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Schmidt, J.M., 1983

Geology and geochemistry of the Arctic prospect, Ambler District, Alaska

Bibliographic Reference

Schmidt, J.M., 1983, Geology and geochemistry of the Arctic prospect, Ambler District, Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 253 p., illust., maps, overlays.


The Arctic volcanogenic massive sulfide prospect is the largest known (40 million ton) deposit hosted by the low greenschist grade, latest Devonian Ambler Sequence of bimodal, basaltic, and rhyolitic volcanic and volcaniclastic rocks, and pelitic, graphitic, and calcareous metasediments. Detailed field mapping, core logging, petrography, X-ray diffractometry, electron microprobe analyses, and whole-rock major element analyses of hydrothermally altered rocks were used to determine the emplacement history and setting of sulfide deposition. The Arctic deposit is a series of semi-massive sulfide lenses with a well developed, horizontally stratified sequence of alteration types, composed of phengite, barian phengite, fluorine-rich talc, barian fluorphlogopite, magnesian chlorite, quartz, carbonates, albite, and cymrite/celsian. Depletion of alkalis and enrichment of Ba, F, and Mg are the major chemical changes of pervasive alteration (.02 km3) developed asymmetrically around the fissure vent for ore fluids. The deposit is low in pyrite, Fe, Pb, and Au, and high in Cu and barite, with moderate Zn and Ag. Strong lateral zonation of metals (Cu overlying the vent, Zn intermediate and Pb and Ag distal) parallels, but is offset from the asymmetric distribution of alteration. Most sulfides were deposited not over the vent but on a relatively flat area and in topographic channels between an elongate vent paleo-high and a carbonaceous mud-bearing basin. No single volcanic unit can be genetically tied to mineralization; the onset of hydrothermal activity was apparently related to fault activity and continued during emplacement of subsequent rhyolites. Low greenschist grade metamorphism was essentially isochemical on a macroscopic scale, and preserved volcanic compositions, the major element chemistry of alteration and the compositions of individual metamorphic, alteration and relict igneous minerals. Mineralization at Arctic was formed along a synvolcanic fault in a tectonically and volcanically active basin in a rifted continental margin, possibly related to an actively spreading oceanic rift.

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