The f O2 -T and f S2 -T stability relations of hedenbergite and of hedenbergite-johannsenite solid solutions

Abstract

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) skarp. s in the southwestern United States, Japan, and Italy were analyzed with an electron microprobe. The clinopyroxenes are members of the hedenbergite (CaFeSi206)-diopside (CaMgSi206)-johannsenite (CaMnSi206) 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 (CaaFe2SiaO•2) 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 ø, 700 ø, and 600øC and Ptotal = 2 kb, utilizing the hydrogen fugacity sensor technique of Chou and Eugster (1976). The terminal oxidation reaction for hedenbergite is: hedenbergite + O2 = andradite + magnetite + quartz. The general equation for log fo2 for this reaction at T and Ptotal is: 26,457 0.087(P-1) log 0Coz)T.e(-4-0.12) -4-12.12 + T 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 fo• over the temperature range of interest.Sulfur fugacities for the analogous sulfur-buffering reaction, hedenbergite + Sa = andradite + pyrite and/or pyrrhotite + quartz, were determined graphically from log fs2-1og fo• diagrams utilizing the experimental fo•-T data for hedenbergite. At Ptotal = 2 kb, pure hedenbergitc can coexist stably with pyrite below 288 ø + 16øC and with pyrrhotite at higher temperatures. With decreasing temperature hedenbergite containing 15 mole percent johannsenite first enters the pyrite stability field at 587 ø + 87øC and Ptotal = 2 kb.Results of this study support theoretical models proposed by Burr (1972a, b) for the stability of clinopyroxene in Ca-Fe-Si skarn deposits.

Chemical compositions of garnet and clinopyroxene and their genetic significances in Yemaquan skarn iron–copper–zinc deposit, Qimantagh, eastern Kunlun

“…This could be explained by the replacement of diopside-hedenbergite by pyrite, pyrrhotite, magnetite, and andradite, which show the change in the system leading to garnet, magnetite and iron sulfides deposition (Fig. 6g, h, and i) according to reactions (1) and (2) (Burton et al, 1982): (2006), the (Fe, Mn)-Mg interdiffusion coefficient is strongly fO 2 sensitive. More oxidizing conditions would favor the formation of andradite and diopsidic clinopyroxene relative to hedenbergite (Gamble, 1982;Gustafson, 1974).…”

“…Due to the lack of more complete experimental data on clinopyroxene solid solutions, it is not possible to offer the stability fields of type II (Di 62 Hd 36 Joh 2 ) and type III (Di 24 Hd 54 Joh 22 ) clinopyroxenes. However, as pointed out by Gamble (1982) and Burton (1982), manganese is far more effective than magnesium at expanding the clinopyroxene stability field. Gamble (1982) showed that at 2 kb, Di 75 Hd 25 coexisting with 4 6 2 3 7 3 5 1 Type II II I II II II II II II Note: And-andradite, Gro-grossular, Alm-almandine, Pyr-pyrope, Spe-spessartine.…”

Characterization of synthetic hedenbergite (CaFeSi 2 O 6 )?petedunnite (CaZnSi 2 O 6 ) solid solution series by X-ray powder diffraction and 57 Fe M�ssbauer spectroscopy

“…Hydrothermal synthesis of hedenbergite was performed by Nolan (1969), Rutstein and Yund (1969), Turnock et al (1973), Gustafson (1974), Kinrade et al (1975), Burton et al (1982), Haselton et al (1987), Moecher and Chou (1990), Raudsepp et al (1990), Perkins and Vielzeuf (1992), Kawasaki and Ito (1994), Zhang et al (1997) and Redhammer et al (2000). The stability of hedenbergite at 0.2 GPa as a function of temperature and oxygen fugacity was determined by Gustafson (1974) and Burton et al (1982). Naturally occurring petedunnite was first described by Essene and Peacor (1987) from Zn skarns in Franklin, New Jersey.…”

Fluctuation model for compositional heterogeneity in skarn clinopyroxenes.

“…Einaudi and Burt, 1982;Einaudi, 1977). Compositional variations within an in dividual grain may even occur, although zona tion patterns within single grains are not always systematic (Burton et al, 1982;Nakano, 1982;Meinert, 1987 Samples analyzed in this study were collected from seven Japanese metalliferous skarn deposits. Most of these were clinopyroxene skarn with small amounts of secondary am phibole replacing clinopyroxene and trace amounts of interstitial calcite and/or quartz.…”