Gold Rat Metal Detectors is located 6/50 Freda Street Upper Mount Gravatt. This is the only store to stock Metal Detectors. Gold Rat Engineering is located 4/16 Redcliffe Gardens Drive Clontarf This is where you will see the largest range of your highbankers, sluices and accessories


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Let's Talk Detector Frequencies and Coils

Let's Talk Detector Frequencies and Coils

Let's Talk Detector Frequencies & Coils

Generally, the lower the frequency used by the detector, the deeper it can penetrate the ground. At low frequencies however, sensitivity to small low conductive targets is reduced. The higher the frequency, the higher the sensitivity to small targets, but it will not penetrate as deeply.

For example, a Minelab GPX 6000 pulse-induction detector (which operates at a frequency of only about 1.2 kHz) and fitted with a 10x5” monoloop coil (the smallest coil currently available for the GPX 6000), will achieve significantly greater depth capabilities in terms of detecting deeper gold nuggets, particularly in highly mineralised ground, than compared to a Minelab Gold Monster 1000 detector with VLF technology (which operates at a frequency of 45 kHz) and fitted with a 10x6” Double-D coil.

Below are the nominated operating frequencies of a range of Minelab metal detectors:

GPZ 7000: “is configured to operate at a fundamental operating frequency of 3.675 kHz

GPX 6000: “Operating Frequency of 1.225 kHz

GPX 5000: "is configured to operate at a fundamental operating frequency of 5 kHz"

SDC 2300: "the transmit frequency is about 3 kHz"

Gold Monster 1000: "uses an intermediate 45 kHz frequency

CTX 3030: "Simultaneous multiple frequency transmission ranging from 1.5 to 100 kHz

MANTICORE: "Simultaneous multiple frequency transmission ranging from 5 to 40 kHz” (as well as the options of 5 selectable individual frequencies of 5, 10, 15, 20 and 40 kHz).   The Minelab Instruction Manual also includes the following information:   “MANTICORE does not have a 4 kHz single frequency setting. Instead, the All-Terrain High Conductors Search Mode provides a Multi IQ+ based mode that can achieve an operating frequency even lower than 4 kHz.

EQUINOX 900: "Simultaneous multiple frequency transmission ranging from 4 to 40 kHz” (as well as the options of 6 selectable individual frequencies of 4, 5, 10, 15, 20 and 40 kHz).

EQUINOX 700: "Simultaneous multiple frequency transmission ranging from 4 to 15 kHz” (as well as the options of 4 selectable individual frequencies of 4, 5, 10, and 15 kHz).

X-TERRA PRO: "Single frequency transmission ranging from 5 to 15 kHz” (options of 4 selectable individual frequencies of 5, 8, 10, and 15 kHz).

[By comparison - Minelab's series of 'Pro-Find' pinpointers (15, 35 & 40) all have an operating frequency of about 11.6 kHz]


Different Coil Configurations
Monoloop coils (often called “Mono” coils) are a popular type of coil for Minelab detectors operating with Pulse-Induction (P.I.) technology. These coils have a winding of wire around the circumference of the coil, which is used to both transmit and receive.

The theoretical detection signal pattern of the Monoloop coil is quasi-cone-shaped, requiring more overlapping. In extremely heavily mineralised grounds they can be more difficult to ground balance, however they tend to provide slightly better depth than the ‘Double-D’ (DD) type of coils.

Many Monoloop coils are elliptical-shaped, and hence have better pinpointing capabilities on small nuggets, than compared to an equivalent sized round coil.

Alternatively, “Double-D” coils (often designated “DD” coils) are a popular type of coil for Minelab detectors operating with Very Low Frequency (VLF) technology.

A Double-D coil has two overlapping wire windings in the shape of two D’s. The benefits of a Double-D coil are stability (especially in heavily mineralised ground), good depth, sensitivity and a thorough search pattern requiring less overlap.

When used on the Minelab GPX 4000/4500/4800/5000 series of detectors, Double-D coils (unlike Monoloop coils) are able to discriminate between ferrous and non-ferrous targets when the Iron Reject function is activated. 

Double-D coils are also more stable when used on wet salt beach sand, and also in electrically noisy environments.

Given the same diameter of coils, Double-D coils tend to be slightly heavier than an equivalent sized Monoloop coil.

Conventional configuration Monoloop (“Mono”) coils, and Double-D coils, would both not operate effectively with the ZVT technology used by the Minelab GPZ 7000 detector. Hence, Minelab invented the ‘Super-D’ configuration coils (sometimes referred to as a “DOD” configuration coil).

The Minelab Super-D coils consist of two symmetric D shaped receive windings (one on the left and one on the right), with a central, oval-shaped transmit winding. This Super-D coils winding geometry provides a double audio response for shallow targets, and a single audio response for deeper targets.

This configuration greatly decreases interference from magnetic soils, reducing ground noise.



The Range of Minelab Headphones

The Range of Minelab Headphones

 The Range of Minelab Headphones

Minelab manufacture an extensive range of about 20 different models of metal detectors, including a range of Very Low Frequency (VLF) technology detectors, and also a range of detectors featuring Pulse Induction (P.I.) technology. 

Nearly half of the range of Minelab metal detectors also include wireless Bluetooth headphones in the detector kit when purchased new.

Gold Rat’ stock a range of Minelab headphones:

Minelab Koss UR-30 (cable) headphones are compatible for the SD/GP/GPX series detectors, and also the SDC2300, GPZ 7000, CTX 3030, Safari, E-TRAC and X-TERRA series detectors.


ML 80 wireless headphones 

The Minelab ML 80 Low Latency (Bluetooth aptX) wireless headphones are compatible for use with EQUINOX 600, EQUINOX 800 and Vanquish 540 detectors. The ML80 headphones will not work with the Equinox 700 and 900.


ML 85 and ML 105 wireless headphones 

The Minelab ML 85 & ML105 Low Latency wireless headphones are both compatible for use with MANTICORE, EQUINOX 900, EQUINOX 700 and X-TERRA PRO detectors. The ML85 & ML105 headphones are not compatible with Minelab Equinox 600/800 detectors – that are compatible with the ML80 or ML100 Bluetooth headphones.




ML 100 headphones for the GPX 6000 detector (Bluetooth aptX Low-Latency).

The ML 100 headphones will operate for approximately 24 hours on a fully charged battery and take around 3.5 hours to charge.

The GPX 6000 can pair and connect with normal (A2DP) Bluetooth headphones and other aptX Low Latency Bluetooth headphones, including the Minelab ML 80 headphones. When connected to aptX Low Latency headphones, there will be a + symbol next to the Bluetooth icon on the detector LCD screen to indicate this.



Waterproof Headphones - compatible for use with MANTICORE, EQUINOX series and X-TERRA PRO detectors.



Minelab Pro-Sonic Universal Wireless Audio System Kit

The Pro-Sonic wireless audio system is compatible with many Minelab detectors including the SDC 2300, GPX series, Gold Monster 1000, GPZ 7000, and X-TERRA series.




Gold Rat’ also stock a range of other brands of headphones that are compatible with Minelab detectors:

MDX150 headphones:

The MDX150 wired (cable) headphones were designed for the Minelab GPX6000 metal detector (not waterproof). They are also compatible with the Minelab Equinox series (600/700/800/900) detectors, and also the new Minelab X-Terra Pro detector. These are an affordable, and comfortable set of reliable wired headphones, with crystal clear audio, and loud volume across the full range of tones. These headphones can not be used underwater.



SteelPhase Pro Series headphones (300 Ohm)

'Phase Technical' is an Australian business that is owned and operated by experienced detectorist and product tester, Nenad Lonic. Nenad Lonic is extremely knowledgeable about Minelab detectors. He is also a manufacturer of quality detector accessories under his own 'SteelPhase' brand name.

The 'SteelPhase' Pro Series headphones are built using high quality aviator muffs for excellent comfort and sound isolation. Highly sensitive 300 Ohm driver elements are used to deliver crisp signal responses. They have a nice bass response as well, for a punchy response on those deep signals.

Designed to be an ideal match to the SteelPhase sP01 'Audio Enhancement System', but will also work direct into detectors which have good volume output such as the GPZ7000, GPX4500/4800/5000, as well as VLF detectors.

The Pro Series features gel/foam filled ear pads and headband, a feature not previously available on prospecting headphones, which offers extreme comfort for extended periods of use.

Adaptor Leads are required for use with the SDC2300, and the GPX6000.



Gray Ghost Amphibian II Headphones

Gray Ghost Amphibian II Headphones are 100% waterproof and totally submersible to a maximum of 200 feet. They feature a convenient foldable design, and are made in the USA.



GPX 6000 Lithium Battery Use Tips

GPX 6000 Lithium Battery Use Tips

GPX 6000 Lithium Battery Use Tips

The rechargeable Minelab Lithium-ion battery for the GPX 6000 lasts for approximately eight (8) hours. It takes about 5 to 6 hours to re-charge from flat. It is rated at a nominal 7.2Volts, with a capacity of 5833mAh (5.83 Ah) / 42Wh. It weighs about 0.3kg. All new Minleab batteries have a 12-months Warranty in Australia.

‘Gold Rat Metal Detectors’ usually have several of the GPX 6000 batteries in stock, and they cost about $240. 


Here are some tips for general use, and also for pro-longing the lifetime, of a GPX 6000 battery:

1. Ideally, keep all loose/spare batteries individually isolated & protected within waterproof & dustproof, sealed plastic bags when not in use. This also prevents a potential short-circuit of the battery and possibly causing a fire. I also store my Lithium batteries within a bubble-wrap pouch – for padded protection against possible impact e.g. via dropping them etc. Broken or cracked battery cases can allow moisture and oxygen to enter the battery and oxidise the Lithium components, causing a heat reaction. This can lead to fires or explosions.

2. Keep the battery terminal contacts clean on both the detector, and also on the battery, otherwise a bad contact may cause the detector to cut off when it is turned on – since the detector control box is unable to receive a consistent & suitable electrical power input to function properly.

Minelab have issued a Product Notice guide - "showing how to effectively clean your detector terminals to prevent low battery run times and intermittent powering off when using your GPX 6000."


3. Always follow proper battery charging procedures, especially with Lithium-ion batteries i.e. if possible, fully charge the battery following each day of use, and also remove the battery from the charger as soon as practicable after it being fully charged (i.e. do not leave Lithium batteries charging & unattended on chargers for tens-of-hours or days after being fully charged). Don't charge Lithium batteries when they are hot.

4. I periodically use a digital multi-meter to check that my batteries are OK – that my GPX 6000 batteries each contain a maximum (fully-charged) voltage of between 8 and 8.2 Volts (a suitable maximum holding voltage – as recommended by Minelab Australia).

5. I have three Minelab Lithium-ion batteries for my GPX 6000 (for when I was gold prospecting in the bush for 7 to 10 days at a time on my regular trips) – which I individually numbered – and I regularly rotated battery usage so that they all were being used & charged equally – for equal longevity.

6. Most days when I am detecting - I change the battery at midday (after about 3 to 4hrs use) and then use another fully charged battery for my afternoon sessions of detecting.

7. Only use the Minelab-recommended/approved chargers for charging GPX 6000 batteries. Otherwise, you may be taking unnecessary risks that could endanger.

8. Don’t leave Lithium batteries within closed/hot vehicles e.g. upon the dashboard exposed to the sun etc.

9. Having a padded control box cover (like the Aussie-made 'Double-D' padded canvas covers) not only protects the detector control box and battery against inpact forces, but it also provides a thermal shield for the battery against direct sunlight and heat radiation.


Minelab provide the following recommendations in their GPX 6000 User Manual (Page 25) with respect to the batteries:

“The battery pack is not waterproof — Do not immerse the battery pack in any liquid or allow water ingress.

• Only charge rechargeable batteries and accessories according to the instructions provided.

• Avoid charging rechargeable batteries and accessories in extreme temperature conditions.

• Remove battery prior to air transportation”

Snake Season

Snake Season

 SNAKE   Season

Spring has sprung ….. and so have the “Joe Blakes” (SNAKES) !

Due to warmer and drier conditions forecast this year in Australia, it is anticipated that snakes may become more active.

Unfortunately, a  man  recently  died  in  Queensland  following  a  snake  bite  incident.

According to official statistics, Eastern Brown snakes are responsible for the majority of snake bite deaths in Australia. Up to 65 per cent of fatalities between 2000 and 2016 were attributed to an Eastern Brown snake, which is found across most of eastern Australia.

A total of  29 PEOPLE  DIED  from various snake bites between 2010 to 2019 in Australia.

About  THREE  THOUSAND (3,000) snake bites are reported each year in Australia.

Queensland Ambulance Service (QAS) data revealed more than 800 people were bitten by snakes in Queensland each year from 2020 to 2022. In January 2023 alone, 97 people were bitten in Queensland.

Six out of 8 (three-quarters, i.e. 75%) of the fatal snake bites in Australia since January 2020 have occurred in Queensland.

A QAS spokesperson said  MOST  SNAKE  BITES  OCCUR  ON  THE  LOWER  LIMBS,  and many happen when people are trying to kill or move a snake.

An adult Coastal Taipan snake has fangs up to about 0.5 inches (12mm = 1.2cm) long. In comparison, an adult Eastern Brown snake has significantly shorter fangs of only approximately 3mm long.

A recent ABC News report included the following information about the Taipan:

"The Taipan is listed as one of the world's most dangerous and venomous snakes. Its venom works fast, attacking the heart, the blood stream and the nervous system, causing paralysis and convulsions while the patient's skin turns black."


Gold Rat sell Aussie-made gaiters (in canvas, or leather), and also snake bite first aid kits.


My tips ..... when it comes to snakes (based on my experience in the bush throughout Australia over several decades):

1.  Always wear fully-enclosed shoes (preferably leather boots), thick socks, long trousers, and also high-quality gaiters - that cover the whole of the lower legs up to the knee. The other advantage of wearing gaiters (as I know from experience) - is that my high-quality gaiters also prevent the penetration of spinifex needles from harming me. 

2.  Never run through high grass.

3.  Never immediately reach down to the ground & move objects such as sheets of iron, logs, timber, boulders etc. - without firstly assuming a snake could be concealed under/behind such objects. Use a long-handled item such as a long stick or pole (or a long-handled shovel) to initially hit-impact such an object to reveal/or rule out the presence of a snake. Also wear thick leather gloves if possible.

4.  Always carry (including when walking) appropriate/quailty First-Aid equipment, and also know  how to apply First Aid properly , and also carry a recently-tested Personal Locator Beacon (or EPIRB)


A question to consider .......

Do Minelab pulse-induction metal detectors, when operating, repel snakes or not ?

I wonder !

Interestingly, I read that scientists from two Queensland universities recently found that some  Australian snakes such as  Taipans,  brown snakes  &  death adders  can hear, as well as sense vibrations.

Have I Found a Meteorite ?

Have I Found a Meteorite ?

Have I Found a Meteorite ?

Below are some links to websites containing informative guides on how to identify a meteorite:


The Differences between a Meteorite, and an Impactite, and a Tektite
In the simplest terms –

A Meteorite is a piece of rock or metal that has fallen to the earth's surface from outer space as a meteor. Not all meteorites consist of metals. Most meteorites (about 95%) that have been recovered on Earth are of the ‘Stony’ type.

Meteorites of high density are heavier, and strongly attracted to a magnet due to the amount of Iron and Nickel they contain. Most of the largest meteorites ever recovered on the mainland of Australia are of the Iron-Nickel Type. These are one of the least common types of meteorites found (only about 4% are of the Iron-nickel type). In fact, the largest recorded meteorite found in Australia was officially discovered in 1966 and it weighed over 12-Tonnes.


A Tektite is a terrestrial molten rock fragment ejected out of the crater during a meteor impacts.

Tektites are small, black blobs that might pass for hardened bits of asphalt but they are actually glassy stones. They commonly take on distinctive regular shapes like teardrops, jelly-beans, dumbbells, and interesting flanged buttons that look like the tops of large rivets with the stems melted off. Tektites are found strewn about on the ground in widely separated "fields" around the world, the largest of which covers most of Australia. Many prospectors find tektites throughout Central Australia and the south part of Western Australia.

An Impactite is a terrestrial rock modified by the high stresses of pressure and temperature during a meteorite impact. It is generally the rocks present in the impact crater. It can also include brecciated rock materials, and shatter cones.

XRF Testing of Metallic Specimens
Some jewellers, and gold/jewellery buyers, use a portable/handheld X-Ray Fluorescence analyser (commonly called an “XRF gun”) to non-destructively test items - in order to determine their metallic composition. XRF guns can cost in the order of tens-of-thousands of dollars to buy.

Accordingly, suspected metallic meteorites can be tested by an XRF gun to quantify the percentages of metals present.
If you have a piece of metal that does attract a magnet, and you want to know if it is an iron-nickel meteorite - then you could try and obtain an XRF analysis for the elements of: Iron (Fe), Nickel (Ni), Cobalt (Co), Chromium (Cr), and Manganese (Mn).

Iron-nickel meteorites will typically contain about: 75 to 95% Fe, 5 to 25% Ni, 0.2 to 2% Co, and less than 0.05 % each of Cr and Mn. The nickel/cobalt ratio in meteoritic metal is usually in the 10 to 25 range. If the metal contains more than 0.05% Chromium or Manganese - then it is NOT a meteorite.

Recently, a customer visited our store to seek advice about some interesting metallic specimens he had detected in Queensland. The customer suspected they may have been iron-nickel meteorites. I was informed that the specimens were very heavy, and very-strongly attracted to a magnet. I was advised that subsequent XRF analysis results indicated the following composition: Fe 97.65%, Mn 1.2%, Ni 0.68% and Cr 0.56%. Based on the absence of Cobalt, and also the Manganese & Chromium contents both significantly exceeding 0.05% - these XRF results indicated that the subject tested specimen is NOT an iron-nickel meteorite.

What is a Widmanstätten pattern ?
The Widmanstätten pattern (also known as Thomson structures) is a distinctive formation of interweaving (cross-hatched) lines that appear in some Iron-Nickel meteorites when a cut & polished cross-section of such a metallic meteorite is etched with weak acid.

The methods used to reveal the Widmanstätten pattern in iron meteorites vary. Most commonly, the meteorite is firstly faceted or cut/sliced, then an exposed face is ground and polished, then cleaned, and etched with an acidic etchant (e.g. a mixture of 1-part Hydrochloric Acid, added to 2-parts Hydrogen Peroxide in a non-metallic container). Then the prepared specimen is washed, and dried.

Can I keep a meteorite I found in Australia ?
That depends on WHEN ?  HOW ?  and WHERE ? you found a meteorite in Australia.
Most of the States (and Territories) in Australia have laws (since from about the 1970’s/1980’s) which deem any meteorites found in those particular States to be the property of the Crown or State.

However, those laws do not apply to other specimens like Tektites (Australites) and Impactites – that are also often formed when a meteorite impacts the ground. Sometimes Tektites and Impactites are also found near an impact crater.

Queensland’s Fossicking laws prohibit the collection of meteorites in Queensland whilst fossicking.

However, it is my understanding that if a person was in Queensland or New South Wales, AND was lawfully permitted to be upon such land AND also permitted to remove any specimens found on such land, AND if by way of chance discovery (e.g. NOT fossicking in Queensland) found a suspected/unconfirmed specimen – then they would be able to take possession of such a specimen.

Can I lawfully sell a meteorite I found/acquired in Australia ?
If you lawfully found a meteorite in Australia, or lawfully acquired a meteorite within Australia – then you are lawfully permitted to sell the subject meteorite within Australia.

In 2021, an ABC News report was published online stating that in 2016 two men, who were reportedly fossicking for gold on Western Creek Station, near Georgetown in North Queensland, detected (using a Minelab pulse-induction metal detector) a meteorite weighing 24.3 kilograms. It was also reported that they sold this Iron-nickel meteorite to ‘Geoscience Australia’ for AU$200,000 (i.e. over AU$8 per gram).

Similarly, to the above 24.3kg meteorite being detected on Western Creek Station – in April 2016 I was informed by an Australian prospector that in early 2016 he was gold prospecting using a Minelab GPX 5000 detector on the eastern part of Western Creek Station (near Georgetown) where he detected a 9.6kg metallic specimen (a suspected Iron-Nickel meteorite) at a depth of about 0.4 metres.

The sale of a meteorite in Australia to an overseas buyer is prohibited without an official export permit pursuant to Commonwealth laws.

Recent Unusual Meteor Sighting in Australia
In May 2023 it was reported that a fireball lit up the sky in Queensland between Mackay and the Gulf of Carpentaria. NASA confirmed that this meteor was the LARGEST recorded over Australia in at least 30 years.

When it exploded, the meteor had an altitude of about 30 kilometres above Blackbull, a small rural locality between the Gulf communities of Normanton and Croydon, in north-west Queensland.

The data also revealed the meteor was travelling at a velocity of almost 30 kilometres per second. Scientists estimated that the meteor had a diameter of about 3.5 metres (of equivalent size to an average caravan) and weighed about 80 tonnes.

My further enquiries with NASA obtained additional data indicating that when the meteor was detected travelling from an altitude of 100km, down to 30km altitude, it was heading approximately Westwards (with an approximate bearing of 280 degrees), whilst falling at an angle of about 40 degrees.

Due to the blue and green colours of the meteor fireball observed, it is suspected by scientists that this meteorite was a metallic (Iron-nickel) meteorite.

Will fragments of this meteorite be found in Queensland ? - including perhaps by someone using a metal detector ?

NASA data also indicates that in recent years (since 2014), there have been several significant fireball events detected in North Queensland.

Where in Australia have meteorites been found ?
The following free on-line search database contains over 700 records of various types/sizes (and photos of) meteorites that were found in Australia over the past few centuries:

In recent years a few large meteorites have been detected on Western Creek Station in North Queensland by gold prospectors using Minelab detectors featuring pulse-induction technology.

Suitable Detectors & Coils for Meteorite Hunting
Meteorites containing sufficient quantities of iron/nickel (including the 'Stony-Irons' Meteorite Types) are detectable with a metal detector. Since metallic meteorite fragments are sometimes strewn about an impact crater, often for many kilometres away from the crater, then a suitable metal detector is a lightweight detector with a large coil – which is ideal for quicker ground coverage, and also greater depth capability.

Many meteorite hunters use a lightweight, Very Low Frequency (VLF) technology detector fitted with a 15-inch DD coil.

Recently released Minelab VLF detectors – which can operate using a variety of DD coils ranging in size from 6 inch diameter up to 15 inch diameter - include the X-TERRA PRO ($499), and Equinox 700 ($1,079), and Equinox 900 ($1,499).

Both Minelab, and Coiltek, manufacture a 15-inch diameter coil for these VLF detectors – priced at about $400 to $450.



Furthermore, in about early December 2023, Coiltek released a new BIGGER coil - the  Coiltek 18" NOX coil  ($470). It is a DD configuration coil of 18-inches (45cm) diameter - that is also compatible with the Minelab Equinox range of detectors, and also the Minelab X-TERRA PRO detector. This coil weighs nearly 1kg, is rated as waterproof to a depth of up to 5 metres, and carries a Coiltek Warranty for 2 years.


You don’t necessarily need a more expensive & heavier Pulse Induction (P.I.) technology metal detector to detect metallic meteorites. Other than a VLF detector, some meteorite hunters also use a lightweight, telescopic walking stick – fitted with a strong, rare-earth magnet attached near the end closest to the ground.

Tektites (which are of a glassy composition) are not detectable by a metal detector. Most are found by sight – often discovered washed down to low points (“sumps”) in the terrain (e.g. gullies or clay pans or salt lakes).

Many prospectors detect so-called “Hot Rocks” throughout Australia. Often many prospectors will simply discard such metallic specimens without further examination of them.

Mostly, such “Hot Rocks” are of terrestrial origin, and sometimes they can contain minerals such as gold. I have detected several hot rocks in W.A., and upon breaking them, I found they contained gold either in the form of a nugget, or a specimen, or concentrated gold mineralisation.

If such hot rocks are very heavy (e.g. like the weight of iron steel), and also highly magnetic, and also have smooth indentations (called “regmaglypts”) on the outer surface - then perhaps the specimen may be more than just a highly-mineralised, terrestrial hot rock ? ….. possibly a meteorite ?

Meteorites containing Gold ? ?
Over the decades, I have heard many explanations/theories (including from experienced geologists and prospectors) on how gold was formed on planet Earth. Most theories relate to terrestrial formation processes. In recent years, some scientists believe that gold was deposited upon earth by meteorites. A study published in 2011 suggested that:

"A massive meteor bombardment 3.9 billion years ago provided most of the gold and other precious metals found near the Earth's surface today"

Interestingly, a United States Geological Survey report published in 1968 stated that:

"The reported gold contents of meteorites range from 0.0003 to 8.74 parts per million. Gold is siderophilic, and the greatest amounts in meteorites are in the iron phases. Estimates of the gold content of the earth's crust are in the range of 0.001 to 0.006 parts per million."