Gold in sea-floor polymetallic sulfide deposits

Univ. Toronto, Dep. Geol., Toronto, ON, Canada

Bulk chemical analyses of 48 samples of hydrothermal precipitates from seven polymetallic sulfide deposits at sea-floor spreading centers in the eastern Pacific reveal distinct patterns of gold enrichment. High gold contents are found in samples from two sulfide deposits: the Axial Seamount (45 degrees 57' N, 130 degrees 02' W), and the southern Explorer Ridge (49 degrees 45.6' N, 130 degrees 16.2' W). A large, 160-kg sample composed of silica, barite, and sphalerite from the top of the Axial Seamount deposit gives analyses up to 6,700 ppb Au, averaging 4,900 ppb Au. Similar material from the southern Explorer Ridge gives analyses up to 1,500 ppb Au, averaging 660 ppb Au. Detailed mineralogical studies together with bulk chemical analyses of a wide range of samples from these two sites reveal strong elemental associations and paragenetic controls on gold deposition. Gold at concentrations of about 200 ppb is associated with high Cu (>1 wt %) and Mo (up to 470 ppm); at concentrations > 800 ppb, with high Zn (>10 wt %), Ba (>3 wt %), and SiO ₂ (>20 wt %); and at concentrations > 1,200 ppb, with high Pb (>0.1 wt %), Ag (>100 ppm), As (>300 ppm), and Sb (50-100 ppm). Samples with the highest gold values from both locations contain late, sinterlike, low-temperature sulfosalts of Pb, As, Sb, Ag, and S in a matrix of amorphous silica. These sulfosalts are the probable repositories of the gold.Published analyses of Au-poor (<200 ppb Au) samples from other basalt-hosted sea-floor deposits (21 degrees N, Galapagos rift, southern Juan de Fuca Ridge, Endeavour Ridge) are high in metals and sulfur but low in silica relative to samples from the Axial Seamount and the southern Explorer Ridge. Hydrothermal deposits in the Guaymas basin are underlain by sediments that are enriched in gold relative to midocean ridge basalts, but samples from these deposits do not reflect this enrichment and give average analyses <200 ppb Au. It is suggested that gold is preconcentrated to about 200 ppb in high-temperature (>300 degrees C), Cu-Fe-rich sulfides and subsequently remobilized by late, sustained, low-temperature (<250 degrees C) fluids and concentrated in SiO ₂ -Ba-Zn-rich precipitates near the surface. Suitable fluid chemistry is required to mobilize the gold and a favorable precipitation mechanism is needed to concentrate it.These observations could explain the occurrence of high gold concentrations in selected ore types of some ancient massive sulfide deposits on land.

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