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Tanco Mine Hydrothermal Secondary Minerals

Last Updated: 30th Nov 2018

By Richard Gunter

Tanco Mine Hydrothermal Secondary Minerals; Tanco Mine, Bernic Lake, Manitoba, Canada

Richard Gunter M.Sc., P.Geo.
5493 Cedarcreek Drive, Chilliwack, British Columbia
pamrichg@shaw.ca


The Tanco Mine was a major commercial producer of tantalum concentrate, ceramic-grade spodumene and cesium formate for oil-well drilling fluid. The ore body is a flat-lying, zoned, highly Ta fractionated pegmatite. The zonation in the pegmatite is grouped into 9 zones (Zone 10 to Zone 90) of which 4 zones contained economic mineralization. Phosphate was a deleterious element in the spodumene concentrate so was actively removed and dumped.

Subsequent to the consolidation of the pegmatite, residual hydrothermal fluids promoted the conversion of the majority of the lithium alumino-silicates from petalite to a spodumene-quartz intergrowth, called SQUI at the mine. The volume decrease of this reaction caused the development of porosity within the SQUI pseudomorphs and the pores were lined with secondary hydrothermal mineral phases. These are lower temperature phases than the primary pegmatite minerals. They often form euhedral crystals with quartz on the cavity linings. The secondary minerals are not of economic interest and so were not actively exploited in the underground workings.

The Tanco Mine had extensive dumps where contaminated material was discarded. These dumps were open to the public during the 1970’s and 1980’s so samples from the mine were collectable. Tours could be arranged for science groups and local mineral clubs so the underground exposures were accessible.

The author was the Industrial Minerals Geologist for the Province of Manitoba from 1984 to 1996 and a member of the Mineral Society of Manitoba from 1985 to 2001 so had access to the dump and the Tanco Mine on many occasions.


Table of Contents:


  • Introduction
  • Location and Geology
  • Minerals
  • Sulphides/Sulphosalts
  • Oxides
  • Carbonates
  • Phosphates
  • Silicates
  • Localities and Parageneses
  • Cavity Formation
  • Conclusions
  • References


List of Figures:


  • Figure 1: Geographic Location of Tanco
  • Figure 2: In-situ Albite-Apatite Cavity
  • Figure 3: Zonation of Tanco Pegmatite
  • Figure 4: XRD Trace of Fluorapatite: Zone 20
  • Figure 5: Acicular Groatite and Platy Whitlockite
  • Figure 6: Cesium-rich Analcime on altered Spodumene
  • Figure 7: Zone 20 and Zone 30 Boundary in Stope Wall
  • Figure 8: Tantalum Ore from the Beryl Stop
  • Figure 9: Alteration of Pollucite


Tables:

  • Table 1: Paragenetic Sequence for Hydrothermal Secondary Minerals



Tanco Mine



Mine site taken from entry road during winter (David Joyce photo).

The head frame for the ventilation shaft is the tall building on the right side; the processing plant for spodumene concentrate and tantalum concentrate is the lower building on the right. The tailings and tailings conveyor are to the left of the processing plant and the mine offices are in the center of the photograph. The mine dump mentioned in the localities section is in front of the processing plant. Bernic Lake is behind the mine buildings.


Introduction:


The Tanco Mine is one of the largest Li-Ta pegmatites in the world and has been developed by an underground mine exploiting tantalum, spodumene and cesium ores from the 1960’s until the present. The deposit has been the subject of extensive investigation as to its origin and the detailed mineralogy of the tourmalines and Ta oxide minerals. The Tanco Mine pegmatite has been subjected to a post-consolidation fluid reaction that converted part of the pegmatite to aggregates of cookeite-purple apatite. Vugs within these aggregates contain rare phosphate minerals for which the Tanco Mine is the type locality.

The secondary vugs often contain or are lined with colourless to light smoky quartz crystals and crystal groups; some of considerable size (Image 1).

Image 1: A floater group of quartz from one of the vugs.


The fluids that produce the secondary suite may be related to the reaction within the Tanco Mine that converts primary petalite (Image 2) to an aggregate of spodumene plus quartz (Image 3) which is an ore used as a base material in ceramics called SQUI.

Image 2: Primary petalite
Image 3: Aggregate of spodumene plus quartz
Image 2: Primary petalite
Image 3: Aggregate of spodumene plus quartz
Image 2: Primary petalite
Image 3: Aggregate of spodumene plus quartz


The minerals for which the Tanco Mine is the type locality are: wodginite (1963); cernyite (1978); tancoite (1980); titanowodginite (1992); ercitite (2000); groatite (2009) and wopmayite (2012).

Image 4: Cernyite, (Brent Thorne photo)
Image 5: Tancoite
Image 6: Ercitite (probable small brown spheres)
Image 4: Cernyite, (Brent Thorne photo)
Image 5: Tancoite
Image 6: Ercitite (probable small brown spheres)
Image 4: Cernyite, (Brent Thorne photo)
Image 5: Tancoite
Image 6: Ercitite (probable small brown spheres)
Image 7: Groatite (acicular, colorless phase)
Image 8: Wopmayite
Image 7: Groatite (acicular, colorless phase)
Image 8: Wopmayite
Image 7: Groatite (acicular, colorless phase)
Image 8: Wopmayite


The two other type species are Ta oxide minerals wodginite and titantowodginite.

Image 9: Wodginite
Image 10: Titanowodginite
Image 9: Wodginite
Image 10: Titanowodginite
Image 9: Wodginite
Image 10: Titanowodginite


Unlike miarolitic gem-bearing pegmatites in California and elsewhere, London (1986), the secondary mineral cavities at Tanco do not contain gemmy tourmaline or beryl. Tourmaline (Figure 11) and rossmanite (Figure 12) are not uncommon at the Tanco Mine but they always occur as embedded white crystals, generally in quartz/albite. Beryl is also embedded white platy crystals except for rare yellow prismatic microcrystals in Zone 20 (Figure 13). The beryl bearing cavities also contain microcrystals of tourmaline (Fe-rich elbaite; Cerny, Ercit and Vanstone 1998) as elongated, olive-green coloured prisms too small to photograph. Since Zone 20 is the first zone to crystallize at Tanco the cavity-forming pegmatite fluids in Zone 50 were not within the tourmaline or beryl Eh/pH fields after crystallization and so crystallized these minerals directly from the melt.

Image 11: Elbaite
Image 12: Rossmanite
Image 11: Elbaite
Image 12: Rossmanite
Image 11: Elbaite
Image 12: Rossmanite
Image 13: Beryl



Location and Geology:


The geological description of the Tanco Mine has been outlined in several publications. Two guidebooks provide the best overview; Cerny, Ercit and Vanstone (1998) and Martins, Kremer and Vanstone (2013). Each gives cross sections of the deposit and descriptions of the mineralogical units found within the various zones, named zones 10 to 90, found in the pegmatite. The secondary mineral accumulations that are the subject of this article are not considered a separate zone as they have no economic potential and are small in volume.

Figure 1: Geographic Location of Tanco (from Martins, Kremer and Vanstone (2013)).


David Joyce has an overview of the mineralogy of the Tanco Mine on his website with a brief description of the collectable minerals. He has provided a photo of one of the in-situ occurrences of the secondary phases (Figure 15). There are numerous cavities of this size and shape in the spodumene zone.

Figure 2: An in-situ pocket of albite-apatite with the red-purple apatite on the left and lower left of the photograph. (David Joyce photo; David’s hand is scale.)


A series of papers on various aspects of the mineralogy of the Tanco pegmatite have been published in the Canadian Mineralogist. The most relevance publications to this article are; Cerny (1972) on secondary minerals from the spodumene-rich zones and Cerny and Harris (1978) on the occurrence of elements, sulphides and sulphosalts at the Tanco Mine. An article in the American Mineralogist by London (1986) on the magmatic-hydrothermal transition at Tanco is also relevant.

The pockets of secondary phases occur primarily in the upper, central and lower intermediate zones (zones 40, 50 and 60). They may be related to the conversion of petalite to spodumene. The diagram for the zonation of the pegmatite is in Figure 3 below.

Figure 3: Zonation within the Tanco pegmatite: Modern zonation adds a 0 to this section; example Zone 2 becomes Zone 20 (Stilling (1998). Stilling does not include Zone 10, the “Border Zone”, which other authors do.



Minerals:



Sulphides:


Sphalerite (ZnS):
Sphalerite is the most common secondary sulphide according to Cerny and Harris (1978). It occurs as brown or yellow grains associated with microcline var adularia (Image 14). The brown sphalerite has an average of 12.1 wt % Cd (3 samples) and the yellow sphalerite has an average of 1.7 wt % Cd (5 samples). There is a large euhedral yellow sphalerite on cookeite displayed in the University of Manitoba mineral collection.

Image 14: Sphalerite.


Other Sulphosalts: Cosalite, Cernyite, Pekoite-Gladite and Sulphides: Bismuthinite and Hawleyite:
The other sulphides and sulphosalts are generally so fine grained that a reflected light microscope is needed to distinguish the phases. Most phases resemble one another so XRD is often required for identification. The sulphides and sulphosalts are not all secondary minerals but two parageneses (4,5C) in zones 40 and 50 and (6,3C) in zones 60 and 30 are cavity and fissure filling. Minor amounts of fine-grained pyrite and calcite coat the quartz crystals in the quartz lined “miarolitic” cavities. (Cerny and Harris (1978)). Their list of cavity and fissure filling aggregates is:

(4,5C) bismuthian antimony, antimonian bismuth, bismuthian, stibnite, arsenic, galena, sphalerite, chalcopyrite, tetrahedrite, dyscrasite

(6,3C) bismuth, bismuthian antimony, pyrrhotite, sphalerite, hawleyite, pyrite, arsenopyrite, cubanite, chalcopyrite, galena, stannite, kesterite, cernyite, cosalite, gustavite, gladite-pekoite, tetrahedrite, freibergite, bournonite,
pyrargyrite

miarolitic cavities (6,3D) pyrite, marcasite


They have no definition of miarolitic cavity in the article and cavities containing hydrothermal phases at Tanco generally also contain euhedral quartz.

Rare and difficult to distinguish sulphosalt from a suite of rare sulphosalts that occur as very minor minerals in the mass of secondary alteration phases. XRD would be required to determine this phase due to both the small grain size and the similar appearing associated minerals. The most common and easily distinguished phases in this paragenesis is colourless to light yellow sphalerite (Image 15).

Some of the phases that form rare macroscopic aggregates and crystal clusters are cosalite, which occasionally occurs as medium sized striated prisms in the secondary phases. As occurs with several other sulphosalts at Tanco the cosalite has widely varying chemistry. Two analyses of cosalite from a secondary paragenesis have (Pb1.409Ag.062Cu.152)1.623(Bi1.979Sb.2147)2.193S5.00 and (Pb1.689Ag.236Cu.019)1.944(Bi1.887Sb.170)2.057S5.00. These are normalized to 5 S (Cerny and Harris 1978).

Image 16: Cosalite.


A similar appearing bladed metallic phase, slightly more silvery in appearance, is a eutectic intergrowth of gladite and pekoite (Image 17). Cerny and Harris (1978) have completed chemistry on these intergrowths and a host Pekoite is Cu.255Pb.142(Bi10.177Sb.822)S16.800 while to co-existing blebs of gladite are Cu.970Pb1.000(Bi4.363Sb.318)S8.363

Image 17: Eutectic intergrowth of gladite and pekoite.


Bismuthinite occurs as bladed crystals in quartz and is similar in appearance to Gladite-Pekoite (Image 18). Cernyite has a similar appearance as well (Image 19).

Image 18: Bismuthinite (Maggie Wilson photo)
Image 19: Cernyite (Brent Thorne photo)
Image 18: Bismuthinite (Maggie Wilson photo)
Image 19: Cernyite (Brent Thorne photo)
Image 18: Bismuthinite (Maggie Wilson photo)
Image 19: Cernyite (Brent Thorne photo)


Hawleyite is associated with Cd-rich Sphalerite among the native elements, alloys, sulphides and sulphosalts that occur as minor phases in the secondary minerals ((Cerny and Harris (1978) and Cerny, Ercit and Vanstone (1998)).

Oxides:


Cassiterite (SnO2; Ta-bearing):
The oxide phase in the cesium analcime alteration zone (Image 20) has been examined by EDS and determined to be Ta contaminated cassiterite with no wodginite or tantalite. The outline of the embedded oxide crystal is consistent with a wodginite crystal. It is possible this is a pseudomorph that has been converted from wodginite to cassiterite during the creation of the cesium analcime alteration. This reaction has not been noted at Tanco before so the circumstances have not been investigated. Cassiterite has been noted as a common mineral in the oxide component of Zone 20 and Zone 90 but it is rare in the other zones.

Image 20: Cassiterite after wodginite.


Simpsonite (Al4Ta3O13(OH)):
Small grains of colourless, bright white fluorescing, simpsonite have been noted in some of the lithiophosphate alteration zones. Simpsonite is a late-stage phase at Tanco, often associated with other rare phases such as hydroxykenomicrolite (Image 21) and rankamaite-sosedkoite (Cerny, Ercit and Vanstone (1998)). It may be more widespread in small amounts within the secondary mineral aggregates as the white fluorescence is often the only method of identifying the colourless grains.

Image 21: Simpsonite associated with hydoxykenomicrolite.


Carbonates:


Calcite (CaCO3):
Calcite is a minor, colourless, late-stage phase associated with quartz crystals. It has minimal Mn and no Fe in the crystal chemistry (see rhodochrosite for details). A colourless crystalline calcite encrustation on quartz crystals can be seen on the top left of Image 22.

Image 22: Calcite (top left).


Rhodochrosite (MnCO3):
Rhodochrosite is a fairly common carbonate in Zone 50, where it can occur intergrown with altering lithiophilite. More often is occurs as an early massive pink phase in alteration zones with lithiophosphate (Image 23) or cesium analcime (Image 24). In open vugs the rhodochrosite occurs as botryoidal masses, often with a brown stain on the surface. The bright pink, fine-grained rhodochrosite masses with cosalite and cookeite (Image 25) are typical of the colourful alteration sequence in Zone 50 when many phases are present.

Image 23: Rhodochrosite with lithiophosphate
Image 24: Rhodochrosite with cesium analcime
Image 25: Rhodochrosite with cosalite.
Image 23: Rhodochrosite with lithiophosphate
Image 24: Rhodochrosite with cesium analcime
Image 25: Rhodochrosite with cosalite.
Image 23: Rhodochrosite with lithiophosphate
Image 24: Rhodochrosite with cesium analcime
Image 25: Rhodochrosite with cosalite.


Chemical analysis of four rhodochrosite samples in Cerny, Ercit and Vanstone (1998) has 1.85 apfu Mn, 0.14 apfu Fe and 0.02 apfu Ca. With calcite having an average of 1.86 apfu Ca and 0.13 Mn there is little overlap within the carbonates.

Zabuyelite (Li2CO3):
Zabuyelite would be considered a hydrothermal mineral but it only survives as crystals within fluid inclusions in late-stage spodumene. It is possible that the bladed mineral surrounded by hydroxylapatite on the wopmayite sample (center of Image 26) was Zabuyelite before it was dissolved. The cavity appears to be monoclinic and the wopmayite sample was found underground and was not exposed to weathering.

Image 26: Possible dissolved zabuyelite.


Phosphates:


Apatite (fluor- and hydroxyl-; Ca5(PO4)3(OH):
There are two distinct parageneses of apatite at the Tanco Mine. The massive pegmatite has an accessory phase that is a deep blue fluorapatite with F of 1.739 apfu. Its occurrence will not be covered here.

The secondary parageneses from Zone 50 contain brown crystalline hydroxylapatite (apatite of Table 1) with F of 0.048-0.144 and an OH of 1.056-1.218 (Cerny, Ercit and Vanstone, 1998). This is the apatite that coats quartz crystals in the secondary cavities. Image 27 and Image 28 illustrate the early deposition of the brown prisms of hydroxylapatite and the association with other phases. This is a paragenesis found later than 1972 as it not represented in Table 1. There are aggregates of hydroxylapatite on the quartz crystals that are epimorphs after a now vanished prismatic phase.

Image 27: Fairfieldite, apatite, cookeite quartz.
Image 28: Apatite, quartz, wopmayite.
Image 27: Fairfieldite, apatite, cookeite quartz.
Image 28: Apatite, quartz, wopmayite.
Image 27: Fairfieldite, apatite, cookeite quartz.
Image 28: Apatite, quartz, wopmayite.


Zone 20 samples have a third type of apatite that occurs as Global radial aggregates in zone 1 belong to a podolite or franconite-type carbonate apatite (Cerny 1972). This differs from the other hydroxylapatite because it is red (Image 29). The photographed sample has been determined as hydroxylapatite by XRD at Manitoba Energy and Mines. It appears to be a pseudomorph after an unknown pre-existing phase.

Image 29: Apatite.


Colourless prismatic fluorapatite also occurs in Zone 20 (Image 30).

Image 30: Apatite.


The corresponding XRD trace is shown in Figure 4. It is different in appearance to the hydroxylapatite and may be a different generation.

Figure 4: XRD trace Fluorapatite Zone 20.


Pink radial hydroxylapatite is the most common of the late stage apatite group minerals. It has not been found coating quartz crystals nor as epimorphs. It is noted as paragenesis 3 in Table 1. A typical vuggy hydroxylapatite-cookeite alteration zone is pictured in Image 31. These are not uncommon in the spodumene-rich portions of Zone 50. Their bright colour makes these zones easy to see on a stope wall (Image 32). The in-situ photograph (Figure 2) is one of these alteration zones with a larger than normal central cavity.

Image 31: Hydroxylapatite-cookeite alteration zone.
Image 32: Cookeite, lithiophosphate, rhodochrosite, dorfmanite, hydroxylapatite.
Image 31: Hydroxylapatite-cookeite alteration zone.
Image 32: Cookeite, lithiophosphate, rhodochrosite, dorfmanite, hydroxylapatite.
Image 31: Hydroxylapatite-cookeite alteration zone.
Image 32: Cookeite, lithiophosphate, rhodochrosite, dorfmanite, hydroxylapatite.


Pink hydroxylapatite occurs with cesium analcime as a rim around a core of massive lithiophosphate (Image 33). This particular lithiophosphate-cesium analcime-hydroxylapatite paragenesis was collected by the author underground in a 1 meter wide vertical alteration zone trending at right angles to the front of the production bench. It was unusual enough for Dr. Peter Cerny to visit the mine and recover additional samples from this location. The stope and floor number were retained because the largest sample collected at the time was donated to the Museum of Man and Nature in Winnipeg.

Image 33: Lithiophosphate, apatite, dorfmanite.


Minor occurrences of pink to orange hydroxylapatite occur as; 5 mm rims around altered montebrasite (Image 34) and as linings of pseudomorph cavities after an unknown phase in Zone 10 (Image 35).

Image 34: Montebrasite, apatite.
Image 35: Apatite.
Image 34: Montebrasite, apatite.
Image 35: Apatite.
Image 34: Montebrasite, apatite.
Image 35: Apatite.


Pink hydroxylapatite was not analyzed in Cerny, Ercit and Vanstone (1998) but its physical properties have been matched to the brown hydroxylapatite in Cerny (1972).

Dorfmanite (Na2HPO4.2H2O):
A colourless, powdery phase associated with lithiophosphate. It was noted at the Tanco Mine before it was characterized as a mineral but was described first in Russia (Kapustin et al. 1980: referenced in Cerny, Ercit and Vanstone (1998)). It has been identified by its bright green fluorescence in SW UV and by XRD on Image 36, where it occurs as part of the alteration rind around the lithiophosphate aggregate. Small amounts of colourless, green SW fluorescing, dorfmanite occurs in other cookeite-hydroxylapatite-lithiophosphate alteration zones, such as the one containing cosalite. It is noted by the green SW fluorescence, otherwise it is difficult to detect.

Image 36: Dorfmanite, etc.


Ercitite (NaMnPO4(OH).2H2O):
Ercitite is a rare type locality mineral from Tanco (Fransolet et al. 2000). Very small, >> 1 mm, dark brown botryoidal hemispheres as the latest phase on the hydroxylapatite and fairfieldite sample match the physical properties of ercitite but have not been XRD confirmed.

Fairfieldite (Ca2(Mn,Fe)(PO4)2.2H2O):
Fairfieldite is a not-uncommon platy mineral in late-stage cavities. The white, platy phase with hydroxylapatite (Image 37) bears a striking resemblance to fairfieldite on brown apatite from the Foote Mine, North Carolina.

Image 37: Fairfieldite, etc.


Groatite (NaCaMn2(PO4)(HPO4)2):
Groatite is a type locality mineral from Tanco (Cooper et al. 2009). The characterization article had a photo (Figure 5) of acicular groatite on rhombohedral to platy tan whitlockite identical to Image 38. A light yellow acicular phase associated with cookeite and quartz also matches groatite but has not been XRD identified.

Figure 5: Acicular Groatite and platy Whitlockite from Cooper et al. (2009)
Image 38: Groatite, whitlockite, etc.


Lithiophosphate (Li3PO4):
A fairly common but not always easily identified phase in the secondary paragenesis. It can occur as colourless prismatic crystals associated with rhodochrosite (Image 39) or as colourless altered aggregates (Image 40).

A rare find was a cavity with euhedral lithiophosphate crystals as the latest phase on a matrix of cesium analcime and rhodochrosite (Image 41). Lithiophosphate is slightly soluble in rainwater and so any samples found on the dumps will have been altered by exposure to the ambient conditions. The euhedral crystals were collected underground so they have not been exposed to groundwater.

Image 39: Prismatic lithiophosphate crystals.
Image 40: Altered aggregate of lithiophosphate.
Image 41: Euhedral lithiophosphate crystals.
Image 39: Prismatic lithiophosphate crystals.
Image 40: Altered aggregate of lithiophosphate.
Image 41: Euhedral lithiophosphate crystals.
Image 39: Prismatic lithiophosphate crystals.
Image 40: Altered aggregate of lithiophosphate.
Image 41: Euhedral lithiophosphate crystals.


Lithiophosphate was considered to be a rare mineral at Tanco for many years but further research (Cerny, Ercit and Vanstone 1998) has shown it to be a common, fine grained, colourless phase in the SQUI aggregates. In these it pseudomorphs primary ambygonite/montebrasite (Image 42 illustrates one of the ambygonite/montebrasite nodules altering to pink hydroxylapatite), alters to lithiophosphate. If the nodule is lithiophilite instead of amblygonite/montebrasite then the Mn from the lithiophilite will be retained and the resulting aggregate will be lithiophosphate- rhodochrosite.

Image 42: Montebrasite, apatite.


Overite (CaMgAl((PO4)2(OH).4H2O):
Tan coloured, 1 mm spheres on whitlockite (Image 43) have been identified as overite in the groatite characterization article, mentioned under groatite.

Image 43: Overite on whitlockite.


Tancoite (HNa2LiAl(PO4)2(OH):
Tancoite is a type locality mineral from Tanco (Ramik et al. 1980). It occurs as small pink crystals associated with lithiophosphate masses (Image 44). The original tancoite was described from dump material where the lithiophosphate had been etched. This sample was collected underground and the tancoite occurs as vitreous 1 mm pink orthorhombic crystals lining fractures in the massive lithiophosphate. The Tancoite crystals have approximately the same colour as the associated radial pink hydroxylapatite but their orthorhombic shape is characteristic.

Image 44: Tancoite, etc.


Whitlockite (Ca9Mg(PO3OH)(PO4)6):
A rhombohedral to platy mineral occurring in secondary cavities associated with acicular groatite and spherical aggregates of overite (Image 45). This paragenesis has been described in the characterization article for groatite (Cooper et al. 2009).

Image 45: Whitlockite, etc.


Wopmayite (Ca6Na3vMn2(PO4)(HPO4)2):
Wopmayite is a type locality mineral from Tanco (Cooper et al. 2013). It occurs as a late-stage, colourless, rhombohedral phase on brown hydroxylapatite (Image 46). On this sample the rhombohedrons are slightly etched. Portions of the hydroxylapatite form epimorphs after a prismatic mineral than might be zabuyelite.

Image 46: Wopmayite on hydroxylapatite.


Silicates:


Cesium-rich Analcime (NaAlSi2O6.H2O; cesium replaces sodium):
Analcime, normally a cesium-rich partial series to pollucite, is the most common of the secondary minerals. It generally occurs on altered spodumene (Figure 6) or rhodochrosite (Image 47). In some cases the cesium-rich analcime occurs in cavities in a box-work-style alteration of spodumene as in the matrix of Image 48. The chemistry of the analcime is outlined by Cerny (1972). His analyses of four samples varied from 6.00 % to 18.00 % Cs2O with an average of 12.35%. He indicated that the analyzed cesium analcime samples were strongly zoned.

Figure 6: Cesium-rich analcime on altered spodumene (David Joyce photo).
Image 47: Analcime on rhodochrosite.
Image 48: Analcime,
etc.
Image 47: Analcime on rhodochrosite.
Image 48: Analcime,
etc.
Image 47: Analcime on rhodochrosite.
Image 48: Analcime,
etc.


Cesium analcime occurs in all but the simplest of the parageneses described by Cerny for the hydrothermal secondary minerals (Table 1; Zone 5 in 1972 is now zone 50). Over the life of the Tanco Mine parageneses 4 and 5 (Table 1) are the most commonly encountered and paragenesis 1 is very rare and localized.

The crystals of cesium analcime are consistent in shape and often in size, over individual samples. Cerny (1972) says: This mineral forms typical {211} trapezohedral crystals with subordinate to missing {100} facets, mostly waterclear, occasionally milky. There is some indication that the crystals may cloud over time after they have been collected.

Cookeite (LiAl4(Si3Al)O10(OH)8):
A light green micaceous mineral that does not have the stiffness of lithian muscovite and occurs lining cavities which muscovite does not. It is a widespread component of the alteration zones in Zone 50. The green micaceous mass on the lower left of the cavity in Figure 2 is Cookeite.

In most cases cookeite occurs with many other phases, for example in Image 49, where cookeite occurs with pink hydroxylapatite and other phases. In some instances, noted as paragenesis 5 Table 1and illustrated as Image 50, it occurs as 5 mm euhedral light green platy crystals associated with earlier-stage quartz and possibly with groatite. I have not seen a chemical analysis of the Tanco cookeite and Cerny 1972 said the cookeite in his study was too inhomogeneous to analyze. The individual hexagonal plates in some cavities (Image 51) should be suitable for analyses.

Image 49: Cookeite with hydroxylapatite
Image 50: Euhedral cookeite crystals.
Image 51: Cookeite, etc.
Image 49: Cookeite with hydroxylapatite
Image 50: Euhedral cookeite crystals.
Image 51: Cookeite, etc.
Image 49: Cookeite with hydroxylapatite
Image 50: Euhedral cookeite crystals.
Image 51: Cookeite, etc.


Microcline var. adularia (KAlSi3O8):
A well-studied late hydrothermal microcline with colourless crystals of adularia morphology is a typical phase in the secondary cavities (Cerny 1972, Cerny and Chapman 1984, Ferguson et al. 1991, Teertstra 1997, Teertstra et al. 1998, Cerny, Ercit and Vanstone 1998). The chemistry of the adularia is 14.09 wt % K2O and 0.00 wt % Na2O (Cerny, Ercit and Vanstone 1998) and it is extremely close to a structural end-member high-sanadine.

Quartz (SiO2):
Quartz is a late-stage phase in the Tanco Mine where it is common enough to form its own zone (Zone 70). It often occurs as massive, colourless to slightly smoky but never rose coloured, surrounding euhedral crystals of silicates such as spodumene (Image 52) and oxides such as tantalite-(Mn) (Image 53). Any sample that looks like rose quartz from the Tanco Mine is eucryptite. Locally in Zone 50 the massive quartz has been stressed to produce a pronounced parting in massive, blocky quartz parallel to {1011} (Cerny, Ercit and Vanstone (1998)).

Image 52: Quartz with spodumene.
Image 53: Quartz with tantalite-(Mn).
Image 52: Quartz with spodumene.
Image 53: Quartz with tantalite-(Mn).
Image 52: Quartz with spodumene.
Image 53: Quartz with tantalite-(Mn).


Throughout the quartz zone and other quartz accumulations in Zone 50 and Zone 60 are vugs lined primarily with quartz crystals, often to large sizes (38 cm across in Image 54). In some cases a quartz crystal grows unattached in the vug and is overgrown by later quartz, using the original quartz as a template. These can be large: Image 55 is a 22 cm floater twin. Its cross-section (Image 56) illustrates the original twin at the base with the light smoky overgrowth at the top.

Image 54: 38 cm quartz vug (Cindy Hasler photo).
Image 55: 22 cm floater quartz twin.
Image 56: Cross-section of floater quartz twin.
Image 54: 38 cm quartz vug (Cindy Hasler photo).
Image 55: 22 cm floater quartz twin.
Image 56: Cross-section of floater quartz twin.
Image 54: 38 cm quartz vug (Cindy Hasler photo).
Image 55: 22 cm floater quartz twin.
Image 56: Cross-section of floater quartz twin.


Only calcite and Tl-enrich pyrite are later than the quartz so there are no gem crystals encrusting quartz crystals at the Tanco Mine.


Collecting Localities and Parageneses:



Since the article by Cerny and Harris (1978) there have been several new phases described from the secondary phases. New parageneses have been found and formerly rare minerals such as lithiophosphate have become noted more often (Cerny, Ercit and Vanstone (1998)). Tanco has been one of the most actively researched Ta-Li-Cs pegmatites. Dr. P. Cerny and Dr. D. London and their associates are the most active investigators. Since the Tanco Mine has been active for many years producing tantalum concentrate, spodumene ceramic raw materials and cesium formate the unweathered mining faces have allowed many examples of pegmatite minerals to be recovered and studied.

The mine used a room and pillar system for underground mapping. Some samples can be located precisely if the room and pillar are known. For many years the geology personnel from Tanco conducted tours of the underground workings. Tour stops in areas of interest (Figures 7 and 8) were the source of many underground samples. More extensive tours took place during the Manitoba Mineral Society Shows in 1991 and 1993. On these tours stope maps would be provided so any samples could be located to the stope and level.

Material, including type samples, has also been recovered from dump material placed outside the gate by the Tanco staff. This dump was accessible to the public and many samples from Tanco were recovered from the dump.

The separate dump for the Tanco Lower Pegmatite was adjacent to the main Tanco dump. The trial mine in the Lower Pegmatite was small and the stopes are flooded so the Lower Mine dump is the only accessible source of samples.

Another location in the immediate vicinity of the mine is the Causeway Occurrence. A few kilometers north of the Tanco Mine the mine road was built across a swamp using crushed rock from Zone 20. The vuggy Zone 20 albite from the edges of the mine road contains: euhedral microcrystals of yellow, prismatic, beryl; colourless prismatic fluorapatite and euhedral microcrystals of Fe-rich elbaite tourmaline. These minerals are not found underground within the Tanco Mine, which did not actively exploit zone 20. The only area where zone 20 is exposed underground is pictured in Figure 7.

The complete paragenesis of Tanco has never been investigated in depth so not all of the phases have been identified. Cerny, Ercit and Vanstone (1998) list phases such as fluorite and metaswitzerite that are known anecdotally but have not been analyzed and crandallite and lacroixite that are very rare.

Table 1: Paragenetic sequence of the hydrothermal secondary minerals in Zone 50 (Cerny (1972))

This paragenetic sequence from 1972 does not reflect the more recent discoveries of late-stage phases within the hydrothermal assemblages. It illustrates that the mineralogy of these cavities is relatively restricted and the different parageneses differ mainly in the absence of late-stage phosphates and late-stage beryl. In most cases the low temperature silicates precipitate first and the carbonates and phosphates precipitate afterward, often simultaneously. Calcite is a minor phase in the paragenesis but it is volumetrically small. There are no calcite filled cavities at Tanco.


Cavity Formation:



The majority of secondary minerals are associated with spodumene-quartz intergrowths (SQUI) pseudomorphed after petalite (Image 57). This is Type A spodumene of Cerny and Ferguson (1972). The crystallization of petalite occurred at or below 700°C and the transformation from petalite to SQUI takes place at 500°C and 3000 bars pressure Small vugs are present in many SQUI pseudomorphs after petalite such as Image 58. These small vugs normally contain only late-stage spodumene blades and small, colourless, gemmy cesium analcime crystals. Cerny and Ferguson (1972) indicate that: The spd + qtz aggregates show a volume reduction in relation to petalite. The resulting porosity can be either filed by later quartz or lined with secondary hydrothermal phases. Larger vugs within or between the SQUI pseudomorphs are much rarer. Based on fluid inclusion homogenization the secondary minerals form between 255°C and 350°C and 1600 bars pressure (London (1986).

Image 57: Spodumene-quartz intergrowth (SQUI) are pseudomorphed after this primary petalite.
Image 58: Vug in SQUI pseudomorph afer petalite.
Image 57: Spodumene-quartz intergrowth (SQUI) are pseudomorphed after this primary petalite.
Image 58: Vug in SQUI pseudomorph afer petalite.
Image 57: Spodumene-quartz intergrowth (SQUI) are pseudomorphed after this primary petalite.
Image 58: Vug in SQUI pseudomorph afer petalite.


The small, 2 to 5 cm, cavities that form within the massive quartz of Zone 70 (Quartz Zone) are often prism faces of incomplete large quartz crystals with green coloured montmorillonite coatings. The can be seen on the back of the stopes in Zone 70. Larger quartz-lined cavities, such as the one that produced the floated crystal described under “Quartz”, are rare.

A different generation of spodumene: as large blades (Image 59); this is Type B spodumene of Cerny and Ferguson (1972) and euhedral crystals in quartz, (Image 60); this is Type C spodumene of Cerny and Ferguson (1972) do not contain vugs nor are they associated with the secondary minerals. The primary spodumene is a lower temperature sequence than the petalite that forms the basis for SQUI pseudomorphs. Fluid inclusions in the primary spodumene do not match those from SQUI. At lower temperature than the formation of SQUI is the formation of eucryptite (Image 61). This reaction also produces no secondary mineral lined cavities though there are rare seams in the massive eucryptite that contain rough euhedral eucryptite crystals (Image 62).

Image 59: Large blades of spodumene.
Image 60: Euhedral spodumene crystals in quartz.
Image 59: Large blades of spodumene.
Image 60: Euhedral spodumene crystals in quartz.
Image 59: Large blades of spodumene.
Image 60: Euhedral spodumene crystals in quartz.
Image 61: Eucryptite.
Image 62: Euhedral eucryptite crystals.
Image 61: Eucryptite.
Image 62: Euhedral eucryptite crystals.
Image 61: Eucryptite.
Image 62: Euhedral eucryptite crystals.



A sequence of Sn-Ta rich aplitic albite, Zone 30 (Figure 7) and in hand sample (Image 63) is considered to be a primary phase related to the crystallization of Li-tourmaline by London (1986). London says: tourmaline-albite-quartz may produce pegmatite pockets by the evolution of aqueous fluid, which expands against largely solidified pegmatite and may ultimately lead to pocket rupture. At Tanco this sequence does not contain cavities or secondary mineral phases. It often occurs adjacent to quartz-rich pegmatite and may have grown within a fluid medium.

Figure 7: Zone 20 (reddish, granular, lower right side) and Zone 30 (white layered) boundary on stope wall (Martins, Kremer and Vanstone (2013)).
Image 63: Wodginite, albite are typical of Zone 30.


Within the main portion of the tantalum ore at Tanco the microcline has been largely altered to a green coloured lithian muscovite and albite. Figure 8 illustrates a normal tantalum ore intersection with white beryl. The beryl is the hexagonal phase at the top of the photograph. The black crystals are the Ta minerals and the green flaky mineral is lithian muscovite. The white phase at the bottom and left side is microcline.

Figure 8: Tantalum ore from the Beryl Stop area (Martins, Kremer and Vanstone (2013)).


There are no cavities in this ore and all of the phases are embedded in quartz or microcline. A hand sample of tantalum ore from approximately the same location is shown in Image 64. It also contains Ta minerals embedded in microcline with major lithian muscovite.

Image 64: Tantalum ore with embedded envelope shaped Wodginite crystals.


The other zones in which there are only massive secondary phases are Zone 80 and Zone 90. Neither zone contains primary petalite or spodumene; zone 80 is massive pollucite (Image 65) and Zone 90 is massive purple “lepidolite”. The sample in Image 66 is coarser grained than average but illustrates the overgrowth of purple “lepidolite” on colourless lithian muscovite. The mica mineral in question is not true lepidolite but a purple lithian muscovite that is distinct from the brown, colourless and green lithian muscovite found elsewhere in the deposit.

Image 65: Pollucite
Image 66: 'Lepidolite' and lithian muscovite.
Image 65: Pollucite
Image 66: 'Lepidolite' and lithian muscovite.
Image 65: Pollucite
Image 66: 'Lepidolite' and lithian muscovite.


The pollucite in Zone 80 has been described in detail by Cerny and Simpson (1978). Late-stage phases are very common in Zone 80 and occur in the form of anastomosing secondary veinlets throughout the pollucite masses. These veinlets are primarily muscovite with minor spodumene and adularia. The density of the secondary veinlets decreases from the edges toward the centre of Zone 80 and so the veinlets were formed after the crystallization of the pollucite.

Figure 9: Alteration of pollucite by later secondary fluids: from https://www.iza-online.org/natural/Datasheets/Pollucite/Pollucite.html


The diagram in Figure 9 illustrates the variability in alteration that takes place in Zone 80. The hand sample in Image 67 is from an area of Zone 80 with little alteration and so is almost pure pollucite. The Zone 80 pollucite has small veinlets of clay minerals but has suffered no weathering as the zone was never exposed to the atmosphere.

Image 67: Pollucite.


The massive lepidolite in Zone 90 is not brittle so it does not carry any shear zones. According to Rinaldi (1970) the lepidolite from Zone 90 is 2M1 muscovite with Li2O between 3.12 % and 3.5%. Green-colourless muscovite at Tanco has between 0.10% and 0.50 % Li2O. The difference in colour between the two types of muscovite can be seen in the overgrowth on image 66
Image 66: Overgrowth of Lepidolite.


The boundary between the violet high-lithian muscovite and the light brown low-lithian muscovite is sharp and indicates that the high-lithian muscovite is a later-stage phase than the low-lithian muscovite. This supports the theory that much of Zone 90 is a secondary replacement of part of zones 40 to 60.

Textural relations within Zone 20 are not known as the majority of the samples come from the Causeway Occurrence. Exposures of the zone are not extensive underground as it has not been the source of economic concentrations of Ta minerals and it contains no spodumene; having beryl as the only characteristic accessory mineral. Stilling (1998) in his volume estimates for each zone considers Zone 20 to represent 30.83% of the total volume of the Tanco pegmatite. Thus it is larger than many of the more economically important zones. The crystal lined vugs within the samples from the Causeway Occurrence are not larger than 5 cm so indicate that portions of Zone 20 will be porous rather than cavernous if they were extracted. The material from the Causeway Occurrence is the product of the original underground shaft excavation; thus it is a random sample of Zone 20.

There are also samples of the Wall Zone (Zone 10) in the dump area. It is a massive unit and has a distinctive black foitite (Selway (1999)) (Image 68) and grey-brown Fe and Mg rich lithiophilite (1.634 Fe, 2.026 Mn and 0.238 Mg apfu; Cerny, Ercit and Vanstone (1998)) (Image 69) in quartz and albite. Zone 10 is a much smaller unit than Zone 20; 0.1% of the Tanco pegmatite (Stilling (1998) and was also not an economic target. It does not contain any of the secondary phases except an occurrence of orange stringy hydroxylapatite (XRD identified) (Image 70) as lining of a pseudomorph after a pre-existing phase (after lithiophilite-amblygonite?). It was also only intersected by the original shaft excavation so the dump material is a random sample.
The secondary minerals at the Tanco mine do not normally form by the dissolution and alteration of phosphate masses, as do many other PO4-bearing pegmatites.

Image 68: Foitite.
Image 69: Fe-Mg rich Lithiophilite.
Image 70: Hydroxlapatite.
Image 68: Foitite.
Image 69: Fe-Mg rich Lithiophilite.
Image 70: Hydroxlapatite.
Image 68: Foitite.
Image 69: Fe-Mg rich Lithiophilite.
Image 70: Hydroxlapatite.


The lithiophilite and amblygonite-montebrasite occur as large, multi-centimeter, rounded single crystals within the pegmatite. The only alteration is a small amount of pink hydroxylapatite on the edges of a few montebrasite pods. There is no darkening of the lithiophilite pods due to the development of Mn-oxides. They all resemble Image 71.

Image 71: High Mn Lithiophilite.


The tantalite-Mn-simpsonite-hydroxykenomicrolite paragenesis (Image 72) has not been considered to be part of the hydrothermal mineral phases (Ercit, Cerny and Hawthorne (1993) though euhedral hydroxykenomicrolite crystals do occur embedded in cookeite (Image 73). Simpsonite is normally an earlier stage phase embedded within massive Ta minerals (Image 74) or occurring separately in albite.

Image 72: Hydroxykenomicrolite, etc.
Image 73: Hydroxykenomicrolite in cookeite.
Image 74: Simpsonite, tantalite-(Mn).
Image 72: Hydroxykenomicrolite, etc.
Image 73: Hydroxykenomicrolite in cookeite.
Image 74: Simpsonite, tantalite-(Mn).
Image 72: Hydroxykenomicrolite, etc.
Image 73: Hydroxykenomicrolite in cookeite.
Image 74: Simpsonite, tantalite-(Mn).



The Ta-cassiterite pseudomorph after wodginite (Image 75) mentioned under cesium analcime has not been noted in the literature before so its relationship to the hydrothermal phases is unknown. The halo of small simpsonite grains surrounding the pseudomorph may be evidence for the mobility of Ta in late-stage hydrothermal fluids.

Image 75: Ta-cassiterite pseudomorph after wodfinite.


One of the conundrums of the Tanco pegmatite is the abundance of Li and B in the hydrothermal fluids. This is expressed as the common occurrence of zabuyelite (Li2CO3) as part of the final fluid inclusion paragenesis (Anderson, Clarke and Gray (2001)). Yet there is a complete absence of late-stage tourmaline in zones later than Zone 20. In all of the later zones the tourmaline occurs as embedded crystals, usually in albite.


Conclusion:



The Tanco Mine and pegmatite in southeastern Manitoba is a large tabular zoned pegmatite that has been a commercial source of tantalum, cesium formate and ceramic-grade spodumene for many years. It has been intermittently mined by underground methods as the pegmatite underlies Bernic Lake. The mine headframe, decline, offices and Ta oxide-ceramic spodumene-cesium formate processing plants are on the north shore of the lake.

Unlike many Ta-bearing pegmatites around the world the secondary cavities in the Tanco pegmatite are formed primarily from the volume loss in the petalite = spodumene+quartz reaction. In some cases this volume loss is infilled by the paragenetically late-stage quartz but in other it is lined with quartz crystals and partially filled by an aggregate of cookeite-cesium analcime-hydroxylapatite-lithiophosphate +/- rhodochrosite. Other rarer phases such as fairfieldite, wopmayite, ercitite and groatite have been identified as the latest phases in the cavities.

The primary pegmatite phosphates lithiophilite and amblygonite/montebrasite are common in the Tanco pegmatite and are not in general altered by later hydrothermal fluids. They do not supply the P for the secondary phosphate phases within the quartz cavities except under the unusual circumstance of fluorapatite after an unknown phase in Zone 20.

The secondary phases at Tanco form at lower temperature and pressure than the normal pegmatite minerals. As mentioned earlier the crystallization in the quartz cavities is approximately half the temperature and pressure of the main pegmatite crystallization. Thus many of the cavities are lined with hydrous phosphate minerals such as hydroxylapatite and lower temperature silicate minerals such as cookeite.

The boundary between the final crystallization of the Tanco pegmatite with the extreme Ta fractionation of the tantalite-simpsonite-hydroxykenomicrolite paragenesis and the creation of the cookeite-cesium analcime paragenesis of the hydrothermal secondary phases is not sharp. There is some overlap with euhedral hydroxykenomicrolite crystals embedded in anhedral green cookeite.


Acknowledgments:



I wish to extend my gratitude to the staff of the Tanco Mine, especially Peter Vanstone, for their generosity and helping me understanding the mine. I wish to thank the staff at Manitoba Energy and Mines for their support and the many XRD identifications. I wish to thank the staff at the Geology Department of the University of Manitoba for help with XRD work. I wish to thank Dr. K. Tait for her assistance with the XRD identification of the fluorapatite. Rob Woodside, Don Peck and Erin Delventhal are thanked for their help with the article.


References:



Anderson A.J., Clarke A.H. and Gray S. (2001) The Occurrence and Origin of Zabuyelite (Li2CO3) in Spodumene-hosted fluid inclusions: Implications for the internal Evolution of Rare-Element Pegmatites: Canadian Mineralogist v. 39 pp. 1513-1527

Cerny P. (1972) The Tanco Pegmatite at Bernic Lake, Manitoba VIII Secondary Minerals from the Spodumene Zone; Canadian Mineralogist V 11 pp. 714-726

Cerny P., Ercit, T.S. and Vanstone P.J. (1998) Mineralogy and Petrology of the Tanco Rare-element Pegmatite Deposit, Southeastern Manitoba; International Mineralogical Association Field Trip Guidebook B6, 74 pages

Ercit T.S., Cerny P. and Hawthorne F.C. (1993) Cesstibtantite-a Geological Introduction to the Inverse Pyrochlores: Mineralogy and Petrology v. 48 pp. 235-255

London D. (1986) Magmatic-hydrothermal transition in the Tanco rare-element pegmatite: Evidence from fluid inclusions and phase-equilibrium experiments; American Mineralogist v. 71 pp 376-395

Martins T., Kremer P. and Vanstone P. (2013) The Tanco Mine: Geological Setting, Internal Zonation and Mineralogy of a World-Class Rare Element Pegmatite Deposit: Geological Association of Canada/ Mineralogical Association of Canada Open File OF 2013-8, 17 pages (and references therein)

Selway J.B. (1999) Compositional Evolution of Tourmalines in Granitic Pegmatites; PhD Thesis, University of Manitoba, 362 pages

Stilling A. (1998) Bulk Composition of the Tanco Pegmatite at Bernic Lake, Manitoba, Canada; M.Sc. Thesis, University of Manitoba, 76 pages.







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Comments

Hi Richard! Let me ask you an explanation: image 1, image 22 and image 52; is it a quartz or a calcite? Why are these images marked differently? Greetings from Italy by Riccardo.

Riccardo Modanesi
3rd Mar 2018 9:47am
Hi Riccardo:

The calcite in all three images are the white crystals on the upper left part of the quartz aggregate. In this article I used the same photo for different purposes rather than refer back to a previous image. It is simpler using that method. The purpose of illustrating the white calcite on quartz was to show that calcite is a late-stage mineral and does not fill cavities. Rhodochrosite and calcite occur in different parageneses at Tanco and are chemically distinct. If you wish to see another calcite photo from Tanco it is on the locality page. I did not use that photo in my article as I did not know where in the Tanco Mine it came from.

Greetings from the Canadian West Coast

Richard Gunter
4th Mar 2018 7:22pm

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