Latitude: 39°46'16"N
Longitude: 105°12'22"W
This is one of only 2 localities open for collecting on North Table Mountain. It is located on the corner of Hwy 58 and Easley Rd. There is a trail going up along the road cut on Hwy 58, which then forks east to the quarry. The Quarry is almost directly above the road cut, at the base of the cliff at the top of N. Table Mountain.
The North Table Mountain mesa contains 4 separate lava flows. These flows are shoshonite (K-rich basalt) porphyry erupted fluidly from vents a short distance to the north. The eruptive sites are now eroded; their remnants are exposed as plugs just south of the Ralston Reservoir. The volcanic activity took place in the early Paleocene, approximately 64 ma. Dating indicates that all 4 flows were erupted during an interval of slightly less than 1 ma (Drewes, 2006).
In recent years the most prolific mineral collecting site has been an old quarry in the capping Flows, 3 and 4, along the south side of the mesa directly above a prominent highway cut constructed in the late 1960’s. This is referred to as the Southeast Quarry. At this site the top 25 feet of Flow 3 are highly oxidized and amygdaloidal. The collectable material is the substantial amount of rubble on the quarry floor containing boulders of all sizes up to some well in excess of 20 tons. Amygdaloidal cavities in the rubble boulders are commonly up to several inches but may occasionally be 8 inches or larger. That after many, many years of collecting there is still much material to attack is a testament to the tenacity of the rock.
In some areas, the famous copper/zeolite-rich basalts of New Jersey for example, alteration and secondary mineral distributions within amygdaloidal horizons, as well as other mineralized structures, are systematic. They reflect many factors; principally permeability, fluid flux, proximity to the fluid source, crystallinity of the country rock, temperature and the changes in these factors through time. This author is not aware of any study of the size distribution of amygdules in the upper part of Flow 3, of systematic variation of alteration or of the distribution minerals within the amygdaloidal horizon at the Southeast quarry or elsewhere in the Table Mountain area.
References
Table Mountain Shoshonite Porphyry Lava Flows and Their Vents,
Golden, Colorado, Drewes, H., 2008, Table Mountain shoshonite porphyry lava flows and their vents, Golden, Colorado: U.S. Geological Survey Scientific Investigations Report 2006–5242, 28 p.
Mineral List
6 entries listed. 4 valid minerals.
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