B2: GEOLOGY AND MINERAL DEPOSITS OF THE NORTHEASTERN ADIRONDACK HIGHLANDS
Philip R. Whitney
New York State Museum
3140 CEC
Albany NY 12230
James F. Olmsted
48 Haynes Road
Plattsburgh, NY 12901
INTRODUCTION
On this trip we will examine three of the most notable features of the Adirondack Highlands. The first stop is at the NYCO wollastonite mine at Oak Hill near Lewis, where wollastonite-garnet-pyroxene skarns preserve a record of a giant Proterozoic hydrothermal system that was fed by meteoric waters and driven by heat from the nearby Westport Dome anorthosite intrusion. The following two stops, at Arnold Hill and Palmer Hill near Ausable Forks, are in the granitoids and felsic metavolcanics of the enigmatic Lyon Mountain Gneiss, host to numerous low titanium magnetite deposits that were the basis of a flourishing iron mining industry in the late nineteenth century. The remaining three stops, one at Jay and two near Elizabethtown, are in metamorphosed anorthosite and its mafic derivatives, and illustrate the structural complexity and lithologic diversity of these rocks which ordinarily appear on maps as undifferentiated blobs.

B2: GEOLOGY AND MINERAL DEPOSITS OF THE NORTHEASTERN ADIRONDACK HIGHLANDS

This is NYCO property.
This is NYCO property; if following this trip on your own, be sure to get permission at the NYCO office in Willsboro.

STOP 1. OAK HILL WOLLASTONITE MINE.
At the first small quarry on L, park on the R well off the road. From here we will walk through the entire mine complex observing and describing the rock units. As you can see the mine is under development so there may be dangerous situations for which care must be exercised. Please use caution and follow instructions with care.
The Oak Hill Mine was discovered by accident during a logging operation, when a skidder exposed white rock which the logger recognized as wollastonite. It is located at a sharp bend in the OBZ, where the structural trend changes from NNE with a gentle W dip to nearly EW with a gentle to moderate S dip. As of June 2002, mine is being actively developed. Two test pits have been blasted in ore, and work has begun removing overlying rock and glacial deposits in order to begin open-pit mining. Additional exposures may be available at the time of the trip, but others may have been removed or covered. Figure 9, constructed from drill core data, shows the complex nature of the ore body.
The lowermost exposures are knobs of skarn consisting largely of orange-red grossularitic garnet, locally with abundant green, diopsidic clinopyroxene and/or veins and patches of sodic plagioclase and quartz. The garnetite encloses blocks of gabbroic anorthosite gneiss, metagabbro, and pyroxene granulite. Some of these blocks have narrow alteration haloes but otherwise are in sharp contact with the garnetite. They appear to be fragments of disrupted layers in what was probably a marble-hosted tectonic breccia, since metasomatically transformed to garnetite (other interpretations welcome!). Garnetite is much more abundant here than at either Lewis or Willsboro. Drill core data indicate that the the proportion of garnetite to ore increases toward the NE, eventually forming a layer of nearly 200', without ore.
Proceeding up-section, several layers of ore are exposed, separated by septa of mafic gneiss and/or garnetite. Garnetite layers are discontinuous; some appear to be megaboudins.
The hanging wall of the exposed ore consists of strongly foliated and lineated gabbroic anorthosite gneiss. This is overlain in turn by a chaotic marble melange with detached blocks of intricately folded calcsilicates. The marble also contains masses of crumbly-weathered skarn consisting largely of black (dark orange-brown in transmitted light) garnet and dark green clinopyroxene. This skarn resembles those associated with magnetite deposits elsewhere in the Adirondack Highlands rather than the garnet-pyroxene layers in the ore. The fact that this marble has apparently escaped extensive metasomatism suggests that the metasomatizing fluids that formed the ore were strongly channelized.
Overlying the marble is mafic gneiss with interlayers of charnockite and coarse, leucocratic augen gneiss, the latter an extreme L-tectonite. Above the gneiss complex, coronitic olivine metagabbro is exposed. The section in the mine is cut by several E-W, nearly vertical faults, one of which exhibits slickensides plunging steeply E. On the N side of the mine road, at the time of writing, bedrock is concealed beneath a thick section of varved glacial lake sediments inter-layered with boulder till.

Crossing north branch of the Boquet River.

Spectacular fault breccia on L.
This is one of the numerous NNE- to NE- trending faults that occur throughout much of the eastern and central Adirondack Highlands. Movement on these faults, possibly associated with the opening of the Iapetus Ocean, began in the latest Proterozoic and continued at least into the Ordovician.

Pokomoonshine State Park.
The prominent cliff is mostly granitic gneiss of the AMCG suite, containing several layers of mafic gneiss that are probably transposed dikes.

Marble xenolith in jotunite
The prominent white spot in the roadcut on the R is a marble xenolith in jotunite and badly contaminated gabbroic anorthositic gneiss. The marble contains abundant graphite, phlogopite and diopside and lesser amounts of chondrodite, grossular, and sulfides. Wollastonite is absent.

Enter hamlet of Clintonville.
Reportedly, in the late 19th century this was a town of 10,000 miners, wood cutters, iron forge workers and their families.

Stop 2A. LYON MOUNTAIN GNEISS; Albite gneiss facies.
Park off the road on R. The recently blasted low cuts on the L are largely albite gneiss with about 30% quartz, 65% plagioclase (ca. An2Ab96Or2) with minor magnetite and/or hematite, and traces of biotite, clinopyroxene, titanite, apatite, and zircon. The fine-grained, equigranular texture is typical of this facies of the LMG. Note the small, nearly isoclinal Z-folds, N 10-20° E lineation, and numerous crosscutting quartz-albite pegmatites and quartz veins. The pegmatites locally contain masses of partially martitized magnetite, possibly remobilized from nearby ore bodies.

Stop 2B FOLDING IN LYON MOUNTAIN GNEISS.
These flat outcrops at the intersection of Arnold Hill Road and a logging road on the L show complex folding in albite gneiss similar to that at the last stop. Abrupt color changes in the rock crosscut the folding and appear to be contacts of more and less oxidized rock, suggesting a post-deformation oxidation event. This area is not far from the site of the Arnold and Nelson Bush Mines, opened in 1830 and operated sporadically until 1906 (Kemp and Alling 1925; Postel 1952). The ore at Arnold Hill occurs as three foliation-parallel layers from 3-25' thick in albite gneiss. The layers were known to the miners as the gray, black, and blue "veins". The gray and black veins consist of magnetite, quartz, plagioclase, chlorite, and hornblende; in the blue vein the ore mineral is martite (Postel 1952).

Small exposure of LMG on R side of road.

Park off road on R.
Walk around the iron gate and proceed S along the unpaved road furthest to the R.

Stop 3. PALMER HILL.
Follow the road approximately 0.3 miles to the top of Palmer Hill. Along the way are several small pavement outcrops exposing heterogeneous LMG showing complex folding and containing numerous small clots of magnetite ore. Were these formed in place, or are they disrupted fragments of a vein of ore emplaced before deformation? At the top of the hill are several large boulders of Potsdam Sandstone (middle to late Cambrian). The Potsdam unconformably overlies the Proterozoic rocks around much of the perimeter of the Adirondack Dome, and is exposed at the surface less than 6 miles to the NE. An abandoned fire tower here has been converted to a cellphone relay. We will then follow a powerline for about 0.1 mile S to the old mine workings. USE EXTREME CAUTION, especially in wet weather. Cross a narrow rock bridge over the open cut and turn L to view the mine workings. Do not go down into the cut; samples of ore will be available. The ore here occurs in a discontinuous layer up to 20 feet thick, striking NE and dipping NW, roughly parallel to foliation on the SE limb of a NE-plunging synform (Postel 1952). It consists of magnetite with quartz, feldspar, and, locally, apatite, fluorite, and andraditic garnet. The host rock is predominantly the microperthite facies of LMG, although drill cores show almost the entire range of LMG lithologies (Postel 1952). Fluorite is a common accessory in the gneiss, and andraditic garnet occurs locally. Mining at Palmer Hill began in 1825 and continued until 1890 (Newland, 1908).

"Lake Placid Granite" quarries entrance on R.
The "granite"here is actually anorthosite.

STOP 4. ANORTHOSITE OF THE JAY DOME.
Walk back across the bridge (Beware of traffic!) and descend the steep bank on the L to outcrops in the river. The river is usually quite low this time of year affording us good looks at several facies of anorthosite and a variety of structural features. Both leucocratic and gabbroic anorthosite are present, as well as a block of coarser gabbroic anorthosite. In one location, a thin layer of pyroxene- and oxide-rich ultramafic rock separates two anorthosite facies, and is offset by parallel small faults. The latter may have formed at relatively high temperatures; foliation in the anorthosite locally shows the effects of drag along the faults. They are subparallel to later, NE-trending brittle faults, some of which are occupied by unmetamorphosed diabase dikes. One such fault-cum-dike offsets a shallow-dipping mylonite zone in the anorthosite. A xenolith of calcsilicate rock in the anorthosite consists largely of diopsidic clinopyroxene. Numerous potholes are present in the outcrop surface.
Walk back across the bridge and, if time permits, up the road beyond the parking area to a glacially polished and striated outcrop showing rounded blocks of coarse gabbroic anorthosite in finer grained, more leucocratic anorthosite.

Several exposures, of calcsilicates, mafic gneiss and contaminated anorthosites
Several exposures, of calcsilicates, mafic gneiss and contaminated anorthosites may be seen along Rt. 9N between Upper Jay and Keene.

STOP 5. JOTUNITE WITH XENOLITHS.
Park on R beyond the guard rails; cautiously cross the road and walk back to a small outcrop on the N side. The rock here is a jotunite containing numerous metasedimentary xenoliths and one xenolith of anorthosite. The metasedimentary rocks are diopside-rich calcsilicates, some with fine, millimeter to submillimeter-scale layering consisting of alternating pyroxene- and plagioclase-rich layers. The layering is normally straight but locally shows irregular folding or chaotic disruption. The general appearance of the rocks is similar to that of stromatolite-bearing metasedimentary rocks in the Balmat area on the northwest Adirondack Lowlands. Is it possible that the layering here is of organic origin? What other processes might be responsible?
Carefully observe the foliation in the jotunite near the anorthosite xenolith. If this is, as it appears, a metamorphic foliation then it suggests that penetrative deformation may affect some rocks and not others. Beware of assuming that a rock that crosscuts foliation in another rock is necessarily younger!
Interestingly, jotunites in the Adirondack Highlands commonly form intrusion breccias, whereas granitoids, olivine metagabbro, and ferrogabbro do not. Why?

STOP 6: "WOOLEN MILL" GABBRO. ANORTHOSITE, SYENITE.
Park on the L (north) side. The best exposures are the stream N of the road near the remains of an old dam. This stop provides another opportunity to study the characteristics and complexity of Adirondack metanorthosite and its associated lithologies. Fine-grained mafic granulites consisting of granoblastic plagioclase, clino- and orthopyroxene, garnet, ilmenite, and magnetite are of ferrogabbroic composition. This rock has been informally called the "Woolen Mill Gabbro". Here it intrudes the anorthosite as a network of thin, discontinuous dikes. At the upstream end of the outcrop, one of these terminates in a small bulb. More extensive exposures of this lithology are found in the roadcut on the S side of 9N just across from the parking area. A slightly coarser version of the same rock is exposed upstream on the opposite bank, in sharp contact with the anorthosite.
Also present are small dikes of coarse syenite, some with rows of pyroxene crystals along their contacts with anorthosite. Both these and the ferrogabbro appear to be intruded along fractures in the anorthosite, sometimes in the same fractures! (Which came first? Or are these composite dikes formed from two immiscible liquids?).