A Distinct Fluid Inclusion Gas Signature for Deep High-Grade Carlin-Type Orebodies

ABSTRACT
The potential to distinguish fault zones that host deep highgrade
gold mineralization using fluid inclusion gas analyses was
evaluated at the Getchell and Twin Creeks deposits, Nevada, and the
Huijiabao project, Guizhou Province of China. Fifty-one samples of
mineralized quartz were collected and fluid inclusion gases were
measured using a quadrupole mass spectrometer.
Gas anomalies are associated with fault zones that host highgrade
gold mineralization at each locality. At the Getchell deposit, the
northeast-trending Turquoise Ridge and the north-trending Getchell
fault zones were characterized by fluid inclusions with average total
gas contents of 19.5 versus 6.3 mole percent, CH4/Ar of 2400 versus
50, CO2/Ar of 3300 versus 430, N2/Ar of 600 versus 100, and CnHn
(hydrocarbons)/Ar of 2.4 versus 0.4, respectively. A comparison of
the data on ternary diagrams and ratios of N2/Ar > 115 indicates magmatic
fluids were restricted to the Turquoise Ridge fault zone, which
hosts the deep Turquoise Ridge orebody. Similar gas data for the
Twin Creeks deposit also document the role of northeast- trending
fault zones as primary conduits for mineralizing fluids. At the
Huijiabao project fluid inclusions contain higher total gas contents
and are enriched in CH4, N2, and CnHn at the zone of intersection
between northwest-trending faults and a regional thrust fault.
Hydrothermal fluids that deposited high-grade mineralization
can also be distinguished from barren hydrothermal fluids based on
gas data. Samples of barren hydrothermal quartz and quartz clasts in
breccias contain fluid inclusions with average total gas contents of
0.8 to 6.9 mole percent, CH4/Ar of 170 to 280, N2/Ar of 50 to 160,
and CnHn/Ar of 1.4 to 1.7.
Fluid inclusion gas analyses have the potential to define anomalies
that represent areas where auriferous fluids were focused during
the formation of Carlin-type gold deposits. A distinct gas signature
is suggested for hydrothermal systems that produced deep
high-grade mineralization, which may have little to no surface
expression.