A widespread Cambrian-Ordovician alkaline magmatic event occurred throughout New Mexico and southern Colorado (Fig. 1, Table 1) and is evidenced by the intrusion of carbonatites, syenites, monzonites, and alkaline granites and associated K-metasomatism (episyenites). The term episyenite, as used by Leroy (1978), is used to describe rocks that were desilicated and metasomatized by hydrothermal solutions. These rocks now resemble syenites. The metasomatic episyenite consists of predominantly K-feldspar with accessory hematite, biotite, and plagioclase; quartz is rare to absent.

Only a few of these alkaline rocks have been dated (Table 1; Loring and Armstrong, 1980; McLemore, 1983; Evans and Clemons, 1988; Ervin, 1998; Riggins, 2014; Smith, 2019; https://geoinfo.nmt.edu/staff/mclemore/projects/documents/Riggins_SEG.pdf), but similar field relationships, textures, mineralogies, and geochemistries of these unmetamorphosed alkaline rocks suggest a common origin. Brick-red Cambrian-Ordovician or older episyenites are metasomatic in origin and overprinted by later Late Paleozoic (437-270 Ma) events, such as the Ancestral Rocky Mountains and even the younger Laramide orogeny. The Cambrian-Ordovician or older metasomatic event appears to be dominated by magmatic fluids, related to either Proterozoic granites or from Cambrian-Ordovician alkaline intrusions. This is followed by a younger, lower-temperature event (due to sedimentary fluids or brines with highly radiogenic Sr and Pb isotopic compositions) that resulted in Late Paleozoic K-feldspar recrystallization or feldspar growth (Smith, 2018). The maximum age of the metasomatism forming the episyenites is between the age of the host granite (~1400 Ma) and the Cambrian-Ordovician Bliss Formation (~500 Ma). Limited isotopic data suggests that episyenites are probably not related to carbonatites, even though carbonatites are present locally.

The Cambrian-Ordovician magmatic event is well documented in southern Colorado and New Mexico (Fig. 1; McLemore, 1987b, 1989; McMillan and McLemore, 2004). Such alkaline magmatism is consistent with continental rift and aborted rift systems, although no Cambrian-Ordovician rift-basin sediments or rift-related structures have been observed for this time period in New Mexico (Fig. 2). Specific geographic boundaries of such a rift would be difficult to observe because of complex overprinting related to the ancestral Rocky Mountains orogeny, Laramide orogeny, and Rio Grande rifting and other tectonic events. Recognition of widespread Cambrian-Ordovician magmatic activity in New Mexico, evidence of relatively rapid uplift and erosion in the Florida Mountains (Evans and Clemons, 1988; Clemons, 1998; Ervin, 1998), and the presence of carbonatites (McLemore, 1983, 1987a) suggest that New Mexico was not a simple passive margin during the Cambrian-Ordovician; but rather experienced sufficient extension to perturb the mantle and initiate magmatism. Thus, we propose that an aulacogen, similar to the Southern Oklahoma aulacogen (Fig. 2; Lambert et al. 1988; McConnell and Gilbert, 1990), existed in New Mexico during Cambrian and Early Ordovician time. Additional studies are underway to fully characterize and evaluate the tectonic history during the Cambrian-Ordovician in New Mexico and southern Colorado.

FIGURE 1: NMGS Carbonates_alkali NM, CO

FIGURE 2: tectonics(OK,TX,NM).jpg

Surface samples of episyenite have low-moderate REE (as much as 3167 ppm total REE), Th (as much as 9721 ppm), and U (as much as 2329 ppm), and some samples have relatively high heavy REE (as much as 133 ppm Yb and 179 ppm Dy), which are important economic commodities. Carbonatites are found at Lobo Hill, the Monte Largo area in the Sandia Mountains, the Lemitar and Chupadera Mountains. Both episyenites and carbonatites are found only at Lobo Hill, and fenitization is common surrounding all of the carbonatites. Compositionally, the carbonatites are sövites, silicocarbonatites, and rauhaugites. U, Th, Nb, and REE minerals are found in some carbonatites. REE minerals are associated with altered amphiboles, magnetite, secondary chlorite, hematite, zircon, and fluorite. Detailed geochemical sampling and drilling are required to fully evaluate the economic potential of episyenites and carbonatites in New Mexico.

In general, the exposed outcrops of episyenites and carboantites in New Mexico appear too small and low grade to be economic for REE, U, Th, and Nb in today’s market. However, drilling and subsurface sampling are required to fully evaluate their mineral-resource potential. The Lobo Hill episyenites near Moriarty are currently being mined for crushed decorative stone, and the Lobo Hill episyenites are approximately 20 m deep and drilling suggests episyenites extends another 20-30 m. Thus, episyenites can be extensive in the subsurface. The relatively high heavy REE in some episyenites in the Caballo and Burro Mountains could be of economic interest, but additional mineralogical, geochemical, and economic analyses are required to fully assess the mineral-resource potential.

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