Origin and evolution of life: clues from ore deposits

摘要

Submarine hydrothermal systems in submerged continental crust must have been common throughout earth history. But exhalative sedimentary (SEDEX) ZnS+PbS-FeS-FeCO_3-Fe_3O_4-FeS_2-BaSO_4 orebodies are rare and only occur in rocks < 2 Ga. Two fluids of contrasting chemistries are concerned; a metal-rich solution and an H_2S-bearing brine. A fault-controlled brine-bearing basin on the sea floor is fed with hundreds of trillions of litres of mineralising fluid that interacts with bacteriogenic sulphide. The ore accumulates on the basin floor. A halocline acts as a semipermeable boundary across which heat and the spent ore fluids disperse in a double-diffusive plume to the seawater sink. If the overlying seawater is oxidising a trail of Fe and Mn is left in the surrounding sediments. If not, these transition elements provide nutrient further afield. The Tynagh deposit, a partially exhalative ZnS+PbS-FeS_2-BaSO_4-Fe_3O_4-Fe_2O_3-Mn_3O_4 orebody, was particularly instructive. Hollow botryoids of iron sulphide found here inspired the idea that life emerged in containers composed, at least in part, of FeS precipitates at a submarine seep.