Volume 1, number 2, February 1999 The Geology of Ester Dome Understanding the geology of Ester Dome is like reading a book with most of the pages torn out. The problem is that the rocks are nearly completely covered with soil and vegetation and cannot be directly observed without first being dug up. Adequate exposures can only be found near the summit and in exploration trenches, roads, and mine cuts. Ester Dome is veneered by relatively young unconsolidated water- and wind-borne sediments. Most people are familiar with them because they are more exposed. You only have to drive past Gold Hill to learn what loess is all about, and local miners have been mining the muck, bones, and stream gravel in the valleys for nearly 100 years. But what is underneath? What forms the core of Ester Dome, and what did we build our town’s foundation on? Well, the best conclusions so far are that Ester Dome is underlain by a variety of rocks named Fairbanks Schist, Muskox Sequence, Birch Hill Sequence, and the Cretaceous intrusives. The Fairbanks Schist, Muskox Sequence and Birch Hill Sequence are all metamorphic in origin, meaning they were originally different types of rock that have since been recrystallized, by intense heat and pressure, to their present condition. The Fairbanks Schist and Muskox Sequence (both composed of various rock types: schist, quartzite, and amphibolite) are moderately metamorphosed. They were subjected to temperatures of up to 500 degrees C and 5 kilobars of pressure. These conditions were presumably caused by deep burial by additional rock that has since eroded away (10-20 km). The Birch Hill Sequence (slate, phyllite, meta-tuff and quartzite) was not changed to the same extent and still resembles the sedimentary rock it once was. Unfortunately, the Fairbanks Schist and Muskox Sequence are very similar in appearance (medium-grained, foliated, and containing different types of mica). Their differences are primarily in age and geochemistry. By correlation with similar rocks in the Alaska Range, it has been determined that the Fairbanks Schist may be Precambrian in age (more than 570 million years). It formed side by side with the earliest life on the planet. In comparison, the Muskox Sequence is much younger (369 million years), its age deduced by the uranium-lead content of the mineral zircon included with the rock. At that time in the history of the Earth, plants were just emerging on land and the oceans were inhabited by very primitive fish. Also by correlation, the Birch Hill Sequence was determined to be just slightly older than the Muskox Sequence. Folding due to intense shearing has deformed all of these rocks. Mark Hall, a former student of UAF, determined in 1984 that they have been creased like pieces of paper as many as five times. At 500º C and 5 kilobars of pressure, rocks will flow like toothpaste. Around 90 million years ago, during the Cretaceous, reign of the dinosaurs, Ester Dome was cut by numerous steep, high-angle faults. These faults are still alive. They are related to an entire network of northeast-trending faults that lie between the Denali and Tintina fault zones, which are still seismically active today. I think most of us can remember the big one centered near Murphy Dome several years ago? That earthquake occurred on one of the larger faults that transect the district. California may have the San Andreas, but Ester has the "Magic Fault." This fault passes right through the town, cutting the bedrock directly under Mark Simpson’s new house on Pebble Drive. It also passes through the Ryan Lode Mine, Grant Mine, Goldstream Valley and the True North Deposit. A lot of gold is associated with this fault, thus the name, "Magic Fault." Soon after they formed, the high-angle faults were intruded in several places by granodiorite and/or mafic dikes. Intrusion is the forcible entry of molten rock into other rock form-ations. Both intrusive rock types have relatively low silica and higher sodium plagioclase contents than typical granite. They are not as prevalent as the metamorphic rocks, but you can find them if you go looking. It is primarily in the faults located near intrusives that people have found lode gold. Lode gold is still in place in the bedrock, versus placer gold that has eroded out and is now found in the stream bottoms. The richest deposits of lode gold on Ester Dome are associated with relatively thick, milky-white, crystalline quartz veins formed within the faults. These veins are typically repeatedly broken and cemented with additional quartz. The gold is commonly visible to the eye and often associated with sulfide minerals. The geology of Ester Dome is not unique to Fairbanks or even to the rest of the interior of Alaska. The entire Yukon-Tanana Upland shares a similar geologic history. The metamorphic rocks forming Ester Dome are widespread and originated in a time long gone and unknown to us. Entire oceans have opened and closed, mountain ranges have formed and slowly eroded away. Some things we will just never know. | ||
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