Abstract

Contact-metamorphosed carbonate rocks occur near a stock of quartz monzonite at Johnson Camp, in Chochise County, Arizona. The metamorphosed carbonate rocks show successive zones characterized by (1) chlorite (locally talc), (2) tremolite, (3) forsterite and diopside, and (4) garnet (with local wollastonite and idocrase), as the stock is approached. The mineralogical and textural changes indicate an origin of the silicate minerals by a fixed sequence of chemical reactions between original constituents of the sedimentary rock, with a negligible amount of material added from an outside source but with considerable interchange of material between beds. Silicate minerals generally did not form in pure carbonate rocks in any zone. The distinctive silicates of the outer three zones formed in abundance only in impure dolomites. Garnet, wollastonite, and idocrase of zone 4 formed only in impure limestones; the mineral assemblages in the once dolomitic rocks remained the same in zone 4 as in zone 3.

The chemical reactions that can reasonably be inferred release carbon dioxide and result in the formation of denser minerals, suggesting a decrease in volume, which is confirmed by stratigraphic measurements showing that the silicated facies is as much as 30 per cent thinner than the unsilicated facies. Structural features resulting from the loss in volume are very obscure because of greater deformation not due to the metamorphism and because of recrystallization during the metamorphic process. The losses in volume must be taken into account in comparing chemical analyses of the unmetamorphosed and metamorphosed facies to determine the gains and losses of constituents.

Metasomatism, which included the formation of copper and zinc replacement deposits that have yielded about $20,000,000 to date, started near the temperature maximum and increased as the temperature fell.

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