Laboratory and field data provide the basis for a chart showing the stability fields of calcite and dolomite in dilute solutions as a function of temperature and Ca/Mg molar ratios. In some areas the Ca/Mg ratios of present ground water are such that they would convert limestone to dolomite if they were heated, as by shallow intrusives. The early pre-ore age of most hydrothermal dolomite is attributed to the heat accompanying the initial rise of magma. Observed differences in amount of dolomite, despite similarity of ores, is readily explained if ground water, rather than magmatic fluid, is the source of Mg.

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