GEOLOGYAND GEOCHEMISTRY OF ORDOVICIANCARBONATITE DIKES IN THELEMITAR MOUNTAINS, SOCORROCOUNTY, NEW MEXICO Virginia T. McLemore New Mexico Bureau of Mines and Mineral Resources Open-FileReport 158 February, 1982 ABSTRACT Ordovician (449k16 m.y.) carbonatites intrude the complex Precambrian terrain in the Lemitar Mountains. They occur as dikes, stockworks, and veins and notare associated with any alkalic rocks or kimberlites. These radioactive carbonatites exhibit igneous textures and intrusive relationships with the country rocks and display the mineralogy and chemistry typical of such rocks. Sodic fenitization is present along some carbona- tite dikes. The Lemitar carbonatites can be grouped1) asxenolith- bearing breccia and microbreccia silicocarbonatite dikes and 2) xenolith-free carbonatites. The xenolith-free carbonatites are subdivided into a) fine-grained calcitic to dolomitic silico- carbonatite dikes, b) sijvite and rauhaug.ite veins, c) ankeritic- dolomitic carbonatite dikes, and d) stockwork carbonatites. Primary magmatic (with igneous textures.) and replacement (with relict replacement textures) fine-grain calcitic to dolomitic silicocarbonatite dikes are differentiated. The Lemitar carbona- tites consistof greater than 50% carbonate minerals with various amounts of magnetite,apatite, hematite, biotite/phlogopite, quartz, and numerous accessory minerals. These rocks are characterized by steep light rare-earth element enriched chondrite-normalized patterns, diagnostic of carbonatites. Similar carbonatites are found in the Chupadera Mountains, Socorro County and the Monte Largo area, Bernalillo County. The Lemitar carbonatites are enriched in uranium, thorium, rare-earth elements, barite, and fluorite; although the economic potential of these carbonatites ii is probably low. The Lemitar carbonatites are similarin texture, emplacement, mineralogy, and chemistry to the mantle-derived carbonatites in the Wet Mountains, Colorado, which suggests that the Lemitar carbonatites have a similar origin. The strong fractionation and enrichment of light rare-earth elementsof the Lemitar carbonatites and the chemical relationship between kimberlites and the Lemitar carbonatites would further support a mantle source. The Lemitar carbonatites probably represent the late phasesof a carbonatite core which may existat depth in the Lemibar-Socorro area. iii TABLE OF CONTENTS Page_ Abstract i Introduction 1 Previous work 3 Methods of study 4 Acknowledgments 5 Definitions and classifications of carbonatites 7 Geologic setting 11 Geology of the Lemitar Carbonatites 13 Age of the Lemitar Carbonatites 17 Petrology of the Lemitar Carbonatites 20 Mineralogy of the Lemitar Carbonatites 38 Geochemistry of the Lemitar Carbonatites 44 Major element chemistry 44 Trace element chemistry 49 Rare-earth elements 54 Alteration 57 Alteration of the Precambrian rocksin the Lemitar 59 Mountains Fenitization of the diorite/gabbro 59 Red feldspathic fenites 62 Fenitized xenoliths 67 Carbonatization of the mafic dikes 68 Discussion 70 Economic Geology 73 Uranium, thorium, and rare-earth elements 73 Phosphate 14 Barite-fluorite-sulfides veins 75 Conclusions 80 Appendix A - Mineralogy and chemical analyses of the 86 Lemitar carbonatites Appendix B - Analytical Procedures 90 References Cited 95 iv TABLES K-Ar age and analytical data 18 analytical data and age K-Ar Mineralogy of the LemitarMineralogycarbonatitesthe of 40 X-raydi.5fraction bastnaesite linesof 41 Distributionelementscarbonatites of in 45 Majorelement chemistrycarbonatites of 48 Traceelement chemistry carbonatites of 51 Classif ication of fenites 58 fenites of Classification Summaryofchemical changes ofthe Lemitar 64 diorite/gabbro 9 Compilation of chemical trends of fenitized mafic 65 rocks 10 Uraniumanalyses ofLemitar carbonatites and 74 feldspathic 11 Silverand lead analyses ofgalena samples 18 i V FIGURES -Page Figure 1 - Map of Socorro County showing area of study 2 Figure 2 - Photographof a xenolith-bearingbreccia 14 silicocarbonatite dike intruding the Lemitar diorite/gabbro Figure 3 - Photograph of a foliated breccia silicocarbonatite14 dike Figure 4 - Photograph of a foliated fine-grained silico- 15 carbonatite dike Figure 5 - Map of New Mexico and Colorado showing Precambrian-19 Ordovician carbonatite and alkalic complexes Figure 6 - Photograph of a xenolith-bearing breccia silico- 21 carbonatite dike Figure 7. - Photograph of a breccia silicocarbonatite dike 23 intruding a mafic dike and the Lemitar diorite/gabbro Figure 8 - Microphotograph of a primary magmatic silico- 23 carbonatite Figure 9 - Photograph of a diorite/gabbro xenolith in a 24 breccia silicocarbonatite dike Figure 10 - Photograph of a fine-grained silicocarbonatite 26 dike Figure 11 - Microphotograph of a primary magmatic fine-grained 28 silicocarbonatite dike intruding the Lemitar diorite/gabbro. Figure 12 - Microphotograph of a replacement silicocarbonatite29 dike exhibiting relict porphyritic texture Figure 13 - Microphotograph of a replacement silicocarbonatite29 dike exhibiting relict subophitic texture Figure 14 - Photograph showing box-work patterns of sijvite 32 veins intruding the Lemitar diorite/gabbro Figure 15 - Microphotograph of a savite vein intruding a 33 replacement silicocarbonatite Figure 16 - Microphotograph of a coarse-grained sijvite vein 33 intruding a replacement silicocarbonatite vi Page Figure 17 - Photograph of a dusky-red ankeritic-dolomitic 34 carbonatite dike intruding the Polvadera granite Figure 18 - Photograph of anankeritic-dolomitic carbonatite 34 dike intruding the Polvadera granite Figure 19 - Photograph of a light brown ankeritic-dolomitic36 carbonatite dike intruding a fine-grained silicocarbonatite dike Figure 20 - Photograph of stockwork carbonatite in the 36 Polvadera granite Figure 21 - Carbonates-apatite-magnetite plot ofthe.Lemitar 39 carbonatites Figure 22 - Total iron-P205 plot of the Lemitar carbonatites46 Figure 23 - Chemical plot showing the relationship between 50 carbonatites, kimberlites, and granites Figure 24 - Chondrite-normalized rare-earth element patterns 55 of the Lemitar carbonatites Figure 25 - Chondrite-normalized rare-earth element patterns 56 of anomalous carbonatites Figure 26 - Photograph showing fenitization of the Lemitar 61 diorite/gabbro Figure 27 - CaO-Na20-K20 plot of the effect of fenitization63 on the Lemitar diorite/gabbro Figure 28 - Microphotograph showing extensive carbonatization 69 of a mafic dike Figure 29 - Sequence of events relating to the intrusion of81 the carbonatites in the Lemitar Mountains Figure 30 - Schematic diagram ofa carbonatite complex 85 vii PLATES (in back pocket) Plate 1 - Outcrop geologic map of the Precambrian rocksof the eastern Lemitar Mountains, Socorro.County, New Mexico Plate 2 - Simplified geologic map of the eastern Lemitar Mountains showing the distributionof ma€ic and carbonatite dikes, mineralization, and sample locations Plate 3 Cross sections INTRODUCTION The Lemitar Mountains are located approximately seven miles northwest of Socorro,New Mexico, along the western edge of the Rio Grande graben (figure1). Most of the area is accessible throughout the entire year via dirt roads leading from the frontage road paralleling Interstate 25. The area ison the Lemitar 7+min topographic map andis owned and administeredby.the Bureau of Land Management (BLM). Radioactive calcareous basic dikes intruding the complex Precambrian terrain were first reported by Stroud and Collins (1954) and Anderson (1954, 1957). Since then a controversy has existed between geologists as to the natureof these dikes. Bonnichsen (1962) and Woodward (1973) identified them as lamprophyres of Precambrianor Tertiary age. McLemore (1980a, b, 1981) called them carbonatites. The economic potentialof these dikes encouraged the New Mexico Bureau of Mines and Mineral Resouces to undertake additional detailed geologic mapping and petrographic, mineralogic, and geochemical studies to determine the nature and extent of these dikes and associated mineralization. The study of carbonatites has recently become important due to their economic potential, their relationship to alkalic and kimberlite complexes, and their relationshipto the nature and origin of silica-poor rocks. Carbonatites may be economically enriched in rare-earth elements (.Mountain Pass, California),U, Th (Wet Mountains, Colorado), Nb(Tron.l.IIil1, Colorado),Fe (Iron Hill, Colorado), Cu, Ti, Mn, Zr, Sr, and Ba (Magnet-Cove, Arkansas). The Palabora complex, a unique carbonatite-alkalic complex in South 2 I I w 0 i 3 Africa, is currently producing copper ore and a number of by- products, including magnetite, baddeleyite, vermicuLite, apatite, and uranothorianite. The relationship between carbonatites and kimberlites may delineate potential areas for the discovery of diamonds and may provide clues for the originof these and other alkalic and silica-poor rocks. Field work in the Lemitar Mountains began during the summer of 1978 and continued through August, 1979 as part of a master's thesis project (McLemore, 1980a,b). Work resumed in September, 1980 after the author joined the staff of the NewBurem Mexico of Mines and Mineral Resources. Previous Work The general geologic setting and the Precambrian rocks of the Lemitar Mountains are described by Darton ,(1928) Lasky (1932), Bonnichsen (1962), Woodward (19731, Chamberlin (1976, 1978a, b), Condie and Budding (19791, and McLemore (1980a,. b)FJoodward (1973) and Chamberlin (1978a) mapped the rocks of the Lemitar Mountains, while McLemore (1980a)