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. 

Economic and Diagnostics Features of Galena

Galena is a metallic mineral belonging to the mineral group known as the sulphides. Its chemical composition is PbS. It is an economic mineral mined for lead, a metal used in making ammunition (its biggest use), batteries, brass and bronze alloys, and as an additive in pesticides. Lead is also used to protect human from X-ray and gamma radiation. Galena has a light silver gray colour, with a distinctive metallic lustre, and is characteristically very heavy because of the lead component (S.G is ~ 7.58). Its streak is dark gray. Based on the Moh's hardness scale, the hardness of galena is 2.5. It has a perfect cleavage in three directions at 90^o angle and a cubic crystal system. The mineral is non-magnetic.

References

Cairncross, Bruce. Field guide to rocks & minerals of Southern Africa. Struik Publishers. Malaysia. 2004.

Dreyer, Lacey. Galena-Mineral Economic Value and Use Reports. Availabe: http://www.emporia.edu/earthsci/amber/go336/reports.htm#galena. Last viewed 12/06/2010.

Highland Valley Copper Mine

Location: Highland Valley Copper Mine, covering a surface area approximately 34,000 hectares, is located in the Highland Valley, 75 Km southwest of Kamloops in central British Columbia, Canada.

Products (Commodities): Copper (primary commodity) and molybdenum.  

Deposit Type: Porphyry copper deposit with only 0.4% copper (grade). The host rock is granitic and contains grains of the ore minerals chalcopyrite, bornite, and molybdenite throughout. These minerals are scattered widely through the granite, and are concentrated in some veins.

Controlling Company: The mine is jointly owned by Teck Resources Limited (97.5% interest) and by Highmont Mining Company (2.5% interest). Teck Resources Limited operates the mine.

History: Operations at Highland Valley dates back to 1962, during the commissioning of the Bethlehem Copper Mine (a three pit copper mine) on the north side of the highway. The mine was developed by Sumitomo Metal Mining Company Limited, a Japanese mining company. The Bethlehem Mine sites were followed by discovery of the Lornex orebody in 1963 and the Valley deposit in 1964. Stripping of the Lornex deposit began in 1970 and the milling of the ore commenced in 1972. Stripping of waste rock of the Valley deposit was not initiated until 1982. The present operation at the Highland Valley mine is a combination of the Lornex mine and mill, the Valley copper orebody and the Highmont mill.

Stage of Mine: Active operation.

Geological Setting: The Highland Valley deposit lies within the late Jurassic Guichon Creek batholiths in Bethsaida phase of porphyritic quartz monzonite and granodiorite. Prominent structural features are the north-trending, west-dipping Lornex fault and the east-trending Highland Valley fault. The deposit’s faults and fractures comprise four main sets with quartz veinlets subparallel to earlier-formed fault and fracture sets.

Mining Method: Highland Valley mine is an open pit, truck-shovel operation.

Major Infrastructure and Equipment: Facilities include the Highland mill and the Valley and Lornex open pit mines. The mill and Valley mine are integrated by two two-kilometre conveyors that feed ore from two large semi-mobile crushers at the mine to the mill. Ore from the Lornex mine is hauled to two permanent crushers with discharge to stockpiles. The ore is processed at the mill using semi-autogenous grinding and conventional flotation processes.

Reserves: Total proved and probable reserves as at December 31, 2009, were 440 Mt with a grading/ton of 0.35% copper and 0.008% molybdenum.

Resources: Total measured resources as at December 31, 2009, were 166.1 Mt with a grading/ton of 0.30% copper and 0.013% molybdenum.  

Market: Most of the copper concentrate is transported by truck to Ashcroft, BC where it is loaded onto railcars. The trains take the material to a terminal on Burrard Inlet in North Vancouver from where it is shipped to smelters in Japan, and other Far East countries. The molybdenum produced at Highland Valley is sold to roasters and trading companies in both Europe and China.

References

Mineral Resources Education Program of British Columbia. Highland Valley Copper. Availiable: http://www.bcminerals.ca/files/bc_mine_information/000078.php. Last viewed 27/06/2010.

Teck Resources Limited. Mineral Reserves and Mineral Resources. Available: http://www.teck.com/Generic.aspx?PAGE=Operations+Pages/Reserves+%26++Resources&portalName=tc. Last viewed 27/06/2010.

Thomaz, C. Highland Valley Mine. Available: http://www.miningweekly.com/article/highland-valley-mine-2006-07-14. Last viewed 27/06/2010.

Whitehead, C. Highland Valley Copper Mine. Available: www.goldtrail.com/.../Highland%20Valley%20Copper%20Mine.pdf. Last viewed 27/06/2010.