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Author: Admin | 2025-04-28
Early as 1855, but for many years it failed to gain traction. An important step to our modern understanding took place in 1966, when William Callahan, a New Jersey Zinc Company geologist, likened the ores to the metamorphic equivalent of the sediments currently forming in brine pools in the Red Sea. Here we have the beginnings of the volcanic exhalative view of our zinc deposits – namely, that they formed as hot, metalliferous brines erupted onto an ancient sea floor. In this view the ores are syngenetic – that is, they formed at the same time as their host rocks. Research since that time has only strengthened this general concept of the origin of the ores.Over the last two decades, the New Jersey Highlands have been thoroughly remapped by geologists of the New Jersey Geological Survey. The new detailed maps, combined with carbon and oxygen isotopic studies and a series of precise age determinations to put the rock units in proper chronological context, have produced a conceptual plate-tectonic model of the evolution of the region. It is now generally accepted that the ore deposits at Franklin and Sterling Hill originated as hot, metalliferous brines that were discharged onto the floor of a shallow sea about 1.3 billion years ago. The sea floor at that time was covered by white carbonate muds, similar in many respects to the Bahama Banks today. As the hot brines came into contact with the much colder seawater, the metals dissolved in them precipitated as small particles that then fell to the sea floor, there to accumulate as a series of metal-rich layers within the thick sequence of white carbonate muds. The original mineralogy of these metalliferous layers is still uncertain, but whatever their nature, metamorphism of these original materials gave us the orebodies that we see today.The illustration below depicts the probable plate-tectonic setting at the time of ore deposition. Here we see, in cross section, an oceanic plate at right colliding with a continental plate at left. Because oceanic crust is significantly denser than continental crust, the result of such collision is that the continental plate overrides the oceanic plate, which then dives or “subducts” beneath the continental plate. Partial melting of the oceanic plate as it descends beneath the continental plate gives rise to a chain of volcanoes (the Losee arc) that shed sediment into a shallow sea. The area of the shallow sea is aback-arc basin, a region beneath which the Earth’s crust is being pulled apart. This extension forms fracture zones deep in the crust that allow hot, metalliferous brines to ascend to shallow depths and erupt onto the sea floor, there to form layered deposits of metal-rich muds. This was the situation about 1.3 billion years ago, when the carbonate muds (later to become the Franklin Marble) and the metal-rich layers within them (later to become the zinc orebodies) were first deposited. About one billion years ago, as the plates continued to move, a second continental plate collided with the
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