Mineral Chemistry
1. Columbite-tantalite mineral chemistry from rare-element granitic pegmatites: Separation Lake area, N.W. Ontario, Canada.
A.G. Tindle and F.W. Breaks#
# Ontario Geological Survey, Precambrian Geoscience Centre, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada.
The Separation Lake area is host to the most important
rare-element pegmatites in Ontario, Canada. They include the Big
Whopper and Big Mack petalite pegmatite systems which potentially
represent the world's second largest lithium deposit of this type.
The pegmatites occur in two distinct clusters adjacent to the
Separation Rapids pluton which is thought to be the source of
the rare-elements. Beryl-type and complex-, petalite-subtype pegmatites
are the most common and a few pegmatites have characteristics
similar to the lepidolite-subtype. This study reveals that columbite-tantalite
in the pegmatites has an extremely wide range of composition from
primitive ferrocolumbite to evolved, almost end-member manganotantalite.
Evidence is provided that melt evolution resulted in increased
fluorine activity (as seen in microlite compositions) and that
in situ fractionation of magma within individual pegmatites
often led to the crystallization of rare-element-enriched, Li
mica-fluorapatite-cleavelandite pods. Zonation patterns seen in
backscattered electron images show primary compositions of columbite-tantalite
were modified by secondary processes related to extreme fractionation
and involving the late stage development of albitic units in individual
pegmatites. This alteration led to recrystallization of columbite-tantalite
and produced compositions with lower Ta contents, but with little
change in Mn content.
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2. Oxide mineralogy of the Separation Rapids rare-element pegmatite group, N.W. Ontario, Canada.
A.G. Tindle and F.W. Breaks#
# Ontario Geological Survey, Precambrian Geoscience Centre, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada.
The newly-discovered Separation Rapids Pegmatite Group, situated in mafic metavolcanic host-rocks, which represent the eastern extremity of the Bird River metavolcanic-metasedimentary belt, contains Ontario's first occurrences of wodginite group minerals (mainly wodginite MnSnTa2O8 and ferrowodginite FeSnTa2O8), the pyrochlore group minerals - stibiomicrolite (Sb,Ca,Na)2(Ta,Nb)2O6(O,OH,F), stibiobetafite (Sb,Ca)2(Ti,Nb,Ta)2(O,OH)7, and yttropyrochlore (Y,Na,Ca,U)1-2(Nb,Ta,Ti)2(O,OH)7, ferrotapiolite Fe(Ta,Nb)2O6 and probably the first occurrence in North America of nigerite (Zn,Fe)(Sn,Zn)2(Al,Fe)12O22(OH)2 from a pegmatite.
It is a rare-element class pegmatite group which divides into eastern and southwestern subgroups and hosts both beryl- and petalite-subtype pegmatites. Columbite-tantalite and cassiterite are the predominant oxide species and these are accompanied by variable concentrations of strüverite (tantalian rutile), gahnite, ilmenite, scheelite, uraninite, sphalerite, pyrite, arsenopyrite, löllingite together with a range of wodginite and pyrochlore group minerals (Mn-, Fe-, Ti- and W-rich varieties of wodginite and Ca-, Sb-, Bi-, U and Y-varieties of pyrochlore).
On the basis of columbite-tantalite compositions, the pegmatites have been divided into an Fe-suite and a Mn-suite. Both beryl and petalite pegmatites occur in each suite. The Separation Rapids pluton is also placed in the Fe-suite. Ferrocolumbite from the pluton is the least evolved of all columbite-tantalite examined from Separation Rapids (low Mn and Ta contents), consistent with it being the parent of at least the Fe-suite of pegmatites.
The Fe-suite include beryl pegmatites within and adjacent to the Separation Rapids pluton, where ferrocolumbite co-exists with ferrowodginite, and with increasing evolution into petalite-bearing pegmatites this is replaced by ferrotantalite and wodginite. In individual pegmatites, columbite-tantalite variation is mainly in Ta/Ta+Nb. Later Nb-Ta mineral crystallization has produced minor microlite, antimonian microlite and stibiomicrolite, usually as a replacement of earlier phases. Cassiterite is the final Nb-Ta-bearing oxide to crystallize. Co-existing garnet, columbite-tantalite and wodginite group minerals from the Fe-suite follow systematic trends in Mn/(Mn+Fe) best interpreted as due to magmatic fractionation.
Pegmatites belonging to the Mn-suite follow a similar pattern
of crystallization to the Fe-suite with early manganocolumbite
followed by manganotantalite, the latter co-existing with wodginite.
Manganocolumbite within individual samples varies appreciably
in Mn/(Mn+Fe), whereas the manganotantalite has variation mainly
in Ta/(Ta+Nb). In cleavelandite-Li-mica-rich pods within one of
the beryl pegmatites, extreme Mn-enrichment has produced near-end-member
manganotantalite compositions accompanied by W-bearing wodginite
(average 5.4 wt.% WO3). Microlite
is an important later phase which is either primary or forms as
a replacement product, mainly after wodginite. The presence of
microlite, Li mica and topaz in Mn-suite pegmatites (and aplites)
indicate they were derived from a more F-rich melt than that which
produced the Fe-suite of pegmatites. Albitization is also more
apparent in the Mn-suite of pegmatites and this perhaps explains
the more erratic compositions of co-existing garnet, columbite-tantalite
and wodginite group minerals when compared to Fe-suite samples.
The wall-zone of the largest pegmatite in the eastern subgroup
(Marko's pegmatite), belonging to the Mn-suite, is unique in hosting
titanowodginite, "ferrotitanowodginite", stibiobetafite
and strüverite. These Fe-, Ti- and Sb- phases are thought
to have developed as a result of interaction of the pegmatitic
melt with banded ironstones and Fe-Ti-rich tholeiitic metavolcanic
host-rocks.
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3. Wodginite group minerals from the Separation Rapids Pegmatite Group, N.W. Ontario, Canada.
A.G. Tindle, F.W. Breaks # and P.C. Webb
# Ontario Geological Survey, Precambrian Geoscience Centre, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada.
Wodginite (ideally MnSnTa2O8), and the rarer species, ferrowodginite (ideally FeSnTa2O8) and titanowodginite (ideally MnTiTa2O8), have been discovered in rare-element pegmatites of the complex type, petalite subtype which occur in the Separation Rapids pegmatite field, N.W. Ontario, Canada. Tungsteniferous varieties of wodginite and an unnamed wodginite ("ferrotitanowodginite") are also described from this locality. The pegmatites intrude a metavolcanic (greenstone) belt between the English River and Winnipeg River Subprovinces of the Canadian Shield where they are associated with a 2643±2 Ma rare-element enriched granitic intrusion the Separation Rapids pluton.
At Separation Rapids, the rare metal oxides follow two distinct evolution paths: (i) ferrocolumbite to ferrocolumbite + ferrowodginite to ferrotantalite + ferrowodginite to microlite group minerals (ii) manganocolumbite to manganocolumbite + wodginite to manganotantalite + wodginite to microlite group minerals. Sequence (i) is thought to have arisen from a relatively F-H2O-poor magma; sequence (ii) from a magma richer in F where extreme Mn enrichment was perhaps achieved through F-complexing. Wodginite group minerals are most often found in the albitic regions or in association with fluorapatite-rich regions of the pegmatites, although magmatic fractionation is considered to be the major process controlling concentration of the rare-elements.
Titaniferous wodginite compositions cannot be explained by simple magmatic fractionation (from a Fe- or Mn-rich wodginite starting composition) and localized interaction of the pegmatitic magma with titaniferous mafic metavolcanics and banded, grunerite - magnetite - chert ironstones is proposed for their origin.
Of wodginite group minerals worldwide, only the "giant" Tanco pegmatite at Bernic Lake in Manitoba hosts wodginites with a spread of compositions comparable to those at Separation Rapids. On the basis of a striking similarity in geological setting, mineralogy and emplacement age with the various rare-element pegmatite groups commencing 40 km west in Manitoba (including Tanco), it is concluded that the Separation Rapids Pegmatite Group constitutes the eastern limit of the Cat Lake-Winnipeg River Pegmatite Field.
An Excel spreadsheet template has been prepared to calculate structural formulae of wodginites. The software requires the user to have a copy of Microsoft Excel (if you require a copy of the template either send a blank formatted floppy disc to the address above stating Mac or PC format, or download a copy direct from here).
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4. Probe-Amph - a spreadsheet program to classify microprobe-derived amphibole analyses
A.G. Tindle and P.C. Webb
A Microsoft Excel spreadsheet program has been developed to calculate structural formulae of microprobe-derived amphibole analyses and determine classification parameters according to the International Mineralogical Association (IMA) scheme for amphibole nomenclature. The program has been tested in the determination of formulae and classification of 138 amphibole analyses from the literature. Of these, the small percentage (14%) of discrepancies were found mainly to be due to (i) differences between measured (wet chemical) and estimated values for FeO and Fe2O3 and (ii) differences inherent in summing structural formula of microprobe data to 23 oxygens and wet chemical data to 24 oxygens. The program has been developed for the Apple Macintosh but a PC version is also available.
Spreadsheet implementations of four published geobarometers based on the total aluminium content of certain calcic amphiboles are also presented. These geobarometers are applicable to metaluminous plutonic (and associated volcanic) rocks containing the assemblage amphibole - biotite - quartz - plagioclase - (orthoclase) - sphene - FeTi oxides and apply to a pressure range of 2.5 to 13 kbars.
The Excel spreadsheet template and wall chart are freely available. The software requires the user to have a copy of Microsoft Excel (if you require a copy of the template either send a blank formatted floppy disc to the address above stating Mac or PC format, or download a copy direct from here).
See Reference List for Publication Details
Please note that since publication of this work, the International Mineralogical Association Commission on New Minerals and Mineral Names, Subcommittee on Amphiboles have met and changed the classification. See Leake et al. (1997) Mineralogical Magazine, vol. 61, pages 295-321 for further details. This is quite a drastic change with 27 existing names being recommended for extinction and many (most?) boundaries between similar amphiboles being redefined.
