Case Quarries, Portland, Middlesex County, Connecticut, USAi

Regional Level Types
Case Quarries	Group of Quarries
Portland	- not defined -
Middlesex County	County
Connecticut	State
USA	Country

The Case prospects lie in the town of Portland, 4.5 miles N. 39° E. of the center of Portland village...

The property is owned by Myron N. Case, Rose Hill, Portland. The Worth Spar Co., Inc., of Cobalt quarried three pegmatites on the property for feldspar from 1933 to 1935. In the summer of 1939 Frank Bajorek of Portland mined the westernmost pegmatite (no. 1 quarry, pl. 43) for feldspar. The Worth Spar Co. prospected the no. 2 pegmatite for sheet mica in August and October 1942. The workings are opencuts that range from 60 to 110 feet in length, 7 to 45 feet in width and 10 to 25 feet in maximum depth. All are flooded.

The pegmatites were mapped by E. N. Cameron and V. E. Shainin in March 1943 and were studied period­ically until December 1943 (fig. 130 and pl. 43). The U. S. Bureau of Mines and Geological Survey cooperated in surface and subsurface exploration of the pegmatites from May to November 1943. E. E. Maillot was in charge of the project for the Bureau of Mines and V. E. Shainin studied the subsurface geology for the Geological Survey. The no. 2 and 3 quarries were pumped, and 14 holes totaling 1,673.5 feet were made by diamond-drill.

The three pegmatites on the Case property lie within 500 feet of one another. They strike north to northeast and differ in direction and magnitude of dip. The pegmatites cut granite-gneiss (Monson gneiss), the foliation of which strikes generally northward and dips 20°-30° W.

The no. 1 pegmatite, westernmost of the group, is 5 to 7.5 feet thick, strikes N. 70 E. and dips 35°-45° SE. It has been quarried for about 100 feet along strike and to a maximum depth of 17 feet. It may have terminated upward a short distance above the rim of the workings. The pegmatite has a border zone 1/2 to 1 inch thick consisting of fine-grained granular quartz, plagioclase, [microcline] perthite, and beryl. The rest of the pegmatite (designated [microcline] perthite-quartz zone on pl. 43) consists of medium- to extremely coarse-grained [microcline] perthite and quartz, with subordinate plagioclase and muscovite, and accessory beryl, garnet, and columbite-tantalite. Muscovite forms small, colorless to gray-green, heavily stained books, irregularly distributed. In general, the texture of this material is progressively coarser toward the center of the pegmatite. There is no clearly defined quartz core exposed, but in places in the central part of the pegmatite there are irregular bodies of quartz, against which the [microcline] perthite crystals are euhedral. Offshoots of the quartz bodies extend outward across the surrounding pegmatite along fractures. In addition, veins of quartz occur along the contacts with wall rock. Debris left in the north end of the opencut suggests the presence of a small quartz core flanked by a perthite-quartz zone similar to that of the no. 2 pegmatite described below.

High-grade [microcline] perthite constitutes at least 50 percent of the [microcline] perthite-quartz zone visible. Beryl occurs chiefly in the border zone and the outermost 12 to 18 inches of the perthite-quartz zone, and is most abundant in the footwall part of the perthite-quartz zone. The crystals range from 1/8 inch to 4 inches in length and from 1/16 inch to 3 inches in diameter. Measurements of all crystals in the cross-section of the pegmatite in the northern face of the quarry indicate a beryl content of 0.15 per­cent. However, the exposures available for measurement are few, and the accuracy of the figure obtained for grade is doubtful. Waste rock on the dump seemed to show considerably more beryl in material derived from the [microcline] perthite-quartz zone than is indicated by the crystal measurements. Measurements of beryl crystals in the footwall part of the border zone indicated 0.41 percent beryl.

The no. 2 pegmatite (mined in the no. 2 quarry) is a tabular lens that strikes N. 17° E. and dips 15° NW. At the surface, it has a strike length of 60 feet. It probably terminates beneath overburden less than 40 feet northward from the quarry. The south edge of the lens plunges southward from the open cut at a moderate angle. One hundred and eighty feet down dip from its surface outcrop, the inferred strike length of the pegmatite is 155 feet. The thickness of the body ranges from 10 feet at the surface to about 16 feet at a point 180 feet down dip.

The pegmatite is distinctly zoned. The border zone, 2 to 4 inches thick, is composed of fine-grained quartz, [microcline] perthite, and plagioclase, with accessory muscovite, garnet, beryl, and tourmaline. The wall zone, 2 to 6 inches thick, consists of medium-grained [microcline] perthite, plagioclase, and quartz, with accessory muscovite, beryl, and garnet. The intermediate zone, 1 to 4 feet thick, is similar to the wall zone but is composed chiefly of quartz and extremely coarse [microcline] perthite. The core averages 5 feet in thickness and is composed of coarsely crystalline milky quartz, accessory [microcline] perthite in scattered large, euhedral crystals, and rare beryl. [Microcline] Perthite crystals in the [microcline] perthite-quartz intermediate zone are euhedral against quartz of the core. The zonal structure was clearly recognizable in 3 of the 4 drill holes that intersected the pegmatite 1, 2, and 5. Cores from holes 5 and 7 were inadequate for the construction of logs.

Beryl occurs in the border and wall zones in scattered crystals ¼ to ¾ inches in diameter and as much as 3 inches long. Some crystals in the [microcline] perthite-quartz zone are 9 inches in diameter and 10 inches long. Measurements on exposures in the quarry (230 square feet) suggest that the average beryl content of the pegmatite is 0.34 percent. On the basis of this figure, and diamond drilling, 53 tons of beryl are indicated and 27 tons are inferred to lie beneath the surface to a depth of 180 feet clown the dip.

The percentage of high-grade feldspar in the no. 2 pegmatite is less than that in the no. 1 pegmatite but [microcline] perthite in the [microcline] perthite-quartz zone is mostly pure and separable by hand sorting.

The no. 3 pegmatite, northernmost of the group, is a tabular lens that strikes N. 25° to 48° E. and dips 63° to 71° NW. The crest of the lens plunges gently northward and southward from the quarry. The strike length of the pegmatite ranges from 70 feet at the surface to at least 350 feet at a level 120 feet down dip from the workings. Along strike the dike is thickest in the center (7 feet) and thins to less than 2 feet both north and south of the workings. The pegmatite was probably not intersected by drill hole 14. It is believed to terminate above the hole but below elevation 240 feet (pl. 43, sec. A-A’).

The dike exhibits a fairly distinct zonal structure in the no. 3 quarry. The border zone, ½ to 1 inch thick, is composed of quartz, [microcline] perthite, and plagioclase, with accessory black tourmaline, muscovite, beryl, and garnet. The wall zone, 1 to 2 feet thick, is irregular and in places absent. It is composed of quartz and plagioclase with various amounts of coarse-grained [microcline] perthite, subordinate muscovite, accessory beryl, tourmaline, and columbite-tantalite. The pegmatite inside the wall zone consists of coarse-grained [microcline] perthite and quartz with accessory beryl and plagioclase. There is no true quartz core, but irregular bodies of quartz are present in the quartz-[microcline] perthite zone. Neighboring [microcline] perthite crystals are euhedral against the quartz bodies. The zonal structure of the pegmatite was recognized in the diamond-drill hole cores, although it was less clearly defined than at the surface.

Beryl occurs in the border zone in crystals less than ¼ inch in diameter and 1 inch long, and in the wall zone crystals as much as 2 inches in diameter and 5 inches long. The footwall part of the wall zone appears to contain more beryl than the hanging-wall part, and the beryl crystals are larger. Counts made on the northern and southern faces of the quarry (250 square feet of pegmatite), before and after exploratory blasting, indicate an average beryl content of 0.22 percent. From this figure and data of the Bureau of Mines, 25 tons of beryl is indicated and 6 tons inferred to lie beneath the surface to a depth of 120 feet down the dip.

The Case nos. 2 and 3 pegmatites are inferred to contain slightly more than 100 tons of beryl, mostly in small crystals. Recovery by hand-cobbing would unquestionably be difficult. Each of the three pegmatites contains limited tonnages of high-grade, hand separable [microcline] perthite, and the no. 2 pegmatite probably contains at least 6,000 tons of coarse milky quartz that appears to be of high purity and could be separated readily by hand.