Silver Mine Park Geology Guide
PEQUEA SILVER MINE At Pequea Township Park
GEOLOGY GUIDE
History. Ore in the Pequea area was known in colonial times but the bulk of the workings visible today probably date from around the time of the Civil War. An 1863 stock prospectus for the “Lancaster Lead Company” attempted to raise $250,000 to begin major ine operations. The prospectus describes extensive and ongoing exploration in 1863 with the sinking of a number of surface shafts but makes no mention of the main underground workings nor of the present open cut excavations. It speaks only of the northerly dip or inclination of the veins indicating that the folding had not been recognized as of the date. Apparently, the stock was sold and most of the present underground workings as well as the surface strip mining aroundm the folds were completed soon thereafter, presumably as part of the lead supply for Civil War misket balls.
Minerals. Around the mine, the most abundant mineral is milky white vein quartz, the host material for the ore. The chief ore malena was galena, a lead sulfide. Characteristics for its identification indluce a silvery metallic luster and a fracture or cleavage into tiny cubes. Galena from this mine included a large number of silver stoms that substituted for lead atoms in the crystal structure. 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.
Rocks. 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.
Mine Workings. This walking tour follows the surface outcrop of the ore veins along exposures that date from Civil War times. The block diagram of Figure 1 shows the east-west trend of these fold hinges and how they crop out as curving lines in the valley wall. This method of digging along a line of outcrop was a simple and cheap way of mining the ore but was obviously limited in extent. For efficient underground workings, most efficient excavations are generally along a horizontal tunnel or adit. To get under the maximum amount of ore, this adit would be driven as low as possible on the valley but just high enough to be above the water table to erase mine drainage. Such considerations would have determined where to start the underground workings. Figures 2 and 3. Apparently, the localization or ore in the fold hinges was recognized by the time underground workings began because the mine adit does not start at the logical location of the vein outcrop but in the black phyllite and heads directly toward the hinge zone. All these details of the underground workings show signs of sophisticated knowledge both of site geology and mining methods, presumably by the “Mr. F.P. Herington, an experienced and highly skilful mining engineer” described in the 1863 prospectus. There are several other mine shafts in the local area supporting the interpretation that these ores occurred in very irregular small and discontinuous pods and pockets. The abandonment of the main mine workings probably resulted from the removal of all ore in the visible pockets in the mine and the decreasing demand for lead immediately following the Civil War.
Geology. Several major chapters in the geologic history of this area are responsible for these deposits.
Stage 1. The story begins about 500 million years ago in Cambrian times when this area was the edge of an ancient ocean (Figure 4A) 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. At location #9 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.
