Type of Document Master's Thesis Author Kar, Adityamoy URN etd-02132009-170725 Title Fluid inclusion and trace element studies of the gem pegmatites of Mt. Antero, Colorado Degree Master of Science Department Geology Advisory Committee
Advisor Name Title Bodnar, Robert J. Committee Chair Eriksson, Susan C. Committee Member Ribbe, Paul H. Committee Member Keywords
Date of Defense 1991-08-15 Availability restricted AbstractThe gem pegmatites of Mt. Antero, Colorado occur in an Oligocene granitic stock containing aquamarine, smoky, clear and milky quartz, phenakite, muscovite, fluorite, garnet, pyrite, topaz and calcite. Aquamarine and smoky quartz contain abundant primary fluid inclusions outlining growth zones, as well as numerous trails of pseudosecondary and secondary inclusions. Salinities and homogenization temperatures (Th) of these inclusions define five distinct groups.
The earliest primary fluid inclusions occur in outer growth zones of gem aquamarines. These inclusions have salinities of 2.6 - 3.9 wt.% and homogenize at 350°- 360°C. The earliest primary inclusions in smoky quartz have salinities of 5.0 - 5.6 wt. % and homogenize at the same temperature as the primaries in aquamarines. Paragenetically later primary and secondary C02-bearing inclusions in aquamarine, and secondary C02- bearing inclusions in smoky quartz homogenize at 350o-360°C. Later secondary inclusions in smoky quartz define two distinct groups based on salinity and Th (Group 1: Th = 285°-355°C, salinity = 6.1 - 7.9 wt.%; Group 2: Th = 215°-245°C, salinity = 3.0- 4.3 wt.%). Salinities and Th's of very latest primary inclusions in aquamarine overlap with those of group 1 in smoky quartz. Homogenization temperature of latest primary inclusions in smoky quartz is 194°-213°C and salinities range from 0.5 - 2.0 wt.%. Secondary inclusions in phenakite and fluorite crystals homogenize at about 270°C and 215°C, and have salinities of 1.3 and 0.5 wt. %, respectively.
The fluid inclusion data, combined with constraints from mineral equilibria, suggest that gem aquamarines formed at approximately 500° to 600°C and - 1 kbar from low to moderate salinity aqueous fluids (2.6 - 6.2 wt. %). The salinity increased during the final stages of aquamarine growth and initial stages of smoky quartz formation, reaching a maximum of about 8 wt. %. During later stages of smoky quartz formation and subsequent fluorite and phenakite growth the salinity continuously decreased. Calcite and the latest quartz crystallized from -0.5 wt. % solutions.
The beryl crystals from Mt. Antero vary in color from dark to pale blue aquamarines to the essentially colorless goshenite variety with some individual crystals showing color zoning. Quantitative electron microprobe analyses showed that iron, which imparts the blue color to the aquamarines, is enriched in the dark blue parts of a gem crystal and comparatively depleted in the paler blue terminated parts of the crystals. Most of the aquamarine crystals showed evidence of growth zoning. Quantitative analyses and X-ray maps generated with electron microprobe show that iron and cesium are depleted in the cores of the beryls and enriched in the rims of the crystals. An exact opposite trend is observed for sodium and magnesium, which are depleted in the rims but enriched in the cores of the beryls.
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