Anhydrite is common in porphyry Cu-Au systems. It usually occurs in cross-cutting veins and it reflects the high oxidation states of their parental intrusions. Near the surface, the anhydrite gets replaced by gypsum.

Wherever you have sulfides, particularly Fe sulfides such as pyrite and pyrrhotite you will find also modifications of Ca sulfate. Pyrite is the sulfide which accommodates most sulfur in its lattice and pyrrhotite is the most vulnerable sulfide when exposed to oxygen in the gossan of all kinds of sulfide-bearing deposits, e.g., porphyry, VMS, black shales ….You might encounter Ca sulfate in voids, fissures etc.

The question whether anhydrite or bassanite, or gypsum will come into being depends on the hydrological system and intrinsic factors such as texture of sulfides and structure of the ore. Simply expressed the lower the degree of crystallinity and X size the more sensitive Fe sulfides are susceptible to decomposition (see e.g. collomorphous pyrite, marcasite, bravoite). Near or under the water level at shallow depth gypsum is the most stable Ca sulfate. If the deposit is uplifted to a significant height above the water table, be it marine or continental anhydrate will be the most stable modification of Ca sulfate. Dehydration may also start as the mineralized system undergoes burial diagenesis or some re-heating during subsequent magmatic or hydrothermal events.

Carbonates at the opposite end of the sedimentary system are highly soluble, more than feldspar and clay minerals which might be able to provide Al for APS minerals and , thus, Ca and S are the logic result of this intimate association of Ca sulfate and Fe sulfides. The latter can also be substituted for by chalcopyrite and sulfur-enriched Cu minerals which albeit retarded respond in a similar way.

There are sulfide-bearing base metal deposits (see Kuroko) where the marginal oxidized rim can also be mined for a profit for gypsum (and baryte).

In the porphyry deposit that I examined (about 5 m.y old) anhydrite in veins was everywhere including the potassic alteration zone. Near the surface it was replaced by gypsum and at higher elevations it was dissolved away leaving cavities in the rock.

In the Archean porphyry deposit there was no anhydrite or gypsum.

Dear Dr. Issigonis,

your field observations bear witness of what I referred to in my post. The Ca sulfate system strongly depends on age and its distance from the water level. When a salt dome is brought into the reaches of the ground water solution takes place and anhydrite gets hydrated. If the salt dome is lingering for a longer period of time not only the halogenides but also gypsum of the caprock undergoes dissolution resulting in collapse breccia with lots of argillaceous residues and subsidences at the surface.

The process can be impeded or even reversed under strongly aridic climatic regimes, e.g., in the Atacama Desert where one of the giant Cu porphyries is located

H.G.Dill

One more question:

Does anhydrate expansion to the gypsum help to the vein opening and widening? Is it documented?