"The Franklin and the nearby Sterling Hill ore bodies are interpreted as originally stratiform deposits of sedimentary origin. This material was recrystallized and markedly deformed by regional dynamothermal metamorphism at the sillimanite grade during late Precambrian time.

Internally, the Franklin ore body is composed of bed-like or lens-like bodies of ore and calcsilicate. They are arranged in a laminated fashion conformable to the synclinal structure of the ore body and to sedimentary structures in the enclosing marble. The individual ore units vary widely in the relative proportions of franklinite, willemite, zincite, calcite, and accessory manganese-bearing silicates of the olivine and chondrodite groups. The sedimentary protore is believed to have been largely carbonatic, with zincite formed by the thermal decomposition of smithsonite or of solid solutions in the smithsonite-rhodochrosite-siderite series.

The calcsilicate bodies are characterized by calcite, manganoan andradite, rhodonite, bustamite, pyroxenes and amphiboles containing much Mn and Zn, barium feldspars rich in Pb and Sr, hendricksite, vesuvianite, wollastonite, and minor franklinite. They represent the recrystallization of relatively highly argillaceous material intercalated in the original sedimentary sequence. In both their detailed mineralogy and their content of minor and trace elements, the calcsilicate bodies show close similarities to metamorphosed sedimentary manganese deposits. The similarities do not wholly extend to the ore units because of the presence therein of much more Zn and much less Si and Al.

Most of the over 230 different minerals that have been identified at Franklin and Sterling Hill are either weathering products or are associated with low temperature hydrothermal veinlets and altered areas. The hydrothermal mineralization is believed to derive from the reworking of the minerals of the primary ore and calcsilicate bodies by a regional metamorphism that was later and of lower grade than the original re-crystallization. Only about 25 minerals are common first stage metamorphic constituents that characterize the original deposit in a genetic sense. Intrusive pegmatites are postore and have had a negligible effect on the mineralization of the deposits."