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The Fanwood quarry: Somerset County New Jersey.

New Jersey's Fanwood quarry has been operating for more than a century. Although not a major producer of collector-quality specimens, it has yielded fine examples of heulandite, stilbite, calcite, chalcedony, datolite and prehnite.

INTRODUCTION

The Fanwood Crushed Stone Company quarry, also known as the Fanwood quarry or the Weldon quarry, is in the Borough of Watchung, Somerset County, New Jersey. Since the late 1890s, the quarry has been owned and operated by the Weldon family. Weldon Materials Inc. produces crushed aggregates, ready-mixed concrete and hot asphalt at the site for the construction industry.

Originally exploiting mostly barren, massive basalt, the operation has greatly expanded over the years, exposing large areas of the quarry's mineralized horizons. Although some interesting and aesthetic specimens of zeolites and associated minerals have been collected there, the Fanwood quarry is not considered to be a major source of high-quality specimens as some of the other New Jersey traprock locations have been. For the most part, the distribution of eye-catching minerals is rather erratic and localized.

GEOLOGY

During the late Triassic and early Jurassic periods, North America split apart from Africa, creating a broad zone of block-faulted uplifts, grabens (down-dropped blocks) and half-grabens (dropped down on one side). The Fanwood quarry is situated in the half-graben known as the Newark Basin, the largest exposed early Mesozoic basin in eastern North America; it extends from southern New York through northeastern and central New Jersey and into southeastern Pennsylvania. The Newark Basin is approximately 190 km long and 50 km wide, and is filled to an estimated depth of 6.8 km with sediments and intrusive and extrusive basaltic rocks (Schlische, 1992; Olsen et al., 1996). In its northern part, the Newark Basin is bounded on the northwest by the Ramapo fault, which separates the late Triassic and early Jurassic deposits within the basin from Precambrian gneiss to the west. The Hudson River borders the graben's eastern flank, while Cretaceous and Tertiary sediments overlap its southeastern and southern margins.

Three basalt flows collectively known as the Watchung Mountains lie within the upper third of the stratigraphic section of the Newark Basin. These formations include, from oldest to youngest, the Orange Mountain Basalt, the Preakness Mountain Basalt and the Hook Mountain Basalt. Each volcanic ridge is separated from the next by an intervening sedimentary unit. The sedimentary beds are composed predominantly of conglomerate, shale, sandstone and mudstone; the majority of these rocks have a reddish brown hue caused by an abundance of iron oxide minerals.

The Fanwood quarry is within the southwestern segment of the Orange Mountain Basalt. Roughly 96 meters of three successive outpourings of lava are exposed in the quarry, all of which dip to the northwest at 10 to 15 degrees, contain numerous high-angle faults, and exhibit the classic colonnade and entablature joint pattern (Laskowich and Puffer, 1990). The term "colonnade" refers to the layer of a flow unit where the basalt fractures into a somewhat uniform series of columns; the term "entablature" refers to a basalt layer which displays a closely spaced columnar joint pattern. The "red beds" of the Passaic Formation underlie the Orange Mountain Basalt; however, the contact between the two has yet to be exposed in the Fanwood quarry.

The principal secondary mineralization is found in the amygdules: gas pockets located in the upper parts of the first, second and third basalt flows and in the adjacent 20 cm of the overlying flows. Secondary mineralization also occurs in fissure veins that transect the massive basalts. These veins are typically filled with calcite, infrequently associated with stilbite or sulfides. The most interesting and exciting mineralized areas encountered in the quarry are in regions where the lower colonnade of the first or second flow arches upward into the overlying entablature. The dome-shaped volcanic structures which form in this event are referred to as diapirs (Laskowich and Puffer, 1990). A diapir is thought to be a cooling feature which reflects the uneven downward movement of the solidification front (Cummings, personal communication, 2009). Locally, in areas where the front proceeds downward more slowly, an upward-oriented dome of lava develops. Gases from adjoining areas, where solidification has already penetrated more deeply, migrate towards the top half of the diapir and are ultimately frozen in place as a cluster of amygdules. The upper interiors of most diapirs contain partially filled vugs of varying shapes and sizes, up to 1.8 meters across, typically mineralized with prehnite and calcite. Since 1985, three diapirs have been identified by field collector Mark Bianchi. The largest was nearly 10 meters wide at its base and 9 meters thick from the bottom of the colonnade to the top of the arch.

[FIGURE 1 OMITTED]

The lower amygdaloid is about 20 meters thick--the thickest of the three layers of mineralized gas pockets. Near the top of this unit is a zone of brecciated scoria, slag-like basalt containing numerous amygdules, all less than 2.5 cm in diameter. Between the angular blocks of scoria, irregular breccia cavities up to 30 cm in size are largely filled with red mudstone, precluding the formation of fine crystal specimens. Just below the rubbly basalt, the degree of alteration becomes less apparent until the rock reaches a relatively uniform dark gray at depth. A myriad of irregularly shaped cavities are dispersed throughout this gradually less altered basalt, which encompasses slightly greater than two thirds of the amygdaloid. Although many of these cavities are either small, partially filled with scoria or nearly solid, large open pockets, around 25 cm or more, are occasionally located.

The Fanwood quarry's middle amygdaloid is approximately 15 meters thick. A layer of greatly altered pillow lava and associated breccia averaging 3.2 meters thick is plainly visible at the uppermost level. The pillow structures clearly indicate that portions of the second flow unit erupted into or under water (Puffer, personal communication, 2008). Isolated cavities, up to 1.3 meters across, sporadically occur in the interior of the pillows. Nearly all of the pockets are badly weathered and show little or no mineralization. However, a very small number are lined with attractive combinations of stilbite, heulandite, albite and calcite crystals. Immediately below the pillow basalt and continuing downward, the vuggy rock is similar in appearance and mineralization to that seen in the equivalent horizons of the lower amygdaloid--with the noticeable difference that albite is not as common as it is in the middle part of the lower amygdaloid.

[FIGURE 2 OMITTED]

In 2005, the upper amygdaloid was unearthed when the quarrying operations reached the rear perimeter of the property. Erosion had stripped away the topmost level, leaving roughly 3 meters of the pocket zone. The cavities there have yielded prehnite and calcite, but the destructive effects of surface water percolating through the basalt have rendered specimen quality poor.

SECONDARY MINERAL PARAGENESIS

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The secondary mineralization at the Fanwood quarry was the result of the hydrothermal alteration of the Orange Mountain Basalt, as described by (Cummings, 1987; personal communication, 2009). The hydrothermal fluids were formation brines high in calcium and sodium sulfates which were derived from the underlying Passaic formation. The fluids circulated upward into the amygdaloids and the vuggy areas of the diapiric structures through faults and joints in the basalt. Late in the Newark Basin's history (around 180 million years ago) there was a period of high heat flow and dynamic fluid movement. This appears to have coincided with the final separation of Africa from North America and a shift in tectonic stress from tension to compression. Steckler et al (1993) proposed that fluid circulation was driven by an artesian system which appears to have briefly increased the temperature of the Orange Mountain basalt to around 220[degrees] C. As the temperature rose, basin brines in openings in the basalt began to react with the host rock, especially the glassy, poorly crystallized parts, which initiated a progressive fluid/rock interaction. This vigorous and evolving process partially replaced some of the basalt's primary chemical components, while others were released to the fluid stage. These chemical components recombined to produce a secondary mineral assemblage that was stable in the hydrothermal environment and that is now seen in the open spaces of the basalt throughout the quarry.

[FIGURE 4 OMITTED]

Cummings (1987) has broken down the amygdaloids into four distinct and discernible stratigraphic zones based on the secondary mineralization expected in each. The spatial distribution of the secondary minerals within these zones reflects the physical characteristics of the basalt, most importantly its permeability, porosity and crystallinity. These factors regulated the fluid chemistry, the fluid flux and the rate and degree of change in the host rock (Cummings, 1987). The progressive alteration in the fluid chemistry produced a paragenetic sequence comparable to that illustrated by Schaller (1932): (1) saline period, (2) quartz period, (3) prehnite period, (4) zeolite period and (5) calcite period.

[FIGURE 5 OMITTED]

The five stages in the paragenetic sequence can be seen in the four zones within the Fanwood quarry's amygdaloidal horizons. Quartz and datolite, two of the earliest minerals to form, are the main constituents inside cavities at the bottom of zone 4 (the lowermost of the four zones). Calcite first appears near the bottom of zone 4 as crystals on quartz and datolite and as fillings of quartz-lined vugs. In the upper section of zone 4, prehnite becomes plentiful in relation to datolite. Since anhydrite crystallized in the initial period of Schaller's sequence, quartz, datolite and prehnite pseudomorphs after anhydrite are found throughout zone 4. An undulating sulfide horizon is present between the top of zone 4 and the bottom of zone 3. Gas pockets within the sulfide horizon range in size from 2.5 to 10 cm and are typically lined with prehnite and datolite. Crystals of calcite, chalcopyrite and rarely galena are scattered atop the prehnite and datolite.

[FIGURE 7 OMITTED]

Zone 3 marks the transition between the earliest-formed minerals of Schaller's periods and the later-formed minerals. Datolite and prehnite are still present; however, albite and calcite become more common, as do the zeolites heulandite and stilbite.

The minerals of the early paragenetic periods are missing from zone 2 in both the mudstone-filled breccia of the lower amygdaloid and the pillow formation of the middle amygdaloid. Mineralization in these two upper stratigraphic sections is limited to heulandite, stilbite, calcite and albite.

Zone 1 (the uppermost of the four zones) is a 20-cm-thick layer at the basal unit of the overlying second and third flows. It contains numerous small pipe amygdules lined or totally filled with albite and calcite.

MINERALS

Albite [NaAlSi.sub.3][O.sub.8]

Albite is ubiquitous in much of the lower amygdaloid and is common in zones 1 through 3 of the middle amygdaloid. While albite is primarily an early-formed mineral seen as thin cavity linings, it also formed following the deposition of heulandite, stilbite and calcite. Crystals are typically white to pinkish orange elongated blades, around 1 mm, in spherical and fanlike groups.

[FIGURE OMITTED]

Analcime [Na.sub.2][(A1.sub.2][Si.sub.4][O.sub.12).[2H.sub.2.O]

Unaltered analcime crystals are unknown from the Fanwood quarry. Hollow albite epimorphs after analcime crystals to 2.5 cm, in combination with heulandite and calcite, have been collected from a cavity in zone 3 in the middle amygdaloid.

Anhydrite [CaSO.sub.4]

Anhydrite has been noted by Cummings (1987) as rare, but no specimens attributable to the Fanwood quarry have been seen by the author. Quartz, datolite, prehnite and albite are found as pseudomorphs after anhydrite within zones 3 and 4 of the lower and middle amygdaloids.

Apophyllite-(KF) [KCa.sub.4][Si.sub.8.][O.sub.20](F,OH).[8H.sub.2]O

Only a handful of apophyllite specimens from the Fanwood quarry are known to exist. Glassy prismatic crystals up to 1 cm on datolite have been collected in zone 4 of the middle amygdaloid.

Bornite [Cu.sub.5][FeS.sub.4]

Bornite is fairly common in zone 3 of the lower amygdaloid, occurring as dodecahedral crystals from 0.5 to 2 mm on albite. Chalcopyrite typically coats bornite, imparting a brassy hue to the crystals.

[FIGURE OMITTED]

Calcite [CaCO.sub.3]

Calcite is abundant in all of the quarry's mineralized structures. It is distributed throughout the amygdaloids starting near the bottom of zone 4 and continuing upward into zone 1. Calcite lines a large percentage of the veins which run vertically through the flows. Diapiric cavities commonly contain calcite alone or in combination with prehnite. Calcite crystals occur in a wide variety of habits, the scalenohedron and rhombohedron being the most common forms. The largest known crystal is 14 cm in diameter, but most are in the 1 to 5-cm range. Colors range from colorless or white to gray, golden yellow, orange and occasionally pink or pale red.

Chalcopyrite [CuFeS.sub.2]

Chalcopyrite is found throughout zones 3 and 4 of the lower and middle amygdaloids as brassy sphenoidal crystals up to 1 cm on datolite, prehnite, albite and heulandite. Several specimens showing butterfly-twinned crystals have been collected. Chalcopyrite also occurs as overgrowths on bornite and infrequently as crystals and small masses within calcite veins.

Chrysocolla [(Cu,Al).sub.2][H.sub.2][Si.sub.2][O.sub.5][(OH).sub.4].[nH.sub.2]O

Chrysocolla forms as a thin turquoise-colored coating on weathered chalcopyrite inside veins.

Datolite [Ca.sub.2][B.sub.2][Si.sub.2][O.sub.8][(OH).sub.2]

Datolite is plentiful in the amygdaloids from the middle of zone 3 downward through zone 4. Datolite is seen as compact masses of granular microcrystals, aggregates of small crystals and isolated sharp crystals, no more than 3 cm across, in combination with prehnite, calcite, albite and zeolites. Datolite varies in color from colorless to milky white, yellow-green and pale green. Crystal surfaces can be lustrous or dull.

Galena PbS

Galena is the rarest sulfide found in the quarry. Crystals are modified gray cubes and octahedrons measuring less than 2 mm, associated with prehnite, datolite, calcite and chalcopyrite.

Glauberite [Na.sub.2][Ca([SO.sub.4]).sub.2]]

Glauberite in its original state has not been found at the Fan-wood quarry. Prehnite pseudomorphs after glauberite are widespread within the diapiric amygdules of the first and second flows. Glauberite molds are uncommon outside of the cavities in diapirs. Within the pockets of the lower and middle amygdaloidal horizons, quartz, datolite and albite have covered glauberite, leaving behind rectangular epimorphs as hollow shells.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Gmelinite ([Na.sub.2],Ca) ([Al.sub.2][Si.sub.4])[O.sub.12].[6H.sub.2]O

Gmelinite is a rare zeolite found only as crystals on datolite, in the basal zone of the lower amygdaloid. The striated crystals, ranging from 3 to 8 mm, are pale pink and highly lustrous.

Hematite [alpha]-[Fe.sub.2][O.sub.3]

Hematite is found in amygdaloidal cavities as druses of silvery black microscopic crystals on datolite, prehnite, albite, calcite, heulandite and stilbite. It is probable that hematite inclusions are the cause of the very infrequent pink color seen in calcite.

Heulandite [(Na,Ca).sub.2-3][Al.sub.3][(Al,Si).sub.2][Si.sub.13][C.sub.36].[12H.sub.2]O

Heulandite is the most common zeolite at the Fanwood quarry, frequently found directly attached to the pocket wall. Crystals are typically coffin-shaped and blocky with varying degrees of luster depending on alteration. Although crystals up to 3.2 cm are known, most are much smaller. Colors include white, gray, greenish gray and shades of brown. Heulandite is commonly associated with stilbite, calcite, albite and (less commonly) prehnite and datolite.

[FIGURE 17 OMITTED]

[FIGURE 18 OMITTED]

Natrolite [Na.sub.2][(Al.sub.2[Si.sub.3])[O.sub.l0].[2H.sub.2]O

Natrolite is an extremely rare zeolite at the Fanwood quarry; specimens were discovered on one occasion as jumbles of acicular crystals, no longer than 1 cm, on prehnite in a diapiric amygdule. The crystals are white and exhibit the classic elongated orthorhombic prism with pyramidal terminations.

[FIGURE 19 OMITTED]

[FIGURE 20 OMITTED]

Prehnite [Ca.sub.2][Al.sub.2][Si.sub.3][O.sub.10][(OH).sub.2]

Prehnite occurs in the amygdaloids distributed between the top of zone 4 and the upper part of zone 3. It forms thin botryoidal cavity linings and small spheroids which are orange, green and yellow-green. It is particularly abundant in the sulfide horizon, where it forms the base on which calcite, chalcopyrite and occasionally galena have grown. The quarry's best prehnite comes from the numerous vuggy openings inside diapirs. Within these structures prehnite is found as botryoidal coatings, isolated spheres and thick carpets composed of prehnite pseudomorphs after glauberite and anhydrite. The color, shape and luster of the prehnite can vary greatly between the pockets. Colors range from a bleached whitish green to a vivid green.

Pyrite [FeS.sub.2]

Pyrite is uncommonly found as lustrous cubic crystals, between 2 and 5 mm, in combination with later-formed minerals such as zeolites and calcite. The crystal faces are typically striated.

Quartz [SiO.sub.2]

Colorless, amethystine and smoky quartz, as well as chalcedony, occur commonly in the bottom half of zone 4 of the lower and middle amygdaloids. Crystals range in size from microscopic up to 1 cm across, normally tightly configured in pockets with only the terminations being visible. Chalcedony is found as translucent gray and off-white botryoidal cavity linings and massive vein fillings; banded agate is also sometimes found. Many pockets at the base of zone 4 contain a layered sequence beginning with chalcedony and followed by quartz crystals, then calcite.

Stilbite [NaCa.sub.2]([Al.sub.5.Si.sub.13])[O.sub.36] [14H.sub.2]O

After prehnite, stilbite is probably the most interesting of the Fanwood quarry minerals for collectors. Stilbite is readily seen in a variety of habits, including single sheaf-like crystals, half and full bow ties, and globular radiating aggregates. The largest spheres can exceed 7 cm in diameter, but most are much smaller. Stilbite is generally golden brown, gray, orange or tan in color. A large percentage of the spherical groups are quite splintery, readily breaking apart into sharp needles. Stilbite generally occurs in association with calcite, albite and heulandite, but it is occasionally found with prehnite and datolite. Some of what appears to be stilbite has been identified by Francis and Metropolis (1985) as stellerite.

Other Species

[FIGURE OMITTED]

Several other mineral species occur at the Fanwood quarry but are of little interest to most collectors. These include pumpellyite and members of the chlorite group, all of which form as tiny clusters of greenish black to black microscopic crystals on other minerals and on cavity walls.

CONCLUSION

During the Fanwood quarry's over-100-year history, the operators have exposed large expanses of mineralized basalt from which scattered collector-quality specimens of stilbite, calcite, heulandite, quartz, datolite and prehnite have been recovered. Several significant discoveries of minerals have been made by collectors. An enormous volume of mineralized rock remains to be uncovered on the property via blasting; thus the potential for finding attractive specimens should continue indefinitely.

Weldon Materials Inc. does not allow general collecting, although occasionally the company will permit state and university geologists to study, photograph and collect inside the quarry. The property is patrolled by security, and management intends to prosecute any and all trespassers.

ACKNOWLEDGEMENTS

I would like to thank Dr. John Puffer, Rutgers University, for his thoughts pertaining to the pillow basalts. I also greatly appreciate the assistance of Warren Cummings, a retired geologist formerly with the New Jersey Department of Transportation, who not only reviewed the article, but contributed information which was incorporated. Thanks also to Mark Bianchi for sharing his insights about the quarry and for the use of his pictures, and to Bob Batic and Brad Plotkin for giving of their time to study and/or photograph their Fanwood collections.

BIBLIOGRAPHY

CUMMINGS, W. (1987) Mineralization at the Fanwood & Summit Quarries, New Jersey. Rocks & Minerals, 62, 150-159.

FENNER, C. N. (1910) The Watchung Basalt and the paragenesis of its zeolites and other secondary minerals. New York Academy of Science, Annals, 20, 93-187.

FAUST, G. T. (1975) A review and interpretation of the geologic setting of the Watchung Basalt flows, New Jersey. United States Geological Survey Professional Paper 864-A:1-42.

FRANCIS, C. A., and METROPOLIS, W. C. (1985) Stellerite: six new occurrences. Rocks & Minerals, 60, 285.

LASKOWICH, C., and PUFFER, J. H. (1990) Volcanic diapirs of the Orange Mountain Basalt, New Jersey. New Jersey Academy of Science Bulletin, 35, 1-9.

MANSPEIZER, W. (1980) Rift tectonics inferred from volcanic and clastic structures. Field Studies of New Jersey Geology and Guide to Field Trips, 52, 314-350. New York State Geological Association Annual Meeting.

MASON, B. (1960) The trap rock minerals of New Jersey. New Jersey Geologic Survey Bulletin 64.

MONTEVERDE, D. H., and VOLKERT, R. A. (2005) Bedrock geologic map of the Chatham quadrangle, Morris, Somerset and Union Counties, New Jersey. New Jersey Geological Survey Geologic Map Series, GMS 04-2, Scale 1:24,000.

OLSEN, P. E., KENT, D. V., CORNET, B., WITTE, W. K., and SCHLISCHE, R. W. (1996) High-resolution stratigraphy of the Newark rift basin (early Mesozoic, eastern North America). Geology Society of America Bulletin, 108, 40-77.

PUFFER, J. H. (1987) The Palisades sill and Watchung Basalt flows, northern New Jersey. Geological Society of America Centennial Field Guide--Northeastern Section, 91-96.

SCHALLER, W. T. (1932) The crystal cavities of the New Jersey Zeolite Region. U.S. Geologic Survey Bulletin 832, 463-503.

SCHLIISCHE, R. W. (1992) Structural and stratigraphic development of tile Newark extensional basin, eastern North America: Evidence for growth of the basin and its bounding structures. Geological Society of America Bulletin 104.

STECKLER, M. S., OMAR, G. I., KARNER, G. D., and KOHN, B. P. (1993) Pattern of hydrothermal circulation within the Newark Basin from fission track analysis. Geology, 21, 735-738.

Frank A. Imbriacco III

1 Fox Hill Road

Edison, New Jersey 08820

fai122@aol.com
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Author:Imbriacco, Frank A., III
Publication:The Mineralogical Record
Geographic Code:1USA
Date:Mar 1, 2010
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