French Creek Mines, St. Peters, Warwick Township, Chester Co., Pennsylvania, USA
|Latitude & Longitude (WGS84):||40° 10' 59'' North , 75° 43' 0'' West|
|Latitude & Longitude (decimal):||40.18333,-75.71667|
|Köppen climate type:||Cfa : Humid subtropical climate|
This mine includes the old Klein (lower) and Elizabeth (upper) mines and the Crossley pits. Located near St. Peters. Dumps have produced specimens to now.
The French Creek iron ores are a “Cornwall type” iron ore deposit (Lapham and Gray, 1972). Many examples of this type of iron deposit, including two very large ones, occur in the east-west neck that connects the Newark and Gettysburg Basins. This neck is filled by late Triassic age clastic and lacustrine sediments and has been heavily intruded by thick sheets of early Jurassic age diabase.
The margins of the neck are complexly faulted. In some places, the diabase intrudes the older country rocks on which the Triassic-Jurassic Basin has been superimposed. In most cases the intruded older rocks are Paleozoic shelf carbonates. In places where a diabase sheet intrudes into the complex basin margin and isolates a mass of carbonate rock above it the magnetite skarn of a Cornwall type iron deposit can develop. However, many aspects of the ore forming process remain to be resolved.
The French Creek ore deposit is a little different because the carbonate rock replaced by magnetite skarn is proterozoic marble (Smith, 1931). In the French Creek area, the southern margin of the Triassic sediments rests on the Precambrian metamorphic rocks of the Honeybrook upland. The Precambrian rocks are graphitic biotite gneiss with a few local pods or bands of marble. The marble occurrences are similar to those in the eastern block of the Reading Prong Highlands in New Jersey and the adjacent Hudson Highlands in New York.
At French Creek Jurassic diabase intrudes the Honeybrook rocks that included two significantly large pods of marble. The smaller marble lens was in close proximity to the diabase while only the lower part of the larger lens was in contact with the intrusive rocks. The marble was replaced to varying degrees by magnetite skarn.
Mineral ListMineral list contains entries from the region specified including sub-localities
55 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
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145 - 201.3 Ma
Age: Jurassic (145 - 201.3 Ma)
Description: Medium- to coarse-grained, quartz-normative tholeiite; composedof labradorite and various pyroxenes; occurs as dikes, sheets, and a few small flows. Includes the dark-gray York Haven Diabase (high titanium oxide) and the slightly younger Rossville Diabase (low titanium oxide). In chilled margins, the Rossville is distinguished from the York Haven by its lighter gray color and distinctive, sparse, centimeter-sized calcic-plagioclase phenocrysts.
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
201.3 - 252.17 Ma
|Mesozoic sedimentary rocks|
Age: Triassic (201.3 - 252.17 Ma)
Comments: Newark Graben System; Newark-Gettysburg Basin
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529.