EDITORIAL & OPINION — GEOLOGY 101 — Zinc-lead-copper VMS deposits, Part 1 (January 11, 1999)

Zinc-lead-copper volcanogenic massive sulphide deposits have also been called Kuroko-type, after the deposits in the Green Tuff Belt in Japan. Aside from these base metals, such deposits also produce precious metals.

As with copper-zinc VMS deposits, zinc-lead-copper VMS deposits consist of a stratabound, generally stratiform, massive sulphide body underlain by a stockwork feeder zone. Although the footwall rocks in the Kuroko VMS deposits are white rhyolite domes, they are typically felsic to intermediate breccia or ashflow volcanic rocks. This composition contrasts with more refractory volcanic rocks of oceanic spreading copper-zinc VMS deposits.

Zinc-lead-copper VMS deposits form as a result of circulation of seawater through the underlying volcanic layer. The felsic volcanic nature of the zinc-lead-copper footwall indicates greater involvement of felsic igneous rocks and, hence, a source for the formation of lead and zinc. The fluids that formed copper and zinc did not have access to rocks with such lead-rich compositions. However, it has been suggested that the absence of lead in greenstone-type copper-zinc VMS deposits reflects the lead-poor nature of the Earth’s crust as it was forming.

The sulphides in zinc-lead-copper VMS deposits exhibit a strong zonation, and the typical model for these deposits defines seven different mineralogical zones. However, the presence of all of these zones is rare, owing to erosion or poor development of individual layers.

These zones include:

• silicious ore — the lowermost zone, consisting of stockwork pyrite, chalcopyrite and quartz;
• pyrite ore — overlies siliceous ore and is stratiform massive pyrite with some veining and disseminations;
• oko or yellow ore — composed of pyrite or chalcopyrite but can also include sphalerite, barite or quartz;
• black ore — overlies oko ore, and consists of sphalerite, galena, chalcopyrite, pyrite and barite;
• barite ore — a chemical sedimentary rock composed of massive barite (calcite, dolomite and siderite);
• chert-hematite — constitutes the top of the sequence; and
• gypsum ore — contains the chemical sedimentary rocks gypsum and anhydrite, and can occur on the edges of the sulphide mound laterally, away from the core.

The ores for copper, lead and zinc are chalcopyrite, galena and sphalerite, respectively.

As in the copper-zinc VMS deposits, the various zonations seem to reflect temperature differences and the outward migration of metals from the site of influx. It appears that anhydrite forms as the first phase of an exhalation system on the seafloor, and is then replaced by sphalerite and galena, followed by chalcopyrite and pyrite. The gypsum, barite and chert-hematite zones can extend laterally some distance away from the sulphide mound. Some divide the siliceous stockwork material into zones consisting of a core of siliceous pyrite that grades into siliceous yellow ore followed by siliceous black ore at the edges.

A typical stockwork zone in a zinc-lead-copper VMS deposit has a quartz-sericite core (frequently with chlorite) that grades into a middle zone with sericite, clay minerals, chlorite and sometimes feldspar, and, finally, into an outer zone with zeolite and clay minerals.

These alteration systems — particularly those consisting of sericite and chlorite, or zeolite and clay — have been known to surround the sulphide mound and envelop the hangingwall for up to 300 metres above the sulphide body, and laterally for up to 1.5 km.

The layers containing massive sulphide mounds can exhibit soft sediment deformation features, indicating that the layers were plastic on the seafloor prior to cementation into solid rock. In some deposits, these sulphide layers moved en masse downslope from the underlying rhyolitic dome, becoming detached from their stockwork feeders.

Such ore horizons are called “transported ores.” “Proximal ores” are in contact with their stockwork zones, whereas the massive sulphide in “distal ores” are not connected to the stockwork.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.