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New Mexico Mineral Symposium — Abstracts

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Minerals from the Pegmatites of theCrystal Mountain District, Larimer County, Colorado

Mark Ivan Jacobson

Denver, CO

Blue apatite crystals, purpurite-heterosite, spodumene, chrysoberyl, tantalite and beryl are some of the minerals specimens available for collecting in the Crystal Mountain district. This is one of the few pegmatite districts in Colorado where neither all the pegmatites have been found, studied, mapped nor all the minerals described. Because the minerals specimens found in this district are not in the $1,000 price range, field collecting is still possible. The Crystal Mountain District is geographically centered around Crystal Mountain in section 26, T7N R72W, about thirteen miles west of Fort Collins and Loveland, Colorado. Most of the district’s pegmatites are found north of Drake in the Big Thompson Canyon and south of Buckhorn Canyon. This area can be located on the Crystal Mountain, Buckhorn Mountain, Drake and Glen Haven 7.5 minute topographic quadrangle maps. Although private property is scattered throughout the area, access via National Forest Service roads is usually possible. Certainly, the pegmatites can be legally accessed by foot. The Crystal Mountain pegmatites occur within 1.7 billion year old Proterozoic high-grade metamorphic phyllites, schists, and gneisses, which are mostly between the andalusite (first appearance) and staurolite (last presence) metamorphic mineral zones. These rocks have been historically called the Idaho Springs Formation, although this name has fallen from favor and has not been replaced. The Longs Peak–St. Vrain batholith and its variant, the Mt. Olympus pluton, is composed of granite and quartz monzonite. This batholith has been aged dated as 1.42 by +/- 30 my, and is part of the Berthoud Plutonic suite in Colorado. The pegmatites are interpreted but are not proved to have been formed from the Longs Peak batholith and of the same age as the granite. Most of the pegmatites in this district are clustered in a thermal synclinal trough that plunges to the southeast, as indicated by metamorphic mineral zones. In general those pegmatites closest to the Longs Peak–St. Vrain granite are most simple with few accessory minerals whereas those furthest away are more strongly zoned with more abundant lithium, beryllium and phosphate accessory minerals. The pegmatites in this district are the most phosphate and beryl rich in Colorado but are poor in rare-earth and radioactive elements, thus suggesting that they are members of the LCT class of complex rare-metal pegmatites. Approximately forty-three minerals have been found in the district. These are:

allanitegarnet (almandine?)triphyllite-lithiophilite
alluaudite series mineralsgraftonite-beusitetopaz
amblygonite-montebrasiteheterosite-purpuritetorbernite- metatorbernite
bismutitemuscovite and Li-muscovitevivanite
chrysoberylphosphuranylitezircon variety cyrtolite

Massive phosphate minerals are common at the Hyatt, Storm Mt., Big Boulder, Crystal Snow, Double Opening, Black Beauty Beryl pegmatites and a few others. Where either iron or manganese end members of heterosite-purpurite are found, ferrisicklerite-sickerite and triphylite-lithiophilite could be found. Metasomatic alteration of these phosphates results in members of the alluaudite series. Primary graftonite-beusite is frequently intergrown along parallel exsolution lamellae with triphylite-lithiophilite but is usually found as heterosite-purpurite and graftonite intergrowths on the surface. Triplite is a primary mineral and is found alone or in association with other phosphates. Vivianite forms bluish coating on fracture surfaces of triphylite or other phosphates. Purple phosphosiderite, massive fine-grained apatite and possibly grungy mitridatite as crusts have been reported from the Big Boulder pegmatite and may be present in the other phosphate rich pegmatites.

Although Thurston (1955) bragged about how rare columbite-tantalite minerals are in these pegmatites, thin blades of these species are numerous enough that its presence should be expected in any pegmatite with albite variety cleavelandite – a variety known from the Bull Elk Beryl Crystal, Big Boulder, Tantalum, Sherwood Place, and Buckhorn pegmatites. Chrysoberyl tends to be present in beryl-rich pegmatites that were emplaced well within the sillimanite metamorphic zones. The Wisdom Ranch and Bull Elk Beryl Crystal contain the most chrysoberyl in the district but the mineral should be expected in many other district pegmatites, especially the thinner ones. Beryl is abundant in many pegmatites as euhedral opaque crystals that vary from white to green, and rarely bluish. Although gem quality beryl crystals have been reported in the literature, their provenance is suspect. In fact, any crystals except for micro blue apatites that show evidence of crystallization in a vug should be suspected of having an origin outside of this district. The alteration of uraninite found within or adjacent to phosphate nodules is responsible for the reports of the other uranium minerals of uranophane, phosphuranylite, vandendriesscheite, autunite, and torbernite in the district. Such minerals have been reported from the Storm Mountain, Buckhorn, Hyatt, and Double Opening pegmatites and may be present in other pegmatites.The most collectable minerals in the district are beryl, schorl, and chrysoberyl. Hard work is still required for attractive specimens but the area to search is large. Since high monetary value specimens are not known from this district, field collectors still have the opportunity to collect interesting species without the distraction of treasure hunters.

Figure 1: Index map showing the major pegmatites within the Crystal Mountain district
pp. 5-6

35th Annual New Mexico Mineral Symposium
November 8-9, 2014, Socorro, NM