Understanding Our Planet Through Chemistry
A great many ore-deposit models are tied to the cause of formation of the deposit. Questions about the environmental conditions related to formation of the deposit are temperature, pressure, source of the metals, and composition of any fluids and gases that transported and formed the ore or associated minerals.
Many crystals in the Earth's crust have formed in some kind of fluid. Small quantities of the fluid that surrounded the crystals during growth are commonly trapped as tiny fluid inclusions within these crystals. In many cases, these fluid inclusions are less than 0.1 mm but record important information about the conditions when the ore was being formed.
Current understanding of movements within continents reveals that throughout the Earth's history periods of large-scale fluid movements occurred in the Earth's crust. Some of these fluid migrations resulted in the deposition of metallic ore deposits and accumulations of oil and gas.
Characteristics of fluid inclusions are extremely variable. In the simplest case, when fluid inclusions cool from the elevated temperature at which they formed, the liquid shrinks and separates into a liquid and a vapor bubble. Detailed microthermometric studies give a reasonable estimate of the temperature at which the mineral was formed. Studies of this type reveal that the inclusions were trapped at temperatures from less than 50°C to over 600°C and at pressures equivalent to what is experienced at the Earth's surface and ranging to what would be found several kilometers deep.
Because of the extremely small size of so many fluid inclusions, determining the composition of the trapped fluids is difficult. First, the total amount of dissolved solids is determined by observing with a microscope the freezing/melting points of the inclusions. The sample is then crushed and rinsed with water. This water is recovered and analyzed by using a sensitive analytical technique to determine the ratios of the elements contributed by the trapped fluid. These ratios are used to calculate the composition of the fluid. The compositions range from aqueous solutions with salt content similar to rainwater to fluids with dissolved solid concentrations of over 60 percent nearly 20 times the amount found in seawater.
Analytical data on fluid inclusions are needed to understand the chemical and physical processes involved in the formation of economic mineral deposits. These data are also critical in understanding modern mineral-deposit models, which promote cost-effective mineral exploration vital to our healthy industrial economy.
Most fluid inclusions contain dissolved gases, and in some environments the inclusions consist entirely of gases. Recently, the USGS has designed a gas quadrupole mass spectrometer (QMS) that will analyze the amounts and chemical identity of gas ions in small gas samples (click here to review details from Chapter IIa on QMS instruments). This instrument is extremely sensitive (8 parts per billion detection) and capable of millisecond speeds of analysis important for gas bubbles as small as 1/100 of a millimeter in diameter.
The QMS is used extensively to study ore- deposit models as well as environmental and geologic hazards. Examples include: identifying carbon dioxide as the responsible gas at the Lake Nyos, Cameroon disaster where 2,000 people suffocated in 1986; tracking atmospheric gases from bubbles in climate- study ice cores of Greenland and Antarctica; tracing dispersal of smokestack emissions and gases of geothermal energy wells and springs.