Mountain Gravel Composition

[Golden_Republic]

Hello all. Hoping someone more knowledgeable will be able to educate me a bit, as I am geologically dumb.

I'll be heading up from Sacramento out toward the Mendocino Forest to visit my grandfather soon - and along the route, in Colusa County, there are numerous turnouts and shoulder areas that contain hills of rock like in the picture. From what I can tell these are shale based formations and possibly formed from prehistoric ocean bed. From what I have researched, the range they are in fall into the California Coast Range. I know the area has been known as a producer of copper and chromium and some gold and other minerals in the past, but I was wondering if  the gravel or larger rocks on these roadsides is worth taking a look at. I was planning on taking my MD and seeing what sounds off at ground level (probably trash upon trash), but wasn't sure about possibly collecting some of the gravel to try and pan it later. If I do decide to collect, I won't be digging into the mountainside at all, just sampling what has eroded away and is at the base of the mountains. 

Based on the composition seen in these pictures is it even worth sampling? Thanks!

[Gold Seeker]

The only way to know if there's gold is to do some sample panning.

That being said you want to look for rounded gravel/rocks, angular rocks have never been in a stream, gold is first formed in hardrock/seams, the rock/seams erodes and the gold washes downhill into streams and gets deposited in various parts of the stream, and time marches on the streambed migrates to another spot leaving rounded rocks and if any gold in the stream it will be with the rounded rock.

So look for rounded rocks in a known gold bearing area will increase your chances of finding any gold left in the area. 

[Geologyhound]

Yes, unless you are lucky enough to find a vein (which won’t be in a fine grained sedimentary rock), then you definitely want to find stream gravels.  In a large outcrop, you may be able to identify multiple flood layers (layers of gravel separated by layers of clay). The top of each clay layer/bottom of each gravel layer would be the best place to check.

[jasong]

If there is quartz around, I'd sample pan some of the dirt around the rocks too if you are in California. Might be a lead to something good in the rock further up. 

Epithermal deposits do host nuggety gold in sedimentary rocks. I don't know the geology out there in the coast range, but Nevada next door is famous for some spectacular crystalline gold nuggets that often comes from veins in sedimentary and metasedimentary rocks (phyllites, shales, slates, sands, limestones, etc). Colorado is another, there it is in shales IIRC. Keep an eye out for pyrite pseudomorphs in the sed rock along with quartz, something I've also seen come with nuggets in AZ so I'm guessing CA too.

In both cases, I've found some pretty rich placer deposits via pan in the dirt and angular rough rocks eroding from outcrop above, this is also how they pocket hunted back in the days before detectors.

I really have no idea what the mineralization and host rock is like out there, you'd have to read up on it. I'm just giving some general advice though and it's up to you to do the research and bootwork. Generally the streams are your best bet, but there can be good gold elsewhere too. Most nuggets I detect for are not in alluvial watercourse gravels, but dry, angular hillside placers known as eluvial deposits. 

[Golden_Republic]

As an update, travelled up this weekend and drove the road I wanted to drive and checked out the rocks. After thinking it over I opted not to take a sample. I'll do some more investigation and may travel up again soon to a different location and check. I appreciate all who chimed in. Some definite education for me on what is what. Appreciated. 

[klunker]

curiosity is a vital component of prospecting. But so is research. I have never heard of detectable gold from that area, but it seems like Homestake Mining had an operation in that general area trying to recover disseminated gold.