The Precambrian iron formation near Atlantic City, Wyoming, lies within a large southwestward-plunging syncline of metasedimentary and metavolcanic rocks. The ore is at least 2,400 million years old, and may have been deposited more than 2,680 million years ago, in which case it is roughly equivalent in age to the older iron formation deposits of the Canadian Shield. The geology of the Atlantic City deposit is similar to that of Precambrian iron formations of the Shield. Magnetite is the chief ore mineral at Atlantic City, and the gangue consists of quartz, actinolite and actinolitic hornblende.Chemical analyses of drill core material average in weight percent: total iron, 33.45; Fe 2 O 3 , 30.75; FeO, 15.49; Al 2 O 3 , 3.85; CaO, 1.31; MgO, 2.80; MnO, 0.09; Na 2 O, 1.22; K 2 O, 0.36; and SiO 2 , 44.13. The values are similar to published averages for Precambrian iron formations from the Canadian Shield.Comparisons of Ca/Mg, Ca/Na, Ca/K, Mg/Na, Mg/K and Na/K ratios from the Atlantic City deposit with various present-day geologic environments and with average crustal percentages has provided chemical information which may help in establishing a mode of origin for the Atlantic City iron formation. Analyses of fumarolic gases, of certain acidic volcanic waters, and of Dead Sea water samples are shown to be statistically similar to analyses of the Atlantic City iron formation. Close similarities in terms of these ratios with average crustal percentages also add significantly to an understanding of the possible genesis of the deposit.It is suggested that the iron formation at Atlantic City, Wyoming, may have been derived through subaerial weathering of moderately low-lying crustal rocks in an atmosphere low in oxygen, and that the leachate was then carried to and concentrated in a closed basin, where it may have received additional contributions from volcanic emanations. The question of whether the basin of deposition was marine or continental remains unanswered.