GEOLOGY 101 — Quartz-Carbonate vein gold deposits, Part 1

Quartz-Carbonate vein gold deposits (also known as mesothermal lode deposits) form along, and are localized to, major regional fault and fracture systems, but are actually located in secondary or tertiary structures. These vein deposits form from hydrothermal (hot aqueous) fluids, which were derived deep in the earth’s crust at a medium geological temperature (250 to 400C).

The fluids use the fault/fracture zones as permeable channels along which to flow from their region of origin until they reach a point wherein any of a number of factors — chemical reactions with country rock and/or changes in the temperature and/or pressure — causes the fluids to precipitate. The gold precipitates out of solution along with the quartz vein material. These regional fault systems develop during the waning stages of continental collision and hence can form at significantly later periods than the host rocks; as such, they are termed “epigenetic.”

The actual host rocks of the quartz-Carbonate veins are affected by these fault/fracture origins and can range from mylonites to fault gouge. Mylonites indicate deformation under confining pressures sufficiently high that the rock recrystallizes to a fine grain size. This is plastic or ductile behavior, and indicates that the vein formed deep in the earth’s crust.

Alternatively, if the fault/fracture cuts a rock at a level close to the earth’s surface, then it does not have the same confining pressure and hence will break into fault gouge.

Typical quartz-Carbonate vein gold deposits consist of quartz veins with gold, pyrite and/or arsenopyrite. The gold is usually pure gold and can be present in textures ranging from solitary grains to grains intimately intergrown with sulphide minerals. In some deposits, gold is present as “invisible” intergrowths with sulphide minerals such as arsenopyrite (that is, the gold is in the crystal lattice of the sulphide mineral). In other deposits, the gold is not pure but electrum — a mineral made up of gold, with 20% to 80% silver.

Quartz-Carbonate vein gold systems are characterized by abundant, typically iron-rich, hydrothermal carbonate alteration assemblages which spread into the host rock from the vein. They represent pulses of fluid which flowed along the fracture/fault plane into the surrounding country rock with which they are not in chemical equilibrium, producing chemical reactions and the resultant alteration halo.

Alteration associated with gold mineralization also involves sulphidation (sulphide halos are a characteristic alteration phenomenon of most quartz-Carbonate vein gold deposits) and potassium metasomatism (potassium is usually enriched in the alteration halo around the veins). These halos overprint pre-existing alteration assemblages in the host rock.

Any rock type can host these vein systems, but, at best, they are developed in mafic rocks such as basalts, greenstones, gabbros and turbiditic shaley sedimentary rocks; this is attributable to the chemical contrasts between host rock and ore fluids. The ore fluids are silica-rich with carbon dioxide and potassium; hence they react best with mafic rocks, which do not contain free silica but which have calcium-iron-Magnesium silicates that can react with carbon dioxide to form carbonate alteration minerals.

Gold abundances are characteristically low in most geological materials. The average crustal abundance of gold is on the order of 3 parts per billion, and generally no single rock type is preferentially enriched in gold.

As a result of the low background contents of gold, a large amount of rock must be affected by the hydrothermal fluids in order for sufficient deposits of dissolved gold to be formed. The general model for these deposits suggests that the associated regional faults have deep roots that extend down to the lower crust. Hydrothermal fluids, which contain gold dissolved from a wide region, are formed, and these are focused up along the faults to higher levels in the crust, where they react with country rock to form lode gold ores.

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