On Feldspars in Pyrometasomatic Ore Deposits

Abstract

In many pyrometasomatic ore deposits, potash feldspar and plagioclase occur witlxskarn minerals. The mode of occurrence of these feldspars is classified into three types, i.e., (1) in banded skarn, (2) in breccia-skarn, and (3) along diopside veins. In general, they exist between biotite hornfels and clinopyroxene zones. From the macroscopic and microscopic observations, it is suggested that these feldspars are formed by the followingreaction :
K₂(Mg, Fe)_6-xAI_x(Si_6-xAI_2+x)O₂₀(OH)₄+6CaCO₃+(12+x)SiO₂=2KAISi₃O₈+xCaAI₂Si₂O₈+(6-x)Ca(Mg, Fe)Si₂O₆+6CO₂+2H₂O
On some assumpsions, the phase assemblage of the right side of the reaction is presumed to be stable at temperatures above 500-600°C and at pressures of 500-2, 000 atm. This condition corresponds to the low-temperature part of the pyroxene-hornfels facies.