Sphalerite & Calcite

The Balmat, Hyatt, Edwards and Pierrepont zinc-lead deposits are hosted by Mesoproterozoic Grenville Series (1120 Ma) marbles in St. Lawrence County in north-western New York State, USA.

The Balmat District are located within an outlier of the Grenville Province on the margin of the Canadian Shield. This orogenic province comprises a series of intense accretionary events that occurred during the Mesoproterozoic in the final stages of assembly of the supercontinent Rodinia, interpreted to have resulted from a continent-continent collision between Laurentia and Amazonia. Following the Grenville accretionary events, a phase of tectonic relaxation occurred and the Canadian Shield margin underwent rifting, erosion and eventually was overlain by a cover sequence of Palaeozoic sedimentary rocks. Paleozoic deposition began with the late Cambrian to early Ordovician Potsdam Sandstone, followed by a limestone-dolostone sequence. Magmatism accompanied both orogenesis and rifting and is represented by numerous igneous intrusions of a range of ages, which have been metamorphosed to varying degrees.

Occurrences of zinc and lead were reported on the Balmat farm as early as 1838, but it was not until 1927. Recently (about 2 years ago), a major find of gemmy, highly lustrous sphalerite crystals associated with calcite and quartz was reported, doesn't like many other sphalerites found in MVP (Mississippi Valley Type) deposits, which are dark red, these crystals display a very rich, bright golden color that contrasts well with the light color matrix. I have seen some major finds of purple calcite from this local and big plates of sphalerite, and I was lucky enough to have a full-size thumbnail for my collection.

Sphalerite on Calcite and Quartz, 3cm, Empire State No. 4 Mine, Balmat, Fowler, St. Lawrence County, New York, USA

Jingnan Zhang Coll & Photo