Abstract

The Headway-Coulee Zn-Pb-Cu-Ag prospect occurs as a series of 13 lenses of vein and disseminated-type sulfides which are associated with hydrothermally altered Archcan mafic lavas and felsic hydroclastic rocks. Mafic lavas form a 4- to 6-km-wide sequence that is locally interrupted by a narrow, lens-shaped horizon (30-200 m X 2 km) of thin to thickly bedded ash-rich tuffs. The felsic tuffs are subdivided into two units which represent two periods of explosive activity.The felsic tuffs and many of the mafic lavas have undergone hydrothermal alteration during the waning stages of explosive activity. Alteration zones and their metamorphosed equivalents both crosscut and are conformable to stratigraphy; they have been divided into six mineralogically and chemically distinct types: least altered, calcite, sericite, iron chlorite, chloritoid, and kyanite.Least altered rocks underlie approximately 50 percent of the area, are all mafic in composition, and have been affected primarily by greenschist facies metamorphism. Calcite-type alteration is the earliest hydrothermal event recognized in the volcanic rocks and forms a 600- by 1,600-m zone which crosscuts stratigraphy. Calcite-rich rocks grade laterally into least altered or sericite-rich rocks; they were formed by the addition of CO 2 and the minor loss of Na, Fe, and Mg whereas Ca remained relatively immobile. Sericite-type alteration postdates calcite alteration and occurs primarily as elongate lenses (up to 1,000 m) within felsic tuffs; it also occurs in anomalous amounts within iron chlorite and chloritoid-rich mafic lavas. Prior to formation of chlorite and chloritoid the sericite alteration zone was 3,000 by 100 to 1,200 m in size and crosscut the stratigraphy. Sericite is thought to have formed from a low-temperature, evolved seawater fluid which circulated only in the very upper portions of the hydrothermal system. At low water/rock ratios this fluid became enriched in K and Si and was heated enough to rise buoyantly along fractures and more permeable lithological units. Water-rock interactions formed sericite and quartz at the expense of plagioclase feldspar.Iron chlorite-type alteration and its metamorphosed equivalent (chloritoid-type alteration) crosscut both calcite and sericite alteration and chlorite replaces sericite within both mafic and felsic rocks. Within felsic tuffs this alteration is characterized by the addition of Fe and Mn and the loss of Na, K, Ca, Si, and CO 2 ; volume may decrease by as much as 30 percent. Within mafic lavas, however, the alteration is characterized by addition of Fe with minor Ca and loss of K and CO 2 ; there was negligible volume change. Iron chlorite is believed to have formed from an evolved, high-temperature seawater fluid which circulated at depth. Interaction with mafic rocks at low water/rock ratios enriched the fluid in Fe, Mn, H (super +) while depleting it in Ca and Na. This acidic and base metal-rich fluid, as it neared the surface, interacted with sericite and calcite-rich rocks to form iron chlorite, iron carbonate, and a hydrous aluminum silicate phase (pyrophyllite?). Within the discharge channel, close to the water-rock interface, the solution extensively leached the ash tuffs to produce a rock composed dominantly of SiO 2 and hydrous aluminum silicate phases. This rock represents a leached cap to the hydrothermal system and a local discharge site. Later greenschist facies metamorphism converted the quartz-hydrous aluminum silicate assemblage to kyanite-quartz; and the iron chlorite plus hydrous aluminum silicate assemblage to chloritoid. Base metals were precipitated below and/or immediately above the discharge site.

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