Cortez Summit Property

Eureka County, Nevada

Eureka County, Nevada

Carlin Gold’s Cortez Summit property is located in the Cortez gold trend.  The property consists of 142 100%-owned unpatented lode claims and is located in the midst of an expanding number of gold deposits.  The company’s primary target is Carlin-style mineralization in a structural and stratigraphic setting similar to that at the nearby deposits, including the Goldrush reserve/resource which is under development and the operating Cortez Hills mine.   The Goldrush deposit is hosted primarily in a favorable interval in the Devonian Wenban Formation, a carbonate unit that is also an ore host to other deposits in the Cortez trend.   Carbonates of the overlying Horse Canyon Formation are also hosts at Goldrush and in some of the other deposits in the district.  The geographical and structural relationships of the mineralized trends are illustrated in Figures 1 and 2.  Deepening of a prior reverse circulation hole with core in 2013 established that the sedimentary rock units at Cortez Summit are stratigraphically equivalent to the mineralized host units at Goldrush.  The presence of favorable mineralized host rock stratigraphy in the vicinity of the important Fourmile structural zone combine with the property’s strategic location to make Cortez Summit an excellent exploration target.  Exploration potential has been augmented by high grade holes at Barrick’s “Fourmile” discovery north of their Goldrush resource and located adjacent to Cortez Summit (Figs 1-4).  Barrick states that these holes have encountered mineralization “well above the average grade of the indicated and inferred resources at Goldrush”, for example 14.3 meters(m) grading 31.7 grams per tonne (gpt) and 5.8m grading 49.6 gpt (Barrick Feb. 22, 2016 news release).   Recent results include 16.6m grading 71.6 gpt and 16.8m grading 57.9 gpt (Barrick July 26, 2018 webcast). Barrick’s Fourmile success represents a different mineralization style than at Goldrush; although still contained in similar lower plate stratigraphic units, it is within the contact metamorphic zone adjacent to the Mill Canyon stock (see Fig. 3). Barrick points out similarities in mineralization style to the Deep Star and Deep Post deposits on the Carlin Trend (Barrick Investor Day webcast presentation, Feb. 22, 2016).  Newly encountered high grade mineralization within the contact metamorphic zone is a positive development for the Company because this contact zone extends on to our property and presents an additional, high grade, target type.

The southwest corner of the Cortez Summit property lies within one mile northeast of Barrick’s Goldrush discovery.  Barrick reports that, as of December 31, 2017, the probable reserve at Goldrush stood at 1.48 million oz grading 8.1 gpt, and the measured and indicated resource was 9.45 million oz grading 9.3 gpt with an inferred resource of 2.3 million oz grading 8.2 gpt, for a total measured/indicated/inferred resource of 11.75 million oz (Barrick Feb 22, 2018 Investor Day webcast).  Barrick has advanced the Goldrush project through the prefeasibility stage, and envisions annual underground production of 450,000 oz at an average all-in-sustaining cost (AISC) of $665/oz.  Barrick contemplates a mine life of 21 years with production beginning as early as 2021.    Development for the twin exploration declines to access the ore body has commenced (Barrick Q3 2017 and Q2 2018 webcasts).  An initial inferred resource is expected at Fourmile in early 2019 (Barrick Investor Day webcast Feb 22, 2018).  Barrick’s Cortez Hills operation is located 4 miles west of the property, just north of the Goldrush exploration decline portal.

Figure 1 - Cortez Trend

Figure 2 – Central Cortez Trend Structural Setting

Carlin Gold identified the area as prospective and staked initial claims in 2007, prior to Barrick’s September 2011 announcement of the Goldrush discovery.   The company was encouraged by initial rock sample geochemistry results from silicified and iron-stained siltstones from the NNW trending Fourmile structural zone on the west edge of the property.  Grab samples collected from this area contained up to 0.447 gpt gold and 307 ppm arsenic (Figures 5, 6).

Two types of mineralizing systems exist on or adjacent to the Cortez Summit property:

  1. Carlin-style sediment-hosted gold.The adjacent Carlin-style deposits (Goldrush, Fourmile, Cortez Hills, Horse Canyon) contain typical structural and stratigraphic controls that characterize Carlin-type deposits in north-central Nevada.These deposits are probably late Eocene (±40 Ma) in age.Most Carlin-style mineralization occurs in the favorable carbonate section that constitutes the ‘lower plate’ of the Roberts Mountain thrust fault system.This is the primary target at Cortez Summit.
  2. Buckhorn-style volcanic-hosted low sulfidation Au-Ag. This style of mineralization is associated with an extensive Miocene epithermal system (14.6 Ma) which is controlled by NNW-trending Northern Nevada rift structures.Nearby examples are Klondex’s (now Hecla’s) operating Fire Creek mine.

Except for a narrow sliver of Paleozoic ‘Upper Plate’ siliciclastic rocks of the Ordovician Vinini Formation poorly exposed on the west edge of the property, the Cortez Summit property is covered by Miocene-age basalts and gravels which postdate the Carlin-style mineralization event.  On the west side of the property, the Vinini Formation is unconformably overlain by a Tertiary volcano-sedimentary sequence consisting of a lower gravel unit and an upper series of Miocene (15-16 Ma) basaltic andesite flows. Drilling by Carlin Gold indicates that the Miocene sequence varies in thickness from zero on the west side of the property to >1,000 ft. (305 m) in the south central portion.  The Buckhorn deposits are primarily hosted by basaltic andesite in this sequence. Dikes, plugs and minor flows of Miocene rhyolite locally intrude and overlie the gravel and basaltic andesite units, including the rhyolite plug exposed in the southwest corner of the property (Figure 5, geology map).

Two intrusive phases are recognized just west of the property including: 1) Jurassic-age quartz monzonite of the Mill Canyon pluton; and 2) probable Tertiary-age quartz porphyry dikes and small intrusive bodies. These quartz porphyry intrusions may be correlative with early Oligocene quartz porphyry dikes which are common at the Horse Canyon, Cortez Hills and other Cortez district deposits.  

Two structural orientations dominate the Cortez Summit area: 1) NNW horst and graben structures that define the Northern Nevada Rift structural trend and control mineralization along the Aspen and Buckhorn mineralized zones, and 2) less well-defined NE-trending structures exhibited by the drainage patterns eroding the basaltic andesite (ie. Willow Creek structure). The Northern Nevada Rift structures associated with Buckhorn-style mineralization potentially represent shallow manifestations of the deep-seated feeder structures involved in older Carlin-style mineralization at depth. A good example of this is the prominent NNW trending Fourmile structural zone that extends for 4,500 feet (1370 m) along the west side of the property and appears to be associated with Carlin-style alteration and geochemistry.  In fact Barrick has mapped alteration in upper plate units along the northern extension of the Fourmile structural zone off the Cortez Summit property and along Fourmile Canyon to the Crescent Valley range front, a distance of approximately 4 miles (6.4 km)(Fig. 5)

In August 2012 a reverse circulation drill program was initiated to establish the presence and depth of the favorable Lower Plate carbonate section on the west side of the property, in proximity to the Fourmile structural zone in the southwest portion of the property. Most of this target area is covered by post-mineral gravel and basaltic andesite.  Where Paleozoic rocks are exposed on the west edge of the property, sampling along the Fourmile structural zone has yielded gold values of 0.447 and 0.263 gpt in altered siliciclastic rocks. Rock sampling along a drill road cut yielded 0.175 gpt gold over 50 feet (15 m) in altered siltstones (Figs 6, 7). 

 

Figure 3 – Barrick’s Fourmile Discovery 

Figure 4 – Barrick’s Fourmile Discovery – High Grade Holes Drilled in Early 2018

 

Figure 5 - Cortez Summit Geology

          

Figure 6 - Cortez Summit Geology - Southwest  

7 reverse circulation drill holes were completed in 2012 to depths ranging between 1500 and 1800 feet (457-549 m), for a total of 11,720 feet (3,573 m).  All holes encountered upper plate Paleozoic sedimentary rocks.  One hole was collared in the Paleozoic rocks and the remainder were drilled into Paleozoic rocks beneath younger Miocene basalts and gravels.  Paleozoic lithologies include grey-black siltstone, with lesser sandstone and chert.   CS12-7 penetrated and bottomed in a 300 foot interval containing variably laminated calcareous siltstones.  

Anomalous gold with Carlin-type pathfinder elements are present in six of the seven holes drilled in 2012.  The following table shows significant gold values (≥ 0.100 gpt Au) encountered in the drill program.

 

HOLE

INTERVAL (FT)

GOLD VALUE (gpt)

COMMENT

CS12-1

1170-1175

0.120

brown sandstone with calcite veinlets

CS12-2

1600-1605

0.101

black siltstone with pyrite and calcite veinlets

 

1700-1705

0.114

Rhyolite intrusive, weakly silicified

CS12-3

275-280

0.120

limonitic sandstone and siltstone

 

590-605

0.267

quartz-sericite-pyrite altered siltstone

 

1110-1120

0.139

silicified siltstone/sandstone

CS12-4

990-995

0.113

Intermediate to mafic intrusive

CS12-5

515-525

0.105

Gravel/fault scarp breccia various clast types

 

990-1005

0.169

Strongly Silicified siltstone/sandstone

CS12-7

840-860

0.149

gray silicified siltstone

Anomalous arsenic zones (>100 ppm) range up to 115 feet (35 m) in drilled thickness and contain values up to 1,530 ppm arsenic, 70 ppm antimony, 3 ppm thallium and 1.8 ppm mercury.  Two holes with the longest anomalous arsenic intervals were drilled 1,900 feet (580 m) apart on the west side of the property, and are located in close proximity to the Fourmile structural zone.  These intervals occur in the upper plate strata above the lower plate Horse Canyon and Wenban formations encountered in the drilling described below.

In September 2013 the company re-entered vertical reverse circulation hole CS12-2 and deepened it from 1,760 ft. (537 m) to 4,032 ft. (1,229 m).  This site was chosen in part because it appears to be located at the inferred structure intersection between the NE-trending Willow Creek structure and the NNW-trending Fourmile structural zone.  Short intervals of anomalous gold values to 0.667 gpt  and Carlin-type pathfinder elements were encountered in hole CS12-2C, including intervals  of  anomalous arsenic values (>100 ppm) ranging up to 85 ft. (25 m) of drilled thickness.  The bottom 392 (120 m) consisted of silty/sandy, laminated, variably carbonaceous limestone, interpreted to be  lower plate strata of the Horse Canyon and Wenban formations.  This material locally displays Carlin-type alteration features such as decalcification, clay alteration and calcite veining.  Carlin geologists are encouraged that favorable lower plate carbonate rocks are present near the projection of the gold-bearing Fourmile structural corridor. 

In November 2016 the Company announced signing of an Exploration and Earn-In agreement with Barrick Gold Exploration Inc. on the Cortez Summit property (see news releases on this site).  Barrick completed a 5,171 ft. (1,576 m) vertical core hole, SJV17-1D, in May-July 2017, located along the east side of the Fourmile structural corridor in the vicinity of Carlin’s shallow holes drilled in 2012.  SJV17-1D encountered a long 567 ft. (173 m) zone at 333-900 ft. (102-274 m) in upper plate rocks consisting of variably clay-altered, iron oxide-stained, brecciated hornfels and lesser mudstone/siltstone, with the most intense clay alteration and iron oxide development at 652-798 ft. (199-243 m).  Associated with this alteration is a very thick interval, 592 ft. (180 m), of highly anomalous arsenic at 328-920 ft. (100-280 m) averaging 480 ppm, with accompanying additional strongly anomalous Carlin-style pathfinder elements including mercury (to 17 ppm), antimony (to 173 ppm) and thallium (to 6.3 ppm).  Gold values in this alteration zone include a 10 ft. (3 m) zone at 694-704 ft. (211-214 m) grading 2.6 gpt.  The favorable lower plate carbonate section was encountered at a depth of 4,234 ft. (1,290 m), although no significant gold values were detected.  On February 15, 2018 Barrick terminated the Exploration and Earn-In agreement, after completing this one hole.

While disappointed that the lower plate rocks encountered in SJV17-1D were not mineralized, the Company is encouraged by the strong alteration and pathfinder elements along with elevated gold values encountered in upper plate rocks adjacent to the Fourmile structural corridor.  The geometry of this “shallow” anomalous zone has not been defined.  It potentially represents the upper level of a mineralized structural conduit, which could be used as a guide to deeper gold mineralization in lower plate carbonate rocks.

The strategic location of the project, along with the similarity in geologic setting to the neighboring Carlin-style deposits, makes Cortez Summit a high quality target for Carlin-style gold mineralization.  Only a small portion of the property has been evaluated, and the Fourmile structural corridor in particular requires more attention.

Figure 7 – Cortez Summit Photo – view to east over north portion of Goldrush resource and Barrick’s Fourmile discovery