TEXTURAL CLUES FOR THE GENESIS OF TOURMALINE IN MAGMATIC-HYDROTHERMAL SYSTEMS

Although tourmaline in sedimentary and metamorphic rocks has been extensively studied, fewer studies have documented textures exhibited by hydrothermal and magmatic tourmaline. Examination of tourmaline-bearing clasts from Tertiary fan gravels and streambeds in northern Sonora, Mexico, has revealed a variety of macroscopic tourmaline textures from Jurassic to Tertiary magmatic-hydrothermal systems.

Samples of very dark-colored tourmaline, schorl-dravite-uvite_ss, have been categorized on the basis of habit, crystal morphology, and relationship to the host rock. Veins range from less than one millimeter to several centimeters in width, are stockwork-like, composed of fibrous radiating to stubby prismatic crystals, and show sharp to diffuse vein-host contacts. Tourmaline breccia-cement occasionally displays the same range of habit and morphology as veins, but the majority of breccia samples are composed of microcrystalline massive to fibrous crystals. Spots are compact, roughly circular or spherical, millimeter to one-centimeter diameter patches of acicular or fibrous crystals. Spots of outward-growing tourmaline crystals appear to originate in the host rock whereas randomly oriented or inward-growing crystals appear to fill pre-existing cavities. Disseminated tourmaline occurs as millimeter-scale euhedral crystals or fibrous crystal clusters. Pockets are composed of discrete clusters of randomly oriented prisms of tourmaline, most commonly stubby but rarely elongate and centimeters in length, associated with euhedral feldspar, axinite, and quartz. Pockets range from two to several centimeters in diameter and are surrounded by a light colored “alteration” halo containing anhedral, disseminated tourmaline. Veins and breccia cement characterize fracture-controlled hydrothermal relationships. Spots, disseminated tourmaline, and pockets are inferred to be of hydrothermal or magmatic origin. Fibrous, radiating tourmaline crystals also occur in pegmatite.

This diversity of textures reflects a variety of geochemical, temporal, and lithologic factors associated with magmatic and hydrothermal processes. Geochemical analyses will permit further refinement of the classification of tourmaline in magmatic-hydrothermal systems.