Structural geology and vein mineralisation in the Callery River headwaters, Southern Alps, New Zealand

Abstract

Three phases of folding have been recognised in the greywacke-Alpine schist transition in the Torlesse terrane rocks of the Callery River headwaters, Westland. The first phase was synmetamorphic and isoclinal (F₁). This deformation resulted in variable textural reconstitution of the rocks (textural zones 2 and 3). Subsequent deformation has disrupted the textural zone boundaries, so that the boundaries have been folded almost isoclinally (F₂) and refolded by an upright antiformal structure (F₃) correlated to Alpine Fault related structures.

All these structures are crosscut by several generations of veins. Three late-stage vein types are recognised on the basis of mineralogy and fluid inclusion studies. Type 1 veins consist of calcite and quartz, and were formed by H₂O-CO₂ fluids (about 12 mole % CO₂) at >270°C and >2–3 km depth. Type 2 veins consist of quartz and calcite with minor biotite and sulphides, and traces of scheelite and gold. These minerals were deposited where hot (>320°C) brines mixed with cooler, less saline waters. Metal deposition involved interaction with pelitic schist. Type 3 veins consist of quartz, calcite, and adularia deposited from low-salinity water which may have boiled at about 240°C.

High regional uplift rates and shallow formation of veins indicate a maximum age of vein mineralisation of about 1.7 million years, and a minimum age for Type 3 veins of about 117 000 years. Mineralisation is believed to have resulted from passage of fluids, at least some of which are metamorphic in origin, induced by high heat flow during rapid uplift of the Southern Alps.

The first appearance of metamorphic biotite in this greywacke-schist transition occurs at the textural zone 2–3 boundary which is at least partly tectonic and has been folded isoclinally. Biotite is also a hydrothermal mineral in Type 2 veins and vein margins. Hence mapping of a biotite isograd in this area was not attempted.

Keywords:

• structure
• metamorphic rocks
• textural zones
• folding
• gold
• mineralisation
• hydrothermal alteration
• fluid inclusions
• Alpine schist
• Torlesse terrane
• Callery River
• Southern Alps