Getting Aquainted With The GPX 6000

[Lanny]

Simon,

You do a stellar job in your write-ups, lots of useful information, and it's fun to tag along as you learn.

All the best,

Lanny

[Valens Legacy]

Another great write up as usual from you Simon!

Glad that you were able to find some of that yellow stuff while checking out the new toy.

Glad to hear that your unit is working as it should for you and I like your new toys also.

Some day I may have to get the 6000, but I think I need to really learn the 800 better.

I need more experience at detecting before spending that kind of money for a new unit.

Good luck on your next outing.

[phrunt]

I was doing some research into why the gold is often found inside the schist and I spend my time smashing it out and came up with this document

Gold In Otago.

Gold occurs in two distinct geological settings in Otago: within quartz veins in the schist (hard-rock gold), and as detrital gold grains in gravels (alluvial gold). The quartz veins which contain the hard-rock gold were formed by hot water (200-400°C) passing through fractures in the schist up to 15 kilometres below surface. The passing hot water deposited the quartz and gold as it rose and cooled through the schist bedrock. Hard-rock gold forms the original source for the alluvial gold, but many alluvial deposits have been derived by erosion of older gravel deposits. Gold is 19 times denser than water, and does not move far in streams unless it is as small particles (less than half millimetre).

The history of gold in Otago goes back about 150 million years, when Otago was mountainous, similar to the Southern Alps of today. The mountain ranges extended from Chatham Islands westwards beyond Wanaka, into Marlborough which at that time was immediately west of Haast. These mountains were made up of schist and greywacke as in Otago and Southern Alps today. Beneath the mountains, gold-bearing quartz veins filled active faults formed by earthquakes which accompanied the rise of the mountains. Similar processes have occurred during many times of fault activity and mountain building over the past 150 million years, and are still occurring today beneath the Southern Alps. Marlborough moved along the Alpine Fault nearly 500 kilometres northeastwards from Otago, taking its gold with it, over the past 20 million years.

As the mountains rose, they were eroded and gold from the quartz veins was concentrated into layers of coarse gravels on the eroded bedrock. Further uplift meant that these gravels were in turn eroded and the gold was reconcentrated. At least 15 kilometres of rock have been removed from over Otago. All that remains of that rock is the widespread alluvial gold, for which Otago is famous, and small amounts of gravel.

[phrunt]

I went back to this spot again yesterday with JW, this time we were both better equipped, he finally got his replacement GPX 11" Coil, it just appeared in the mail with no notice it was even coming!   He's now put that funky tape all over the top like others have seeing it was a new coil he wanted to protect it from scratching to bits.  I haven't done that with mine, if I wear a hole in the top of it Minelab can replace it for me under warranty.

We arrived and I zoomed straight over to where I found the two nuggets in the bedrock last week but this time I was using the GPZ and 8" X-coil.  I had slammed that little area last week with the 6000, spent almost half a day on a small area of bedrock making sure I'd done every inch of it so this was to see if the extra depth on small nuggets I know the 8" coil gives would find me another one.  It turns out it wasn't the extra depth that found me another one, it was the smaller size.

It is hard to see in this photo but above the scoop is a crevice in the bedrock, below the scoop are 3 big rocks (it's on a downhill slope so my aboves and belows may sound confusing by the photos)  those rocks were positioned either side of the crevice making a bit of a bridge across it, I was able to poke the 8" coil up under them and got the signal, quite clearly too, it was very obvious and I'm sure the GPX would have hit it too, if the 11" coil could have fit up under the rocks, but of course it couldn't.  So in this instance the find was more about coil size than detector performance.

This was the little nuggie

So, I checked the rest of the area and the 6000 had missed nothing else, even though I'm confident the 8" is deeper on small gold for this to be realised the gold has to be in that 20% or so range outside of the GPX depth and within the 8" depth so for me to find a nugget in that exact scenario and know the nugget was found because of that scenario is probably not overly likely very often.  I'm never the best at covering every inch of ground 100% effectively too so I'm unlikely to say the GPX missed this but the GPZ got it, I just know which I have more confidence in.

I walked over to JW who was detecting about 20 meters away with his 6000 at the perfect time, he had a target that was likely not to be a pellet, too deep for a start for a lead pellet and not a booming signal by any means so I hung around for the recovery as I was sure he had gold.  It took quite a while to get it out, the GPX is difficult to pinpoint so he was messing around a bit, I offered to go get the GPZ to recover it but he persisted, practice makes perfect I guess 🙂 He eventually got it out and into the scoop and narrowed it down to the last bit of soil in there and said damn, a pellet as there was this round little rock in the scoop.  I said nope! look there, I saw a little glisten of gold.  Indeed it was, a pretty small bit for it's depth too.

With that mission complete I switched back to the GPX and went off exploring new to me areas, not new areas as there were dig holes evidence everywhere from JW and some other person that's been going to the area. 

I spent an hour or two scouting around digging and avoiding pellets and struggling with EMI and then I detected over the top of flat piece of schist and had a signal, I scraped the surface off it to clean off the plant life and soil and the signal was still there, very faint but consistent,  Some of these schist rocks have a green or iron type stuff inside them so I wasn't sure if that was setting it off, the GPZ gets troubled by the green ones so I was going to blame that but I thought I'd try lift the rock out just in case, I used my pick to leaver it up and it snapped in half but I was able to move the half away and the signal was indeed under the rock.  I recovered it and took a little video.

I was pretty happy with that, yes I think the GPZ would have got it too, but I'll never know.

JW came over shortly after while I was recovering a pellet and said it was lunch time so we went and had lunch, he was up to 3 nuggets now from memory to my 2.

I decided I'd had enough of the 6000 for a while, I just enjoy using the GPZ more, I love how stable it seems compared to the GPX, I'm now running the sensitivity on the GPZ at 20 all the time, I used to run it on 18 as I felt going to 20 made it less stable and I prefer stable but now after running the GPX then 20 on the GPZ feels incredibly stable by comparison.

My other mission was to go work right under the power lines, this is a big advantage of the GPZ, it just does the job and the 8" is awesome under power lines.   I don't need to drop my sensitivity or put on a DD coil to work there, I can run my normal settings which are maxed out normal/high yield/20 gain and continue on as if they're not there.

I walked up there and found JW's scoop on the ground, he'd lost it when he walked past to another spot so I put it in my pocket for when I next saw him, I had no idea where he now was I couldn't see him.  I normally listen out for the pick noise hitting the bedrock but couldn't hear it so he was quite far away.

I continued on detecting under the lines and was recovering lots of little tiny shards of metal and I was wondering if they were something to do with the power lines when they installed them, a digger bucket or something.  I also ended up getting a nice target sound, a boomer really but it's depth continued down to the bedrock.

A nice size nugget, at first when I dug it I thought it was junk, one side of it is covered in iron stone or something, very weird for around here to have a polluted nugget.

That's so weird for a NZ nugget from what I've seen!

The other side looks almost normal.

This was when I look up from where I found it.

The dig hole to the right of the coil.

And a little video I took after the recovery

I was using the ML-100's on the GPZ using a MPow aptX LL transmitter and oddly the hiss I'm always complaining about with these headphones on the 6000 doesn't exist with this setup, I'm now wondering if the hiss is caused by interference to the Bluetooth transmission from the GPX, it's obviously not a fault with the headphones, so weird.

My biggest of the day!   Shortly after finding this and covering that small bit of open ground under the power lines I swapped back to the GPX, I took it up to this spot but didn't have my DD with me and tried the 11" but it was far too unstable, even on lowest manual sensitivity I just couldn't justify using it there but I persisted and checked the spot anyway and found nothing I'd missed, the only pellets it found were ones in my scrape and rejects and no other targets I'd missed.

And my junk for the day

The tiny little junk targets at the top were with the GPZ and they were the little bits I suspect are from the digger bucket when they cleared the area to put in the power lines.

I'm going to give the GPX a rest now I think, I'll start using it again once I get an aftermarket coil for it and see if I like it better then.  I was going to get a NF and the Coiltek and now I think I'll just get one and see if I find it any better for my needs then before buying the second one, no need to throw more money at it unless I start to enjoy using it as that's what it's all about, if you don't enjoy doing it there is no point doing it and I really enjoy the GPZ.

JW ended up with 5 nuggets for the day to my 3 but because of my bigger one I might have the most weight.

 

[GotAU?]

Nice writeup Simon, thanks! Say, when you mentioned avoiding pellets with the GPX, what were you doing and listening for? If possible, can you post an example video with the sound of doing that with your GPX? 

[phrunt]

23 minutes ago, GotAU? said:

Nice writeup Simon, thanks! Say, when you mentioned avoiding pellets with the GPX, what were you doing and listening for? If possible, can you post an example video with the sound of doing that with your GPX? 

Yea sure I'll film it next time, I avoid them with the GPZ too as with the 8" you get the double blip as you pass them, it's mostly an odds game, pellets are often on or near the surface, with the GPX it's pretty easy to avoid them in this scenario as it really ROARS on them when they're right next to the coil, so if I do a few scrapes with my pick and signal is no longer in the same place and moved into my dig pile then it's less likely to be gold, combine that with the GPX losing the roaring target signal as soon as the pellet has any depth by being buried in my dig pile under a few inches of soil and the GPX signal becoming much more faint or even disappearing on some smaller lead pellets then I write it off as a pellet. Gold tends to keep its signal strength better than lead pellets.  We don't have many surface type nuggets around here unless on bedrock so this only works when there is a decent amount of top soil on the bedrock.

As you can see by how many pellets I dug a lot of them are too deep for this trick to work.  These were all pellets that survived past the first few scrapes and could have possibly been gold.  I rejected just as many as I dug I would guess.

[GotAU?]

11 minutes ago, phrunt said:

Yea sure I'll film it next time, I avoid them with the GPZ too as with the 8" you get the double blip as you pass them, it's mostly an odds game, pellets are often on or near the surface, with the GPX it's pretty easy to avoid them in this scenario as it really ROARS on them when they're right next to the coil, so if I do a few scrapes with my pick and signal is no longer in the same place and moved into my dig pile then it's less likely to be gold, combine that with the GPX losing the roaring target signal as soon as the pellet has any depth by being buried in my dig pile under a few inches of soil and the GPX signal becoming much more faint or even disappearing on some smaller lead pellets then I write it off as a pellet.  We don't have many surface type nuggets around here unless on bedrock so this only works when there is a decent amount of top soil on the bedrock.

As you can see by how many pellets I dug a lot of them are too deep for this trick to work.  These were all pellets that survived past the first few scrapes and could have possibly been gold.  I rejected just as many as I dug I would guess.

Thanks for the help, Gerry is going to have to put you on his payroll!

[phrunt]

7 hours ago, GotAU? said:

Thanks for the help, Gerry is going to have to put you on his payroll!

I use products he doesn't sell 😄 It'd be like me working at a Ford dealer wearing a Toyota T-shirt 🙂

I was just watching this video by abenson and I would not trust my pellet rejection method in his area, it seems there is a lot of small shallow near surface gold where he is hunting and my method would likely lose a lot of gold there as where we are these nuggets are not on the surface like he's finding them, it works here but certainly not there.  It was interesting to see he has a lot of the same troubles as I do with the 6000 like the speaker and even quite reasonable size small nuggets near disappearing in dig out piles if they end up on edge in the piles or towards the bottom of the pile.  This behaviour is so different on the GPZ.   I can see why people hunting in this area he is at would love the GPX over a stock GPZ, it's hyper sensitive to small surface targets so these nuggets would stand out.