Formation on the sea floor of the Hellyer volcanogenic massive sulfide deposit

Abstract

New fluid inclusion data from the stockwork veins beneath the Hellyer orebody, a relatively undeformed, Late Cambrian polymetallic volcanogenic massive sulfide deposit, allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide. The earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies. Subsequent vent fluids, buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater, mixed with the cooler brine. During and after filling the basin to the level of its lowest outlet, density inhomogeneities within the brine pool were reduced by double-diffusive convection across the brine-seawater interface and internal boundaries, and mixing by currents within the brine pool. Estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage (avg 200°C, 11 wt % salinity), assuming a steady or quasisteady state, had been established, and using volume fluxes similar to those observed in modern oceans, indicate temperatures ≪100°C. Sulfides (mostly pyrite, sphalerite, and galena) precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor, a pattern that would have continued whether or not a true steady state was reached. If the early fluids were buoyant in seawater they ponded after mixing, forming a hybrid brine pool that would have required longer periods to homogenize. At the peak temperature (≥300°C) and volume flux the salinity of the vent fluids fell to <7.5 wt percent, but the flow paths would have been maintained unless the brine pool had been drained. At this time that part of the massive sulfide body over the vent (the Cu core) grew by dilatational veining together with leaching, cavity infilling, replacement, and recrystallization. Chalcopyrite was deposited both in the Cu core and probably at the same time in a layer over the flanking massive sulfide. In the final stage as temperature fell and salinity rose (avg 250°C, 11 wt % salinity), the fluid oxidation potential increased so that barite precipitation dominated, resulting in a formation of a discontinuous barite-rich cap over the massive sulfide. The brine pool model explains several features of the Hellyer ore that differ from those of the ores of the Hokuroko basin, namely, the large metal content, high Zn/Cu ratio, low aspect ratio, absence of chimney fragments, presence of mineral banding, and high barite sulfur isotope values, features seen in many other massive sulfide orebodies.