Lithia Pseudomorphs of the Eastern Mt. Apatite Pegmatites, Auburn, Maine.

By Douglas Watts, Augusta, Maine.

[Note: this article is an 'open file' work in progress. Constructive comments and suggestions are welcome.]

Despite a century of intermittent commercial and recreational quarrying and exposure, the eastern group of lithia pegmatites at Mt. Apatite in Auburn, Maine have never been well studied. Virtually all material available has been collected and reported by avocational, non-scientific enthusiasts. Collected material indicates (a) the Maine Feldspar Quarry pegmatite underwent an extreme late stage crystallization process wherein the lithia pegmatite was completely altered to muscovite, quartz and epitaxial albite and (b) the pegmatite is much richer in primary lithia phosphates and iron and arsenic sulphides than previously understood. The latter suggests the mineralogical diversity of the MFQ pegmatite is more due to complete melting of the host metasedimentary rock than fluidic, 'squirted' melt from the presumed parent Sebago batholith. For no other good reason, this would explain why the Sebago pluton and the southern end of the Mooselookmeguntic pluton are associated with expansive networks of mineralogically diverse pegmatites; and the Lucerne, Deblois and Bottle Lake plutons and others to the east have aureoles which are monotonous, chilled and mineralogically barren.



King et al. (2000) define the eastern group of Mt. Apatite pegmatites as those lying east of the Hatch Hill Road in Auburn, Maine, as opposed to the western group, ie. the Pulsifer, Wade, and Keith quarries which lie just west of the Hatch Hill Road. The principal named eastern quarry excavations are the Greenlaw and Maine Feldspar quarries, both of which are deep enough to intersect the local water table. The slope of Mt. Apatite immediately to the west of these two quarries contains several other large excavations and dozens of small test pits. These are usually steep-walled, 20-30 feet deep, generally strike E-W, and do not intersect the local water table.

Typical of this unnamed group is what is informally called the Greenlaw Extension quarry, which is narrow and lenticular and strikes NNW-ESE just west of the N-S trending Greenlaw quarry. Its length is about 250 feet with headwalls up to 40 feet high but its width narrows to 20 feet at its base. Since the cut is dry, pegmatite exposures are observable from top to bottom. Wall and floor relations indicate the pegmatite was mined for its nearly pure microcline and quartz core zone. One hanging wall near its eastern end is pure quartz with several enormous (0.5-1 m) well-formed cream-colored microcline crystals still intact. Accessory minerals are sparse and consist of 0.5 cm columbite crystals, almandine (to 1 cm) and water clear 2-5 mm fluoapatite crystals perched in miarolitic vugs in microcline. Lithia minerals are absent.

Of the eastern group, the Greenlaw and Maine Feldspar quarries are the only ones which contain documentable, substantial lithia minerals. Of the two, the Greenlaw pegmatite is more sill-like and shallow, while the MFQ pit is cylindrical and deeper, with a 30 foot headwall on one side which still shows some pale blue beryl masses in its face. Photos of both quarries during active mining in 1913 by Douglas Sterrett of the USGS show them both to be quite shallow and sill-like, with a max. depth of 15-20 feet below the surrounding tree-line. Sterrett in 1914 mentioned one quarry was worked to 'nearly fifty feet in depth.' This presumably is the deep cylindrical pit today referred to as the Maine Feldspar Quarry.

For the purposes of this article, the author makes the ad hoc assumption that the dumps directly adjacent to and just a few yards north of the Greenlaw quarry pit contain waste rock exclusively from Greenlaw; and that the 'giant pile' due east and down hill from the deep, water-filled MFQ pit came exclusively from that pegmatite and not the Greenlaw pegmatite. By making this delineation, one can make fairly confident generalizations and distinctions about the character of each in situ pegmatite.

The MFQ pegmatite had sufficient volume to produce massive piles of feldspar and quartz rich waste rock, including one conical pile nearly 80 feet tall and 150-200 feet wide at its base. The dumps adjacent to the Greenlaw pegmatite are much smaller in volume and tend to be more scattered as thin (5-10 feet) layers in what is now mature deciduous forest. While shards of gemmy tourmaline in MFQ dumps are uncommon (and still highly sought after by ardent 'dump diggers'), examples are encountered frequently enough to suggest the MFQ feldspar operators of the 1920s and 1930s were not too picky about blasting out non-pockety, lithia-rich areas in search of ceramic-grade soda feldspar, the principal commercial product of the quarry.



Perhaps due to its larger size and greater depth, the MFQ pegmatite is more mineralogically diverse than the Greenlaw pegmatite and the other unnamed pits just to the west and upslope. One divergence is that the MFQ is noticeably richer than Greenlaw in sulfide minerals, particularly pyrrhotite, arsenopyrite and pyrite, with scarcer and smaller masses of sphalerite. Sulphide masses can be as large as a tennis ball and are nearly all anhedral 'blobs' or irregular stringers in heavily stained microcline and quartz.

The lithia-related component in the two quarries differs as well, with the Greenlaw pegmatite much richer in cleavelandite, lepidolite and elbaite, ie. a more classic western Maine lithia pocket zone assemblage. Cleavelandite is comparatively scarce in the MFQ pegmatite. A noticeable component of the Greenlaw pegmatite is what is best called 'fizzy' albite, wherein large masses of nearly pure white albite are riddled with 0.5-1 cm vugs displaying well-formed epitaxial clear albite crystals with occasional minor columbite, zircon and microlite. A variant of this fizzy, miarolitic albite fabric occurs in a small unquarried exposure of the Greenlaw pegmatite just to the west of the water-filled pit. At this exposure, the albite is friable, lightly iron-stained and occasionally studded with small, bright blue apatite crystals of 2-5 mm. Most of these apatites have a longish 'beryl' shaped columnar habit rather than a tabular habit. This identical association occurs at the nearby Berry-Havey quarries in East Poland, Maine.


A prime difference between the Greenlaw and the Maine Feldspar Quarry pegmatites is a substantial iron, manganese and lithium rich phosphate zone at the MFQ. This is observed today by hand-sized irregular masses of lithiophilite, montebrasite, olive green manganoan fluorapatite, rhodochrosite and cassiterite, along with highly altered spodumene laths up to 12 inches in length. Secondary mineralization of some of the lithiophilite masses produced micromount-sized euhedral examples of landesite, eosphorite, siderite, rhodochrosite, mitridatite, ludlamite and strunzite. 5-10 mm sphalerite and pyrite are present in some lithiophilite masses. Triplite is found sparsely in several lithiophilite masses. Assignment of this lithia phosphate zone to the MFQ pegmatite is assured due to where all the specimens were found: in the center of the enormous conical dump pile several hundred yards south of the pegmatite pit. Based upon the transport methods used during feldspar mining (wheelbarrows), the concentration of these lithia phosphates in one small (15 foot) area of the dump indicates they most likely all came from a single blast in the MFQ pegmatite.

That said, a second concentration of lithia phosphates at the MFQ was located at the steep southern toe of the large conical pile, including a large slab (14 x 8 x 3 inches) which is a near 50/50 percent mixture of anhedral cassiterite and montebrasite with a large 'slug' of brown lithiophilite with minor triplite. Small zircon bipyramids embedded in the cassiterite are moderately radioactive and several 1-2 mm 'spots' of altered uraninite are evident.

None of these associations are evident in the Greenlaw pegmatite and associated dumps; they appear unique to the MFQ pegmatite and were only found by the author by chance in 1995-1996 at the top of the MFQ giant conical dump pile. This one chance 'find' in the rubble of the MFQ considerably increased the number of confirmed minerals in the eastern Mt. Apatite group, especially in the arena of lithia phosphates and secondary alteration minerals. Except for a brief apocryphal listing in Morrill (1958) of 'reddingite' at the MFQ, primary lithia phosphates were unreported and undocumented from the eastern Mt. Apatite group [cinnamon brown lithiophilite had been previously confirmed at the Pulsifer quarry]. The lithiophilite exhibits a wide range of color, size and texture with some masses that appear to be nearing the Mn/Fe triphylite boundary. By the same token, other masses of lithiophilite appear trending toward manganese end-member stage, given that they are intimately intergrown with masses of pure rhodochrosite.

While the Greenlaw and MFQ pegmatites are clearly related by sheer proximity, the abundance of primary lithia phosphates at the MFQ and their absence at Greenlaw indicates a substantial chemical and petrogenetic difference, enough in my opinion to warrant calling them two separate and distinct pegmatite bodies. The MFQ pegmatite appears far more enriched in both iron and manganese. Its use of lithium, as lithiophilite and montebrasite and spodumene rather than as schorl and elbaite, suggests the Greenlaw pegmatite was perhaps a 'stringer' or outlier to the MFQ core. An analog to this relation might be seen in the relation between the Nevel or 'Twin Tunnels' pegmatite at Plumbago Mountain, Newry and the tourmaline rich Dunton Gem Pit.

It should be noted that at least 80 percent of the dump volume associated with the Maine Feldspar Quarry pegmatite is inaccessible to examination due to the very large size and height of the dump piles. The instability of the piles and their steep angle of repose makes it impossible to dig a 'hole' into them greater than six feet without it collapsing. This results in a situation where the upper 5-8 foot thick 'skin' of the dump surface is being sporadically and annually re-dug and re-explored while about 80 percent of the actual pile is still as it existed in the 1910s, 1920s and 1930s. [At the much shallower and tabular Greenlaw dumps, one can dig all the way down to the boundary line of the dump and natural forest soil and find old cigarette packs and soda can pull-tabs.]

In general, the mineral complexity of the eastern group of Mt. Apatite pegmatites tracks fairly close to their relative size, with the largest contiguous body, the MFQ pegmatite, displaying the highest degree of complexity and diversity in mineralogy, crystal size and fabric. Or to put it another way, nearly all of the mineral fabrics and associations seen in the smaller pegmatites are observed in the MFQ pegmatite, but numerous minerals, associations and fabrics in the MFQ pegmatite are not seen in the smaller, adjacent pegmatites. Again, since all these associations are only observed through 'dump' specimens, one has to make the assumption that material from each pegmatite was kept substantially segregated during removal of the pegmatite during blasting and sorting and that 'Greenlaw' material was not randomly mixed with MFQ material 90 years ago. Given the crude tools used in feldspar mining at the site from 1900-1940 (wheelbarrows and hand carts), this assumption appears to be fairly robust. The 1913-1914 photographs by D. Sterrett, reproduced in King (2000), show that commercial feldspar mining at the eastern Mt. Apatite group was a very simple process in which large, pure masses of merchantable feldspar for use as ceramic glaze was systematically 'cobbed' and separated by hand with small sledges and chisels from the blasted matrix. Everything that was not considered merchantable feldspar [and not containing gem tourmaline or showy quartz crystals] was unceremoniously wheeled by hand carts to the 'dump.' In contrast to the MFQ, the Greenlaw 'dumps' indicate a much more careful parsing of material, presumably because that pegmatite was much richer in gem tourmaline and mining activities there were much more focussed on recovering tourmaline.



The MFQ Muscovite Replacement Body

Both the MFQ and Greenlaw pegmatites show partial to extensive late-stage replacement in their lithiated zone. The Greenlaw pegmatite shows extensive replacement and alteration of gem elbaite with a coating of 1 mm golden yellow cookeite/muscovite rosettes and 1-2 mm clear quartz crystals. The MFQ is different in that it contains a large volume of lithiated pegmatite which has been completely replaced by greenish-white muscovite in a solid state fashion, leaving behind intact pseudomorphs of the original minerals and their presumed albitic ground mass.

What is unique about the MFQ replacement zone is how completely it has been altered. Cabinet sized pseudomorphs of schorl, spodumene and garnet can be identified, but only by their exterior crystal shape. The crystals themselves been completely altered into a tough, fine-grained olive green to greenish-white muscovite fabric. Substantial parts of the replacement zone are devoid of any recognizable pseudomorphs, but are dominated by odd, pell mell aggregations of 1-4 cm 'poker chip' stacks of greenish white pseudohexagonal muscovite prisms. These stacks of pseudohexagons bear no visual resemblance or lineage to the minerals they presumably replaced. In several instances, this greenish replacement muscovite appears to have completely replaced and pseudomorphed masses of white to yellow muscovite in the original pegmatite body -- creating muscovite pseudomorphs after muscovite itself. What is also odd is that the well-formed pseudohexagonal green muscovite 'poker chip stacks' are invariably about 3 cm in diameter. While a few are smaller and a few are larger, the modal size of these pseudohexagons (n=100) seems to gravitate right on 3 cm to the point that they look 'stamped out' by some crazy Paleozoic muscovite machine.



The only remote analog of this occurrence I have observed in Maine is from specimens collected at the Emmons Quarry in Greenwood, Maine during active blasting and re-exploration in the 1990s by Ray Sprague and Tony Wielkiewicz. At pockets at the Emmons Quarry, they encountered numerous slightly curved masses of silvery muscovite arranged into circular to pseudohexagonal pillars which looked like they had been pounded flat with a ballpeen hammer or nail set awl. Like the MFQ material, the best of these pseudohexagons showed themselves to be 'bootstrapped' into their large form by 1-2 mm hexagons aggregating themselves into a much larger, but still uniform euhedral hexagonal shape.

Taking all of the extant specimens into context, a basic pattern begins to emerge. First is that all minerals except muscovite are gone except for epitaxial coatings of quartz and albite after muscovite. Second is that the muscovite has a unique greenish-white coloration not found except in the replacement body. Third is that in parts of the replacement body, pseudomorphs of the original minerals exist in large size, but in other parts they are wholly absent: whatever minerals did exist have lost their chemical identity as well as their exterior form.

Quartz is the major outlier to this alteration event. In the replacement matrix, quartz is present as very small epitaxial coatings of clear crystals (1-2 mm) as well as large, complex crystals up to 20 cm in length and 8-10 cm in diameter. Parallel growth and epitaxial growth on large crystals is common, with the latter forming 'birds nests' of thousands of tiny quartz xls engulfing and consuming larger crystals. A theory consistent with observation is that the large quartz crystals formed in pockets well before the muscovite alteration event and were then attacked, corroded and agglomerated by these 'mosquito hordes' of epitaxial quartz on their surfaces.

A few macroscopic observations are useful here. First is that the colored elbaite pocket zones at both Mt. Mica and the Greenlaw quarry at Mt. Apatite are marked by extensive alteration to quartz, cookeite and muscovite after crystallization of the elbaite; at both sites this late stage alteration led to the 'loss' of what would have been otherwise outstanding euhedral, gemmy elbaite crystals.

Second is an inverse correlation between primary, massive lithia mineral abundance (ie. lithiophilite, montebrasite, spodumene) and gem elbaite. On this topic, Frank Perham states regarding his mining at Mt. Mica in 1965 (see King, 2000 at p. 273):

"If you chanced to find any montebrasite-amblygonite nodules round the edge of a pocket, embedded in lithiophilite, etc. around the edge of the edge of the pockets, that would be the 'kiss of death.' Not that phosphates were bad, it was just that you could guarantee that you wouldn't find a piece of gem tourmaline in the pocket. It got to the point that if you'd have found 8-10 of these montebrasite nodules about golf ball to tennis ball size embedded right in the edge of the pocket, your heart would sink."

Dump material at the MFQ corroborate Frank Perham's observations from the working face of Mt. Mica in 1965, ie. aggregations of primary lithia minerals at the MFQ dump inversely correlate to elbaite abundance in the same and presumably adjacent matrix. The most commonplace explanation for this inverse correlation is that primary lithia minerals, such as massive nodules of montebrasite and lithiophilite, ended up being the 'consumers' of lithium at the expense of colored elbaite.



Dating of the Muscovite Replacement Phase

Late stage, partial replacement of schorl to muscovite is evident at the MFQ as well as a small test pit 1/4 mile west of the Greenlaw pegmatite. In the latter example, large well-formed 3-8 cm schorl crystals in a clear quartz matrix are partly altered to white muscovite in their internal fabric and surface. At this same outcrop, large, subeuhedral crystal masses of schorl are intimately mixed with pyrite masses and white muscovite.

At the MFQ, several softball sized schorl cross-sections are half to almost completely pseudomorphed into fibrous olive green muscovite and studded with 0.5-1 cm cubic octahedra of pyrite on their original crystal faces. These relations are interpreted as an intermediate stage of a late alteration which permeated much of the pegmatite. The late stage melt appears to have been enriched in sulfur, which caused iron to be removed from the previously solidified schorl crystals and redeposited as euhedral pyrite. In the latest replacement stage at the MFQ, that marked by 'poker chip' stacks of pseudohexagonal muscovite, the replacement appears to have been so complete that all accessory minerals and elements were mobilized and did not recrystallize back onto the matrix.

From these relations, I assume that the green muscovite pseudohexagonal phase at MFQ must have been the latest and final stage of the melt for several reasons. First, the mineral assemblage is the most simple in the pegmatite: it had to be either the earliest stage or the latest stage, and it was clearly not the earliest stage based upon the large pseudomorphs present. Also, there appears to be no stage 'after' this stage, ie. minerals epitaxially deposited onto the muscovite, quartz and albite matrix. A logical question is where all of the accessory elements went when the pegmatite mass was fully replaced by muscovite, epitaxial albite and quartz. They had to go somewhere: lithium, for example. Or boron. Or iron and manganese. Or zirconium. I do not have an answer.

Conclusions

The low, erosion-resistant edifice known as Mt. Apatite in west Auburn, Maine comprises an extensive, sill-like collection of coarse pegmatite outcrops of several square miles in extent, all with similar fabric and mineralogy. By addition of the nearby Berry-Havey pegmatite in East Poland, Maine [which is nearly identical to the Mt. Apatite group in mineralogy and fabric], the Mt. Apatite 'district' is the most areally extensive complex pegmatite zone in the state. Only a small portion has been excavated, first as surface exposures in cow pastures for gem tourmaline pockets exposed by glacial scouring, and later for commercial soda and potash feldspar for ceramic glazes. The collapse of the Maine feldspar industry by WW II caused active mining to cease at Mt. Apatite. This was not due to the exhaustion of commercially suitable feldspar deposits, but more to the comparatively lower acquisition and transport cost at feldspar deposits in other parts of the U.S. As an 'industrial' mineral, the opportunity cost window for ceramic grade feldspar is quite narrow, meaning that even a small difference in transport and excavation cost can make an otherwise viable deposit economically unattractive. Economies of scale also favor very large operations and excavations over small operations and scattered deposits. Future mining development of the eastern group of Mt. Apatite pegmatites is precluded because the City of Auburn decades ago acquired most of the pegmatite zone east of Hatch Hill Road as a public recreation area, with mineral collecting limited to hand tools. While this has meant the foreclosure of any potential for future blasting and exhumation of the pegmatites, it has also preserved the pegmatite complex from residential housing development, which would foreclose forever all opportunity for collecting and study. The City of Auburn's foresight has also resulted in the eastern Mt. Apatite pegmatite group being the only lithium-rich pegmatite complex in Maine which is fully open and accessible to the general public for specimen collecting and scientific study. For these reasons, the City of Auburn did an extraordinary thing, in my opinion, by acquiring and preserving this entire property. It is a gift that keeps on giving.

As important as all that is, what's still left to ponder is where the Mt. Apatite pegmatite group came from. As far as I know (and please correct me if I'm wrong), the pegmatite group has still not been radiometrically dated. The only dating which has been done nearby is from the Sebago Lake pluton to the southwest, which by the most recent dating efforts shows a crystallization age of approx. 293 Ma. The 'old school' of thought is that the Mt. Apatite pegmatite group is a stringer of the Sebago pluton melt which intruded at long distance into the local metasedimentary country rock in Auburn. But absent any radiometric dating of minerals in the pegmatite itself, this is a reasonable but totally untested guess [I think it's turtles all the way down]. An alternate theory to the 'pegmatitic fluid squirt' from the Sebago pluton is that the Mt. Apatite pegmatites were wholly or primarily created by the complete melting and recrystallization of the metasedimentary country rock with limited 'pure' Sebago igneous melt involved.

In a 2008 NSF poster paper, Paul Tomascak of SUNY-Oswego and Gary Solar of SUNY-Buffalo, suggested a fairly radical revision to the traditional delineation and geographic scope of the Sebago Lake pluton, wherein they identified a 'core' granitic pluton centered at Sebago Lake surrounded by a distinct and much larger and more complex melt zone to the north, west and east which they labelled the Sebago Migmatite Domain. Of relevance to the Mt. Apatite pegmatite group is to what extent it received actual melt from the Sebago Lake core pluton or is comprised of metasedimentary rock which was completely melted. Absent trace element data and radiometric dating of the pegmatite itself, these are open questions. The pegmatites of the Mt. Apatite district are all closely related since they display similar mineralogy and fabrics [most notably seen in the extreme similarity of fabric and minerals at the Berry-Havey pegmatites in East Poland and the eastern Mt. Apatite group, even though they are ~5 miles apart.] Is this similarity due to a shared igneous source or the similar composition of a metasedimentary melt?

What I am willing to say is that the extensive sulphide mineralization in the Maine Feldspar Quarry pegmatite suggests a substantial melting and contribution from metasedimentary rock due to the sulphide minerals present (pyrrhotite, pyrite, arsenopyrite) and the presence of thin, but highly sulfidic lenses in the regional metasedimentary host rock (Central Maine Sequence). The mineral fabrics themselves suggest sulphides were introduced fairly late in the melt, ie. the sulphides are usually massive and anhedral and in most examples appear to have corroded and eaten 'into' the quartz and microcline. A type specimen (shown above) displays what best can be described as a 'swirly goop' of distinct bands of pyrrhotite, pyrite and arsenopyrite in otherwise massive, blocky microcline.

A second tell is gathered by relations in truly metamorphic rock at locations approx. 15 miles east and northwest of the Mt. Apatite pegmatite group, at Upper Parish Hill Road in Turner, Maine and at Sabbattus Mountain in Sabbattus, Maine. At Turner, metasedimentary rock has been raised to high sillimanite grade with 1-2 mm flakes of pure, silvery graphite mixed with sillimanite. At Sabbattus Mountain, large (4 cm) masses of fibrous white sillimanite are intergrown in quartz. Both examples give a clue to the extreme level of heating which occurred in the general area. The giant grossulars in calc-silicate lenses at the Pitts-Tenney deposit in Minot, several miles west of Mt. Apatite, offer another nearby example of extreme heating. How is this high sillimanite grade heat related to the paragenesis of the Mt. Apatite pegmatites? Is it? This is where cooling closure dating would be very helpful, since regional Acadian orogeny heating occurred 50-70 Ma earlier than emplacement of the Sebago pluton.