By James C. Rakestraw, PE, CFM
In Part 1 of this series we examined the pre-plate tectonic view of uniformitarian geology. In this post, we will look at what is termed the geologic column. The geologic column is an important to geologists and any geologic study of an area will list the geologic column for that area. A geologic column starts with oldest observed rocks in a study area and goes upward to the most recent. The best-known geologic column is that of the Grand Canyon.
The Journey of a Skeptic, Step 2, Gilman, Colorado and the Eagle Mine:
Gilman was a company town that was constructed by the New Jersey Zinc Co., the owner of the Eagle Mine. The town is a situated on a cliff about 600 feet above the Eagle River. Interest in Gilman continues; it is the subject of ghost town Facebook pages and YouTube videos. Wikipedia has an article on Gilman which gives a good overview of the place.
My job at the Eagle Mine was to go underground and record the geology that was exposed as mining was taking place. Then I updated the mine maps at the mine office. The mine was a consolidation of earlier mines by the New Jersey Zinc Co. It was a fabulous property that produced over 250 million dollars’ worth of, copper, silver, gold, and lead over its productive life which ended in 1984.
Before going further, I need to emphasize two very important things:
- Gilman is posted: NO TRESPASSING!
- Never enter a mine unless it is a tourist attraction: KEEP OUT – STAY ALIVE!
The geology of the Eagle Mine was simple but fascinating. It is fully described in a chapter in the book “Ore Deposits of the United States, 1933-1967” (published by the American Association of Mining, Metallurgical and Petroleum Engineers in1968). The geologic column at Gilman is exposed in a deep canyon of the Eagle River. At the base column (elevation about 8900) is Precambrian basement rock, which consists of schist (a metamorphic rock). The schist was intruded by granite (an igneous rock). The basement rock was overlain by 200 approximately feet of the Cambrian Sawatch Quartzite. The basement rocks and the Sawatch Quartzite are separated by the Great Unconformity. Above the Sawatch Quartzite are layers of shale, sandstone, limestone, and dolomite. The most significant sedimentary layer is the Leadville Limestone which is the host rock for most of the ore deposits. Above the Leadville Limestone was the Belden Shale and the Gilman Sill. Above the Gilman Sill are great thickness of later sedimentary rocks. All the rock layers of the geologic column dip to the east at an angle of about 10 degrees.
The Great Unconformity is observed only in a few places in the world; the most notable is at the Grand Canyon in Arizona. At Gilman, it is possible to walk up to the Great Unconformaity and place your right hand on the Precambrian basement rocks (3.0 billion years old) and your left hand on the Sawach Quartzite (0.5 billion years old). There is nothing between these totally different rocks that differ in age by 2.5 million years.
Although the Great Unconformity may be viewed in only a few places. It is listed frequently in mining geology literature – mining geologists are able to see what is underground as well as what is on the surface. In mining literature, the most commonly listed rock above the Great Unconformity is quartzite, a metamorphic rock with the chemical composition as sandstone (SiO2). The difference between sandstone and quartzite is that sandstone breaks around quartz grains while quartzite breaks across them.
The geologic column at Gilman contains layers of sedimentary rocks of different geologic periods all separated by “flat gaps.” The term “flat gap” is used describe how two different types of rock rest on top of each other with no erosion surface between them. At Gilman, the first flat gap is between the Precambrian basement rocks and the overlying Cambrian Sawatch Quartzite. There are flat gaps between layers of sandstone, limestone, dolomite and shale all the way up to the rocks of the Pennsylvania period at the top of the geologic column. In contrast to the “flat gaps” the Eagle River carved a canyon about 2,000 feet deep from rocks of the Pennsylvania Period into the Precambrian basement rocks.
The ore deposits of Gilman are found in four situations:
- Fissure veins are found in the basement rocks. These veins end at the Great Unconformity. The oxidized portions of the veins produced high-grade gold and silver; several small mines recorded some production.
- A zone of high-grade gold ore was found in the Rocky Point horizon of the Sawatch Quartzite. Several small mines recorded some production.
- “Chimney deposits”, of which there are several, extend from above the Precambrian to the Belden Shale, which overlays the Leadville Limestone. The mineralization of the center of chimney deposits consisted on pyrite (FeS2), chalcopyrite (FeCuS2), silver, and gold. Sphalerite (ZnS) is found at the edges of the chimneys. The chimneys may have ascended higher in the geologic column, but the Belden Shale blocked the rising mineralized solutions. As a result, the mineral deposits spread out laterally under the Belden Shale to form “Manto” deposits.”
- Manto deposits were found at the upper portions of the Leadville Limestone and under the Belden Shale. They extend laterally from the chimney deposits. Nearest the chimneys the manto deposits consisted sphalerite, and further from the chimneys galena (PbS).
The mineralogy of the Gilman ore deposit displays a zonal pattern based on temperature. Zonal patterns of ore depositions are developed to one degree or another in several mining districts – most notably Butte, Montana. The central (highest temperature) portion of the Butte ore deposit contained molybdenum rich mineralization surrounded by rings that were rich in copper, then zinc, lead, then silver, and finally, manganese.
I was thankful for my firsthand encounter with the Great Unconformity and a geologic column that extended from the Precambrian into the Pennsylvanian eras. However, I started to formulate questions:
- Why was there are Great Unconformity? How can uniform processes create a single Great Unconformity?
- Why is there a “flat gap” representing 2.5 billion years between the Precambrian basement rocks and the Cambrian Sawatch Quartzite? Wouldn’t over two billion years of rainfall produce some topography?
- Why are gaps between sedimentary rocks of different geologic periods flat?
- How could the Eagle River, a relatively small stream with a relatively small drainage area produce such a wide and deep canyon at Gilman?
My time in Gilman was cut short by my “Call of Duty” to the US Army Engineer School at Fort Belvior, Virginia. I looked forward to returning to Gilman, after my time in the Army, but God had other plans.