FRANKLIN AND STERLING HILL NEW JERSEY: THE WORLD'S MOST MAGNIFICENT MINERAL DEPOSITS
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SOROSILICATES AND CYCLOSILICATES INOSILICATES PHYLLOSILICATES TECTOSILICATES AND SILICATES OF UNKNOWN STRUCTURE
ELEMENTS SULFIDES ARSENIDES ANTIMONIDES AND SULFOSALTS OXIDES AND HYDROXIDES HALIDES AND CARBONATES
SULFATES BORATES TUNGSTATES AND MOLYBDATES ARSENATRES ARSENIDES PHOSPHATES AND VANADATES UNNAMED MINERALS


General discussion

 

Regional geology

 

Local geology

 

Major formations and rock units

 

The local magnetite deposits

Regional geology  

The definitive recent works on the regional geology of the area are those of Smith (1969), Drake (1969, 1984, 1990, 1991), Puffer (1980), Rankin et al. (1989), Volkert and Drake (1986), Drake et al. (1991), and Read in Rankin et al. (1989). The area is part of the Reading Prong of the New England physiographic province. Drake (1990) stated:

“Rocks in the Franklin-Sterling Hill district have a basement of quartz keratophyre, trondhjemite, spilite, tonalite, and enderbite interpreted to be an oceanic suite. This basement is overlain by a sequence of biotite-quartz-feldspar gneiss (graywacke), marble, amphibolite (pillow basalt in part), and potassic feldspar gneiss that is interpreted to be largely meta-arkose, in part with an exhalative component. This association of rocks is only compatible with a rift environment. I suggest that these rocks were deposited in a rift trough on oceanic crust along a continental margin. The ocean lay to the east (present direction) of the New Jersey Highlands as rocks in the external basement massifs and Adirondacks to the north and south are similar to those here, and metavolcanic rocks core the Baltimore “gneiss-domes” to the southeast (Drake et al., 1991). Later closing of the ocean resulted in Grenvillian orogenesis and the emplacement of synkinematic granitoids, which were generated in a continental arc at about 1090 Ma [million years ago].  Grenvillian orogenesis in this area apparently occurred over a long time-span, as synkinematic granite in the Hudson Highlands of New York has been dated at 1134 Ma (J. N. Aleinikoff, written communication, 1989). Orogenesis had ceased by 1020 Ma when post-kinematic granite related to post-collision uplift was emplaced.”

 
 
 
  Figure 8-4. Geologic map of the Franklin mining district; this is the “Special Map.” Illustration from Palache (1935).  
   

 

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CHAPTER 8. REGIONAL AND LOCAL GEOLOGY OF THE FRANKLIN - STERLING HILL AREA