Hi Rich,
Mineralized Soil (Hot Ground) is the sort of ground that Native Gold (ie Nuggets) commonly occurs in.
The X-Terra 70 is one the handful of detectors which can handle highly mineralized soils. It's also one of the very lightest detectors which is an attractive benefit over some of the much heavier (though more dedicated) gold detectors.
When detecting for gold in America, Canada or South America, the issue of highly mineralized soil is not as much of a problem as it is in Australia where the soil is apparently far richer in other annoying minerals. So Australian soils tend to be classified as some of the "Hottest" soil in the world.
To hunt for Gold, you need to use a detector that is considered suitable for Mineralized Ground. Not all detectors can be used for nugget hunting since the vast majority of detectors are best suited to coins. Whilst ANY metal detector will find gold on the surface, you need one that can read through the mineralized soil in order to detect nuggets underneath and this is where just a handful of detectors can do the job. The X-Terra 70 is one of those detectors. The GPX 4000/4500 and other detectors like the GP series are considered "Dedicated Super Gold Detectors" and the Sovereign GT from Minelab is also in the same class of detector (as is the X-Terra 70) which can actually punch through that mineralized soil and detect a nugget.
The type of coil you use DOES make a big difference. The accepted favorite is a DD (Double D) coil which is named for the fact that the concealed emission cable inside the coil cover forms the shape of the letter 'D' back to back... which can form an ellipse or a disc shaped coil. The frequency closest to 20khz is considered perfect for punching through the mineralized soils on the Goldfields to detect nuggets. In the case of the X-Terra coils, the 18.75khz coils are considered the 'highest frequency' coils available in the range (sometimes referred to a Low Frequency compared to other older machines) and therefore are the best choice for those hunting for nuggets since they read deeper through the noisier soils than coils of other frequency.
Now I use an X-Terra 70 and I tend to prospect with a friend who uses a generic detector. We're in Australia. I've watched him pass right over the top of a target with his coil and miss a small gold nugget that I spotted... I then ran over it with my X-Terra 70 which signaled loudly. He was mortified at missing it although I gave him the nugget as a memento. This convinced him that he needs a better detector, one that can indeed read the ground better as his had been confused by all the noisy soil and even moisture nearby.
The GPX 4000/4500 is the top of the line Super Gold Detector and as such, it's a little (a lot) more expensive than the X-Terra 70. But with greater expense comes greater weight and slightly more complexity in using it. The GPX detectors are completely unsuitable on the beach for example. Sure, they'd work at picking out a signal for a buried beach target but they're really ONLY made for gold detection and not for beach combing. The X-Terra 70 on the other hand is designed to be used on the Beach AND on the Goldfields. It even sports a Beach Mode plus a Prospecting Mode.
Now Mineralized soil (Hot Soil) can be composed of nearly any other mineral that would give other detectors a hard time. A detector NOT designed for hot soils is going to be squawking at all sorts of sounds every couple of feet or so. These sounds can be caused by so-called Hot Rocks (ironstone etc) or by Soil that is impregnated with leeching minerals from a nearby source. Australia has some of the "worst" soils in the world, in my opinion. The hotrocks and hot soils cause many detectorists to dig non-existent targets and the best way to avoid this is to correctly use the Noise Cancel and Ground Balance features on your detector to make sure you only detect legitimate targets.
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Pictures I took recently showing red soil in Sofala and Yellow soil in Hill End. Both soils in the pictures produced LOTS of gold nuggets. Sadly, I got there about a hundred years too late. The red soil in the top picture is part of a Mulloch heap and was dug by early prospectors in 1853. The yellow soil at Hill End in the bottom picture has some recent holes in it from detectorists hunting for nuggets.[/size]
Since Gold and Iron are read the same way by any detector, you may find yourself detecting and digging iron horseshoes and lots of nails on the Goldfields.
Iron and gold often occur in quartz and even together so that a gold nugget may look rusty in appearance before cleaning. You'll need to be prepared to dig many non-gold targets in order to find what you want.
Now the major benefit for the GPX detectors over the X-Terra 70 is that they're more powerful and can detect somewhat deeper. They use Pulse Induction (P.I.) too which is a slightly different type of detection than the X-Terra 70 uses.
Mineralized soil is often a brighter color than typical dirt. Gold nuggets can be washed many kilometers from the source that they formed in but in many instances, you need to be scanning soil that is rich in other minerals like tin, silver and iron ore. Iron will make the soil rather red in color. But not all red dirt contains gold of course. Various clays (Including one called Pipe Clay) can contain nuggets and this clay often contains minerals which give it a yellow color. Other times it's grey. You want to look for gold where it has been found before. Then try to find places where fewer people have been scouring. The newer machines like the GPX will often find nuggets were other machines have found nothing or have been used to pick the place clean in the past.
The X-Terra 70 has the ability to "Track" the ground and this allows it to cancel out sections of mineralized soils as you slowly move across them. Without this feature, you would need to rebalance your machine each time you moved onto different ground. It can also display numbers to show you how much the soil is changing under your coil. Handy if you strike a target but want to know if it's legitimate or just a pocket of enriched soil.
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A picture I took from my car. This is the remnants of a Reef Mining operation which my road cut through. The large chunk of quartz had been shattered nearly 150 years ago by miners trying to find gold inside the quartz reefs. That rock now sits in my Garden.[/size]
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Typical Hot Rocks stuck to my magnet. Iron Ore tends to stick to magnets should you carry one on the end of your pick. Some other targets from that particular day included the obligatory horseshoe and .303 bullet.[/size]
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Many nuggets look like this: A combination of Gold and Iron Stone. Many Prospectors remove the Ironstone with Acids.[/size]
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Hematite, Quartz & Gold. Again, the iron oxide stains this Specimen with a rustic iron color. Much like the soil it was found in.[/size]
Whilst the X-Terra works fine in the Gold Fields, a person making a living as a Gold Prospector is probably going to want to look at a more dedicated machine for the job such as the GPX 4000/4500. There are other Minelab models like the Eureka Gold or the SD 2200v2 which also work well for professionals and all these machines have the advantage of larger coils to allow for detecting deeper nuggets. But the main advantage that the GPX detectors have over the X-Terra 70 is that they are quiet and stable over wavering and faint targets. The X-Terra can make a rising pitch sound over clustered minerals under the ground yet the GPX will completely ignore the same signal. This means you'll tend to dig more false targets with the X-Terra than the GPX. The GPX can also sense deeper into the ground which means half a day might be dedicated to digging a single hole ...which may (or may not) result in a gold target.
Many X-Terra owners find small specimens and small gold which the GPX detectors ignore due to the size and location (under water) so owners of the GPX 4000 sometimes keep an X-Terra 70 handy as a backup machine for certain areas. Why do they do this? Because the X-Terra 70 has many of the features typical of the high end detectors. That's why they're so popular.
A PROBLEM FOR BOTH DETECTORS: BOTH the GPX 4000 AND the X-Terra 70 can be fooled by soil that is saturated by minerals from nearby water runoff (usually on the side of a hill or a sloped area). Whilst one can eventually distinguish this type of soil, both detectors will signal a legitimate target. But both detectors can still be fooled. Best way to determine if this is what you've found is to scan around slightly uphill or downhill and if the signal blankets the area, it's a good bet that this is what you've found.
