Clay

Practical Placer Recovery

19 posts in this topic

I've recently completed sampling on a small (~ 30 ac) private gold placer prospect. I'd like any experienced feed back you may have on recovery methods for the type of material we are processing.

 

On the ground situation:

1. No local water supply - yet, possible well in the future.

2. Good access by privately created and maintained roads.

3. No local electric supply - generators only.

4. 2-4 man operation only - all experienced hard workers.

 

The placer property sits on the edge of a laccolith. There is an exposed contact point from the intrusion on the property. The exposure is from a very old hand mining operation. The rest of the placer consists of 6 - 9 foot deep colluvium. The colluvium is a course mix of local material and has  fairly consistent layering exposing similar layers of Quaternary mixed gravels primarily distinguished by their exposure to fire (2 layers) and a migrating weak caliche deposit about 3 foot  thick on average.

 

The ore rock is a Biotite Quartz Latite. The colluvium is composed in significant part of coarse weathered angular and decomposed ore rock.

 

Distribution is remarkably even throughout the colluvium. The fire layers and Caliche hold slightly greater values but the entire deposit exhibits similar values no matter where sampled. Bedrock seems to hold greater value in fines but lesser values in grain + values.

 

Sampling was initially done by small sample (2-4 yards) hand methods. Very consistent results led to digging 10 trenches to bedrock. Each trench is being micro sampled and then the sorted material from the trench is being drywashed.

 

Current recovery is ~ 1.5 gram per yard with Keene 151- material screened to 8. Coarse mixed karat gold with about 8% attached gangue - smallest recovered fractions are generally +/- 1 grain.

 

On examination with 60X stereo microscope classified tailings show multi grain to sub-grain gold  attached to latite and feldspathic particles of fairly regular crystal form - typical coarse decomposed quartz - feldspath. This attached gold more than equals in weight the gold already recovered in drywashing.  Wet methods recover a greater percentage but still miss the gold attached to larger latite pieces.

 

So the question is...

 

What is the most efficient way to recover those attached, unrecovered values considering the size of the operation and the existing limitations?

 

All replies are appreciated.  :)

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What you didn't mention was how much material and if your group has access to money and are willing to invest.

 

The uniform values not concentrated on bedrock implies its residual.

 

If you have no water, you are limited to dry methods - period. As you pointed out, its hard to recover very fine gold dry. Dry washing just cant do that efficiently.

 

I don't know how well attached everything is in the ore. Is crushing required to liberate all the gold? Some placers / residual materials are that way.

If so, I'd be looking into some sort of ball mill set up to easily flow the material through. You are not really trying to pulverize the material to minus 100 mesh, just to bust it up.

Ball mills can do that. You can do short term ball mill tests to see how well it will work and how much milling it will take. I would do significant testing before committing any significant money. Testing is way cheaper than spending a bunch and later finding it does not work. However it probably needs to all be done wet. Ball mills can be run dry, but they are extremely dusty that way and I think you would have screen plugging problems and in order to recover those fines you are freeing, you need water anyway.

 

So is there enough material to make it worthwhile to invest 50 to 80 K to put in a well, ball mill and water handling system? Another 20 k to buy a loader to dig material, etc. - always easy to spend more money....

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Thanks for the reply Chris.

 

The estimated paying placer deposit is ~82,000 cubic yards. Right now there is about 28 yards of classified dry processed material stockpiled with about 178 yards of unclassified material ready to classify and process.  This is the material from the 10 sample trenches.

 

Equipment is being purchased or leased once the need is defined. The budget can be expanded if a profitable result can be pre established. A well is already on the to do list but it's priority on the list is what we are trying to determine. We also need to decide whether to go with a 2" or 4" - big price difference since bedrock is just six foot down and the well will bottom out around 240 ft.

 

This project is a bit unique since the partners want to establish family homes in conjunction with their full time artisanal mining. They have no desire to mine the entire placer. They want to maintain the character of the land to the greatest extent possible. Mining a small rich placer and following up with exploration of the hardrock potential over the next 30 - 100 years would be a very agreeable situation.

 

You are right that the deposit is primarily residual. We have narrowed the source down to a feature about 800 ft x 70 ft. All indications are that an elluvial placer is trapped in a significant portion of that feature. Considering the age and extent of the deposit there is a very good chance of productive stockwork in place.

 

Right now the issue is those 10 open trenches. We need to work the material and refill and stabilize the trenches. There will be no further mining of these areas. Being frugal miners we don't want to throw half the gold back in those trenches unless the cost of processing outweighs the value. The entire sampling project is to determine mineral values and lowest tech productive processing methods that fit the site and material.

 

Water can be hauled. Wells can be drilled. Dust can be dealt with. It all depends on what makes the most sense for the longest term. A loss while processing these trenches will matter little if the experience produces the lowest impact and highest return on this multi-generational project. The value being sought here is not a good fit with the concept of highest return in the shortest time.

 

We have considered a small continuous feed stamp mill. These are favored by many locals. We would prefer to go with something less sonic. Our neighbor is running several different types of livestock on his 2,000 acres and my own experience tells me livestock and the rumble of stamps is not a good combination. Being a good neighbor is critical to the long term success of this endeavor.

 

I know little of ball mills or their feed rates and capacities. When you consider that about 70% of the material will need to be crushed feed rates and throughputs along with water volume may make this part of the recovery uneconomical. That is what I am trying to determine. We did run some rough tests with a small ancient cement mixer and some river rock. The gold was freed quite nicely with this and a few other tests. The mother rock is quite evenly decomposed.

 

I hope that answers your questions Chris. I appreciate your feedback and that of anyone with experience in this sort of deposit.

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Well, I have met folks with similar comments and thoughts about their property and what I would say is that if developing a series of pretty home sites, disturbing the property as little as possible and keeping it "as is" are the priorities for their land, then what they want to do is really not comparable with serious mining, especially on a 30 acre parcel. If you had a hundred acres or more, than maybe you could segregate the operations and residences to different parts of the property. However at 30 acres, if you are mining, it will likely that operations and mining pits will be visible and hearable from one or more of the properties - though dust is controllable you will be right up close to the operation and the home will get a lot of dust - if only from the blowing wind on the disturbed surface area. Additionally, keeping the neighbors happy in an area focused on ranching pretty much means no larger scale mining.

 

Perhaps the solution is just to do a little dry washing and metal detecting from time to time on a recreational basis, and not worry about the more serious mining part of things. Take the money to be invested and develop the homesites and put in a community well and good, well graveled road access. In the long run that seems like what they would be happiest with.

 

The cement mixer is the same concept as a ball mill, but the ball mill is designed to flow material through and you can adjust it to feed and process material fairly quickly. Although Ball mills are not exactly quiet, they are less noisey than a stamp mill which is a pretty extreme noise generator. The problem with stamps is that you have a hard time flowing the material through if it only needs minor crushing. Still in the end, I think "none of the above" will be the answer that best fits their priorities.

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Clay...

 It sounds to me like they want more 'hobby mining' any anything else.  

I would suggest they not worry about the locked-up values and just play with

the easily recoverable stuff.

 

 If it were me I suppose I would haul in the water so a recyle system can be used

 until a well was drilled, so a wet recovery can be used.

Then build, or have built, a custom small trommel that incorporates a longer "scrub" section

to help liberated those locked-in values.

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Thanks for the advice folks.

 

I'm seeing I wasn't very clear about the size of this private property.  :blink:

 

The placer deposit is ~30 acres - the private property is much larger. The area to be mined is only about 1.5 acres. The deal to acquire this property has been more than 20 years in the making - this placer will be mined. These folks are full time small scale multi generational hard rock miners. This isn't their first rodeo but the situation is new to them. They have never encountered a significant placer covering their lode discovery. They are just looking for experienced suggestions on this particular type of deposit.

 

I'll pass on your suggestions about hobby mining if you like but I'm thinking there might be a lot of laughter coming from the equipment shed when I do.  :lol:

 

We have considered a trommel Dick but I'm not sure it could break out enough of the material to be worthwhile. Have you had good luck with a trommel breaking out small gold still attached to the mother rock? We will be building settling ponds eventually but first there must be a water supply and a production method. Is it possible to scale down a trommel setup for testing? I would think larger trommel loads would produce very different results than small loads on the same material? If I'm wrong about that we can make up a small trommel in the shop for testing.

 

These folks are favoring a stamp mill as that is what they know how to operate. The advantages include an easy transition to hard rock and the ability to process material for other local miners. The drawbacks are already stated. Skinny nervous livestock are not going to please the neighbor. They are feeling a bit lost without the ability to use familiar tools and methods.

 

Chris if you could point me towards some info on small production ball mills perhaps I could investigate that possibility? It may be that it's not worthwhile trying to wet process the material from the trenching but the main eluvial placer shows high values in small attached gold in the test samples. We had hoped to develop a recovery method on the trench material before moving on to the main deposit.

 

Water continues to be discussed. We had considered purchasing a water truck and hauling to the site. Unfortunately seasonal rains would cut down on the number of loads until we improve access with gravel. Time and money are pointing to drilling a well sooner than later as being the better option. Until we determine processing methods the quantity of water needed can't be determined. Catch 22.

 

This project will be on hiatus for the next few weeks while equipment is being cleaned up and transported from an east coast operation they have been running for several years. I hope to have some choices to investigate when everyone is back on site.

 

Thanks again for your insight guys. It is appreciated.  :)

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I'm confused with this statement:

"We also need to decide whether to go with a 2" or 4" - big price difference since bedrock is just six foot down and the well will bottom out around 240 ft"

Are you talking about the water well? You will not get much water from a 2" or 4" well. How do you know it will bottom out at 240'? Has it already been drilled or is that a financial decision?

If the difference between a 2" and a 4" well might be a deal breaker, a 6" is probably out of the question. It would be a bummer to drill a tiny well and not get enough water.

Are there other wells in the area? How deep and what size are they?

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Hi Harry,

 

I'm hardly an expert on wells but I have had a few drilled through the years. I'll try to give you what information I have to answer your questions.

 

I see you are from northern California. I'm guessing you don't have any problems flowing an 8" well there. When I was farming in the San Juaquin valley we thought nothing of drilling a 2200 foot 12" well and irrigating right off the wellhead (expensive and lots of red tape but the water was there). Things are very different in the desert southwest than they were in California.

 

In the area the property is situated there are three watertables. The closest is in shale and is a pretty consistently 145 foot down with less than 100 foot to the base. The second is near the bottom of that same shale formation (about 600 - 800 foot down) and is off limits because of the demonstrated possibility of piercing the thin and irregular base and draining the aquifer into the permeable region below - bad juju. The third aquifer is large and about 2200 - 2800 feet down. That aquifer is controlled by one of the State's regional water districts and is effectively off limits to individual wells.

 

The upper aquifer is our only real possibility to draw from. Although the shale is permeable the flow rates are not high. The shale is very weak in this zone and too big of a bell will collapse and with it your investment. There are several other existing wells in the immediate area including an overambitious 10" well that collapsed within the first month. Too much drawdown and you risk the possibilty of temporarily dropping your neighbors watertable or collapsing your well.

 

The typical solution to these very real and regionally common problems is to bank your well water in storage tanks, or in the case of mining in settling ponds. These commonly seen features in the desert are not designed to provide static line pressure but to provide flow rates beyond what your well can deliver at the head.

 

The expense of the well is not a deal breaker in itself but, as you can see, there is a tradeoff between storage capacity and well capacity. Without knowing your water requirements it becomes difficult to estimate storage size versus replenishment rates. It all comes down to actual need when dealing with wells in these arid regions.

 

I know this method of water well management is foreign to most areas of the country where 100 gpm flows are considered standard household flows. I managed a small ranch in the Dragoon mountains where the difference between 22 gallons per hour flow and 46 gallons per hour flow made the difference between the sustainability of ~ 80 head of livestock and two households. I'm pretty sure most areas of the country would be surprised to find those sort of flows are even a possibility.

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Looks like you have done the research....

 

I guess even if you don't get much water, you can fill your storage tanks over several days(or weeks).

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 "The area to be mined is only about 1.5 acres".

 

  Well, I think it would be tough for me to justify a mill setup for such a small placer deposit.

 

 

 

"We have considered a trommel Dick but I'm not sure it could break out enough of the material to be worthwhile. Have you had good luck with a trommel breaking out small gold still attached to the mother rock? We will be building settling ponds eventually but first there must be a water supply and a production method. Is it possible to scale down a trommel setup for testing? I would think larger trommel loads would produce very different results than small loads on the same material? If I'm wrong about that we can make up a small trommel in the shop for testing."

 

  I honestly don't know if a trommel would, but your mention of using an old cement mixer with a favorable result suggests it might .

Seems to me you could build a small test trommel plant, although I imagine the larger scale version would work even better at liberating

the gold.

 

 

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Clay you have a headache on this one.

 

You have a valuable deposit that is located sans water.  The oldtimers knew the problem all too well.

 

You'll messup the local water supply by extracting water for mining,  don't go there.

 

Either extremely heavy water or dry concentrates need to be trucked.  In my experience with Nevada placers,  trucking the concentrates is best.  Many successful operations are currently in operation hauling concentrates to a seperate mill.  But is it worth it on your deposit? 

 

Size down a sample transportation operation and see what pencils out.  You can probably work with an already established mill.  A trucking business is a whole different breed of challenges.

 

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Thanks for the info Geowizard.

 

We have come up with a method to boost recovery to 4.2 grams per yard. We are building the doodlebug in our shop during the cold season. The well will be drilled next week.

 

We are not mining in the basin and range or Arizona. The Sulphur Springs Valley is the poster child of water mismanagement. What happened to Soldiers Hole and surrounds is nothing short of criminal.  The fact that some of that damage continues under the current management scheme doesn't speak well for the efficacy of government control of our water resources. I'm not sure I would even attempt to drill a water well for mining purposes in Arizona under the current scheme.

 

We have prior water rights to both the county and State. Without revealing too much, this is private land created by a royal grant and adjudicated by the Court of Private Land Claims more than 100 years ago. The Supreme Court has declared the adjudication to be complete and final. We can legally take water and mine as needed.

 

Personal responsibility towards our neighbors wishes and needs has always been a prime consideration in our plans. All those matters are now settled after nearly 20 years of fencepost negotiation. That is why we are just now drilling, building and mining on the property.

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Clay,

 

I was tasked a few years ago to design a lead mill near a residential area north of Willcox.

 

- Geowizard

 

Was that Pierce Carson's project?

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"Practical"?

 

An excellent choice of words! "Practical" does have certain constrains. :)

 

"Practical" makes it an interesting question! It's a question of physics, economics and social expectation.

 

- Geowizard

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"Tightening of the Screws" The "social expectation" side of mining...

 

Over the past 30 years, placer miners have experienced a tightening of the screws.

 

Even simple life forms move when poked with a sharp object! No, we can't move the mine. We can improve the margin of "practicality" by selecting appropriate areas to mine where the public and policy makers consider mining to have importance and make it a policy to reduce permitting costs in both time and money. :)

 

- Geowizard

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