Magnetite was mined extensively from the rocks of Harriman State Park and the entire Highlands region from colonial times until the mid-19th century. The magnetite was smelted with a flux of limestone from the local Wappinger Group limestones of Cambrian age to form iron ore. The region was the major source for iron in the colonial, revolutionary, and early nation periods in American history. The great chain that was strung across the Hudson River to stop the British ships during the Revolutionary war was made from iron from the Harriman magnetite deposits.

FORMS OF MAGNETITE DEPOSITS
The magnetite deposits take on several forms with several origins. The one described here was extensively studied and applies to several major mines including Hogencamp, Pine Swamp, Bradley, Surebridge, and Greenwood Swamp among others. These ore deposits form long sheet-like bodies that fill fractures. These bodies cross the main layering in the gneisses. Some other types of deposits are interlayered with the gneissic layering. Still others are associated with magma intrusions.

MAGNETITE VEINS
Late in the history of a major strike-slip faulting event, large dilatant fractures opened along the margin of the faults. Hot metamorphic fluids flushed through these fractures depositing minerals along the walls. The minerals that fill the veins vary with the rock type where the veins deposits occur. Minerals common to all deposits in order include pyroxene (clino and ortho), hornblende, phlogopite-biotite, and magnetite. Some deposits include early scapolite, apatite, and sphene and late calcite, extensive hornblende and late quartz, and late sulfides including marcasite, pyrite, chalcopyrite, and pentlandite. These veins can be traced for up to 5 miles from one magnetite deposit to another. Isotopic dating of these veins indicates that they formed about 923 million years ago.

Magnetite veins surrounded by a pyroxene mass in Hogancamp Mine in Harriman State Park