Abstract
Introduction
Geologic setting of the deposits
　Empire Mine (and Princess Mine)
　Monte Civillina
　Valle del Temperino
Description of samples
　Empire Mine (and Princess Mine)
　　Microscopic description
　　　Clinopyroxene
　　　Bustamite
　　　Rhodonite
　　　Andradite
　Monte Civillina
　　Microscopic description
　　　Clinopyroxene:
　　　Rhodonite
　Valle del Temperino
　　Microscopic description
　　　Clinopyroxene
　　　Bustamite
　　　Other phases
　Clinopyroxene-rhodonite assemblages
Mineral chemistry
　Clinopyroxenes
　Bustamite
　Rhodonite
　Andradite
Genetic considerations
Acknowledgments
References

Abstract

Samples from the Pb-Zn-Cu skarns of M. Civillina (Italy), Valle del Temperino (Italy), and Empire Mine (New Mexico, USA) have been analysed for their pyroxenes and pyroxenoids. The samples were collected immediately adjacent to the marble-skarn replacement front. All contain manganiferous pyroxenoids and manganeserich Ca-pyroxenes. The pyroxenes from each deposit form distinct groups of compositions within the diopside-hedenbergite-johannsenite triangle, with no apparent miscibility gap. Diopside contents usually are below 15 mole percent. Fibrous bustamite occurs as monomineralic zones in the Empire and in the Temperino deposit. Although rhodonite may be a primary phase in some samples from the Empire Mine, it is commonly of secondary origin in the Empire Mine and in the Civillina deposit. Its formation from manganiferous
clinopyroxenes is either due to increasing Mn activity in the hydrothermal skarn solution or to higher X(CO2) in the vapour phase. When rhodonite is formed within clinopyroxenes as submicroscopic lamellae that eventually replace the whole host crystal, resulting compositions lie in the miscibility gap between rhodonite and bustamite. Textural relations indicate the replacement reaction: johannsenite + CO2 = rhodonite + calcite + quartz. Equilibrium temperatures for this reaction have been calculated by using estimated thermochemical data for johannsenite, giving a T(eq)= 385℃ for X(CO2)= 0.1 at P(tot)= 1 kbar. Taking into consideration the reduced activity of Mn in rhodonite and of Ca in calcite, both buffered by
the johannsenite, the temperature is increased for about 15℃ at X(CO2)= 0.01. At lower temperatures, where johannsenite is stable, the X(CO2) is confined to values below 0.01. Despite the mineralogical similarities of the three deposits differences in the development of the manganiferous skarns can be depicted.

Fig. 8. T−XCa diagram of the system CaO-MnO-SiO2-CO2 for low X(CO2) and low pressures. Constructed from data by Maresch and Mottana (1976), Abrecht and Peters (1980), and Peters et al. (1980) Fig. 11. Possible phase relations in isobaric-isothermal sections in the system CaO - MnO - (Fe,Mg)O - SiO2-CO2 constructed from data by Abrecht (1980) and Brown et al. (1980). Circles represent compositions of samples from Empire Mine (E), Valle del Temperino (T), and M. Civillina (C). Black arrows indicate compositional trends suggested by analytical data and reactions inferred from textures. The white arrows refer to possible paths of skarn development at the temperatures T1, T2, and T3 taken from Fig. 8. These temperatures are only relative and demonstrate development within single deposits Abrecht.(1985)による『Manganiferous pyroxenes and pyroxenoids from three Pb-Zn-Cu skarn deposits』から