This article is part 3 of our discussion about the Big River Gold Exploration Series and will be its conclusion. In part 3 we will discuss our general sampling philosophy, and thoughts as well as the final determination for this section of river. Part 3 of the discussion covers the last 2 videos in the YouTube series. You can find them at the bottom of this article. And if you havent read parts 1 and 2 of this discussion you can find them here: Big River Gold Exploration Series Discussion : The Prospecting Geologist Big river Gold exploration, discussion: part 2 : The Prospecting Geologist %
Sampling Philosophy
Sampling for gold paystreaks can be one of the hardest and most frustrating parts of prospecting. This is especially the case on big rivers, in areas with no information about the gold. You are basically starting from scratch with only geology, and maps to help point you in the right direction. Our initial sample site was chosen by fate and turned out to have at least enough gold to pique our interest and wonder if other spots nearby held more.
Our initial test spot was on bedrock that had a near horizontal dip. This doesn’t make the best catches for gold. So, our initial sampling plan included hitting nearby bedrock formations that would have more favorable conditions to catch gold. But first we decided to expand sampling around our initial test area. This is mainly to see we hit an anomaly or if the gold gets better or worse. For the most part the gold stayed fairly consistent in the area of our first test. This was a bit disappointing as it wasn’t enough to meet our requirement of at least 1 gram of gold per day.
Upriver Phyllite Sample
With our initial sampling area not showing anything better, it was time to expand our area. Our new area of interest to sample was a few hundred yards upriver where the bedrock changes from Triassic siltstone to phyllite with abundant quartz veins. Anytime you have a change in lithology, it’s worthwhile to sample in that area as it’s a zone of weakness where mineralization could happen.
We did a number of samples on both the upriver and downriver sides of the contact zone. The phyllite bedrock looked amazing for catching gold and had some nice hardpack on top of it. But all but one of the samples showed more than a spec or two. But even the sample that showed slightly more gold was a very poor showing. What was interesting was that the better sample was taken on the conglomerate side of the contact. This bit of info points to that the gold maybe coming from the conglomerate and not the phyllite.
Additional siltstone area Samples
As we came back down river, the center of the river and the left side still needed some samples taken in the siltstone area. The center to left side of the river in this section is pretty much exposed bedrock. But it still needed sampled since there could be good gold stuck in the cracks of the bedrock or in the small lags of gravel.
We did about 4 or 5 samples in this part of the river. Any gravel material we found was all loose, and most all the cracks were shallow and devoid of material. The top mat cleanouts showed little to no gold in each sample. And with that we were down to one last area to check.
Downriver Phyllite Sample
The last area we needed to sample was where the siltstone and phyllite are in contact downstream from our initial sample area. When scouting the river down this way it was obvious that bedrock becomes very shallow, and most of the gravels disappear. We picked a spot that would be roughly in line with our first good sample. The hope was that this was part of the gold line within this section of the river. Within this sample area there are visible quartz veins sticking up out of the gravels. But upon dredging down to bedrock, the bedrock was very smooth and didn’t provide good catch areas.
We ran two dredges for about an hour in this area and both cleanouts produced little to no gold. There could be a number of reasons for this. The main one being that with the change in bedrock, the phyllite is a harder rock and causes a set of minor rapids just below this sample. This could have caused this general area to be scoured during high water and deposited downstream of the rapids. However due to the long set of rapids we are not able to test the area below the rapids easily. Eventually we may plan to try and sample the downstream area.
Conclusions
While more sampling could be conducted in this area, based on our strategically placed samples I would doubt that this area could produce our minimum of one gram a day with a 4-inch dredge. The gold found in this area seems to be coming out of the conglomerate which is probably a very low-grade paleo placer.
So, this series crosses off one of the three areas where a newly found source may cross the river. Two more areas need to be sampled on the river to determine if the new source crosses it at all and forms a placer deposit that has never been mined. I did get some encouraging results when I did a short-day trip on the next section of river we will sample. You can see that day trip in this video: https://youtu.be/FmJf2jpCXi8
I hope everyone enjoyed this series and found it informative! Thanks for reading and watching!