Garnets in Montana diatremes; a key to prospecting for kimberlites

Abstract

Introduction 1 Techniques of color classification 2 Summary of samples 2 Geology of the Williams diatremes 8 Inclusions from deep levels 9 Garnets from garnet peridotites 9 Megacryst garnets 9 Garnets from granulites, mafic granulites, and mafic amphibolites 19 Garnets from other inclusions in the Williams diatremes 19 Garnet-bearing inclusions in other diatremes 19 Garnets from Precambrian rocks in the Little Rocky Mountains 20 Colluvial garnets from the Williams diatremes 21 Comparison of Williams colluvial garnets with garnets from known sources 23 Panned concentrates from other diatremes 31 Big Slide diatreme 31 Bullwhacker Coulee diatreme 31 Conclusions 31 References 32 FIGURES 1. Geologic map of the Williams diatremes and vicinity, north-central Montana 10 2. Diagrams showing Ca-Mg-Fe in colluvial garnets, megacrysts, and garnets from inclusions in the Williams 1 and 4 diatremes 14 3. Histograms showing Cr203 content of color groups of peridotitic and colluvial garnets in the Williams diatremes 17 4-8. Diagrams showing: 4. Weight percent CaO versus weight percent Cr203 for garnets in the Williams 1 and 4 diatremes 18 5. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) for garnets in the Williams 1 and 4 diatremes 20 6. Chromium versus aluminum cations per 12 oxygens for garnets in the Williams 1 and 4 diatremes 22 7. Ca-Mg-Fe in garnets from localities other than the Williams diatremes 29 8. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) in garnets from localities other than the Williams diatremes 30 Contents Ill IV Contents TABLES 1-6. Analyses of: 1. Colluvial garnets in the Williams 1 and 4 diatremes 3 2. Garnets from fresh garnet peridotites in the Williams 1 and 4 diatremes 12 3. Garnets from altered garnet peridotites in the Williams 4 diatreme 13 4. Garnet megacrysts in the Williams 4 diatreme 24 5. Garnets from other inclusions in the Williams 1 and 4 diatremes 25 6A. Garnets from the Big Slide 1 diatreme 26 6B. Garnets from the Bullwhacker Coulee diatreme 27 6C. Garnets from other inclusions, Precambrian bedrock, terrace deposits, and Paleocene sedimentary rock 28

Garnet peridotites from Williams kimberlites, north-central Montana, U.S.A

“…Analyses of the six color groups (purple, pink, red, red-orange, orange and light orange) of colluvial garnets from the Williams 1 and 4 diatremes (Hearn and McGee, 1982) show that 20 of 21 purple garnets contain more than 2.4 percent Cr203, which is more than any of the red-orange, orange or light-orange garnets, and 16 purple garnets contain more Cr203 than any red or pink garnets (fig. 7).…”

“…7). All of the peridotitic garnets with more than 4.0 percent 0^03 are purple, the typical color of Cr-rich pyrope which aids in the field recognition of peridotitic garnets in prospecting for kimberlites (Leighton and McCallum, 1979;Dawson, 1980;Hearn and McGee, 1982). Peridotite garnets with less than 4.0 percent 0^03 are red or red-orange.…”

Garnet xenocryst analyses; potential for diamonds in the Williams Kimberlite, north-central Montana and the Lake Ellen Kimberlite, northern Michigan

“…Xenocryst garnets from the Williams and Lake Ellen kimberlites have Cr-rich compositions typical of peridotitic garnets and Cr-poor compositions typical of granulitic,.amphibolitic, or eclogitic garnets (Hearn and McGee, 1983;McGee and Hearn, 1984). At both localities, deep purple color seems to correlate with Cr-rich peridotitic compositions (Hearn and McGee, 1983;McGee and Hearn, 1984). To look for the subcalcic, Cr-rich garnets that are associated with diamond elsewhere, purple garnets were preferentially selected from panned concentrates obtained from bag samples collected on the outcrops at each locality.…”

A transect of metamorphic rocks along the Copper River, Cordova and Valdez Quadrangles, Alaska: A section in <i>The United States Geological Survey in Alaska: Accomplishments during 1982</i>

“…Additional support for a high-pressure origin for Bushy Point garnets comes from two well-studied (Hearn and McGee, 1983). Another xenolith is an amphibolite included in a latite flow in Chino Valley, central Arizona (Arculus and Smith, 1979).…”

“…At pressures less than about 12-13 kbar, garnet became unstable and began to react with the melt to form plagioclase and a more calcic garnet rim. Garnet is preserved (Hearn and McGee, 1983); asterisk, amphibolite inclusion from latite flows in Chino Valley, Ariz. (Arculus and Smith, 1979); upright triangles, Ecuador eclogite (Feininger, 1980); squares, Oregon eclogite (Ghent and Coleman, 1973); diamonds, Western Alps eclogite (Ernst and Dal Piaz, 1978). Open symbols represent garnet compositions produced in experimental studies; circles, 18-26 kbar data on andesitic and basaltic compositions (Allen and Boettcher, 1978;Allen and others, 1975); squares, 30 kbar data for tonalitic compositions (Stern and Wyllie, 1973); the arrow with temperature ticks represents Stern and Wyllie's calculated trend of garnet compositions at 30 kbar.…”