mound-shaped deposits formed in a way similar to that of modern massive sulfide deposits - via production of a hydrothermal mound formed by successive black smoker chimneys. Deposits that have formed in environments dominated by sedimentary rocks or highly permeable volcanic rocks can show a tabular morphology that mimics the geometry of the surrounding rocks. VMS deposits have an ideal form of a conical area of highly altered volcanic or volcanogenic sedimentary rock within the feeder zone, which is called the stringer sulfide or stockwork zone, overlain by a mound of massive exhalites, and flanked by stratiform exhalative sulfides known as the apron. The stockwork zone typically consists of vein-hosted sulfides (mostly chalcopyrite, pyrite, and pyrrhotite) with quartz, chlorite and lesser carbonates and barite. The mound zone consists of laminated massive to brecciated pyrite, sphalerite (+/-galena), hematite, and barite. The mound can be up to several tens of metres thick and several hundred metres in diameter. The apron zone is generally more oxidised, with stratiform, laminated sulfidic sediments, similar to SEDEX ores, and is generally manganese, barium and hematite enriched, with cherts, jaspers and chemical sediments common. Metal zonation Most VMS deposits show metal zonation, caused by the changing physical and chemical environments of the circulating hydrothermal fluid. Ideally, this forms a core of massive pyrite and chalcopyrite around the throat of the vent system, with a halo of chalcopyrite-sphalerite-pyrite grading into a distal sphalerite-galena and galena-manganese and finally a chert-manganese-hematite facies. Most VMS deposits show a vertical zonation of gold, with the cooler upper portions generally more enriched in gold and silver. The mineralogy of VMS massive sulfide consists of over 90% iron sulfide, mainly in the form of pyrite, with chalcopyrite, sphalerite and galena also being major constituents. Magnetite is present in minor amounts; as magnetite content increases, the ores grade into massive oxide deposits. The gangue (the uneconomic waste material) is mainly quartz and pyrite or pyrrhotite. Due to the high density of the deposits some have marked gravity anomalies (Neves-Corvo, Portugal) which is of use in exploration. Alteration morphology Alteration haloes developed by VMS deposits are typically conical in shape, occur mostly stratigraphically below the original fluid flow location (not necessarily the ore itself), and are typically zoned. The most intense alteration (containing the stringer sulfide zone) is generally located directly underneath the greatest concentration of massive sulfides, within the footwall volcanic sequence. If the stringer