Page 119 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 99

2.18 Distribution of metalliferous sediment and brine pools in the Red Sea at
the early stage of seafloor spreading (derived from Mitchell and Garson, 1976).

ranks with the larger ancient orebodies found on land and is typical of an ore
formed on the seabed, which may eventually be recycled in a subduction zone or
obducted to form accreted terrain.
Potential source rocks on the seafloor range from mid-ocean ridge type
basalts and clastic sediments to lavas of intermediate composition (basaltic-
andesites, andesites) in intraoceanic back arcs. Felsic volcanics (dacite) are
typical for young intracontinental back-arc rifts. Samples from white-smoker
chimneys in the Lau back arc contain the first examples of visible primary gold
(up to 18 ppm) discovered in polymetallic sulphides at active vents. Other finds
include a deposit with epithermal characteristics in the crater of a submarine
volcano 2,200 m below sea level in the Woodlark Basin, where seafloor spread-
ing interacts with the continental crust of Papua New Guinea. Gold-rich (up to
24 ppm Au) polymetallic concentrations with veins averaging up to 3.8 ppm Au
have also been identified in rifted continental crust resembling Kuroko type