USGS Professional Paper 1697 - Alaska Resources Library

thermally altered to siliceous and adularia phases. Au is either
disseminated throughout the hydrothermally altered rocks or
is concentrated in small quartz veins. The adularia phases also
locally contain Au. Placer Au deposits, as at Kolchanskoe,
Ulskoe, and Oemku, are associated with the Au-Ag epithermal
vein deposits. In addition to the Au-Ag epithermal vein deposits,
the Lower Amur metallogenic belt includes few small W
skarn, porphyry Cu, and Sn greisen deposits, which are all
hosted in or near Paleogene alkaline granitoid rocks.
Mnogovershinnoe Au-Ag Epithermal Vein Deposit
The large Mnogovershinnoe Au-Ag epithermal vein
deposit (fig. 93) (Zalishchak and others, 1978) consists of
hydrothermally altered, adularia-sericite-quartz vein-like
zones as much as 800 m long. The zones contain a series of
adularia-quartz veins and veinlets. Some ore bodies consist of
rhodonite-carbonate veins, and lenses of skarns and sulfides.
The ore minerals include pyrite, marcasite, gold, argentite,
Au- and Ag-tellurides, galena, sphalerite, chalcopyrite, and
freibergite. The ore minerals constitute as much as 1 percent of
veins. The Au:Ag ratio is 1:1. The deposit is hosted in Paleo-
Cross section
A
Map
Surficial deposits (Quaternary)
Leucogranite (Paleocene)
Granodiorite porphyry (Paleocene)
Granodiorite (Paleocene)
Subvolcanic intrusive rocks
(Late Cretaceous and Paleocene)
Lava flows and extrusive rocks
(Late Cretaceous and Paleocene)
Sandstone (Early Cretaceous)
B
A
Early Late Cretaceous Metallogenic Belts (100 to 84 Ma; figs. 79, 80) 193
0 2 4 km
Quartz-adularia
vein
Replacement
zone
Fault
Contact
cene andesite-dacite that is genetically related to a multiphase
intrusion of highly alkaline granitoid rocks. K-Ar isotopic
studies indicate an age of mineralization of 49 to 69 Ma. During
formation of local Au-bearing skarns, which presumably
formed during intrusion of Paleogene subalkaline granites, Au
was remobilized (Ivanov and others, 1989). Placer Au deposits
are associated with the Au-Ag epithermal vein deposits.
Belaya Gora Au-Ag Epithermal Vein Deposit
The medium-size Belaya Gora Au-Ag epithermal vein
deposit (Mel’nikov, 1978) consists of disseminated and
stockwork-type Au-Ag ore that occurs in extrusive bodies of
subalkalic rhyolite-dacite and explosive breccia of an Eocene-
Oligocene igneous complex. Alteration minerals are quartz
(50 to 90 percent), kaolinite, dickite, sericite, hydromica, and
adularia. The ore minerals are gold, silver, argentite, pyrite,
marcasite, chalcopyrite, sphalerite, galena, hematite, and cinnabar.
The ore assemblages are Au-quartz and Au-sulfosaltssulfide-quartz.
Gold distribution is highly irregular and the ore
bodies do not have clear boundaries. The deposit extends to
100 m deep.
B
Figure 93. Mnogovershinnoe Au-Ag epithermal
vein deposit, Lower Amur metallogenic belt, Russian
Southeast. Schematic geologic map and cross section.
Adapted from Ratkin (1995). See figure 79 and table 4
for location.