The Sterling zinc-manganese deposit crops out on a hilltop immediately west of the Wallkill Valley. This hill is called Sterling Hill. The deposit is shaped like a trough, the axis of the trough oriented about 30° east of north and plunging northeastward at an angle of 45° from the horizontal. Because of this plunging trough shape, the deposit resembles a fish-hook in both map and cross-sectional views. This is illustrated in figure 1.

The subparallel limbs of the trough, designated the west limb and the east limb, dip 55° to the southeast. This means that the east limb is over-turned, which is to say that the lowermost ore layer (see below) faces upward. The bottom of the trough is called the keel. A thickened wedge of mineralization joins the approximate mid-point of the west limb with that of the east limb. This is named the crossmember. The barren marble interior of the deposit is called the core. At depth the northern portion of the east limb has been displaced northward by faulting. This isolated block of mineralization is known as the North Ore Body.

The longer east limb of the Sterling deposit ranges up to 1/2 mile in length. Its average thickness is 10 to 12 feet, but is locally as thick as 30 feet. The shorter west limb is similar in thickness. The crossmember ranges up to 100 feet in thickness, but an average value is closer to 50 feet. The North Ore Body bottomed at a depth of about 2,550 feet. Total tonnage of the Sterling deposit was about 18 millions short tons at an average grade of 20% zinc. This is exceptionally high-grade in comparison to most of the world’s zinc deposits. The deposit is also unusual for containing relatively little lead or sulfur. Both of these are undesirable in the smelting and refining of commercial-grade zinc metal. About 14 million tons of the deposit had been mined prior to mine closure in 1986. Before erosion the deposit may have totaled as much as 48 million tons.

Visitors to the museum may observe all of the different portions of the deposit except the North Ore Body. Remnants of the east limb are present in the stripped wall above the adit opening, as well as outcrops south of the Edison Tunnel portal. The east limb is also exposed underground in the Landmesser tunnel workings. The west limb may be observed underground in the Rainbow Room. It also forms the west wall of both the Passaic and Noble pits. The keel is exposed along the south wall of the Noble pit. The crossmember is cut by the Edison tunnel workings underground and is also exposed in the north high-wall of the Passaic pit.

More than 350 different mineral species have been found in the Sterling Hill and Franklin ore bodies and in the adjacent Franklin Marble. Several others remain to be described. This list comprises nearly 10% of all known mineral species and represents the most diverse mineralogy of any mineral deposit on earth. Similarly, just over 80 of these species fluoresce in ultraviolet light, the largest number of fluorescent specie occurring at any mineral deposit in the world. These have delighted mineral hobbyists and school children for decades. Finally, 35 minerals are known only from Sterling Hill and Franklin - this too is among the largest number of specie unique to a single locality.

Among Sterling Hill’s minerals about 25 are primary constituents of the mineralization. Three - franklinite, zincite, and willemite - comprise the ore minerals. Franklinite and zincite are virtually unknown except at Sterling Hill and Franklin. Although willemite has been found at a number of localities around the world, it has been mined as an ore mineral only at the small Boquira deposits in Bahia, Brazil and at the two big ore bodies in New Jersey. Except at Sterling Hill, Franklin, and a small mineral deposit in the San Bernardino Mountains of California willemite has also always been found in the weathering environment. The reader should be starting to understand some of the factors, which make Sterling Hill and Franklin so unique.

The mineralization at Sterling Hill is in part layered or banded. Lowermost in the west and east limbs is a thin layer, rich in zincite. Above the zincite-rich layer is a much thicker band, rich in franklinite, and above that a similarly thicker band in which willemite is the dominant ore mineral. Recalling that the east limb of the deposit is overturned, the zincite-rich layer is structurally uppermost in that part of the deposit; the willemite-rich layer on the bottom. Layering within the limbs of the deposit may be observed in the Rainbow Room (west limb) and in the outcrops immediately south of the Edison Tunnel portal (east limb). This layering of the mineralization is conformable with (subparallel to) layering in the adjacent Franklin Marble. This bears on the origin of the deposit. The crossmember is not so much layered as zoned. Near its join with the west limb is the Central Zincite Zone; in the center is the Black Willemite Zone; and near the crossmember’s junction with the east limb is the Brown Willemite Zone.

Many hypotheses of origin have been proposed for the Sterling Hill deposit. Early theories centered around an epigenetic origin, which is to say the mineral deposit was formed after the formation of the rocks hosting it.

Today, geologists believe that the Sterling Hill deposit is syngenetic. The deposit was formed together with the host Franklin Marble. The deposit is imagined as an original, flat-lying, hot spring accumulation - for readers who have visited the park, one may make a very loose analogy to the tufa mounds of Yellowstone - hosted by limestones in a shallow ocean basin. Deposits of this kind are called exhalites. The original minerals would have comprised zinc-manganese-bearing silicates and oxides. These precursor minerals are collectively called the protolith. The Sterling Hill protolith is a contrast to most metal-bearing hot spring deposits, in which the metal-rich species are generally sulfide compounds.

The metamorphism taking place about 950 million years ago, which was described by Volkert, transformed the protolith into the present zincite, franklinite, and willemite bearing assemblages. Present layering represents compositional layering in the original hot spring-deposited material. Host limestones became the familiar Franklin Marble. Simultaneously with metamorphism, the deposit was deformed; folding while it was actually sinking into the plastic, but lower density marble. This sinking and folding brought the deposit into its present structural configuration.

As noted above, this high-temperature “Grenville” metamorphism accounts for the origin of about 25 mineral species, the primary constituents of the ore. What about all the others, which make the deposit so wonderful and unique? Some species are probably attributable to retrograde or lower temperature metamorphism; they were formed while the metamorphic system was cooling down. Some quartz, feldspars, scapolite, and other minerals, which occur in bodies rimming pyroxene blocks near the ore body core, may have formed during lower temperature metamorphism. Though it is unclear how many distinct events were involved, many other species were formed much later in response to alteration by hot-water solutions, entering the deposit along fractures. Finally, minerals like hemimorphite, hydrozincite, the manganese-oxide phases, and others were formed during surface weathering.

Although the protolith of the original exhalite may have been unusual, mineral deposits formed by hot spring activity are not themselves uncommon. Similarly, mineral deposits, which have been modified by metamorphism, are not rare. Most of the earth’s great iron ore deposits and many gold deposits have been formed in part through metamorphic processes. The later alteration events, which have formed so many of Sterling Hill’s rare and unusual minerals, are exceedingly commonplace. Sterling Hill’s unique and wonderful character is thus the sum of a number of fairly ordinary events, taking place at a single location.