In Volcanogenic Massive Sulfide (VMS) deposits, the sulfides are deposited in the upper part of the hot spring, as the chimneys themselves ("mound"). Part of these systems are rocks termed "exalites": deposits around the mound from the cloud of hot water leaving the vent. These exalites have facies, from proximal to distal, that display a wide range of compositions. These deposits can be used as a vector towards the sulfide deposits in the proximal facies. Typically, these deposits are largely silica, either as iron-bearing cherts, various colored jaspers, clean quartz, and banded iron formation. They can also contain gold and other economic resources. In deposits that have been metamorposed, the exhalite deposits also include "dirty" marbles: calcite with lots of Ca-Na-bearing amphiboles, other Ca-Na-minerals, and silicate minerals. Pretty rocks that I usually call "calc-silicate marble" on my maps. I've never seen much in the way of an explanation of the petrogenesis of marble in association with VMS exhalites, I've always assumed a CO2 component to the mineralizing fluids (either from magma released water or as an alteration product within hydrothermal waters), but now it has been observed in the modern environment.

Here's a simple example of the VMS deposit model:
http://www.geocities.com/ijkuk/ik_vms.htm

It doesn't show the facies of the exhalites,(which are often variable from deposit to deposit) but you'll get the picture of how exhalites form.

good stuff, thanks Adam.

jms