• Sleeper has many geologic features of epithermal gold deposits of low-sulfidation type worldwide, although the early-stage breccia-stockwork mineralization rich in sulfide minerals is an unusual adjunct to the more typical sulfide-deficient veins.
Low-sulfidation deposits are of particular exploration interest because they host many of the world’s bonanza-grade , >1 oz/t, ore shoots that invariably sustain some of the industry’s lowest-cost gold mining operations.
• Notwithstanding several substantial exploration programs at Sleeper, both during and subsequent to the open-pit mining activity, the district is considered to have been only
incompletely tested. The potential areal and depth extents of low-sulfidation gold districts were apparently not fully appreciated and, hence, not factored into the exploration strategies employed.
• Additional potential is believed to remain in the Sleeper project area, mainly beyond the area that was extensively drilled previously. The prime target is a bonanza-grade
vein, broadly parallel to the Sleeper vein but with greater persistence both along strike and down dip. Accompanying breccia-stockwork gold mineralization may also be of
interest, especially if the contained sulfides are thoroughly oxidized.
• Extensive discussions during this assignment, involving input from ten experts on the Sleeper district, reached the consensus view that five specific high-priority
exploration targets may be defined in the Sleeper district. Four of them appear to lie beneath relatively shallow (<150 m) overburden, whereas the fifth is the range-front structural zone that is exposed at the foot of the Slumbering Hills, immediately east of the Sleeper pit. Three of the targets are entirely untested, whereas the other two appear to have been subjected to only preliminary drill testing.
• Recent three-dimensional compilations of geologic, geochemical, and geophysical data sets, some not available before, were used to highlight the five exploration
targets. These data sets will soon be merged into a single database, which may be used to further refine the targets preparatory to drill testing.
• A disciplined drilling approach is recommended, with each target being tested using fences of inclined RC holes that overlap one another so that any veins present cannot
fail to be intersected. Most targets will require a minimum of three drill fences assuming that the first fence across each target provides adequate encouragement.
• The proposed exploration program does not address two large portions of the extensive land package, namely the outcropping Slumbering Hills area and the westernmost concealed area where overburden thicknesses are inferred to exceed 150m. Both these areas may contain low-sulfidation gold mineralization of potential interest, but are assigned lower priorities than the area selected for immediate attention.
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INTRODUCTION
At the request of Shawn Kennedy, the writer spent three and one-half days reviewing the exploration potential of the Sleeper project in Humboldt County, Nevada, on behalf of X-Cal
Resources Ltd. Technical briefing sessions were held in Reno and at the Sleeper exploration office, where maps, hand samples, and selected drill core were also inspected. A final
technical session was convened in Winnemucca.
The review benefited from the participation of a group of individuals who have had long associations with the Sleeper project, namely Keith Blair (geological consultant), Vic
Chevillon (Placer Dome Exploration), Rich Histed (New Sleeper Gold LLC), Shawn Kennedy (President, X-Cal Resources), Larry Kornze (geological consultant), Larry Martin (New Sleeper Gold LLC), Win Rowe (geological consultant), Ken Snyder (geological
consultant), and Jim Wright (independent geop hysical consultant). The three-dimensional geochemical visualization of the Sleeper deposit provided by Robert Jackson (independent
geochemical consultant) was also valuable.
This report summarizes the geologic model for the previously mined Sleeper gold deposit and its environs preparatory to an assessment of the exploration potential of the Sleeper
project. The review process resulted in the selection of specific targets meriting further work, including drill testing, along with the elaboration of a systematic exploration approach.
SLEEPER GEOLOGIC MODEL
Regional setting
Sleeper is one of several low-sulfidation epithermal gold deposits localized by the northnorthwest- striking North Nevada rift, the site of active extension and compositionally
bimodal (basalt-rhyolite) volcanism during the mid-Miocene. The rift zone occupies a back-arc setting and has been linked by some investigators to the effects of mantle plume activity.
Midas and Mule Canyon are the currently exploited low-sulfidation epithermal gold deposits in the North Nevada rift, with Ivanhoe and perhaps Fire Creek earmarked for production.
Sleeper lies beyond the main rift zone, but may be localized by a subsidiary parallel axis of rifting marked by a prominent linear magnetic anomaly comparable to that defining the main
rift.
Stratigraphic setting
The Sleeper deposit is hosted by a shallowly east-dipping sequence of faulted volcanic and volcaniclastic rocks of mid-Miocene age. The association of rhyolitic and andesitic to
basaltic units suggests a bimodal association. The most important gold mineralization, including the bonanza-grade ore for which Sleeper was particularly famous, is contained by
the Sleeper rhyolite, which appears to comprise both intrusive and extrusive, tuffaceous units. Where observed during this visit, the intrusive rhyolite is a homogeneous, devitrified
rock displaying consistent flow foliation parallel to the district-wide dip attitude. If this observation is confirmed more widely, it suggests that the rhyolite has a sill-like geometry more appropriate to a subsurface cryptodome than a flow-dome complex.
Radiometric dating suggests a close temporal relationship between the intrusive rhyolite and gold mineralization.
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These rhyolitic rocks are underlain by an andesitic to basaltic unit, which includes distinctive amygdaloidal flows, which, in turn, overlies a fine-grained volcaniclastic and/or air fall succession. The latter is in unconformable contact with a folded metasedimentary formation assigned a Permo-Triassic age, which crops out widely in the Slumbering Hills immediately east of the Sleeper deposit. The Miocene and older rocks beneath the Sleeper rhyolite appear to be less favorable hosts for gold mineralization, probably because they do not sustain brittle fractures as readily as the brittle rhyolite.
The main Sleeper vein and all accompanying mineralization beneath the pediment west of the range front were concealed beneath lacustrine sediments and alluvial deposits of postmineral timing. The post-mineral sequence thickens progressively westward, with drill-hole evidence supplemented by a magnetotelluric geophysical interpretation provided by Jim
Wright showing that thicknesses probably only exceed about 150 m to the west of an imaginary north-south line drawn just west of the tailings disposal area.
Structural setting
Gold mineralization at Sleeper, and in the Slumbering Hills to the east (e.g., Alma and Jumbo prospects), appears to be confined to a northwest-trending structural corridor (Fig. 1), which may be considered as a zone of basement weakness that acted as a fundamental control on the localization of gold mineralization. At the scale of the Sleeper deposit, a component of this northwest system interrupts the continuity of the main Sleeper and Wood veins, near the middle of the Sleeper pit (Fig. 1), and appears to have acted as a transfer structure at the time of gold introduction.
The gold mineralization at Sleeper coincided with an episode of broadly east-west extension, which gave rise to a set of north- to north-northeast-striking normal faults, the most
important of which display west-side-down displacement. The principal fault in the Sleeper district, marked at surface by a broad zone of brecciation and shearing, is located at the range front where it places outcropping Permo-Triassic metasedimentary rocks to the east against the concealed Miocene volcanic succession to the west. Subsidiary faults, which may be considered as hanging-wall splays, have opposite vergence. The normal faults may have listric geometries, although this has yet to be fully confirmed. The principal gold
mineralization, with a vertical extent of only about 100 m, appears to be controlled mainly by the west-dipping normal faults, either where they juxtapose Sleeper rhyolite and the andesitic to basaltic flow unit or, at shallower levels, where they cut the rhyolite itself. East-dipping faults are also mineralized, as exemplified by the West Wood breccia (see below). There is a suggestion that the main ore shoots coincide with the fault segments that underwent
maximum throw.
Three-dimensional visualizations of multi-element geochemical data (provided by Robert Jackson) and blast-hole assays for gold and silver (provided by Vic Chevillon), generated
using GoCAD pattern-recognition software, show that additional structural directions influenced gold deposition in the Sleeper deposit. Clearly, east-west and northeast structures
contiguous with the main north-striking vein structure were also locally dilated at the time of mineralization. There is a suggestion that the northeast structures underwent minor sinistral strike-slip motion during mineralization, thereby resulting in the weakly sigmoidal shape defined by the Sleeper and Wood veins. The strike-restricted, steeply plunging geometries of the highest-grade gold mineralization at Sleeper suggest control by intersections of the northstriking
and transverse structures.
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It is clear that the current structural architecture of the Sleeper district existed at the time of the gold mineralization. Nevertheless, an undetermined amount of post-mineral displacement has also taken place, although this is apparently fairly limited in the zone of the known gold
mineralization. In the Sleeper pit during mining, for example, a post-mineral normal fault was clearly visible in the footwalls of the Sleeper and Wood veins.
Alteration features In common with many low-sulfidation epithermal vein systems, illite alteration accompanies
the main gold mineralization at Sleeper. The lower-temperature zones peripheral to the main gold mineralization are characterized by smectite according to the results of an ASD
spectrometer survey of selected drill core. The spectrometer also reportedly detected ammonium-bearing minerals, including buddingtonite, in proximity to the main veins, where
adularia is visually prominent.
The upper parts of the gold-mineralized zones contain abundant kaolinite, which has destroyed much, but not all, of the pre-existing illite and smectite. The kaolinite occurs within the zone of supergene sulfide weathering, averaging about 100 m thick, as well as beneath it in the underlying sulfide zone. While some of this kaolinite, including local latestage
cavity fillings of massive kaolinite in the veins, may be attributed to the effects of downward-migrating fluids that originated in the overlying steam-heated environment, much
of it is believed to be of supergene origin. Supergene oxidation of the abundant iron sulfides associated with the gold mineralization (see below) would have generated abundant acidic solutions capable of widespread kaolinization, both above and below the water table existing at the time.
Larry Martin provided samples of altered rocks diagnostic of the steam-heated environment, which existed between the paleo-water table and paleosurface at the time the Sleeper system was active. Vuggy chalcedony and opal, in which cavities are lined with kaolinite and minor cinnabar and metacinnabar, were reportedly obtained from a shallow RC hole in the Bedrock Casino area, immediately northwest of the Sleeper pit, whereas the powdery cristobalitebearing rock rich in native sulfur out crops immediately east of the pit, where this writer observed similar material in situ during the early stages of the Sleeper mining operation.
These occurrences of steam-heated alteration are interpreted as the basal erosional remnants of a formerly thicker, blanket-like horizon that capped the entire gold-bearing zone. The
thickness of this former steam-heated horizon cannot be determined with any degree of certainty, although 50 m might be a reasonable estimate given the structurally depressed
setting of the Sleeper district.
Gold mineralization
In marked contrast to most low-sulfidation epithermal gold districts, Sleeper hosts two distinct albeit closely associated mineralization types: sulfidic breccias and stockworks and sulfide-deficient chalcedony-adularia veins.
Only the latter type is characteristic of most low sulfidation
deposits. The hydrothermal breccia ore and its transitions to stockwork-style mineralization are characterized by the introduction of abundant pyrite and marcasite intergrown with
chalcedony. The latter, where unoxidized, is gray to black in color due to fine impregnation by the iron sulfide minerals. The iron sulfides typically constitute 10-15 volume % of the |
