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Mine Exploration Forum

Author Galena concentrates question
John Mason

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Galena concentrates question
Posted: 17/03/2017 08:58:49
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Working on a manuscript and it reminded me that I had not properly responded to a discussion in the WMS Newsletter It was about whether galena concentrates from mines like Darren contained tetrahedrite or not. George Hall thought not but it hinges entirely on one thing to which I cannot readily find an answer.

In typical 19th Century galena concentrates, what was the maximum grainsize? Anyone know?
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John Lawson

Joined: 09/12/2010
Location: Castle Douglas Dumfries & Galloway

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Galena concentrates question
Posted: 17/03/2017 20:31:12
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John I do not think that there can be a definitive answer to that question.
You see in some mines, especially those of the 'bonanza type', some galena was found in distinct pure masses.
Under the conditions that prevailed at that time these pieces would be literally, hauled out as they were and put on the pile to go to the smelter.
To go back to what I think was your original question, could tetrahedrite become mixed with a galena concentrate?
This primary would depend the two minerals. SG.
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Manicminer

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Galena concentrates question
Posted: 17/03/2017 22:18:11
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It would depend on the crushing equipment used. They would not want to crush too fine as the galena would go to slime and be hard to recover with simple jigs and buddles.

Having walked the surface workings in some of the mid Wales mines it would appear that they crushed to about 5mm down to dust.

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Graigfawr

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Galena concentrates question
Posted: 17/03/2017 22:31:45
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The following is written with a mid Wales perspective dominant but the broad principals would apply to many mining areas.

Run of mine orestuff was only crushed down to the maximum size necessary as over-crushing resulted in losses of finer grains. The physical nature of the inter-growth of galena, blende and gangue crystals greatly influenced this. In C19, limited power supplies and limited machine capacities, coupled with cheap labour, resulted in a fine-tuning of crushing and sizing, so most mines of any size marketed concentrates in a variety of grain sizes.

Substantially pure galena, whether lumps or dust, might be bagged underground.

On surface, a good deal of hand picking of run-of-mine and hand spalled lump galena and blende occurred; this would vary considerably in grain size, from multi-inch cube probably down to less than 1 in cube.

Broadly the remainder was crushed and jigged, and the fines buddled, with additional machinery of increasing sophistication being introduced in the second half of the century, but to keep this post simple, I'll limit it to the classic jigging and buddling, and will mainly focus on the dressing of galena rather than separating blende from galena.

Roll crushers had a propensity to allow some over-size lumps through and when such lumps forced the rolls further apart than optimum, a range of grain sizes fell through. In theory the raff wheel would catch the over-size and recirculate them, but raff wheels were not universal although they were pretty standard in mid Wales - hence the taller crusher houses in mid Wales compared to some other fields. The nominal roll gap would be set by experience at the individual mine, to what had proven to minimise dressing losses: it is not possible to talk of a single standard particle size produced by all crushers.

Jigs were capable of treating galena bearing ore from 1 5/8 in down to 1/32 in and the speed of agitation was increased when treating finer particles, and the finer material passed to adjacent jigs with smaller mesh sieves. One scheme of meshes for a group of jigs was: 7.5 to 10mm, 5 to 7.5mm, 3.25 to 5 mm, 2 to 3.25mm, 1.25 to 2mm, 0.75 to 1.25mm, 0.5 to 0.75mm, 0.25 to 0.33mm. This would produce concentrates of eight different grain sizes.

Rejects from the coarse jiggers would be recrushed; rejects from the finest jiggers would rarely pay further treatment. The rejects from the middle jigs would be settled in slime pits, sometimes tromelled to remove over size material and contaminants (wood, iron, etc) and then mixed with water to form a consistent fine stream of ore and water to feed buddles.

Products of buddles were classed as heads (marketable), middles (needing further buddling) and tails (usually waste but sometimes worth further buddling). Textbooks seem to focus on the percentage Pb in these grades rather than mentioning grain sizes, but from the fill of excavated buddles that I have sen, some of which definately contained the last run of orestuff treated, rather than post-disuse infill, I suggest that the heads would vary from fine grit to fine sand with coarse sand predominating, the middles being finer, and the tails finer still.

So the concentrates marketed from a first buddling would be coarse sand to fine grit; concentrates marketed from a rebuddling of middles (becoming the heads of a second set of buddles) would be mainly coarse sand sized. In these contexts I would judge the grit to be, say 3 to 5mm, and the coarse sand to be, say 2 to 3mm. Nevertheless, there would be a proportion of grains down to fine sand and even coarse silt mixed in.

What the foregoing summarises is that concentrate varied enormously in grain size and that a single mine would market a range of grain sizes, reflecting the removal of concentrate from the series of dressing processes at the earliest stage it became marketable, so avoid the expense (and ore losses) of unnecessary further dressing, whilst freeing up the dressing floor's capacity to treat the downstream grain sizes and grades.

I appreciate the basis of your question which is: would crushing and dressing continue to the point where it (intentionally or unintentionally) separated tetrahedrite-rich particles of ore from non-tetrahedrite rich (i.e. mainly galena) particles of ore. The sizes of the grains of each mineral, the nature of their intergrowth, and - importantly - whether particles of ore did, or did not, tend to fracture along tetrahedrite/galena grain boundaries, would affect behaviour during crushing and dressing. Also, the amount of, and grain sizes of, inter-grown gangue minerals in the tetrahedrite-rich zones of the vein would affect behaviour during dressing: relatively plentiful gangue would reduce the density of grains; sparse gangue would increase the density of grains.

Failing the impossibility of observing and sampling a representative cross-section of vein (nothing readily workable would have been left in sight), examining a range of vein fragments and also dressing discards may provide useful clues in these regards.

Bear in mind that the Ag grades reported in the Min Stats are unlikely to provide the full picture: there seems to be significant under-reporting and also a good deal of generalisation. Further, the published annual figures for a single mine can amalgamate ore from veins (or areas of veins) that might carry significant tetrahedrite, with others that carried much less. What is unknowable is the proportion of concentrate of varying grain sizes that made up the annual grand total - tetrahedrite would survive best in larger concentrate grain sizes and run the greatest risk of being separated out during dressing and being discarded in the very smallest grain sizes.

John, please feel free to pm me to discuss further.
IP: 176.24.114.185 Edited: 17/03/2017 22:52:23 by Graigfawr
John Mason

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Galena concentrates question
Posted: 18/03/2017 01:58:08
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Thanks all!

I think I can be fairly confident that tetrahedrite did indeed make it into galena concentrates at mines like Darren, South Darren etc. I'll try to explain why.

The mineralisation at these mines did not involve any appreciable blende: the amount I have obtained in repeated sampling studies over 30 years amounts to a matter of less than 100 grammes. Zinc in the A1-c assemblage that these mines exclusively worked is confined to tetrahedrite. The best galena occurs (historical accounts) as the steel-ore variety, in which polished sections show that the galena has recrystallised into an finely, essentially equigranular groundmass and inclusions (including tetrahedrite) occupy triple-point positions between galena grains. These triple-point inclusions are tens to a few hundreds of microns across. The tetrahedrite contains up to 20 wt% Ag from electron microprobe analyses. In the dressed ore grain-sizes mentioned by Graigfawr, it is clear that such inclusions would have largely made it through to concentrates.

What the miners did with the coarser tetrahedrite is an interesting one, since centimetre sized masses of it (10cms on occasion) were formerly common at these mines, when they still had tips. I find it hard to accept they simply ignored it - tetrahedrite or fahlherz being well-known from Cornwall and the German mines at the time and recognised as valuable. But I have found no reference to either name in the reports from the time.

It's a pity that I didn't tackle this while George Hall was still with us, but I was dealing with a serious health crisis in my own family at the time. Got a bit preoccupied, in other words.

I can however see where he was coming from: in The Gold Mines of Meirioneth he covers 19th Century ore-dressing at the Tyddyn Gwladys mine (Dolgellau Gold-belt) in some detail, where tetrahedrite occurs associated with pyrargyrite, both being obvious in polished sections of galena-bearing veinstone. However, at T-G, the mineralisation is quite different from anything in the Darren mines. It is a horrible fine-grained variably galena-sphalerite-arsenopyrite-pyrite-pyrrhotite-chalcopyrite mixture that would have to be pulverised to death if any one mineral species could be milled to a half-decent concentrate using the technology of the time. In such circumstances I can see how the silver-bearers could indeed be lost in the process, given their relatively low specific gravity - and from the accounts quoted by George, their tendency to float on the meniscus - "like a mirror". In general, the Dolgellau ores have always been problematic in terms of base-metal recovery.

Another interesting and this time unresolved problem in Central Wales concerns the Llanfair Clydogau ores - these belong to a different and much younger (A2) mineral assemblage to that worked at the Darren mines. Both old reports (especially from when John Taylor was there) and modern petrology come to the same conclusion: galena (with minor chalcopyrite) studded through a highly siliceous matrix that must have had to be pulverised finely to dress. Yet the concentrates from there carried up to 2000 g/t Ag. The nature of the silver-carrier in these ores remains elusive: polished sections have drawn a blank, though the ore is a devil to polish because the galena (very soft) occurs in a fiendishly hard matrix. Another unresolved Central Wales metallogenic problem. There are still a few MSc projects in this neck of the woods!



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Manicminer

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Galena concentrates question
Posted: 18/03/2017 09:10:17
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The tetrahedrite at Tyddyn Gwladus and Cwmheisian Issa/Isaf is extremely hydrophobic and can be easily collected by crushing a few specimens and floating the tetrahedrite in a pan. Fancy a day out ?


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John Mason

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Galena concentrates question
Posted: 18/03/2017 10:02:25
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Yes wouldn't mind another look at those mines, whenever it stops raining. Although the general sequence of mineralising events is similar to the rest of the Gold-belt, they represent an antimony-silver "blip" for the area. Tetrahedrite occurs at Gwynfynydd and the Foel Ispri mines but in nowhere near the same quantities, although I found it in Gilbey's PhD collection polished sections. A bit like Central Wales where there's a general sequence of events but with stark local enrichments of certain elements (such as Co and Ni in the district N and E of Talybont). IP: 31.52.225.223
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