Evidence for lateral fluid flow in vent-distal SEDEX Zn-Pb deposits and districts

Abstract

The concept of dense, metalliferous hydrothermal fluids exiting from seafloor vents and flowing across the seafloor to accumulate as stratiform deposits in depressions (collector basins) was introduced by Sato (1972) mainly in reference to volcanic environments. The exhalative concept was later applied to the sedimentary exhalative environment (SEDEX) to describe bedded or laminated tabular sulfide-rich bodies in carbonaceous shales or other fine-grained clastic rocks (Carne and Cathro 1982). Based on major differences in geological parameters, two subsets of the SEDEX class of Zn-Pb deposits have been proposed (Sangster 2002): (a) vent-proximal and (b) vent-distal. Whereas the former are directly underlain by a discordant feeder pipe and surrounding alteration zone, vent-distal deposits lack this distinctive component, are neither underlain nor directly connected to a feeder complex, and related hydrothermal fluids are therefore considered to have flowed away from the source vent. Simple tank experiments (Sangster 2002) and genetic models (Large et al. 1998) have since validated the concept but strong field evidence has not been assembled in support of the theory. Volcanogenic massive sulfide (VMS) exhalative deposits, almost all of which are vent-proximal, are distributed in district clusters roughly surrounding a volcanic center (Sangster 1980). In contrast, vent-distal SEDEX districts comprise deposits occurring in a relatively narrow stratigraphic interval distributed in a straight or gently curvilinear array along strike. For purposes of this discussion, a SEDEX district is taken to comprise three or more deposits. Stand-alone vent-distal deposits are formed when the volume of a collector basin exceeds the cumulative volume of incoming hydrothermal fluid. Because vent-distal deposits do not form a plume upon exhalation but, rather, flow along the seafloor, stand-alone deposits are therefore completely contained in a closed basin and the entire metal content of the hydrothermal fluid must precipitate in the basin. For this reason, stand-alone deposits do not form a vent-distal district. Districts form when the volume of a collector basin is less than the cumulative volume of the incoming fluid thereby forcing fluid to decant out of the basin. The decanted fluid then flows down-slope until entering a second collector basin, the basin is again filled to overflowing, and the process is repeated. Herein asymmetric metal zoning and its causes are presented as evidence of fluid migration in three stand-alone ventdistal deposits and in four districts. Metal zoning is expressed as 100 × Zn/(Zn + Pb) and referred to as the “zinc ratio” (Huston and Large 1987).