The Pakeagama Lake Pegmatite: Continued Field and Laboratory Investigation of Highly Evolved, Complex-Type, Petalite-Subtype Rare-Element Mineralization in the Berens River-Sachigo Subprovince Boundary Zone
By F.W. Breaks(1), A.G. Tindle(2), and S.R.Smith(2)
(1)Precambrian Geoscience Section, Ontario Geological Survey
(2)Department of Earth Sciences. The Open University, Milton Keynes, United Kingdom
From the Ontario Geological Survey (OGS), Open File Report 6000: Summary of Field Work and Other Activities, 1999 (Release date: November 8, 1999)
The Full report is on Houston's website (Research Reports section):
houstonlakemining.com
The following key points should be noted as it leaves no doubt in my mind as to the potential for a significant discovery:
INTRODUCTION
- "Field-based and laboratory data amassed from the Pakeagama Lake pegmatite (Breaks, Tindle and Smith 1998), located 160 km north of Red Lake, has revealed one of the largest and most highly evolved rare-element-mineralized pegmatite systems in the Superior Province of Ontario. The chemical and mineralogical database in this project comprises 1648 electron microprobe analyses, 50 bulk chemical analyses, 21 analyses of potassium feldspar and X-ray diffraction mineral identification work conducted upon 30 samples. These data collectively represent the largest for any single pegmatite mass studied to date in northwestern Ontario."
- "A follow-up field investigation of three weeks duration was undertaken to:
ú further characterize the internal mineralogical zonation in terms of mineralogy and chemistry
ú assess the distribution of economically important minerals such as tantalite, wodginite and pollucite
ú select oxide minerals such as columbite-tantalite group for U-Pb age determination (see Smith, Tindle, Kelley and Breaks, this volume)"
- "In order to facilitate this investigation, shallow soil and organic materials, which obscured several important internal zones of the pegmatite system, were extensively removed by a Wajax© Mark 3 high-pressure centrifugal fire pump. This work was followed by extensive application of bleach in order to remove tightly adhering organic film. The extraction of nineteen, 1 m-long channel samples and an additional 10 bulk samples was undertaken from two areas of the pegmatite not previously sampled (Figure 26.1)."
DETAILED GEOLOGY OF THE PAKEAGAMA LAKE PEGMATITE
- "The geology of the Pakeagama Lake pegmatite was initially described by Breaks, Tindle and Smith (1998). However, the overall dimensions of the main exposure of the pegmatite have been increased to a minimum strike-length of 260 m from 250 m, with the northwestern and southeastern parts open along strike. It is possible that the pegmatite system extends an additional 300 m to the southeast beyond Locality 98FWB-48 and under clay cover to Locality 98FWB-34 on Pakeagama Lake. The latter locality contains aplite sheets, up to 1 m thick, hosted in granite of the Pakeagama Lake pluton. An economically interesting Ta2O5 value of 0.07% was documented in the tourmaline-apatite aplite (Table 26.1), and electron microprobe analysis confirmed the tantalum minerals, ferrotantalite, ferrotapiolite and microlite (see Table 26.2). The estimated width of the dominant, northwest striking arm of the pegmatite system has been revised to 30 to 70 m from the previous estimate of 10 to 70 m (Breaks, Tindle and Smith 1998, p. 171).
Beryl-green tourmaline-lepidolite-quartz wall zone
- "The quartz-rich wall zone is considered as an important specific zonal target for tantalum, rubidium and cesium. Significant levels of rubidium and cesium were detected in lepidolite by preliminary electron microprobe analysis (Rb2O: 1.84 to 2.94 weight %; Cs2O: 0.26 to 0.54 weight %). The inference that this zone represents a good exploration target is further supported by bulk analysis (98FWB-35 in Table 26.1) which indicates an economically interesting Ta2O5 value of 0.038% accompanied by significant Rb2O of 1%, representing the highest bulk value encountered to date in any rare-element pegmatite in northwest Ontario. The highly evolved chemistry of the wall zone is also indicated by significant levels of Li2O (1.93%) and F (1%), and extremely low ratios of K/Rb (4.5) and Nb/Ta (0.26). The Nb/Ta reveals a high degree of fractionation of Nb relative to Ta as the value for the quartz-rich wall zone departs significantly from the crustal average ratio of 11.4 and closely approaches the bulk Nb/Ta ratio of the Tanco pegmatite (0.141: Stilling, Cerny and Vanstone 1999)."
Pollucite-bearing vein system
- "The chemical, physical and crystallographic characteristics of the Pakeagama Lake pollucite and its alteration phases are presently under investigation by Professor P. Cerny, Department of Geological Sciences at the University of Manitoba. "
MINERALOGY
1. COLUMBITE-TANTALITE GROUP
- "The (highly evolved manganese-rich) group contains the largest number of electron microprobe analyses and consists of highly evolved, manganese-rich compositions (Mn/Mn+Fe = 0.926 to 0.962) mainly confined to the southeastern part of the pegmatite system. The analyses are marked by a generally appreciable enrichment in tantalum (weight % Ta2O5 = 31.57 to 72.58; Ta/Ta+Nb = 0.471 to 0.770). This group contains the most evolved tantalite composition thus far documented in the pegmatite system (sample 99PKL-2: 72.58 weight % Ta2O5; Ta/Ta+Nb = 0.77; Mn/Mn+Fe = 0.928) which occurs in a montebrasite-lepidolite-plagioclase-quartz rock from the wall zone). Wodginite (see below), the chief ore mineral for tantalum at the Tanco Mine, Bernic Lake, Manitoba (Cerny, Ercit and Vanstone 1996), also occurs in this sample."
Ferrotapiolite
- "This mineral has been found only locally to date in the Pakeagama Lake pegmatite system and contain very high levels of Ta2O5 (61.89 to 82.57 %). However, its presence in aplites in association with ferrotantalite and manganotantalite at Locality 98FWB-34, as briefly described above in reference to the second field of columbite-tantalite mineral compositional variation, merits future exploration attention to such rock units as viable hosts for economic tantalum mineralization ."
Stibiotantalite
- "This rare mineral has not been previously documented in Ontario and only comprises ten localities on a global basis (Foord 1982, p. 221-222). In the Pakeagama Lake pegmatite, the mineral has to date been only found at one locality (98FWB-46 in Figure 26.1), namely the spodumene-quartz core zone as discrete grains that are partly altered to stibiomicrolite and native antimony. Levels of weight % Ta2O5 range from 48.62 to 51.36 %; a representative analysis is given in Table 26.3. Stibiotantalite can be of economic significance as it constitutes an ore mineral for tantalum at the Greensbushes Mine in western Australia (P. Cerny, University of Manitoba, Winnipeg, Manitoba, personal communication, 1999)."
2. WODGINITE GROUP
- "The Pakeagama Lake wodginite (see Table 26.3) closely correspond to the ideal mineral composition in the wodginite classification quadrilateral of Tindle, Breaks and Webb (1998, p.641),as atomic ratios of Ta/Ta+Nb and Mn/Mn+Fe both exceed 0.9. "
EXPLORATION POTENTIAL
- "The detailed documentation of a variety of tantalum-rich minerals coupled with the presence of pollucite renders the Pakeagama Lake pegmatite and adjoining area one of the best exploration targets for tantalum and cesium in northwestern Ontario."
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DRT |
