Formation (Genesis) of Porphyry Copper Deposit

Idealized model (based on magmatic-hydrothermal process) for the generation/formation of porphyry copper deposits is summarized in the stages given below.

Stage 1

As shown in Fig. 1, the first stage in the formation of Porphyry copper deposit is the intrusion of a sub-volcanic magma to a depth ~ 4 km. The magma type is I-type (e.g. granadorite I-type magma) and thus has high volatile contents (H₂O, CO₂, Cl, etc).

Fig. 1 Schematic illustration of I-magma intrusion

Stage 2

In the second stage, the sub-volcanic magma chills against the country rocks, thus crystallizing magma close to the country rock (see Fig. 2).

Fig. 2 Schematic illustration of magma crystallization

Stage 3

In stage three, magmatic fluids (hydrothermal fluid or water – volatile content) separate during the crystallization (Fig. 3). This process is known as the second boiling.

Fig. 3 Schematic illustration of separation of magmatic fluid

Stage 4

In this stage, pressure starts to build-up as the magmatic fluid boils to form steam, producing increase in volume (Fig. 4). This process is known as the first boiling.

Fig. 4 Schematic illustration of pressure build-up

Stage 5

In stage 5, the pressure generated by first boiling results to the fracturing of the crystallized magma and country rocks as the pressure build-up is greater than pressure of the country rocks (Fig. 5).

Fig. 5 Schematic illustration fracturing and formation of stock work

Stage 6

In stage 6, the fracture of the crystallized magma and country rocks results to rapid fluid escape into the fracture network known as stock work; increased crystallization temperature causing magma to crystallize more rapidly to produce porphyritic texture; deposition of ore mineral in the stock work, as the magmatic fluid contains copper mineral (Fig. 6). This stage is also part of second boiling.

Fig. 6 Schematic illustration of porphyritic texture formation

Stage 7

In this stage, the magmatic fluid may undergo phase separation into low density vapour and brine phases. The vapour many travel further out into the fracture network while dense brine will tend to pond at the top of the intrusion. It is during this period of interaction that the potassic alteration develops close to the core of the system and propylitic alteration further out.