Page 118 - Handbook of Gold Exploration and Evaluation

98 Handbook of gold exploration and evaluation

Discoveries of evolving gold-bearing polymetallic sulphide deposits in
marine settings, made initially as a result of submarine investigations at mid-
ocean ridges, have similarly been extended to other parts of the marine
environment including volcanic and tectonic settings in shallow marine, island
arc environments. Smaller marginal basins developed between island arc and
nearby continental masses are somewhat analogous to the larger oceans in
having spreading axes which involve separation of the lithosphere and upwelling
of magma from the mantle to form new oceanic lithosphere. Differences in
chemical composition of seafloor metal-rich sulphides and their precious metal
contents are shown in Table 2.1 in relation to samples derived from a range of
mid-ocean ridges and back-arc spreading centres.
A polymetallic sulphide ore body located in the Atlantis Deep, due west of
Mecca in the Red Sea, represents the style of mineralisation that may occur at an
early stage of seafloor spreading during the opening of an ocean basin (Fig.
2.18). The metalliferous muds of this deposit are estimated to contain 30 million
tonnes of iron, zinc and copper, some 6,000 tonnes of silver and 50 tonnes of
gold at an average grade of about 2 ppb Au (Rona, 1986). The Red Sea deposit

Table 2.1 Bulk chemical composition of seafloor polymetallic sulphides from mid-
ocean ridges and back-arc spreading centres

Mid-ocean ridges Back-arc ridges

Volcanic- Sediment- Intraoceanic 3 Intracontinental 4
hosted 1 hosted 2

N 890 57 317 28
Fe (wt%) 23.6 24.0 13.3 7.0
Zn 11.7 4.7 15.1 18.4
Cu 4.3 1.3 5.1 2.0
Pb 0.2 1.1 1.2 11.5
As 0.03 0.3 0.1 1.5
Sb 0.01 0.06 0.01 0.3
Ba 1.7 7.0 13.0 7.2
Ag (ppm) 143 142 195 2766
Au 1.2 0.8 2.9 3.8
1
Explorer Ridge, Endeavour Ridge: MainVent and High Rise Fields, Axial Seamount: AHSES and
CASAM, Cleft Segment: N and S Fields, East Pacific Rise: 11ëN, 13ëN, 21ëN, 7ë30 S. 16ë45 S.
0
0
0
18ë30 S. 21ëS. Galapagos Rift,TAG: Active Mound, Mir and Alvin Zones, Snake Pit Mid-Atlantic
Ridge 24.5ëN.
2
EscanabaTrough, Guaymas Basin.
3
MarianaTrough, Manus Basin, North Fiji Basin: KingsTriple Junction,White Church,Vai Lili, Hine
Hina Fields.
4
OkinawaTrough,
Data compiledby Geological Surveyof Canada and Freiberg Universityof MiningandTechnology,
Germany.