Roger Bond

Thesis: The Mineralogy and Geochemistry of the Kasaan Peninsula Iron-Copper-Silver-Gold Skarns, Prince of Wales Island, Southeastern Alaska

Support: SeaAlaska Corporation, University of Utah

Abstract

Stratabound magnetite-sulfide deposits of the Kasaan Peninsula, Prince of Wales Island, Alaska occur as replacements of andesitic volcanics, calcareous matrix conglomerate, and marble of the Silurian-Ordivician Descon Formation. The age of diorite-granodiorite intrusive suites, that may be the cause of mineralization, have been documented at 400 and 100 m.y.

Mineralization can be divided into three stages. The anhydrous-silicate stage is dominated by the formation of diopside, hydrogrossular, and hematite. The oxide stage is dominated by epidote, hydrogrossular, hornblende, and massive magnetite, which often replaces hematite. The sulfide stage is characterized by epidote, chlorite, pyrite, chalcopyrite, and electrum (1.7 to 100.0 atomic % Ag). Pyrite frequently replaces magnetite during this stage of mineralization. Minor supergene mineralization includes chalcocite, limonite, cuprite, and native copper.

Silver to gold ratios range between 143:1 to 1:2.3. Chalcopyrite-rich samples contain the highest silver and gold values. Skarn samples may contain up to 64 % Fe, 25 % Cu, 99 ppm Ag, and 22 ppm Au. Trace elements associated with precious metal mineralization are Cu, Zn, Cd, Ni, Co, ± Cr, Bi, and As.

Microthermic analysis of sulfide stage quartz and epidote yields homogenization temperatures of 195±24°C and 224±26°C respectively, and salinities of 18.4 and 10.2 equivalent weight percent NaCl. Oxygen isotopic composition of magnetite, calcite, and quartz provides provides a temperature estimate of 317°C for the oxide stage of mineralization. d18O values of magnetite relative to SMOW range from -2.4 to +1.7 permil. This requires a fluid with a d18O value of 5.8 to 9.9 permil at 300°C and 3.8 to 7.9 permil at 500°C. Alternatively, the d18O value of magnetite could be controlled by the minerals which it has replaced. Estimation of other geochemical parameters of the hydrothermal fluid for the oxide stage (T = 325°C) include log[aCa+2/(aH+)2] = 7.8, log[aMg+2/(aH+)2] < 5.2, log fCO2 = -1.27, log fO2 = -29.1 to -35.8, and log fS2 < 9.5; for the sulfide stage (T = 250°C) they are log[aCa+2/(aH+)2] = 7.3 to 9.1, log[aMg+2/(aH+)2] = 5.9 to 6.3, log CO2 = -0.69 to -2.47, log fO2 = -34.9 to -40.2, and log fS2 = -11.1 to -14.5.

The deposits of the Kasaan Peninsula have a high degree of similarity with the precious metal enriched iron-copper skarn class of Ettlinger and Ray, 1989. These characteristics include dioritic intrusives, island arc/back arc basin assemblage country rocks, hosting of the mineralization in carbonate and volcanic units, micron sized gold, and ore mineralogy and textures. The trace element suite, and the temperature, pressure, and salinity estimates are consistant with the calcic iron skarn class of Einaudi et al.,1981. The d18O value of the mineralizing fluid is in agreement with those proposed for skarn mineralization by Rye, 1984.