GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF EPITHERMAL GOLD MINERALIZATION ALONG THE VITI LEVU LINEAMENT, FIJI

The ENE-trending Viti Levu lineament (VTL) is a prominent structural feature that extends from Viti Levu through Vanua Levu, the two largest islands in Fiji. It is characterized by seven Pliocene shoshonitic and high K calc-alkaline volcanic centers and the three largest gold deposits in Fiji (Emperor - 11.5 Moz; Tuvatu - 300,000 oz; Mt. Kasi - 232,000 oz), and likely formed by extension resulting from the initiation of the Vanuatu trench. The Emperor and Tuvatu ore systems show considerable resemblance to each other: 1. The Emperor deposit occurs along the margins of the Tavua volcano whereas the Tuvatu deposit may occur adjacent to an eroded shoshonite volcano. 2. Both deposits are spatially related to monzonite intrusives and extrusive rocks of shoshonitic affinity of almost identical age (~5.2 to 4.6 Ma) and compositions. 3. Both occurrences show an overprint of a low-grade porphyry Cu system by low-sulfidation, epithermal gold-telluride mineralization in flat lying veins, steep faults, shatter zones, stockworks, and hydrothermal breccias. Gold tellurides and V minerals were deposited at approximately 250^oC from boiling moderately saline fluids; and 4. O and H isotope compositions of Emperor and Tuvatu ore fluids both overlap waters from arc magmas and subduction-related volcanic vapors. Previously published values of d³⁴S of sulfides (-15.3 to 3.9 per mil) from Emperor are similar to those obtained here from the Tuvatu deposit (-15.3 to -3.2 per mil). Sulfur was derived locally from monzonites. The Mt. Kasi ore system superficially resembles those associated with the Emperor and Tuvatu deposits because gold (including precious metal tellurides) precipitated in narrow veins and hydrothermal breccias in basaltic and andesitic flows, breccias, and tuffs at around 250^oC. However, in detail it differs since it is a high-sulfidation epithermal gold system that shows no apparent spatial relationship to porphyry-style mineralization.