Australian (ASX) Stock Market Forum

Geologists discussion thread!

Jump to new
Ultrabasic is the same as ultramafic, just an older terminology, though it is still used occasionally. Ultramafic rocks are igneous rocks characterised by low silica (usually <45%) and high magnesium (>18%). The term ultramafic is a garbage bin term that covers a variety of rocks that fit the above parameters.

Ultramafic rocks are originally composed of varying percentages of olivine and pyroxene, commonly chromite and sometimes plagioclase (feldspar), plus numerous other accessory minerals. Olivine is MgO rich and by comparison pyroxene is MgO poor so olivine rich rocks are higher MgO and are considered more ultramafic.
common rock types are:
Dunite: olivine rich (minor to no pyroxene) - very high MgO
Peridotite: olivine+pyroxene - generally high to mod MgO
Pyroxenite: pyroxene dominant - low MgO.

Ultramafic rocks form as either intrusive bodies crystallising at depth (often forming layered mafic-ultramafic intrusives) or as volcanic rocks that erupt and solidify at the surface (komatiites).

Nickel sulphides can be associated with volcanic or intrusive styles:
The volcanic type (Komatiite) comprise the high grade Kambalda style nickel deposits (Long, Miitel, Cosmos) and low grade disseminated Mt Keith style (there is some evidence now that at least some of these are high level intrusives). They are characterised by being hosted by high MgO rocks.
The intrusive associated deposits are hosted in what is called differentiated ultramafic or mafic-ultramafic dykes where the minerals segregate by various processes producing layers of different rocks. Usually with the higher MgO varieties at the base. Nickel is usually found near the base of these deposits. The rocks will have a range of MgO. Deposits include Radio Hill, Carr Boyd, Sally Malay. Voisey's Bay and the West Musgrave Nebo and Babel are also associated with differentiated mafic intrusives. These intrusives can also contain PGE mineralisation as well as chromite layers and are oftem much more copper rich than their komatiite counterparts.

Ok now that I have laid a bit of ground work, onto your question Spaghetti:
From the information you have provided it is hard to make a judgement. If the exploration is for komatiite hosted Ni then drilling into low MgO pyroxenite is probably not a good sign. Though, if they are exploring a differentiated dyke it may not be a negative. It also depends on the scale of the plan, they may be showing a large area and have a schematic drill collar placed on it. It also depends on what their target criteria. It may be conceptual, they may be following up on surface geochem or have a geophysical target or existing drilling. Lateritic nickel usually forms from high MgO rocks that have high corresponding Ni, it is unlikely they would be targeting a pyroxenite for Ni-laterite.

Hopefully that may shed a bit of light on the subject for you.

Hi jman, was a good idea to get the thread going, I will help out where I can.
The flies sure are friendly, I would like to shake the hand of the man who had the brainstorm to mix sunscreen and insect repellent together.
 
Hi Derty

Would have posted this question yesterday, but I was too busy chain-smoking about 35 packets of cigarettes and resisiting the urge to drink the pain away after watching the market eat itself alive.

You my friend, being the resident ASF laterite guru might be able to help me out here. :D

It seems to me that "laterite" is a term often used and abused by numerous Geo's. To top it off, almost as many researchers seem to have studied laterite profiles as have offered classification schemes!...

The original definition of laterite came from Buchanan (1807) who described a softish regolith material cut in Kerala India, to make building bricks. Ollier and Rajagura (1989) describe this material as typical vesiscular mottled saprolite, hard enough to maintain shape while being cut from the ground with an axe.

The original 'laterite' of Kerala seems to occur in a deeply weathered profile. But these days, most Geo's seem to use the term 'laterite' to refer to the Fe-oxide cemeted cap of a weathered profile, which it is clearly not if the material from Kerala is indicative of what laterite actually is....

So what's your take on this?

In terms of your nickel sulfide exploration work, do you differentiate between laterite and ferricrete? (According to Taylor and Eggleton, ferricrete is defined as near-surface masses of regolith cemented by Fe-oxides and oxyhydroxides). Most of the Geo's I work with would generally reserve the term 'laterite' for the Fe-oxide cemented cap of a weathered profile, which to my mind, is incorrect useage. This isn't to say you can't have 'laterite' profiles cemented by ferricrete! :banghead: I'm a bit of a lone ranger sticking up for the good ol' ferricrete Derty, appreciate your thoughts on this subject.

Regards
jman
 
jtb said:
Roger that Jman.

Shouldn't that be sitting in the air-conditioning in the 'cruiser with your Engel listening to an ipod though?;)

Don't let any of those smelly drillers do unspeakable things in your sample bucket .............

As the hatchetman said- great idea for a thread, look forward to it.

Whatyousee - loved the freudian slip thing :) I'm going to use that
Click to expand...

Aaaah!...

Ya dirty bugger!...:D That was a knife straight through my heart!

Funny you should mention the dirty drillers to me, one lot managed to slip a slice of beetroot into my sieve sample once, which I dutifully sieved and thought I had stumbled on a new and exciting form of mineralisation (for about 3 nannoseconds)..much to their amusement :rolleyes:

jman
 
Laterite guru, hehe, probably the first and last time i will be called that.

I may disappoint you somewhat here jman. The terminology for much of the regolith, especially the upper part is a dogs breakfast. The term laterite as coined by Buchanan referred to a soft ferruginous material that could be cut by knife or axe that hardened into bricks when dried. The original laterite would be called mottled zone these days from what i have read. As you said, today the term laterite is generally referred to the material that has been indurated by iron oxides and oxyhydroxides, usually in the upper parts of the profile.

Lateritisation as a process for me involves the cementing of weathered or transported material with Fe-oxides. I try to avoid the term laterite in regolith descriptions, though if I do use it it is always in reference to the insitu or weathered material. Analogous with the duricrust or ferrugenous upper saprolite/pedolith.
I use ferricrete to describe Fe-cemented transported material, usually Fe-cemented pisolitic or nodular ironstone gravels. Typical of the resistive bands you see at the top of some hills or breakaways as a result of topographic inversion.

I primarily used to use Cliff Ollier's and Louisa Lawrance's definitions of the regolith zones for field mapping and logging. Though these days I try to adhere as close as I can to the CRC LEME terminologies as I believe there needs to be some polarising of the terms used in industry and CRC are the leaders in the field atm and I have no real problem with their models.

So I guess I probably line up with most of the other geo's you work with :)
 
derty said:
Laterite guru, hehe, probably the first and last time i will be called that.

I may disappoint you somewhat here jman. The terminology for much of the regolith, especially the upper part is a dogs breakfast. The term laterite as coined by Buchanan referred to a soft ferruginous material that could be cut by knife or axe that hardened into bricks when dried. The original laterite would be called mottled zone these days from what i have read. As you said, today the term laterite is generally referred to the material that has been indurated by iron oxides and oxyhydroxides, usually in the upper parts of the profile.

Lateritisation as a process for me involves the cementing of weathered or transported material with Fe-oxides. I try to avoid the term laterite in regolith descriptions, though if I do use it it is always in reference to the insitu or weathered material. Analogous with the duricrust or ferrugenous upper saprolite/pedolith.
I use ferricrete to describe Fe-cemented transported material, usually Fe-cemented pisolitic or nodular ironstone gravels. Typical of the resistive bands you see at the top of some hills or breakaways as a result of topographic inversion.

I primarily used to use Cliff Ollier's and Louisa Lawrance's definitions of the regolith zones for field mapping and logging. Though these days I try to adhere as close as I can to the CRC LEME terminologies as I believe there needs to be some polarising of the terms used in industry and CRC are the leaders in the field atm and I have no real problem with their models.

So I guess I probably line up with most of the other geo's you work with :)
Click to expand...

Thanks for the info derty,

Agree that the CRC LEME chaps/chapettes are probably the only ones who really know what they are talking about, a few of the senior exploration Geo's here are of the same opinion. I guess if you use the original Buchanan definition the mottled zone and even the saprolite zone could be referred to as part of the 'laterite profile'... obviously current useage is not compatible with this model however.

Interesting to hear your useage of the ferricrete term, I would generally reserve this term for relative or absolute accumulation of Fe-oxides or oxyhydroxides in the duricrust. "Relative" being the removal of other solutes in the cap leaving a relative accumulation of these minerals, and "absolute" meaning the transport via water of these components from point A to B eg. slope wash down slope to a topographic low, or transport in groundwater....arguably this could be interpreted as "transported" ferricrete as well since the cementing minerals are sourced from another location!

The guys here are pretty big on identifying the base of tranpsorted material in the profile, and we use the term "transported laterite" for this, sometimes it is clear we are drilling through a paleochannel we we intersect stransported sands and gravels etc. but I still find it pretty tough picking transported material in the top 1-15m sometimes....

jman
 
Great thread idea jman. Good to see someone in the industry willing to share info and help us mug traders.

I was reviewing some anns from a stock I own (PEN) and was hoping you could shed some light on the nature of this ore type. From reading previous posts I'm thinking this is refractory ore?

DDHQ010 intersected 40.5 meters from 9.5 meters (down-hole depth) of
0.85g/t Au, including 2.5m of 4.48g/t Au from 43m within a zone of quartzpyrite-
carbonate veining. Deeper intervals of similar grade mineralisation
were intersected between 52-59.5m and 77.5-91m. Individual assays
range up to 9.14 g/t Au and variability of repeated assays indicate that
coarse gold may occur in some intervals.

The ann also referred to this as epithermal quartz veining.

Any help would be greatly appreciated:)
 
Excellent thread jman!! Keep it going. Thanks to everyone who's contributing their time and knowledge and also to those showing an interest in learning more. This is one area that I know little of. It's very encouraging to see the topics being discussed in the typically helpful ASF spirit.

<edit> It's important to make sure that this thread doesn't turn into a 'co anncts' thread where jnr mine anncts are discussed at the expense of theory and practice- the stock specific threads are probably best for the former. Illustrative examples of course are another story but it's important to strike a workable balance.

RichKid
moderator
 
Just quickly, I had a look at the announcement and they have assayed using fire assay which gives you a total gold value (i.e. both refractory and non-refractory gold). So from this you cannot make a call if there is any refractory gold. To determine refractory vs non-refractory analysis needs to be cyanide leach with the residue analysed with fire assay. The assay value for the cyanide leach will tell you non-refractory gold and the fire assay residue will tell you the refractory component.
 
Thanks derty,
I was under the impression that the presence of pyrite indicated refractory ore, but it's obviously more complicated than that:eek:
So would it be correct to say that these results aren't worth much at this stage, especially as the grades are fairly low? Metallurgical testing would be needed to determine feasability before more exploration?
Hope you don't mind my stupid questions, just trying to gain a basic understanding.

Rich Kid - This is an old ann and just trying to put some theory into practice:)
 
Ta Derty,

I was thinking along similar lines, again a metallurgist would be great to have on board. I don't see how you could look at an ore sample and determine whether it is capable of being free-milled (non-refractory) or not. If the gold is hosted in the pyrite, and the pyrite is disseminated, not granular, then perhaps most of the gold (>92-93%) could be recovered with conventional leaching. But ultimately, the recovery testing process you proposed would be the only way to definitively answer the question

jman
 
In reply to your previous regolith post jman (apologies for the delay, have been in Esperance for the long weekend), I used the topographic inversion exposures of ferricrete as an example. When I see massive iron accumulation in transported material, often lake sediments I also call that ferricrete too.

I sympathise with your guys looking for the Recent/Tertiary - Archaean boundary. At times it can be a trying experience, especially when the distance between what you are comfortable calling transported and Archaean is 10m or so. I have logged over 120km of aircore through most parts of Lake Cowan and quite a bit on Lake Lefroy. The southern palaeodrainages are often reducing below 20 or so metres and it makes the spotting of the boundary quite a bit easier. However up north it is my understanding that the drainages are usually oxdised and where you have an oxidised overprint over the unconformity it will make the task of locating it all the more difficult. You end up feeling a bit like Sherlock Holmes, looking for little shreds of evidence that will allow you to make a call one way or another.

***********

Now on a different track, I just wanted to mention a useful way to look at interpreting intersections. Quite often you will see an intersection reported as XXm @ YYg/t including AAm @ BBg/t, where BB is quite a large number. This usually means that the remainder of the intersection is quite poor.
What you can do is convert the intersection into gram metres and that will allow you to easily work out the metal distribution for the remainder of the intersection.

I will use the PEN intersection from daggs as an example. The intersection is 40.5 meters from 9.5 meters (down-hole depth) of 0.85g/t Au, including 2.5m of 4.48g/t Au from 43m.

First you need to convert both intersections to gram metres, this is easily done by multiplying the interval thickness by the grade e.g.

40.5m at 0.85g/t = 40.5 x 0.85 = 34.4 gram.metres Au
2.5m at 4.48g/t = 2.5x 4.48 = 11.2 gram.metres Au

Then to work out how much the remainder of the intersection contains you subtract the intersection widths and the gram.metre intersections and divide the gram.metres by the intersection e.g.

40.5m - 2.5m = 38m
34.4 g.m - 11.2 g.m = 23.2 g.m
then 23.2 g.m / 38m = 0.61g/t Au

So the remainder of the intersection contains 38m at 0.61g/t Au.

If you look around you will find some excellent examples of this where the remaining metal in the intersection is pathetic. This can me done for any commodity. If the metal is presented as % the resultant calculation is metre percent. Have fun you will be amazed at some of the huge intersections some companies will present all carried by one small high grade intersection.
 
derty said:
Now on a different track, I just wanted to mention a useful way to look at interpreting intersections. Quite often you will see an intersection reported as XXm @ YYg/t including AAm @ BBg/t, where BB is quite a large number. This usually means that the remainder of the intersection is quite poor.
What you can do is convert the intersection into gram metres and that will allow you to easily work out the metal distribution for the remainder of the intersection.

I will use the PEN intersection from daggs as an example. The intersection is 40.5 meters from 9.5 meters (down-hole depth) of 0.85g/t Au, including 2.5m of 4.48g/t Au from 43m.

First you need to convert both intersections to gram metres, this is easily done by multiplying the interval thickness by the grade e.g.

40.5m at 0.85g/t = 40.5 x 0.85 = 34.4 gram.metres Au
2.5m at 4.48g/t = 2.5x 4.48 = 11.2 gram.metres Au

Then to work out how much the remainder of the intersection contains you subtract the intersection widths and the gram.metre intersections and divide the gram.metres by the intersection e.g.

40.5m - 2.5m = 38m
34.4 g.m - 11.2 g.m = 23.2 g.m
then 23.2 g.m / 38m = 0.61g/t Au

So the remainder of the intersection contains 38m at 0.61g/t Au.

If you look around you will find some excellent examples of this where the remaining metal in the intersection is pathetic. This can me done for any commodity. If the metal is presented as % the resultant calculation is metre percent. Have fun you will be amazed at some of the huge intersections some companies will present all carried by one small high grade intersection.
Click to expand...

Great bit of information there. So many companies must use higher grade intersections to boost the average grade along the strike. Crazy!!!

Anyway I am starting my science degree majoring in geology in about 2 weeks. So this is a great thread to start me of on.
 
Controls on Gold Endowement: Shear Zone Comparison

This might be interesting for all you gold bugs out there, I stumbled across this little review a few days ago. This work is relatively recent, but may in fact revolutionize the way in which companies conduct their exploration programmes.

(*Cough* I'll try not sound like I'm speaking Polish here)

The guts of this 'new' theory is that the key to gold deposition is that a variety of rock types are required at the depositional site. This will lead to local changes in competency contrast which basically means different rocks in this local area will react differently when a stress is applied to them. Some might flow like a toffee, while others might fail by brittle fracture.

This is all looking very nice for gold minerlisation now, because this competency contrast will lead to strain incompatibility. Some large competent blocks within the shear zone will be rotated according to the orientation of the stress field and will actually focus the gold bearing fluids as these deep-sourced fluids try to find a way around these blocks, utilising more permeable pathways. Additionally, the variety of rock types can also lead to chemical gradients in which the gold may become saturated in solution and start precipitating out.

Also bear in mind that there are also many structures within shear zones that may "trap" the mineralization, ie. it has no-where else to go, such as antiformal fold closures (fold looks like a rainbow). Compare this to synclinal forms which have little/no ability to trap gold (fold looks like an upside down rainbow).

In the case of the Golden Mile, many of these ideas in the model can be bourne out by observation. A particular low permeability group of rocks called the Black Flag sequence are thought to have acted as low permeability seals for rising fluids, and deflected the fluids toward the higher permeability Boulder-Lefroy Shear zone! The ultimate regional fluid focussing mechanism!!

Apparently, after combining all of these factors, the reseachers concluded that it was most likely that the Pressure/Temperature conditions required for ideal Au minerlisation corresponded to greenschist facies metamorphism (erm...well this will be a particular pressure and temperture regime where a certain group of minerals will be 'formed').

If we imagine that we ramp up the pressure and temperature and go into a higher metamorphic grade (amphibolite facies), this will be bad news. Basically the host rocks will all become 'similar' to each other, leading to vast areas of monotonous geology, with little competency contrast and therefore little or no ability to focus fluid flow. This is because fluids will be pretty much dispersed throught the system so mineralisation cannot take place.

Bear in mind that this model seems to work well for large areas of the Archean Yilgarn Craton, (around Kal and beyond), but there are many other deposit types around the world of younger age, such as placer deposits, Carlin-Style deposits and SEDEX deposits to name a few, which will undoubtedly have their own tailor-made models, and that this model may not necessarily be applicable in other geolical settings.

Hope this was some help
jman

References:

Controls on Gold Endowement: Shear Zone Comparison. Ch. 2.6: Weinberg, R.F., Groves, D.I., Hodkiewizc, P., van der Borgh, P., Hydrothermal Systems, Giant Ore Deposits, Yilgarn Atlas Volume III UWA Gold Module, Part I, AMIRA Project P511
 
First, I think it's great that these geos take time to answer questions to help us fellow ASFs.

I have a (2) question about Airbourne EMs.

1. What is the turn around time from the actual recording of data to reading a report of the data?

2. Is the data accurate alone, or enhanced by dilling results?

Thanks in an advance for any feedback!:)
 
Birdster said:
First, I think it's great that these geos take time to answer questions to help us fellow ASFs.

I have a (2) question about Airbourne EMs.

1. What is the turn around time from the actual recording of data to reading a report of the data?

2. Is the data accurate alone, or enhanced by dilling results?

Thanks in an advance for any feedback!:)
Click to expand...

Hi Birdster,

Not sure about the tat for EM surveys these days, there are specialist geophysical companies who usually do this kind of work, but it could be anywhere from 5-6 weeks I imagine b4 the data is collected, analyzed and worked into a draft report for the mining company.

I guess in terms of accuracy, it depends on the grid resolution that the data is based upon. For example, a 50m by 50m grid pattern will not be defined as a 20m by 20m pattern. The low res pass is good for regional targeting on a broad scale however, as it can sometimes pick out regional structures such as shear zones and define the geology in a general sense. The other factor would probably be the rock types, as it can sometimes be very hard to distinguish between two relatively similar density materials that exist in close proximity to each other, as they can give a very similar 'signal', such as a sediment and a granite for example.

Another common problem is surface scattering of the EM data by tertiary transported regolith material, which can essentially 'swamp' the signal of the underlying basement rocks. The image has a distinctive fuzzy look, and can almost be impossible to interpret.

Hope this helps

(Derty probably knows a lot more about this actually :D)

jman
 
RichKid said:
Excellent thread jman!! Keep it going. Thanks to everyone who's contributing their time and knowledge and also to those showing an interest in learning more. This is one area that I know little of. It's very encouraging to see the topics being discussed in the typically helpful ASF spirit.

<edit> It's important to make sure that this thread doesn't turn into a 'co anncts' thread where jnr mine anncts are discussed at the expense of theory and practice- the stock specific threads are probably best for the former. Illustrative examples of course are another story but it's important to strike a workable balance.

RichKid
moderator
Click to expand...

Cheers for the heads up RichKid,

Yeah people seem to be enjoying the thread so far which is really great. It's a bit of a 2-man crusade at the moment though :D, hopefully we can get some more geo's onboard and start expanding our knowledge base too.

I think we'll try to keep things fairly general and use hypothetical examples where possible, and I agree, the best place to discuss and pick apart specific company info released to the market should be the relevant company thread itself.

Cheers
jman
 
i'll put in my :2twocents too: As jman said the time to get the report back will depend on a lot of things. 4 to 8 weeks for a report and modelled data is likely, though the geophysicist should be able to supply a draft summary of any main anomalies within a week or two. It will depend on the density of the data collected, the size of the survey and how busy the geophysicist is.

Now Birdster you said it was airborne EM, which is an ElectroMagnetic technique that looks for conductive bodies in the upper part of the crust. It is a really nifty technique. The are two main components used for the survey. A transmitter coil/loop (TX) and a receiver coil/loop (RX). With airborne EM the TX is usually a wire loop strung between the tips of the wing, the nose and the tail and the RX (called a bird) is towed on a cable behind the plane.
How it works is; a current is passed through the TX which creates a magnetic field, the magnetic field penetrates into the earth and induces a secondary current in any conductive bodies it intersects (the materials that usually conduct in the crust are salt, graphite and sulphides). The TX current is then switched off and now the magnetic field that is produced by the current in the buried conductor induces a current in the RX. The RX will measure the strength of the magnetic field and how fast that field decays. Better conductors will decay slower. (that is the basics of EM, surface and down hole EM all use the same principle). The survey produces a whole heap of squiggly lines that the geophysicist interprets, a bit like very scientific tea leaf reading :p:

Now when you ask is the data accurate, I assume that you mean can you look at it and tell what is down there?
What the data tells you once the geophysicist has modelled it is that there is a conductive body, you will also usually know it's depth, size, orientation and conductiveness. You will not know what is causing it. In nickel exploration the majority of identified conductors are sulphide bearing graphitic shales (and if they were worth money I would be a rich man) and at them moment there isn't a way I know of to tell these from bona fide nickel sulphides unless there is some surface expression such as a gossan. The only way to really tell is to drill a hole a have a look.

Airborne Em has pro's and con's over surface techniques.
cons
- Because the RX is mobile it will not see the vary late time conductive bodies.
- As the magnetic field attenuates rapidly away from the source the field will not penetrate to the depth a suface EM survey will. Some surface techniques are claimed to penetrate up to 500m down, though usually 150m-250m is the norm. Airborne will be around 150m ish and can be much less dependant on several factors, and may not even be able to penetrate past the weathering to detect the fresh sulphides.
- The weathered part of the rock often contains saline waters, these are weakly conductive and is called the conductive overburden. The currents in the conductive overburden will rapidly attenuate the TX mag field and will often render airborne surveys useless. In Western Australia the ground waters south of Menzies are typically saline and those north relatively fresh. As such the usefulness of airborne EM south of Menzies is limited.
Pros
- fast and covers very large areas quickly
- per hour it is MUCH more expensive though for line kilometre of survey it is usually cheaper than surface EM
- survey does not have accessibility issues that surface techniques will due to terrain and vegetation e.t.c.

Airborne EM is great when used in the right conditions and a very expensive waste of time when not. Anomalies are usually followed up with surface EM to better define the conductor before drilling. Surveys can be flown by aeroplane or helicopter.
 
Thanks Jman and derty. Very well explained.

FYI, I asked is because EXM is conducting the Airborne Em. (ann'd 16/1) They have had ann Positive drill reports previously with neg. results :confused:

But thats the whole Stock Market in a nut shell ATM :banghead:
 
Hey geo's - I see this thread has fallen silent for awhile which is a pity as much was learnt via the excellent contributors.

I have a coal question which may or may not belong on this thread but anyway -here goes. What does it mean when a coal deposit has a high ash content? Is this an issue for a thermal coal deposit. I know that it is not great for coking coal.

Would appreciate anyone's feedback on this.

Cheers
B'man
 
Top