There are a lot of specific methods for gold placer exploration depending on geolology and geomorphology environments. Please send more details

Dear Pavel

Hello

My thesis in about placer gold deposit in northeast of Iran.I have about 200 sample form some borehole. 

I have a lot of data but I dont know how should I work on that. I think is better than work on grade and gathering place for this deposit.

I want to best method for achieve to this processing.

Dear Pouria,

here is a link that reviews the techniques for exploring gold placers

http://riosuerte.com/in-the-beginning-there-was-gold/exploration-and-evaluation-methods-to-find-gold-in-alluvial-deposits/

and also two doc. that overview different methods for placer exploration in general

Hope these informations will be useful for you

Kind Regards, Anicet

Dear Anicent

Thank you for your respond.

You are welcome Pouria,

Please find also attached a paper that I have authored about gold in West Africa. You will find some useful techniques too and refs therein. Regards Anicet

Dear Pouria, I hope Anciet's respond fits your requirements. If you have specific questions in future  do not hesitate to send me message. 

Dear Mr. Salami,

The sample management can be achieved in tripartite approach:

Given you boreholes (spiral drilling, auger drilling, Bangka drill?) have been sunken at a regular spacing you need to create a litholog so as to come to grips with a facies or environment analysis. If supplementary downhole measurements are made with respect to the gamma spectrometry or any other wireline log the better. It will help you figure out the issue of the host environment.

I do not know if you can carry out a direct analysis of rock samples at outcrop or drill core with handheld devices providing you with digital data.

The mineralogical and sedimentary parts should involve a grain size analysis and morphological studies. Samples should be along the drill core (?) taken as channel samples. CAMSIZER are an effective device to do this job. The main focus should be placed on the heavy minerals. The more detailed you analyze these minerals the better will be results of your provenance analysis (from heavy mineral mounts under the petrographic microscope through the SEM/WDX-EDX or EMPA).

For gold you need rather "big samples" to avoid the nugget effect.

The chemical part involves some elements relevant to constrain the origin of gold in the primary and secondary deposits. Apart from Au it is Ag, Cu, As, Sb, Sn, PGE, Nb/Ta, Zr, Ti, REE which may provide valuable information.

It is difficult at the moment to provide you with a more detailed manual, because of lack of information on the host and potential source environments.

Best regards

H.G.Dill

Dear Pouria,

To illustrate some of the issues addressed by Mr. Dill, please find also a pdf copy of a paper that develops geomorphological and erosion patterns of a West African subregion, which is known for artisanal gold mining of placers, and hillslopes mantled by lateritic pediments. Regards, Anicet

Dear Pavel

You kind to me I hope use all of your experience and knowledge.

Dear Anicet

Thank you for your paper and I will study these days, if had any question I will get your time and use all of your experience and knowledge.

Best regard.

You're welcome Pouria. With the two papers I provided you I think you have matter to explore for your own research. Do not hesitate if you have further qestions about these two papers and the methods developed in there...Best wishes Anicet

Dear Anicet

Im so happy you help me and I try to do best.

I will connect with you.

Best regard

Hi Pouria. How deep? Overlain by thick clays or not? Nature of placer ore (eg sand, coarse lithic or quartz gravel? Distal or proximal, lag placer or depositional? True placer or is it palaeoplacer? Regional exploration of palaeovalleys or simply localised drill-testing (or other detailed testing) of a specific valley or paleovalley? If you are a bit more specific we can possibly direct you better.

So your data suffice for a scientific approach for future research, because we are dealing with the gold deposits associated with sediments.

 So whether it is gold sediments made regarding license to extracted.

I think first you need to identify a source rock near to your placer area. Most placers are found in sandstone, so you need to sieve your sand first and then analyse for gold in the fine fraction of sand. If you got anomaly (e.g. 1 ppm) then you can explore further up and downstream

Good luck

Dear Martin

The thick of layer is 10 to 15 meter, and nature is sand. This is distal fluvial and true placer. I have some samples from boreholes  with high gold grade.

you know method for fluvial placer gold deposits? I mean I want some method for estimate grade and network for placer.

Best regard.

Dear Mr Nasir

My hard rock is granitoid and my source is sand and conglomerate. in my sample the gold grade is more than 2 ppm. I want some methods for gold exploration in this deposits.

Best wishes.

Dear Anicet

I red your paper and I have got some data. But I have one question my deposits are in sand and conglomerate, Do you know how is tonnage and grade estimates in alluvial deposits?

Best regard.

Are the gravels and sands consolidated or unconsolidated?  Is the gold fine grained or coarse grained?

Mr Salami

Evaluating a gold placer requires an understanding of the stratigraphy of the deposit and sampling requires a good knowledge of the gold grain size distribution and frequency.

Develop a database that describes the individual weights of the gold grains to provide the sampling parameters for each of the definable stratigraphic units in the deposit - if the drainage has developed under flow regimes without violent floods and frequent mass movement of material there will likely be three environments of deposition, each with characteristic gold grain size distributions that reflect the characteristic energy of those environments.  These are the high-energy channel bottom gravels, the intermediate energy gravels deposited over the high-energy material as the channel migrates through time, and flood plain deposits over the first two units.

The coarser gold grain size distribution will be found with the high-energy (coarser) gravels, the intermediate grain sizes will occur in the intermediate energy gravels, and the finer and flatter gold grains will dominate the over-bank sediments.

This is a simplified sequence - climate fluctuates with time and periods of abundant water and high energy sediment transport may alternate with periods of little activity.  These climate variations repeat through time so there may be multiple sedimentary packages superimposed one upon the other;  that must be sorted out for each drainage system.

Sampling placers can be problematic - particularly if the gold is coarse grained.  Fine gold, if present in economically interesting concentrations, is less difficult since the frequency of 'events' must be high.  Such material does not suffer from the probability of inclusion in relatively small samples, such as those recovered by drilling.

Every primary distribution of valuable component (gold grains) will have an adequate sample size and samples that do not have an expected average of at least five significant events will introduce understatement and often zero results - not because the stratigraphic unit is barren but due to the probability of the 'event' of interest not occurring in the particular sample. [A significant event is a grain of a size that contributes an important part of the total gold in the stratigraphic unit.  See attached slide. This is an example of a reasonably common sort of distribution - given here as  what might be expected.  Coarser and finer distributions are frequently encountered.  This one truncates at about 149µ on the fine limit and somewhere between 2mm and 3mm on the coarse end of the distribution.]

Sampling must be confined to individual sedimentary units since the concentration of gold is a function of the energy applied during deposition, as is the gold grain size distribution.

Drill samples are restricted by the inside diameter of the casing coupled with the thickness of the sedimentary unit.  And it is advisable to keep individual samples to under one metre in length even if the sedimentary unit is thicker.  In one African placer 15cm diameter Banka drilling (~18L/m) understated the actual grade by about three to four times due to 60% of the total gold being coarser than 1mm and the larger grains were under-represented in the sample population.

Reliable gold grain size distributions will allow determination of the adequate sample size for each sedimentary unit and the robustness of any resource estimate based on any collection of field samples.  Remember that a resource estimate must be generated one stratigraphic unit at a time - it is not safe to assume one grain size distribution is representative of an entire package - see above.

Mechanical treatment of samples to recover the contained gold is recommended chemical or fire assay methods.  Treat all the sample and physically recover the gold grains.  I there is a large contribution from very fine gold grains in the panned concentrate the concentrate can be assayed to destruction and the gold recovered reported in milligrams - not in grams per tonne, since this is misleading - even meaningless.  Coarse grains larger than ~300µ can be managed - the finer ones with the help of a binocular microscope; as the grains become larger they will be easier to handle.  Fine grains can be weighed in groups and their average contribution in milligrams calculated.  +300µ grains can be weighed individually - to +/- 0.1mg is usually close enough.  When one has developed a good understanding of the grain weight distribution, screening and counting is often all that is required for a continuing understanding of the gold distribution from one part of the placer to another - and there will be changes in the gold grain size distribution because of distance from the source and changes in total applied energy that may relate to change in gradient of the stream.

We have applied this approach and were usually +/- 10% between predicted and mined gold - quite often we were +/- 5%.  It may be rather tedious and time consuming - it requires patience and practice, but the reliability of the results is gratifying.

Good luck.

Dear Anciet

Thank you for your respond, your respond is completely and I will try to use your knowledge.

Best regard.

Dear Martin

Gravels and sands are consolidated and golds are fine grained in this area.

Best wishes.

OK, Pouria, hope this will be useful for you, but you should know that it is required to collect the deepest layers of the alluvial deposits, which generally host heavy minerals and gold. Regards

Dear Anciet

I have some data from layers with gold grade. If you like I can send it for you with location  and other data.

Good luck

For  more information shall receive research books Au sedimentary deposits find all the information you need.

All the best

Dear Mr Hysein

Thank you for your respond.

Best wishes.

Dear Mr Tilsley

Thank you for guidance, and your description is quite useful and understandable.

I hope use all of your information in my project.

Best regard.

Apologies for delay. Attached are some references.

It is also worth reading:

Maney Publishing. in Applied Earth Science. Applied Earth Science, Volume 124, pp 136-146; doi:10.1179/1743275815y. ... Graphical representation of production results versus estimates in placer mining. R. H. T. Garnett.

which was published last year. It discusses results from various types of placer but has some gold examples (and the principles are the same). What it shows is that estimating place grades is very difficult, with issues like pay streaks and nugget effects, longitudinal versus transverse grade estimation variations. Estimated (E) grades versus actual recovered (R) grades vary hugely, even when estimates are made using bulk sampling of dredge faces. E:R can vary from 2:1 to 1:2 or more, and certain patterns in this are partly dependent on whether deposits are high grade or low grade.

Good luck with your work Pouria

Mr Salami

I submit an edit to clarify one of my previous points.

"Mechanical treatment of samples to recover the contained gold is recommended rather than chemical or fire assay methods.  Treat all the sample and physically recover the gold grains.  If there is a large contribution from very fine gold grains in the panned concentrate - say less than 250µ - the concentrate can be assayed to destruction and the gold recovered reported in milligrams - not in grams per tonne, since this is misleading - even meaningless"

Sorry for the omissions.  Hope this improves the sense of the comment.

Dear James

No problem sometimes occur for every one.Thank you for your edit.

Good luck. 

Other issues come into this, including how gold will be finally recovered (eg water-gravity circuit only or not), and whether the deposits are truly placer and not palaeoplacer and have not seen prolonged weathering. I am not familiar with the Iranian weathering history, but here a major proportion of the gold can be secondary, not the original detrital particles (at depths of 20 to 40 m for example, in Cainozoic palaeovalleys).  In some cases very little gold remains in the original sand/gravel and much now exists at a few meters greater depth in the floor rocks of the palaeovalley. Some exists relacing microbial filaments (golden bugs!), and pebbles and cobbles can be penetrated by supergene gold that precipitates within them, giving them grades as much as 5 - 40 g/t Au.

I doubt this is relevant to the placers being discussed though. but it is good to be aware that this can occur.

Mr Salami and MJH

This is probably drifting off topic in regard to modern placers and their evaluation but -

Although gold has been considered historically to be relatively immobile in the weathering environment, more recently there is a broader recognition of evidence of mobility and re-concentration at and near surface, and as suggested by Frimmel (2014 Society of Economic Geologists, Inc. Special Publication 18, pp. 1–000), probably at depth.  [See attached]

I have recovered beautiful specimens of dendritic gold from the weathering zone in semi-arid tropical environments - with 'leaves' of gold a few microns thick in cracks in massive quartz;  alluvial nuggets that when sectioned show alternating rings of desert varnish and gold; and have observed large grains of gold - some 500mg to over 1000mg - in lower greenschist grade metamorphism Huronian quartzites where they were hydraulically incompatible with the hosting medium to fine grained quartzite.  The latter must have grown in place.

In North Africa the gold concentration in the weathering zone of quartz/carbonate veins often showed enrichment some metres below surface coincident with reappearance of unleached calcite - this suggests a pH related reaction and, in that particular case, some response to the presence of manganese dioxide which  - after Goldschmitd - may have acted as a catalyst in the process, and which we were able to simulate in the laboratory.  

Boyle, Geol. Surv. Canada, Bull 280, goes into great detail and describes a variety of geochemical responses of gold in the weathering environment.  I am unable to find the reference at the moment, but Boyle also observed apparently electrolytic gold and silver in ice-filled stopes at Keno Hill in the Yukon - apparently a rather rapid development since his samples of the ice were taken from stopes mined ~30 years earlier in permanently frozen (permafrost) ground.  An interesting observation in this case is that the ice was derived from moisture in ventilation air circulated through the mine workings.  There is, of course, essentially no ground water circulation in permafrost.  The electrochemical behavior of elements in the weathering environment is little studied and has some fascinating aspects - but that is another digression.

Modern placers and younger paleoplacers - the semi-consolidated ones - usually show only evidence of mechanical rather than chemical controls on concentration.  Therefore, at least initially, pay attention to the depositional/erosional characteristics of the drainage being evaluated, restrict sampling to unit-by-unit limits and process data on the same basis - unless the gold grain size distributions of some of the various units prove to be the same - sometimes they are similar, but never assume that is the case.

There is good reason to take large samples - tonnes rather than kilograms - to establish the characteristic gold grain size distribution and to design the sampling program to respond to that control.  In evaluation of placers all samples should be 'assayed to destruction' - that is if you take - for example - 5 cubic metres - the entire sample must be processed with no sub-sampling to yield a 'representative sample'.

Fortunately, placers are often quite co-operative in this regard.  Sink pits by hand or use a mechanical excavator.  Build an easily portable washing plant similar to a production plant, and treat your samples rather as small-scale production runs.  The data generated will - if the equipment is run properly - be reliable descriptors of the gold distribution and concentration in each sedimentary unit studied.  Put all these data packages together and you will have an amazingly reliable estimator.

If done properly placer evaluation is relatively easy compared to evaluation of the same distribution and concentration in hard rock.

Just remember that "Sampling requirements are set by the ore - Not by the engineer"

Have fun!

Dear Martin

Your respond is very useful I try to use it.

Good luck 

Dear James

Thank you for your respond. Do you have any article about new method for gold exploration? I like to describe step by step.

Good luck 

What James mentions has been studied extensively here for 15 years or so - a wealth of modern papers, especially through CRC-LEME (free downloads). ie. Coperative Research Centre - Landscape Evolution in Mineral Exploration. I;ve published a little on it, with SEM photos - if anyone is interested in this please advise and I will give some references.

Mr Salami,

The evaluation approach I mention is probably not 'new' in that some of us in the industry have been applying it for several decades.  Descriptive papers describing its application probably exist, but there seems to be a general reluctance to 'give away secrets' but perhaps due more to the struggle to survive in a tenuous industry than any other reason.  Prof. Hughes may know of some that you will find useful.  If he were to give us some instruction on obtaining the publications he mentions, that would be greatly appreciated.

To repeat, the first step is to understand the stratigraphy of the drainage and to be able to distinguish between the high energy gravels, the intermediate energy gravels, and the over-bank deposits.  We find that many 'simple' placers will show a reasonably uncomplicated general package - if it is too chaotic it is unlikely to be economic - but expect the high energy and intermediate energy 'units' to be made up of multiple remnants.  The high-energy material will normally lie on bedrock, with some frequency of eroded and not subsequently replaced areas usually filled with intermediate energy gravels or fine sediments or clays as often seen in-filling meanders.  Intermediate energy gravels generally overlie the high-energy deposits - sometimes on bedrock if there was erosion or non-deposition of the basal unit.  Every stream will have characteristic patterns depending on such things as slope, discharge history, surrounding topography and rock-types. 

In rugged topography there may be other disruptions to the concentration cycle.  If your drainage has high gradient and narrow valleys one frequently finds evidence of high-energy debris flows that tend to destroy alluvial concentrations and make mining difficult or uneconomic.  One of the best illustrations of this sort of (undesirable) event is given in this video:

https://www.youtube.com/watch?v=51C7vEAVbxk

Study it carefully - and confine your exploration and evaluation to drainage that does not show evidence of experiencing such mass movement!  Pay some attention to the older deposits through which the active slurry is moving.  That arrangement of boulders and smaller elements is evidence that mass movements are part of the sedimentation history and suggest difficult if not impossible mining of transient concentrations. 

Broader valleys and stream gradients in the order of 2% are a better target for development and preservation of concentrations, and usually more easily mined.

Successful placer mining operations generally require significant gold reserves from which the gold can be recovered at a profit over a mine life measured in multiple seasons with production at a truly industrial scale.  The failure rate is high, often due to under-financing perhaps more often than unreliable resource estimation - but I have seen both, sometimes on the same project. 

Nothing beats hands-on experience through which you come to understand the deposit(s).  I assure you that they are not difficult in spite of a reputation for being so.  Stratigraphy first: Sampling using samples of adequate size for the primary distribution of gold second - and results interpreted stratigraphic unit by stratigraphic unit.  You will be working with measured gold weights from measured gravel weights which reduces the potential for problems often described as "the Nugget Effect" but which are the inevitable result of failure to meet the sampling requirements of the gravels involved.

Most of the thousands of tonnes of alluvial gold production from Victoria came from valleys with gradients less than 2% - some only 0.4%. I think gradient is only one of many factors determining where preservation is best (eg gold grain size, flow volume). And ease of mining depends on mining method. But I am not familiar with the situation in Iran....

"The nugget effect should include any random errors incurred by sampling procedures as well as inherent variability of the ore deposition itself. Hence the term ‘nugget effect’ was coined when considering the likely difference between a (say) gold sample with a nugget in it and one immediately next to it with no nugget in it" (Isobel Clark 2010).

So nugget effect is affected by between-sample variation which can be so large as to make a deposit nearly incapable of being sampled adequately (i.e. you would effectively have to mine the deposit to get its grade).  Measured gold mass and measured gravel mass only accurately give you the grade of your sample, not of your gold reserve. In  many cases one can get a reasonable result if the gold is fairly fine-grained, as is common, but in places like Victoria that was commonly not the case (eg even some of the quartz vein systems individually containing hundreds of tonnes of gold, from which the alluvial gold was derived by erosion in Victoria, can have a MEAN gold grain size of 4 mm, gold nuggets are up to 72 kg and nuggets in the size hundreds of ounces are common and still being found fairly often). Victoria is not BC is not California is not Iran, so there are dangers in general rules of thumb being extrapolated elsewhere.

Nevertheless, as the reference I gave in AES J shows in its study of case histories (such as Yuba California and Alaskan gold), recovered grade over the history of a deposit that is not highly "nuggety" (in the sense of containing actual gold nuggets) can lie in the range of a half to twice the estimated grade, even with careful sampling. Malaysian tin placers reportedly had R/E above 1.5 for decades. High grade placer deposits apparently have a tendency to be overestimated, low grade deposits underestimated. Since the latter is more common, I imagine not too many people cry about their underestimation.

I think you have enough  answer for your problem with gold.

Dear Mr. Hyseni

Yes your idea is correct, but Im confuse in something that time and experience will solution it.

 Dear Mr Tilsley

Thank you for your complete answer. I try to use all of method for my study area.

Im first man that work in this deposits in my country, I dont forget your guides.

Best wishes.

Dear Mr hughes

Thank you for your guide and points. Im so happy you answer my questions.

You improve my knowledge.

Good luck.