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Objections to current pegmatite theory

Last Updated: 20th Jul 2015

By William C. van Laer

OBJECTIONS TO CURRENT PEGMATITE THEORY

OPEN VS. CLOSED SYSTEMS:
-The conduits needed to provide the fluids necessary for crystallization as well as the exit conduit needed to evacuate the pocket are not evident; no such supply openings are to be seen in miarolitic cavities, and none have been evident in virtually every pegmatite pocket I have seen and/or opened. There MUST be physical evidence of these “conduits” like brecciated & cemented zones (as proposed by Sinkankas), altered zones, fault openings, slickenslides, cuts transecting previously solidified pegmatite or granite. None of these are found in evidence, so the “outside” source of hydrothermal fluids is highly suspect.
-The source and nature of such fluid is highly suspicious as well: how is it that an “outside” source of hydrothermal fluid has the specific chemistry that is identical to the final phases of pegmatite/miarolitic growth, and not of some more general content, like just plain quartz-microcline-albite? Given that the mineral species of virtually all pockets represents both the primary granitic constituents along with species associated with pegmatite pockets like topaz, elbaite, beryl, zinnwaldite, apatite, spodumene, axinite, titanite, etc. How does this fortuitous fluid come to exist and be emplaced into the exact specific circumstances? Both the source and nature of the hydrothermal supply is highly dubious; it is FAR more likely that the source is from within the pegmatite and from confined rather than open source conditions.

-In the final consolidation phases of pegmatite crystallization and solidification, it is clear that some systems undergo rupture from increased internal pressures. Two manifestations of this phenomenon would be: 1) fracturing around the pocket zone or pocket, restricted to an area within distances corresponding to the overall effect of those conditions, and 2) overall failure of the entire pegmatite body, in which case any fluids (magmatic or otherwise) would be squeezed into any openings, fractures, or faults that transect the inner zones, where magmatic or hydrothermal fluids have not yet solidified and remain mobile. These are the so-called “replacement bodies” of Cameron, Jahns, McNair, and Page (1949). There is more than substantial evidence of this; as a matter of fact, this failure and subsequent rupture of the rock is to be found in many if not most pegmatites, and in all those that have pockets or vugs (at least to some degree). Some miarolitic cavities exhibit no rupture, while others clearly do. I only disagree with the term “explosive” as being a little overdramatic, especially since rupture and failure are far more accurate explanations from an engineering standpoint. Furthermore, a truly explosive event would most likely cause total destruction of the entire contents of a pocket, as illustrated by the use of explosives near pockets. The dissociated nature of some pocket contents can be explained by other mechanisms, especially those related to cooling and shrinkage or even phase change volume differences, which are far more likely to be the actual solution than an explosive event. I do agree that the rupture of some pockets may actually be explosive in nature, but it is clear that if this occurs, the pressure exerted is directional and away from the pocket, that is, the pressure is from within the pocket fluid and exerted on the surrounding rock away from the opening, not internally (although internal pressures are clearly at work as well).
-Dissolution is a complex set of events that cannot be easily explained by confining the mechanism to mere “solubility”. There are numerous physical and chemical forces at work here, and the process of dissolution can be very selective, despite one author’s objections to the terminology. Perhaps other descriptions for these complex processes could be adopted into etymology. It should not be dismissed because of a few select examples that only show that the process is not well understood (and therefore obviously should be studied in detail & written up).




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Discuss this Article

25th May 2008 23:30 BSTRob Woodside Manager

Thank you for an interesting post.

Recently I have been wondering just what the difference is between pegmatites and mariolitic cavities? My first thought was open vs closed as you mention, but how different are the minerals? My guess is they are the same, but perhaps a more knowledgeable person could comment. Some years ago there was a discussion on the size of mariolitic cavities. The literature suggested on the order of cm, but field collectors had been in meter sized cavities. Could pegmatite dykes just be very large mariolitic cavities?
Consider the emplacement of a large igneous intrusion. The ratio of the intrusion's surface area in contact with country rocks to the total volume of the intrusion must be quite small. This leads one to think that the hydrothermal fluids in the intrusion have had little contact with the country rocks and the component of fluid that has, makes a skarn. So what ever is in the fluids presumably has the same origin as the intrusive. Supposedly the contents of these final fluids contains all the elements that don't crystallize easily. So we get garbage can structures of Tourmaline, garnet, beryl, rare earth phospates, Niobates and Tantalates, etc. Since the fluids are close to equilibrium with the intrusive, one should also expect to find the cations of the intrusive itself in the final fluids. I thought this last point would differentiate a pegmatite in an alkaline syenite from one in granite.

The surprising number of Beryllonites from Afghanistan and Pakistan currently on the market got me thinking about this. All of these Beryllonites have the same columnar habit and look like they came from the same pocket!!! Other localities produce quite different Beryllonite habits. My first thought was: a rare mineral with the same habit must be from the same pocket or locality. Consider the rare mineral Brazilianite. It has essentially two habits, equant and prismatic, so that would argue for only two pockets, instead of the dozens of localities and thousands of pockets that do host Brazilianite. Brazillianite has only aluminum and sodium as cations, so I would suspect its occurence in an alkaline intrusive pegmatite, such as Mont St. Hilaire. But it is not there??? Why not??? Maybe I am neglecting the temperature and pressure of deposition and perhaps St Hilaire crystallised outside the stability field of Brazillianite?

Now that we have defined minerals as chemical structures, it would be nice to return mineralogy to its geological roots with an understanding of these hydrothermal solutions and how the elements get segregated in them to produce the minerals we find. I would greatly appreciate any comments that others may have.

8th Jul 2008 14:56 BSTWilliam C. van Laer Expert

Rob:
I can only address your first question, concerning the difference between a miarolitic cavity and a pegmatite pocket: from my field observations, which amount to literally thousands of miarlitic cavities and many hundreds of pegmatite pockets, there is clearly a difference, but there are some exceptions along that thin red line that blur the facts. A pegmatite is a definable body, with a length, width, and thickness, and often a measurable attitude (dip angle and direction). Most pegmatites are either epigenetic or syngentic, and of those there are interior and exterior pegmatites. Despite the observations of others (and here I refer to the article on the mineralogy of the sawtooth batholith by Boggs and Menzies in the Mineralogical Record, May-June 1993) I find there are epigentic pegmatites within some granitic inntrusives, and they are not always syngenetic. That is, some are clearly injected into the granite that has already crystallized or solidified, most likely from an adjacent source of granite magma that has yet to solidify.

But the difference can be observed, if you look at the pegmatites of Colorado (i.e., the amazonite-bearing pegmatites of the Pike's Peak batholith) or the pegmatites of the Boulder batholith of Montana, these are distinct bodies with clear definitions and boundries. Many of these exhibit zones or margins of aplite that completely enclose the pegmatites. This can also be seen in many other exterior pegmatites. The pockets are openings in the pegmatitic zones, and are often bordered by zones of graphic granite intergrowths, or are adjacent to massive quartz cores, especially between and under where large feldspar crystals grow into the quartz cores.

In contrast, a miarolitic cavity is an ABRUPT OPENING in what is otherwise solid granite. It may show some pegmatitic texture surrounding the vug opening, but there is no definable body of pegmatite that it is associated with. The interesting thing to note here is that where many miarolitic cavities are found, such as the Sawtooth batholith and other miarolitic granites in Idaho and Montana, there are also some distinct pegmatites, some with pockets, so the geological definition becomes somewhat indistinct. I have hundreds of photographs of miarolitic cavities in situ, and many also of pegmatite pockets (you can add photos from other authors like Sinkankas here) that show these relationships. After over 30 years of field collecting and picture-taking, I was looking at all these pictures and realized there was a common thread amongst all of them, and this lead to a lengthy paper yet to be published, but evaluated by several experts in the field for credibility.

I hope to add to my photos some of these pictures, certainly the best of them, to further illustrate these features.

28th Jun 2011 20:34 BSTCurtis Wilbur Expert

I've often wondered about the relationship between Miarolitic cavities in granitics versus "pockets" in pegmatites. Your discussion of open versus closed systems suggests an interesting model that could explain both. As a geologist you are of course familiar with Bowen's reaction series. If the volatiles, along with the smaller ions, "travel" locally along the crystallization front, they will end up coalescing in localized areas. If those localities are small enough, the remaining crystallization will occur there (and you have a miarolitic cavity). But if the material has a large enough volume, the surrounding structures in the host rock may not be able to constrain the fluid. Here the story gets a little sketchy. How then does the fluid generate a fracture that it is able to then flow into, creating the pegmatite. The pegmatites in Pala are highly constrained by the host rock, and all angle in roughly the same direction. There are many cases where a succession of pegmatites layer on top of one another. As far as I know none interpenetrate, and they all may be largely epigenic. It could be suggested that the original shape of the fluid body as it formed from the crystallization front may be planer - simulating a dike. In any case, once the pegmatite came into its dike-like shape, for whatever reason, the same "crystallization front" effect could operate on the material there, ultimately forming pockets or bands of final materials. To say "final" is perhaps a misnomer though, as the reaction series for pegmatite pockets is often unique - depending entirely on the components left behind and in what amounts. (original spod material may be entirely replaced by later regrowth of tourmaline). Your argument of a closed system suggests that the boron and/or beryllium may be resident in the pocket fluid the entire time (or alternatively able to travel along crystal boundaries and accumulate in pockets over time.)
 
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