The Ertsberg copper, Estudios de cobre,,

The Ertsberg copper, Estudios de cobre

1. ↑ Figure 1 - Mineralized bedding replacement
Located in the Papua Province of Indonesia, the Ertsberg deposit lies within a
mountainous region known for its rugged terrain and complex geological history.
It is positioned near the large Grasberg mine, making it part of a prolific
mining district that yields substantial copper and precious metals. The geology
of the immediate Ertsberg area is dominated by skarn deposits, a product of
contact metamorphism where intruding magma interacts with carbonate rock,
creating a mineral-rich environment through chemical replacement.
by Michael Thomsen, Chairman at North American Strategic Minerals Inc.
The Ertsberg copper
skarn deposit
A geologic and mining story
like no other
Significance of the Ertsberg
deposit in mining geology
The Ertsberg deposit stands as a prime example of copper skarn
mineralization and has contributed greatly to the geological under-
standing of such deposits. Its unique textures, for example the chal-
copyrite-replaced foraminifera and brecciated structures, make it a
focal point of study for skarn formation processes and post-mineral
structural modifications. Additionally, Ertsberg’s history illustrates
the challenges and rewards of mineral exploration in remote and geo-
logically complex regions.
For mining engineers, drillers and geologists, the Ertsberg deposit
provides valuable lessons in resource estimation, mining design, and
↑ Figure 2 - Crackle breccia ↑ Figure 3 - Foram
/EXPLORATION & MINING GEOLOGY
the importance of recognizing geological processes. Its position in
the mining world is emblematic of the resource potential that skarn
systems hold, as well as the intricate geological processes that create
such deposits.
Structural characteristics
of the orebody
The Ertsberg deposit has a vertical, ovoid structure, with approx-
imately one-third of the orebody exposed above ground and the
remaining two-thirds extending below ground level. Its vertical
orientation in such a remote and rugged area adds to the deposit’s
extraction complexity, necessitating innovative mining techniques.
The mineralized zone is largely composed of chalcopyrite and mag-
netite, with unique textures and structural features resulting from
various geological processes.
Mineralization and ore textures
The Ertsberg deposit exemplifies a typical skarn replacement of the
original silty carbonate host rock. During mineralization, fine-grained
sediment bedding layers were massively and selectively replaced by
primarily chalcopyrite and magnetite, resulting in detailed textural
patterns. Below are some notable ore textures found in Ertsberg:
1. Replacement textures: In typical skarn deposits, primary host
rock minerals are gradually replaced. Figure 1 from Ertsberg
demonstrates this process, where fine-grained sediment layers
are intricately replaced by chalcopyrite and magnetite. This quiet,
systematic replacement showcases the skarn’s mineralogical evo-
lution, a defining feature of this copper-rich deposit.
2. Crackle breccia: Another distinctive feature is the presence of
crackle breccias within the orebody. Figure 2 highlights a brecciated
texture with angular fragments of chalcopyrite encased in an opal-
ine silica cement. Crackle breccias form when post-mineralization
structural forces fracture the orebody, creating angular fragments
that are later cemented by secondary minerals. This texture pro-
vides valuable insights into the geological history of tectonic forces
acting on the orebody post-mineralization.
3. Fossiliferous bedding and foraminifera: The host lithology of
Ertsberg includes fossil-bearing layers with large foraminifera,
particularly Discocyclina. During mineralization, these foramin-
ifera were replaced by chalcopyrite and magnetite, resulting in
elongated, wispy textures within a calcsilicate groundmass. This
fossilized texture, visible in Figure 3, serves as an important marker
of pre-existing biological activity within the host rock, overprinted
and replaced by mineralization processes typical of skarn systems.
4. Shattered chalcopyrite zones: Ertsberg’s orebody also displays
post-mineral shattering in certain zones. Figure 4 shows massive
chalcopyrite fragmented by structural activity, likely due to tectonic
stresses following mineral deposition. This texture is unique and
provides evidence of the post-mineral deformation processes that
have reshaped parts of the orebody.
5. Downstream cobble: Figure 5 is a fist-sized cobble found several
kilometers downstream from the deposit, containing angular frag-
ments of chalcopyrite, magnetite, and hematite embedded in a
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↑ Figure 4 - Shatter breccia ↑ Figure 5 - Cobble
siliceous matrix. This cobble offers a unique clue to the deposit’s
presence upstream and the dispersal of its materials over time.
Such cobbles serve as indicators of upstream mineralization,
and in this case, offer a spectacular bit of evidence of what is
outcropping upstream.
Mineralizing processes
The Ertsberg deposit owes its formation to complex interactions
between hydrothermal fluids and carbonate host rocks. As magma
from deep within the Earth’s crust intruded into the sedimentary
layers, high-temperature fluids rich in metals permeated through,
chemically replacing the original rock with mineralized zones rich in
copper and other metals. The chalcopyrite and magnetite present in
Ertsberg are products of this process, typical in copper skarn deposits,
where iron-rich minerals interact with sulfur and other metal-rich
solutions, precipitating valuable ore minerals.
Skarn mineralogy
and zoning
Skarn deposits often exhibit zonation, with different minerals con-
centrated at varying distances from the intrusion. At Ertsberg, chal-
copyrite and magnetite dominate the inner zones, while the presence
of other skarn minerals (e.g. garnet and diopside) in surrounding areas
suggests zoning reflective of temperature and chemical gradients.
This zonal distribution aids geologists in interpreting the fluid path-
ways and the thermal history of the deposit, enhancing the under-
standing of ore genesis within the deposit.
Post-mineralization structures
Ertsberg’s orebody shows clear signs of structural modification after
mineral deposition. The presence of crackle breccias and shattered
chalcopyrite zones speaks to significant post-mineral tectonic
activity. These structural disruptions likely occurred during regional
tectonic shifts, which fractured and re-cemented portions of the ore-
body, creating unique mineral textures. The opaline silica cement in
crackle breccias further indicates secondary mineralization events
that occurred after the primary copper and iron mineralization.
Conclusion
The Ertsberg deposit, a marvel of geological processes, offers insights into
the formation and evolution of skarn-type copper mineralization. From
its detailed replacement textures to post-mineral brecciation, Ertsberg
encapsulates a rich history of geological activity. This deposit remains a
significant case study for geologists and mining engineers, exemplifying
the mineral wealth that skarn systems can yield and the complexity of their
structural and mineralogical characteristics. For those involved in mining
geology, understanding Ertsberg is essential, offering both inspiration and
knowledge that is fundamental to the field. C
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