Fields' Copper Mine (Field Mine), Warrenville, Washington Valley, Somerset Co., New Jersey, USA
Latitude: 40°36'N
Longitude: 74°29'W
Ref.: NJ State Geologist's Annual Rpt. (1897):81; The Minerals of New York City & Its Environs, New York Mineralogical Club Bull., Vol. 3, No. 1, Manchester, J.G. (1931): 97. A copper mine located near Warrenville and about 4½ miles NNW of Bound Brook. Deposit is in green, gray & black carbonaceous shale. Featured a shaft. No specifiic species listed but fossils are cited as occurring here. NOTE: The description of the shale is consistent with the hornfelsic sedimentary rocks immediately underlying the Watchung basalt and commonly host to copper mineralization. This locality is near the Pluckemin and American copper mines.
As with many older sites the descriptions are often vague and need to be taken with caution. There is no evidence that the bed(s) that host the copper immediately beneath the Orange Mountain Basalt were carbonaceous. However, the Washington Valley is underlain by the Feltville Formation which contains a prominent carbonaceous unit, the Washington Valley member. There are a small number of copper occurrences associated with both the breccias at the upper contact of the Orange Mountain basalt and the carbonaceous shale (and locally limestone) that closely overly the basalt. Copper deposition is probably localized where fractures transecting the basalt had a relatively high permeability. Some of the connate brines from Passaic Formation, which were responsible for the Chimney Rock, American and other copper deposits, reached the upper contact of the OMB with enough copper intact to permit deposition of minor amounts. Copper deposition required chemical reduction of sulphate to sulphide. In the basalt the reduction was accomplished by the oxidation of ferrous iron to ferric iron, just like at the lower contact of the OMB. In the Washington Valley member copper deposition probably resulted from a reaction of the oxidized copper-bearing fluid with either pyrite or sulphate reducing bacteria in the shale. This is the typical deposition mechanism in Red Bed Type copper deposits.
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