Silver and gold? EarthCache
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Pequea Silver Mine is located 4 miles south of Lancaster along the Pequea Creek. Silver occurs in galena which is concentrated in quartz veins and along joints in the Vintage dolomite just below the Conestoga phyllitic limestone. The Pequea silver mine was worked from before the Revolutionary War to 1875. A minor amount of mining was done about 1900.
Galena deposits often contain significant amounts of silver as included silver sulfide mineral phases or as limited solid solution within the galena structure. These argentiferous galenas have long been the most important ore of silver in mining.
About 500 million years ago in Cambrian times this area was the edge of an ancient ocean and lasted for about 50 million years. The city of Lancaster lay at the edge of a great limestone bank, somewhat like the barrier reefs of Australia today. North of Lancaster the thick bedded, shallow water limestones, we see today, were deposited in the quiet marine backwaters. To the south of Lancaster, are the thin-bedded sandy and muddy limestones and black shales of the Conestoga Formation. These are the continental slope deposits of material washed or collapsed into deeper water from off the limestone bank. Along Silver Mine Road is an outcrop of one of the submarine slide materials. This contains boulders of the shallow water limestones which slid off the bank and down the submarine slope into this area. The Vintage Dolomite constitutes the shallow water deposits which were later overwhelmed and covered by the black shales and limestones of the Conestoga Formation. The deeper ocean waters were rather stagnant so that organic carbon was preserved to cause the deep blue color of the Conestoga limestones and the black color of its shales. The stagnant waters also preserved some of the sulphur, iron, lead, zinc, and silver collected by primitive animals living in those seas. This is probably the original source for the pyrite cubes (iron sulfide) that occur throughout the area, as well as the ore minerals.
Around the mine, the most abundant mineral is milky white vein quartz, the host material for the ore. The chief ore was galena, a lead sulfide. Characteristics for its identification: a silvery metallic luster and a fracture or cleavage into tiny cubes. Galena from this mine included a large number of silver atom. These silver atoms warped the crystal structure and caused pronounced curvature of the galena’s cleavage surfaces. When the Pequea ore was smelted, a significant amount of silver was also recovered. Analyses are reported to have included 10 to 16 ounces of silver to the ton, or about $50 per ton at present prices. Unfortunately for modern mineral collectors, the mine was so thoroughly worked in its day and the old dumps were so thoroughly picked over by later mineral collectors that finding even a tiny fragment of galena is now quite rare. Small amounts of sphalerite (zinc sulfide) also occur in the ore.
The two most abundant host or country rocks are: (1) massive, light gray dolomite of the Vintage Formation (Dolomite is a magnesium-rich limestone.) and (2) black, limey phyllites and limestones of the Conestoga Formation (Phyllite is a recrystallized shale.) Both formations were deposited in the sea between about 500 and 450 million years ago. From about 450 to 250 million years ago these dolomite and shale formations were folded and fractured at great depth. During this time hot waters moved through them to emplace the quartz and galena. During deformation, the dolomite was brittle and fractured to provide openings for the hot waters whereas the shale flowed and acted as a seal. For this reason the galena bearing quartz veins are now concentrated in the dolomite where the hot waters were trapped just beneath the impermeable shale cover. Since that time erosion has removed the former mountains to expose these deeper roots of the once mighty Appalachians
Posted coordinates take you to nearest parking at Pequea's Silver Mine Park where you will find 11 specimens.
In order to claim a "find", please submit the following--
1) Take a photo of yourself with specimen #8 and upload with your log.
E-mail (do not post) the answers to these questions to my account:
2) What is the "mark" in specimen #6 and why is it there?
3) What is specimen #4 famous for?
4) What are the marks that specimen #7 are known for?
5) Through research, determine how the silver that was extracted at Pequea came to be in the galena. What historic and geologic events occurred that caused this?
Additional Hints
(Decrypt)
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