A PROPER APPROACH TO PROSPECTING OF PEGMATITES E

A PROPER APPROACH TO PROSPECTING OF PEGMATITES FOR GEMSTONE: A GUIDE TO SMALL SCALE MINERS IN MALAWI.

Grain W.P. Malunga MIMM

ABSTRACT

Gemstones are precious and semi precious stones which are suitable for cutting as gems. Common gemstones in Malawi belong to tourmaline, garnet, quartz, beryl and corundum. Most of these gemstones are found in proterozoic pegmatites of northern Malawi

Gemstone mining in Malawi is a recent activity and mainly involves mining of aquamarine. Most mines emerged after discovery of the stones in top soil as float. Most prospectors have spent a lot of money due to lack of proper prospecting, evaluation and grading of these stones before taking them for marketing.

1.0 INTRODUCTION

Pegmatite is a term applied to graphic granite which is an extremely coarse grained rock containing the worlds largest crystals which can be choicest mineral specimens as well. The dominant mineral constituent is alkali feldspar associated with quartz and white mica. Accessory minerals include lanthanides and gemstones. The northern region of Malawi has the largest dyke swarm of granitic pegmatites extending into Zambia and Tanzania/Mozambique (trending NE)

Of specific interest in the pegmatites, for Malawi prospectors are gemstones stones which are suitable for cutting as gems. Well-known gemstones, here, include tourmaline, garnet, quartz, beryl and corundum, although some topaz may be found. Below are the constituents of the minerals mentioned above:

TOURMALINEIts gem varieties are verdelite, a light green variety owing its colour to ferrous iron, pink rubellite whose colour comes from the presence of

manganese, dravite, rich in magnesium giving it a brown colour And indicolite which is blue probably due to the presence of iron

and manganese.

Gemstone Workshop: 12 -13 September 2002 1

GARNET

The famous gem variety of garnet is- rhodolite bearing a rose like colour corresponding with the presence

of 2 pyrope and 1 almandite,- and almandite which is sometimes called almond stone.

QUARTZ

Known almost to every prospector or small scale miner (SSM) in this category is amethyst although other varieties listed below exist.

- smoky quartz having a brownish colour due to the presence of aluminium and the effect of radio activity bombardment.

- rose quartz, having rosy colour due to the presence of manganese and /or lithium, sodium and titanium.

- amethyst, a purple or bluish-violet quartz having its colour from the presence of iron.

- blue quartz, this is blue due to the inclusion of tiny needles of tourmaline,

- citrine which is yellow quartz- and clear or colourless quartz known as rock crystal.

BERYL

- aquamarine, this is clear transparent bearing pale green yellowish green or bluish green colours. The most common gem stone in the region.

- emerald is grass green in colour. Although mostly restricted to metamorphic paragenesis, it has been observed in Mzimba pegmatites (Chatupa J.; 1977).

- heliodor; this is amber or golden yellow.- morganite, a pink variety.

The last three gem stones are rare in the region. Emeralds have been found in Mzimba while a few heliodors have been found in Chitipa-Karonga area.

CORUNDUM

Although it is restricted to syenitic pegmatites, its occurrence has been noted in granitic pegmatites.

- ruby is dark red and rich in chromium.


- sapphire is common and has a clear blue colour although white, pink, yellow and green varieties exist. The blue sapphire owes its colour to the presence of iron and titanium while the yellow and green varieties are a result of varying amounts of ferric and ferrous iron in them.

- topaz is used as a gem especially when it is yellow in colour.

1.2 HISTORICAL BACKGROUND OF GEMSTONE MINING IN MALAWI

Most SSMs have gone straight into mining without carrying out systematic prospecting but have discovered the stones in their gardens or farms as float. This is mainly true for Euthini, Mpherembe, Kafukule and Mtantha areas in Mzimba District. Some people with the knowledge of the Mines and Minerals act have applied for Non Exclusive Prospecting Licences (NEPL) and eventually applied for Mining Claims (MC) to start mining. Others have used the NEPLS to sell their stones and sometimes to start mining. The rest have picked the stones and kept them in pots in their houses not knowing where to sell them.

A majority of the miners and prospectors have had their knowledge of gemstones from Zambia but have lacked an economical approach to prospecting. This has cost them a lot of money. Those who have nearly exhausted their money before finding any gemstone have managed to pull through by buying stones from the people with NEPL or the local farmers who have kept their stones in pots.

This paper tries to advise on the proper approach to prospecting for gemstones so that maximum economy is exercised to save money and time; and indeed to educate people on the proper way of mining.

2.0 PROSPECTING PROCEDURES

2.1 AREA SELECTION

After being issued an NEPL by the Commissioner for Mines and Minerals, Area selection should be affected and this involves desk studies using geological bulletins and unpublished technical reports produced by the Department of Geological Survey (GSD). Maps are studied to locate pegmatite occurrences.


Since NEPLs are issued for the whole district, area selection aims at selecting target area for prospecting within the district. These areas are usually metamorphic regions intruded by pegmatites of granitic affinities in which the gemstones are found.

It is recommended that satellite photos (landsat image) and aerial photos be used n tracing lineaments or ridges. These are usually associated with pegmatites. The only problem here is that most SSMs are not qualified enough to do remote sensing, not to mention that these photos are expensive.

In conclusion, area selection enables people to select areas where they are going to do field investigations. The tools to be used during area selection are geological reports and maps, landsat imagery and aerial photos and their accessories such as stereoscopes, tracing papers, pens and pencils and mathematical sets.

2.2 FIELD INVESTIGATIONS

The equipment associated with field investigation/work comprises of, camping gear, a geological hammer, a compass, a camper’s axe, panga knives, a field note book, aerial photos, hand lens (Magnification x6) and a hardness testing hard steel knife or pencil.

The existence of pegmatites can be known through observation of float material, shrubs and trees, colour contrasts, topography and sampling of scree:

o float material exists in form of gemstones themselves as for the Euthini and Mtantha cases for aquamarine. Look for quartz, tourmaline and mica floats.

o Most buried pegmatites are over lain with grass or shrubs. Close examination may reveal outcrops of quartz or mica.

o Look out for colour contrasts. Different rock types weather to form topsoil with different colours. Hornblende biotite gneisses may weather to form reddish sandy clays while pegmatites (quartz feldspar + mica and tourmaline) may weather to form whitish to brownish sandy clays. For pegmatites the colour contrasts may spread in form of irregular segregations and linear shapes mixed with quartz.


o Topography: generally pegmatites are more resistant than their host rocks. This results into formation of pegmatites ridges occurring in form of irregular segregations.

o Sampling of scree covering stones of a hill may lead to knowledge of whether a hill or cliff is a pegmatitic body or not.

o Stream sediments if panned may contain some gemstones or pegmatitic mineral assemblage

This may help identify pegmatites within the vicinity.

In conclusion, field investigation should involve identification of float material, vegetation type, soil colour contrasts, topography, scree and panned concentrates from stream sediments.

The field team should consist of at least a prospector and two chain men.

2.3 EVALUATION AND ASSESSMENT

Once the pegmatites have been identified, trenching and pitting commences in order to see if they bear gemstones.

o trenching should always be across pegmatites to save time and money. The trenches should be spaced at a distance of about 10 metres. Pitting should usually be done where soil colour contrasts exists just to reach weathered debris or bed rock.

The depth of the trenches and pits will be determined by the nature of material found. If trenches and pits are dug up to 1.5m and no gemstone is found, it may be worthy abandoning them. If weathered or “rotten” gemstones are found, dig until you find no more as it is possible that at greater depths good gemstones may be recovered-even if water table is reached. After gemstones have been found and sampled, the samples must be examined to see if they are valuable.

o examination of samples concerns mainly an establishment of grade of material. This is through establishment of colour, cloudiness, size, hardness and refraction.

o Gemstone colour (chroma or hue) range from colourless to black. The more brown and grey colour is the less desirable. This is worthy observing.


o Cloudiness of gemstones is an effect of the existences of inclusions and pits. This can be observed by exposing the stone to light. The way a gemstone grew can best be done through testing with ultra violet or infra Red rays. Infra Red rays works better. Inclusions are usually made of minerals such as rutile and tourmaline; and liquids. Remember inclusions and pits should be as few as possible. Clarity is very important.

o The bigger the size of a gemstone the more valuable it is. It should be born in mind that if a stone is faceted, it usually has 20% of its original weight (recovery = 20%)

o Hardness can help identify mineral. For example Beryl has a hardness ranging from 7.8 corundum 9, quartz 7, garnet 6 – 7 .5 and Topaz 8.

o Refraction is a more advanced way of testing samples. This is through the use of a refractometer. With the instrument you can also determine their optical character (uni axial or bi axial) and their refractive index:

Corundum :RI = 1.76 – 1.77Beryl :RI = 1.57 – 1.58Quartz :RI = 1.54 – 1.55Tourmaline :RI = 1.62 – 1.64Topaz :RI = 1.61 – 1.64

After testing, the gemstones must be graded. Here is a simple chart for grading:

GRADING

COLOUR

1

2

3


FI Fairly IncludedLI Lightly IncludedMI Moderately

IncludedHI Highly IncludedEI Exclusively

Included

4

FI LI MI HI EI

INCLUSIONS

Remember: Graded stones will fetch more money than ungraded stones.

The following is a list of equipment which will be needed during evaluation and assessment of pegmatite deposits.

Trenching and Pitting : 2 Crow bars: 4 14lb hammers: 4 Mortar pans: 2 Muttock picks / Hole picks: 2 Hoes: 2 Chisels: 4 Shovels: 1 Campers axe: 2 Panga knives: 4 Safety goggles

Testing of samples : “Human eyes” and colour chart ultraviolet/infra red view cabinet Hardness pencilsRefractometer/Sodium lampHand lens/Gemolite

3.0 REVIEW OF PROJECT COST

Before beginning prospecting operations a financial assessment of the project must be given in form of estimates. Main items of considerable importance are the cost for prospecting equipment, salaries and wages, transport (to and from field) administrative overheads such as stationery and telecommunications and subsistence allowances.

These costs must be reviewed to find the actual expenditures so that future prospecting should have viable expenditures to avoid running out of money or over expenditures.


4.0 PEGGING CLAIMS

Once one has discovered a deposit of the gemstones specified in his NEPL, he is entitled to peg a claim and apply to the Commissioner for Mines and Minerals for his or her claim to be registered. If the deposit is to large for SSM a mining licence should be applied for.

Registration of claims has the advantage that one has the right to mine minerals in the claim without external interference or has legal protection for the ownership of the minerals he is mining.

It is recommended to seek services of experts to peg one’s claim. Much of the pegging is carried out by qualified personnel from the Department of Mines (D.M). Other experts/surveyors can be obtained from the Geological Survey Department (G.S.D.), Agricultural Development Divisions (ADD) and the Surveys Department (SD).

REFERENCES

DEER W., HOWIE R. and ZUSSMAN J. (1978): An Introduction to the Rock-Forming Minerals. Longman, London.

MALUNGA G.W.P (1989): Njira yofufuzira ma gem stones. Paper presented to Small Scale Miners Seminar; November 1, 1989, Mzuzu.

Author’s Address

Grain W.P. MalungaDepartment of minesP.O. Box 251LilongweMalawi

E-mail: [email protected] [email protected]


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A PROPER APPROACH TO PROSPECTING OF PEGMATITES E

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