Minerals Report - International Seabed Authority

with a high concentration of gold was taken from the Lau basin. That sample
had about 28ppm of gold, or 28g of gold. He said that in general the
mineralogical composition of back-arc sulphides differed from mid-ocean
ridge sulphides because the rocks, which are leached at back-arc spreading
centres have a higher content of lead, arsenic and antimony. The sulphides
produced therefore included lead sulphides (galena), some sulphosalts,
arsenic sulphides, copper-arsenic-antimony sulphides and gold.
He pointed out the gold found in these deposits is primary gold,
hosted in iron-poor sphalerite. He also pointed out that the discovery of gold
in these deposits has created interest in the gold potential of seafloor massive
sulphide deposits in back-arc areas, resulting in the analysis of a number of
previously collected samples from other back- arc areas for their gold content.
With regard to gold grades from different spreading centres, Professor
Herzig said that at deposits located at mid-ocean ridges, based on the analysis
of 1,200 samples, the typical average grade of gold is about 1ppm. He noted
that this average is also typical of the so-called ophiolite-hosted massive
sulphides deposits on land. Such deposits he further noted were not typically
the deposits mined on land. The important types of massive sulphides for
land-based mining are those deposits that occur in felsic to intermediate rocks
in a back-arc environment. He said that this is also true for the seafloor where
back-arc deposits are the deposits of interest with respect to gold and base
metals. In this regard, based on fewer samples, he said that the average grade
of gold in back-arc deposits was 30g per tonne, with maximum gold values of
55g per tonne.
Professor Herzig informed participants about a very recent discovery
on the Conical Seamount. He said that this deposit is neither a mid-ocean
ridge deposit nor a back-arc deposit but a seamount in a fore-arc
environment. He also said that from analysing 40 samples taken from this
deposit, an average gold grade of 26g per tonne had been established for the
deposit. The maximum gold value found in samples taken from this deposit
is 230ppm gold or 230g per tonne.
At this point in his presentation, Professor Herzig embarked on the
subject of resource assessments of seafloor massive sulphides. In this regard,
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