Clinopyroxene in mineralized skarns appears to place local chemical controls on ore mineral precipitation by entering into oxygen- and sulfur-buffering reactions in which garnet is an end product. Prior to investigating these reactions experimentally, the compositions of clinopyroxenes and garnets from ten polymetallic (Zn, Cu-Pb-Zn, Fe-Cu, Zn-Cu, and Fe) skarns in the southwestern United States, Japan, and Italy were analyzed with an electron microprobe. The clinopyroxenes are members of the hedenbergite (CaFeSi 2 O 6 )-diopside (CaMgSi 2 O 6 )-jo-hannsenite (CaMnSi 2 O 6 ) solid solution series with the hedenbergite component dominating and ranging from 36 to 95 mole percent, the diopside component from 0 to 41 mole percent, and the johannsenite component from 2 to 31 mole percent. The garnets are predominantly andradite (Ca 3 Fe 2 Si 3 O 12 ) with traces of Mn (0.3 to 2.6 wt %).The oxygen- and sulfur-buffering reactions involving pure hedenbergite in skarns have been studied at 800 degrees , 700 degrees , and 600 degrees C and P total = 2 kb, utilizing the hydrogen fugacity sensor technique of Chou and Eugster (1976). The terminal oxidation reaction for hedenbergite is: hedenbergite + O 2 = andradite + magnetite + quartz. The general equation for log f (sub O 2 ) for this reaction at T and P total is:log (f (sub O 2 ) ) (sub T.P) (+ or -0.12) = - 26,457/T + 12.12 + 0.087(P-1)/T.Experiments were also performed utilizing the same sensor technique to evaluate the effect of Mn on the hedenbergite oxygen-buffering reaction. The microprobe compositional analyses of naturally occurring skarn clinopyroxenes were used to constrain the starting material chemistry of the hedenbergitic phase. Addition of 15 mole percent johannsenite to the clinopyroxene raises the stability field of hedenbergite by about 1 log f (sub O 2 ) over the temperature range of interest.Sulfur fugacities for the analogous sulfur-buffering reaction, hedenbergite + S 2 = andradite + pyrite and/or pyrrhotite + quartz, were determined graphically from log f (sub S (sub -2) ) -1og f (sub O 2 ) diagrams utilizing the experimental f (sub O 2 ) -T data for hedenbergite. At P total = 2 kb, pure hedenbergitc can coexist stably with pyrite below 288 degrees + or - 16 degrees C and with pyrrhotite at higher temperatures. With decreasing temperature hedenbergite containing 15 mole percent johannsenite first enters the pyrite stability field at 587 degrees + or - 37 degrees C and P total = 2 kb.Results of this study support theoretical models proposed by Burt (1972a, b) for the stability of clinopyroxene in Ca-Fe-Si skarn deposits.

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