Whole-Rock Oxygen Isotope Traverses Across Gold-Bearing and Barren Structures, Lone Tree Complex, Nevada

The Lone Tree Complex includes the Lone Tree, North Peak,
and Trenton Canyon systems. These fine-grained gold deposits are
variations of Carlin-type deposits having structurally controlled ore
zones. Both gold-bearing and barren structures have hydrothermal
alteration centered along faults, with modal percent of alteration
minerals increasing towards the structure. This alteration mineral
assemblage is dominated by quartz, fine-grained white-phyllosilicates
(sericite), oxidized pyrite, and locally, clays and chlorite.
Whole-rock oxygen isotope traverses across gold-bearing and
barren structures record isotopic alteration resulting from the deposition
of alteration minerals and water-rock exchange. All gold-bearing
traverses have depletions at the fault zone relative to background
values, while barren traverses exhibit either an enrichment (North
Peak) or a depletion (Lone Tree) at the fault zone. Gold-bearing
structures are associated with greater oxygen isotope alteration
(change relative to background values) than the barren structures.
Quartz associated with gold-bearing structures at Lone Tree, North
Peak, and Trenton Canyon has average d18O values of 12.4 ä, 19.5
ä, and 8.6 ä, respectively. In contrast, quartz associated with barren
structures at Lone Tree and North Peak yields average d18O values
of 19.5 ä and 23.6 ä, respectively. The calculated d18O of fluid
traveling through gold bearing structures at the three deposits ranged
from 0.0 to 8.0 ä, while calculated d18O of fluid associated with
barren structures at Lone Tree and North Peak ranges from 12.5 to
13.1 ä.
Calculated oxygen isotope compositions and fluid inclusion
data indicate multiple sources of fluid for deposits of the Lone Tree
Complex. A regional, barren formation water is suggested by the
ranges in salinity (3.0-18.0 eq.wt. % NaCl), oxygen isotope composition
(8.5-13.1 ä), and homogenization temperatures (170-310¡C).
At Lone Tree, a magmatic source for gold-bearing fluid is suggested
by the range in homogenization temperatures (280-400¡C) and
salinity (12-39 eq.wt. % NaCl), and is supported by the calculated
d18O of the fluid (6.5 ä). Trenton Canyon gold-bearing fluid has a
calculated d18O of 0.0 ä, indicating an evolved meteoric fluid,
which varies in homogenization temperature (210-350¡C) and salinity
(2.6 to 5.7 eq.wt. % NaCl). Gold-bearing fluid at North Peak has
a calculated d18O value of 8.5 ä. This fluid varied in homogenization
temperature (170-270¡C) and salinity (3.7-14.0 eq.wt. % NaCl).