MESEL SRHDG DEPOSIT AND LOW-SULFIDATION GOLD VEINS IN THE RATATOTOK DISTRICT, NORTH SULAWESI, INDONESIA

The North Arm of Sulawesi is a classic oceanic island arc that includes porphyry Cu and volcanic-hosted epithermal Au-Ag deposits. The 8 x 4 km Ratatotok Au district hosts SRHDG deposits and low-sulfidation veins that contain > 88 t Au. Most of the Au is in the Mesel SRHDG deposit (65 t Au @ 7 gm/t). Both styles of mineralization are hosted in Miocene carbonate rocks deposited in a NE-trending graben. The carbonate sequence was deposited on, and later covered by, andesitic volcanic and volcaniclastic rocks. The carbonates were gently folded along E-W axes and intruded along ENE to E-W structures by porphyritic andesites that are in part sill-like. Similar intrusions are dated at 4.3 to 3.4 Ma. Quartz-calcite veins (e.g Lobongan) are in karsted, massive limestones along NW and ENE-striking faults. Veins are gradational with, and locally overprint stratabound, karst-controlled silicification. They are irregular, crustiform, banded, with drusy quartz, coarse calcite, visible Au, and have low concentrations of As, Sb, and minor Cu, Zn, Pb, Ag. Fluid inclusions in quartz indicate the veins formed at T=270-158°C from low-salinity, CO2-bearing, boiling fluids. At Mesel, SRHDG ore is mainly in altered carbonates below and adjacent to andesite intrusions at intersections of E-W and NW-striking faults. Carbonates are silicified, decalcified, dolomitized, and have anomalous As, Sb, Hg, and Tl and low base-metals (Cu+Zn+Pb < 100ppm). Sub-micron Au is in disseminated As-pyrite that rims early pyrite. Open-space minerals include orpiment, realgar, cinnabar, drusy quartz, stibnite and late dolomite or calcite. Andesitic rocks are sulfidized and argillized, have similar trace elements, and introduced K. Fluid inclusion data from drusy quartz (Th=170°C) and late calcite (Th=119°C) record the waning stages of the system. Stable isotopic data on ore-related minerals suggest ore fluids consisted of magmatic water that evolved by interactions with limestone (water dD = -54 to -74‰ and d18O = 5 to 7‰, CO2 d13C = -1‰, H2S d34S = 0 to 5‰). Late calcite records the influx of meteoric water (dDwater = -24‰, d18O = 12 to 15 ‰, d13C = 0 to 2 ‰). We conclude that Mesel is intrusion-related and suspect that the quartz-calcite veins formed by boiling of similar ore fluids.