I think we will find as many opinions on this subject as there are
members of geo-metamorphism.
As Stefan points out, the 'type locality' for skarn is not demonstrably
a magmatic-hydrothermal/metasomatic occurrence but many still use the
term in that context. I prefer to use it for descriptive not genetic
nomenclature, but many would disagree. I also use 'calc-silicate' as a
descriptive name, and distinguish the two loosely on the basis of the
thermodynamic variance of the system. Calc-silicate rocks in my mind
tend to be mineralogically complex rocks, while skarn rocks tend to
have few minerals. Skarn also tends to be coarse-grained relative to
calc-silicate. Skarnoid is commonly used for calc-silicate rocks
within a skarn deposit; the redundancy in this usage is between
calc-silicate and skarnoid. Within skarn deposits I use skarnoid for
rocks devoid of carbonate and calc-silicate for rocks that contain
'primary' carbonate minerals.
Clearly, a detailed description is better than any of these names!
Greg
On Saturday, December 21, 2002, at 04:54 PM, Stefan Nicolescu wrote:
> As Eric pointed out, the term skarn is purely descriptive, with no
> genetic implication(s), just like "calc-silicate rock". It barely
> describes a calc-silicate mineral assemblage w/hedenbergite, grandite,
> wollastonite, vesuvianite, scapolite, etc. (in Mg-skarns with
> forsterite, diopside, etc.) and a whole plethora of hydrated
> alteration products (amphiboles, epidote, micas, other
> phyllosilicates, etc.). It can form from any protolith(s) that can
> provide Ca, Si, Fe, Mg, Al, etc., be that intrusive-carbonate
> contacts, marl, altered basalt, impure carbonates, etc. which went
> through high-temperature metamorphism, irrespective of the heat source
> (magmatic, burial, regional metamorphism, etc.). The confusion made
> very often is between “skarn” and “skarn deposits”. One should be
> aware that many skarns are actually barren. In most cases when
> mineralization is present it is actually post-skarn, mainly related to
> overlapping, lower-temperature (than skarn) hydrothermal events. Thus,
> depending on the scale of the system considered, no large scale
> metasomatism, or mass-transfer is required to get skarn, although the
> classical view was/is that skarn is formed only in contact-
> metamorphic/metasomatic environments. One should bear in mind that the
> region where from the term was introduced into the geological
> literature more than 125 years ago – Bergslagen, Sweden – has skarns
> formed in a regional metamorphic environment, with no traceable
> magmatic source. It is obvious that the question why should one
> use “skarn” instead of “calc-silicate rock” is on everybody’s mind.
> Besides my life-long involvement w/skarns, I personally prefer “skarn”
> for its brevity and because I suspect it has priority. I confess I
> don’t know who coined “calc-silicate rock” or when did this happen,
> but I guess it was after Törnebohm introduced the Swedish mining
> term “skarn” (meaning "refuse" or "trash" in old Swedish, i.e.,
> useless to ore miners) to the geological community in 1875.
>
> To comment on Pietro's message, the term skarnoid is used in regional
> metamorphic environments to describe Fe-poor calc-silicate assemblages
> in relatively fine-grained rocks. However, as skarn is already used
> only in descriptive sense, the use of skarnoid seems redundant.
>
> As pointed out in my first message to Marcello, more details on
> skarns, terminology, geochemistry, genesis, etc. is available in Larry
> Meinert's 1992 paper "Skarns and skarn deposits", Geoscience
> Canada, v. 19(4), p. 145-162. Lots of information is also available on
> Larry's web site: http://www.wsu.edu/~meinert/skarnHP.html. An older
> overview paper on the subject: Einaudi, M.T., Meinert, L.D., and
> Newberry, R.J., 1981, Skarn deposits: Econ. Geol., 75th Anniv. Vol.,
> p. 317-391.
>
> As for the timing of carbonate (calcite or dolomite) formation in
> skarns and its abundance (see Eric's earlier message on the subject),
> it is true that carbonates do often occur as subordinate, late phases,
> in many cases in veins cross-cutting previous assemblages. However,
> there are numerous skarns in which marble is abundant and by no means
> late: euhedral calc-silicates are nested in dominant marble,
> suggesting they formed at the expense of the carbonate, at least syn-
> if not post-carbonate recrystallization.
>
> So much about this. I wish you all a blessed holiday season and a
> great New Year, with even more exciting and challenging discussions on
> the list(s)!
>
> Stefan
>
>
_________________________________________________
Gregory M. Dipple, Associate Professor
Dept. Earth and Ocean Sciences, University of British Columbia
Vancouver, BC CANADA V6T 1Z4
voice: +1 604.822.2624
fax: +1 604.822.6088
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