VMS Basics: The Significance of Anconia’s Zac and Atlas Discoveries
Volcanogenic Massive Sulfide (VMS) deposits are a principal source of zinc, copper, silver, lead and gold worldwide. These deposits occur in clusters or camps. In each camp they are typically evenly spaced (2-6km). Each has from 3-10 deposits, typically with one giant (>30mt) and two medium sized (10-20mt) deposits, as well as several smaller (1-5mt) deposits. Anconia’s Zac and Atlas discoveries represent two such clusters. The following is a description of the formation of a VMS deposit with specific reference to the ZAC deposit.
The mode of formation of VMS deposits is probably the best understood of all base metal-producing deposit types, in part due to extensive research on major districts such as Noranda and Bathurst, and in part because of the discovery in 1979 of actively forming VMS deposits on the modern seafloor. All such deposits form in areas of the seafloor that are (or were) volcanically active, and under extension. The essential components for forming (and therefore predicting) such deposits are:
- Volcanic rocks that formed in a marine environment;
- Evidence of a local near-seafloor heat source;
- Evidence of a “break” in volcanic activity, usually represented by a thin sedimentary sequence commonly called “exhalite”.
The key characteristics are illustrated in Figure 1, with each identified by number, with reference to the Atlas and Zac camps:
- Heat source (1): These are typically 500m to 3 km below the prehistoric seafloor, and are the “engine” that caused trapped seawater to circulate in convection cells, and also the source of heat that released the metals from the rocks. This was intersected in two of the 2013 drill holes at Zac (ZAC-13-04 and -05), and is a gabbro sill.
- Reaction zone (2): This is the section of rocks below the deposit, that underwent a high-temperature reaction (380-400oC) that caused the inherent trace levels of copper (Cu), zinc (Zn), lead(Pb), silver (Ag), gold (Au) and sulfur (S) to be dissolved from the rocks into the trapped seawater. These “reaction zones” are rarely identified at the exploration stage, but have tentatively been identified at Atlas/Zac just above the gabbro sill noted above.
- Impermeable sub-seafloor barrier (3): As the hot, metal-rich water rises, cold “fresh” seawater descends to replace it. Where the two meet, various minerals (quartz, chlorite) form a seal that isolates the hot water from the overlying cold seawatercharged rocks. The lower zone (2) then begins to heat up to >380oC, causing almost all of the metals from below the barrier to dissolve into the seawater.
- As the hot water rises to near the seafloor and begins to cool, it encounters an area of high permeability. Because copper is less soluble than zinc, it preferentially precipitates in this zone, filling fractures and inter-fragment void space. This is the “stringer zone’ (5), a zone of copper- and somewhat gold-enriched veins and disseminated sulfides that is highly altered. This was encountered in several holes in the Atlas and Zac areas, including ZAC-13-05.
- The remaining fluid, now somewhat depleted in copper, is discharged onto the seafloor, where massive cooling occurs, causing the remaining metals to be deposited. This forms the Massive Sulfide zone (6) (in the Atlas and Zac trenches), comprised of zinc, silver, lead, and iron sulfides. At approximately 100m in core length, the stringer zone at ZAC-13-05 is considered quite substantial and COULD indicate the presence of a larger zone of massive sulfides.
- Some of the fluid escaped into the overlying seawater, forming the “black smoker vents” observed in many seafloor rift areas. These plumes of “smoke” (actually particles of zinc and iron minerals) may rise as much as 450m, and are dispersed over 100s (or more) meters as a thin “blanket” around the vents. These form part of the “exhalite’, a unit of volcanic detritus, mud and/or chert and iron formation that commonly caps and surrounds the deposits. The abundance of base metals and silver in these exhalite units is used as a guide or “vector” to locate hidden deposits. In the Atlas area, these are in iron formation and sediments.
Finally, the Archean volcanic rocks in the Atlas and Zac areas are highly deformed and recrystallized, with folding causing the deposits to be steeply dipping and possibly displaced. The stringer zone, which was intersected in only two holes, ZAC-13-05 and Comaplex VL98-07, is probably laterally displaced from the massive sulfide zone, which has not been drilled.