Hydrothermal and Metamorphic Environment main content.

Hydrothermal and Metamorphic Environment

Part of Allison and Roberto Mignone Halls of Gems and Minerals.


The development of a mineral deposit may involve more than one kind of environmental condition. The world-famous and valuable zinc-manganese-iron ore deposit 50 miles northwest of New York, in the Franklin-Sterling Hill area of New Jersey, is an excellent example. About 230 mineral species, a record number, have been found in a complex geologic situation that probably involved both hydrothermal and metamorphic mineral-forming activities. Samples of the ore rock, its contained ore minerals, and other outstanding mineral specimens are displayed here.

Audio Transcript
These three cases deal with a collection of minerals that geologists think may be the result of the interaction of processes at work in more than one kind of environment. In all probability, the minerals shown here were developed through the processes involved in both the hydrothermal and metamorphic environments.

Approximately 50 miles Northwest of New York City, in New Jersey, is a very famous mineral locality the town of Franklin. It's an area where zinc had been mined until 1954. Zinc mining still continues near Franklin, at Sterling Hill in Ogdensurg.

What makes the Franklin Sterling hill locality unusual is the vast variety of mineral species that have been found here — some 230, which is close to being a world's record. Additionally, many of the minerals, such as the Franklinite, the zincite, and the willemite, have not been found in any abundance elsewhere in the world.

And to make the area even more exciting, quite a few of the minerals that are known from the Franklin locality spectacularly change their color when brought under the influence of an ultraviolet lamp, a property known as fluorescence. A sample of the ore-bearing rock is specimen number 1. And in somewhat of a cornucopia fashion, there spills from this sample the great variety of minerals that you see here. Although only 89 specimens are on display, do appreciate the fact that over 130 additional minerals species could be displayed.

The Franklin area is perhaps one of the best-known mineral occurrences in the world, and how fortunate we are here, in the New York City region, that it is so close at hand. While geologists do not agree completely just how this diversified mineral body came to be, it seems, in all probability, that both the hydrothermal and metamorphic activities were involved a very, very long time ago in Earth's remote history. In fact, these minerals are thought to have formed sometime in the order of 1 billion, 200 million years ago.