Diamond-drilling Exploration of the Beecher No. 3- Black Diamond Pegmatite Custer County, South Dakota GEOLOGICAL SURVEY BULLETIN 1162-E Diamond-drilling Exploration of the Beecher No. 3- Black Diamond Pegmatite Custer County, South Dakota By J. A. REDDEN CONTRIBUTIONS TO ECONOMIC GEOLOGY GEOLOGICAL SURVEY BULLETIN 1162-E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS Page Abstract___________._______________________.__ El Introduction.___________________________________ ________ 1 Description of pegmatite units_____________________ _____.-__ 2 Structural geology_________________________________________________ 9 Economic appraisal of the exploration____________________________ 10 References. ___ __________________________ 11 ILLUSTRATION Page PLATE 1. Outline map and sections, Beecher No. 3-Black Diamond peg- matite______________________________________ In pocket TABLE Page TABLE 1. Diamond-drill logs, Black Diamond pegmatite.____________ E3 m CONTRIBUTIONS TO ECONOMIC GEOLOGY DIAMOND-DRILLING EXPLORATION OF THE BEECHER NO. 3-BLACK DIAMOND PEGMATITE, CUSTER COUNTY, SOUTH DAKOTA By J. A. REDDEN ABSTRACT Diamond-drilling at the Beecher No. 3-Black Diamond pegmatite, Ouster County, S. Dak., has provided information that modifies and supplements findings reported previously (Redden, 1959). Two zones not exposed at the surface were found during the drilling: a quartz-albite-perthite-muscovite-pegmatite zone and a quartz-albite-perthite-spodumene pegmatite zone. Previous concepts of the structure near the surface require no significant change, but the new data make possible a greatly improved interpretation of the structure at depth. The most notable change is the recognition of a narrow constriction in the pegmatite at a depth of 60 to 100 feet. The drilling results are not encouraging in an economic sense. The beryl-rich deposit exposed at the surface was not recognized in the drill core, but the quartz- albite-perthite-muscovite pegmatite zone is the most favorable for further ex­ ploration. The known beryl deposits are all above the newly discovered constric­ tion and along a low-plunging roll on the east side of the deposit. Some zones containing spodumene were cored, but they are either low in grade or altered and iron stained. INTRODUCTION A report on the Beecher No. 3 Black Diamond pegmatite, Ouster County, S. Dak., including a map and inferred cross sections, was published in 1959 (Redden, 1959). In the fall of 1960, the pegmatite was core-drilled by the Lithium Corp. of America under Office of Minerals Exploration contract 6,083. This core-drilling provided a partial test of the previously interpreted structure and the inferred distribution of beryl; it also supplied added detail on the subsurface character of the deposit. Six BX core holes, totaling 1,740.9 feet, were drilled in the deposit. One split of each core was logged by the writer (table 1), although the first part of the core from hole 2 was unfortunately ground to a minus 0.5 inch and the textures destroyed before it was examined. El E2 CONTRIBUTIONS TO ECONOMIC GEOLOGY Core recovery in pegmatite was generally good. The drill core of the country rock was not available for logging. These data permit the construction of new cross sections of the pegmatite, as shown in plate 1. The writer wishes to thank the Lithium Corp., of America and its representatives, especially J. N. McClure and F. F. Clarke, for permission to publish the results of this work and to examine chemical assays. DESCRIPTION OF PEGMATITE UNITS At the surface, the Beecher No. 3 Black Diamond pegmatite contains five distinguishable zones (Redden, 1959, p. 543-548). From the contact inward they are (1) quartz-muscovite-albite peg­ matite (border zone), (2) albite-quartz-muscovite pegmatite (wall zone). (3) quartz-albite-muscovite-perthite-beryl pegmatite, (4) perth- ite-quartz-albite pegmatite, and (5) perthite-quartz pegmatite. Accessory minerals include beryl, tourmaline, spodumene, and apatite. In general, the muscovite content decreases and perthite increases inward from the contact. Zone 3 is discontinuous, and zone 5 occurs only in the thickest parts of the pegmatite. Fracture fillings containing spodumene and quartz cut across all zones. Details of the mineral character and distribution of these zones, and the general geologic setting of the pegmatite, are included in the published account (Redden, 1959). All zones except zone 3 were cut by one or more of the drill holes (table 1). In addition, two new zones were noted. One of these, which apparently is the downward extension of zone 3 consists of quartz-albite-perthite-muscovite pegmatite; the other, which lies in the center of the pegmatite body consists of quartz-albite-perthite- spodumene pegmatite. The distribution of all zones, and their inferred relation to the surface exposures, are shown in the cross sections. Zone 1 (border zone) was recognized in cores from holes 1, 3, and 4. At the surface it occurs along most contacts between pegmatite and country rock, but it is absent at about half of the subsurface contacts. The zone is normally 0.1 to 0.2 foot thick, and because it is so narrow, it is combined with zone 2 in some of the logs (table 1). The chief minerals are quartz, muscovite, and albite. Zone 2 (wall zone), cut by all six drill holes, ranges in thickness from 1 to 7 feet. The mineral character is similar to that at the surface. The zone consists dominantly of albite and quartz, which occur in nearly equal amounts, and it also carries about 10 percent muscovite. In holes 4, 5, and 6, the wall zone (as well as parts of adjacent inner zones) has as much as 10 to 15 percent of arsenopyrite(?) in veinlets or disseminated throughout the core. TABLE 1. Diamond-drill logs, Black Diamond pegmatite [Tr, trace] Volume percent 0-50.0 _ . ____ . _ . Overburden and schist. No core. 50.0-245.1 _______ . 245.1-245.2.-- __ I 45 45 10 Grain size loss than 0.1 in. 245.2-252.0 - 2 30 Tr 62 8 Blue phosphate Grain size about 0.5 in. at 251 ft. QKO A-OR7 7 3a 40 5 50 5 Beryl. I- Muscovite is in 0.5-in. books except at 266 ft where books are larger Tourmaline... .. 267.7-279.0.. 4 OC 50 25 Tr Tr 0.8 in. grain size. o 279.0-295.0 4 40 7 50 3 Spodumene-- !zj O Tourmaline - Tr Some of albite is sugary textured. Log of other core split reports a ?S-in. crystal ol beryl at 290 ft. ^ 6 70 15 5 5 Spodumene- ...... I 5 Perthite is only slightly perthitic and could more aptly be called H 295.0-310.5- . Unidentified Tr microcline. Fresh and altered spodumene crystals as much as phosphate 2.4 in. long. Part of albite is sugary textured. I mineral. > 310.6-329.0. 4 35 7 55 3 Spodumene-. ..... 1 * a H3 329.0-331.6.. ........ 4 20 60 20 13 331.6-333.1~ 2 30 60 10 Fine grained. Thin border zone at end. Schist contact is at 25° N* to core. 333.1-387.0...... Micashist. Not logged. Hole end 387.0 ft. CO CO TABLE 1. Diamond-drill logs, Black Diamond pegmatite Continued Mode (percent) Interval (feet) Zone Other minerals Additional data Quartz Pert bite Albite Musco­ vite Hole 2 Volume percent 0-50.0 .............. 60.0-104.0 104.0-110.5.. 2 40 50 10 Tr covery. 110.5-127.0. 3a 40 5 50 5 Tr that in surface exposures of zone 4 in southeast part of pegmatite except core has more muscovite and less perthite. Core recovery, 50 percent. 127.0-128.0.. 3a 65 5 30 filling. 128.0-131.6 i FP(?) 60 30 10 131.6-150.0 3a 30 5 60 5 150.0-155.5.. ...... ..... 3a(?) 10 80 10 155.5-156.6. 2 30 65 5 156.6-157.7..... 2 25 70 5 161.0-162.0 FF(?) 50 30 20 162.0-190.0- 3a(?) 30 65 5 Spodumene... I - 173.7 and 173.9 ft. Small rounded grain of beryl between 183 and 186 ft where core recovery is only 50 percent. 190.0-196.0. 6(?) 50(?) 25(?) 25 a fracture filling. 196.0-216. 3a 25 65 8 1 Tr Fine grained. Gneissose structure in some of core. Core brecciated I 216.0-225.0.. _ 6(?) 40 25 10 15 Tr measuring 1.5 by 2 in. at 225 ft. Could be a series of spodumene- b earing fracture fillings. 225.0-234.0. _ .. ....... 3a(?) 40 20 40 234.0-236.6. 2 15 65 . 15 5 Schist contact is at 63° to core. 236.6-239.0.,-.. 239.0-240.1.. 2 Pegmatite. Same as 234-236.6 interval. No contact angles avail­ able. 240.1-241.1. 241.1-241.8- Pegmatite. Same as 234-236.6 interval. 241.8-255.0- Schist country rock. Hole ends 255.0 ft. Holed 0-104.6..... __ ........ 104.6-105.0..... _ ..... 1 65 35 Average grain size about 0.1 in. Schist contact is at 80° to core. 105.0-110.9 55 in 15 110. 9- 111.1....... ...... 1 40 40 20 cd 111.1-123.9 123.9-124.0 .......... 1 60 40 124.0-129.0......... 2 50 40 10 3 in. in diameter. 129.0-132.3 3a 40 40 20 132.3-141.1 5 15 80 5 141.1-142.6 i FF(?) 95(?) 5(?) co very. 142.6-186.9 5 10 85 5 Tr The hole was in the same crystal of perthite for at least 15 ft.