of 

2 feet, were collected at horizontal intervals of five feet along the 

length of the trench. No laterite zone was present. Table 6 shows the 

average lead and zinc content of soil and weathered shale from this 

trench.

TABLE 6. Distribution of lead, and- zino in weathering profiles over known lodes

Lode

Ameri.-. _ . __ . __ ..

Palm Wine ................

Zone

Soa.. .............................

Weathered shale............. . ....

Soil... 

Number 

of samples 

analyzed

10 

10 

6 

6 

6

Depth 

(inches)

6 

24 

6 

24 

96

Parts per million

Pb

7,500 

5,300 

7,800 

1,140 

3,700

Zn

1,090 

1,460 

166 

91 

425

A similar study was made at trench A (figs. 5 and 6) near the north 

end of the Palm Wine lode, where a narrow vein of galena was exposed.

The principal feature displayed by this trench was the occurrence at a 

depth of 6 inches of a laterite zone about B1/^ feet thick. Weathered

shale lay immediately below the laterite. Sets of three samples, one 

of surface material, one of laterite, and one of the -weathered shale at

the bottom of the trench, were collected at horizontal intervals of 10 

feet. The average lead and zinc content of the soil, laterite, and

weathered, shale in the six profiles east of tKe vein is given in table 6.

These data conform in general with the common observation that

lead is enriched and zinc is impoverished in soils, as compared with the 

parent rock. At the Palm Wine locality, the content of both lead and
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 79

^-^f Sample data. 

1 on fig 10

\v~

Ujl ^

//'

Sample data 

I on fig 7

EXPLANATION 

Approximate location of Ameri lode

Swamp

Exploration trench 

Soil-sampling traverse

Sample data \ 

on fig 4 *

Base map from survey by Nigerian 

Lead-Zinc Mining Company

100 0 

I . . i . t I____i_

500 Feet

FIGURE 3. Map of Ameri lode.
80 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

East

Z 2 500

ZINC

^Weathered shale, 

96 in. depth

8-in. galena lode 

at 120 in.depth

I________I

50 Feet 

J________I

FIGURE 4. Vertical distribution of metal over Ameri lode.

zinc in the laterite is less than that in the associated soil and weathered 

rock, although at least the lead content is well above background for 

laterite over unmineralized rock. From this, it may be concluded that 

for reconnaissance geochemical exploration in well-drained areas 

samples of surface soil analyzed for either lead or zinc will be just 

about as effective in locating buried lead-zinc lodes as samples of 

material from greater depth, in spite of a certain amount of leaching or 

enrichment in the course of weathering.

A similar study was not made of the distribution of lead and zinc 

in a clay-type weathering profile over a known bedrock sulfide deposit. 

Such an experimental study would be well worth making, if a place 

could be found where clay soil of definitely residual origin overlies 

a known lode.

LATERAL DISTRIBUTION" IN RESIDUUM

TRENCHES OVER, PALM WINE AND AMERI LODES

The data presented in figures 4 and 6 also show a lateral dispersion 

of lead and zinc from a relatively narrow sulfide vein over a distance

of several tens of feet and each side of the vein, both in the soil and in

the underlying laterite and weathered rock. In the Ameri trench, a

higher than normal metal content occurs in the soil across a width of at 

least 55 feet 5 at the Palm Wine trench, the minimum width is 90 feet,

Within these limits the distribution pattern of metals in the soil, 

laterite or weathered shale does not appear to correspond in detail with

the location of the veins -where they are exposed in the trenches. From

this, it may be concluded that samples of lateritic soil collected at

intervals of fifty feet would have been adequate to show the existence 

of either of these veins; samples taken at closer spacing, however,
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 81

200 o 100 <\/00

o °\ o X)

10,000 \750 400

Base map from survey by Nigerian Lead-Zinc Mining Company 

100 0 200 Fei

EXPLANATION

A _.:£..

Swamp

SO

500 . -'" --" 

Soil-sample location. Line of equal lead content Line of equal zinc content 

Upper figure is zinc in parts 

per million; lower figure is 

lead in parts per million

FIGURE 5. Geocbemical soil survey, Palm Wine lode.
82 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS 

West East

<n 10,000

o(r 

< uj

UJ 0.

LEAD

in.goleno lode at 36 in. depth

 Weathered shale, 

24 in. depth ZINC

l; 6 in. depth

in. galena lode at 36 in. depth

50 Feet 

j______I

FIGUKB 6. Vertical distribution of metal over Palm Wine lode.

would not have provided very much additional information as to the 

exact location of the veins.

TRAVERSE ACROSS AMERI LODE

Figure 7 shows the distribution of lead, zinc, and antimony in 

lateritic soil samples collected at 50-foot intervals across the Ameri 

lode, at a point 300 feet north of the ternch discussed above (fig. 3). 

Because of contamination of the surficial material by nearby excavations, a significant sample could not be obtained directly over the vein. 

The metal Content Of soil samples on each side of the vein indicate that 

the lead and zinc anomalies here are in the order of 200 feet wide> as 

compared with a lode width at this point of less than 10 feet, The 

economic threshold of this anomaly is about 100 ppm for both lead 

ftlld ZinC, The distribution of antimony in soils shows no apparent 

correlation with the location of tk© locU.

TRAVERSE ACROSS PALM WINE LODE

Samples from a traverse across the Palm Wine lode, the location 

of which is shown as Section A-A' 0n figure 5, were analyzed for lead, 

zinc, copper, cobalt, nickel, silver, arsenic, and antimony. As shown 

in figure 8, the lead and zinc anomalies here are more than 100 feet 

wide, on the assumption of economic thresholds of 1,000 ppm for lead 

and 100 ppm for zinc. Of the other metals, only silver and antimony 

formed significant distribution patterns. Silver content ^as o.e PPm 

in the sample containing maximum lead and zinc, and less than 0.2 ppm 

in all the other samples. The antimony pattern is very similar to the 

patterns of both lead and zinc. The copper content of these sampler
INVESTIGATIONS IN .NYEBA LEAD-ZINC DISTRICT, NIGERIA 83

Surface contamination; 

no sample taken

Approximate location 

of Ameri Lode 0.2

400 Feet 

_I

FIGURE 7. Metal content of soil over Ameri lode.

shows very weak correlation with the location of the lode. Cobalt 

content was 10 ppm in the sample directly over the lode, and less than 

10 ppm in all others. Nickel content was less than 30 ppm and arsenic 

less than 10 ppm in all samples.

GRID OVER PAUM WINE LODE

Near-surface soil was sampled on 50-foot centers in an area 600 by 

1,000 feet adjoining the Palm Wine lode (fig. 5). Most of this area 

is well drained and mantled with lateritic soil, although a broad stream 

channel flanked by clay soil crosses the north part of the map area.

The southern part of the lode had at one time been mined by opencast methods, undoubtedly resulting in some scattering of metal-rich 

material and contamination of the soil in the immediate vicinity. 

Soil samples were not taken unless the ground appeared to be undisturbed, and thus less likely to be contaminated. In spite of the uncertainty regarding contamination, it is believed that the pattern 

appearing on the map is probably not very different from what would 

have been found if the lode had never been worked.

Inspection of the data presented in figure 5 shows that the 100 ppm 

Zn contour and the 1,000 ppm Pb contour effectively outline the location of the known lode and its probable extensions. The width of the 

metal-rich zone thus defined ranges from 100 to 250 feet wide. Reconnaissance traverses sampled even at 100-foot intervals therefore would 

not have failed to locate the Palm Wine lode. A unique feature of the
84 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

West

20,000

10.000-

5000

East2000

1000

500

200

100

20

100

Approximate location of Palm Wine lode 

0 400 Feet

FIQOBE 8. Metal content of soil over Palm Wine lode along section A A'.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 85

Palm Wine area is the high lead background in the soils, averaging 

almost 500 ppm, as contrasted with a normal background of 30 to 

50 ppm Pb. The significance of the local high background in lead 

will be discussed later in connection with the distribution of metals in 

rocks.

The belt of clay soil in the northern part of the Palm Wine area is 

interpreted as residual rather than alluvial. The anomaly in lead 

and zinc extends from the area of lateritic soil northward into the 

area of clay soil, with no discontinuity at the boundary except for a 

decrease in the maximum lead and zinc content. The gradational 

character of the bottom contact of the clay profile with underlying 

weathered shale where it was exposed in exploration trench B (fig. 

5) also indicates a residual origin for the clay cover. Furthermore, 

if the clay were alluvial, one would expect that the anomalous pattern 

of metals in the clay would curve to the northeast following the direction of stream drainage.

The lead and zinc content of the soils north of the stream channel 

is markedly different from that to the south. Both the geochemical 

anomaly corresponding to the Palm Wine lode and the area of high 

lead background end at the stream channel. This abrupt change in 

the geochemical pattern suggests that the stream here may be coincident with an important fault, contact, or other bedrock structure.

DISTRIBUTION IN ALLUVIAL COVER

A trench across the suboutcrop of the Ameri lode in the poorly 

drained swampy area flanking lyinu stream (fig. 3) exposed a 6- to 8- 

foot section of clay cover believed to be of alluvial origin.

The lode where it is exposed in the trench is about 6 feet wide, and 

dips 70° to the west (fig. 9). Both the lode minerals and the shale 

country rock appear to be relatively unweathered. On one side of 

the lode, the shale country rock is overlain by a wedge of conglomeratic material containing rounded fragments of sulfide minerals, 

gossan, and shale cemented with limonite. A knife-edge contact separates the clay cover from both the bedrock and the limonitic conglomerate. Except for the sharp basal contact of the clay, the profile 

is superficially indistinguishable from that observed in the residual 

clay exposed in the trench near the Crocodile prospect. The surficial 

foot or so is even-hued gray clay, overlying a mottled red and gray 

clay which extends to the surface of bedrock. Although the clay 

cover shows no immediately apparent sedimentary structure, careful 

probing with a blade indicated the presence of horizontal layers an 

inch or so thick containing relatively gritty material. Scattered, 

rounded fragments of weathered shale, gossan, galena, and charcoal up to an inch in diameter are distributed apparently at random 

throughout the clay. The fragments of charcoal show relict growth
WEST

. ' . PRESENT SURFACED ' .' -.'. . 3W ;... . .'. ' . : . '. -.' / IE :. 

T . ' .: -iw . . . -.1::

FIGURE 9. "Vertical section showing distribution of lead and zi
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 87

rings indicating that they are the carbonization product of wood 

rather than of roots. The sharp basal contact of the clay and the 

presence of charcoal are interpreted as good evidence that the clay 

deposit is of sedimentary origin and is made up of alluvial material 

brought into the basin by sheet wash from the surrounding higher 

ground. It is probable that this deposit was laid down as part of the 

catastrophic cycle of soil erosion that followed the clearing of the 

forests several hundred years ago.

Vertical channel samples of the clay profile were collected by cutting 

out shallow grooves in the wall of the trench with a pocket knife, and 

placing all cuttings from the section sampled in a container. In this 

manner, 11 profiles were sampled in sections of one foot or less. Analytical results from these samples are shown in table 7 and figure 9; the 

profiles are numbered according to the distance in feet east or west 

of the center of the vein.

These data show a systematic distribution of both lead and zinc 

with respect to the suboutcrop of the lode. In sectional view, the lead 

as outlined by the 200-ppm contour is distributed both in a chimneylike pattern extending upward from the lode, and in a layer extending horizontally along the surface of bedrock for about 10 feet east 

of the east contact of the lode. The lines of equal zinc content form 

more of a domelike pattern over the suboutcrop of the ore. In addition to the dome pattern, a layer of clay containing more than 300 

ppm zinc extends horizontally along the surface of bedrock east of 

the lode for at least 20 feet.

TABLE 7. Lead and zinc content of alluvial cover over Ameri lode

Feet above bedrock

Lead and zinc content, parts per million for indicated distance from 

lode, in fnet

24 W 14 W 6 W 0 6E 16 E 26 E

Lead .

7-8.. 

6-7 

5-6....... ...............

4-5 

3-4 

2-3 

1-2..... ............. ...........

0-1 

50

50

50

50

50

50

70

50

50

90

50

50

50

30

50

70

50

125

600

500

330

330

500

2,000

120

50

100

200

70

550

50

50

70

. 100

100

200

1,500.

40

50

50

50

50

. 50

70

60

Zinc

7-8 

6-7..---..........- ........

5-6.... ....... .............. ....

4-5 

3-4 ........... .... ......... .

2-3 . 

1-2 

0-1 

100

100

100

100

100

100

100

100

100

125

200

100

100

120

150

150

325

200

400

500

600

1,500

3,250

100

100

200

400

300

100

100

150

200

250

300

400

75

50

75

50

200

250

350

750
88 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

The systematic relationship of the distribution of lead and zinc to 

the position of the suboutcrop of the sulfide lode strongly suggests 

that these anomalies have been developed by movement of metal from 

a source in the lode up into the overlying clay, where it has been fixed. 

If the age of the alluvial deposit, as inferred, is about 400 years, the 

rate of dispersion of lead and zinc in the formation of superimposed 

halos of this kind would be fairly well established.

The width of the halo studied in the Ameri trench is hardly more 

than the width of the vein itself. If 300 ppm zinc is taken as the 

threshold, the zinc anomaly 2 feet above the surface of bedrock has 

a width of about 10 feet. Thus in exploration, the location of a geochemical indication of a vein 6 feet wide covered by recent alluvium 

would require a maximum spacing of samples of not much more than 

5 feet.

To test this conclusion, a series of samples were collected across 

the lode 100 feet south of the trench (fig. 3J. Samples were taken at 

5-foot intervals and from a depth of 4 feet below the surface. The 

total depth to bedrock was not known. Figure 10 shows the distriLead

East

0.5

10 40 Feet

FIGURE 10. Metal content of alluvium over Ameri lode.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 89

bution of lead, zinc, and antimony in these samples. By analogy with 

the relationships found in the trench, it may be assumed that the position of the subout crop of the vein coincides with the peaks on the 

curves for lead and zinc. Copper, cobalt, nickel, silver, and arsenic 

were also determined in these samples, but showed little or no correlation with the pattern for lead and zinc or with the probable location 

of the lode.

The sample holes were made with a standard soil auger. A satisfactory soil auger may be built by brazing or welding a standard woodauger bit to a section of light cast-iron pipe, to the top of which 

is fitted a cross piece to serve as a handle. If necessary, additional 

sections of pipe may be attached for greater depth. One African 

assistant was able to collect 12 samples from a depth of four feet 

in a 6-hour working day. At the time of .the experiment, the clay 

was fairly dry and hard; with moist, pliable clay, it is probable that 

a somewhat better rate could be achieved.

As a result of the experimental work in the Ameri swamp, the following conclusions are offered regarding the behavior of lead and zinc 

in alluvial clays:

(1) Zinc and, to a lesser extent, lead move up into alluvial cover 

from suboutcrops of sulfide-bearing lodes.

(2) Subsurface samples at a spacing not greater than the width 

of the lode will under favorable conditions show a pattern of zinc and 

lead that can be used in locating buried lead-zinc lodes. Surface 

samples at 50-foot spacing, as used in reconnaissance on residual, lateritic cover, are not adequate for locating lodes under alluvial cover.

(3) Clay cover can be identified as alluvial rather than residual in 

origin if a sharp basal contact with the underlying rock can be 

observed, and if the clay contains obviously exotic fragments. Otherwise, alluvial clay cannot be positively distinguished from residual 

clay by any method known to the writer.

DISTRIBUTION IN ROCKS ADJOINING VEINS

Natural exposures of unweathered shale are seen only in deep river 

channels, which are lacking in the immediate vicinity of the mines 

of the Nyeba district. It was therefore not possible to get samples 

for the determination of the background metal content of completely 

unweathered shale known to be unmineralized. The average metal 

content of the two samples of partially weathered shale from areas 

believed to be unmineralized is 67 ppm Pb and 160 ppm Zn, as determined by quantitative colorimetric analysis. It must be remembered 

that, because of the relative mobility of zinc in the weathering cycle 

compared to that of lead, these samples of weathered shale may be 

somewhat, lower, in zinc and higher in lead than the parent shale.
90 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

The lead content of residual soils in the area of the Palm Wine lode 

is substantially higher than normal. In the area covered by figure 

5, the average lead content of the 156 samples taken south of the claycovered swamp and outside the anomalous area over the lode, is 475 

ppm Pb, as compared with a background for soil of between 30 and 

50 ppm. The average zinc content of the same samples is 65 ppm, 

or essentially the normal background content for lateritic soils. North 

of the clay-covered swamp, the lead content is not substantially above 

background. Inasmuch as the surface of the ground south of the 

swamp slopes fairly uniformly to the northwest, it is extremely 

unlikely that very much of the excessive lead in the residual soils was 

derived by erosion of lead-rich material from the main lode. Contamination of such an extensive area by human activities seems equally 

unlikely. The possibility remains, therefore, that the lead-rich residual soils reflect a higher than normal lead content of the parent shales.

To determine the relation of the metal content of soils to that of 

the unweathered bedrock in the vicinity of the Palm Wine lode, samples taken of the core from bore hole 1 (fig. 5) were made available 

to the writer by the mining company. Each sample represents a split 

of the contents of the core barrel each time the drilling tools were withdrawn from the hole. The samples were digested in nitric and hydrofluoric acids, and the lead and zinc content of each sample estimated by 

the rapid dithizone procedure; in addition, weighted composite samples were prepared and analyzed by quantitative colorimetric methods. 

Table 8 shows the analytical results.

For the section west of the ore zone, the quantitative value for lead 

on the weighted composite sample is 185 ppm, which is significantly 

above background but not nearly as high as the 475 ppm average Pb 

content of the residual soils. For the section east of the ore zone, 

the lead content is 66 ppm, which is not substantially different from 

the assumed background for unmineralized shale. The zinc content

of both, sections is well above the assumed zinc background, and contrasts sharply with the relatively low zinc content of the soils. The

bore hole samples are, of course, a very inadequate representation of

the rocks of the area, a fact that may account for some of the discrepancy between the rock and soil data. Part of the discrepancy

may also be the effect of the demonstrated tendency for lead to be

enriclled. and. for zinc to be impoverished, in the course of weatheringIn conclusion, the drill core data do not wholly support the idea that

the abnormally high lead content of soils in the vicinity of the Palm

Wine lode is a reflection of a primary dispersion of lead in the shale

bed rock; nevertheless, in the absence of evidence to the contrary, the 

existence of a primary anomaly in this area seems to be the most 

reasonable working hypothesis.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 91

TABLE 8. Distribution of lead and zinc in Palm "Wine bore hole 1

(Detailed Log)

Depth (feet)

From

0

37

. 38.6

41.6

43.3

47.0

48.6

50.0

52.3

53.6

54.6

56.2

60.5

61.0

63.6

65.0

68.9

70.0

71.0

72.9

73.9

80.0

85.0

87.5

on i OCT. 4-

91.5

94.8

97.2

100.0

101.5

102.5

105.9

107.1

107.7

108.8

110.1

111.2

112.3

113.0

116.5

118.5

120.4

120. 8

122.5

124.8

128.1

130.3

131.6

132.6

135.6

137.3

141.3

143.9

145.9

148.0 

151.6

156.2

160. 1

162.1

164.0

To

37

38.6

41.6

43.3

47.0

48.6

50.0

52.3

53.6

54.6

56.2

60.5

61.0

63.6

65.0

68.9

70.0

71.0

72.9

73.9

80.0

85.0

87.5

89.1

01 yj.. uH 

94.8

97.2

100.0

101.5

102.5

105.9

107.1

107.7

108.8

110.1

111.2

112.3

113.0

116.5

118.5

120.4

120.8

122.5

124.8

128.1

130.3

131.6

132.6

135.6

137.3

141. 3

143. 9

145.9

148.0

151.6 

156. 2

160. 1

162.1

164. 0

166. 9

Feet of 

core

37.0

1.6

3.0

1.7

3.7

1.6

1.4

2.3

1.3

1.0

1.6

4.3

0.5

2.6

1.4

3.9

1.1

1.0

1.9

1.0

6.1

5.0

2.5

1.6

2.4

3.3

2.4

2.8

1.5

1.0

3.4

1 1. 9 £i

0.6

1.1

1.3

1.1

1.1

0.7

3.5

2.0

1.9

0.4

1.7

2.3

3.3

2.2

1.3

1.0

3.0

1.7

4.0

2.6

2.0

2.1

3.6 

4.6

3.9.

2.0

1.9

2.9

Parts per million

Lead

1,666

100

50

70

50

50

150

100

400

200

100

100

50

3,000

50

200

200 50

120

600

50

60

70

50

70

150

100

750

250

1 20, 000

1 10, 000

'60,000

i 80, 000

30

300

50

50

50

50

650

1,250

50

70

50

50

50

70

70

50

30

50

50

50 

70

50

70

70

70

Zinc

300

150

300

350

300

400

200

500

350

800

350

600

500

1,000

600

700

400

200

200

300

200

300

200

150

250

300

400

200

400

1 5, 000

1 30, 000

1,000

800

800

300

350

200

600

400

250

100

100

150

100

70

150

200

200

150

150

100 

200

200

150

150

150

Description

Noncoring.

Intercalated shale and sandstone.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Barren siderite with sparse marcasite.

Intercalated shale and sandstone.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

No core .

Intercalated shale and sandstone.

Do.

Do.

Do.

Do.

Marcasite and siderite at end of run.

Barren siderite and marcasite with few spots of

galena.

Do.

Siderite and marcasite with some galena and sparse

sphalerite.

Siderite with spotty galena, sphalerite, chalcopyrite,

 minor marcasite.

Barren siderite, with minor marcasite and shale.

Sandstone with sphalerite, marcasite, and siderite.

Intercalated shale and sandstone.

Gray sandstone.

Do.

Do.

Do.

Do.

Do.

Intercalated shale and sandstone.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Shale with minor sandstone.

Do. 

Intercalated shale and sandstone.

Do.

.Do.

Do.

Do.

Summary

Weighted averages of data from detailed 

log

Depth (feet)

From

37.0 

112.3

To

102.5 

106.9

Parts per million

Lead

239 

97

Zinc

335 

193

Quantitative determinations on 

weighted composite samples

Parts per million

Lead

185 

66

Zinc

380 

178

Assay values for lead and zinc furnished by Nigerian Lead-Zinc Mining Company.
92 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

VEGETATION

The present series of experiments did not include geochemical exploration by collection and analysis of vegetation. Webb and Millman 

(1951) carried out a fairly extensive investigation on the lead, zinc, 

and silver content in vegetation as a guide to ore in the Nyeba district, 

and the reader is referred to their report for details. Their conclusions are quoted below in full (Webb and Millman, 1951, p. 499):

Marked increases in the concentrations of lead and zinc in plants can be correlated with proximity to lead-zinc vein deposits. Using a spectrographic analytical technique, lead is found to be the more satisfactory index element. In 

addition, silver has a limited value as a "pathfinder" element.

The anomalies, which in places attain widths of several hundred feet, are considerably wider than the responsible mineralized zones. Several interrelated 

factors contribute to the nature, width and magnitude of the observed anomalies 

over known mineralizations. In addition to certain botanical considerations, 

these factors include (a) width and tenor of the mineralization near the surface, 

(b) the mineralogy and degree of oxidation of the ore, (c) mobility of the metals 

in the zone of weathering, (d) topographic relief, (e) disturbance by mining, and 

probably other factors. Their relative importance is peculiar to each locality.

In the area examined, several species of trees of the savannah vegetation 

possessing a convenient collective distribution provide suitable sampling points. 

The data are incomplete, but apart from minor parochial and specific variations, 

all the species examined seem to behave as normal accumulator plants towards 

lead, zinc, and silver. Although the background concentrations of these metals 

are usually rather higher in leaves than in twigs, both parts of the plant give 

comparable anomalies over mineralization. Twig samples are found to be more 

convenient in practice and, subject to the results of control work, it seems probable that anomalies would be detectable in twigs at most times of the year.

Much of the work was carried out in areas where the lodes have been disturbed, 

mainly by shallow opencasting. A smaller number of analyses indicate that appreciable anomalies are also detectable over essentially virgin, sub-outcropping

veins. The results suggest that, in favourable circumstances, a hidden economic 

lode could be expected to promote increased metal concentrations in suitable 

plants for some appreciable distance on either side of its sub-outcrop. Fluctuations in the biogeochemical profiles due to botanical and other factors, not 

directly related to mineralization, should not in general obscure an anomaly 

associated with an economic- deposit. However, aitnougn such actuations are 

usually relatively small, occasional higher values may have a deceptive significance.

It is considered that the systematic determination of the metal concentrations 

in plants shows promise of providing a useful supplementary aid to mineral exploration in the Nigerian lead-zinc belt and in other areas where similar favourable conditions prevail.

From the practical point of view, further orientation studies might be undertaken, combined with the investigation of other analytical techniques and a comparison of the relative advantages and requirements of the plant and son methods

of geoohemieal prospecting, in addition, there is a wide field for research in the

basic principles and processes involved,

The writer collected soil samples along Traverse A-B, across the 

southerly projection of the Ameka lode, as shown in Webb and Millman's plate 3 (fig. 2 of this report for location). These samples all
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 93

West 

isoo r

.CC 1000

tc <I a

o

CO

2

O

ID

500

East 

-1150

EXPLANATION

Lead content in soil

Lead content of oven-dried 

twigs of Afzelia, Africana (from 

Webb and Millman, 1951)

- 100

FIGURE 11. Comparative lead content of soils and plants near Ameka lode.

 consisted of residual lateritic soil, and were well outside the area of 

disturbance caused by mining activity around the Ameka lode. Figure 

11 shows the comparative lead content of soils and plants 

along this traverse. These soils were also analyzed for zinc, silver,

 copper, cobalt, nickel, arsenic, and antimony. The correlation between 

the zinc content of plants as determined by Webb and Millman and 

the zinc content of the corresponding soils is not as well marked as 

that of lead. The silver content of all the soils was below the 0.2 ppm 

sensitivity of the analytical procedure, so that no comparison was possible for silver. The other elements either showed no systematic correlation with the lead, zinc, and silver content of the plants (copper, 

cobalt, antimony) or were present in amounts below the sensitivity 

of the analytical method (nickel, arsenic). This fragmentary bit of 

information indicates that, at least for lead, the pattern of distribution 

in the plant species sampled by Webb and Millman is similar to that 

in the supporting soils.

It would be premature to hazard a guess as to the relative economics 

of geochemical plant surveys as against soil surveys until cost data 

for the plant work could be worked out on a basis strictly comparable 

with that of the soil survey. The majority of workers familiar with 

both methods of approach are generally of the opinion that soil work 

is more economical where it is applicable, and that plant sampling 

should be saved for problems where for some reason the study of soils
94 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

fails to provide the desired information. However, the relative economy and effectiveness of the two methods is a question still actively 

being debated, and the final answer must await further research.

WATER

Webb and Millman (1950) in summarizing their experimental work 

on geochemical water surveying in the ISTyeba district, conclude that 

springs issuing from the lodes contained relatively high concentrations of heavy 

metals (undifferentiated zinc, copper, and lead). The addition of these natural 

lode waters to the main drainage system caused significant increases in the metal 

content of the stream waters, the magnitude of these increments being subject 

to considerable diurnal variation during the rainy season. 

Many of the samples reported by Webb and Millman were from 

springs or surface water in the vicinity of the Ameka lode, which has 

been extensively excavated and exposed to accelerated oxidation and' 

solution by ground water passing through the old workings.

Webb and Millman also sampled surface water in the vicinity of 

the Ameri No. 2 lode, now known as the Ikwo (E'kwo) lode (fig. 2), 

where increments of metal from oxidizing sulfide minerals in old mine 

workings are probably negligible. Their results follow:

Sampling point.

125 ft upstream from lode- _ ....................

Heavy metals 

(ppm)

.1'

In an effort to reconfirm and extend the earlier work of Webb and 

Millman, the writer attempted to analyze similar samples of surfacewater by the same method (Huff, 1948). With almost all of the water

samples collected, however, the dithizone solution oxidized rapidly to

a dirty-green or yellow color, and with many of the samples the carbon tetrachloride emulsified on shaking so that no clean, two-phase

Separation was obtained. After some experimental work on the procedure, the water sampling program was abandoned.

Interferences of this kind have been commonly attributed to organic

impurities in the water. The difference in the response of the same

teSt in .the liaildS Of different workers probably lies in the season of

the year when the samples were collected. Webb aild MlllmaH COlleCted tlielr Samples during the rainy season when presumably the

content of organic refuse in the water was not high enOUgh1 to CaUS& 

interferences, whereas the author's samples were taken at the beginning of the dry season when streams and pools were stagnant and 

highly charged with putrifying organic matter.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 95

EXPERIMENTAL GEOCHEMICAL SOIL SURVEYS

With the exception of the Ameri lode, every known lode in the 

Nyeba district gave some surface evidence for its existence in the form

 of fragments of limonite, sulfate-coated galena, or quartz in the 

surface rubble and the material of the yam heaps. As has already 

been pointed out in connection with the soil erosion problem, it is

 quite likely that all the veins of any importance in the district in areas 

of lateritic weathering have already been discovered, as a result of 

the very thorough and intensive scouting for this type of evidence by 

African prospectors in the employ of the mining companies. In the 

well-drained parts of the Nyeba district, therefore, conventional methods of prospecting appear to be quite adequate, and geochemical prospecting methods are hardly needed, even though they can be shown 

to be outstandingly effective. The experimental soil sampling surveys described below Avere carried out more for the purpose of determining the economics of geochemical work manned principally by 

African labor than of making any new mineral discoveries. There is 

good reason to believe that the cost and probably the effectiveness of 

geochemical soil surveying in the tropics would not be substantially

 different in areas of deeper soil cover, where conventional methods of 

prospecting would be of little or no value.

SAMPLING METHODS

As a result of the field experiments already discussed, it was concluded that samples of soil collected at a depth of 6 inches and at 100- 

foot intervals were most suitable for reconnaissance work. Although 

samples of 100-foot intervals would have been quite adequate to locate 

all the major anomalies, a 50-foot sample interval was used in the

 experimental surveys for the sake of gathering additional confirmatory data on the optimum spacing. An all-African field crew of eight 

men was trained for this work. Surveying and sampling equipment 

included a prismatic compass mounted on a tripod, a 100-foot link

 chain, a survey pole, and machetes for cutting brush and digging 

holes. The crew consisted of a party chief, a compass man, two men

 on the chain, one lead man with the survey pole as a foresight target,

 one man to dig the sample holes, and two men to cut survey lines. 

Sample holes were prepared by chopping and cleaning out a hole 6 

inches deep with the end of a machete, and loosening a handful of 

earth from the bottom of the hole which was left in place to dry. 

Samples were collected a day or two later, after they had dried sufficiently to be sieved. Sieving was done at the sample site with an 

80-mesh stainless-steel sieve, the +80 mesh fraction discarded, and 

about 3 grams of the fines retained for analysis. Samples were collected in 3- by 5- inch manila envelopes sealed with fold-down metal 

tabs. Sample numbers, which were written on the outside of the
96 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

envelope, were assigned in the order of collection, regardless of the 

type of material sampled or the nature of the problem. Description 

and location of the samples were entered in the notebook.

On this basis, the African crew averaged 1,500 feet of traverse, or 

30 samples, per 6-hour working day. Much of this time was taken 

up with cutting brush; in open country, the crew could average 3,000' 

to 4,000 feet (60 to 80 samples) per day. The principal factor governing the productivity of the crew appeared to be the length of 

traverse and the amount of brush to be cut, rather than the number 

of samples. The party chief for the job was a member of the permanent junior staff of the Nigerian Geological Survey Department,, 

who was given complete responsibility for all aspects of routine sampling work. The only participation by Europeans in the sampling, 

program was in laying out the problem and in studying the geology 

or surface features of the terrain after the lines had been cut and the: 

samples collected.

* All of the soil sampling was done in the dry season, when the' 

loosened earth in the sample holes was dry enough for sieving in thefield after exposure to the air of a day or two. In the rainy season, 

it would be necessary to collect the samples wet, and dry them later 

in an oven before sieving. An added difficulty with moist samples isthat the glue in standard containers or envelopes will soften so that 

many samples will be lost in transit, and all samples must be repackaged when dry. Sample containers made with waterproof glue have 

been put on the market, but the writer has had little experience with 

them. Another alternative is to wrap the wet sample in waterproof 

paper or metal foil before inserting it in the envelope. Because of 

the difficulties in handling wet samples, it would, in general, be best 

to avoid the problem completely by planning the program so that all 

soil sampling can be done in the dry season.

An unanticipated difficulty was the destruction of sample envelopes 

by white ants. A large part of the supply of envelopes in the mining1

company's laboratory at Nyeba were thus destroyed during 6 months* 

of storage.

APPBAISAL OF PROSPECTS BY GEOCHEMICAL METHODS

R. G. Bogue in the course of his geological investigations found

Scattered, fragments of gossan and. the remains of an ancient opencast

pit at a point about 3 miles north of the Igbaja dispensary, and about

30 miles north Of Nyeba (fig, 1). He suggested that geochemical 

methods be used for appraisal of the prospect and for determination of the size and continuity of the bedrock deposit, if any existed.

Four parallel sample traverses were laid out, one across the ancient 

workings, one to the south, and two to the north of them, as shown on 

figure 12. The traverse across the old workings shows a very definite-.
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 97

70 SO 50 50 SO 50 50 50 50 50 SO 50 50 SO 50 50 150 

OOOOOOOOOOOOOOOOO 

30 50 50 50 30 30 30 30 30 30 30 30 30 30 30 30 130

50 SO SO 50 SO SO SO 50 SO SO SO SO 50 SO SO SO 70 

OOOOOOOOOOOOOOOOO 

50 20 50 50 50 50 150 800 75 50 30 30 30 30 30 30 30

SO SO SO 30 SO

O O O O O

50 50 -20 20 20

60 

O 

30

Soil sample location. 

Upper figure is zinc in parts 

per million; lower figure is 

lead in parts per million

Samples contaminated 

with material from dump

'lOOOJlSO 150 70 100 150 200 100 70 50 o-^ ooooooooo

70 50 50 50 50 20 20 50 20 20 50

SO SO SO SO 50 50 70 70 70 100 

OOOOOOOOOO 

20 20 20 70 70 100 150 130 50 50

FIGURE 12. Geochemical soil survey, Igbaja prospect.

geochemical anomaly in zinc, although the pattern may have been 

somewhat disturbed by contamination from the old workings. Theother traverses show no outstanding pattern in zinc, although there 

are variations in the lead content that may indicate minor sulfides in 

the bedrock. No strong pattern of lead and zinc distribution similar 

to that found at the Palm Wine lode is apparent, and it may be safely 

concluded that the chances of an important sulfide deposit at this 

locality are slight.

Sample collection required one day of field work on the part of" 

the African sampling crew, and one day of analytical work in the 

local laboratory. As a result, it was possible in two days to eliminate 

this occurrence as a likely prospect with considerable certainty.

RECONNAISSANCE EXPLORATION

An experimental geochemical reconnaissance survey of the area 

between the Ameri and Palm Wine lodes was conducted for the purpose of gaining experience with a large-scale surveying and sampling:
WEST 

5000 r

1000

500

AREA OF 

FIGURE 5

IKWO LODE.

* 500- 

F5 1000 :

FJQURB 13. Reconnaissance soil traverse between Palm 
INVESTIGATIONS IN NYEBA LEAD-ZINC DISTRICT, NIGERIA 99*

program entrusted entirely to African labor. The work was laid out 

as a series of parallel east-west sample traverses, along which soil samples were collected at 50-foot intervals. Figure 2 shows the location of 

sample traverses. Through an unfortunate accident, most of these 

samples were destroyed; the data from those that were saved, however, are shown on figure 13. On this traverse, it is interesting to 

note that the Ameri and Ikwo lodes as well as the area of lead-rich 

soils adjoining the Palm Wine lode are all indicated prominently in 

the geochemical curves. In addition, two unexplained geochemical 

anomalies were brought to light by the data from this traverse.

CONCLUSIONS

The following conclusions are believed to be accurate for the Nyeba 

district, and are probably valid for many other areas in the tropics:

1. The lead and zinc content of 3-gram samples of the minus 80 

mesh fraction of lateritic residual soil collected at a depth of 6 inches 

and a spacing of 50 feet may be depended on with confidence to indicate the presence of lead-zinc veins in the bedrock in the immediate 

vicinity. Samples collected at 100-foot intervals would be adequate to 

locate the Palm Wine and Ameri lodes. Significant anomaly values 

are 100 to 200 ppm for zinc and commonly 500 to 1,000 ppm for lead,, 

over a background of 50 ppm for zinc and 30 ppm for lead.

2. Sampling and analysis of residual clay soils at similar depth 

and spacing will probably serve to locate veins in the immediate 

vicinity.

3. The lead and zinc content of samples of recent alluvial clay 

deposits taken from two to three feet above the surface of bedrock, at 

a spacing not greater than the width of the lode, will under someconditions show the location of suboutcrops of lodes containing galena 

and sphalerite.

4. The lead and zinc content of limonitic gossan may provide 

evidence for base-metal mineralization where the primary sulfides 

have been completely oxidized.

5. African labor can be trained to do all but a minor part of the 

routine work of sample collection and analysis.

UNSOLVED PROBLEMS

It seems to be true of almost every investigation in a new field that 

more problems are brought to light in the course of the work than 

are solved. Among the most important unanswered questions raised 

by the present work in Nigeria are the following:

1. Would the geochemical halos of lead and zinc determined by 

sampling near-surface soils be as well-developed if soil erosion had 

not removed the indigenous weathering profile in the Nyeba district ?
100 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS

2. Can reliable and simple criteria be worked out for distinguishing residual from alluvial clay cover ?

3. What is the mechanism whereby zinc and lead migrate upward 

into transported cover from veins in the underlying bedrock?

4. Can a test for metals in water be developed that is suitable for 

;all natural water regardless of organic contamination?

5. How does the cost of geochemical soil work compare with plant 

work as a method of geochemical reconnaissance exploration ?

REFERENCES CITED

Almond, Hy, 1953, Determination of traces of cobalt in soils: Analytical

Chemistry, v. 25, p. 166-167. 

Almond, Hy, and Morris, H. T., 1951, Geochemical techniques as applied in recent

investigations in the Tintic district, Utah: Econ. Geology, v. 46, p. 608-625.

 Cooper, J. R., and Huff, L. C., 1951, Geological investigations and geochemical 

prospecting experiment at Johnson, Arizona: Econ. Geology, v. 46, p. 

731-756.

 du Preez, J. W., 1948, Laterite: a general discussion with a description of Nigerian 

occurrences: Bull. Agricole du Congo Beige, v. 40, p. 53-66.

'Elkins, H. B., 1950, The chemistry of industrial toxicology: John Wiley and Sons, 

New York, 406 p.

STarrington, J. L., 1952, preliminary description of the Nigerian lead-zinc field: 

Econ. Geology, v. 47, p. 583-608.

Fersman, A. Ye, 1939, Geokhimicheskiye i minerologicheskiye rnetody poiskov 

poleznykh iskopayemykh [Geochemical and mineralogical methods of prospecting for mineral deposits]: Akad. Nauk SSSR, Moscow. (English translation of Chap. IV, Special methods of prospecting, by Lydia Hartsock and 

A. P. Pierce, released as U. S. Geol. Survey Circular 127, 37 p., 1952.)

"'Guillemain, 1910, Die Eisenindustrie der Eingeborenen Kameruns: Koloniale 

Rundschau, Jahrg. 1910, no. 1, p. 15-25.

-Harbaugh, J. W., 1953, Geochemical prospecting abstracts through June 1952:

U. S. Geol. Survey Bull. 1000-A, p. 1-50.

Hawkes, H. E., 1948, Annotated bibliography of papers on geochemical prospecting for ores: U. S. Geol. Survey Circular 28, 6 p.

- 1950, Geochemical prospecting for ores, in Trask, P. D., Applied sedimentation, John Wiley and Sons, New York, p. 537-555.

1952, What geochemistry is and what it can do: Min. Cong. Jour., v. 37,

no. 9, p. 55-58. 

"ISuff, L. C., 1948, A sensitive' field test for heavy metals in water: Econ. Geology,

v. 43, p. 675-684. 

Lakin, H. W., Almond, Hy, and Ward, F. N., 1952, Compilation of field methods

used in geochemical prospecting by the U. S. Geological Survey: U. S. Geol.

Survey Circular 161, 34 p. 

:Lakin, H. W., Stevens, B. B., and Almond, Hy, 1949, Field method for the

determination of zinc in soils: Econ. Geology, v. 44, p. 296-306.

'Levering, T. S., Sokoloff, V. P., and Morris, H. T., 1948, Heavy metals in altered 

rock over blind ore bodies, East Tintic district, Utah: Econ. Geology, v. 43,

p. 384-399.

Mackay, R. A., 1950, The lead and zinc veins in the Lower Cretaceous of Nigeria:

Proc. Int. Geol. Cong. (XVIII Session), Ft. VI, p. 217-220. 

'Minerals Yearbook, 1949, U. S. Bureau of Mines, p. 483,1212,1292.
INVESTIGATIONS EN NYEBA LEAD-ZINC DISTRICT, NIGERIA 101

Morris, H. T., 1952, Supergene and hydrothermal dispersion of heavy metals in 

wall rocks near ore bodies, Tintic district, Utah. Part II, Primary dispersion patterns of heavy metals in carbonate and quartz monzonite wall 

rocks: Econ. Geology, v. 47, p. 698-716.

Uankama, K. K., 1947, Some recent trends in prospecting: chemical, biogeochemical, and geobotanical methods: Mining and Metallurgy, v. 28, p. 

282-284.

Riddell, J. E., 1950, A technique for the determination of traces of epigenetic 

base metals in rocks: Quebec Dept. Mines, Prel. Rept. 239.

Roberts, R. O., 1953, Geochemical investigation in Nigeria in 1948 and 1949; 

Colonial Geoi: and Min. Res., v. 3, No. 4, p. 347-357.

Sandell, E. B., 1950, Colorimetric determination of traces of metals: 2d ed., 673 

p., New York, Interscience Publishers, Inc.

:Sergeyev, Ye. A., 1941, Fiziko-khimcheskiy metod poiskov rudnykh zalezhey 

[Geochemical method of prospecting for ore deposits] Materialy Vses. 

Nauchno-Issled. Geol. Inst, Geofizika, sbornik 9-10, p. 3-55, (English 

translation by V. P. Sokoloff in Selected Russian papers on geochemical 

prospecting for ores. Released for free distribution by the U. S. Geological Survey, February 1950).

Tattam, C. M., 1930, The lead-zinc deposits of Ogoja Province: Geol. Survey 

Nigeria Ann. Rept., p. 14-23.

 1943, A review of Nigerian stratigraphy: Geol. Survey Nigeria Ann. 

Rept., p. 27-40.

Webb, J. S., and Millman, A. P., 1950, Heavy metals in natural waters as a 

guide to ore; a preliminary investigation in West Africa: Inst. Min. Metallurgy Bull. 518, p. 3-16.

 1951, Heavy metals in vegetation as a guide to ore: Inst. Min. Metallurgy, Trans. v. 60, pt. 2, p. 473-504.

INDEX

Page

Abstract. _ 51

Acknowledgments for aid . . 52-53,67

Agricultural practices.. . 64

Ameka West prospect, results of tests.. 67

Analytic methods, requirements.. 54,67

Anomalies, primary, at Palm Wine lode. 90

primary in overlying rock.. . 54

occurrence of metals in....__ 55

"wall rock" halos.... 54

secondary, superimposed halos... 56 

vegetation_______---------- 55,92

significant values_________ - 99 

width............... .. - 82

Antimony, distribution. _... 82 

Arsenic ... ........ - 83,93

Background_________________.. 56,90 

Bibliography.... ..... ................. 100-101

Biogeochemical anomalies. See Plant anomalies. 

Bogue, R. G., and Reynolds, R. R., quoted.. 58-61

Catastrophic cycle of soil erosion_.__.. 65,87,88 

Cobalt..- 83,93

Colorimetric tests..._________ 67-68 

Contamination, effect on geochemical investigations____________-. 57 

sources.........__.__...__... 66-67,83

Copper, distribution-.._______... 83,93 

Crew......._....__-.----..-..... .- 95,96

Crocodile prospect, results of tests... . 66

Determinations, accuracy.____..-.. 74 

comparison of field with Denver Laboratory....... . 72-74

total and average number... 72

Earlier geochemical investigations .... 

Enrichment, systematic during weathering...

67

Field work. . 96-98

Galena, resistance to oxidation .. 76 

Geochemical prospecting, defined ... 56 

Geologic mapping, handicaps..... 61 

Geology 58 

Gossan. . . - 76,99

Igbaja prospect, survey at____.._ 96-97 

Igneous rocks. .... .. 58 

Iron industry, ancient. 65

Juju pools ... ---- - 61,62

Laboratory field...__.... .. . 69-70

Laterite.-.-------.-..------------------------ 63

Lead, compared to zinc in vegetation analysis. 92,93 

difficulties in field determinations... 72,73 

estimation of-.-..... .. 71

range in background 56 

Lead mining, native__..-. .. .......

Limonite ......... - 76

Mineralogy ------__...- .... . 60-fll

Mobility of elements..- 66

Nickel. 93

Ore deposits, of Nyeba district, mineralization. 69 

occurrence..._..____________ 58 

wall rock alteration____ _.. ____ 69-60

Palm Wine lode________________ 90

Plant anomalies.._______________ 55

Preliminary investigation... _. _ _ ___ 57 

Poorly drained areas, possibilities of new

discoveries____________ 66

Primary minerals ______________ 60

identification in gossan. _________ 75

Purpose of investigation _ _____ ___. 52

Reagents....... .__. __.. 68,70,71

Reconnaissance survey, experimental____ 97-99 

Relative solubility of metals..________ 66 

Residual soils, lead content, Palm Wine lode.. 90

Samples, alluvium..______________ 87 

bore hole...... ...-. ._............ 90

digestion of________________ 71 

gossan. ...__________ 76 

methods of collecting.___________ 96-96 

rate of taking.__________ ___ 89 

shipment of. ___________ 74-76 

slag-....-.-. -. .. . 65-66 

spacing of.. -.... . 81,83,88,95,9<4 

surface water...______________ 94 

twig.-.-...-...-.-....-.. . .-___ 92 

unmineralized rock____________ 76-77 

Seasons . ... . 62,94,96 

Sedimentary rocks ____________ 58 

Shale, unweathered, outcrops of.______ 88 

Silicifled wall.rock, protection of lodes by__ 75 

Silver, distribution __.__..____ 92,93 

Soil, alluvial . 85-87

clay.

content of lead and zinc contrasted ___ 78 

fragmental zone._____________ 62 

importance in investigations_._____ 66 

laterite...__.-..____________ 63 

Soil and plant surveys, costs compared. ___ 93-94 

Soil auger, use in sampling.__________ 88 

Surflcial factors, importance in mineral investigations. ______.___ 61 

Systematic distribution of elements, lacking in

siliceous schists._________ 77 

lack of some at Ameri lode._______ 89 

 in-vertical profiles, Ameri lode______ 87-88 

in weatheredUateritic rocks.'J______ 76

Threshold . . 66,88,90 

Topography .. 61

Vegetation.... . ........ .......

geochemical exploration by analysis of -

Weathered bedrock..______.______ 63,64 Weathering profiles, in poorly drained areas.. 63-64 

in well-drained areas..___...._.._ 62-63 

Webb, J. S. and Millman, A. P., quoted 58,92-94 

Well-drained areas, doubtful possibility of new

discoveries _._._ .. 66,95

Zinc, estimation of '. .... 71

lower values in rapid determinations. __ 74

range in background, - 67

removaljfrb.ni'Salution.. . _... . 56

threshold"at "Ameri lo'ae:.. . . 9o

103

O