Under a Gold Blanket: Discovery Guide Tours Famed Mine in Minas Gerais

by “Cretaceous” Chris Wells

The City of Ouro Preto, “Black Gold”. It was the largest City in Latin America for a while in the 18th Century, during the Brazilian gold rush. Today the city is famous around the World for it’s preserved, Baroque Architecture.

The city of Ouro Preto, “Black Gold.” It was the largest City in Latin America in the 18th century during the Brazilian gold rush. Today the city is famous around the world for its preserved Baroque architecture.

Brazil is a beautiful country, but not in the same way as the Florida Keys, Hawaii, or Aspen. Everywhere in Brazil you have a wonderful dichotomy between the grotesque and the graceful. The entire country is like an impressionist painting — up close you see sloppiness and imperfection, but if you stand back, all of the colors and textures come together to create a stunning portrait, a portrait based in reality, not contrived. Throughout my July 2015 trip through the state of Minas Gerais, in the mountainous interior of Brazil, I had been awed by the natural beauty of the country, and also with the artificial splendors. I was always amazed by what people who have so little can create with what they do have.

The scenic train ride to the city of Mariana, where the mine museum was. Minas Gerais is famed in Brazil for its natural beauty

The scenic train ride to the city of Mariana, where the mine museum was. Minas Gerais is famed in Brazil for its natural beauty.

I was basking in the adrenaline and the charm of exotic travel, but the grotesque crept back into my perception as I sat in a rickety old mine car, suspended above the mouth of a mineshaft on a track with a forty-five degree slope. The rusted mining equipment and dilapidated offices and supply sheds had blended nicely with the mountainside when I first viewed them from across the valley during the train ride over, but up close it was less than dazzling. The only thing keeping us from sliding hundreds of feet into the earth was a steel cable hooked up to a winch probably as old as my grandfather. There were no seat belts in the cart, and the angle of the tracks was so steep, I had to press my feet against the seat in front of me to keep from sliding off. There were no other people visiting the mine — it was only Fernanda and I — and I couldn’t help but wonder if there was a sinister reason for that emptiness. I was a bit nervous as the little old lady manning the craft announced in Portuguese that we were about to descend, but then the buttons were pressed, the car slipped slowly and smoothly into the darkness and it wasn’t that bad.

he entrance to the mine. Until only a couple decades ago, miners would make this descent every day.

he entrance to the mine. Until only a couple decades ago, miners would make this descent every day.

The trip down was noisy. The wheels of the cart screeched against the steel tracks, and every once in a while the cart would jump on a connection between two rails and snap back down loudly. The sound would bounce off the solid stone walls around us and pound our eardrums. The shaft was dark. Lamps, bare bulbs budding from a ragged wire that crept like a vine along the walls of the shaft, emitted a honey-colored glow that lent the place a very intimidating and volcanic atmosphere. But it was actually quite cool down there. The earth insulated us from the sun up top; we were no longer subject the conditions on the surface.

Our descent into the earth, in a simple old cart that used to ferry the miners down to their work.

Our descent into the Earth in a simple old cart that used to ferry the miners down to their work.

Beside us, embedded in the rock walls of the mine, ran a sliver of milky white quartz. The shaft followed it until we reached the bottom of the mine. At the bottom, the path flattened out and the rails ended. Men were laboring with shovels, loading rocks into a pile for transport back to the surface. The mine was not active anymore; these were simply employees of the museum that owned the mine clearing debris for the safety of the guests. Their store-bought, not-too-dirty clothes belied their fortunate position as men who did not permanently work in a mine. Still, they looked tired and unhappy, and generally ignored our presence, which I don’t blame them for. I have often said that I can do physical labor or customer service, but not both.

Milky-white bands of quatz running through the stone walls of the mine.

Milky-white bands of quartz running through the stone walls of the mine.

The vein of quartz that we had followed down was thicker at the bottom of the shaft.  Here it became apparent that it was not just a small sliver of quartz we had seen, but a thin cross-section of an entire layer blanketing the Earth for who knows how far in every direction. Originally, it would have formed as a flat layer, but like a massive, restless sleeper, the Earth had shrugged its silvery blanket during bouts of tectonic activity. The quartz layer had been folded into the Earth’s crust, resulting in a slumped, angled descent beneath the surface. This is why the mine goes so deep. It follows the descent of the quartz vein. The Portuguese had mined near the surface in the 18th century, but with their limited technology, they didn’t go very deep. The shaft we visited was dug much more recently.

Bands of quartz, along with other minerals, lined the walls.

Bands of quartz, along with other minerals, lined the walls.

The mine wasn’t a quartz mine. The reason this vein was so important is that very often gold is found in quartz. Down in the mine, we could see no gold, but it was there, trapped in the quartz. The mining companies had cleared the shafts with explosives, creating a maze of passages spreading in all directions. To keep these shafts from collapsing under the millions of tons of sediment above, they left columns of rock standing, like pillars in an Egyptian temple, throughout the galleries. Like the walls of the mine, these pillars had diagonal layers, alternating between rock and quartz, like a layered cake. In these columns, quartz and gold still rested, impossible to get to without collapsing the mine.

At the Bottom of the mine, passages like this one meandered in every direction. There were some passages that went deeper, but those had been flooded.

At the bottom of the mine, passages like this one meandered in every direction. There were some passages that went deeper, but those had been flooded.

And although the gold itself was invisible, pyrite (fool’s gold) was everywhere, so in places the mine shafts really did look like they were covered in gold. There were also tourmaline and garnet, a menagerie of natural splendor, though none of them were of gem quality. The good stuff would have been plucked out and sold. Museums around the world buy specimens from Minas, even our museum. A notable portion of the pieces in our Cullen Hall of Gems and Minerals came from the region. Our hall is what inspired me to travel there in the first place. Our great collection of gold does not come from Minas, but like the mine I visited, our gold was found in quartz veins. We have a wonderful, natural gold “sculpture” called “the dragon,” a sliver of gold that actually looks like a rearing reptile with spread wings. We have other pieces of gold in all sorts of abstract, contorted shapes as well. Since gold doesn’t grow (unfortunately), the way it acquires these weird forms is by being trapped in quartz. As the quartz grows, it manipulates the gold inside into all sorts of interesting shapes. So originally our gold was trapped in quartz crystals, and was extracted by dissolving the quartz in a mild acid that does not harm the gold. A similar process is used to extract gold from quartz in mines like the one I visited.

My girlfriend and I.

Fernanda and I, overlooking the valley.

Editor’s Note: “Cretaceous” Chris is a Discovery Guide for the Houston Museum of Natural Science.

Dumb as a rock? A lumpy bit of earth reveals a geological timeline in Seymour

Editor’s note: Today’s blog comes to us from paleontologist and field volunteer Neal Immega.

You all know by now that the museum has a dig in Seymour where we are finding fabulous Permian fossils, including the toothy Dimetrodon and the weird boomerang-headed salamander Diplocaulus. We don’t dig in just one place; lots of people go prospecting for new sites (or maybe they are just looking for a private spot to do their business).

A previous blog topic was on a weird lump that turned out to be a caliche ball. Today we have another lumpy rock to look at. This specimen was collected by geologist Gretchen Sparks, who brought it in just to plague me.  Let’s see just how much information we can squeeze out of it:

CrossBeds

At first glance, it’s just a rock showing cross-bedded sandstone with low-angled bedding, doubtless caused by water deposition in the Permian creek that crossed our digging area. This is pretty normal stuff. We see cross-bedding everywhere at the dig site, because the sandstone is durable and stands in relief.

But why is it lumpy? The bulge in this picture is not exactly standard:

Profile

It gets better. The rock is too heavy to be only quartz. A heavy, light-colored sandstone is likely to be cemented by barite (barium sulfate). Let’s cut the rock in half and polish the face.

cutLumpLabeledThis is turning out better than I expected. You can see a seam of barite cutting the nodule vertically in half. The sandstone shows horizontal layers which correspond to the cross-bedding.

What about the red-colored area? The area we are working in North Texas is called the Permian Red Beds because everything got oxidized from prolonged exposure to the atmosphere. It was a really dry time, and the critters stayed close to the Permian creek which deposited these sediments. It is good for us because the fossil remains are concentrated in a small area (our dig site is just about the size of a tennis court).

Let’s consider this possible sequence of events.

1.    250 million years ago, sandstone was deposited in a creek. It is all cross-bedded.

2.    The sand grains were likely coated with iron oxide and thus turned red.

3.    The sandstone was buried by maybe 1000-plus feet of additional rock.

4.    Shales deeper in the geological section were heated by the normal geothermal gradient to hundreds of degrees and adsorbed water was squeezed out, taking with it the barium that was also adsorbed (from the ocean) on to the clay surfaces. The water moved vertically along cracks in the rock.

5.    When the barium reached the rocks we are digging in, the barium precipitated because the pore water is very “hard” with dissolved gypsum. The barium reacts with the local sulfate, producing barium sulfate which is essentially insoluble in water. It is the ultimate “hard water” scum.

6.    The barite precipitated as the vertical seam and filled the pore space in the red rock.

7.    A whole lot of rock was eroded in the next 250 million years to bring us to the present and the rocks back to the surface.

8.    10 to 100-thousand years ago or so, North Texas was in an Ice Age, and it was really wet with lots of vegetation. Decomposing vegetation created a reducing environment which dissolved iron right out of the rock. Barite is very chemically resistant, and this lump could have been at the surface through part of the ice age. The iron could have partially leached out of the lump at that time. You can see that the leaching went deeper into the lump where there are horizontal fractures in the rock.

All this from a lump of rock. Thanks, Gretchen!

Rock Steady!

In addition to the wonderful movies and exhibitions we put on at HMNS, we also offer unique learning experiences. Our paleo hall is a great place to pick up new skills, such as sifting through seashells millions of years old or learning how to cut gemstones.

Most days throughout the summer, a volunteer is stationed in our Paleo hall cutting facets in rock and explaining to the interested visitors how to carve the perfect gem.

Our volunteer starts with a mineral, usually quartz. They set the stone to the holder, and using a protractor measure out the angle at which they are going to grind their quartz. The spinning wheel, which is a diamond gritted lap (diamond is the hardest natural substance known and can cut through anything) is then used to grind the stone and create a facet.

This lap is used in the final stages,
to polish the almost finished stone.

After one side is cut, the stone is rotated so another side can be ground down. Our volunteers use different laps to make larger cuts, or to polish the stone as it nears completion.

Check out the beautifully cut quartz stones pictured below. Notice that the gems can be cut into different shapes with a different number of facets.

An experienced volunteer can craft one of these in a little over an hour.

Interested in becoming a volunteer and learning how to cut gems yourself, or how to lead tours or get more behind the scenes opportunities? Contact Sibyl Keller at 713-639-4656 or check online here.

Want to learn more about gems, diamonds and jewelry? Don’t miss Faberge: Imperial Jeweler to the Tsars, at HMNS until July 25.

100 Years – 100 Objects : Elbaite (on Quartz)

The Houston Museum of Natural Science was founded in 1909 – meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from Joel, the Museum’s President and Curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world, that we’ll be sharing here – and on hmns.org – throughout the year.

main-elbaite-on-quartzTourmaline Queen Mine, San Diego County, California.
North America has produced some extraordinarily beautiful specimens of elbaite, a member of the tourmaline group, but the most admired are the bright red-pink crystals with blue caps found in 1972 at the Tourmaline Queen mine. The 24-cm example pictured here is the finest of the 33 major specimens recovered and is therefore the finest North American tourmaline. The lustrous, lusciously colored, undamaged pair of crystals at the top grow from an undamaged quartz crystal and are accompanied by smaller tourmaline crystals. It has been nicknamed “The Rabbit Ears.”

Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see larger and more detailed images of this rare specimen – as well as the others we’ve posted so far this year – in the photo gallery on hmns.org.