Experimental determination of the ZnFe 2 O 4 -ZnAl 2 O 4 miscibility gap with application to franklinite-gahnite exsolution intergrowths from the Sterling Hill zinc deposit, New Jersey

Abstract

The Sterling Hill zinc deposit is a regionally metamorphosed stratiform oxide-silicate deposit enclosed in the Precambrian Franklin marble. Spinels, which consist of oriented exsolution intergrowths of gahnite (G) (ideally ZnAl 2 O 4 ) and franklinite (F) (ideally ZnFe 2 O 4 ), were collected at four locations in the Sterling Hill mine. The bulk chemical composition of these spinels and the composition of the phases which constitute the intergrowths were determined by broad beam and point analysis, respectively, with the electron microprobe. These data show that the original homogeneous high-temperature spinels were as aluminous as F 80 G 20 .The miscibility gap in the system ZnFe 2 O 4 -ZnAl 2 O 4 was determined experimentally by hydrothermal methods. Homogeneous synthetic spinels on the join ZnFe 2 O 4 -ZnAl 2 O 4 were prepared by dry reaction of the oxides at 1,380 degrees + or - 50 degrees C. Equimolar mechanical mixtures of two synthetic homogeneous spinels with the compositions F 90 G 10 and F 10 G 90 were sealed with water in separate platinum capsules. These, in turn, were sealed in gold capsules and subjected to P (sub H 2 O) = 1 kbar between 700 degrees and 900 degrees C for a maximum period of 72 and 11 days, respectively. The oxygen fugacity was controlled by the hematite-magnetite buffer (hematite packed between the gold and platinum capsules). Equilibrium was approached from two directions, namely, by exsolution of synthetic homogeneous spinels and by reaction of the spinels in the mechanical mixtures. An excellent fit with the experimental data was obtained by solution modeling (asymmetric Margules model) which gives a miscibility gap with a calculated consolute temperature of 957.4 degrees C at X gahnite = 0.525.Integration of the electron microprobe analyses of the natural intergrowths with the experimentally determined miscibility gap indicates that the minimum peak temperature attained during regional metamorphism at the Sterling Hill mine was 760 degrees C.