FORMATION ON THE SEA-FLOOR OF THE HELLYER VOLCANOGENIC MASSIVE SULFIDE DEPOSIT

New fluid inclusion data from the stockwork veins beneath the Hellyer orebody, a relatively undeformed, Late Cambrian polymetallic volcanoge nic massive sulfide deposit, allow semiquantitative modeling of the be havior 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 tha n seawater and ponded in a basin identified by sedimentological and st ructural studies. Subsequent vent fluids, buoyant with respect to the brine pool and subsequently at higher temperature with respect to seaw ater, 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 brin e-seawater interface and internal boundaries, and mixing by currents w ithin the brine pool. Estimates of likely temperatures and salinities in tile brine pool for the first main mineralization stage (avg 200 de grees C, 11 wt % salinity), assuming a steady or quasisteady state, ha d been established, and using volume fluxes similar to those observed in modern oceans, indicate temperatures much less than 100 degrees C. Sulfides (mostly pyrite, sphalerite, and galena) precipitated from the vent fluids after mixing with cooler basin fluid settled from the gra vity current to the basin floor, a pattern that would have continued w hether or not a true steady state was reached. If the early fluids wer e buoyant in seawater they ponded after mixing, forming a hybrid brine pool that would have required longer periods to homogenize. At the pe ak temperature (greater than or equal to 300 degrees C) and volume Aw; tile salinity of the vent fluids fell to <7.5 wt percent, but the flo w paths would have been maintained unless the brine pool had been drai ned. At this time that part of the massive sulfide body over the vent (the Cu core) grew by dilatational veining together with leaching, cav ity infilling, replacement, and recrystallization. Chalcopyrite was de posited both in the Cu core and probably at the same time in a layer o ver the flanking massive sulfide. In the final stage as temperature fe ll and salinity rose (avg 250 degrees C, 11 wt % salinity), tile 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 ba rite sulfur isotope values, features seen in many other massive sulfid e orebodies.