Round River Rock in the San Domingo area

[SandomingoJim]

Hello
I have a question for those that are familiar with the San Domingo gold placer area of Arizona. I have noticed over the years of prospecting the area the gravels are predominantly angular and not rounded by water action. Although surrounding areas not too far away show much rounded river rock on the surface. On occasion I will find a nice smooth cobble amidst the hard pack angular gravels which makes me wonder if an old river channel lies beneath the newer volcanic deposits. I would love to hear anyone's thoughts on the subject.

Thanks
Jim

 

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[Terry Soloman]

After watching the weather coming off the Colorado Plateau and out into the Santa Domingo, I'm positive there is an ancient riverbed, maybe several. Just look at the alluvial fans coming off the Plateau.

 

[smokeythecat]

You are most likely correct.

 

[SandomingoJim]

Thanks Terry. Makes good sense to me. Finding a water worn smooth cobble with all the angular rocks makes be think some of the newer deposit layers are thin in some places. Much AU to ya.

 

[Clay Diggins]

The San Domingo district sits right at the southern core of an ancient enormous mountain that 250 million years ago covered most of what would become south central Arizona. (To give some time perspective the Rocky Mountains are only 20 million years old) The more erosion resistant minerals and elements like gold have been concentrated there as the mountain has eroded. That's why you have success prospecting there. That's why there is an almost random pattern to the mixing of heavy and resistant minerals on the surface.

The core of the mountain is still there and erosion is still ongoing but it's mostly imperceptible because the base of the mountain is covered with past erosion materials. If you follow the Little San Domingo down to where it enters the Hassayampa river bed the depth to bedrock is just 16 feet. Just a few miles down the Hwy near Surprise the depth to bedrock is 11,200 feet. That's because what's left of that huge mountain is still there buried under it's own eroded rock. The difference in depth to bedrock demonstrates the steepness of the slope of the mountain that's buried there.

The Valley of the Sun (Phoenix) and several other southern valleys are filled with as much as 17,000 feet of the erosion products from the south side of the mountain but most of the eroded materials actually went to the north. There was so much erosion that the finer materials (sand) formed huge sand dunes and sandy plains extending into what is today Utah, northeastern New Mexico and western Colorado. This sand was deposited on top of a base of sea sediments. That sand consolidated into sandstone and limestone. Those northern erosion deposits were eventually uplifted more than 2 miles as a single unit about 20 million years ago during the Miocene Epoch. This is the area now known as the Colorado Plateau.

Now that you have a little perspective on this process you can begin to understand just where the minerals and rocks are coming from. Essentially most of the material in the LSD has already been roughly sorted by specific gravity and resistance to erosion. There are other interim sources initiated by much more recent vulcanism and surface enrichment through supergene processes but those are only localized modifications of the overall erosion processes.

Being a desert area flowing water erosion has little to do with formation of the placers you are working. The coarseness of the gold is direct evidence of the lack of water or significant movement after being eroded from the vein. Most of the rocks and minerals you encounter in the LSD were eroded from the country rock thousands of feet above the present terrain over millions of years. They haven't traveled very far laterally.

As you noted most of the loose rock in this region is angular. There are of course some stream formed cobble deposits but those are almost always very recent in geological terms and are limited to the obvious bends, benches and static areas in the beds of the washes. The Hanson placer at the common junction of San Domingo wash, the Hassayampa river and Hwy 60 was enriched by the small volcanic dike intersecting that junction. That allowed the "stream" cobbles from San Domingo to accumulate before the dike was eroded through to the river. Another such obstruction deposit was formed in the box canyon further up the San Domingo where the failed dam is now.

Assuming rounded rocks are the result of being water worn is a basic mistake many prospectors make. There are a lot of reasons different types of rock become rounded. The most common causes are exfoliation in medium to fine grained rocks like granite and basalt and spherodial weathering. Wind erosion plays a big part in the local near surface deposits and has also created local eolian placers in the LSD. As I'm sure you have observed the exposed portions of even hard rock material is often more rounded than the buried portion of the same rock - wind erosion is the explanation. It's not unusual to find rounded clasts of volcanic material that were rounded in travel from the vent as well as existing angular fanglomerate rocks being rounded by abrasive vibration of the loose volcanic material. In other words round rocks do not necessarily mean that moving water was involved.

The odd round rock found in among the angular rock could be the result of the above processes or it could be the result of the erosion of an ancient stream deposit once much further up the now eroded mountain. If you have a real interest in just how any particular rock took on a round form start by taking a slice and determining just what type of rock is involved. In many cases you will discover minerals that are subject to chemical, structural or wind rounding of the material.

If you study the actual known geology of this area you will see there are no verifiable signs of any intact but buried ancient river system. When the Laramide Orogony came around (fairly recent in geologic terms) there were no rivers to be covered by the volcanics. You may find some small local incidences of past streambed materials covered in volcanic materials but you won't find any major gold bearing river systems hidden under volcanic flows like you do in the Pacific provinces.

I hope that helps?

Heavy Pans

 

[SandomingoJim]

Thanks Clay for a very informative answer. I can see that a good study of the local geology is what I need. LSD has always intrigued me and I enjoy learning. Much Au to ya!!

 

[Bejay]

People fail to think of the earth in geologic terms of time and happenings. It is more than one can often comprehend! Just imagine the time and effort it takes to sort it all out....as stated in Clays posting on this subject. Yet someone did.

Bejay

 

[Terry Soloman]

Ahh heck!