Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 96

Six large SEDEX deposits are located in two Proterozoic basins in north-
ern Australia. They are very large (14-150 mt) and contain very high grades
(average 16% Pb + Zn with significant Ag contents). The Mt Isa deposit was
discovered in 1923 and mining started 8 years later leading to a major town in
a remote semi-arid part of Australia. The HYC deposit was discovered
30 years later, in 1955, in the more northerly McArthur basin. The name
HYC comes from a remark made at the time of discovery: realizing that they
had found a major deposit, one geologist turned to the other and said “you
have always wanted to name a mine; Here's Your Chance”.
The deposits are stratiform and located in intracontinental rifted basins
commonly adjacent to major syn-sedimentary faults. The host rocks are
dolomitic siltstones and shelf carbonates and the ores are finely laminated,
as shown in photos (see also Fig. 1.6). The ore deposits all formed within
a relatively short period between 1650 and 1575 Ma.
Opinions differ about the origin of the deposits. The fine banding is very
similar to sedimentary bedding and this, together with other features of the
ores and their geological setting led Stanton, an influential Australian geolo-
gist, to propose in the 1960s an “exhalative-sedimentary” origin, challenging
the prevailing view that such deposits were epigenetic and related to granitic
intrusions. Other geologists question this interpretation and argue that the
textures, mineralogy and chemical compositions of the ores point to their
having formed through replacement of pyritic sediments by metals preci-
pitated from hydrothermal fluids. There is agreement, however, that the
two processes - sedimentation from exhalative fluids and diagenetic replace-
ment - took place at the sea floor or at shallow levels in the sediment pile.
Canada, are Proteroic but others, including major deposits like Red Dog in Alaska
and Mehdiabad in Iran, as well as geologically interesting examples like the Irish
deposits and Meggen in Germany, are Paleozoic.
A characteristic feature of the deposits is the fine grainsize of the original ore
minerals. Compare, for example, the three Australia deposits listed in the Table: in
the Broken Hill orebody the grains are coarse, up to centimeter sized, in the Mt Isa
deposit the average grainsize is 100-500 m m, and in the McArthur River deposit, it
is less than 10 m m. The difference in grainsize is due largely to the degree and grade
of metamorphism that affected the deposits after their initial formation. The coarse
grains of the Broken Hill deposit result from recrystallization during the high-grade,
granulite-facies metamorphism that affected this deposit; the finer grains of the
Mt Isa deposit are influenced by the sub-greenschist metamorphism of this region;
and the minute grains of the McArthur River deposit probably are those of the
original sedimentary ore minerals (This difference in grainsize strongly influences
the viability of the three deposits. As mentioned in Chap. 2, the coarse Broken Hill
ores are easily mined and refined, in contrast to the ultra-fine McArthur River ores,
which for many years were unable to be exploited).