Gold-Mineralized Eocene Dikes at Griffin and Meikle: Bearing on the Age and Origin of Deposits of the Carlin Trend, Nevada.

ABSTRACT
Field relations, isotopic ages, and petrographic and chemical
data of dacite dikes suggest that Carlin-type Au mineralization at the
Griffin and Meikle deposits is of Eocene age and closely linked to
contemporaneous igneous activity. An 40Ar/39Ar age of 39.21 ± 0.12
Ma on biotite phenocrysts from mildly argillized dacite and a U/Pb
zircon age of 38.1 ± 0.8 Ma from mineralized dacite from the Griffin
deposit confirm the Eocene emplacement age of several porphyritic
dacite dikes that intrude the Post fault zone from north of Meikle to
the Betze-Post deposit. The dacite dikes are variably altered and
locally mineralized in both the Meikle and Griffin deposits. Sericite
(1M and 2M1 illite) in dacite is associated with fine-grained pyrite
and arsenopyrite and formed during the earliest and most Au-rich
paragenetic stage. Illite-quartz mixtures from dacite that contains 3
to 9 ppm Au and 3 to 55 ppm Ag yielded 40Ar/39Ar total gas ages of
between ca. 40 and 46 Ma, older than the ca. 39 Ma age of the dikes.
The older apparent sericite ages have resulted in part or entirely
from recoil loss of 39Ar from fine-grained illite on irradiation.
Nonetheless, the similarity of apparent illite ages and the fact that
they are only a few m.y. older than biotite and zircon ages suggest
that sericitic alteration and associated Au mineralization at Meikle
and Griffin closely followed dike intrusion.
Late hydrothermal activity at Griffin produced comb quartz
veinlets containing Ag-Sb sulfosalts and native silver and still later
barite veinlets and vug fillings. Paragenetic and geochemical relationships
of ore-grade dacite from Griffin and Meikle indicate that
Au was deposited with fine-grained As-bearing Fe sulfides similar
to other deposits of the Carlin trend and that a later stage resulted in
the precipitation of minerals containing more Ag, Sb, Se, Mo, ± W,
but with less S and only minor Fe.
Alteration in dacite ranges from most intense quartz, sericite,
and pyrite near ore zones to intermediate quartz, illite-smectite,
kaolinite, and arsenopyrite, to more distal smectite, carbonate, and
kaolinite. This zoning is also reflected in Fe-bearing minerals, which
vary from pyrite, to arsenopyrite, to Fe-bearing carbonate and oxide
phases.
Multi-element analyses show progressive gains in pathfinder
elements, including Au, Ag, S, As, Hg, Sb, and Tl, with greater
intensity of sericitic alteration. Other elements that were added during
quartz-sericite-pyrite alteration include Se, Mo, W, and Ni. In
contrast, Na, Ca, Sr, Mg and Mn were removed; K was leached to
a lesser degree. Variation in the Fe/Ti and Si/Ti of mineralized
dacite indicate that Fe and Si were mobile and in part added during
mineralization.
Mineralized rocks of the various deposits of the Carlin trend
have many common paragenetic and geochemical features that support
common ore-forming processes and which argue against multiple,
unrelated periods of Au deposition. The spatial and temporal
link between 40 to 36 Ma intrusive activity and mineralization at the
Meikle, Griffin, Betze-Post, Genesis, Deep Star and Beast deposits
suggests that Eocene magmatism was critical in the formation of Au
deposits of the Carlin trend.