FLUID FLASHING AND POLYMETALLIC VEIN FORMATION ABOVE A SHALLOW PORPHYRY PLUTON: PETROGRAPHIC EVIDENCE FROM THE SUNNYSIDE DEPOSIT, COLORADO, USA

The Miocene age, Sunnyside deposit represents one of the most productive precious and base metal vein deposits in the San Juan Mountains of southwest Colorado. The polymetallic ore occurs in veins that exhibit crustiform banding and sulfide mineral assemblages that are typical of intermediate sulfidation (IS) epithermal deposits.

To advance understanding of this deposit, fluid inclusion petrography, microthermometry, and optical CL analysis was conducted on representative samples from six types of polymetallic veins recognized in the mine workings as well as drill core ~500 meters below the workings. Quartz-sulfide veins in drill core below the workings resemble A, AB, B, C and E veins in well-studied porphyry Cu-(Mo) deposits. Fluid inclusions contained in A and AB vein quartz have been affected by post-entrapment modification. However, unmodified secondary planes of fluid inclusions consist dominantly of vapor that coexists with heterogenous silicate melt inclusions (HSMIs). Polymetallic veins from the workings have unique characteristics, including a diverse metal suite of Cu-Zn-Pb-W-Te-(Ag-Au-Bi), fluid inclusions with high homogenization temperatures (325-330°C), low salinities (1-3 wt% NaCl), and an absence of boiling assemblages. These features are unlike those found in most IS epithermal deposits. The presence of ore mineral dendrites and silica recrystallization textures such as mosaic quartz and feathery quartz in these veins indicate that ore and gangue minerals precipitated as fluid flashed to vapor.

The sequence of vein types at Sunnyside reveals a cooling trend where the earliest vein type formed under lithostatic conditions within the two-phase field of the H₂O-NaCl system, while the later vein types formed under hydrostatic conditions. In the hydrostatic regime, distinct episodes of hydrothermal fluid flashing occurred that produced the six vein types in the workings. The veins straddle the porphyry to epithermal transition.