| Abstract: | Abstract The King Island Scheelite Mine lies in the contact aureole of a granodiorite stock. Its open cut and numerous drill cores expose a contact metamorphosed and metasomatized series of finely interbedded argillaceous and calcareous sediments, with interleaved flows of picrite-basalt and basic pyroclastics, the scheelite ore being limited to two limestone horizons. The range and gradation in composition of the original rocks has resulted in an unusual variety of meta-morphic rocks, including forsterite-phlogopite-spinel-tremolite hornfels, anthophyllite-cordierite hornfels, biotite hornfels, actinolite hornfels, a variety of calc-flinta, and marbles. The original sedimentation gave rise to a rapid alternation of limestone and shale, many times repeated, and during metamorphism these rocks reacted with each other to produce narrow bands of calc-flinta. Subsequent pyrometasomatism selectively converted the greater part of the marble beds to scheelite-bearing andradite skarn, leaving the various hornfels and calc-flinta very little affected. The replacement of the marble was a volume for volume process, and the conversion of 1,000,000 tons of marble to average grade ore involved the introduction of about 350,000 tons of SiO2, 250,000 tons of Fe2O3, 55,000 tons AI2O3, 30,000 tons of H2O and 82,500 tons of CaO. The temperature of the contact metamorphism attained over 500° C, and the rocks cooled to about 400° C. before the pyrometasomatism occurred. The rocks giving rise to the various hornfels underwent varying degrees of contraction during metamorphism, whereas the limestones probably expanded during metamorphism, and became more permeable to solutions. |
