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Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4
Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4
Geological Survey of Canada, 3303 – 33rd St. NW, Calgary, Alberta, Canada T2L 2A7
Corresponding author: e-mail, atomkins{at}ucalgary.ca
The timing of mineralization at the Hemlo gold deposit has
long been a controversial issue. We present arguments that use partial melting
of the ore assemblage to explain the mechanism of ore mobilization and resolve
problems with the model of premetamorphic mineralization. The results have
implications for mine exploitation and exploration strategies. We show that the
ore mineral assemblage underwent partial melting during middle-amphibolite
facies metamorphism (600°–650°C, 6–7 kbars), primarily through breakdown
of stibnite and arsenopyrite in a high-
S2
environment. Concurrent deformation led to segregation of the resulting Sb- and
As-rich sulfosalt melt. Interaction between this melt and a range of unmelted
sulfides led to further melting and incorporation of other elements into the
melt. The gold-bearing melt was mobilized from compressional high-strain sites
into dilational domains such as boudin necks and extensional fractures developed
in competent lithologies. Ore minerals that did not melt significantly (pyrite,
molybdenite, pyrrhotite, and sphalerite) were not extensively mobilized and
largely remained within high-strain compressional domains. This segregation of
melt from residue thus resulted in the observed heterogeneous distribution of
ore minerals within the deposit. Crystallization of the sulfosalt melt produced
a diverse suite of ore minerals, dominated by stibnite and realgar and
containing an array of rare sulfosalts, native elements, intermetallic
compounds, and tellurides. Some sulfosalt melt persisted to low temperature
(<300°C), allowing continued small-scale, localized mobilization during late
deformation. Although gold occurs at moderate concentrations within the
compressional high-strain domains, it is particularly concentrated in the
dilational domains, a consequence of its mobilization within a sulfosalt melt.
Our model of partial melting of the ore mineral assemblage with consequent
mobilization explains how ore minerals that are unstable even at greenschist
facies conditions came to be hosted in structures that formed at or near the
peak of amphibolite facies metamorphism.
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