The growth of crystals is never perfect and as a result samples of the fluid in which the crystals grew may be trapped in tiny cavities usually <100 um in size. These are called fluid inclusions and can be divided into various types. Primary inclusions formed during the growth of crystals provide us with samples of the ore-forming fluids. They also yield crucial geothermometric data and tell us something about the physical state of the fluid, e.g. whether it was boiling at the time of entrapment.
The principal matter in most fluid inclusions is water. Second in abundance is carbon dioxide. The commonest inclusions in ore deposits fall into four groups.
1. Type 1: moderate salinity inclusions, are generally two phase consisting principally of water and a small bubble of water vapour which forms 10-40 % of the inclusion (see Fig.). The presence of the bubble indicates trapping at an elevated temperature with formation of the bubble on cooling. Heating on a microscope stage causes rehomogenization to one liquid phase and the homogenization temperature indicates the temperature of growth of that part of the containing crystal (provided the necessary pressure correction can be made). Sodium, potassium, calcium and chlorine occur in solution and salinities range from 0-23 wt % NaCl equivalent. In some of these inclusions small amounts of daughter salts have been precipitated during cooling. These include carbonates and anhydrite.
2. Type II: gas rich inclusions, generally contain more than 60 % vapour. Again they are dominantly aqueous but C02 may be present in small amounts. They appear to represent trapped steam. The simultaneous presence of gas-rich and gas-poor aqueous inclusions is good evidence that the fluids were boiling at the time of trapping.
3. Type III: halite-bearing inclusions, have salinities ranging, up to more than 50%. They contain well formed, cubic halite crystals and generally several other daughter minerals, particularly sylvite and anhydrite.
4. Type IV: C02-rich inclusions, have C02:H20 ratios ranging from 3 to over 30 mol %. They grade into type II inclusions and indeed there is a general gradation in many situations, e.g. porphyry copper deposits between the common types of fluid inclusions.
Perhaps one of the most surprising results of fluid inclusion studies is the evidence of the common occurrence of exceedingly strong brines in nature, brines more concentrated than any now found at the surface. These are not only present in mineral deposits but are common in igneous and metamorphic rocks. Many, but not all, strong brine inclusions are secondary features connected with late-stage magmatic and metamorphic phenomena such as the genesis of greisens, pegmatites and ore deposits as well as wall rock alteration processes such as sericitization and chloritization. They are compelling evidence that ore-forming fluids are hot, saline aqueous solutions. 'I'hey form a link between laboratory and field studies, and it should be noted that there is strong experimental and thermodynamic evidence which shows that chloride in hydrothermal solutions is a potent solvent for metals through the formation of metal-chloride complex ions, and indeed inclusions that carry more than one per cent of precipitated sulphides are known.
In recent years, probably more attention has been given to the study of fluid inclusions than to any other method of determining points on the geologic thermometer. The theory behind inclusion thermometry is simple. It is assumed that the partly filled vacuoles were completely filled with a single fluid phase when the mineral formed. If the inclusion is more than half liquid at room temperature, the ore fluid was hydrothermal, if more than half gas, the ore fluid was pneumatolytic. The temperature at which the subordinate phase disappears - the homogenization ternperature - marks a lower limit for the temperature of minera] genesis. When a specimen is heated, a liquid inclusion will expand until the liquid occupies the entire vacuole, and a gaseous inclusion will simply lose its liquid fraction. Further heating should cause the vacuole to burst. This theory is substantiated by the study of fluid inclusions in artificial crystals formed under controlled conditions.
Two methods are used to study homogenization temperatures of fluid inclusions. If the mineral is transparent, the specimen is heated on a rnicroscope stage, where the homogenization can be observed directly; in fact, if the vacuole has a regular shape, the ratio of liquid to gas can be estimated, allowing the temperature of homogenization to be calculated from thermodynamic considerations. The homogenization temperature should be considered a minimum temperature of formation for the mineral, provided no leakage exists. Indirectly, the temperature can be studied by heating the specimen until the vactioles burst, or decrepitate. A curve is obtained by plotting the successive decrepitation points of different vactioles. Theoretically, the mineral could not have formed at temperatures and pressures above the decrepitation conditions existing at the time of the experiment; hence a maximum temperature of formation is obtained. The decrepitation method is now largely discredited, since the values obtained reflect mineral strength, as well as other variables.
In spite of the wide use of fluid inclusions, the value and rcliability of this geothermometer are still debated. Decrepitation temperatures are significantly influenced by rock pressures; as a result it is necessary to estimate the depth of burial at the time the mineral was formed. Some fluid inclusions may be introduced into rocks and minerals after the minerals were deposited; hence they may be entirely unrelated to the processes that formed the minerals and may not give an index of the temperature or composition of the mineral-bearing fluid. It is argued that the contents of the vacuoles may leak out and hence give no accurate indication of the original constituents or of the original temperatures and pressures. If the fluids do leak out, it is also likely that they may leak in under other circumstances. Indeed it has been that water can diffuse into and out of the vacuoles. Supporters of fluid inclusion geothermometry acknowledge these criticisms but believe that at least in some of the harder, noncleavable minerals such as beryl, leakage is negligible, and that the liquid or gaseous inclusions furnish a fair sample of the original fluid. They also think that under favorable conditions the presence of leakage can be detected by statistical analyses, because the primary or undisturbed inclusions should give less erratic results than those that have leaked or received additions of fluid. The numerous criticisms leveled at the decrepitation method, or even of the use of vacuoles in any way, emphasize the dangers of assuming that all vacuoles contain original ore-bearing fluids. Nevertheless, fluid inclusion studies do give relatively accurate results for artificial minerals; furthermore, the possibility exists that secondary and primary fluid inclusions can be distinguished. These facts suggest that careful determinations may after all give meaningful results.
Assumptions and Limitations
One of the fundamental assumptions in using fluid inclusion data to study mineral genesis is that the inclusions have behaved as closed systems since their formation. It has been demonstrated that H diffusion into and out of fluid inclusions during metamorphism or laboratory heating could significantly modify the chemical compositions of the inclusions. Diffusion will be most rapid at high temperatures and when the hydrogen fugacity difference between the inclusion and its surroundings is large. Failure to recognize or expect diffusion problems could result in flawed reconstruction of the formation conditions of some minerals and ores.
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