Geology, alteration, and mineralization of the Kougarok Sn deposit, Seward Peninsula, Alaska

Abstract

At the Kougarok deposit tin mineralization occurs in association with a Cretaceous granite complex. The tin mineralization is hosted in both granite and pelitic schist country rock.The most significant concentrations of cassiterite occur in a late zinnwaldite granite body which cuts older granite phases of the intrusive complex. The body has a subhorizontal lower contact and an irregular upper contact which merges into higher level dikes and plugs. The intensity of hydrothermal alteration increases upward in the body; successive alteration assemblages, in order of increasing destruction of igneous texture, consist of sericite-tourmaline, brown zinnwaldite-sericite, white zinnwaldite-sericite, and quartz + or - tourmaline greisen. In general, the grade of tin mineralization parallels the intensity of the alteration, ranging from less than 0.10 percent in the sericite-tourmaline zone to over 1.0 percent in the quartz + or - tourmaline greisen. Highly anomalous arsenic (greater than 1,000 ppm), anomalous lead (up to 1,500 ppm), and silver (up to 30 ppm) are associated with tin grades greater than 1.0 percent. Tantalum and niobium, in tantalite-columbite, are also enriched (up to 900 ppm Ta and approximately 600 ppm Nb) upward in the zinnwaldite granite but vary independently of tin.Schist adjacent to and above the zinnwaldite granite contains veinlets and fractures with hydrothermal alteration selvages. Alteration zones are distinguished by the alteration selvages on quartz and/or tourmaline veinlets and hairline fractures in schist, which change from tourmaline enveloped by chlorite in a zone proximal to altered granite, to axinite enveloped by chlorite in an intermediate zone, and to chlorite selvages in a distal zone. Tin grades in quartz and/or tourmaline veinlets in the proximal zone commonly exceed 0.10 percent and give way outward to widespread halos of tin grades above 0.01 percent; tin mineralization is weaker or absent in the intermediate and distal alteration zones in schist.Other granites and schist beneath the zinnwaldite granite body are altered and locally contain anomalous tin. Evidence that the zinnwaldite granite body cuts altered and mineralized older granites implies that hydrothermal activity and tin introduction commenced prior to the intrusion of the late zinnwaldite granite body.