Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 93

project. Initial reserves were estimated at 690 million tonnes grading 2.58%
copper. The Guggenheims had also developed a process for extracting copper
from low-grade ores and in 1912 organised the Chile Exploration Company
(Chilex) to mine the deposit. Chilex purchased heavy equipment such as
steam shovels (imported from the Panama Canal) and helped build the port at
Tocopilla and a 90-mile aqueduct to bring water in from the Andes.
Production started in 1915 and reached 135,890 t in 1929, the year of the
Great Depression when demand fell. Companies owned by Guggenheim Bros
ran the mine until 1971 when Salvador Allende government nationalized the
Chilean copper industry. Since then, Codelco (Corporaci´n Nacional del
Cobre de Chile) has mined the deposit.
sampled in fluid inclusions in quartz and other gangue minerals, extend from the
magmatic field well into the field of meteoric fluids indicating that the latter are also
involved in the ore-forming process.
Putting this all together leads to the following model.
1. A granitic magma is emplaced as a series of pulses into a magma chamber high
in the crust, beneath a volcanic edifice. Each pulse cools and partially
crystallizes, and as it does, a hydrous fluid phase separates from the silicate
magma. The separation of this phase results from one or both of the following
processes; (a) the drop in pressure attendant on ascent of the magma decreases
the solubility of water in the magma and (b) crystallization of the magma as heat
is lost to the wall rocks causes the water content of the residual liquid to build up
until it eventually exceeds the solubility limit. Escape of fluid increases the
liquidus of the granitic magma, causing the remaining liquid to crystallize
rapidly around already crystallized minerals, creating the porphyritic texture
characteristic of these deposits. The fluid phase may also migrate up through the
silicate liquid, to concentrate at the upper part of the intrusion.
2. The fluid escapes from the inner still liquid interior and moves through
fractures in the surrounding solidified carapace and onwards into the wall
rocks. As it does so it cools, and it reacts with the wall rocks to form the
characteristic alteration that surround all porphyry deposits. The ore metals
are transported in the fluids, most probably as chloride or sulfate complexes;
as the fluid cools and as its composition changes through reaction with the
wall rocks, the stability of the complexes decreases. The ore metals are then
precipitated in fractures and within the alteration zones surrounding the
granitic intrusion.
3. As the magma intrudes it heats up groundwater in the surrounding rocks, setting
up convection cells surrounding the intrusion. The heated groundwater mixes
with and reacts with the magmatic fluids, diluting and cooling them and
accelerating the precipitation of ore minerals.