9 22 10
play

9/22/10 VISUAL FEATURES OF KIM Kimberlitic Indicator Minerals Maja - PDF document

Feb 16, 2023 •158 likes •473 views

9/22/10 VISUAL FEATURES OF KIM Kimberlitic Indicator Minerals Maja Kiridzija Knowing the micro world leads to the macro discovery OBJECTIVE At the end of todays activities you will be able to identify and separate KIM grains from

  1. 9/22/10 VISUAL FEATURES OF KIM Kimberlitic Indicator Minerals Maja Kiridzija “Knowing the micro world leads to the macro discovery” OBJECTIVE At the end of today’s activities you will be able to identify and separate KIM grains from the regular till samples in 30 minutes. AGENDA 1. Introduction to KIM (30min) Definition and list of all KIM KIM size fraction Procedure for KIM observation General visual features of KIM Importance of KIM visual features 2. PG, EG, IL, CR, CD and OL visual properties (30min) Visual features Morphologies and abrasion Kimberlitic vs Non Kimberlitic Similar minerals Sources of non kimberlitic Indicator minerals Review quizzes Break 3. Practicing KIM identification under the binocular microscope (60min) Kimberlite concentrates Teaching sets and teaching material Extraction of KIM from KIM concentrates Different exercises: separation of PG from EG, CR from IL, etc. Break 4. Microscopic observation of till concentrates spiked with KIM grains (60min) KIM observation (30 min) Recovery of spiked grains Conclusion Evaluation of the course 1

  2. 9/22/10 Name the KIM in the photo Picroilmenite Eclogitic Garnet 1 6 Chrome diopside 2 7 Olivine 3 Picroilmenite Picroilmenite 8 9 Chromite 4 Peridotitic Garnet Chromite Peridotitic Garnet 5 10 LIST OF KIM 1. Kimberlitic garnet Peridotic (PG) Eclogitic (EG) 2. Kimberlitic Chromite (CR) 3. Picroilmenite (IL) 4. Chrome Diopside (CD) 5. Kimberlitic Olivine (OL) OTHER KIM (not examined this time) • Kimberlitic orthopyroxene (enstatite) • Kimberlitic zircon • Diamond 2

  3. 9/22/10 KIM DEFINITION • macrocrysts and megacrysts in kimberlite/lamproite • easily liberated from their host rock • sufficiently dense to be concentrated by gravity • abundant compare to diamonds • durable during transportation • distinct by their visual and geochemical features SIZE FRACTIONS OF KIM • Most common in regional till sampling: – 0.25mm to 0.5mm • Why this size fraction? – To ensure the abundance of KIM during till sampling – To provide good range of KIM for geochemical studies • >0.5mm useful for studying KIM surface features ( abrasion ) KIM OBSERVATION time consuming! binocular microscope, tweezers or brushes, petri dishes, scratch pencil indicator collector, KIM observation skill and patience “IF IN DOUBT PICK IT OUT!” lines quarters spirals 3

  4. 9/22/10 GENERAL VISUAL FEATURES OF KIM • Lack of crystal shape (with exceptions) • No inclusions (except of other mantle minerals) • Sculptured surface features : resorption, orange peel, coating, pitted surfaces, cockscomb, bumpy surface • Specific color : fire, bright emerald, bright pink/purple/ red/orange KIM SURFACE TEXTURES from M. B. MCCLENAGHAN AND B. A. KJARSGAARD A) Cr-pyrope with dark green-grey adhering kimberlite (k); B) Cr-pyrope with dark green-grey kelyphite rims (kp); C) Cr-pyrope displaying sub-kelyphite orange peel texture on surfaces, outlined by dashed line; D) Cr-pyrope displaying sub-kelyphite frosted surfaces; E) Cr-pyrope displaying fresh, angular broken surfaces; F) Cr-diopside displaying rounded (r ) to subrounded shape, adhering kimberlite (k), as well as broken angular surfaces (a); G) and H) chromite displaying octahedral to suboctahedral (resorbed) shapes. WHY IS IT IMPORTANT TO KNOW THE VISUAL FEATURES OF KIM? • To be able to distinguish KIM among background and other similar minerals in the core, till, stream or other concentrates • To be able to provide a quick and statistically viable selection of KIM for further geochemical analyses • To possibly determine: – proximity of the kimberlitic source – difference among indicator trains – diamond potential of their source 4

  5. 9/22/10 KIMBERLITIC GARNET Pyrope, Peridotitic and/or Eclogitic garnet Mg, Fe,Ca, Al silicate with elevated Cr, Ti, Na VISUAL FEATURES • Isotropic • Purple/Orange color with “fire” • Lack of crystal shape • No polycrystalline aggregates • Kelyphitic rim • Resorption • “Orange peel” MORPHOLOGIES OF KIMBERLITIC GARNET 1. Kimberlitic garnets with kelyphitic rims 2. Fine resorption surfaces 3. Euhedral kimberlitic garnets (very rare) 4. Intensive surface “etching” 5. Frosted kimberlitic garnets 5

  6. 9/22/10 Kelyphitic rim Fine resorption Euhedral 6

  7. 9/22/10 Surface etching Frosted PERIDOTITIC vs ECLOGITIC GARNET PERIDOTITIC ECLOGITIC • Cr 2 O 3 >2% • Cr2O3<2% • “purple garnets” • “orange garnets” pink-purple to blue and pale pink to pale orange and orange red with red fire orange with orange fire (“megacrystic garnet”) 7

  8. 9/22/10 “Megacrystic garnet” (elevated TiO 2 ) COLOR SCHEME TYPICAL COLORS 1. Blue-purple to deep purple (Peridotitic) 2. Pink-purple (Peridotitic) 3. Red (Megacrystic) 4. Orange garnet (Eclogitic) 8

  9. 9/22/10 UNUSUAL COLORS 1. Green kimberlitic garnet (Dry Bones kimberlite, Canada) 2 . “Dirty” kimberlitic garnet (Ranch Lake kimberlite, Canada) 3. Bright pink “G10” type kimberlitic garnet 4. White kimberlitic garnet (Dry Bones kimberlite, Canada) KIMBERLITIC vs NON-KIMBERLITIC GARNETS KIMBERLITIC NON-KIMBERLITIC no crystal shape often crystal shape (rounded and/or fractured) no inclusions possible inclusions kelyphitic rim no kelyphitic rim (wholly or partially present) sculptured surfaces no sculptured surfaces (resorption, etching, “orange peel”) not magnetic slightly magnetic 5 peridotitic garnets mixed with non kimberlitic garnets PG PG PG PG PG 9

  10. 9/22/10 5 eclogitic garnets mixed with non kimberlitic garnets EG EG EG EG EG MINERALS SIMILAR TO KIMBERLITIC GARNET 1. Common garnet PG 2. Pink zircon PG 3. Pink corundum (Ruby) 4. Pink spinel PG 5. Staurolite EG 6. Gahnite NON ABRADED vs ABRADED NON ABRADED ABRADED • Preserved surface • Not preserved or features: partially preserved surface features • absence of kelyphitic rim • kelyphitic rim • polished resorption • resorption features, etching • etching • abraded fractured edges • sharp fractured edges 10

  11. 9/22/10 NON ABRADED vs ABRADED fresh slightly abraded well abraded proximal distal NON ABRADED PERIDOTITIC GARNET Kelyphitic Mixed with kimberlite rim Rounded with coarse Sculptured resorption surface NON KIMBERLITIC GARNET SOURCES • minette • melnoite • eclogite lenses in ophiolites • granulites • pegmatite, • granite, etc. 11

  12. 9/22/10 Kimberlitic garnet review quiz 1 Kimberlitic garnet has a metallic luster FALSE 2 Crystal shape is typical for kimberlitic garnets FALSE 3 Green colored garnets can be kimberlitic TRUE 4 Inclusions are frequent in kimberlitic garnets FALSE 5 A kelyphitic rim is one of the characteristics of kimberlitic garnets TRUE 6 Megacrystic garnets are indicative of good diamond potential FALSE 7 "G10" garnets are a bright red color FALSE 8 Megacrystic garnets are an orange red color with red fire TRUE 9 Lamproitic and kimberlitic garnets have the same visual features TRUE 10 Eclogitic garnets are easily mixed up with epidotes FALSE PICROILMENITE Mg Ilmenite Mg,Fe,Ti,Oxide with elevated Cr, Mn, Al VISUAL FEATURES • Opaque • Lack of crystal shape and cleavage • Coating (leucoxene, perovskite, anatase) • “Bumpy” surface • Conchoidal fracture • Black scratch powder • Lack of inclusions and adhering grains 12

  13. 9/22/10 PICROILMENITE MORPHOLOGIES 1 . Fractured picroilmenite (brittle, sharp, conchoidal fractures, no surface preserved) 2. Matte, smooth surface picroilmenite (could be similar to chromite except for the metallic and not glassy luster) 3. Coated picroilmenite with a“bumpy surface” (leucoxene, rutile, anatase and/or perovskite coatings) 4. Polygranular picroilmenite (with or without coatings, multiple crystal growth, rare but present) Fractured Smooth surface 13

  14. 9/22/10 Coated surface Polygranular PICROILMENITE vs ILMENITE PICROILMENITE ILMENITE • lack of crystal shape • crystal shape • visible cleavage and • lack of cleavage layering • lack of inclusions • inclusions present • conchoidal fractures • flat fracture along cleavage • lack of holes or any • possible presence of holes depressions and depressions • often coated • rarely coated 14

  15. 9/22/10 MINERALS SIMILAR TO PICROILMEITE 1. Common Ilmenite 5.Columbite/Tantalite 2. Magnetite 6. Euxenite 7. Rutile 3. Hematite 8. Amphibole 4. Chromite Mix of picroilmenite and chromite 3 picroilmenite and 3 chromite CHROMITE PICROILMENITE 15

  16. 9/22/10 1 picroilmenite and amphibole IL NON KIMBERLITIC ILMENITE SOURCES • Picroilmenite: basalts ultramafic lamprophyres carbonatites • Common ilmenite: gabbros norites anorthosites, etc Picroilmenite review quiz 1. The picroilmenite is the most distinctive kimberlitic OXIDE mineral 2. Genetically the picroilmenite in a kimberlite is a MEGACRYST 3. Morphologically picroilmenite may appear as a SINGLE or POLYGRANULAR crystal 4. A typical surface feature of the picroilmenite is COATING 5. The picroilmenite has a BLACK streak 6. Morphology of picroilmenites is used in ABRASION studies 7. Compared to the common ilmenite, the picroilmenite does not have a CRUSTAL SHAPE and INCLUSIONS 8. Layers on fractured surfaces are typical for COMMON ILMENITES 9. The picroilmenite has a METALLIC luster 10. The picroilmenite has CONCHOIDAL fractures 16

Recommend

More recommend