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Abstract

Scheelite mineralization at Sangdong is probably related to a late
Cretaceous episode of igneous activity following the Daebo Orogeny in the
Korean peninsula. It occurs. in stratabound skarns that replace Cambrian
limestones of the Myobong Slate or the overlying Pungchon Limestone, and
also in a number of associated quartz veins.
These sediments in the Sangdong area overlie the Jangsan Quartzite and
Precambrian schists and lie in the southern limb of the Hambaeg Syncline,
striking N 70-80°E and nipping 25-35°NW. The M1(6 m thick) and F.(0.3 to
0.8 m thick) orebodies are stratiform, extend about 1.2 km along strike
and down dip, and grade laterally to interbedded limestones in the Myobong
Formation. H1 orebody has an irregular shape, bounded on the bottom by
the Myobong slate and on top by Pungchon limestone, with thickness varying
from 10 to 100 m. The M1 and F. orebodies show a generalized zonal distri
bution
of the major component minerals: a central quartz-mica zone is
surrounded by an amphibole-rich zone, which in turn is enveloped by a
pyroxene-garnet zone. Scheelite is highly concentrated in the quartz-mica
zone where the grades-average 6 % WO3. No zonation and little mica is
observed in the H1 orebody.
Relics of pyroxene-garnet skarn in the quartz-mica zone, relict blocks
of limestone in the pyroxene-garnet skarn zone, and small-scale veins and
fractures with quartz-mica skarns rimmed by amphibole and the pyroxene
garnet skarn, demonstrate the sequential metasomatic replacement of lime
stone. Early garnet-pyroxene±wollastonite skarns are replaced by late
pyroxene-garnet skarns, then amphibole skarn, and finally, mica skarn.
Geochemical analyses, fluid inclusion and stable isotope data assist in defining
fluid composition and source, and P - T conditions. δ180, δ34s
and δ13c of ore minerals indicate a derivation from magmatic fluids, and
the high temperatures indicated by the mineralogy and fluid inclusions (up to 600c)
strongly suggest a magmatic heat source. Though drilling
has not encountered
granite to a depth of 500 m below the mine, a
granitoid pluton is inferred within
a kilometre of the deposit.
Tectonic fracturing may have caused a change from lithostatic to
hydrostatic pressure(from 800 to 300-400 bars), while precipitating
pyroxene-garnet at 350-500°0, and the fluid.boiled. Later in pyroxene
garnet growth, lithostatic pressure was restored, possibly because of
fracture sealing by mineral precipitation. In the M1 and F. ore zones,
hydrous skarns formed mainly during the non-boiling phase, growing outwards
from a central feeder zone marked by a concentration of footwall veins.
The skarn assemblages appear to approach equilibrium with the hydrothermal
fluid, justifying the use of phase equilibria. Fluid inclusion evidence
indicates that the major solution species were NaCl, KCl, CaC12 and MgCl,
with an average salinity of about 1 m NaCl. There was also localized
development of co2-rich and more saline fluids.
The f02- fs2 conditions were close to the pyrrhotite-magnetite-pyrite point with
f02 increasing slightly_ towards the central core, and 5s::s = 2.5 x 10-3 m. Part of
the mica zone is muscovite-rich and probably lower
temperature( 350°c); the pH
range estimated for this assemblage, assuming mK+=0.1, is between 4.2 and 5.9.
Under these conditions, tungsten was transported mainly as HW0-4 and at pH = 4,
the maximum aCa++ in he fluid to maintain 2 ppm W in solution in equilibrium with
scheelite is 10-7. at 350°0.
Double diffusive mechanisms may have maintained steep temperature and
chemical gradients at the margins of the skarn zones, with the central,
hot, dilute magmatic plume in contact with an outer envelope of hot, saline
groundwater. Local and occasional mixing, or influx, of groundwater may
have given rise to the saline fluids identified locally in fluid inclusions.
The quartz veins appear to have formed in similar P-T-X conditions,
with wolframite- and molybdenite-bearing veins forming at higher
temperatures tnan sulphide-bearing veins.
Late fluid circulation involved formation of calcite and local hematite, with
δ13ctvalues indicating non-magmatic fluid.

Item Type:	Thesis - PhD
Authors/Creators:	Moon, Kun Joo
Keywords:	Scheelite, Tungsten ores
Copyright Holders:	The Author
Copyright Information:	Copyright 1983 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Bibliography: l. 276-284. Thesis (Ph.D)--University of Tasmania, 1983
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