USGS Professional Paper 1697 - Alaska Resources Library

composition magmatic bodies (with approximate K-Ar ages
of 70 Ma). The first stage consists of grossular-wollastonite
skarn and concentrically zoned, finely banded aggregates with
numerous finely crystalline datolite and druse-like accumulations
of danburite crystals in paleohydrothermal cavities. The
second stage skarn consists predominantly of long, radiated
hedenbergite and andradite with coarsely crystalline datolite,
danburite, quartz, axinite, and calcite. The arcuate nature of
metasomatic mineral zonation in the skarn is interpreted by
Shkolnik and others (2003) as replacement of stromatolite
build-ups in the host Triassic Limestone. The complex layering
are relict biogenic features of the stomatolite-bearing
limestone. An Ar-Ar age for orthoclase indicates an age for the
late-stage skarn assemblage of 57 Ma. The silicate mineralogy
of the early-stage skarn is similar to that in Pb-Zn skarn deposits
in this same region. B isotopic studies indicate a magmatic
source for boron (Ratkin and Watson, 1993). The deposit is
very large and had been mined from 1970’s to present. The
deposit produces more than 90 percent of all borate in Russia.
The Dalnegorsk open-pit mine at the deposit is prospected to
the depth of 1 km.
Clastic sedimentary rocks (Early Cretaceous)
Sedimentary breccia with small fragments of
limestone and chert (Early Cretaceous)
Siliceous shale and siltstone (Triassic, Jurassic,
and Early Cretaceous)
Olistolith of bedded limestone (Triassic)
Datolite-danburite-wollastonitehedenbergite-andradite
skarn
Mafic dike (early Tertiary)
Mafic dike (Cretaceous)
Fault
Contact
Early Late Cretaceous Metallogenic Belts (100 to 84 Ma; figs. 79, 80) 185
Nikolaevskoe Pb-Zn Skarn Deposit
The Nikolaevskoe Pb-Zn skarn deposit (fig. 86) (Garbuzov
and others, 1987; V.V. Ratkin, this study; Vasilenko and
Valuy, 1998) consists of large, layered ore bodies formed in
a giant olistolith of Triassic limestone that is part of an Early
Cretaceous accretionary-fold complex. The skarn occurs at the
contacts of limestone with hosting siltstone and sandstone and
with overlying felsic volcanic rocks of a Late Cretaceous to
Paleogene post accretionary sequence. Small ore bodies also
occur in limestone blocks in the volcanic rocks that were torn
off the underlying basement. The ore minerals are dominantly
galena and sphalerite that replace an earlier hedenbergite skarn
near the surface, and, at depth, replace a garnet-hedenbergite
skarn. Subordinate ore minerals are chalcopyrite, arsenopyrite,
pyrite, pyrrhotite, fluorite, and Ag-sulfosalts. K-Ar isotopic
studies indicate an age of mineralization between 60 and 80
Ma. A 60 K-Ar Ma isotopic age is obtained for an unmineralized
basalt dike that cuts the deposit, and a K-Ar age of 70-80
Ma is obtained for a mineralized ignimbrite that overlies
the olistolith. The deposit is currently being mined. Average
0 500 m
Figure 85. Dalnegorsk B skarn deposit, Taukha metallogenic belt, Russian Southeast. Schematic geologic
map. Adapted from Nosenko and Chernyshov (1987). See figure 79 and table 4 for location.