Hi,
I have a Gold Bug 2, a GMT, and a GPX-4500.
There is no way my GPX or any of the other Minelab units I've owned (SD2200, GP 3000, GP 3500) can come close to the Gold Bug 2 or GMT for the smallest gold. Sure the GPX can hit gold under a pennyweight. But I can hit gold weighing less than a 10th of a grain (not gram) with either the GB2 or GMT. Gold you can roll around on a small coil on my GPX and get no signal. This is inherent in the design of PI units which have a pulse delay wherein targets below a certain point experience a signal decay rate that simply will not allow them to be detected. An IB (induction balance) detector, what most people refer to as VLF these days, is able to detect these targets that a PI will never be able to detect. Anyone who thinks otherwise does not understand the inherent limitation imposed on a PI unit by the very way in which it works. In theory the pulse delay can be shortened to the point where these tiny objects can be detected, but then you also lose all the properties that make a PI better than an IB detector, like the ability to see through hot rocks.
I just did a post on why I own both the GMT and Gold Bug 2 at
http://forum.treasurenet.com/index.php/topic,252713.0.html
But there is always a catch. In extreme mineral conditions hot VLF units suffer more than lower frequency units. In low mineral ground there in no question the GMT or GB2 will be the better choice for the tiniest stuff. But as you crank up the mineralization you do reach a point where the PI units are going to be the better choice. A person with the Minelab and a small coil will get better depth on small gold than a person sporting a high frequency VLF unit if the mineralization gets bad enough. Super sensitivity does you no good if you can't get any ground penetration. In severe soil a Gold Bug 2 must have its sensitivity reduced so much to handle the ground (otherwise it will overload) that you give up all that hot performance. The fact unfortunately is there will be small gold there that no detector will find as by ignoring the ground you also end up ignoring gold that runs down into the ground range. Yes, at the smallest end gold and iron overlap and so tuning out the ground also causes certain gold to be tuned out.
For the same reason a lower frequency VLF unit can have an advantage in high mineral ground. The lower frequency MXT or Lobo, etc. are inherently less sensitive to tiny gold but also handle bad ground better.
Which is why I own all three. There simply are no cut and dried answers in metal detecting. If you only detect one area you can come up with one best solution. But I get around a lot. If I head south of Anchorage I get into super low mineral conditions and lots of small gold. Time for my Gold Bug 2 and 6" coil. But head up north to Petersville, and you get into graphitic slate that the GB2 and GMT call gold nuggets. Time for my GPX. And if I head up to Ganes Creek, there are hundreds of deep pieces of iron and steel that will bring a PI user to their knees in exhaustion. Time for a big coil on my GMT. Or Maybe an MXT or an F75.
Now back to the original question - the gold mentioned was specifically mentioned as "wheat grain size". There is no doubt a Minelab with a small coil can hit gold that small. So the question boils down to ground conditions. For milder ground I'd go GMT or Gold Bug 2. The hotter the ground, the more sense a lower frequency unit like the MXT or a PI like the Minelab makes, and the original question mentions hot ground and hot rocks. So I'm going with the PI guys on this one. But the SD2100 is now no longer available new unless a dealer has some old stock. The Sd2200v2 is slightly over the mentioned budget of $1500.00. So for new PI you'd be looking at the Garrett Infinium or White's TDI. Or a better solution might be a used SD2200 which you should be able to get for under $1500.00
The real problem with the internet is that we all live in different places with different types of gold, different ground conditions, and different amounts of trash in the ground. Everyone chimes in with what works best for them in their location and the fact generally is that everyone is being honest and each is absolutely correct for their given location. But it leads to interesting arguments over this is better than that when there really is no way the question can clearly be answered without taking in all the factors in any given location. And lots of times something as simple as how much money a person has to spend is a real factor. Sometimes you just have to do the best you can with whatever you can afford. I never thought I'd see a day when a detector sold for over $5000!!
Steve Herschbach
http://www.akmining.com/mine/steve.htm
P.S. Interesting side subject but related is the problem inherent in hunting salt water beaches. Salt water is conductive, so a detector must be designed to ignore the salt effect. PI detectors are inherently good at this. The problem is the salt signal overlaps that of small gold or platinum items like ear rings or thin gold chains. So there are items on a salt water beach that are undetectable with current technology. In order to detect them, the detector would also have to detect the salt signal.