Applications of mass spectrometry in economic geology and environmental geochemistry: Gas composition of inclusion fluid from ore deposits and sources of lead pollution in lake sediments.

Abstract

Quadrupole mass spectrometry was used to analyze the gas composition of small amounts of inclusion fluid (${<1}\times10\sp{-4}$g) from ore deposits. Methods were developed to evaluate factors that affect the gas composition including: decrepitation versus crushing to release fluid, calibration of the detector for mass fragmentation and relative sensitivity to different gases, adsorption, gas release from sources other than inclusion fluid, and generation of gases when inclusion fluid is released in vacuum. Gases from volcanic fumaroles are similar in composition to those of inclusion fluids from ore deposits (Sn-W, Cu-Mo and acid-sulfate) that formed near magma chambers. Gases from geothermal wells are more similar in composition to inclusion fluid from ore deposits (chimney-manto, epithermal vein and hot spring) farther from magma chambers. The source(s) of gas in inclusion fluid originate from processes including metamorphism, degassing of magma, equilibration of the atmosphere with meteoric water, and radioactive decay. Differences in gas composition of inclusion fluid from successive paragenetic stages in epithermal veins reflect the process(es) involved in mineral precipitation. Using gas concentrations of inclusion fluid to control metal speciation, computer simulations can replicate the ore grade and mineralogy found in epithermal veins in nature. The grade and type of ore precipitated during boiling and cooling simulations depends on the concentration of H$\sb2$S and the saturation state of Au in the fluid. Adding oxidized or CO$\sb2$-rich magmatic gases to a hydrothermal fluid saturated with precious metals will result in precipitation of ore-grade mineralization, whereas adding reduced or H$\sb2$S-rich magmatic gases will not. Anthropogenic lead from different sources can be identified through the use of lead isotope ratios in lake sediments from the Great Lakes region. Lead from natural sources dominated the lead flux to lake sediments until 1850. Anthropogenic sources of lead to lake sediment were dominated by processes associated with (1) deforestation from 1850 to 1900, (2) coal and ore use from 1900 to 1930, and (3) lead from gasoline combustion from 1930 to the mid-1970's. Discharges from municipal and industrial effluents are now the major sources of anthropogenic lead in lake sediment.