Might you take a quick crack at explaining the chemical complementarity between sulfate and sulfide to someone who had 2 semesters of Inorganic and 2 semesters of Organic about 25 years ago? To others: this relates to Zuben's excellent LINK above I know what these definitions mean, but can't see how the creation of one would aid the other: sul·fate (sŭl'fāt') pronunciation n. A chemical compound containing the bivalent group SO4. sul·fide (sŭl'fīd') pronunciation n. A compound of bivalent sulfur with an electropositive element or group, especially a binary compound of sulfur with a metal.
I'm a Chem Engr not a chemist so I'm more a plumber than a molecule man.
However, a basic explanation to your inquiry above can be found here. Follow the hot links within the definition to learn more specific information.
The cold seep in your post above refers to the ambient temperature of the deep portions of the ocean that surround the very small hot spots created by the vents. If I recall correctly, the temperature of the seawater is about -30F but the pressure from the weight of the seawater at that depth causes the freezing temperature down, so liquidity persists.