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Significance of Tourmaline-Rich Rocks in the Grenville Complex of St

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Significance of Tourmaline-Rich Rocks in the Grenville Complex of St Significance of Tourmaline-Rich Rocks in the Grenville Complex of St. Lawrence County, New York U.S. GEOLOGICAL SURVEY BULLETIN 1626-C Chapter C Significance of Tourmaline-Rich Rocks in the Grenville Complex of St. Lawrence County, New York By C. ERVIN BROWN and ROBERT A. AYUSO A study of tourmaline compositions and their possible use as indices of mineralization U.S. GEOLOGICAL SURVEY BULLETIN 1626 CONTRIBUTIONS TO THE GEOLOGY OF MINERAL DEPOSITS DEPARTMENT OF THE INTERIOR DONALD PAUL MODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE: 1985 For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 Library of Congress Catalog-card No. 84-600189 CONTENTS Abstract Cl Introduction Cl Purpose of study C2 Stratigraphic and lithologic setting C2 Petrography and summary of the bulk composition of tourmaline-bearing rocks C3 Sampling C7 Analytical procedure C7 Results of compositional studies Cll Tourmaline end members in nature Cll Compositional variation of tourmaline in North Gouverneur area Cll Calcium content of tourmalines C20 Substitutions in hypothetical structural formula C20 Mineralized areas and tourmaline compositions C22 Discussion C25 Comparison with the Balmat-Edwards district C29 Tourmaline genesis in the North Gouverneur area C29 Terranes showing most favorable implications for containing stratabound sulflde deposits C31 References cited C31 FIGURES 1. Index map of St. Lawrence County, N.Y. C2 2. Generalized geologic map showing sample localities for this study C4 3. Photographs of tourmalinite, brecciated tourmalinite, granular patches of black schorl, and schorl prisms C6 4. Photomicrographs of granoblastic tourmalinite and tourmaline-bearing rock C7 5. Distribution of selected elements in rocks from the North Gouverneur area, New York C8 6. TiO2-FeO/(FeO + MgO) plot for tourmalines in lithologic units from North Gouverneur, N.Y., compared with those from Sullivan Mine, British Columbia C19 7. Ternary plot of Na2O-MgO-FeO for tourmaline from North Gouverneur, N.Y., and the Sullivan Mine, British Columbia C20 8. CaO-Al2O3 plot for tourmalines from North Gouverneur, N.Y., compared with those from the Sullivan Mine, British Columbia C21 9. Al2O3-FeO/(FeO + MgO) plot for tourmalines from North Gouverneur, N.Y. C22 10. (Na + Fe)-Al plot for tourmalines from North Gouverneur, N.Y., compared with those in rocks from Sullivan Mine, British Columbia C23 11. (Na + Ca)-Al plot for tourmalines from North Gouverneur, N.Y., compared with those in rocks from the Sullivan Mine, British Columbia C24 12. (Na + Mg + Al)-Ca + 2Fe plot for tourmalines from North Gouverneur area, New York, compared with those from Sullivan Mine, British Columbia C25 13. Ca-Na plot for tourmalines from North Gouverneur, N.Y., compared with those from Sullivan Mine, British Columbia C26 14. Al2O3-Na2O/(Na2O + CaO + K2O) plot for tourmalines from North Gouverneur, N.Y., compared with those from the Sullivan Mine, British Columbia C27 Contents iii TABLES 1. Summary of petrographic features of tourmaline-bearing rocks from the North Gouverneur area, New York CIO 2. Electron microprobe analyses and structural formulas of tourmalines in the North Gouverneur area, New York C12 3. Chemical analyses of selected tourmaline-bearing rock types in the North Gouverneur area, New York C17 4. Semiquantitative spectrographic analyses of tourmaline-bearing rocks in the North Gouverneur area, New York C17 5. Semiquantitative spectrographic analyses of tourmaline in Beaver Creek area, New York CIS iv Contents Significance of Tourmaline-Rich Rocks in the Grenville Complex of St. Lawrence County, New York By C. Ervin Brown and Robert A. Ayuso such as a low TiO2 content, variable Na/(Na + Ca + K) ratios, Abstract and the antipathetic correlation of Na and Ca might also be Feldspathic quartzite and metapelite of Middle Pro- useful in discriminating tourmalines associated with ore terozic age north of Gouverneur, N.Y., contain abundant deposits. dravite-uvite (magnesian tourmaline). These rocks, more Ore bodies at the nearby Balmat-Edwards mining district than 1,000 feet thick, are regionally metamorphosed to the are in marbles associated with rocks of evaporitic origin. The upper amphibolite facies, are pyritic, and locally contain boron- and scapolite-rich rocks under consideration in this porphyroblastic scapolite. The rocks are underlain by talc- study possibly also are related to an evaporite-producing tremolite schist and a thick sequence of calcitic and environment. Similar rocks that have an evaporite origin are dolomitic marble, and are overlain by quartz, calc-silicate associated with stratabound sulfide deposits at many places. carbonate rock, and a pyroxene scapolite unit. All rocks are part of the metasedimentary Grenville Complex. Individual tourmaline-rich layers in the quartzite are INTRODUCTION as thick as 3 cm and contain as much as 50 percent fine­ grained, brownish-green dravite. Subjacent talc schist con­ Tourmaline is a common accessory mineral in tains fine-grained amber dravite, and silicated marble locally rocks of the Grenville Complex in the Grenville Low­ contains brown porphyroblasts of uvite. Gneissic granite, lands of St. Lawrence County, N.Y. It occurs dissemi­ pegmatite, and migmatitic segregations cutting the nated in a variety of metasedimentary rocks ranging tourmaline-bearing quartzite, gneisses, and schists are rich in from marble to feldspathic gneiss, and also in black schorl. Tourmalines from the tourmaline-rich quartzite and pegmatitic segregations and granites that intrude the other metasedimentary and metaigneous rocks were ana­ tourmaline-bearing metasediments. Although tour­ lyzed by means of an electron microprobe. The compo­ maline is normally present only as an accessory mineral, sitions of tourmalines in the North Gouverneur area are it is very abundant in certain rock units in belts mainly clearly a function of the bulk composition of the rock. Tour­ in the Richville 7.5-minute quadrangle (fig. 1). Gushing maline compositions in the quartzite have a wide range in and Newland (1925, p. 24-25) called attention to rocks, FeO/(Fe+MgO) ratios, from 0.15 to 0.58, which are distinct particularly along the Oswegatchie River west and north from the ratios for tourmalines in granitic and pegmatitic of Richville, N.Y., that contain 35 to 50 percent tour­ rocks that range from 0.55 to 0.75. Tourmalines in the area maline. Such rock units are lithostratigraphic sequences have Na2O contents that range from 0.85 to 4.25 weight per­ consisting of feldspathic quartzite and quartz-feldspar- cent. Aluminum in all tourmalines ranges widely from about mica granofels having layers that contain as much as 50 25 to 37 weight percent AI2O3. The most heterogeneous tour­ maline compositions are in the quartzites, although most percent fine-grained tourmaline, and are classified as compositions cluster in the dravite-uvite solid solution "tourmalinite." series. Substitutions involving Na, Ca, Mg, Fe, and Al result in In 1966 Harold M. Bannerman (1972) mapped the the following compositional schemes: Na+Fe = Al+va­ belt of microcline-tourmaline-quartz gneiss along the cancy, and Na = Ca. Such coupled substitutions probably Oswegatchie River (fig. 2), where at many outcrops the represent valid constraints for tourmalines from North rocks are spectacularly brecciated in a zone of intense Gouverneur. faulting. In an attempt to determine the significance of The abundance of magnesian tourmaline in the metasedi­ these tourmaline-rich rocks, and as an aid to future mentary rocks of the North Gouverneur area is comparable research, Bannerman had several chemical analyses to that of tourmaline-rich rocks associated with sediment- made of rocks from this zone (Brown, 1980, analyses 45 hosted massive sulfide deposits for example, Sullivan through 71) and indicated the locations of tourmaline- Mine, British Columbia. A recently described large lead-zinc deposit at Dugald River, Australia, is associated with rich rocks on his map of that area (Bannerman, 1972). tourmaline- and scapolite-bearing rocks similar in many re­ The present study was in part instigated by his concern spects to those north of Gouverneur. Compared with known about the significance of these rocks, and by his encou­ tourmaline compositions in the Sullivan Mine area, tour­ ragement that further research be done. malines from North Gouverneur have comparable FeO and Subsequent geologic mapping by C. E. Brown in MgO values. We suggest that other tourmaline components an area a few miles to the northwest of the Oswegatchie Tourmaline-Rich Rocks in Grenville Complex, St. Lawrence County, N.Y. C1 4 in composition and are believed to be the result of boron ' *$!&$& . V^t?*f | *f3$ir"f?vJ-' " ^ ***^ ^ and magnesium introduced syngenetically or diageneti- - ^ \- { f- *^-, ^^ J , f cally by hydrothermal solutions. Because of these rela­ | .; 4j$fwn»- <*>* \ tionships, Ethier and Campbell (1977) and Slack (1980, | '- ^rif f^ \ 1982) suggested the use of tourmaline as a prospecting \ \_,.k _ guide for stratabound sulfide deposits. Brown (1983) speculated that the anomalously 1 7 1 *' KST LAWRENCE COUNTY it abundant tourmaline in the vicinity of North Gouver- neur indicated depositional conditions possibly related 1 ,-?* 1 ; to massive base-metal sulfides. The study reported here WSJM/ Richville | * was undertaken to determine the composition of tour­ | ? r" ',,, quadrangle l^" * malines in the metasedimentary
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