(NOTE: This is the second in a series of three posts on copper mineralization in skarn.)
As a geology student several years ago, I had wondered why copper (chemical symbol Cu) shows up where it does. I had wondered how copper gets to where it gets. I had found that copper could mineralize in rocks in certain areas of what is called skarn.
These queries are turning out to be yet another chapter in my endlessly provocative geological quest: How did that get there? With this WATCH FOR ROCKS copper mini-series, I attempt to answer these myriad questions in ways that won’t compel anyone to leave the building.
My previous post addressed two burning questions:
First: What the heck is a skarn?
Second: What does quartz monzonite (a type of igneous intrusion present in my southern Utah study area) have to do with the skarn?
So now we arrive at the Third question — How are skarns related to copper mineralization, anyway?
Again, skarn is a type of rock formed by contact metamorphism and metasomatism of carbonate rocks (generally limestones) where hot, acidic, silica-rich fluids are driven from an igneous intrusion to react with the carbonates of the surrounding rock. This is where hydrothermal veins enter the picture. Skarn is considered to have formed along fractures in the “country or host rock” or bedrock.
Most major ores of important metals such as copper, lead and silver are sulfides. Sulfides are most commonly found in hydrothermal sulfide deposits, either in veins or disseminated throughout the deposit. While native copper was a source for the metal early in the development of human civilization, now most copper is extracted from sulfide minerals.
What is an ore?
“It is the naturally occurring material from which a mineral or minerals of economic value can be extracted at a reasonable profit.”
Fourth questions (a two-fer!) — How do sulfides form? How do we get copper from them?
In a nutshell: Transition metals (such as Fe (Iron), Zn(Zinc), Cu (Copper), Pb (Lead), Co (Cobalt), and others) bond with Sulfur (S) by forming molecular orbitals that share electrons to satisfy the valence requirements of the transition metal atoms.
Yikes! Chemistry rears its ugly head!
Alas, there’s more…
Sulfides and related minerals are characteristic of hydrothermal vein and replacement deposits.
Essential features of hydrothermal systems include:
— A source for the metals and other elements precipitated by the water
— Migration pathways
As you might suspect, these features (of hydrothermal systems) have features:
1— Water can be evolved from magma, be released during metamorphism (I could really go on about this one…), originate as rain or snow (meteoric), and that all-time favorite, be trapped in sediment pores (connate).
2— Heat is often provided by the igneous intrusion; it also can come from depth.
3—There are several ideas for a source of metals and other minerals precipitated by the water. Metals, sulfur, and other elements in the water may be derived from the crystallizing magma or they may be leached out of a large volume of country rock by the water.
The presence of the metals may also come about through ionic diffusion (ack!) from an unknown source (porphyry deposit?) or by gaseous emanation (but of what, from where?).
4— Fluids commonly flow along migration pathways through rocks along fractures, faults, and normal pore spaces.
5— Minerals may precipitate either by a) filling void spaces or by b) replacing other minerals in the rock through which the fluids flow. As the fluids migrate away from the crystallizing magma, they can encounter chemically reactive rock such as limestone, which may trigger precipitation of metal ions from solution.
Circulating fluids can scavenge elements from the rocks like a liquid Pac-man swimming through a stream. They can dissolve previously produced mineralization and re-precipitate those minerals.
The ultimate result here may be a progressively concentrated volume of valuable minerals.
Next post – the final question along with pictures of magnificent minerals:
What are the relationships between copper and many other magnificent minerals such as magnetite, chalcopyrite, cuprite, and chrysocolla?