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BillA
01:48:24 Sun
Jan 31 2010

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'black sand' of 100% magnetite !

A while back in a thread related to Costa Rica I mentioned that the black sand in Bahia Drake was ~50% magnetite and Joe S Indy said "Jeez, that's a lot". Well I was wrong, what I just tested is 100% magnetite; it is all magnetic.

I took a 30kg sample from a 'seam' ~300' long and 3' thick on the beach; nominal wet bulk density ~2.3 but it clumps so the real value should be higher. The black sand is 'pure' in layers 1/3 to 1" thick with alternating layers of ~50/50 black sand to blonds; no idea how thick it goes. Cleaned 200gms with a magnet twice and ALL of the black sand (+98%) is magnetic.

There are 2 other local beaches with large deposits I will check out. No apparent gold so far.

Perhaps Bill or Harold will comment if this is common in other parts of CR. I never saw such in the US.

Bill

  
JOE_S_INDY
03:05:33 Sun
Jan 31 2010

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Re: 'black sand' of 100% magnetite !

Geez, Bill!

Magnetite has a SG of 4.9 - 5.2

http://www.minerals.net/mineral/oxides/magnetit/magnetit.htm

So in the great scheme of things unless the grain size is much larger than the Gold's, the Gold is somewhere else.

In some places, there is such a disparity in SGs between all the heavies and the blond sand that all the heavies move together, and concentrate together, while the blonds just cover them at the tail end of storms.

I would imagine that Gold might be in a layer below the Magnetite - somewhere.

Let us know how you do.

Joe S


  
Zooka
05:08:20 Sun
Jan 31 2010

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Re: 'black sand' of 100% magnetite !

What kind of magnet? Hematite will follow a super magnet (rare earth neomidium).
-Z

  
BillA
05:41:34 Sun
Jan 31 2010

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Re: 'black sand' of 100% magnetite !

not neos, that's for sure
Jobsmart 16lb magnetic pickup tool - $4.41 #51115
lol

Joe
the bulk density will be a lot less than the described mineral density deoending on particle size distribution, blah, blah

even at a very low tide I'll only get down 6' at best due to a river 40' away, worth trying ?

Bill

  
JOE_S_INDY
16:50:39 Sun
Jan 31 2010

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Re: 'black sand' of 100% magnetite !

Bill,

You sure have an interesting one here!

For starters, any magnet will pick up Magnetite. Large specimens of Magnetite (that are also the magnetic variety) are "Load Stone" and natural magnets themselves. I do use "Neodymium" Magnets to remove Magnetite, Hematite and ferro-Platinum from my super concentrates - and the classified, removed, material is then a simple pan-out.

Now, a tidal area with all this Magnetite and a river 40 feet away, no less ----- Yowser! . :eeekyellow:

Cribbing could help to maintain some of a previously dug hole to allow deeper than 6 feet - but the danger of collapse and getting trapped is too great. :gonetoofar:

Dredging (actually hydraulic excavating) is a possibility - but the water table and soft sides will still make that a loosing venture, too. :gonetoofar:

The only thing that comes to my feeble old mind would be pipe, hose, sluice box and a pump.

A 10 foot piece of cheap 4" PVC with a "T" just stuck on the end (oriented to have a single discharge horizontally and a through hole vertically).
A 10 foot piece of 2 or 3" PVC pipe able to be connected to the pump's discharge through a valve.

Dig a starter hole (? feet deep) and insert the 4" with the 2 or 3" inside it. Pump water, furiously, into the inside pipe and bore your way down, pushing the casing down as best as you can. (Assuming no rocks or buried trees you should be able to bore down 8 maybe 10 feet.)

Bring the discharge water (and sand) from the horizontal discharge from the "T" to your sluice box and run the discharge through it.

NOT a true sample since in this "Eleutherian (sp?) Tube" the larger grain sizes of Gold should not carry up a max of 10 feet. If you find any fine Gold at all it should be worth further interest.

I've heard that boring with "Heavy Water Flow" through a pipe was a way that some miners' post holes were quickly 'dug' in the past.

When you're done, if unsuccessful, you might be able to pull the "rig" and re use it elsewhere.

I don't know if it'll work 100% - but it might be worth a try (if nothing else it's a great campfire story).

Joe S



---
Wiser Mining Through Endless Personal Mistakes
 
 
billcosta_rica
16:51:47 Sun
Jan 31 2010

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Re: 'black sand' of 100% magnetite !

bill
one beach close by my house has a lot of black sand. up at the hi-tide mark there are stringers that run several feet deep. then a heavy mix on down the beach. the river do not have a huge amount black sand. I don't know what the% of magnetic is. it isn't 100%. also the beach sand here is barren of gold. talked to a guy that ran a 8" dredge in dominical area that said the bland sand was so heavy he could not get his riffles to clean and finally gave up. Rio tigre has black sand with gold, but I don't know the % of blacks. the beach placers around zancudo are not real heavy in blacks.

bill-cr

  
kringle_mining
00:02:08 Mon
Feb 1 2010

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Re: 'black sand' of 100% magnetite !

Post some pictures of the regular beach sand at taken through a microscope
What percentage of qtz. is the sand on your beach
Or is your sand Arkosic in composition?
How far from the Mountains is your beach?
Or is your beach beneath an escarpment or cliff face?
Are there any volcanoes in costa rica? If so how far are they from your beach?

  
BillA
01:22:51 Mon
Feb 1 2010

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Re: 'black sand' of 100% magnetite !

lost my post, grrrrrrr

no scope here, I will see if my old camera does macro tomorrow
- would say the lt brown sand is about all quartz
- no sedimentary or conglomerates that I've seen about, I'll photo some of the veining in the exposed rocks on the beash by my house
- from the beach to a steep good sized hill (the local cemetery) is ~300'
- the Rio Sierpe runs parallel to the beach ~50' behind it for a mile or so, the magnetite appears only in a portion of the beach that is not over-washed at high tide
--volcanoes ?? lol CR is a string of old and current volcanoes, there is the remains of a very old caldera ~5mi up the Sierpe; most of the gold in CR has come from this area, it is literally all over (why I came here 26yrs ago)

Bill

  

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