CARLIN-LIKE GOLD IN THE IDAHO BATHOLITH – EVIDENCE FROM THE YELLOW PINE DEPOSIT

The Yellow Pine district is an historic producer of Au, Sb, Hg, and W and is an active exploration area. To advance understanding of its origin, new and previous results were synthesized and compared to Carlin-type Au deposits (CTD) in Nevada.

The deposits are localized by damage zones along, and NE-splays from, a N-striking fault that transects 94-83 Ma intrusions of the Idaho batholith and metasedimentary rocks in roof pendants. The 51 Ma age of adularia associated with As-pyrite in one deposit, 47 Ma age of post-ore dikes, and position of the district 5 km west of an Eocene caldera suggest that Au mineralization is related to the onset of magmatism and extensional faulting in the Challis volcanic field.

In proximal Au ore zones, igneous minerals are replaced by quartz, adularia, sericite (±NH₄⁺), As-pyrite and arsenopyrite (asp). Carbonate minerals and smectite are distal or late. Ore-related veinlet types include: early fine-grained As-pyrite, asp, ± adularia and quartz with dark CL, later coarse-grained stibnite ± dolomite and drusy quartz with bright CL, and distal pink dolomite or calcite. Gold resides in As-pyrite and asp.

Mass transfer studies of Au ore show that Al, Ti, and Fe were immobile, Na, Ca, Sr, Mg, and Mn were depleted, K, S, As, and Au were strongly introduced, and Sb, Tl, Hg, Ag, Se, Cd, Te, Cu, W, and Bi were moderately introduced. Though the As content of ore and ore-stage pyrite is similar to that in CTD’s, the Au content is lower. The immobility of Fe and introduction of S, Au and As indicate that Au ore formed from H₂S-rich fluids by sulfidation of Fe in the batholith.

Liquid-rich fluid inclusions (FI) in ore-related quartz and stibnite have T_h of 233 to 160°C, P_h of 28 to 6 bars, salinities of 11 to 4 wt. %, and low gas contents. The lower T_h and presence of vapor-rich inclusions in stibnite suggest it precipitated by cooling and decompression. H, O, C, and S isotopes of FI, quartz, sericite, carbonates, arsenian pyrite and stibnite suggest that magmatic fluids mixed with meteoric water.

Although the Eocene age, association with magmatism and extension, and processes of ore formation at Yellow Pine are Carlin-like, the ore fluids were less acidic and had a lower sulfidation state. These differences may be due to ore fluid evolution in the Idaho batholith rather than in carbonaceous, pyritic, sedimentary rocks as in Nevada CTD.