MINERALOGY, GEOCHEMISTRY, and CHRONOLOGY of the CABALLO and BURRO MOUNTAINS REE-BEARING EPISYENITES Annelise M
Total Page：16
File Type：pdf, Size：1020Kb
MINERALOGY, GEOCHEMISTRY, AND CHRONOLOGY OF THE CABALLO AND BURRO MOUNTAINS REE-BEARING EPISYENITES Annelise M. Riggins1, Nelia Dunbar2, Virginia T. McLemore2*, Matthew Heizler2, William McIntosh2 and Kwame Frempong3 1Earth and Environmental Sciences Department, New Mexico Institute of Mining and Technology, Socorro, NM, 87801 2New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM, 87801 3Mineral Engineering Department, New Mexico Institute of Mining and Technology, Socorro, NM, 87801 * Presenting author ABSTRACT PURPOSE LOCATION AND FIELD RELATIONSHIPS Extraordinarily potassium feldspar-rich, brick-red rocks, termed The overall objectives of this research are to Apache Gap, Caballo Mountains episyenites, in the Caballo and Burro Mountains, New Mexico, address the following questions: have anomalously high concentrations of U, Th and TREE • What are the mineralogy and chemistry of the (2,329 ppm, 9,721 ppm and 1,378 ppm, respectively). Some episyenites? Where are the REE and other episyenites contain high HREE (as much as 133 ppm Yb and elements found? 179 ppm Dy). Episyenite is a term used to describe altered • What is the origin of the episyenites? Are they rocks that were desilicated and metasomatized by alkali-rich magmatic or metasomatic or both? fluids solutions and may be similar to fenites. Field and electron • What is the mineral resource (U, Th, REE, Nb, microprobe investigations of outcrop distribution and Ga, Zr, Hf, Y, other) potential of the episyenites mineralogical textures suggest episyenites formed by and associated veins in the Caballo and Burro interaction of K-rich metasomatic fluids with Proterozoic granitic Mountains? host rocks, resulting in K-feldspar-rich rocks with bulk Episyenites in the Apache Gap Xenolith of granitic schist Vuggy texture along surrounded and cut by episyenite. compositions containing up to 16 wt.% K2O. The secondary area, unconformably overlain gradational contact between feldspars are significantly less fractured than primary igneous by the Bliss Formation (looking episyenite and Caballo north). feldspar, display no perthitic textures, and contain micron-size DEFINITION OF EPISYENITES Brick red episyenites are granite hematite inclusions. The most reddened episyenites are found in a fault block in he term episyenite is used to describe altered composed largely of interlocked K-feldspar crystals, which T the Apache Gap area, rocks that were desilicated and metasomatized by display no relict igneous texture. Electron microprobe near the base of the alkali-rich fluids solutions (Leroy, 1978; Recio et al., Longbottom Canyon, Caballo Mountains investigation of rocks with the highest concentrations of U, Th, Caballo Mountains and REE indicate complex mineralogy associated with fluid 1997). These metasomatic rocks in the Caballo Mountains were erroneously called syenites and alteration, particularly of primary magmatic mafic silicates, now Dike-like alkali granites, but are actually metasomatic in Simplified geologic map of Proterozoic rocks and replaced by a combination of secondary chlorite, hematite, episyenite in origin and not primary igneous rocks. Elsewhere in Cambrian-Ordovician episyenites in the Caballo carbonate, apatite, rutile, synchysite, priorite, xenotime, thorite Longbottom the world, these alkali-rich metasomatic rocks are Mountains, New Mexico (modified from Condie and uranophane. Synchysite is the major host of LREEs, up to Canyon area 63 wt.%, while HREEs are concentrated dominantly in associated with uranium and thorium deposits and Budding, 1979; Seager and Mack, 2003; xenotime, up to 16 wt.%, and priorite, up to 9 wt.%. The (Costi et al., 2002; Condomines et al., 2007; Cuney McLemore et al., 2012). geochemical and textural characteristics of these rocks are et al., 2012), gold deposits (López-Moro et al., consistent with alteration of host granite by fluids related to 2013) and tin-tungsten deposits (Charoy and alkaline or carbonatitic magmatism. In the Caballo Mountains, Pollard, 1989; Costi et al., 2002; Borges et al., Southern Red Hills, Caballo Mountains Sharp contact between timing of metasomatism is constrained to be older than late 2009), but unmineralized episyenites are found as More than 25 lenticular to elongate pods, lenses (<100 m long, episyenite and schist in Cambrian as episyenite clasts occur in the C-O Bliss Sandstone well (Petersson and Eliasson, 1997; Recio et al., <10 m wide), narrow pipe-like and dike-like bodies (<2 m wide, Burro Mountains Longbottom Canyon area that unconformably overlies metasomatised basement. One 1997; Hecht et al., 1999). Episyenites are similar to 400 m long) of episyenites are found scattered throughout the such clast contains xenotime with the same chondrite- rocks formed by fenitization and could be called Protereozoic Cablallo Granite (1487 Ma, Amato et al., 2012) in fenites by some geologists. Fenitization is the Geologic map of the normalized REE pattern as episyenite-hosted xenotime found in the Southern Red Hills Episyenites are found in alkali-metasomatism associated with carbonatites Longbottom Canyon other areas of the mountain range, constraining mineralization Proterozoic Burro Granite (1460 or alkaline igneous activity (Le Bas, 2008). area, Caballo to be older than late Cambrian, as well. Direct dating of the Ma, Amato et al., 2011) and 40 39 However, we are reluctant to use the term fenite for Mountains metasomatism using the Ar/ Ar method on sub-milligram Jack Creek Granite (1461 Ma, these rocks in the Caballo and Burro Mountains fragments of metasomatic K-feldspar yield complex and Rämö et al., 2002) in the because a carbonatite or alkaline magma source is intriguing age results that may be related to multiple fluid- northern Burro Mountains not found at the surface in this area. alteration events. In the Caballo Mountains age spectra range (Hedlund, 1978a, 1978b, 1978c, from nearly flat to highly disturbed with total gas ages (TGA) 1978d; McLemore et al., 2000) between ~40 and 460 Ma. Individual fragments with flat spectra PARAGENESIS from single samples vary in TGA by ~140 Ma (approx. 320 to 460 Ma). However, one sample from the Burro Mountains yields Uranophane crystals in a plateau age at approx. 540 Ma that may record late Cambrian fractures in episyenite metasomatism caused by an alkaline or carbonatitic intrusion in the subsurface. INTRODUCTION Geologic map of the Southern Red Hills area, Worldwide, alkaline igneous complexes similar to carbonatites Caballo Mountains. and metasomatic rocks found in southern Colorado and New Mexico (Fig. 1; McMillan McLemore, 2004) are well known for Geologic map of the Evening Star Orthoimage from GoogleEarth potential economic deposits of REE, U, Th, Nb, Zr, Hf, Ga, and Canyon area in the Burro looking north at an elongate other elements (Long et al., 2010; McLemore, 2012). Brick-red, Mountains showing dike-like Dike-like episyenite forming exposure of episyenite in the K-feldspar-rich rocks, called episyenites are found in the episyenites along fractures in Caballo Mountains in southern New Mexico that contain ridges in Caballo Granite in Evening Star Canyon area. Southern Red Hills Burro Granite (1460 Ma, Amato et anomalous concentrations of REE, U, Th, and other elements. al., 2011) Similar episyenites are found elsewhere in New Mexico and Simplified paragenesis of episyenites in southern Colorado and are thought to be part of a Cambrian- the Southern Red Hills as determined Ordovician magmatic event that is documented throughout from field relationships, petrographic southern Colorado and New Mexico. This event is Adit in episyenite in observations, and electron microprobe characterized by the intrusion of carbonatites, syenites, Proterozoic Caballo studies. monzonites, alkaline granites, and mafic dikes, and is Granite. associated with K-metasomatism (i.e. fenites and episyenites) and Th-REE-U mineral deposits. Carbonatites are found in the Lemitar and Chupadera Mountains, Lobo Hill, Monte Largo and FIGURE 1—Cambrian-Ordovician carbonatite, Sharp contact between episyenite The rapakivi texture is preserved southern Colorado (Wet Mountains, Iron Mountain). It is alkaline, and mafic igneous rocks and episyenites 1461 Ma Jack Creek in Proterozoic schist (~1600 Ma) within the episyenite, suggesting possible that the episyenites are related to alkaline or in New Mexico and Colorado. Rapakivi Granite (Rämö carbonatite plutons at depth and are possibly related to this et al., 2002) a metasomatic origin. widespread Cambrian-Ordovician magmatic event. MINERALOGY, GEOCHEMISTRY, AND CHRONOLOGY OF THE CABALLO AND BURRO MOUNTAINS REE-BEARING EPISYENITES MAJOR MINERALOGY AND CHEMISTRY GEOCHONOLOGY REE CHEMISTRY AND MINERALOGY MINERAL RESOURCE POTENTIAL Field relationships, such as the apparent Whole rock geochemistry indicates that episyenites are overall enriched in We used the electron microprobe to examine mineralogical characteristics of truncation of episyenite by Cambrian- total REE as compared to host granite, and that some episyenites are In general the exposed episyenites in the Caballo and Burro Mountains are too small episyenites, in order to better understand the process by which they form. Of Ordovician Bliss Sandstone (COb) (top HREE enriched, and others are LREE enriched (left). Most fall between and low grade to be economic in today’s market. However, drilling and subsurface particular interest are samples like the one shown below, which exhibit a gradient image) and presence of red, altered