Discussion of inclusion survival in sheared quartz

 

Ranjan Ray

Bruce Yardley

David Vanko

Carlos Coelho

Jeremy Richards

Brian Marshall

Anne-Marie Boullier

Kris Piessens

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From: Ranjan Ray

Dear fluid inclusionists,

I am a research scholar and fairly new to the field of fluid inclusions. Presently I am working on quartz veins which are highly sheared. Can anybody please suggest to me any literature or provide me any information related to the effect of shearing of quartz veins on the fluid inclusions.

Any information will be helpful to me.

Ranjan.

RANJAN RAY( ranjan@gg.iitkgp.ernet.in)
Department of Geology and Geophysics
IIT,KHARAGPUR
West Bengal- 721 302
India

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The short response to Ranjan Ray's query is that there is not much fluid inclusion literature on sheared quartz veins because they don't have many inclusions! The example with which I am most familiar is the Otago schists of New Zealand. There, veins at very low grades show original growth textures of the quartz and there are both primary and secondary inclusions including many that are large enough to study. However with increasing metamorphism and deformation (which means the lowest greenschist facies there), the same generations of quartz veins have a saccharoidal texture and the only inclusions are trails of extremely small (sub-micron) size secondary inclusions related, presumably, to cracking of the veins during exhumation. This is discussed briefly in Smith & Yardley, Journal of Metamorphic Geology v.17, p.173-186 (1999). Since significant deformation of quartz veins leads to recrystallization and expulsion of inclusions as well as cracking and trapping of secondaries (which may subsequently be annealed out) I don't think this is untypical. In other words, even if you find inclusions in sheared or recrystallised veins that can be measured, it will prove extremely difficult to identify when they formed, and there is growing evidence that most will prove to be deep penetrating groundwaters from the final stages of exhumation, rather than participants in the metamorphic process. N.B. All fluid inclusions in metamorphic rocks, whatever there age and origin, can always be made to yield isochores that pass through the supposed peak conditions of metamorphism. I hope this is not too negative - of course there have been valuable studies of fluid inclusions in metamorphic quartz veins - but where veins are deformed I believe the odds are against being able to extract useful information about the metamorphic fluids.

Bruce Yardley

Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK
Tel. 0113 233 5200 Fax 0113 233 5259
B.Yardley@earth.leeds.ac.uk

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Bruce Yardley is correct in noting that metamorphism wipes out the earlier fluid inclusion record in quartz. We studied quartz veins from the volcanic section of the Josephine ophiolite. With the notion that these originated on or beneath the seafloor, we wanted to find juicy samples of hydrothermal seawater. Instead, we found abundant petrographic evidence for subgrain development, with scant tiny inclusions commonly restricted to the subgrain boundaries. The Josephine experienced regional metamorphism ranging from prehnite-pumpellyite facies in the north to greenschist facies in the south.

Dave

David A. Vanko
Professor and Department Chair
Department of Geology
24 Peachtree Center Blvd.
P.O. Box 4105
Georgia State University
Atlanta, GA 30302-4105
ph. 404-651-2272
fax 404-651-1376
dvanko@gsu.edu

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Bruce Yardley is correct when he says that in deformed quartz with saccaroidal textures the only inclusions that someone can observe are the ones in trails (secondary inclusions). However, there are some cases where the quartz grains are not completely recrystallized, like quartz porphyroclasts with mortar textures. The core of these clasts may have not experiened complete recrystallization, and thus they may contain primary fluid inclusions. There are a number of examples cited in the literature (Garba and Akande, 1992, Mineral Deposita, 27: 249-255; Xavier & Foster, 1999, Chem Geol, 154:133-154; Walshe, 1999, Research Review CSIRO Exploration and Mining, pp. 24-26; Schmidt-Mumm et al., 1997, Min Deposita, 32:107-118; Schmidt-Mumm, 1999, Terra Nostra, 99/6:259-260 for the most recent ones) that discuss these inclusions, especially the deficient water carbonic inclusions in auriferous quartz veins in shear zones.

From: Carlos Eduardo Coelho <ccoelho@pppg.ufba.br>

Organization: UFBA/Centro de Pesquisa em Geofmsica e Geologia

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Dear Ranjan et al.,

Bruce Yardley takes an excellent conservative standpoint, which is that all inclusions in such sheared environments should be considered secondary until proven primary.

Carlos Eduardo Coelho points out that under some circumstances primary inclusions can locally be preserved in sheared rocks, but that it is up to the individual worker to seek these examples out, and document them well.

My favourite example of this is in a paper by Jayanta Guha et al. (1991, Economic Geology, v. 86, p. 667-684), where in Figure 2 they show a wonderful photograph of a euhedral quartz crystal wholly enclosed in a pyrite grain from the Sigma mine, and containing a cluster of undeformed primary fluid inclusions. The hard pyrite has protected the quartz crystal and its inclusions from deformation, while the surrounding material is sheared to destruction.

A temptation when studying systems like this is to measure all the inclusions one can see, and then try to sort out the primaries from the secondaries later. Usually, this just results in a broad cloud of data from which little can be discerned. Clearly a better approach is to be much more discriminating in the first place, and starting with sample selection in the field. To me, the dozen or so numbers reported by Guha et al. for their pyrite-protected inclusions are far more valuable than an indiscriminate data set of 1000+ measurements of unknown provenance.

Good luck,

Jeremy Richards

Jeremy P. Richards, PhD
Associate Professor, Economic Geology
Dept. Earth and Atmospheric Sciences
University of Alberta
Edmonton, Alberta
Canada, T6G 2E3
Tel: (780) 492-3430 (messages: 3265)
Fax: (780) 492-2030
E-mail: Jeremy.Richards@ualberta.ca

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I can only endorse the views of Bruce and others that regional metamorphism of quartz veins GENERALLY destroys the first-formed (vein crystallisation) inclusions, COMMONLY preserves decrepitated or leaked secondary inclusions along recrystallisation grain boundaries, and TYPICALLY carries late-formed (uplift-related) populations of secondaries.

Many of these points are made and discussed in Marshall, Giles and Hagemann in SEG Reviews volume 11 - there is a strong rumour that it will appear before the end of the millennium!!

Brian.

Professor Brian Marshall, Tel: +61-2-9514 1775
Department of Applied Geology, Fax: +61-2-9514 1755
University of Technology - Sydney
P O Box 123 Broadway,
NSW 2007,
AUSTRALIA.
Email: Brian.Marshall@uts.edu.au

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Dear all,

I agree with the point made by Jeremy Richard: a few well constrained microthermometric measurements are better than a thousand unconstrained ones. I would like to add a point for the discussion: if the host crystal is deformed, we will not be able to say anything on the density of the fluid without a very careful textural study. This has been documented by experimental works (Papers by L. Sabouraud, by A.Pecher and myself, by R. Bodnar and his team, by R. Bakker etc...). However, the chemistry of the fluid could remain almost unchanged. We lost the P-T message but the chemical signature remains.

Regards,

Anne-Marie

Anne-Marie Boullier
C.N.R.S. - Laboratoire de G=E9ophysique Interne et Tectonophysique
BP 53X
38041 GRENOBLE CEDEX 9
France
Tel +33 4 76 82 81 16
Fax +33 4 76 82 81 01
Anne-Marie.Boullier@obs.ujf-grenoble.fr
HTTP://www-lgit.obs.ujf-grenoble.fr

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Dear Ranjan, Dear all,

Although I do not have the broad and profound experience in Fluid Inclusion Research of the former respondents, you might still be interested in some of the results, largely unpublished, of the project I have been involved in for the last couple of years.

We worked on a mineralization in Belgium, which turned out to be of hydrothermal syntectonic/synmetamorphic origin, and in order to specify the conditions of formation, we did some FI research. Quartz veins were emplaced during deformation, often along the cleavage planes, and were strained by non-coaxial deformation, although maybe not enough to become nice examples of 'sheared quartz veins'. Anyway, during this further deformation sulphides precipitated in the veins, sometimes indeed encapsulating or cementing quartz fragments, like in the examples mentioned by Carlos Eduardo Coelho and Jeremy Richards. The veins and vein fragments themselves contain abundant and often large inclusions, but I haven't found a single growth zone to prove their primary formation. Instead, they are clustered or randomly distributed.

However, other evidence for an early formation of the inclusions can be found in the recrystallization zones around sulphide grains, recognizable as clear and virtually inclusion free rims of quartz (bordering the sulphides) that 'cross-cut' the inclusion rich areas. The FI's therefore must have formed either primary or early secondary, i.e. prior to the emplacement of the sulphides. This was quite a satisfactory result for us, as this linked the inclusions to the mineralizing tectono-metamorphic event.

Basically, I just wanted to mention this as an example where 'primary' fluid has been preserved in spite of considerable deformation of the veins, and where the presence of sulphides was helpful in one way or the other.

Cheers,

Kris

Kris Piessens
K.U.Leuven
Celestijnenlaan 200C
B-3001 Heverlee
BELGIUM
Tel.: 016/32 72 97
Fax: 016/32 79 81
kris.piessens@geo.kuleuven.ac.be