ORIGIN OF HYDROTHERMAL AND LOW TEMPERATURE DOLOMITE

Laboratory data and field evidence provide compositions of water that are in equilibrium with dolomite and calcite from temperatures of about 20° C to 400° C. From these data it is possible to construct a chart showing the stability fields of calcite and dolomite in dilute solutions as a function of temperature and Ca/Mg molar ratios; this chart indicates that most non-saline groundwaters have insufficient magnesium in them to allow the formation of dolomite unless they are abnormally hot, or have passed through a magnesium silicate environment. Nevertheless, in some areas present ground-waters contain substantial amounts of magnesium and the Ca/Mg molar ratios are such as to indicate that limestone would be converted into dolomite at higher temperatures. If such areas were above or near intrusive centers or shallow hearths where magma laid quiescent for some time, the rocks and their contained waters should have heated up as the temperature gradient between surface and hot intrusion became stabilized, and a hyperthermal area such as we find in many parts of the world today would result. A convective circulation would start in permeable rocks and this would provide large volumes of magnesia for conversion of limestone to dolomite, where the ratio of calcium to magnesia was appropriate. Age relations of hydrothermal dolomite in various Cordilleran districts show that the dolomitic alteration in nearly all is quite early, and this would suggest that it formed in response to the initial rise of magma in the district well before ores as such were deposited at the horizon of the dolomite. Most of the puzzling features of the hydrothermal dolomite associated with ore deposits are readily explained if the source of the magnesia is primarily groundwater, the composition of which varies from district to district. The difficulty of providing an adequate source of magnesia from magmatic differentiation in rocks that contained very little magnesia is avoided. It also explains the occurrence of hydrothermal dolomite in areas remote from ore deposits but in which there are deep reaching fractures or other permeable channels that could conduct hot groundwater from depth. © 1969 Society of Economic Geologists, Inc.