m British Geological Survey

TECHNICAL REPORT WG/94/7 Mineralogy and Petrology Series

CHARACTERISATION OF GOLD FROM MERSING, JOHORE, MALAYSIA M T Styles, D J Bland, and P D Wetton

A Report prepared for the Overseas Development Administration under the ODA/BGS Technology Development and Research Programme, Project 9Ul

ODA Classification Subsector: Others Subject: Geascience Theme: M i n d Resources Project Title: Alluvial Gold clmaderisation in Exploration planning Reference numb-: R5449

BibIiognrphic refrence: M T Styles, D J Bland, and P D Wctton Characterisation of gold from Mersing, Johore, Malaysia BGS Technical Report WG194ff

Keywordr: Gold, mineralogy, electron microprobe. Malasysia

Front cover illustration: Collecting alluvial gold samples at Telok Bangka, Musing, Malaysia

0 NERC 1994

Keyworth, Nottingham, British Geological Survey, 1994

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TABLE OF CONTENTS

EXECUTIVE SUMMARY

List of figures

List of tables

1. INTRODUCTION

2. SAMPLE COLLECTION

3. SIZE AND SHAPE STUDIES

3.1. SEM Characterisation 3.1.1. Samples MA18-20 3.1.2. Sample MA22

3.2. Size Analysis 3.2.1. Samples MA18-20 3.2.2. Sample MA22

4. ELECTRON MICROPROBE STUDIES OF ALLUVIAL GOLD

4.1. Quantitative electron microprobe analysis of gold grains.

4.2. Electron microprobe analysis of inclusions in gold grains

5. CONCLUSIONS

6. ACKNOWLEDGEMENTS

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EXECUTIVE SUMMARY

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This study of gold from the Mersing area of Johore, forms part of a wider study of c oold from Malaysia and elsewhere, carried out by the ODNBGS Technology Development and Research(TDR) project 'Alluvial gold characterisation in exploration planning' (Project 92/1 ,RS549). At Mersing gold is found in quartz veins in the Permo-Carboniferous shale sequence, a type of mineralisation found in several other parts of Malaysia. Particular to this area, gold is also reported from palaeo- placer deposits in Jurassic conglomerates that overly the sequence of shales, a unique occurrence in Malaysia.

Samples were collected from several localities but it was not possible to obtain gold in bedrock, only from alluvial deposits.

Lahoratory studies were carried out to characterise the gold and comparisons were made with gold from other areas studied.

Studies of the s i x and shape of gold grains showed increased deformation and abrasion with increased distance from the likely source. There was some evidence to sugzest that thcre was more than one original size population of grains, one "coarse", greater than 0. lmm and the other fine. This is not a large difference and it is possible that they could all be derived from a single source.

Studies of the composition of gold grains showed that two populations were present. Most had "high" silver, 8- 1Swt% (920-850 fineness), while a few had low silver, less than 2wt%- (980 fineness). These two groups do not match the different size groups.

Studies of microscopic inclusions inside the gold showed that there are possihly three different sources for the gold studied:

1. A mesothermal, sediment-hosted, quartz-vein type mineralisation. This is the dominant type and has been found in several other parts of Malaysia.

2. A minor component with a mafichltramafic rock association.

3 . A minor component of red-bed type, unconformity-related mineralisation. This could be from the conglomerates and may be the same as that previously reported as palaeo-placer type.

The likelihood of multiple sources for the alluvial gold in the area means that exploration programmes must take account of this to maximise the chances of locating the parent ore bodies.

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List of Figures.

Figure 1. Simplified geological map of peninsular Malaysia showing the distribution of primary gold occurrences and the Mersing study area(1ocation 5).

Figure 2. Field sampling localities.

Figure 3. Secondary electron micrographs of gold grains.

Figure 4. Reflected light photomicrographs of inclusions in gold grains.

List of Tables.

Table 1. Size measurements for samples MA18-20.

Table 2. Size measurements for sample MA22.

Table 3. Electron microprobe analyses of gold grains from samples MA18-20.

Table 4. Electron microprobe analyses of gold grains from sample MA22.

Table 5. Electron microprobe analyses of inclusions in alluvial gold grains.

Table 6. Atomic proportions and identifications of inclusions in alluvial gold grains.

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CHARACTERISATION OF GOLD FROM MERSING, JOHORE, MALAYSIA.

1. INTRODUCTION.

This report contains the results of work carried out on the ODA/BGS Technology Development and Research (TDR) project 92NR5549, entitled 'Alluvial Gold Characterisation in Exploration Planning'. This study involves collection and laboratory study of gold from various developing countries. The aim of the project is to characterise gold grains from bedrock and associated alluvial deposits to establish whether variation in bedrock gold is inherited by the alluvial gold. Further, if this is the case features found in alluvial gold might be used to identify possible source terrains. This information can be used to improve the effectiveness of gold exploration by targeting certain rock types or geological settings at the planning stage of detailed exploration programmes. This characterisation covers the study of morphological features such as size and shape, chemical composition and mineral inclusions within the gold.

Gold is being studied from five areas in Malaysia, where the gold is hosted in mixed sequences of shales, volcanic and volcaniclastic rocks and limestones, but in the Mersing area some of it may be derived from palaeo-placer deposits. This had been reported by Aw Peck Chin and Loh Chiok Hoong(1989), geologists working for the Geological Survey of Malaysia (GSM) and it was thought this would provide an interesting contrast with the other areas.

Similar studies are being made in Ecuador (Styles et al. 1992), Zimbabwe and Fiji.

2. SAMPLE COLLECTION.

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Mersing is located on the southeast coast of Peninsular Malaysia, in Johore state approximately 70 km north of Singapore, Figure 1. Gold has been reported in two distinct settings in the area, quartz veins in the Permo-Carboniferous shale sequence, similar to other localities studied in Malaysia and supposed fossil placer deposits in the overlying Jurassic conglomerates. Samples were collected from five sites, four in the shale sequence and one from the conglomerates. Gold had previously been reported from most sites during GSM geochemical surveys. Field sampling was carried out with Mr A G Gunn of the BGS/GSM gold project and geologists from GSM Kuantan office.

Kuala Mayang ( Sample number MA 18) was an alluvideluvial gold mine that had recently closed (Figure 2a). Ore had been extracted from surface workings on the hillsides and alluvium in the valley bottom. The exposed rocks are pyritiferous shales and fine sandstones cut by a complex system of quartz veins. There is abundant iron oxide/hydroxide much probably from the alteration of pyrite. Gold could not be found in bedrock and a sample from the mine tailings was taken.

Material from Air Merah (MA 19) and Telok Bangka (MA 20) comprised alluvial samples from small streams flowing through small holdings and plantations, where gold had previously been found in GSM regional surveys (shown on front cover).

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The island of Pulau Belanak is located a short distance offshore and is accessible at low tide. It is the best locality for the Jurassic conglomerates, and the sequence consists of very coarse conglomerates interbedded with siltstones and sandstones. The conglomerates are very poorly sorted and crudely bedded and contain angular fragments up to 0.5m in size, though 5-20cm is more typical. They are cut by ferruginous veins and layers that are of replacement origin. Gold is reported from these rocks and a fossil placer origin has been suggested. The fragments in the conglomerate are essentially the same as the nearby rocks of Permo-Carboniferous age and could easily be formed by local erosion and reworkmg. The grain size seemed very coarse for placer deposits to have formed. We found no visible gold but panned the detritus from crevices between the outcrops (MA21) (Figure 2b). Samples of panned beach sand, from the crevices between the Permo-Carboniferous strata, were also collected a couple of km to the north,(MA22) and gold was recovered. We also hoped to get samples from the original survey, from staff who had done the work and are still at GSM.

Gold assay of bedrock samples, did not show the presence of gold and attempts to locate material from previous GSM work proved unsuccessful. The samples collected in this area were thus largely alluvial samples and not suitable to fulfil all the aims of the project in the absence of bedrock gold. Neverthleess, they were characterised for comparison with alluvial gold from other areas and to see if any information about their origin could be deduced.

3. SIZE AND SHAPE STUDIES

3.1 SEM Characterisation.

Samples of alluvial gold from all localities were examined by making measurements of the size and shape of the grains and studying their chemical composition and included minerals. The samples were hand-picked under a binocular microscope to separate the gold from other heavy minerals present in the concentrate. The grains were placed on double sided sellotape, mounted on a glass slide and the slide then carbon coated. The first stage of examination, by scanning electron microscopy (SEM), was then carried out. The grains present in each sample were examined by SEM to provide a ‘visual’ morphological description. These observations are presented in a summary below to give a typical description of grain shape and surface features for each sample location.

3.1.1. Samples MA18, MA1 9, MA20.

The gold from these samples showed a moderate level of deformation and abrasion in the morphologies present. The grains retain an overall irregular outline, modified by folding and rounding of their edges. Figure 3a shows a grain typical of these samples.

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The vertices of the grains showed deformation in all cases, usually visible as flattening and smearing of the protruding section against the main body of the grain. The surface of the gold was irregular with pockets of undeformed surface present in depressions. The areas protected within these depressions showed a porous or sponge-

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like surface at high magnification which is likely to be representative of the original surface texture. Progressing through the samples in number order the level of deformation observed generally increased and scratching of the grain surface became noticeable at higher magnifications. This is consistent with the grains being progressively further from the source and deposited in the beds of minor streams.

3.1.2. Sample MA22.

The grains present in this sample displayed a higher level of deformation than in the preceding samples. The outline morphology of the grains is more rounded with most of the irregularities abraded away or folded into the main body of the grain. Many grains showed an elongated, lozenge type form in which their surfaces appear smoothed by abrasion and folding. The surface of the gold showed heavy abrasion and scratching visible at relatively low magnifications. An example of the gold from this sample is shown in figure 3b. This grain shown is one of the more rounded examples present in this sample and the heavy abrasion of its edges and surface are clearly visible and the resulting elimination of the original surface structure is evident. These observations are consistent with gold found in beach deposits where there would be continual working of the grains and differ from the less modified morphology of the gold from a stream placer deposits, described above, where there would be little reworking after initial transport.

3.2 Size analysis.

Measurement of the size characteristics of the samples was performed using a petrological microscope coupled to a video camera to capture images for input to the image analyser. The samples were mounted on microscope slides and the microscope operated in transmitted light mode to provide silhouettes of the grains for image analysis purposes. The grains were mounted such that their two major axes lay in the image plane of the microscope so as to present the largest possible cross-section to the analyser. This was done to maximise the validity of data obtained by a 2-dimensional technique operating upon 3-dimensional subjects. The image analysis routine developed features individual grain identification allowing size and shape data to be cross-referenced to results obtained by EPMA. It is then possible to explore correlations between sample populations suggested by chemical and morphological techniques.

The size characterisation was made by measuring area, DMAX, DMIN, perimeter, FCIRCLE and FSHAPE for each grain in the sample. These are segregated into primary measurements which are taken directly from the grain and derived measurements, calculated from the primary measurements.

DMAX and DMIN are the maximum and minimum diameters of the grain respectively, automatically selected from 32 measurements of the grain diameter made at an angular resolution of 5.7'. FSHAPE is a simple aspect ratio for the grain arrived at by division of the minimum diameter by the maximum diameter of the grain. Objects with low values of FSHAPE have elongate shapes whilst high values signify objects which are equant. The function FCIRCLE is a measure of the circularity of the grain, defined by the equation

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FSHAPE= 47tArea Perimeter2

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where a circle returns the value 1. In addition to indicating the circularity of the object, FCIRCLE is also a function of the edge roughness. This plotted against FSHAPE provides a basic morphological characterisation. For example consider a set of grains where the aspect ratio (FSHAPE) remains approximately constant but the value of FCIRCLE decreases. This behaviour represents a population where the general shape of the grains remains the same but the irregularity of their perimeters is increasing.

Analysis of the measurements was made by plotting the data sets in the form of s- curves, constructed by plotting the data in ascending value order for each of the samples. When plotted in this manner different populations have a tendency to plot as lines of differing slope. e 3.2.1. Samples MA18-20.

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The area data for these samples, (Table l) , shows grains with an observed size range of 0.009-0.382mm2 and an average grain size of 0.089mm2. The size ranges for individual samples vary considerably. Sample MA 19 consists of predominantly small grains ranging from 0.030-0.100mm~, average area 0.041mm2. Sample MA18 is dominated by grains of larger areas, size range 0.045-0.382mm2 and average size 0.11 2mm2, whilst sample MA20 contains grains which are spread across these two ranges with an average grain size of 0.074mm2. Comparison of the s-curves for the primary data sets show that three populations may be present. Grains from each of the samples are spread across these populations indicating similar sizes and morphologies present at all three sample sites. The populations seen on each curve are composed of the same groups of grains but grains appear in differing positions within the groups. This possibly shows that there are several size populations within the source.

The circularity values (FCIRCLE) have a range from 0.125-0.759 showing the overall grain morphology tending towards elongate. The range in aspect ratio values from 0.389-0.830 and shows the grains vary from elongate to near equant. The plot of FSHAPE/FCIRCLE shows most of the grains to be of irregular outline with elongate shapes. There are exceptions to this, shown by the two points which lie outside of the main point grouping of the plot. These grains, MA18.12 and MA18.14, have high aspect ratios but low circularity values. This indicates grains with irregular perimeters which have a regular morphology, tending towards a rectangular shape. The secondary data s-curves show only tentative population groups with very minor inflections present on the curves. These populations are composed of differing grains for the two plots obtained and show no correlation with the groupings seen in the primary data curves and are not included in this report.

Sample MA22.

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The data for gold grains from this sample (Table 2), show that they are on average smaller than those from MA 18-20 described above. The area data shows a size range of 0.018-0.219mm2 with a mean grain size of 0.073mm2 split into two probable

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populations. The majority of the grains lie in a small size grouping with an area range of 0.018-0.081mm2 and form the first population. The larger grains have areas in the range 0.103-0.219mm2 and form the second population. This trend is carried through in the remaining primary data s-curves though the perimeter data s-curve(not shown here) does not show the same magnitude of inflection seen on the other plots.

The s-curves generated from the derived parameters showed differing trends both to the primary data curves and to one another. The FCIRCLE data shows a clear inflection and probable two populations whilst the FSHAPE data shows an even curve with only minor steps present. When plotted together these parameters show two grain populations within which there is considerable variation in circularity of the grains but much less variation in the aspect ratio, FSHAPE. The first population, roughly half of the grains, have values for FCIRCLE and FSHAPE between 0.4 and 0.65. This population lies in the region of the plot where grains feature moderately rough or irregular edges on grains which are tending towards elongate proportions. The second population comprises of grains with high values of FCIRCLE and of FSHAPE which reflects an overall more circular or equant and smooth morphology, these are generally the larger grains.

4. ELECTRON MICROPROBE STUDIES OF GOLD

4.1. Quantitative electron microprobe analysis of gold grains.

All the gold grains were mounted in epoxy resin on glass slides and then ground down to reveal a cross section through the grain and the surface polished for further study. The polished grain mounts of gold were examined on a reflected light petrological microscope and then analysed using a fully automated computer- controlled Cameca SX50 electron microprobe. Quantitative analyses have been made on all gold grains and included minerals using the wavelength dispersive system under operating conditions of 30 kV high voltage and 20 nA beam current. Sixteen elements were analysed for each point. The in-house standards used were mostly pure metals apart from, pyrite for S, arsenopyrite for As, HgTe for Hg and Te and PbF2 for Pb. An analysis was made in the core of each grain and, if a rim or other feature such as a white patch could be distinguished optically once the grain had been carbon coated, this was also analysed. Inclusions of sulphide, selenide etc. were also analysed. Silicate inclusions, principally quartz but also orthoclase and rutile, were noticed and the identity checked by qualitative analysis but these inclusions were not quantitatively analysed.

The results for samples MA18-20 have been grouped together as in the shape study, and the results are given in Table 3. The only constituent of any magnitude apart from gold is silver and the variation of silver is plotted as an "S" curve. This shows that apart from two very low values they plot as a single line with silver contents ranging from 6-14% but with no marked inflection points. This suggests that they could all be part of a single population, and as there is a range of values from each locality, this shows that the range is not a secondary effect due to transport.

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The results for MA 22 are given in Table 4. and are broadly similar to the previous samples with the majority of grains falling in a single population with between 9- 15% Ag. The analyses with very low Ag values from both samples are mostly from the rims of grains and thin "tracks" of "pure" gold along a subgrain boundary. This is a phenomenon that has been found in many localities and is thought to be due to the later deposition or redeposition of pure secondary gold possibly associated with the leaching of primary silver-rich gold. The evidence from the bulk composition of all the gold grains from all sites does not show any other clear separate groupings and suggests that apart from the low-Ag gold they all could be part of one population and could all have a similar source. These two populations identified on the basis of silver content do not match those identified on the basis of size and shape.

Most grains contain low levels of mercury and there are other very low concentrations of other elements recorded. The latter being very close to the theoretical limits of detection, should be treated with great caution and little significance attached to them. A more detailed study of the trace elements in gold has been made by laser-ablation ICPMS, where detection limits close to lppm are achieved. This will be the subject of a separate report.

4.2. Electron microprobe analysis of inclusions in gold grains.

Inclusions of other minerals, mostly sulphides, have been found in gold grains from all sites and examples are shown in Figure 4. The inclusions are often very small, only a few microns in size, which makes quantitative microanalysis difficult as the electron beam tends to excite X-rays from the surrounding gold as well as the inclusion. The actual analyses are shown in Table 5 and the recalculated atomic proportions, corrected for gold from the host are given in Table 6 along with an identification of the mineral phase. In general it is desirable to have at least 20 grains from each site to give a reasonable number of inclusions to give an idea of the proportions as well as range of types of inclusions. This was not achieved in this case and hence interpretation must be treated with caution.

The inclusions from MA 18-20 are mostly base metal sulphides and sulpharsenides, pyrite, arsenopyrite and galena, an assemblage that has been seen in the other gold deposits from Malaysia and other parts of the world. It is generally associated with sediment hosted mesothermal quartz veins but is not particularly diagnostic of a particular type of deposit. Sample MA 19 also contains the mineral tiemannite, HgSe, and a complex selenide that is dominantly Hg but also contains Ag and Bi. The latter inclusions can clearly be seen to be multiphase, Figure 4a, but they are so small, only a micron or so in size, that analyses cannot be made of the individual components. The Ag could be contained in the mineral naumannite AgSe and Bi in selenobismutite, Bi2Se3 but it is also possible that they are mixtures of the various metals in complex selenides. These are rare and unusual minerals. Most similar inclusions have been found in our previous studies of gold in southwest England, (Leake et al 19931, where tiemannite is associated with a whole suite of selenide minerals. This particular association of gold with with selenides is interpreted as deposition from oxidising solutions outside the stability field of sulphides connected with the Permian erosion surface, a type of "red-bed I t mineralisation. It is interesting

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that at Mersing there are also red bed sediments above an unconformity and a similar origin of the gold is possible. The gold grain containing the selenides is that with the lowest Ag, 0.6596, which also suggests that it belongs to a different mineralisation event to the majority of gold grains. This suggests that the gold with sulphides is not from the same type of mineralisation as the gold with selenides. It is possible that the palaeo-placer type of mineralisation previously recorded could be in-situ red bed mineralisation but as we could not obtain samples of this type this cannot be checked. The gold from MA 19 may have a mixed source, both red bed and mesothermal quartz veins.

The inclusions in MA 22 are also pyrite and arsenopyrite, with possibly altered arsenopyrite, similar to the other sites. Galena is absent but chalcopyrite and "pentlandite" are present. The "pentlandite" is rich in cobalt, similar to nickel and cobalt minerals found as inclusions in gold in north Wales and Ireland (Leake et al 1993) where a connection between gold mineralisation and basic or ultrabasic rocks is likely. This suggests that these grains from Mersing might have a similar association. This is type of gold mineralisation that has not been recognised in the other locations in Malaysia.

The study of inclusions suggests gold from three different styles of mineralisation 1) a typical mesothermal vein type with base metal sulphides, 2) a mafichltramafic type and 3) a red bed type of mineralisation. This multiplicity of types was not obvious from studies of size and shape.

5. CONCLUSIONS

1. The study of gold from the Mersing area was a small part of the wider study of gold from Malaysia. Various samples were collected but it was not possible to obtain bedrock gold. Only studies of alluvial gold could be carried out. Studies in other areas, eg Lubuk Mandi, Terengganu (Henney et al 1994), have shown that many features in alluvial gold are inherited from its bedrock source.

2. Size and shape studies showed a range of features, particularly that gold grains become more deformed and abraded with increasing distance of transport from the source and that those from a beach environment were more rounded and abraded than those from streams. The general conclusion is that there is some evidence to suggest that the gold belongs to two original populations, those coarser than 0. lmm and those finer.

3. Studies of the composition of the gold showed that there are two populations, most in a group with Ag contents ranging from 8-15% which is thought to be the primary gold mineralisation. A second group with much lower Ag, less than 2%. is thought to result from a later phase of secondary gold deposition. These two populations do not match those identified on the basis of size and shape.

4. Studies of the inclusions in the gold were somewhat hampered by the small numbers of gold grains recovered and the uncertainty of interpretation must therefore be taken onto account. The results show that three types of gold mineralisation may be present:

1. The dominant type is a mesothermal sediment-hosted, quartz-vein type mineralisation indicated by the presence of pyrite, arsenopyrite and galena. This has been found in several other parts of Malaysia.

2. A minor component with a mafic/ultramafic rock association, indicated by the presence of "pentlandite".

3. A minor component of red-bed type, unconformity-related mineralisation indicated by the presence of tiemannite(HgSe) and other selenide minerals. This also corresponds with some of the low-Ag gold.

4. The latter two types of gold mineralisation are not widely known in Malaysia.

5. This has important consequences for gold exploration as there could be multiple sources of gold in this area. The particular types of exploration techniques employed might be modified to suit the type of gold mineralisation being sought. For example the sampling stategy to investigate an unconformity or bodies of mafic rock would be different from that to investigate structuraly controlled quartz veins and shear zones.

6. ACKNOWLEDGEMENTS We would like to thank Mr A G Gunn and the GSM/BGS gold project for assistance in planning and discussions connected with this work. GSM, particularly Mr Rohimi and staff from the Kuantan office, provided extensive support for the field sampling programme which made the work possible.

7. REFERENCES

Aw Peck Chin and Loh Chiok Hoong. 1989. Lithogeochemistry of the Mersing area, Johor and its implications for gold exploration in the eastern belt of peninsular Malaysia. Proceedings of the 20th Geological Conference, Vol. 1. pp 1-24.

Leake, R.C., Bland, D.J. and Cooper, C. 1993. Source characterisation of alluvial gold from mineral inclusions and internal compositional variation. Trans. Instn Min. Metall. (Sect.B. earth science) 102.B65-82.

Henney, P.J., Styles, M.T., Bland, D.J. and Wetton P.D. 1994. Characterisation of gold from the Lubuk Mandi area, Terengganu, Malaysia. British Geological Survey Technical Report WC/94/2 1.

M.T.Styles, M. Perez-Alvarez, D.J. Bland, P Wetton and J. Naden. 1993. Charaterisation of gold from Ecuador. British Geological Survey Technical Report WGl92148.

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Figure 1. Simplified geological map of peninsular Malaysia showing the distribution of primary gold occurrences and the Mersing study area(location 5).

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Table 3. Electron microprobe analyses of gold from samples MA 18-20

1- MA1 914

lMAl9/5 185.42 MA19/5 track 95.17 LFL

84.12 IZEFLTZ IMA 1 9/9 188.24

tMA20/12 187.35

IMA20/15 188.84

1.90 I I I I 0.08 I 9.46 I 0.02 I 0.08 I 14.45 0.20 8.1 1 0.02 0.12 0.1 1 9.54 0.04 0.06

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Table 4. Electron microprobe analyses of gold from sample MA 22

Blank = not detected

Silver "S" Curve

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