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Pegmatite classification?
January 12, 2012 04:38PM
ca    
What is the current status of what is a pegmatite? Most references say; "a coarse grained igneous intrusive rock", however some pegmatites are clearly hydrothermal. Are the hydrothermal pegmatites no longer considered to be pegmatites or has the definition of intrusive changed to include hydrothermal "intrusion" not just magmatic?
Re: Pegmatite classification?
January 12, 2012 05:26PM
us    
I've always considered the term "pegmatite" or pegmatitic as being a textural term. A qualifying term should preceed the term - "granitic pegmatite". Since one of the mechanisms producing a pegmatitic intrusion is an abundance of segregated water - under pressure and at high temperature - it would seem to me that ALL pegmatitic dykes have a definite hydrothermal aspect to one degree or another.

Don
Re: Pegmatite classification?
January 12, 2012 06:37PM
ca    
Hello Don,
Thank you for your reply. Certainly in the literature there does not seem to be any disagreement as to the coarse grained nature of a pegmatite, just the rest of it is not clear. My confusion comes from terminology commonly used to describe these things, some of which you have used here. For example the term " Pegmatitic intrusion" - does this mean that the term intrusion does not have to mean a magmatic intrusion? "Pegmatitic dykes" My understanding of what is a dyke is that it is a relatively thin steeply inclined magmatic intrusive, as such wouldn't "pegmatitic dyke" be an oxymoron? Or is there such a thing as a hydrothermal dyke?
Re: Pegmatite classification?
January 12, 2012 07:04PM
at    
London, D. (2008): Pegmatites. Canadian Mineralogist Special Publication 10, 347 pp.

&

[pegmatopia.ou.edu]
Re: Pegmatite classification?
January 12, 2012 08:22PM
A dike may be classed pegmatic in the sense that it is course-grained, as opposed to a fine-grained basalt dike.
avatar Re: Pegmatite classification?
January 12, 2012 08:46PM
us    
As pointed out by Uwe, the best definition I've seen is in London (2008) an awesome publication, highly recommended, can be purchased from the Min. Assoc. of Canada web site. [www.mineralogicalassociation.ca]
On page 4 he discusses the original meaning, then gives a redefinition and explains why. His recommended "descriptive definition suitable for field identification" is:

"an essentially igneous rock, commonly of granitic composition, that is distinguished from other igneous rocks by its extremely coarse but variable grain size, or by an abundance of crystals with skeletal, graphic, or other strongly directional growth-habits. Pegmatites occur as sharply bounded homogenous to zoned bodies within igneous or metamorphic host-rocks."

He is purposefully ambiguous regarding "essentially" - leaving room for some hydrothermal effects, and "mostly of granitic composition", leaving room for non-granitic mineral assemblages (zones) within (such as lithium mineral assemblages) and rarer ultramafic, mafic, syenitic and carbonatitic (aka "calcite vein dike") igneous rock compositions. But not ambiguous about the textures and directional growth habit portions.

That's the short version, if you are seriously interested in pegmatites, you need the book.
Re: Pegmatite classification?
January 12, 2012 09:46PM
ca    
Thank You Uwe and Harold, I will look into it.

Hello Nate,

Have never heard the term pegmatic before, why not just say coarse grained and leave any possibility of confusion with a pegmatite out of it?
avatar Re: Pegmatite classification?
January 12, 2012 09:49PM
us    
Reiner,

There are lot's of different pegmatite bodies out there... some deposits or formations appear more like veins, lenses, pipes, dykes or pods. Some pegmatites are very complex, and contain zones of mineralization that range from greisen to migmatite and granite within the same body. Although pegmatites are usually found within the outer margins of intrusive type rock bodies, there is no conclusive evidence that points to the veins themselves having been intruded through faults or joints within the country rock. Structural control features within the host rock usually factor into the genesis of the typical formation, but most pegmatites appear to be the distinct product of metasomatism.

Scott
Re: Pegmatite classification?
January 13, 2012 01:45AM
ca    
It would appear that pegmatite dyke is not an oxymoron [rruff.geo.arizona.edu] will have to see what the 2008 publication has to say.
avatar Re: Pegmatite classification?
January 13, 2012 02:33AM
us    
A dike or dyke is igneous by definition. So the term "pegmatite dike" merely states that the type of igneous rock in that dike is a pegmatite. There is no such thing as a hydrothermal dike as hydrothermal rocks are not igneous but formed from aqueous solutions, hence once crystallized they are typically called veins. Some pegmatites may undergo a late hydrothermal phase when crystallizing as they become more water rich due to precipitation of magmatic minerals from the mix, but the initial intrusion was magma and the bulk of the rock is still igneous.

I think Nate meant to say "pegmatitic" earlier, not "pegmatic".

Scott, after reading London (2008), I dont see how you can state that "most pegmatites appear to be the distinct product of metasomatism"; they are igneous melts. Pegmatites may be internally metasomatized by some of the last fluids to crystallize (forming pseudomorphs of some earlier crystals) and the pegmatite magma may metasomatize the host rock to some degree, but pegmatites don't originate from metasomatism.
avatar Re: Pegmatite classification?
January 13, 2012 03:22AM
us    
Sandstone dikes aren't igneous, but I digress...
avatar Re: Pegmatite classification?
January 13, 2012 08:31AM
us    
Harold,

There are many hypothesis out there, and that's good.

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. Because metamorphism usually requires water in order to facilitate metamorphic reactions, metasomatism and metamorphism nearly always occur together.

Most modern geophysical studies in southern California have concluded that the age of the country rock and pegmatites are coveal. The textural and structural differences of the pegmatite and immediately surrounding country rock are most likely a result of complex metasomatism from subduction-fed magmatism. The process is thought to be caused by shallow intrusive injections of diabase or gabbro into highly water saturated felsic granites such as granodiorite in the upper continental crust, thereby metamorphosing and chemically altering the rock along proto-rift zones and ancient rift zone margins, resulting in the formation of concentric bands or long belts of xenolithic gabbro and simple aplitic to complex zoned pegmatic rocks.

Since the term 'dike' is an intrusion into an opening cross-cutting fissure, this would imply that it is always younger than the rocks that contain it - but since the pegmatites in our area are the same age as the rock, and they are often closely associated with stringers, stockworks and greisens, the use of the term vein to describe these features seems more accurate... at least most of the time. It's important to note, that many tourmaline-bearing hydrothermal quartz and gold veins are closely associated with tourmaline-bearing aplites and pegmatites.

Cheers,

Scott



Edited 4 time(s). Last edit at 01/13/2012 10:30AM by Scott L. Ritchie.
Re: Pegmatite classification?
January 13, 2012 02:59PM
ca    
Good discussion!
What prompted me to post this thread was that I was trying to come to a clear understanding of what is meant by the term "Pegmatite Dike" when used as a name for a rock. If one had in drill core a short (1-2m) coarse grained felsic igneous rock intersection with parallel contacts, within granodiorite, considering what as been said thus far, how would one determine whether it is a "pegmatite dike" as opposed to a xenolith ? Unless one could distinguish zoning parallel to the contacts there wouldn't be any way of knowing would there?
Assuming it were not a xenolith, I also have trouble with the use of the term dike as it would not be possible to rule out a sill based on only a single drillhole and no outcrop, so wouldn't the term "dike" be presumptuous? Secondly, it seems to me that the term pegmatite in itself is not a rock type but a textural/genetic term that requires an adjective such as Granite to be a proper rock type. What do you think?
Along this same line, how about a fine grained felsic igneous rock intersection rather than a coarse grained one, any way of knowing whether it was a dyke or xenolith?
avatar Re: Pegmatite classification?
January 13, 2012 03:30PM
us    
Scott:
All of the pegmatites I have experience with (New England, New York, New Jersey, Maryland, Virginia, Colorado, Canada) are clearly cross-cutting dikes from igneous intrusions. As a geologist, I find what you describe to be very interesting [any papers you could recommend?], but appears to be limited to your area and apparently does not fit London's definition of most pegmatites. London carefully included the section of his definition that says, "an abundance of crystals with skeletal, graphic, or other strongly directional growth-habits [emphasis added]." He says only igneous melts can produce this texture, not hydrothermal or metasomatic rocks. Anyway, you clearly know far more about the fascinating geology of southern California than I do, and London knows far more about pegmatites than I do. Take a look at the book, maybe he's missed something regarding the rocks you describe?
Thanks
Harold
Re: Pegmatite classification?
January 13, 2012 04:40PM
us    
Be careful about the age issue. Sometimes the age referred to for the host rocks is the metamorphic age rather than the original age of formation of what later became metamorphic rocks. The metamorphic age and the age of pegmatite formation can be very close, but the age of the original rocks prior to metamorphism is usually older.

When I did my Masters thesis on pegmatites in 1996, the emerging science was swinging away from the idea of pegmatite-forming melts being water saturated. Lots of studies point in that direction now. Some of the best indicators are that when pegmatites intrude ultramafic rocks like dunite, most of the water in the melt gets taken up by the formation of hydrous minerals like biotite and amphiboles along the wallrock contact. But when the volumes are considered there is not enough water represented in those minerals for the melt to have been saturated with water. They are undersaturated at the time of emplacement. The mechanisms that lead to coarse crystallization have more to do with the temperatures of the melt and of the enclosing host rocks, and additionally the presence of incompatible elements like fluorine, boron, and phosphorus.

Pegmatites can reach water saturation late in their crystallization but they do not always do so. When it happens it can give rise to the pockets we love so much, but it is a late thing.
Re: Pegmatite classification?
January 13, 2012 06:03PM
ca    
Hello Keith:

"when pegmatites intrude ultramafic rocks like dunite, most of the water in the melt gets taken up by the formation of hydrous minerals like biotite and amphiboles along the wallrock contact." How would they know that? I imagine that this may be true in some cases, but I can't imagine that the water content of all melts would be the same. My impression so far is that there is a great deal of variation in how these can form, with no hard and fast rules.
avatar Re: Pegmatite classification?
January 14, 2012 12:01AM
us    
Hey Reiner,

Here is another link to a popular pegmatologist page:
PIG

If your summers were not so busy, you could take a class in pegmatology:
[homepage.mac.com]

And here is a paper from a leading pegmatologist about different classifications:
PEG Classifications

Although it is region specific, my favorite pegmatite document contains a lot of general information:
South Platte PEGs
The authors seem to use "pegmatite dike" to mean a small linear 'wannabe pegmatitic' region containing large quartz and feldspars crystals, but nothing else interesting.

I would also suggest that pegmatite occurrence is not due just to water, but any other chemical species such as fluorine or lithium which can lower the granitic melt's solidification temperature and allow larger crystals to grow.

Regards,
Dean Allum
Re: Pegmatite classification?
January 14, 2012 02:53AM
us    
Reiner,

That's a really good question. They can know that because they can calculate the water content of the hydrous minerals and compare it to the other introduced constituents. It's called mass balancing and it is a common practice when melt-rock or rock-rock interactions take place. A dunite, for instance, would contain miniscule amounts of potassium and water, so after the pegmatite intrudes, the amount of potassium and water in the biotite can serve as a reference for how much potassium and water there was in the original melt. It involves some assumptions about volumes, but when balanced across numerous elements the conclusions can be pretty tight, to within a weight percent of the correct water content with a pretty small error bar. And since solubility of water in granitic melts is well understood as a function of temperature and pressure it has become apparent that the melts are water undersaturated at the time of initial intrusion. They can evolve to water saturation later, but many don't because they begin crystallizing hydrous minerals like micas that take up a lot of the water, or the water goes into the wallrocks altering the mineral assemblages there.

Hope this helps.



Edited 1 time(s). Last edit at 01/14/2012 04:04AM by Keith Wood.
Re: Pegmatite classification?
January 14, 2012 02:53AM
us    
And yes, other elements like boron, fluorine, and phosphorus can play a big role in the crystallization dynamics. But they are often absent in pegmatites, so there are other factors that can lead to pegmatite formation, among which are the temperatures of the host rocks, the temperatures of the melts, and the composition of the melts..



Edited 1 time(s). Last edit at 01/14/2012 02:56AM by Keith Wood.
Re: Pegmatite classification?
January 14, 2012 03:53AM
us    
Harold,

London did a lot of research at the Little Three mine in So Cal, so he classifies the So Cal PEG's as magmatic intrusions as well.

Reiner,

Pegmatite dike is like saying a coarse grained dike...., you are right that you would need a rock ID based on mineralogy to completely name the dike. You should look for directional growth from the contacts with the host rock or chill margins to better determine if you have intersected a dike or something else in your drill hole.

And Keith is correct. The only recent (in the past 15 years) research all agree that granite pegmatites (or any other for that matter) are not enriched in volitiles but instead are a funtion of super saturation (undercooling) and fluid flux boundaries along growth fronts within the melt. Much bulk chemistry has been done. Metasomatism is a pretty old term and is really only applicabile with respect to the late stage fluid saturated cental portions that occur during the final stages of elevated temperature mineral alteration. And stating that the water was taken up by early formed hydrous minerals can be easy if it can be determined that the walls of the intrusion crystallized early and biotite or amphibole, etc..., is restricted to intrusion margins. Then if the core of the intrusion is full of anhydrous minerals it can then be deduced that the available water was taken up by early hydrous minerals.
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