Geology of an ancient porphyry copper deposit,
Karas region, Namibia

Being a geologist is great!  The job can require travel to some amazing places.  As geologists, we often study things unimaginably large - tectonic plates thousands of kilometers across moving at the speed that fingernails grow for hundreds of millions of years.  The mountains don't come to us, so we go to the mountains.  The mountains of southern Namibia are breathtaking.

This is a view of about half of the field area.  There are a few roads along ridges and in the valleys - rough four-wheel-drive trails, really, but mostly passable.

This is an outcrop of felsic volcanic rock that's a small part of the thicker pile of intermediate volcanic rocks called the Orange River Group.  From a distance, the felsic volcanic rock looks like it could be a rock that was chemically altered by geothermal waters like those that deposited the copper two billion years ago (a process called hydrothermal alteration).  Alas, there's more than one reason a rock can appear bleached - this was just an unusual rock type that broke the monotony of thick andesites.

There are several large quartz veins in the region.  Big quartz veins like this form when hot water (called hydrothermal fluid - hydro = "water," thermal = "hot") flows through giant fractures in the rock (called faults).  The hydrothermal fluid has dissolved elements in it that deposit as minerals on the sides of the fracture - in this case quartz and chalcopyrite.  Rainwater has oxidized the chalcopyrite in this outcrop to form bright green secondary copper minerals that paint some of the surfaces in this photo.

The secondary "oxide" copper deposits on the rock surfaces is easier to see in this photo.  This was a little old mine dug by German prospectors a hundred years ago.  They dug this rock by hand and then dissolved the copper off the surfaces using mild acids.

This is a close-up of the blue-green "oxide" copper that coats the rocks.  It's a common feature in copper deposits, but very rare in our field area because the copper in this deposit is mostly many meters beneath the surface - beneath the level of oxidation.  This mineral is chrysocolla - a copper-bearing clay mineral that sticks to a person's tongue.  It looks a lot like turquoise, but chrysocolla is not useful as a gem because chrysocolla is soft and crumbly, and swells when wet.  Turquoise is hard and waterproof.  (The brown nubs at the bottom of the photo are my gloved fingers for scale.)

This may look like some sort of black plant fossil, but these branching black patterns are actually mineral dendrites.  These particular dendrites are a copper oxide called tenorite.  Dendrites are a weathering product - not particularly indicative of anything we were studying, but none-the-less an interesting little natural wonder.

This is entrance to an old mine blasted into the mountainside.  We did not go in to explore for safety reasons.  Abandoned mine adits like this may look inviting, but they're dangerous places.  Abandoned mines can kill a person in a lot of different ways - rock falls, cave-ins, poisonous gasses, falls into open stopes, wild animals, etc.  The dangers far outweigh the value of any geologic data we may have been able to observe, so we looked at the rocks at the entrance, and moved on to more enlightening outcrops.

Saave demonstrates excellent hand lens technique (one of my pet peeves is poor hand lens technique).  Saave knew the field area like the back of her hand.  She'd hiked every ridge, every canyon, and every slope - and knew every rock there.

Jewels did a great job on the trip.  She worked hard, kept her eyes open, asked lots of questions, thought about what she saw, and adapted well to Namibian culture. Here, she's measuring the strike of the foliation of bedding (volcanic ash layers) in a rhyolite in the Orange River Group.

Geologists need to know both the direction a bed of rock is oriented (the "strike") and the angle the bed is inclined into the ground (the "dip").  Here, Jewels uses the inclinometer of her Brunton compass to measure the dip of the volcanic bedding

There are many dark patches of rock like this on the landscape.  Any deviation from the typical country rock of an area attracts the eye of a geologist.  Could this be a different body of rock?  Some sort of hydrothermal alteration?  A field of black lichen?  There's only one way to find out!  Get on that outcrop for a closer look!

The black rocks turned out to have a very smooth, shiny appearance.  It's a strange way for black basalt to break (that's usually more angular with a slightly concave fracture).

Hammering away at the surface reveals the regular country rock andesite beneath the black stuff.  The black stuff is a mineral coating.  The geologic term for this is "desert varnish."  Desert varnish forms when water dissolves the trace amounts of manganese and iron from a rock, then re-deposits it on the surfaces of the rock when the water evaporates in the desert heat.  These black patches on the landscape probably formed where springs bubbled mineral-laden groundwater up to the surface - especially during the rainy season.  There are many disciplines within the field of geology - including hydrogeology, which studies how water moves about the earth.  We have an excellent hydrogeologist/geophysicist at Kutztown University (Dr. Laura Sherrod).  It would have been handy to have her there to help out.

The walls of this canyon have many bleached zones that are places where hydrothermal alteration (the water that carried and precipitated the copper in the deposit) chemically changed the rock.  The copper deposit in this field area looks very favorable.

Moses (left) and Neil (right) investigate a body of hydrothermally-altered rock to see if there is evidence of copper mineralization.  Again, Neil demonstrates great hand lens technique.  The Tech geologists were very good.

Jewels shows off a zone of highly-fractured rock rich in the mineral goethite.  Goethite forms by weathering of pyrite and chalcopyrite.  This is good looking rock for mineral deposits geologists like me.

These light-colored, subvertical bands within the andesite caught my eye.  They are more resistant to weathering by rain and wind, suggesting they're harder and/or more chemically "stable" than the rest of the rock.  If this was a granular rock, I would have thought these might be deformation bands, but these volcanic rocks aren't granular.  These are actually veins in which the rock was reinforced by precipitation of minerals from hydrothermal fluids.  Many things in geology have superficially similar appearances, but can have very different origins and thus different meanings.  Geology is a science that demands attention to detail.  The deeper one looks, the more one sees.

More veins in the andesite - these showing a very continuous nature, which tells us something about the stress environment in which the rocks fractured.  The continuous, planar nature indicates they formed in a brittle environment.  The parallel pattern suggests they formed in a tensional stress environment rather than one that forms conjugate shears (compression).  The close spacing suggests the fractures may have formed in succession, with earlier fractures being "healed" by mineral deposition, which strengthened the rock enough to allow a new fracture to form nearby.

Exploring rocks in the field generally takes geologists off the beaten path.  The only trails in the field area are game trails (places where animals preferred to walk and so packed the ground down a little more.)

The mineral deposit had been explored by geologic drilling in years past.  Geologic exploration drilling uses a drill bit with a hole in the middle.  The rim of the drill bit is hardened steel impregnated with diamonds.  As the drill bit cuts down into the stone, the rock in the middle of the hole slides up through the center of the drill bit and is retrieved by the drillers.  The drill core is stored in boxes like this one for detailed study by geologists and chemical analysis to determine the copper and gold contents.  The green plastic blocks record the depth from which the rock was retrieved.  There were no active drills on the site while we were there, but we could study core drilled in the past.

Teck cut their core using a rock saw.  The cut surface, when wetted, reveals a wealth of textural and mineralogical information about the rock.  One half of the drill core was preserved for visual study and a permanent record of the geology of that spot, and the other half was sent to an assay lab for chemical analysis.  The rock in this photo came from 92 meters depth - roughly the length of a football field.  The bottom of this hole was more than four times this depth.

Geologists like Saave carefully study the drill core, measuring vein and fracture characteristics, rock type, hydrothermal alteration type, and minerals present on paper logs.  These logs are then typed into a computer database to create a complex, three-dimensional computer model of the deposit.  Geologists and engineers use computer models to quantitatively assess the economic value of rock.

We collected rock samples during our field traverses.  To better see the textures in the rock and to save on shipping weight, Jewels and I use a rock saw to cut the samples into manageable pieces.  The water spray keeps the saw blade cool as it wears through the rock.

After a night of rain, purple flowers popped up everywhere!  Here, Moses, Jewels, and Saave discuss their observations of the outcrop on which they stand.

This is certainly one of the most beautiful field sites in which I've ever worked.  Sometimes I had to stop working just to look around and take the scenery in.  A geologist who does not appreciate the aesthetics of the environment is hollow indeed!

Working together in the field builds bonds between people.  Michael, Saave, Jewels, and Moses formed a good friendship.

On to Namibian wildlife

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