Walking tour for the Harding pegmatite mine


Regional geological map, Road map


Walking tour for the Harding pegmatite mine

Stop 1: (Top of North Knob)

Stop 2: (Quartz knoll and prospect pit on East Ridge)

Stop 3: (Quartz knoll at east end of quarry)

Stop 4: (Bench at east end of quarry)

Stop 5: ("Spotted rock" face, west end of quarry)

Stop6: (North wall of west entry way to quarry, north of Stop 5)

Stop 7: (Road-cut in access road)

Stop 1: (Top of North Knob)

Pegmatites are widespread magmatic rocks that form confined bodies of various shapes and sizes. Pegmatite magmas are volatile rich fractions derived from other magmas, commonly a crystallizing granitic pluton. The derivative magma, or pegmatite magma, is enriched with volatiles and because of density differences these bodies of magma can travel large distances from the parent magma. Often no parent magma is readily apparent for a pegmatite district; that is the case here. When a pegmatite magma reaches temperature and pressure conditions that allow it to crystallize, the isolated body begins cooling from the walls inward. Fractional crystallization proceeds with incomplete reaction between early-formed crystals and the remaining melt. This process enriches the liquid with volatiles, of which water is the most important. Sequential crystallization of minerals forms distinct lithologic units in the consolidated pegmatite body. These zones sub-parallel the contact of the pegmatite with the surrounding country rock. During the final stages of crystallization, the liquid becomes highly corrosive. The liquid at this stage can often replace previously consolidated pegmatite. Mineral replacement (pseudomorphism) and replacement bodies are common. During all stages of crystallization the liquid can fill fractures in consolidated pegmatite or escape into the country rock. Zones, replacement bodies and fracture fillings are the three types of lithologic units that are found in many complex pegmatites. Pegmatite magmas crystallize at a depth of 4-10 kilometers in the crust.

The Harding pegmatites intrude a complex terrane of Precambrian metamorphic rocks. The pegmatites were emplaced approximately 1330 m.y.B.P. along a complicated boundary between amphibolite to the south and quartz-muscovite schists to the north. Several generations of granitic plutons intrude the area, Cerro De Los Arboles (to the SW), Cerro De La Cruz (to the S) and Cerro Alto (to the E). No genetic link between any of the plutons and the pegmatites has been established. Most of the pegmatite bodies are lensate dikes ranging from a few meters in exposed length to as much as 350 meters. All are granitic in composition. Some are completely homogeneous and others display internal zoning, replacement bodies and fracture fillings. The main dike, which we shall refer to as the Harding pegmatite, strikes along the East Ridge in the form of a large flat disk. The thickness varies from 1 meter in areas to the east to as much as 25 meters in the quarry. The dike dips gently (10° to 15°) to the south. Looking toward the quarry, we can see the exposed thickness of the western portion of the main dike. The hanging wall contact between the overlying amphibolite (dark colored) and the pegmatite (light colored) is clearly visible. The foot wall contact with the underlying schists is exposed for a short distance in the main entry way to the quarry. We are standing on a dip slope remnant of the dike. Pegmatite is exposed on much of the south slope of North Knob. The eastern extensions of the dike are complicated by pinching, swelling and branching. The down plunge portions of the dike in the western part are known from extensive core drilling done by the U. S. Bureau of Mines in 1943 and 1948. This exploratory work extends the down plunge portions of the dike for 500 meters. The up plunge portions of the dike, beyond where we are standing, have been eroded away. Eight lithologic units can be distinguished in the main dike. These include continuous zones and discontinuous replacement bodies, but not fracture fillings. The main dike is disk-shaped so that zones appear as layers across the quarry face. The layered appearance is enhanced by the asymmetric zoning. The zones comprising the footwall are not the same as those in the hanging wall. The internal structure is best exposed in the main quarry, but first we will examine exposures along the East Ridge.

Because pegmatite magmas are volatile fractions, they host a unique and unusual collection of elements. The conditions of crystallization allow these elements to form minerals that are rare in other rocks and crystals that are gigantic in size. Many of the world's largest crystals are found in pegmatites. The Harding pegmatite is no exception. About 50 minerals occur in the main dike. This number is not astounding; some pegmatites in New England have over 100 different mineral species. The Harding is famous as the world's largest deposit of microlite, a complex tantalum oxide. The bulk the dike is composed of six major minerals: quartz [SiO2], albite [NaAlSi3O8], microcline [KAlSi3O8], muscovite [KAl2(AlSi3)O10(OH)2], lepidolite [K(Li,Al)3(Si,Al)4O10(F,OH)2], and spodumene [LiAlSi2O6]. The four principal accessory minerals are beryl [Be3Al2Si6O18], garnet [(Mn,Fe)3Al2(SiO4)3], microlite [(Na,Ca,U)2(Ta,Nb)2O6(OH,F)] and tantalite-columbite [(Fe,Mn)(Ta,Nb)2O6].

Stop 2: (Quartz knoll and prospect pit on East Ridge)

We have just traced the strike of the upper contact of the pegmatite with the country rock. Here we stand on a quartz outcrop. This is part of a continuous zone composed almost entirely of quartz, the quartz zone. This zone is a few centimeters thick to as much as several meters thick in the down plunge portions of the western part of the main dike. At the prospect pit rim you are standing on the hanging wall contact of the pegmatite; the country rock has been eroded away. The pegmatite body dips to the south. Beneath your feet you will see three units. An upper thin continuous band a few centimeters thick composed of a relatively fine-grained aggregate of quartz, albite and muscovite forms the border rind. Underlying this think band is another layer composed of slightly coarser aggregate of the same minerals. Several accessory minerals not seen here are locally abundant. They include microcline, apatite, lepidolite, tantalite-columbite and beryl. In the thick western portions of the dike, beryl frequently forms immense anhedral crystals with dimensions measured in meters. This border rind and wall zone are collectively known as the berry zone. Beneath the berry zone, we encounter a cleavelandite layer. Cleavelandite is a variety of albite, a sodium plagioclase feldspar. Here we see typical examples; the cleavelandite forms radiating aggregates of lusterous white or cream colored curving plates. This cleavelandite unit, composed almost entirely of cleavelandite, is a distinct lithologic unit. The unit forms discontinuous masses throughout the dike. Beneath the cleavelandite unit, we see a new assemblage of minerals. The long white blades or laths are spodumene; the matrix is quartz. This unit is the quartz-lath spodumene zone. Spodumene is a lithium pyroxene found exclusively in granitic pegmatites. These white elongate crystals are typical. When unaltered, they frequently are here, spodumene displays two good cleavages parallel to the length of the crystal. We have now seen four of the eight lithologic units comprising the dike. The three zones, the beryl zone, the quartz zone and the quartz-lath spodumene zone, form the upper portions of the dike; the cleavelandite unit is more often in the core of the pegmatite.

If we visualize the crystallizing pegmatite magma, the border rind in contact with the cooler country rock is the first to crystallize. Its fine-grained texture is typical of a chilled margin. Quartz crystallizes next, not all the quartz in the magma, just a zone of quartz. Many minerals will occur in several units. As crystallization continues we expect to see an increase in crystal size toward the center of the dike. In general, and in this outcrop, this is observed. The late stage liquid may, however, replace zones of consolidated pegmatite and disrupt this trend.

Stop 3: (Quartz knoll at east end of quarry)

We have just walked down part of the dip slope remnant of pegmatite blanketing North Knob. By the repeated appearance of pegmatite and country rock down the slope you begin to appreciate that the external morphology of the dike is very complicated. The contacts are not smooth, gently curving surfaces. The dike is a disk only in generalized form.

Again we are standing on the quartz zone. From under our feet the quartz zone can be traced onto the quarry face where it is much thinner. The beryl zone is above and the quartz zone is below, a similar sequence of units observed at Stop 2. The hanging wall contact is abrupt and discordant. Reaction between country rock and pegmatite is minimal.

A different unit, forming the shelf directly in front of us and the two knolls in the quarry, lies beneath the quartz-lath spodumene zone. This lithologic unit is composed of an even-grained aggregate of microcline, spodumene and quartz with lepidolite, lithium muscovite and albite. The texture of this unit, "spotted rock," is markedly less coarse than overlying zones and displays abundant mineral replacement. "Spotted rock" forms a lobe extending down the center of the dike. Its maximum breadth is exposed in the quarry, extending form the east to the west end. "Spotted rock" contains numerous accessory minerals, such as apatite, microlite, tantalite-columbite and spessartine.

The mining operations as seen form here appear to be irregular and haphazard. This is due to the primitive mining techniques (small crew and mule) and the several episodes of mining. Quartz outcrops, similar to what we are standing on, first attracted prospectors in search of gold. Lepidolite, a lithium mica, was found instead and used in the glass industry. Joseph Peyer mined the lepidolite from 1919 until 1930. The quarry was excavated and the main adit, a southward extension of the main entry way, was driven. The next episode of mining, 1942-1947, produced the world's largest production of microlite, an unusual ore of niobium and tantalum. 10,000 kilograms of microlite concentrate containing an average of 68% Ta2O5 and 7%Nb2O5 were produced. During this period the quarry was enlarged and three adits on the east side of the main adit were driven. The last episode of mining, 1950-1958, produced great quantities of beryl, a beryllium cyclosilicate. Three adits to the west were driven along the hanging wall contact where large masses of beryl were encountered. The Harding mine was the country's leading beryl producer in 1950 and 1951. Annual beryl production during the peak years exceeded 13,000 kilograms. Total beryl production during this period was 622,000 kilograms of ore containing 11.2% BeO and 167,000 kilograms containing 5.5% BeO. The underground working are very DANGEROUS. DO NOT ENTER ANY OF THE UNDERGROUND WORKINGS.

Stop 4: (Bench at east end of quarry)

Here we see the effect of a late stage residual liquid reaction with the consolidated quartz-lath spodumene zone. Laths of spodumene are replaced by fine scales of rose muscovite. The shapes of the laths are still recognizable (pseudomorphs), but in other areas the spodumene crystal form is obliterated. Cleavelandite replaces the quartz matrix. This replacement unit forms discontinuous masses throughout the core of the dike. The aggregate of rose muscovite-cleavelandite is another of the eight lithologic units. It is evident that replacement units often transect other zones and do not necessarily parallel the contacts between primary zones. At this time, most of the pegmatite magma has crystallized and very small amounts of residual liquid can cause extensive replacement by reaction with previously consolidated zones, as in the "spotted rock" unit.

Stop 5: ("Spotted rock" face, west end of quarry)

The top of this pinnacles is the hanging wall contact. The country rock has been removed. We see zones as layers: the beryl zone (top), the quartz zone (middle) and the "spotted rock" (bottom). Note the absence of the quartz-lath spodumene zone. You can see how the "spotted rock gets its name. The white "eyes" are spodumene; the matrix is mostly microcline so impregnated with lepidolite and lithium muscovites that it is pink. "spotted rock" locally grades into masses of pure lepidolite. Many of these lepidolite masses have been mined out. Lepidolite is characterized by compact, finely crystalline, lilac to purple masses or by curving platelets in quartz or cleavelandite. Often it is impossible to distinguish lepidolite from rose muscovite, but in general the rose muscovite is more red than purple in color, and it does not form large masses like the lepidolite. The dark glassy specks in the "spotted rock" are uranium rich microlite crystals. Microlite is common throughout the "spotted rock," but the economic occurrences are found at the base of the quartz-lath spodumene zone. There the microlite is lighter in color and in dense aggregates of tiny crystals.

Another mineral difficult to identify is beryl. Here a mass is exposed in the wall zone. All the beryl is white to light pink in color and almost always without crystal form. The masses resemble quartz and feldspar. The beryl has a resinous or greasy luster and often a basal cleavage that distinguishes it from quartz and feldspar. The beryl mass in the wall zone displays a broad flat cleavage surface.

Stop6: (North wall of west entry way to quarry, north of Stop 5)

The replacement of microcline by micas in the "spotted rock" masks the original character of that zone. Exposed in the north wall of the west entry way are masses of perthitic microcline that are not replaced. Microcline is a potassium feldspar that forms large blocky crystals in pegmatites. The mineral displays two good cleavages at approximately right angles. The color ranges from white to tan. Perthite is an intergrowth of potassium and sodium feldspar. All the microcline is perthitic. This perthitic microcline unit is another of the eight lithologic units. This zone occupies much of the lower portions of the dike. We have now seen seven units. Keep in mind that as the pegmatite magma was crystallizing, zones were forming simultaneously in both the top and bottom of dike.

Stop 7: (Road-cut in access road)

In passing through the main entry way we have walked past the footwall contact of the dike with the underlying schist. Here the footwall contact is also exposed on part of the dip slope remnant that rests on North Knob. The lower contact is sharp and discordant. We can trace this contact to the basal contact exposed in the main entry way by a curved plane. The basal unit here is a mixture of the perthite zone (as seen at last stop) and aplite. The aplite zone here is composed of fine-grained albite and forms large continuous units in the base of the dike. The aplite zone constitutes another lithologic unit. The eight lithologic units are: the beryl zone, the quartz zone and the quartz-lath spodumene zone comprising the top portions; the "spotted rock," rose muscovite-cleavelandite, and cleavelandite units, all interior units; and the perthite zone and the aplite zone forming the bottom of the dike. Keep in mind these lithologic units are generalized. They do exist, but gradations between units are common. When looking at an exposure it is not always possible to identify the unit, especially in the eastern extensions.

As pegmatite magmas crystallize the rest liquid is enriched in volatiles, so we should see a specific paragenesis of mineral chemistry reflecting the changing composition of the melt. More volatile rich minerals should occur in the interior portions of the pegmatite body. This is not apparent with ubiquitous minerals like quartz and feldspars, but is better displayed by the rarer minerals. For example, the micas here have the following paragenesis: green muscovite (in the wall zones), then lepidolite (interior units), and finally rose muscovites (interior units). Trace element chemistries in nearly all the minerals reflect the volatile enrichment.

Walking tour designed by Bryan Chakoumakos.