Regional prospecting for ores based on heavy minerals in glacial till

Journal of Geochemical Exploration, 7 (1977) 1--19 1 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

R E G I O N A L P R O S P E C T I N G F O R O R E S B A S E D O N H E A V Y M I N E R A L S IN G L A C I A L T I L L

N.H. BRUNDIN and J. BERGSTR(~M

Geological Survey of Sweden, Stockholm (Sweden)

(Received May 12, 1976; revised and accepted September 10, 1976}

ABSTRACT

Brundin, N.H. and BergstrSm, J., 1977. Regional prospecting for ores based on heavy minerals in glacial till. J. Geochem. Explor., 7: 1--19.

A new heavy mineral technique suitable for regional prospecting of ores within previously glaciated areas has been developed at the Geological Survey of Sweden. The method is based on sampling and investigation of glacial till instead of the traditional alluvial sediments. A mechanical device including a riffle box is used for quick pre-concentration in the field instead of manual panning, and the analyses are carried out by instrumental chemical methods (mainly X-ray fluorescence). Mineralogical analyses based on mineral recognition and grain counting are used to a limited extent in that only chemically anomalous samples are investigated mineralogically.

The method provides significant information with sampling densities of 0.05--0.1 sample per km 2 in reconnaissance surveys and 3--5 samples per km 2 in follow up work to delimit the anomalies.

Results so far obtained, show that the method is applicable for weathering resistant minerals for example scheelite, wolframite, cassiterite and chromite preserved in the till, but sulphide mineralizations can also be indicated.

It should also be noted that this method has proved of great potential in prospecting surveys in areas chemically contaminated by human activity, such as industry and agriculture.

INTRODUCTION

Concen t ra t ion o f weather ing resistant heavy minerals by panning of alluvial sediments is p robab ly the oldest m e t h o d used by man in recovery and prospect ing o f ores. Dur ing the last decades, p rospec t ing based on concen t r a t ion and s tudy of heavy minerals has p robab ly been used mos t systematical ly in the Soviet Union, bu t has also been e m p l o y e d in the U.S.A., Canada and France (see, e.g., Par fenof f et al., 1970; Hu t ton , 1950) .

Exp lo ra t ion based on heavy minerals in clastic weather ing p roduc t s is usually carried ou t in s t ream beds for three obvious reasons:

(1) In s tream beds the resistant heavy minerals are in places pre-concent ra ted , thus making it possible to use a relatively small sample for ob ta in ing enough heavy minerals fo r fu r ther investigation.

(2) Water for wet concentration of heavy minerals is always available. (3) In stream beds the resistant heavy minerals are transported as bed load

and can be detected far from their source, a feature useful especially in regional prospecting.

PRELIMINARY STUDIES BASED ON ALLUVIAL SEDIMENTS

In 1967, one of the authors (N.H.B.) made a systematic study of the possibilities to use heavy minerals in stream sediments for prospecting purposes in central Sweden. About 1000 stream sediment samples from different mineralized areas were collected and the heavy mineral fraction was extracted with tetrabromoethane (TBE, density 2.96 g/cm 3 ). The heavy minerals were divided into magnetic, weakly magnetic and non-magnetic minerals, and these three fractions were separately investigated microscopically and chemically. The results were not encouraging as in most of the samples the heavy fraction contained a large proportion of precipitated limonite, disturbing the magnetic separation, making microscopic studies difficult and the result of chemical analyses unreliable. Further, it was often difficult to find sufficient amounts of fine-grained stream sediments to obtain adequate samples. This was especially the case in rapid streams and in swampy areas.

During the field season of 1969 the Geological Survey of Sweden (SGU) made an at tempt to use heavy minerals in stream sediments for prospecting purposes in northern Sweden. The experiences were more or less the same as those in central Sweden and, therefore, this type of prospecting has now been generally discarded.

INVESTIGATIONS BASED ON GLACIAL TILL

In Sweden the bedrock is extensively (up to 97%) covered by glacial drift, mainly till composed of unsorted material. The till consists partly of pre- glacial weathering products but mostly of material from solid rock, loosened and disintegrated by the action of the previous continental ice. When passing the outcrop of an orebody, the continental ice distributed the loosened parts of the ore, partly as boulders, often in the shape of a fan from the ore. The boulders can at times be transported far (tens of kilometres) from their source in the bedrock. These facts have made boulder tracing to a very important method for ore exploration, particularly in Fennoscandia (Hedstr5m, 1894; Sauramo, 1924; Grip, 1953). In the "boulder fans" smaller ore fragments and weathering-resistant ore grains are also present. Therefore, it was concluded that a heavy mineral fan, like a boulder fan, could serve as a first indication of mineralization in accordance with the assumptions of, e.g., Dreimanis (1957), Lee (1963) and Shilts (1971, 1973).

Based on the above, SGU carried out preliminary tests on glacial till in northern Sweden during the field season 1970. These tests were partially made in regions where scheelite mineralizations are known. Scheelite was chosen as

a representative heavy mineral since it is easily recognized in ultraviolet light. One-litre samples were taken from the C-horizon of the till at intervals of about 200 m along the roads in a particular region. The samples were panned by hand and, after removal of magnetite, the scheelite grains in the heavy mineral concentrates were counted under ultraviolet light. The average number of scheelite grains in the samples within the mineralized regions was 25 with peaks above 100 and 220 grains as a maximum. In other regions, considered barren as regards scheelite mineralization, the average number of counted scheelite grains was three, with nine as a maximum.

Thus, there is a large regional contrast between the two types of surveyed regions in the scheelite content of the till. The results also showed that it should have been possible to obtain indications of the presence of scheelite mineralization with much larger sampling distances than 200 m.

The results encouraged further methodological development. It was decided to develop a field concentration method for large samples which would be more suitable than manual panning. Larger samples were considered desirable in order to obtain sufficient amounts of concentrates for further investigation and also for decreasing sampling errors.

METHOD ADOPTED BY THE GEOLOGICAL SURVEY OF SWEDEN

The method for regional prospecting purposes, based on heavy minerals in glacial till will be shortly described.

Sampling and pre-concentration in the field

Ten-litre samples from the C-horizon of the till are collected mainly from road-cuttings and transported to places suitable for pre-concentration. Each sample is first wet-sieved on a 5-mm sieve and the coarse fraction discarded. The minus 5-mm fraction is collected in a 50-1itre tub filled with water, from where it is removed in water suspension by means of a suction nozzle moved along the bot tom of the tub. The suction is caused in the nozzle by an inversed water jet from a pump taking water from outside the tub. The material is transported from the tub to a sluice box in a flexible tube. The material remaining on the sluice box constitutes a pre-concentrate of heavy minerals weighing about 1500 g. The pre-concentration process also cleanse the grain surfaces and washes the clay particles off from the concentrate. The pre- concentrates are sent to the Geological Survey Laboratory for further treatment and investigation.

To obtain comparable results in the pre-concentration, it is important that the water flow and the inclination of the sluice box are kept reasonably constant in different runs. In this manner, from 30 to 50 pre-concentrates can be produced by two persons per working day.

FLOW SHEET 1 FOR HEAVY MINERAL PROSPECTING

SampL*ng

GLacial t~LL

IF S,evir~ 5ram . . . . . . ~ [ N a t ~ n a t , Smm~ Waste E _ _ ~L . . . . . . . ,

L ~:itmat<Smm D2

Wo ,] Suctio ~n" dredge - - ~rf_~W j Waste R , and sluice box K

Retained on L s l u i c e b o x

!

Drying

Sieving 0.5 mm . . . . .

)Haterial<OSmmJ Separation wi th ~ ]

tetrabromoethone (2.96g/crib) IHateriaL<296 Waste

I Material>2.961

L A _ Separation with B hand magnet O R

T o R . - - _ Separation wi th Y ..... ~ranz magnet 0.5 amp.

w ; O . . . . . . . . Separation with R Franz magnet 1 amp. K

_ _ Separation with Franz magnet 1.5 amp.

I

_~ Separation with ronz motet 2.0amp.

__ k fractions | [ maternal ~ l<O.5mm<Smm Kept for further

~ ~ l ~ investigat ions Subfroction No. 1 2 3 4 ~ 5

Chemica I. and mineralogical analysis

Fig. 1. Flow sheet 1, first applied by SGU in heavy mineral prospecting. The different stages in field and laboratory work are shown.

Laboratory work

Treatment ofpre-concentrates. At the laboratory, the pre-concentrates are dried and sieved on a 0.5-ram sieve and the minus 0.5-ram fraction is further treated. The reason for sieving on 0.5 mm is that the minus 0.5-ram till fraction has proved to consist mainly of monomineralic grains, which in the further treatments will give purer subfractions, making investigation of them easier and more reliable. The fraction with a grain size less than 0.5 mm is separated with TBE in two fractions. The fraction having a density less than 2.96 g/cm 3 (consisting mainly of quartz and feldspar) is discarded. The other fraction with a density of more than 2.96 g/cm 3 contains practically all ore minerals of economic interest and also many industrial minerals. After the separation with TBE, the heavy mineral fraction is divided into subfractions.

Two different ways of further sample preparation have been tried at large scale, according to the flow sheets shown in Figs. 1 and 2. These will be discussed separately below.

Flow sheet 1 (Fig. 1) is the flow sheet first used on about 2800 samples. The heavy fraction from the separation with TBE, after removal of magnetite, is separated in five subfractions according to the magnetic susceptibility of the different minerals. This is effected by five consecutive runs through a Frantz Isodynamic Separator specially modified at the Survey. Successively increasing currents of 0.5, 1.0, 1.5 and 2.0 amperes are used. The inclination forwards and sidewise is kept constant at 15 ° . Thus, besides magnetite, four weakly magnetic and one "non-magnetic" subfraction is obtained. The subfractions are numbered in Fig. 1 and Table I. The table also shows their average weights.

The six subfractions are investigated by X-ray fluorescence and optical emission spectrometry and partly with mineralogical analyses. For these investigations an amount of about 1 g is needed.

In practical work, flow sheet 1 has shown some serious disadvantages. Firstly, in too many cases the 1 g required for investigation was not obtained, which particularly applies to subfractions 5, 4 and 1. A second disadvantage is that weakly magnetic ore minerals, which stay in subfractions 2 and 3 with

TABLE I

Numbers of subfractions with average weights according to flow sheet I (Fig. 1 ).

Subfraction Average weights from No. 2800 samples (g)

1 isodynamic separation at 0.5 ampere 2.7 2 isodynamic separation at 1.0 ampere 8.7 3 isodynamic separation at 1.5 amperes 12.6 4 isodynamic separation at 2.0 amperes 3.3 5 "non-magnetic" residue 1.8 6 magnetite 5.9

FLOW SHEET 2 FOR HEAVY MINERAL PROSPECTING

~mp~ing

I Gtacial,tiLL ! ~_ 10 Liters

F Sieving 5mm I E L [Hat~-iat<5m-m~

w 0 Suction d r e d g e ~ OverflOw lWaste R i and sluice box -1 K~ ~

[ Retained on sluice box

J . . . . . . . . . . . . . . . . . . . . . . . E . . . . . .

Dryling

Sieving 0.Smm . . . . . . . .

'Hoteriat< 0.Smm

Separation with L j ~ Hoteriat<2.96j Waste tel robr omoet hone (2.¢~gcrrTj

L A B O R A T O R Y

W O R K

Separation with Fran~magnet 0.7 amp. J

~ - ~ f Non magnetic WeakLy mognetlc~ Wtognet=tel ] material I I material | I material

~ _ J ~_ _ >~1.31 _J ~_ >2.t6<3,31

fraction No. 5 z, 3 1 2

v

Chemic(][ and mineral.ogicnl, analysis

erio['2."l r

Separation with --- hand magnet

I

Weakly and non m~n~ic mtrl,.

Separation with |Wlakt~gndno~, [ Irnogn~ic rr~rt.~, ~magnet¢ mere. k | >3,31 J dimethyLiodide | >2.t6<3.31 ]

1 (3m/~31 1 i ~ Separation with

~-ranz magnet 1,3 amp

t Hoteriot 0.srnndSmm

Kept for further investigations

Fig. 2. Improved flow sheet 2, now appl ied by SGU in heavy mineral prospect ing. The dif ferent stages in field and l abora to ry work are shown.

amphiboles, pyroxenes and garnets, are so diluted by these minerals that their presence is difficult or impossible to detect . A third limiting factor is that isodynamic separation does not give such a distinct mineralogical split between the subfractions as separation based on heavy liquids does. It was,

therefore, felt that a reduction in the number of isodynamic separations had to be carried out in favour of another heavy liquid separation. A new flow sheet where the weaknesses of flow sheet 1 are reduced has been worked out.

Flow sheet 2 (Fig. 2) has been applied to 9500 samples collected in 1973--1975. It should be observed that this flow sheet is identical to flow sheet 1 up to, and including, the removal of magnetite. After this, however, the heavy material from the TBE separation is further separated according to density by the use of dimethyl-iodide (density 3.31 g/cm3). In this way, two heavy mineral fractions are obtained, one with a density of 2.96--3.31 g/cm 3 and the other with a density higher than 3.31 g/cm 3 . The former contains practically all amphiboles and the latter most of the ore minerals, with a result that the dilution of ore minerals will be considerably decreased.

Each of the density fractions is then separated on a Frantz Isodynamic Separator or by strong permanent magnets. For the lighter fraction (2.96-- 3.31 g/cm 3 ) the magnetic strength applied allows practically all amphiboles to enter a weakly magnetic subfraction, leaving a "non-magnetic" residue. This is obtained with 1.3 amperes on the Frantz Separator. For the heavier fraction (density > 3.31 g/cm 3 ) the magnetic strength is chosen so that most of the garnets and minerals with a higher magnetic susceptibility enter the weakly magnetic subfraction, thus leaving a residue mainly consisting of epidote and less magnetic minerals in a "non-magnetic" fraction. This is achieved with a current of 0.7 ampere on the separator. With the procedure described for flow sheet 2, five subfractions are obtained (Table II).

Using this flow sheet, experience has shown that the weight of any subfraction very rarely falls below the 1 g required for chemical and mineralogical investigations.

Experience has shown, that when applying flow sheet 2, some important

TABLE II

Numbers of subfractions with average weights according to flow sheet 2 (Fig. 2)

Subfraction Average weight from No. 3872 samples (g)

1 d = 2.96--3.31 g/cm3; isodynamic separation at 1.3 amperes 9.0

2 d = 2.96--3.31 g/cm3; "non-magnetic" residue 8.5

3 d > 3.31 g/cm 3 ; isodynamic separation at 0.7 ampere 3.8

4 d > 3.31 g/cm3; "non-magnetic" residue 9.4

5 magnetite 9.5

TABLE III

Typical heavy minerals in the different subfractions of flow sheet 2

Subfraction Heavy minerals No.

1 amphiboles, pyroxenes, biotite 2 apatite, fluorite, magnesite, tourmaline 3 garnets, ilmenite, chromite, hematite, wolframite 4 epidote, titanite, rutile, zircone, scheelite, baryte, cassiterite, monazite,

columbite, tantalite, pyrite, chalcopyrite, sphalerite, galena, molybdenite, gold

5 magnetite, pyrrhotite

minerals are collected in the different subfractions in the manner indicated in Table III.

Chemical and mineralogical analysis. At an early stage, it was decided to rely as much as possible on instrumental chemical analysis of the subfractions. The reasons for this decision were:

(1) The analytical capacity of the Central Laboratory was large enough for this increase in load wi thout additional equipment.

(2) The results are not so dependent on individual skill and thus more objective than those obtained by mineralogical analysis.

(3) The chemical analysis is quicker and cheaper than mineralogical analysis. (4) Chemical analysis gives some information not obtainable with mineral-

ogical investigations. Thus, the presence of abnormal contents of certain trace metals (Ni, Zn, Cu, Cr) in magnetite or amphiboles, only detectable by chemical analysis, might indicate mineralizations.

The chemical analysis is carried out by X-ray fluorescence spectrometry for the elements U, Y, Th, Zr, Zn, Rb, Nb and W {see, e.g., Leake and Aucott , 1973). By optical emission spectrometry in a quantometer , equipped with a tape machine, the following elements are determined: Cu, Pb, Zn, Mo, Mn, Fe, Ni, Mg, V, Ti, Cu, Ba, Sr, Ag, Bi, As, Sn, Be and Cr.

It is, however, not possible to rely entirely on chemical analyses. Three reasons are:

(1) Some elements, which are major constituents in interesting minerals, are not included in the routine analyses mentioned above. Examples of such elements are Au and Pt. Native gold and platinum must, therefore, be looked for under the microscope. Another example is F, present in both apatite and fluorite. Fluorite must, therefore, be determined by mineralogical analysis.

(2) In many cases, an ore metal is a consti tuent not only in ore minerals, but also in other minerals which are economically unimportant. If anomalous values for an ore metal are found, it is, therefore, necessary to verify the presence of ore minerals by mineralogical studies.

(3) Sometimes, an ore metal can be a const i tuent of two or several ore minerals. It is of importance to know which ore mineral, or minerals, are present and this can only be found ou t by mineralogical analysis.

R E P R O D U C I B I L I T Y OF R E S U L T S AND R E C O V E R Y

For an exploration method based on concentration of heavy minerals, it is of considerable importance to know the reproducibility of results and the recovery of heavy minerals. Of these two parameters, the reproducibility must be regarded as by far the most important. If just sufficient heavy minerals for further investigations are obtained, recovery is of less interest, when compared to reproducibility of results.

Reproducibility of results

For obtaining figures for the reproducibility of results, a series of tests was made with carefully mixed 100-1itre samples. In each test, ten runs according to flow sheet 1 were made with material from the mixed samples. Typical results from such a test with ten runs is given in Table IV.

TABLE IV

Statistical parameters for 10 runs with a composite sample using the sluice box and sample preparation according to flow sheet 1

Min. Max. Mean Stand. Coeff. of (g) (g) (g) dev ia t ion var ia t ion

(%)

Material r ecovered o n sluice b o x 1298 1398 1347 30.6 2.3

Material < 0.5 m m 477 581 520 22.3 4.3 Heavy m ~ e r a l s

> 2.96 g /cm 3 22.1 29.8 25.2 2.3 9.2 Magne t i t e 3.1 4.2 3.4 0 .30 8.8 Sub f r ac t i on

1 1.0 1.2 1.1 0 .03 2.8 2 2.1 2.4 2.3 0 .10 4.4 3 12.8 16.1 13.5 1.47 10.9 4 1.9 4.0 3.4 0 .57 16.8 5 1.2 1.9 1.5 0 .12 7.8

Experience has shown that the coefficients of variation are far lower in the test runs, than those obtained in statistical t reatment of results from samples taken at different sampling sites (cf. Table VI). Thus the reproducibility is considered acceptable.

Recovery of heavy minerals

With sluice box, like in all methods for concentrat ion of heavy minerals based on running water, there is an inevitable loss of heavy minerals. One type of loss depends on heavy minerals included in large polymineralic grains, mainly consisting of lighter minerals. This type of loss is disregarded here as further concentrat ion in the laboratory is made on the fraction less than 0.5 mm, mainly consisting of monomineralic grains. Another type of loss consists of small monomineralic grains of heavy minerals, which are kept in suspension of the moving water and discarded as overflow with water and lighter minerals. This type of loss decreases with increasing density of the heavy mineral.

In order to check the recovery of the sluice box, two test series were performed. The pre-concentrates from the sluice box were panned manually and the magl~etite contents of these concentrates were compared with the magnetite contents of those panned directly from the original material. The results showed, that the average recovery of magnetite from the panned sluice box pre-concentrates was about 70% of that obtained by panning of the original material. Of course recovery would be bet ter for minerals with higher densities.

A second series of tests on recovery was made. Here, 10 litres of till, after sieving on 5 mm, was run over the sluice box. The pre-concentrate as well as the material in the overflow, was collected separately. The material in overflow was rerun on the sluice box and a new pre-concentrate recovered. This was repeated until, after the four th run, no material remained. The contents of magnetite in the pre-concentrates from the different runs were determined and the results are given in Table V.

The figures indicate, that after repeated runs a recovery comparable with that of manual panning can be obtained. Because of the time factor involved, and because of the large amount of pre-concentrate obtained with three runs it was, however, found most practical to operate with only one run. Also a recovery of 70% of the magnetite from the original sample was achieved on the sluice box in the first run. An impor tant factor in the decision of using one run only was the good reproducibili ty of results demonstrated in Table IV.

T A B L E V

R e c o v e r y o f m a g n e t i t e f r o m a s a m p l e d u r i n g f o u r c o n s e c u t i v e r u n s

Ma te r i a l o n Mate r i a l M a g n e t i t e P e r c e n t o f s lu ice b o x < 0 .5 m m (g) t o t a l r e c o v e r y (g) (g)

1s t r u n 877 517 3.2 2 n d r u n 8 2 4 542 1.1 3 rd r u n 8 9 0 2 5 5 0 .3 4 t h r u n 10 2 0.1

70 24

6 0 .1

11

STATISTICAL PARAMETERS AND TREATMENT OF ANALYTICAL DATA

General parameters

Table VI presents the m a x i m u m , min imum and mean values, the s tandard devia t ion and the coef f i c ien t o f var ia t ion for the weight o f p re -concen t ra tes and subfract ions , ob ta ined in using f low sheet 1. In all, 2800 samples were included in the calculations.

A compar i son o f Table VI with Table IV, present ing the same paramete rs for r epea ted runs o f mixed mater ia l f rom one sampling site, shows tha t the s tandard deviat ion and coef f ic ien t of var ia t ion in Table VI far exceeds tha t in Table IV. This indicates tha t the natural var ia t ion is greater than the variat ion when using the m e t h o d for f rac t iona t ion o f the samples.

TABLE VI

Statistical parameters for different sample points (regional surveys) using the sluice box and sample preparation according to flow sheet 1

Min. Max. Mean Stand. Coeff. of (g) (g) (g) deviation variation

(~)

Material < 0.5 mm Heavy minerals

> 2.96 g/cm 3 Magnetite Sub fraction

1 2 3 4 5

120 1596 506 210.7 41.7

0 311 39 28.1 72.1 0.1 117 5.9 8.0 135.6

0.1 46.7 2.7 2.7 101.8 0.2 150.6 8.7 8.7 83.4 1.5 113.6 12.6 12.6 86.7 0.7 46.7 3.3 3.6 110.0 0.1 59.0 1.8 2.0 112.0

A cor responding compar i son has no t been made regarding f low sheet 2. Similar results should have been ob ta ined because the ope ra t ion p rocedures fo r f r ac t iona t ion of the original sample in the two f low sheets were the same, i.e., use o f sluice box , hand magnet , heavy liquids and Fran tz I sodynamic Separator .

Significance o f analytical data and their treatment

When chemical analysis o f the subf rac t ion is chosen as the main opera t ion for judging the possible exis tence o f minera l iza t ion in the n e i g h b o u r h o o d o f the sampling si te , it is necessary to keep in mind tha t an ore meta l can exist in two d i f fe ren t manners in the ana lyzed material .

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(1) As one of the main components in the chemical compounds which form an ore mineral, like Pb in galena (PbS), Sn in cassiterite (SnO2), W in scheelite (CaWO4), etc. Such ore minerals are generally missing or are very rare in normal samples. Their presence indicates mineralization in heavy mineral exploration.

(2) As a trace element, in small concentrations, partly replacing one of the main elements in the crystal lattice of common rock-forming minerals. To these common rock-forming minerals belong not only the main components of rocks, such as quartz, feldspars, amphiboles, pyroxenes, micas, garnets but also accessory minerals, such as magnetite, ilmenite, titanite, rutile, zircone, monazite. All these rock-forming minerals, except quartz and feldspar, enter the heavy concentrate. Examples of ore elements in heavy, rock-forming minerals are: Cu and Zn in amphiboles, pyroxenes and mica, Sn and W in mica, Sn in titanite, Cr in ilmenite and U in zircone.

In heavy mineral prospecting the content of ore elements in rock-forming minerals is generally not of the same importance as the presence of the ore minerals themselves.

It is not possible to judge from a total chemical analysis to what extent the concentration of an ore metal determined in a heavy subfraction comes from ore minerals or from heavy rock minerals. Therefore, it is necessary, that a flow sheet for separating the pre-concentrate in subfractions is designed in such a way that ore minerals and common rock minerals as far as [)ossible fall into different subfractions. In this respect, flow sheet 2 is better than flow sheet 1, because in the former the amphiboles are generally removed from the ore minerals by separation with dimethyl-iodide. A complete separation is of course not possible.

From the above it follows that the total content of a metal, determined by chemical analysis, Memt , is composed of two factors: (1) the content in common, rock-forming minerals, MeR ; (2) the content in ore minerals Meo. Of these factors, generally only Meo = Metot-Me R , is of interest for prospecting purposes.

Supposing, that the main mineralogical composition of a subfraction is reasonably constant in a large prospecting region, the median concentration from a great number of analyses can be regarded as a figure for Mez and thus the value for Meo can be calculated for any sample.

The obtained figures for the concentrations of Me o in subfractions are not directly comparable for different samples, because the weight of the subfractions vary considerably. In order to obtain comparable figures for the content of an ore metal in ore minerals, Me o has to be multiplied by the weight of the subfractions. Then comparable figures, expressed as the weight of an ore metal in the subfractions, are obtained.

APPLICATIONS OF THE METHOD

Investigations of heavy minerals in glacial till has given the SGU a new tool in exploration for new mineral deposits. Mainly, this tool is suitable to indicate

13

areas mineralized by valuable heavy, weathering-resistant ore minerals, such as scheelite, wolframite, cassiterite, chromite, tantalite, pyrochlor, magnetite, ilmenite, hematite, platinum, gold, diamond, fluorite, apatite and barite.

It is important to note that in areas of heavy coverage of the bedrock by glacial till, mineralized occurrences of the above, excluding magnetite and ilmenite, are difficult or impossible to indicate by geophysical methods. Further, boulders containing these minerals are much less noticeable in the field than those containing base metal minerals.

Of special importance for Sweden, with its highly developed alloyed steel industry, is that heavy mineral exploration is very effective in indication of ores containing the important ferro-alloy metals W, Cr, Nb and Ta. The Swedish steel industry is now practically totally depending upon import to cover its needs of ferro-alloy metals and thus very sensitive for shortages and increases in price. Therefore, a domestic production of raw materials for production of ferro-alloy metals should be highly valued.

To what extent sulphide mineralization can be indicated with the sampling depth now used, is not yet quite clear. Earlier work has shown that sulphide mineralization (Pb, Zn, Cu, Mo) is indicated in the heavy minerals of the till (Brundin, 1969). These results are not conclusive as they were based on samples from much larger depths, where chances for sulphide minerals to survive weathering are larger. Certain anomalies of sulphide ore metals have been obtained in SGU's current work, but they have not yet been followed up and, therefore, definite conclusions will not be drawn here. The advantage of sampling the till at depth has been demonstrated from Canada by Gleeson and Hornbrook (1974).

Reconnaissance surveys

The heavy mineral exploration based on glacial till is, and will be applied with different sampling densities depending upon the purpose of the prospecting campaign.

A low sampling density (10 samples per 100 km: ) is used for a land-wide exploration for new ore fields which, regardless of known geological features, is intended to cover the Precambrian shield of Sweden in about 15 years. Isolated orebodies can very well be missed with this low sampling density, but much valu-ble information regarding mineralization in geologically little known regions will be obtained.

The use of 10-1itre samples creates a problem in the transportation of samples from sampling sites to a suitable place for pre-concentration. There- fore, as a first step, sampling is based on the existing road net in which samples are collected by car-borne teams (two persons) with intervals of about 3 km, preferably from road-cuttings where the till is easily obtainable. When 30--50 samples have been collected, they are transported to a suitable place for pre- concentration. In regions with a well-developed road net, the sample sites are selected in accordance with the required sampling density.

14

A sampling team can either collect or pre-concentrate 30--50 samples per day. An average production of 15 samples per day and team, corresponding to 150 km 2 can be calculated with. If there is a less developed road net which is the case especially in northern Sweden, the required sampling density cannot be reached by the use of cars. In these regions other vehicles such as boats, amphibia vehicles and helicopters have been used in sampling and transportation.

During the years 1971--1975 altogether 300,000 km 2 have been sampled, and several anomalies have so far been found.

Regional follow up surveys

Medium sampling density (1--2 samples per km 2) is chosen in the following cases:

(1) As a second step in checking anomalies received from reconnaissance surveys, in order to study their infrastructure and draw conclusions regarding individual mineralizations.

(2) As the first step for exploration in regions, where mineralization is known to exist, either as outcrops or as boulders.

(3) As a first step in the investigation of regions with specific rock types which can be suspected to carry mineralizations. Of special interest is the investigation of granite areas, which may have generated mineralizations of cassiterite and/or molybdenite and tungsten minerals. This type of investigation can replace or complement bedrock geochemistry with the same purpose.

ADVANTAGES AND LIMITATIONS OF THE METHOD

The greatest advantage of heavy mineral prospecting is that it makes it possible to indicate mineralization of certain weathering-resistant ore and industrial minerals, which gives no, or only weak, response when geophysical and/or other geochemical methods are applied.

Regarding minerals more easily decomposed by weathering than those mentioned above, the possibilities of heavy mineral exploration are more restricted. Examples of easily weathered ore minerals are the sulphides of the base metals, molybdenite and uraninite. It is believed that most of the original grains from such ore minerals have disappeared and that, therefore, the anomalies caused by them are weakand local, although some of these minerals have been found in glacial till in the present study. In mineralization containing these minerals, there are, however, often other more resistant minerals, which can cause anomalies of larger extension. These weathering- resistant minerals can be secondary minerals formed from the primary ore minerals, such as cerussite and anglesite from galena and smithsonite from sphalerite. They can also be weathering-resistant accessory primary minerals containing the ore metal. An example is Zn-spinel, gahnite, which often is present in Zn ores.

A considerable advantage of heavy mineral techniques based on investigation

15

of glacial till (C-horizon) in prospecting compared to conventional geochemical exploration is that it is totally independent of contamination caused by mining industries and other human activity. This is of special importance in areas like central Sweden where mining and smelting have continuously been carried out since the 12th century.

Experience has shown, that heavy mineral anomalies in the till generally have very large extension, often several square kilometres, and that mineralization, therefore, can be indicated with a very low sampling density. This compensates for the comparatively high cost for t reatment and analysis of samples in the laboratory and makes it possible to cover large areas at a low cost. Thus, regional exploration can be carried out without regard to known geological features, making possible discoveries of ores in unexpected areas and of unexpected types. This is important in Sweden, where outcropping bedrock is rare, thus making detailed geological mapping difficult, and sometimes impossible at a reasonable cost.

Also it is important that in areas, anomalous with regard to an ore mineral, the study of other heavy minerals can give indications of the paragenesis of the mineralization causing the anomaly. Thus conclusions can be drawn regarding the ore type to look for in further exploration.

In some cases, the presence of a valuable mineral can be revealed by identification of another mineral in paragenetic, but not chemical, relation- ship with the valuable mineral. An example of this is the garnet pyrope, which indicates the presence of kimberlite, which rock might contain diamonds. To look for diamond itself in normal heavy mineral concentrates is next to meaningless because of the scarcity of this mineral even in rich deposits.

CONCLUSIONS

It has been found, that in large parts of Sweden, glacial till is a bet ter sampling medium than stream sediments in regional heavy mineral prospecting, because the latter is not always available and is often contaminated with limonite.

Heavy mineral studies are a very valuable complement to geophysical and geochemical methods in regional exploration for ores and deposits of industrial minerals.

It has been found suitable to produce a pre-concentrate in the field which is further concentrated in the laboratory and split in several subfractions based on differences in the physical properties of the minerals (density and magnetic properties). The subfractions are investigated by chemical, as well as mineralogical analysis, and the application of these two methods should be balanced so that the highest possible efficiency is reached at the lowest possible cost.

The total cost per sample will be relatively high, but this will be compensated for by the fact that a low sampling density can be used in reconnaissance exploration, because of the large extension of the anomalies. The cost per unit area, therefore, will still be very low compared to most other regional exploration methods.

16

Previously the SGU started a nationwide prospecting campaign based on organic stream sediment samples (Brundin and Nairis, 1972) and has included since 1971 a similar heavy mineral exploration programme. This is based on samples of glacial till collected preferably from road-cuttings at intervals of 1--3 km. The anomalies found are followed up in a first step by closer sampling along roads and extension of the sampling into the terrain surrounding the road anomalies.

CASE HISTORY

The Ostans]5 scheelite region

Different types of anomalies could be delineated as resulting from the first field seasons in 1971--1972, when a large-scale and systematic heavy mineral prospecting campaign was carried out in an area of 300,000 km: . Several unexpected mineralizations were indicated, among them a region with tungsten (scheelite) in northern Sweden that encouraged to further prospecting efforts.

• " c~o. "g~ "

:7' ]

, ; M : H j

/ f2"

, ~ i H E A V Y MINERAL M A P ', L

! 5" LEGEND i i 5cheelibl content bolld

on groin counting Contour i n t ~ L 1_ ] • 2_J

• ~0 Sampling point with nurnbo" of scheeiite grains

\ ~ Dwt~ct~ of glac~ transpoet

10 201~

Fig. 3. Results from scheelite grain (20--500 urn) counting in a reconnaissance survey. Figures are given only for anomalous sampling sites.

17

HEAVY HINERAL HAP

LEGEND

W conterlt I :~ud on c h e w ~ onotysc.s Contour intervoJ.s

[ t [ I I f, 10-6g 910 1300 2WO Y/O0 /.'/00

. ~ Som~ing po~t with tungsten content

~ Direct~ of glnc~ tronspect

0 10 ZlOkm

Fig. 4. Tungsten content (X-ray fluorescence analysis) of the 20--500-pm fraction in the same reconnaissance survey as shown in Fig. 3. Figures are given only for anomalous sampling sites.

0 8kin

. •

x-,. N • <

Fig. 5. (a) Anomaly from lower right corner of Fig. 3, based on scheelite grain counting. (b) Anomaly from lower right corner of Fig. 4, based on X-ray fluorescence analysis.

18

Figs. 3 and 4 show part of the area sampled in 1971. In the legend of Fig. 3 the figures indicate the number of scheelite grains in subfraction 5 according to f low sheet 1 (Fig. 1). Fig. 4 shows the same region where the figures in the legend indicate the chemically (X-ray fluorescence spectrometry) determined tungsten content in the same subfraction. The content of tungsten was calculated by multiplying the weight of the fraction by the analyzed tungsten concentration after subtracting a background value which was put at the 50th percentile (median value) of the concentration value of a large number of samples (5000).

A comparison of the two maps shows the striking similarities between the configuration and the position of the anomalies, despite the fact that one is based on mineralogical, and the other on chemical analyses. In the lower right corner of the maps the most important scheelite anomaly is situated. In Fig. 5a and b this anomaly is shown in detail. It can be concluded that the sample points with the highest figures from scheelite grain counting coincide with the highest content of tungsten according to the chemical analyses.

The anomaly was further investigated with follow-up work by decreasing the distance between the sampling points along the roads and also including some smaller roads. This second step was also combined with sampling be-

N

~307'I ',

,: ,'LL ,

L~I-- •" - HEAVY MINERAL MAP I l l3 \ ,/

LEGE o • ~ ,rL_V.._. ~co°,°o, ~ 0 oo

1300 L300 6700 x lO- g

~ " ~ 5171 Sompt)n£ Ix~nt with

155252 \ ~ \ "~ ~ \\p

~ transport

2119 • ScheeLite mineroLiz ed • boulders

0 2 C 6 8 km " 3oo9 ; /~

Fig. 6. Very high tungsten content s obta ined in a f o l l o w up work on a n o ma l y s h o w n in Fig. 5a, b. F inds o f boulders conta in ing sob ,e l i t e are also indicated.

19

tween the roads in order to delimit the anomaly, which resulted in the dispersion of the original anomaly into several smaller ones (Fig. 6).

As this anomaly was prominent in relation to other tungsten anomalies, it was decided to carry out boulder hunting in the area. It could be established that the scheelite was bound to quartz veins as fissure fillings in a medium- grained granodiorite. So far, two boulder fans have been located, one with approximately 200 boulders and the other with around ten (Fig. 6). From the boulder hunting, it has been concluded that the scheelite content increases with the increased tectonization of the bedrock. Further investigations of the area will include geophysics, and the sampling of till and bedrock by percussion drilling in order to delimit the mineralization in the bedrock.

ACKNOWLEDGEMENTS

The methodological and scientific development of this new heavy mineral prospecting technique was initiated by Dr. N.H. Brundin and it has been financed by the Geological Survey of Sweden. During the work Dr. C.-A. Nilsson, head of the Geochemical Prospecting Section, to whom we are greatly indebted, has supported the survey with enthusiasm and also critically read the manuscript. We also want to thank the colleagues at the Survey involved in this project who have been of great help to us.

REFERENCES

Brundin, N.H., 1969. Some experiences in geochemical and heavy mineral prospecting. Q. Colo. Sch. Mines, 64: 89--94.

Brundin, N.H. and Nairis, B., 1972. Alternative sample types in regional geochemical prospecting. J. Geochem. Explor., 1: 7--46.

Dreimanis, A., 1957. Heavy mineral studies in tills of Ontario and adjacent areas. J. Sediment. Petrol., 27: 148--161.

Gleeson, C.F. and Hornbrook, E.H.W., 1974. Semi-regional geochemical studies demonstrating the effectiveness of till sampling at depth. In: J.L. Elliott and W.K. Fletcher (Editors), Geochemical Exploration 1974. Elsevier, Amsterdam, pp. 611--630.

Grip, E., 1953. Tracing glacial boulders as an aid to ore prospecting in Sweden. Econ. Geol., 48: 715--720.

Hedstr6m, H., 1894. Studier 6ver bergarter fr~n mor~'n vid Visby. Geol. F6r. FSrh., 16: 247--274.

Hutton, O., 1950. Studies of heavy detrital minerals. Bull. Geol. Soc. Am., 61: 635--710. Leake, R.C. and Aucott, J.W., 1973. Geochemical mapping and prospecting by use of rapid

automatic X-ray fluorescence analysis of panned concentrates. In: M.G. Jones (Editor), Geochemical Exploration 1972. Institution of Mining and Metallurgy, London, pp. 389--400.

Lee, H., 1963. Glacial fans in till from the Kirkland Lake fault: a method of gold exploration. Geol. Survey Can. Paper, 63--45.

Parfenoff, A., Pomerol, C. and Tourenq, J., 1970. Les Min~raux en Grains. Masson, Paris, 578 pp.

Sauramo, M., 1924. Tracing of glacial boulders and its application in prospecting. Bull. Comm. Geol. Finl., 67: 1--37.

Shilts, W.W., 1971. Till studies and their application to regional drift prospecting. Can. Min. J., 92: 45--50.

Shilts, W.W., 1973. Glacial dispersal of rocks, minerals and trace elements in Wisconsinan till, southeastern Quebec, Canada. Geol. Soc. Am., Mere., 136: 189--219.

Documents

Regional prospecting for ores based on heavy minerals in glacial till