B. Singoyi et K. Zaw, A petrological and fluid inclusion study of magnetite-scheelite skarn mineralization at Kara, Northwestern Tasmania: implications for ore genesis, CHEM GEOL, 173(1-3), 2001, pp. 239-253
The Kara magnetite-scheelite deposit is located about 40 km south of Burnie
in northwestern Tasmania. The deposit consists of a number of orebodies an
d the total mineable reserves are 1.7 Mt of magnetite at > 30% Fe and 0.3 M
t of scheelite at 0.52% WO3. The major orebodies at Kara are hosted by the
Ordovician Gordon Limestone at the southern end of the Devonian Housetop Gr
anite, adjacent to the granite or separated from it by the Ordovician Moina
Sandstone. At least four paragenetic stages of skarn formation and ore dep
osition have been recognized:
Stage I Clinopyroxene +/- garnet +/- vesuvianite +/- wollastonite +/- quart
z +/- scheelite,
Stage II Garnet-vesuvianite-magnetite +/- scheelite +/- apatite +/- quartz,
Stage III Magnetite-amphibole-epidote-fluorite-quatz +/- chlorite +/- garne
t +/- vesuvianite +/- scheelite +/- carbonate +/- pyrite +/- clinopyroxene,
and
Stage IV Hematite +/- fluorite st calcite +/- quartz.
Fluid inclusions in clinopyroxene from Stage I homogenize at 460-620 degree
sC (mode 520 degreesC). Stage II gave homogenization temperatures of 349-57
8 degreesC (mode 500 degreesC) from scheelite and 360-570 degreesC (mode 50
0 degreesC) from vesuvianite. The Stage I clinopyroxene gave a salinity ran
ge of 10.0-12.0 equiv, wt.% NaCl with only one low value of 2.0 equiv. wt.%
NaCl. The Stage II mineral assemblages also showed moderate salinities of
12.0 and 17.8 equiv. wt.% NaCl from scheelite and vesuvianite. Fluid inclus
ions in quartz, scheelite, calcite and fluorite from the main scheelite min
eralization phase, Stage III, homogenize at 230-360 degreesC with a mode at
300 degreesC and salinity varies from 0.2 to 19.8 equiv. wt.% NaCl.
The mineral paragenetic and fluid inclusion studies indicate that magnetite
-scheelite mineralization at Kara was formed as a proximal skarn assemblage
in carbonate host. The skarn formation and ore deposition occurred in stag
es starting with prograde anhydrous clinopyroxene-garnet metasomatism at re
latively high temperatures, > 500 degreesC. This early assemblage was repla
ced by the subsequent retrograde deposition of hydrous mineral phases (e.g.
amphibole, epidote and chlorite) as temperatures decreased to 300 degreesC
or less.
Deposition of scheelite predominantly occurred in association with hydrous
minerals (e.g. amphibole in Stage III). The high-temperature and moderate-t
o-high-salinity ore fluids in early anhydrous skarn formation (Stages I and
II) are consistent with fluids of magmatic origin. In comparison, the lowe
r temperature and variable salinity from high (19.8 equiv. wt.% NaCl) to as
low as 0.2 equiv. wt.% NaCl in Stage III suggests a possible involvement o
f low-salinity meteoric waters mixing with high-salinity fluids originating
from the nearby granite in the later hydrous skarn and ore formation. (C)
2001 Elsevier Science B.V. All rights reserved.