Discovery and geology of the JB Zone at the REN property, Elko County, Nevada


The REN property is located on the northern Carlin trend, Nevada. Centerra
(U.S.) Inc. and its predecessors, in joint venture with Homestake Mining Company
(now wholly owned by Barrick Gold Corp.), began exploring the deep potential of
REN in 1995 and in 2000 encountered a significant intercept in hole RU-24C grading
39.4 g/t Au over 36.6 m beginning at a depth of 828.4 m (1.148 oz Au/ton over 120 ft
beginning at 2,718 ft). Discovery of the nearby JB zone followed in 2001 and 2002
when several adjacent drill holes returned high-grade gold intercepts, including
21.3 g/t Au over 19.8 m in hole RU-49C (0.620 oz/ton over 65 ft). The discovery holes
targeted the area beneath a hill consisting of strongly altered upper plate rocks near
an intersection of major structures, including the north-striking Ren (Post) fault. A
large arsenic soil anomaly is located north of the JB zone. Anomalous rock chip gold
values to 0.75 g/t were obtained from upper plate rocks in the area, although not
directly above the JB zone.
A block model estimate completed on four separate zones of mineralization at
REN in December 2004, using an 8 g/t cutoff grade and an assay cap of 40 g/t, identified
indicated resources of 1.873 M tonnes grading 13.1 g/t for 24.5 tonnes of contained
gold (2.065 M tons grading 0.382 oz/ton for 791,000 oz), plus inferred resources
of 1.261 M tonnes grading 12.7 g/t for 16.0 tonnes of contained gold (1.390 M tons
grading 0.370 oz/ton for 516,000 oz). The JB zone is the largest of the four zones discovered
to date, containing over half of the total resource. It extends from 700 to
960 m below the surface (2,300 to 3,150 ft) and is a typical Carlin-type deposit, hosted
predominantly in dolomitic to calcareous siltstone and mudstone of the Devonian
Popovich Formation in the lower plate of the Roberts Mountains thrust.
The Popovich Formation has been structurally disrupted and ranges from 0 to
over 120 m (0 to over 400 ft) in thickness. It overlies the massive Silurian-Devonian
Bootstrap limestone. The contact between the Popovich and Bootstrap is prone to
karsting and collapse brecciation. A low-angle fault separates the Popovich from the
overlying, strongly sheared, Devonian Rodeo Creek unit which is approximately
200 m (650 ft) in thickness. The Roberts Mountains thrust zone separates the Rodeo
Creek from about 500 m (1,700 ft) of overlying Ordovician through Devonian strata
of the upper plate.
Structural controls on gold mineralization in the JB zone are complex. A firstorder
control is low- to moderate-angle, north-dipping faults related to a duplex of
favorable Popovich host rock, and the associated low-angle fault along the Popovich-
Rodeo Creek contact. The mineralized zone strikes east-west, parallel to and in the footwall of the high-angle MBX fault. The highest-grade intercepts align north-south
along the east margin of the zone and are likely controlled by a north-striking fault.
The JB zone is composed mostly of replacement and breccia styles of Carlin-type
mineralization associated with decarbonatization and related collapse brecciation,
weak to moderate silicification, sulfide deposition, and drusy quartz + barite veining
within a structurally thickened section of Popovich Formation. Late-stage realgar
occurs immediately above high-grade gold mineralization in two holes. Locally, highgrade
values occur in altered lamprophyre sills and dikes. An early event of collapse
breccia development and intense silicification unrelated to the main gold stage is
widespread along the Bootstrap-Popovich contact. Some of the high-grade gold mineralization
in the JB zone is found immediately above this silicified breccia.

SKU: 2005-23 Category:

Additional information


Primary Author

Vance Spalding






Deposit Type


Exploration Method