High Grade Zinc Ore from Franklin, New Jersey
Contributed by: Michael Crawford
Date: Jun 27th, 2025
Locality: Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 10 x 11.5 cm
Description:
This is a specimen of 100 percent zinc ore from the Franklin Mine, Sussex County, New Jersey. The specimen contains three ore minerals: green willemite (Zn2SiO4), red zincite (ZnO), black franklinite (Zn2+Fe3+2O4). Only willemite fluoresces and its green fluorescence is very bright under all UV wavelengths. The shortwave emission spectrum has a maximum of 524 nm.
The specimen exhibits distinct banding with one of the three minerals dominating a band. The banding may be the result of the original deposition of the ore deposit. The Franklin and Sterling Hill ore bodies were precipitated from high temperature fluids in a back arc basin on top of sedimentary carbonate deposits that eventually became the Franklin Marble. Willemite, zincite and franklinite represent the original minerals precipitated and the layers represent separate expulsion episodes of hydrothermal fluids with changing chemistries that favored the precipitation of one ore mineral over the others. Carbonate deposition continued during the hydrothermal events and afterwards which left the ore bodies enclosed in carbonate deposits.
At a later date, the flat lying ore deposits and carbonates were deeply buried, deformed into an overturned syncline, and strongly metamorphosed by very high temperatures and pressures. The carbonates were metamorphosed into the Franklin Marble. The banding in this specimen could also be the result of shearing in the ore beds due to folding of the deposits into the overturned syncline under high heat and pressure. The shearing in the limbs of the fold can produce a gneissic (layered) texture. The layering in this specimen could be depositional, the result of deformational shearing, or both.
Summary of luminescence responses:
Willemite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green