Abstract

Aqueous and non-aqueous inclusions in 84 samples of various minerals from a wide range of geologic environments were studied with the freezing stage in order to gain an insight into the range of concentrations and compositions of fluid inclusions. Inclusions in most Mississippi Valley-type ore minerals contain highly concentrated saline solutions, showing freezing temperatures between --23.4 degrees and --10.5 degrees C; minerals from ore deposits of more typically hydrothermal affiliations mainly show temperatures of --9.4 degrees to nearly 0 degrees C; and inclusions in quartz crystals from sedimentary, metamorphic, and igneous rock environments show a wide range of freezing temperatures. Inclusions in pegmatite minerals in particular vary over a wide range, from the most concentrated solutions found in any inclusion (> 40% salts) to fairly dilute solutions (< 5% salts). Most inclusions in quartz from Swiss Alpine-type veins, and from Brazilian quartz veins and pegmatites, show freezing temperatures in the range --8.5 degrees to --2.5 degrees , and considerable free CO 2 . Other geologic environments sampled include Colombian emerald, pegmatitic topaz and fluorite, the Triassic traprock zeolite assemblage, and sedimentary halite beds.Not all the phenomena exhibited by inclusions at low temperature are completely understood at present but several crystalline hydrate phases, such as NaCl.2H 2 O and CO 2 .5 3/4H 2 O (structural formula 8CO 2 .46H 2 O), are shown to be stable in inclusions of appropriate composition even at temperatures above 0 degrees C, and probably exist in the inclusions in natural rocks in polar regions. More significantly, the formation and recognition of such phases aid in establishing the gross composition of individual inclusions far too small for chemical analysis.The data obtained are useful in a variety of ways, such as: discriminating among gas, liquid, and supercritical fluid, and among liquid water, liquid oil, and liquid CO 2 in inclusions; improving precision of the pressure corrections applied to inclusion filling temperature determinations; proving the general lack of leakage into or out of inclusions; estimating the minimum pressure at the time of deposition of certain samples; verifying the lack of extraneous solid crystallization nuclei in the inclusions and hence their formation from exceedingly quiet, clean solutions; determining the total equivalent NaCl concentrations and some information concerning the composition of the fluids from which ores have formed; and determining changes in the composition of the fluids bathing a single crystal during its growth, and at certain times throughout its history.

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