Ontario Geological Survey Open File Report 5966 Rare-Metal Exploration Potential of the Separation Lake Area: an Emerging Target for Bikita-Type Mineralization in the Superior Province of NW Ontario 1997 ONTARIO GEOLOGICAL SURVEY Open File Report 5966 Rare-Metal Exploration Potential of the Separation Lake Area: an Emerging Target for Bikita-Type Mineralization in the Superior Province of NW Ontario by F.W. Breaks and A.G. Tindle 1997 Parts of this publication may be quoted if credit is given. It is recommended that reference to this publication be made in the following form: Breaks, F.W. and Tindle, A.G. 1997. Rare-metal exploration potential of the Separation Lake area: an emerging target for Bikita-type mineralization in the Superior Province of Ontario; Ontario Geological Survey, Open File Report 5966, 27p. e Queen’s Printer for Ontario, 1997 e Queen’s Printer for Ontario, 1997. 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Bag 3060, Hwy. 101 East, South Porcupine, Ontario P0N 1H0 This report has not received a technical edit. Discrepancies may occur for which the Ministry of Northern Development and Mines does not assume any liability. Source references are included in the report and users are urged to verify critical information. Recommendations and statements of opinions expressed are those of the author or authors and are not to be construed as statements of government policy. If you wish to reproduce any of the text, tables or illustrations in this report, please write for permission to the Manager, Publication Services Section, Ministry of Northern Development and Mines, 933 Ramsey Lake Road, Level B4, Sudbury, Ontario P3E 6B5. Cette publication est disponible en anglais seulement. Parts of this report may be quoted if credit is given. It is recommended that reference be made in the following form: F.W. Breaks and A.G. Tindle, 1997. Rare--metal exploration potential of the Separation Lake area: an emerging target for Bikita--type mineralization in the Superior Province of Ontario; Ontario Geological Survey, Open File Report 5966, 27p. iii Contents Abstract ................................................................................... ix Introduction. ................................................................................ 1 Geological Setting ........................................................................... 2 Rare--metal mineralization .................................................................... 3 Big Whopper Pegmatite System........................................................ 4 Internal Zonation .................................................................... 4 Mineralogy ................................................................................. 7 Structural Geology ........................................................................... 8 Comparison to Bikita--type rare--metal mineralization............................................... 9 Economic Geology ......................................................................... 10 Petalite ........................................................................... 10 Tantalum ......................................................................... 11 Cesium ........................................................................... 13 Exploration for rare--metal pegmatite mineralization............................................... 13 Search for parent granite............................................................. 14 Ta--Nb bulk chemical variation........................................................ 18 Lithium lithochemistry.............................................................. 18 Metasomatic selvedges.............................................................. 21 Recommendations for future mineral exploration................................................. 23 References ................................................................................ 24 TABLES 1. Summary of mineralogy in rare--element pegmatites of the eastern subgroup, Separation Lake area ... 5 2. Representative electron microprobe analyses of oxide minerals from the southwestern pegmatite subgroup of the Separation rapids group .................................................. 9 3. Average Li20 contents and calculated percent petalite across important pegmatites of the Big Whopper rare--metal system. ........................................................... 10 4. Range of Ta205 in Ta--Nb bearing oxide minerals from the Separation Lake area ................. 11 5. High grade tantalum bulk analyses from the Separation rapids Pegmatite Group .................. 11 FIGURES 1. Location of Separation Lake area........................................................ 2 2. General geology of the Separation Lake metavolcanic belt and adjacent part’s of English River and Winnipeg River subprovinces........................................................... 3 3. Rare--metal pegmatite zones and mineral occurrences of the southwest subgroup, Separation Lake area 6 4. Classification quadrilateral for Separation Lake area wodginites.............................. 12 v 5. Suggested exploration scheme for rare--metal mineralization in glaciated Archean shield terrain..... 15 6. K/Rb versus Cs variation in potassium--feldspar from peraluminous granite and related rare--metal pegmatites of the Separation Lake area.................................................. 16 7. Fields of K/Rb versus Cs variation for potassium--feldspar from peraluminous granite plutons considered to be parental to rare--metal pegmatites in the Cat Lake--Winnipeg River Pegmatite Field.......... 17 8. Variation of rubidium and cesium in potassium--feldspar within the Separation Rapids pluton....... 19 9. Ta versus Nb in various units of rare--metal Pegmatites and peraluminous granites of the Separation Lake area .............................................................................. 20 10. Lithium variation in mafic metavolcanic host--rocks to rare--metal pegmatites of the eastern part of the Southwestern Pegmatite Subgroup...................................................... 21 11. Cesium in metasomatic selvedges in southeastern rare--metal pegmatite subgroup................ 22 PHOTOS 1. Strongly recrystallized petalite from Pegmatite #7, Champion Bear claim--group.................. 7 2. First occurrence of pollucite found in the Separation Lake area............................... 13 vii ABSTRACT Rare--metal class pegmatites of the complex--type and petalite--subtype represent the most desirable target for tantalum, cesium, rubidium and ceramic quality petalite in Archean terrain settings. However this depos- it--type is one of the most difficult explorationtargets owingto lackof responsiveness to airborne geophysical systems and the difficult and exotic mineralogy of the pegmatites. Geological, mineralogical, structural and some preliminary chemical features are given for the Big Whopper pegmatite system, discovered by the au- thors in 1996 and which is now the largest petalite pegmatite system in Ontario. Current economic interest is focussed upon the petalite potential which retails at US$270 per ton of con- centrate.The widest part of the BigWhopper Pegmatite averages 37% petalite over 60 m which is comparable totheworld’s premierepetalitedeposit at the BikitaPegmatite of southern Zimbabwe.The tantalum potential is also considered significant as wodginite, the chief ore mineral for Ta at the Tanco Mine of southeastern Manitoba,is not onlywidespreadintheSeparationLakearea,butalsoexhibitscompositional variationunlike any other pegmatite group on a global scale. Cesium, a metal of growing economic importance, also has high exploration potential as pollucite, the only ore mineral for the metal, has recently been verified in the area. A