Mode of Occurrence of Skarns in the Goyomatsu Mine, Nara Prefecture

The Goyomatsu magnetite skarn deposit consists of large and small irregular massive bodies interlayered in upper Chichibu formations (limestone, sandstone, slate etc.), which have been metamorphosed by Miocene Omine acidic rocks (granodiorite).On the basis of mineralogic observations and thermodynamic data, the principal conditions of formation of the skarns have been determined. Results are as follows:1) Field evidence reveals that in the deposit hedenbergite is predominant in and near the granodiorite, and andradite is predominant in and near the limestone. This contrasting occurrence appears to show that the μO2 in the limestone gradually increased away from the granodiorite-limestone contact, assuming these skarns were formed at nearly the same stage.2) Generally, andradite rarely coexists with actinolite in skarns. This field evidence iswell in agreement with thermodynamic data, which shows that the andradite+actinolite phase cannot be a stable assemblage at constant temperature and pressure asseen in Fig. 3.3) Fig. 3 suggests that in general, under constant μCO2 conditions, the following mineral zonation formed with increasing μO2: zones of hedenbergite+actinolite, hedenbergite+andradite, and andradite, respectively. However primary assemblages of hedenbergite+actinolite havenot been observed. On this point, we need more detailed examinations.4) It is also found that in the skarns, hedenbergite has sometimes altered into aggregates of actinolite, calcite and quartz. Assuming constant temperature and pressure it can be interpreted that such alteration was caused by an increase in μCO2 at low μO2 environments.