New Mexico Mineral Symposium — Abstracts

Point of Rocks Mesa, in Colfax County, northeastern New Mexico, is formed by a 100-m-thick phonolite sill of mid-Tertiary age. The relatively high abundance of sodium, fluorine, zirco-nium, niobium, thorium, and other elements in this silica-poor igneous rock has led to the formation of at least 45 different minerals both in the matrix of the rock and as free-growing crystals in small (typically a few centimeters in diameter) miarolitic cavities.

The host rock and the suite of minerals at Point of Rocks are similar to those of a number of well known mineral localities throughout the world. These localities have in common the presence of silica-undersaturated igneous rock that is rich in alkalies, volatile components (water, carbon dioxide, fluorine, chlorine), and rare metals. The host rocks typically include varieties of nepheline syenite, of which phonolite is the fine-grained equivalent, and they contain some or all of the following as major rock-forming minerals: orthoclase, albite, nepheline, sodalite, analcime, aegirine (acmite) or aegirine-augite (acmite-¬augite), and arfvedsonite. The accessory minerals that make these rocks mineralogically distinctive include eudialyte, cancrinite, titanite, astrophyllite, catapleiite, elpidite, epididymite, lorenzenite, murmanite, narsarsukite, neptunite, polylithionite, rinkite (mosandrite), steenstrupine, and many others.

The best known North American locality of this type is Mont St-Hilaire, Quebec, a 9-km² composite intrusion of gabbro, nepheline syenite, and sodalite syenite. It is one of about ten alkalic plutons forming the Monteregian Hills, an alignment of Late Cretaceous (about 110 m.y.) intrusions along a 200-km east-west trend in southern Quebec lying in the Ottawa graben at its intersection with the St. Lawrence continental rift zone. The unusual minerals at Mont St-Hilaire are most abundant in pegmatitic veins that cut the syenite and in miarolitic cavities as much as 2 m in diameter. New minerals were produced by hydro-thermal alteration of the primary minerals, leading to a total of more than 150 known species.

About 20 plutons of late Precambrian age lie within a 180- by 80-km region along the coast of southwest Greenland centered near Julianehaab. These plutons compose the 1,000- to 1,250-m.y.-old Gardar igneous province. They include alkali gabbros, syenites, and alkali granites. Three areas within the region are notable for their diverse mineral suites. 1) At Ivigtut, about 130 km west of Julianehaab, a small cryolite-siderite body is associated with explosive brecciation of a peralkaline granite stock. 2) The Ilimaussaq intrusion is about 25 km north of Julianehaab and covers about 135 km2; it is cut in two by Tunugdliarfik Fjord, and its southern half lies at the head of Kangerdluarssuk Fjord. It is a composite, layered intrusion including augite syenite, "naujaite" (sodalite nepheline syenite), "kakortokite" (mafic acmite-arfvedsonite-eudialyte nepheline syenite), "lujavrite" (trachytic arfvedsonite-acmite nepheline syenite), quartz syenite, and alkali granite. The unusual minerals are most abundant in pegmatites that cut the nepheline syenites and in zones of pneumatolytic or hydrothermal alteration. Kvanefjeld, on the northwestern edge of the Ilimaussaq intrusion, is one site at which many minerals have been found. 3) The Igdlerfigssalik intrusion, covering about 150 km², is about 30 km farther to the northeast, at the head of Tunugdliarfik Fjord, near the town of Igaliko. One notable mineral site in this intrusion is Narsarsuk, a level plain on the side of Igdlerfigssalik Mountain. The intrusion is composed of augite syenite and nepheline syenite, and the minerals are most abundant in pegmatitic segregations in the augite syenite. At least 50 different minerals have been found in the Igdlerfigssalik-Narsarsuk area, and about 35 at Ilimaussaq.

The Kola Peninsula lies mostly above the Arctic Circle in the northwestern part of the U.S.S.R. It is composed mainly of Precambrian rocks, within which are two large, alkalic intrusions of late Paleozoic (Hercynian, 290 m.y.) age. The Khibina (Khibiny) massif, occupying 1,327 km², is in the central part of the peninsula, about 150 km south of the port of Murmansk. Just east of the Khibina massif is the Lovozero massif, covering 650 km². A smaller, satellite intrusion (Soustova, 38 km²) lies just south of the Khibina. The Kola Peninsula also contains a number of older alkaline ultramafic and gabbroic igneous complexes of middle Proterozoic and of Caledonian (middle Paleozoic) ages.

The Khibina massif is composed of multiple, concentric intrusions of nepheline syenite, alkali syenite, "khibinite" (coarse-grained, eudialyte-bearing nepheline syenite), "rischorrite" (nepheline syenite with poikilitic texture of nepheline enclosed in microcline perthite), alkalic pegmatites, alkali gabbros, lamprophyres, and others. The Lovozero massif, which is interpreted as a layered intrusion rather than a ring complex like that of Khibina, includes nepheline-, sodalite-, nosean-, and analcime-bearing syenites, "lujavrites," alkalic pegmatites, and lamprophyre dikes. Rare minerals in the two massifs are developed in pegmatites or pegmatitic segregations, in several stages of pneumatolytic or hydrothermal alteration, and in altered zones near the outer contacts. About 200 mineral species are known from the Lovozero intrusion, and probably somewhat fewer have been found at Khibina.

In comparison to these other alkalic intrusives, the Point of Rocks phonolite takes a relatively subordinate position; it is thinner, less extensive (approximately 8 km²), and younger (about 20 m.y.), and its mode of emplacement as a near-surface sill also differs. The composition and volatile content of the phonolite caused it to develop a diverse suite of minerals, but the rela¬tively rapid cooling and crystallization of this near-surface body kept it generally fine-grained, restricting the size of the iniarolitic cavities, preventing the development of late-stage pegmatites, and limiting the extent of hydrothermal alteration. Point of Rocks is part of the Chico Phonolites,,a complex of flows, sills, and dikes with an aggregate exposed area of about 65 km², situated within a larger province of basalt flows, trachytes, andesites, and dacites. It is reasonable to assume that the source of the phonolites was a larger body of alkalic magma that crystallized deep below the surface, and that the next few hundred million years of erosion of this part of the High Plains may well expose an alkaline intrusive complex to rival those of Quebec, Greenland, and Kola.