Gold prospectors have won many fortunes and there are many smaller finds that have gone undocumented. Here is a general introduction plus a few tips for gold prospectors or would be gold prospectors.
EQUIPMENT
The equipment available for a prospector is varied. This includes metal detectors, dryblowers and hydraulic concentrators of various shapes and sizes. While metal detectors remain the most popular tool, knowledge of other equipment is useful, especially if the prospector wants to expand his activities. Some of the basic types of equipment are described here.
Accessories
Useful accessories include the geological pick, prospecting pick, compass, times ten hand lens, safety glasses, pen knife, sample bottles and bags, hand auger and gold pan. A geological pick can be used to dig holes and split rocks while metal detecting, or collect rock samples for identification or analysis. Safety glasses are used to protect the eyes when sampling or splitting rocks. A compass is necessary when prospecting away from known tracks and landmarks. A hand lens is useful for examining fine gold and minerals. The hardness of a mineral can be tested using a pen knife. A stainless steel pen knife has a hardness of 6 1/2 on Moh's hardness scale (1-10). Sample bottles and bags are used to store fine gold and samples. They should be labelled and a list written up so locations won't be forgotten. For sampling alluvium and soils, a barrel type hand auger is useful. A gold pan is used to separate fine gold from concentrates. A pencil magnet is used to test for magnetic minerals.
Metal Detectors
V.L.F. detectors with ground balancing are the best type of detectors for prospecting. The new models of prospecting detectors have better depth and sensitivity than many of the old types. They are most useful for detecting small nuggets, which would have been missed by the old detectors. Garret, Minelab and Whites prospecting models are popular. Garret detectors have had widespread success on WA's goldfields.
Metal detectors will respond to any type of conductive or magnetic material. The metal detectors transmitting coil produces a primary electromagnetic field. When a conductive object encounters a primary electromagnetic field, currents flow through the surface of the object, called eddy currents, each producing its own secondary electromagnetic field. The secondary electromagnetic fields distort the primary electromagnetic field. A receiving coil within the metal detector receives the distorted primary electromagnetic field signal, ultimately producing an audio signal in response to the strengths of the secondary electromagnetic fields. Materials of greater conductivity produce larger and stronger secondary electromagnetic fields, and therefore audio signals, than smaller objects. For a given conductivity, the sizes and strengths of the secondary electromagnetic fields are controlled by the surface area facing the primary electromagnetic field rather than density or mass of the object. Larger surface areas produce larger fields and responses.
Manufacturers of metal detectors classify targets as either metal or mineral. Metal targets include all conductive, non-ferrous metals. Examples are silver, gold, copper and aluminium. Mineral targets consist of ferrous metals, magnetic minerals and conductive ground minerals. Examples of mineral targets are steel, iron, magnetite, iron oxide ground minerals and wet salt. These have lower conductivities than most metal targets. "Hot rocks", often encountered in the field, are concentrated forms of conductive iron oxide.
The audio signal produced usually varies according to the type of target. Gold tends to produce short, sharp signals while ferrous objects produce broad signals. A double blip will be produced on long thin objects, such as wire or nails. It is wise to do a bench test of different objects to determine their different responses. Some objects that can be used are a nail, silver coin and gold ring. With field experience, audio response from various targets, including "hot rocks", will become familiar so that identification will be easier.
Ground conditions affect the operation of metal detectors. Heavily mineralised and dense ground conditions cause the primary electromagnetic field to compress, resulting in loss of depth. Wet ground allows greater penetration of the primary magnetic field, providing better depth. Magnetic and conductive minerals (mostly iron oxide minerals) in ground soil produce background signals that can mask target objects. The ground cancel is used to decrease the effect of minerals in mineralised areas.
The following steps should be followed when tuning a manual ground cancelling detector; such as a Garret or Whites prospecting metal detector: 1. Switch on and allow to stand for 10-15 minutes to stabilise batteries. 2. Check battery condition. Batteries must be in good shape for the metal detector to work properly. 3. Place tuning on automatic. 4. Place into V.L.F. mode. The V.L.F., or ground cancelling mode, should always be used for prospecting. 5. To begin with, sensitivity should be placed on half. It can be increased or decreased according to ground conditions. If it is placed on minimum, small nuggets will be missed, therefore, always place it on the maximum allowable setting. 6. Discrimination should always be at zero. 7. Adjust tuning audio so that a humming sound is barely audible. 8. Compensate for ground conditions. To compensate for ground conditions, raise and lower the search coil (from 60cm high to 15cm low). As the search coil is lowered, the audio signal will either increase or decrease in strength. If the ground cancel knob is in its midway position then it can be turned backwards or forwards to compensate for the increase or decrease in signal strength. The ground cancel knob should be moved one complete turn each time. When the audio signal remains constant as the searchcoil is raised and lowered, the ground has been compensated for. The ground cancel will have to be adjusted as ground conditions change.
Headphones should always be worn when prospecting otherwise the batteries will drain quickly. Signals are also easier to comprehend with headphones. Some prospectors have an audio boost fitted to amplify small signals, while also suppressing very loud signals. Some audio boosts also operate with a more sensitive tone. They definitely make it easier to detect small signals. Hipmounts reduce strain when detecting for long periods or using large coils.
Dryblowers
Dryblowers are mainly used to recover fine gold in areas where there are no water supplies. Nuggets are more efficiently located by using metal detectors.
A dryblower consists of a hopper/classifier overlying an inclined riffle tray fitted with an air blower. Today's dryblowers are motorised. They range in size from small, easily portable units, less than half a metre high, to large units that can process 20 tonnes of material per hour or more.
Dry material is fed into the vibrating hopper/classifier which removes the coarsest material. All material small enough to pass through the classifier falls into a riffle tray underneath. The heavy fraction is separated using a combination of vibration and air being blown upwards to remove the dust. Finally, heavy concentrates are removed by lifting up the riffles and sweeping the concentrates into a pan. Gold is recovered by panning the heavy concentrates.
Vibrostatic dryblowers use a combination of static electric charge, air flow and vibration to collect gold. They only differ in that the gold is precharged and later attracted to assist in retaining the gold in the riffle box. This allows damp material to be processed.
Dryshakers consist of a hopper/classifier overlying an inclined riffle tray. Dry material is placed into the hopper /classifier which removes the coarse material using vibration. All fine material passes into the underlying riffle tray. High frequency, short vibrations displace light material over the riffles and out of the tray. Gold is retained by the riffles. Air blowing is not utilised by dryshakers.
Hydraulic Concentrators
When a water supply, such as a bore or stream, is available hydraulic concentrators are used to recover gold. Water can be recycled in dry areas to reduce consumption. Modern hydraulic concentrators are driven by petrol engines. The simplest of these is the rocker cradle. It consists of a hopper over a tray fitted with riffles, all mounted on a rocker.
Washdirt is fed into the hopper, the base of which contains small holes to prevent pebbles and boulders from passing through. The discarded coarse material should be examined for nuggets. Water is poured into the hopper and carries the fine material onto the riffle tray and over the riffles. At the same time, the cradle is rocked. Eddy currents form behind each riffle, the decrease in current velocity trapping heavy minerals. Matting covers the base of the riffle tray to help trap heavy minerals. Most gold will be trapped behind the first few riffles. The angle of decline of the riffle trays must be adjusted according to water flow and the amount and type of sediment. Too steep a decline will result in gold being washed away. A decline that is too shallow will have the riffles becoming choked in sand, preventing settling of gold. When the matting behind the riffles fills up, the riffles can be lifted and the matting removed. Finally, the heavy concentrates should be panned to remove any gold.
Various types of hydraulic concentrators; such as, gold screws, knelson concentrators, jigs and shaking tables can be used when large amounts of washdirt are to be treated. It should be noted that all clay material must be thoroughly dissaggregated before processing to prevent the formation of clay balls. Puddling and log washing machines are specially designed for this purpose.
Gold Pans
Gold pans are made from metal or plastic. Plastic pans are easier to maintain and just as efficient as metal pans. The pan should be large (about 40cm in diameter) and contain riffles along its side to help trap gold. A pan with riffles along one half of its side is preferable to a pan with riffles along its full circumference. This allows easy collection of gold and concentrates after panning. Metal pans are often greased and should be degreased by holding over an open flame or washing in hot, soapy water.
To use a goldpan, a layer of washdirt 3/4 inch thick is placed over the base of the pan. Rest the pan in water and rake fingers back and forth to loosen and separate material. Tilt the pan and rake coarse material to the top end, letting the fines fall back. Remove this coarse material. Next, shake the pan from side to side to help the heavy minerals settle on the bottom of the pan. Repeat these two steps four or five times. Now, place the pan in water and tilt so the fine material accumulates just under the pans edge. Remove from water and tilt back, allowing a wave to form. Tip forward again, letting the wave travel forward to carry the top material out of the pan. Next, shake the pan from side to side again. Place in water and tilt, so the light material remains just under the pans edge. Remove the pan from water and tilt back, then forward, resulting in a wave carrying the top material out of the pan. The previous few steps should be repeated until only a tablespoon of fine material is left. A gentler wash action is required as the amount of wash dirt remaining decreases. Finally, swirl the remaining washdirt on the base of the pan so contents fan out and gold specks will be visible. Gold specks can be collected with a damp finger and placed in a sample bottle filled with water. A teaspoon is useful for collecting gold when large amounts are present.
Black, magnetic sands can be removed using a magnet. Place the magnet in a plastic bag so that black sands are collected on the outside of the bag. Now, the magnet can be removed and the black sands will fall away. This prevents a buildup of black sands on the magnet. A sieve is useful to initially separate coarse material from the fine fraction. Automatic gold pans, or concentrating wheels, are an alternative to manual pans for separating gold from fine alluvium and concentrates. The wheel contains riffles which pass from the pan's edge to its centre. It is set at an angle so that washdirt remains at the lower edge of the wheel. Water is added to the centre of the wheel by a jet spray. The circular motion and spiral action of the wheel cause gold grains to migrate towards the centre of the wheel where they pass through a hole to a collecting bottle underneath. They have electric 12v motors which operate from batteries.
A new type of hydraulic concentrator, called the mini gold concentrator is replacing conventional gold pans for treating small samples of alluvium, eluvium and colluvium. The concentrator can treat twenty panloads of washdirt in the same time an expert can wash a single panload using a conventional pan. By following simple instructions a beginner can easily master the separation of gold from a shovelful of washdirt.
The unit consists of a removable dish with sieve resting on top of a lower settling pan, all clamped inside of a twenty litre bucket. To use, fill the bucket with water. Place the entire assembly into the bucket and fasten with wingnuts. Add washdirt to the upper dish and agitate the dish in a circular motion. All large material is retained in the upper bowl by the sieve. This material is discarded by removing the upper dish and sieve. Small material passes through the sieve into the lower settling pan. Agitation washes the light material over the top of the settling pan with the help of agitation blades and a helical scraper blade. Gold and heavy minerals settle to the base of the retention bowl where they remain until panning is finished. Panning continues until the bucket fills with the discarded washdirt. Finally, the unit is removed by undoing the wingnut fasteners and the bucket is emptied then refilled to start the process over again. Any gold in the retention is removed and placed in a sample bottle. To recycle water, the entire bucket with concentrator is placed in a large container so overflowing water is collected until ready for reuse. Small in size and weighing only 3.5 kg it is easily transported.
Sample Mill
The sample mill is used to crush rock samples before testing for gold. It is powered by a petrol motor for portability. A sample is placed in the hopper which feeds the pulverisers, reducing the sample to powder. Ideally, the sample mill should be adjustable so that the desired grain size can be obtained. Most sample mills have hardened steel jaws. These can produce fine steel filings that show up in the residue when panned. When more than one sample is processed, residue from previous samples carries through. Therefore, a gold bearing sample followed by a barren sample will give positive gold results in the barren sample. When accurate results are required, the mill can be cleaned by grinding quartz between samples (sometimes, particularly with ironstone's, this is not effective).
A cheaper alternative to the sample mill is the dolly pot. A dolly pot consists of two parts: a mortar and a pestle, both of large dimensions (eg. 1 litre). It is used for crushing hand samples. Samples are broken into small pieces with a hammer, then placed in the dolly pot for crushing.
Analytical Instruments
Today, the options available to the prospector for analyzing rock and mineral samples are numerous and sophisticated. Depending on the results required, techniques such as polarized light and electron microscopy; x-ray diffraction; and chemical analysis using various spectrometric methods are available.
Polarizing microscopy is the best method for identifying and examining most rocks and minerals. By observing a section of a rock or mineral with a polarizing microscope the texture, structure and mineralogy of the sample can be determined. From this information an identification can be made and the origin determined. This information is of use during mining and prospecting. For routine use, lower cost alternatives are stereo microscopes or high power pocket microscopes.
For analyzing the composition of individual minerals emission spectroscopy (ICP) or electron microscope (microprobe) analysis is carried out. Ores containing submicroscopic gold particles within their crystal lattice are analyzed with a microprobe to determine which ores are the gold carriers and where the gold is sited.
Chemical analysis of a rock or mineral sample for gold is called assaying. For most prospectors, a low cost, moderately sensitive technique is adequate. For most gold bearing samples requiring accurate determination of the gold content fire assaying is the most common method but not necessarily the cheapest. Modern fire assaying techniques can determine grades as low as 1g/tonne and starts at prices of about $12.00 per sample. In samples containing minute trace amounts of gold, more sophisticated methods are preferred.
For the geochemical explorationist who is searching for trace amounts of gold, indicating the presence of a hidden orebody, the latest analytical techniques are almost mandatory. Atomic absorption spectrometry (AAS) , induced coupled plasma (ICP) and even mass spectrometry have detection limits in the parts per billion or less and are the preferred choice. Analytical costs are higher for these methods although bulk sampling and multi-element analysis bring the costs down.
GOLD AND ITS ORES
A mineral profitably mined for its metal content is called an ore mineral, whether it is an element, such as gold, or a compound of two or more elements, such as the sulphides and tellurides. A knowledge of the properties of gold and its ores is necessary for correct identification. This information is also necessary for selecting and controlling the mining and ore processing equipment. Visual examination of a sample is usually sufficient to reduce the number of possible identities to a few, if not a single identity. Gold is most commonly found in its elemental form, with varying amounts of silver, copper and iron as impurities but also occurs in ores; such as, the sulphides and tellurides.
Beginners sometimes experience problems when identifying gold, most commonly confusing with similar minerals; such as pyrite, chalcopyrite, pyrrhotite, pentlandite and gold coloured mica. With experience, there should be no difficulty identifying gold except when it is extremely fine grained or microcrystalline. In these situations, gold cannot be easily observed and requires examination with a microscope.
The most distinctive properties of gold are its gold-yellow colour, metallic lustre, softness, high specific gravity and gold-yellow streak. Other minerals with a similar colour and lustre are often mistaken for gold. Pyrite, chalcopyrite, pyrrhotite, pentlandite and gold coloured mica are the minerals usually mistaken for gold. By keeping in mind the properties of gold each of these minerals can be eliminated. Gold is the only mineral that will easily scratch, leaving a residue of gold-yellow powder. Gold is malleable while the rest are brittle, will break and flake when struck with a hammer. When fine and placed in a pan of water, gold will sink rapidly and refuse to move, the rest will sink slowly and swirl easily. Gold occurs in grains whereas mica is flaky.
Gold also occurs as microscopic and submicroscopic particles within sulphide minerals; particularly pyrite, chalcopyrite, arsenopyrite and pyrrhotite. All of these are common within veins and zones of hydrothermal alteration and replacement. They occur as macroscopic and microcrystalline grains and crystals.
Pyrite is brass-yellow in colour with a metallic lustre and greenish-black streak. Often, it forms perfect isometric crystals in cubic or polyhedral form.
Chalcopyrite is also brass-yellow with a metallic lustre and greenish-black streak. It is easily confused with pyrite but forms tetragonal crystals instead of isometric cubes and polyhedrons. When exposed to air it often tarnishes to iridescent or deep blue. In some situations, a chemical test for copper using concentrated nitric acid may be necessary to distinguish it from pyrite.
Arsenopyrite is silver-white to steel grey with a metallic lustre and greyish-black streak. When crystalline, it exhibits monoclinic crystals usually in prismatic form. When struck with a hammer arsenopyrite often gives off a garlic smell.
Pyrrhotite is brass-yellow or brownish-bronze with a metallic lustre, greyish-black streak and orthorhombic crystals. Pyrrhotite is easily identified using a pencil magnet as it is distinctively magnetic.
Gold also occurs in compounds of gold and/or silver with tellurium. The tellurides, sylvanite and calaverite are mined for their gold content. They are quite rare, however, have been mined in Kalgoorlie as ores of gold.
Calaverite is brass-yellow to silver-white with a metallic lustre, yellowish to greenish grey streak and monoclinic crystals that are often striated.
Sylvanite is silver-white to steel grey with a metallic lustre, black streak and monoclinic crystals. The hardness of calaverite is 1 1/2 to 2 and of sylvanite 2 1/2 to 3.
GOLD Au
Colour: Gold yellow to pale yellow
Lustre: Metallic
Hardness: 2.5 to 3
Specific Gravity: 19.3 to 15.6
Fracture: Ductile and malleable
Streak: Gold yellow
Best Field Characteristics: Gold yellow colour, high SG, gold yellow streak, softness.
Similar Minerals: Pyrite and chalcopyrite have a greenish-black streak, will sink slowly and swirl in a pan of water when fine whereas gold will sink rapidly and refuse to move. They are brittle: will break and flake when touched with a knife but won't scratch. Gold is malleable and will scratch easily. Once gold has been seen and held, future identification will be simple.
Gold also occurs as submicroscopic particles within sulphide minerals, particularly pyrite, chalcopyrite, arsenopyrite and pyrrhotite. All of these are common within veins and zones of hydrothermal alteration and replacement. They occur as macroscopic and microcrystalline grains.
Pyrite is an iron disulphide.
PYRITE FeS2
Colour: Brass yellow
Lustre: Metallic
Hardness: 6 to 6.5
Specific Gravity: 4.9 to 5.2
Fracture: Uneven/brittle
Streak: Greenish-black
Crystals: Isometric, in cubes and pyritohedrons. Also occurs massive and in anhedral grains.
Best Field Characteristics: Colour, streak and cubic crystal form.
Chalcopyrite is a copper iron sulphide.
CHALCOPYRITE CuFeS2
Colour: Brass yellow
Lustre: Metallic
Hardness: 3.5 to 4
Specific Gravity: 4.1 to 4.3
Fracture: Uneven/brittle
Streak: Greenish-black
Crystals: Tetragonal, usually massive and in anhedral grains.
Best Field Characteristics: Colour and streak
Arsenopyrite is an iron arsenide sulphide.
ARSENOPYRITE FeAsS
Colour: Silver white to steel grey
Lustre: Metallic
Hardness: 5.5 to 6
Specific Gravity: 6 to 6.2
Fracture: Uneven/brittle
Streak: Greyish-black
Crystals: Monoclinic prismatic. Also massive and in anhedral grains.
Best Field Characteristics: Colour and crystals.
Pyrrhotite is an iron sulphide with small amounts of nickel and cobalt.
PYRRHOTITE Fe1-xS
Colour: Yellowish to brownish bronze
Lustre: Metallic
Hardness: 3.5 to 4
Specific Gravity: 4.6
Fracture: Uneven/brittle
Streak: Dark greyish-black
Crystals: Orthorhombic, also massive and anhedral grains.
Best Field Characteristics: Pyrrhotite is magnetic.
The tellurides are compounds of gold and/or silver with tellurium. The tellurides, sylvanite and calaverite are mined for their gold content. Calaverite is a ditelluride of gold. Sylvanite is a telluride of gold and silver. These are not common.
CALAVERITE AuTe2
Colour: Brass yellow to silver white
Lustre: Metallic
Hardness: 2.5 to 3
Specific Gravity: 9.1 to 9.4
Fracture: Uneven/brittle
Streak: Yellowish grey
Crystals: Monoclinic prismatic with striations. Also in anhedral grains.
Best Field Characteristics: Streak and striated crystals.
SYLVANITE AuAgTe4
Colour: Silver white to steel grey
Lustre: Metallic
Hardness: 1.5 to 2
Specific Gravity: 8.2
Fracture: Uneven/brittle
Streak: Black
Crystals: Monoclinic prismatic. Also in anhedral grains.
Best Field Characteristics: Hardness and streak.
Gold can be described according to its natural size and nature of occurrence. Based on these, gold occurs in six main forms:
(1) Large pieces of free gold >2mm in size that are known as nuggets.
(2) Pieces of gold and gangue (quartz, ironstone etc.) known as specimens.
(3) Coarse to fine grains of free gold 2mm to 150 microns that are visible to the naked eye.
(4) Microcrystalline gold 150 to 0.8 microns in size only visible with a microscope.
(5) Submicrocrystalline particles of gold that occur in the crystal lattice of certain sulphide ores.
(6) In compounds with tellurium.
All types show various degrees of crystallinity from rounded grains (eg. alluvial) with no crystal faces through subhedral grains with some crystal faces (hydrothermal) to crystalline grains with well developed crystal faces (hydrothermal and supergene gold). In most situations, gold is found in rounded forms, however, where open space crystallisation has occurred, such as in supergene environments, crystalline gold is common.
Nuggets are well known to metal detector operators. While many nuggets are almost pure gold, impurities of iron and quartz are common. Nuggets that have been chemically deposited or altered in the weathering profile are often intergrown with ironstone.
Large grains and veinlets of gold intergrown with quartz are derived from quartz reefs and lodes and are referred to as specimens. These are also well known to metal detector operators.
Free grains of gold that are visible to the naked eye are either intergrown with gangue in primary deposits or as loose grains within secondary deposits. Machinery is required to separate gold grains from unwanted gangue. Fortunately, the high specific gravity of gold enables it to be effectively segregated and concentrated using low cost gravity methods, such as jigs, sluices, shaking tables etc.
Microcrystalline gold is common within primary deposits. Grains of gold are disseminated and intergrown within a quartz gangue or locked within sulphide minerals. Coarse grains can be liberated by crushing and grinding followed by concentration using gravity concentrators. If the ore consists of very fine grains extraction with sodium cyanide or amalgam is necessary.
Gold contained within sulphide minerals is present as small grains and particles within the crystal lattice of the mineral. Many primary deposits consist of disseminated grains of pyrite, chalcopyite, arsenopyrite and/or pyrrhotite containing significant amounts of gold and intergrown with gangue minerals. Sulphide minerals cannot be concentrated by gravity methods due to their low specific gravity. Froth flotation is common, followed by treatment with sodium cyanide to remove the gold. Such mining methods are expensive and can only be used on large deposits, however low grades can be worked.
Gold also occurs in compounds of gold and/or silver with tellurium. The tellurides, calaverite and sylvanite are mined for their gold content. They are quite rare, however, have been mined in Kalgoorlie.
5.0 GOLD ENVIRONMENTS
Gold occurs in alluvial, eluvial, supergene, quartz vein and stockwork, shear related and hydrothermal replacement deposits. In the general sense, alluvial refers to eluvial, colluvial, fluvial and lacustrine deposits but is restricted to the traditional meaning of stream and lake deposited gold here. Alluvial, eluvial and supergene deposits are secondary deposits formed by reworking of primary deposits. Quartz vein and stockwork, shear related and hydrothermal replacement deposits are primary deposits formed by the direct precipitation of gold from hydrothermal solutions originating in the earth's interior. Alluvial and eluvial deposits are collectively known as placer deposits. Large, continuous quartz veins are known as quartz reefs and all other large primary deposits are usually referred to as lodes. Alluvial deposits are formed by the mechanical accumulation of grains, derived from pre-existing rocks, in streams and lakes. Eluvial gold is deposited on the surface by the downward movement of material, via gravity processes, from the source which is situated above. Supergene deposits result from "in situ" weathering of mineralised bedrock which leaves behind a residue of weathered bedrock, primary and secondary ore in the weathered profile. Quartz veins are formed from hydrothermal solutions which intrude the country rock along fractures and faults. Lodes consist of a closely spaced network of quartz veins and veinlets. Shear related deposits form during shearing of the host rock along planes of stress. The associated hydrothermal solutions form gold bearing alteration haloes around the shear zones. Hydrothermal replacement deposits are formed when hot aqueous solutions react with and replace the host rock.
Alluvial Deposits
Alluvial deposits consist of hydrodynamically accumulated gold by streams and lakes. They occur on the surface, just below the surface or deeply buried. Ancient stream channels that are deeply buried are called deep leads.
Gold and heavy minerals, such as magnetite, ilmenite, zircons etc. have high specific gravities; therefore, they will be transported within the base of flowing currents where they will be trapped by irregularities in the channel base or changes in current velocity. In present day channels, the heavy mineral fraction, including gold, will accumulate in pools and in cavities, fractures, depressions, behind ridges and boulders present in runs between pools. Gold will also occur in buried channel alluvium below the present river bed. Basal channel deposits will contain the most gold. These rest upon the bedrock. Other channel base deposits can occur between the surface and bedrock where they are marked by beds of coarse sediments, pebbles and conglomerates. Gold and heavy minerals will be much finer grained than the light fraction. This is due to their density and size relationships, expressed as their hydraulic ratio. Consequently, fine gold and small gold nuggets will be found with coarse sediments, pebbles and conglomerates.
Another area of heavy mineral accumulation is the point bar. A point bar is formed on the inside of a bend in a meandering stream. Current flow is strongest on the outside of the bend, decreasing inwards. As a result, heavy minerals will drop out of suspension on the inside of the bend, or point bar, where current flow is least. As the stream migrates laterally, increasingly finer grained material is deposited until the channel is finally covered by fine grained alluvium. Stream channels that migrate laterally form widespread alluvial deposits that may contain gold in the abandoned channel base or point bar.
Eluvial Deposits
Eluvial gold is deposited by gravity processes on the surfaces of hills, rises and flat lying areas. Rainfall assists by carrying the surface material, or float, downslope. Eluvial deposits consist of the unconsolidated rock fragments and soil lying on the surface. It is derived from quartz reefs and other mineralised deposits (supergene, quartz reef and lode) located above. Deposits of transported material containing gold also form on the surface of hillsides where it is concentrated at changes in gradient, such as, the base of a hill. Technically, this hill wash is referred to as a colluvial deposit but is included with eluvial deposits here.
Supergene Deposits
Supergene deposits include both secondary and primary gold that occur in the weathering profile from "in situ" weathering of an orebody. It consists of chemically altered primary grains and nuggets, secondary grains and unaltered primary gold which may overly auriferous bedrock. Supergene gold, as it is popularly known, is the chemically precipitated gold grains and nuggets deposited within surface ironstone's, including laterite, of the weathering profile. Aqueous solutions travelling through the weathering profile transport and concentrate the gold element at or above the water table. Chemically reworked and physically transported primary grains and nuggets are present in the surface and near surface laterite and soil. Secondary gold, formed by chemical precipitation, is dispersed within the surface laterite and deeper saprolite of the weathering profile. Below the water table, unaltered primary gold, within the orebody may be present. Rich deposits, such as the "Rabbit Warren" gold find, near Leonora, have been found by the metal detecting prospector in WA.
Quartz Reefs and Stockworks
Auriferous quartz veins and stockworks containing free gold are keenly sought after by prospectors. Quartz veins originate from hydrothermal solutions being injected along fractures and faults in the country rock. The source of these hydrothermal solutions varies. They may be sourced from rising magmas that crystallise to form igneous rocks. The solutions left over are injected into fractures and faults overlying the igneous bodies. They may also originate from a deeper magma source or metamorphism of the surrounding country rock.
Fractures and faults cut the country rock at various angles and in various patterns. Consequently, the infilling quartz veins cut the country rock according to the pattern of fractures. A concentrated network of gold bearing quartz veins forms quartz stockwork deposits. Widely spaced networks of quartz veins are known as vein sets. Saddle reefs form when quartz veins are concentrated in the apex of an anticline.
Quartz veins are classified as hypothermal (high temperature), mesothermal (medium temperature) or epithermal (low temperature) veins. Hypothermal veins are deposited at great depths (>3600m). Epithermal veins are deposited near the surface ( Gold is not only present within the quartz vein itself but also in the altered zone of wall rock associated with quartz veins. Gold occurs as free grains in quartz veins and submicroscopic particles within sulphide minerals. The auriferous sulphide minerals are concentrated in the altered zone of wall rock adjacent to quartz veins and within the quartz veins themselves.
In the Yilgarn Block, most auriferous quartz veins are contained within mafic rock types (particularly meta-basalts, meta-dolerites, amphibolites) within volcanic dominated greenstone belts. Ultramafics and felsic volcanics also contain gold deposits (in fact, all rock types are represented). Auriferous quartz veins are mainly controlled by shear zones and faults, particularly where faults cut competent (brittle) beds, such as dolerite, contained within less competent country rock. Vein type mineralisation occurs at Kalgoorlie, Leonora, Wiluna, Cue, Mt. Magnet, Sandstone, Marble Bar etc..
Other
Shear related, Banded Iron Formation hosted and hydrothermal replacement deposits also occur (listed in decreasing abundance). Shear related gold mineralisation consists of alteration haloes (a form of replacement) around zones of intense deformation (shear zones), formed from the reaction of hydrothermal solutions with the wall rock. Gold is present as submicroscopic particles within sulphide minerals that occupy the alteration haloes. Quartz veining can also be present.
B.I.F. (Banded Iron Formation) hosted deposits are an example of host rock control, being restricted to a B.I.F. unit. They contain either replacement style or auriferous quartz vein mineralisation. In replacement style B.I.F. deposits, hydrothermal solutions transport the gold element along faults, forming auriferous deposits by replacing magnetite and carbonates within B.I.F.. At Hill 50, near Mt. Magnet, gold is concentrated along northeasterly trending faults cutting the Banded Iron Formation. Gold is present as submicroscopic particles within sulphide minerals plus/minus free grains. The sulphide minerals replace carbonates and magnetite within B.I.F.. Auriferous quartz veins, within B.I.F., occur in the same fashion as those described under Quartz Reefs and Stockworks. These deposits are entirely restricted to a host B.I.F. unit.
With hydrothermal replacement deposits, hydrothermal solutions react with and replace the host rock, forming massive or disseminated gold deposits. In the massive style these typically preferentially replace a specific bed. This style is called stratabound as it is restricted to a single bed, or stratum. These can occur in combination with the deposit styles described above.
6.0 PROSPECTING METHODS
In the early days, prospectors adapted their equipment to environmental conditions so that dryblowers were used in dry areas and hydraulic concentrators in wet areas. Today, metal detectors have superseded the dryblower as the major prospecting tool. The gold pan and sample mill also have their uses.
Metal Detecting
The abundance of iron oxides on the surface of W.A.'s goldfields caused many problems for the first metal detectors. This led to the introduction of ground cancelling machines in 1975. They proved effective and became popular, although there are still areas where ground cancelling machines cannot operate.
The metal detecting prospector is concerned with alluvial, eluvial, and supergene gold. In the Yilgarn and Pilbara Blocks, these occur in linear greenstone belts. Areas that have been dryblown by the early prospectors mark surface gold producing districts. Many nuggets have been found on and adjacent to these dryblowing patches. Together with the geology, they should be regarded as initial guides to metal detecting areas.
Alluvial gold can be found in the small seasonal streams that cut these areas. Basal channel deposits concentrate heavy minerals and are the most prospective deposits. Laterally migrating streams that change course regularly will contain gold in the abandoned channel base and point bar. These deposits will occur in the present day stream channel and immediately adjacent ground.
Eluvial gold can be found on low hills, rises and flat lying areas adjacent to the above locations. These are often covered with quartz and ironstone rubble. Eluvial deposits are concentrated at a change in gradient, such as the base of a hill.
Supergene deposits are found on low hills or flat lying areas that have developed laterite profiles over bedrock. The occurrence of supergene gold is difficult to predict since it is controlled by a complex combination of processes. It is generally present above weathered orebodies where it is concentrated and deposited by certain solutions travelling through the weathered zone. Secondary gold occurs in the surface laterite and deeper saprolite of the weathered zone (laterite profile) and consists of dispersed crystalline grains. Chemically altered and physically transported primary grains and nuggets, derived from the original orebody, occur in the surface and near surface with the secondary deposits. These are the main targets for metal detector operators. Weathered bedrock is also often covered by thick sequences of transported overburden (sand sheets, alluvium and colluvium). This material should be avoided as it has been diluted and mixed. The prospector should also beware of laterite profiles developed over alluvium and colluvium instead of bedrock.
In most situations, alluvial, eluvial and supergene deposits will only form over bedrock or residual laterite profiles. Exceptions to this occur when alluvial and eluvial systems are fed from these areas or where deeply buried ancient river channels exist.
The beginner should locate ground that is not heavily contaminated by iron oxide or ironstone nodules that play havoc with the detectors audio. Even so, the ground cancel will have to be adjusted as the prospector moves over new ground. Audio drift or badly erratic audio signifies that the ground cancel needs adjusting. If the ground cannot be compensated for the prospector should move to a new area.
"Hot rocks" are always encountered by the prospector. These are concentrated forms of magnetic or conductive iron oxide that behave in a similar fashion to gold. Mostly, they will give broad signals. To test whether a "hot rock" contains appreciable amounts of gold, switch to the ferrous target identification mode of your detector. With other detector types that do not have a ferrous target identification mode, the hot rock can be cracked open and both halves tested. If both halves give the same response, it can be discarded. Of course, it may not contain any gold, it may just be a lump of iron oxide.
Gridding is employed to comprehensively cover a section of ground. After a nugget has been found, the area should be gridded and explored thoroughly. This is done by marking a rectangular grid with a pick or trailing a chain. A grid is formed by marking the corners of a 10m by 5m rectangle. Next, the ends of the rectangle are marked off in one step (1m) intervals. Detecting is started at one corner and continues along the length of the rectangle. When this is completed, the operator moves to the next grid mark and follows this lengthwise so that he eventually moves across the whole of the rectangle in 1m intervals. Even when an area is gridded it is possible to miss gold. The best solution is to slow down and detect carefully.
Dryblowing and Hydraulic Concentrating
Dryblowers and hydraulic concentrators are used to recover fine gold and nuggets. Consequently, alluvial, eluvial and supergene deposits, which are most likely to concentrate fine gold, are the main targets.
Alluvial deposits are restricted to present day stream channels and immediately adjacent ground. The latter is deposited by migrating stream channels that change course regularly (being deposited in the abandoned channels). Basal channel deposits usually contain most of the gold. These are marked by conglomeratic or coarse grained beds in the subsurface or along deeply cut banks. Places to look for alluvial gold include creeks and gullies along hill sides and in depressions between hills. Eluvial deposits occur on hillsides and in depressions between hills.
Loaming
Loaming is the technique of systematically sampling and testing soil for particles of gold. Loaming is carried out to locate and test gold deposits and trace shows back to their source. Loaming using gold pans was widely employed by the early prospectors. Today, sampling machines can be used instead of gold pans to test soil samples for gold. Automatic gold pans (concentrating wheels) and small, portable dryblowers are two examples.
Prospecting for Quartz Reefs and Other Deposits
Reef prospecting involves locating gold bearing quartz veins. Most of the accessible reefs have probably been found by early prospectors and explorationists; consequently, remote and poorly outcropping reefs are more likely to be found. Today, in the short term, this form of prospecting is not as rewarding as metal detecting.
Surface weathering of outcropping quartz reefs distributes gold away and downslope from the reef, resulting in the formation of alluvial and eluvial deposits. Consequently, it is possible to trace the alluvial or eluvial deposit upstream and upslope until the source reef is located. Often, the reef has been completely weathered away, leaving only alluvial and eluvial deposits.
Once a quartz reef is located, it may be rewarding to follow the reef along its length searching for auriferous locations. Gold concentrations can increase and decrease along the length of a quartz reef.
In areas that are poorly exposed, reef prospecting is mainly restricted to the low hills and rises, where outcrop is best. In deeply weathered areas, the surface expression of quartz reefs will be in the form of supergene deposits (described previously). The presence of gossan is an indicator to an underlying orebody. Gossan is the weathered product of an orebody and is stained various colours from the oxidation of ore minerals. It generally consists of iron oxide minerals with a relict box work texture left behind after the removal of cubic pyrite. Since pyrite is often associated with gold deposits, gossan may indicate the presence of an orebody.
Within greenstone belts, mafic rock types should be targeted as the most likely host rocks. Meta-basalts and meta-dolerites are common host rocks; however, virtually all rock types are represented. Auriferous quartz veins are mainly controlled by faults and shear zones. The major regional faults and shears are barren of gold mineralisation. Secondary (and later) faults and shears, leading off the regional structures, contain major quartz reef and lode deposits. Alteration haloes around quartz veins and structures (faults, shears and fractures) are indicators to gold mineralisation (particularly the presence of iron sulphide minerals). Gold is present as submicroscopic particles in sulphide minerals (pyrite, pyrrhotite, chalcopyrite, arsenopyrite) plus/minus free grains in veins.
The best method for correctly identifying sulphide minerals, particularly microcrystalline grains, is polarized light microscopy (petrography). A petrography laboratory routinely does this type of work for a moderate price.
Whenever quartz veins or zones of alteration are encountered in the appropriate geological environment they should be sampled. In some cases, fresh bedrock will not be preserved in outcrop. Laterites, the weathered product of fresh rock, are most common. In some situations, it is sufficient to sample laterite, provided the laterite profile is residual (overlying bedrock) and unmodified, since gold is fairly chemically immobile and resistant to chemical weathering, some residual gold will usually be preserved. This will vary from area to area according to the degree and type of weathering. One disadvantage is that the original rock texture is obscured by weathering; therefore the prospector cannot be certain of the rock type being sampled. Once the sample is obtained, a sample mill or dolly pot is required to crush the sample. The sample can then be panned to determine weather any free gold is present; or preferably, samples can be assayed by a lab (this would not be of interest to the small scale prospector). If the sample gives a significant result, it can then be examined microscopically to determine the nature of the ore (whether as free gold grains or in specific sulphide minerals).
Geochemical Prospecting
Prospectors with some vision and adequate resources prefer geochemical testing of soils and rocks to the loaming technique. Geochemical sampling can identify and locate deposits with poor surface signatures, such as, when gold particles are present in insufficient quantities or coarseness to show up in a gold pan or concentrator. Geochemical prospecting is carried out to locate hidden orebodies that are without visible surface indications or to define the location, distribution and size of a known deposit. With this type of prospecting, samples are collected and sent to a lab where they are analyzed for gold and elements associated with gold (pathfinders, particularly As). This type of prospecting can identify a variety of deposits- quartz reef and lode, supergene, hydrothermal replacement etc.. Soil sampling is done to locate and analyze the distribution of alluvial and eluvial deposits or locate anomalies that overlie hidden orebodies. Geochemical sampling of outcrops can be done to determine their gold content. Soil sampling or stream sediment sampling can be carried out to analyze gold or pathfinder elements. For detailed evaluation of prospects, contour maps can be drawn to show the distribution of elements. These may show the distribution of alluvial and eluvial deposits or the location of anomalies, indicating the presence of an orebody (where elements are most concentrated): for example, a reef.
Geochemical sampling permits accurate estimation of the grades and reserves of a gold deposit.
For some deposits containing microscopic gold (some shear related, hydrothermal replacement, quartz reef and stockwork deposits) geochemical analysis is the only method able to identify them.
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