Fluid Inclusion Evidence for Gold Mineralization Related to Decalcification at the Pipeline Mine, Nevada

ABSTRACT
Pipeline is a sediment-hosted, disseminated gold deposit
located 120 km southwest of Elko, Nevada. Gold mineralization is
argillization-hematite-silicification-gold in decalcified Roberts
Mountains Formation. Subvertical quartz-sericite-pyrite-gold-calcite
veins occur at depth. The vein quartz and ore quartz were studied
by fluid inclusion microthermometry and gas analysis, and mineralization
was dated. Our objective was to study decalcification and
related gold mineralization.
Vein and ore quartz contains aqueous Type 1 inclusions and
CO2-dominant Type 2 inclusions. Quartz Type 1 inclusion salinities
range from 5 to 25 eq. wt. % NaCl and homogenization temperatures
(Th) range from 179¡ to 265¡C. The most saline Type 1 inclusion
have anomalous calcium-rich fluids with Ca/Na ratios of 0.8 to 1.4.
Type 2 inclusions are about 1:8 H2O-CO2, have salinities about 0.5
eq. wt. % NaCl and Th ranges from 285 to 312¡C. Bulk fluid inclusion
gas analyses show that CO2 varies from 0.5 to 8 mol. %. Carbon
dioxide strongly correlates with H2S, which varies from 0.03 to
0.00001 mol. %, and CO2 inversely correlates with salinity. Fluid
inclusion N2/Ar ratios are >100 indicating magmatic volatiles.
Dating vein sericite by the 40Ar/39Ar method indicates quartz veins
are Early Cretaceous.
Calculated trapping pressure of quartz inclusions is about 2 kb.
The pressure calculation suggests mineralization occurred at 8 km,
which is consistent with an Early Cretaceous age. Calculated pH and
gold solubility in low and high salinity fluids is respectively 4.5 and
201 ppb Au, and 5.9 and <2 ppb Au. Similarity of vein quartz and ore quartz inclusion fluids indicates they represent a common mineralizing event. The data indicate a mineralizing fluid chemistry that could both dissolve calcite and transport gold. They document a change in fluid chemistry that is consistent with solution of calcite and formation of pyrite. Our data are best explained by a slightly-acidic, 300¡C, lowsalinity, high-CO2, high-H2S, high-Au ore fluid with a significant magmatic component that migrated up fractures and entering sheared Roberts Mountains Formation at a depth about 8 km. This fluid dissolved calcite and formed pyrite resulting in a Ca-rich bicarbonate, high-salinity, low-CO2, low-H2S near-neutral fluid. The decrease in H2S resulted in decomplexation and precipitation of gold from solution. Weathering has oxidized the ore-body leaving gold associated with Fe-oxides.