breccia.
Gold values are typically modest in the breccia ore alongside the Sleeper vein, although bonanza-grade intersections have been obtained recently from similar material at the West Wood breccia prospect. The Ag/Au ratios are generally somewhat higher than in the sulfide-poor veins.
The chalcedony-adularia veins tend to be rather irregular and impersistent structures (Fig. 1), displaying crustification and colloform textures in common with most low-sulfidation epithermal veins. The sulfide content probably does not exceed 3 volume %, most of it comprising silver-bearing minerals. Spectacular, coarse-grained visible gold, giving rise to multi-ounce assays, characterized the veins, and in the Sleeper vein occurred as semicontinuous colloform bands. Although the hypogene electrum was shown to have lost some of its silver content as a result of supergene weathering, the observed distribution and grain size of the visible gold are inherited hypogene features.
Most of the chalcedony-adularia veins appear to transect the breccia-stockwork mineralization and, hence, are younger. Nevertheless, the occurrence of banded chalcedony vein clasts, some containing visible gold, in breccia near some of the crosscutting veins shows that there was some temporal overlap between the two mineralization types. It is speculated that the breccias, emplaced as a result of fluid-overpressuring events in the hydrothermal system, tapped fluids containing a greater magmatic component, a proposal supported by the exceptionally high Mo (up to 0.4 %) and U (up to 11 %) contents in part of the West Wood breccia. If the suspected tourmaline or dumortierite observed in samples of the small Blue vein, in the western part of the Sleeper pit, are confirmed, the boron required for their precipitation may also have a direct magmatic origin.
EXPLORATION POTENTIAL
General considerations
Notwithstanding the extensive exploration, including a total of >400,000 m of reverse circulation (RC) and core drilling (in >3,000 holes), conducted by Amax, X-Cal Resources, and the New Sleeper Gold/X-Cal Resources joint venture in the Sleeper district, additional gold potential is believed to still exist. The drilling programs carried out by Amax appear to have confirmed that the Sleeper and Wood veins lack both along-strike and down-dip extensions of any consequence. The mineralized zones defined by X-Cal Resources and New Sleeper Gold/X-Cal Resources at West Wood and Facilities demonstrate that additional auriferous structures are present, although these two bodies are clearly subeconomic on a stand-alone basis. As documented in a recent report by Jeff Hedenquist (December 2005), most of the drilling to date has been shallow and did not penetrate more than approximately 250 m vertically into bedrock as well as being largely confined to the immediate vicinity of the Sleeper pit. Recent experience in comparable low-sulfidation gold districts, such as Midas in Nevada and El Peñón in northern Chile, highlights the fact that major high-grade veins continue to be discovered after many years of intense and well-directed exploration effort and several hundred thousand meters of exploratory drilling. Such discoveries typically result from improved understanding of the district geology.
The exploration recommendations made below are presented in the context of the current knowledge of low-sulfidation epithermal gold districts worldwide. They take a broader view of the likely overall dimensions of the Sleeper district as well as expanding the depth interval over which economic gold mineralization might be anticipated. Nevertheless, for well founded practical and economic reasons, all recommended targets are restricted to the eastern parts of the pediment, where no more than about 150 m of post-mineral cover are present (see above). The work conducted since the cessation of mining at Sleeper has resulted in an enormous increase in geologic knowledge of the district, much of which has yet to be
brought to bear on target definition. The current database, which is close to being finally compiled (see below), will underpin future exploration efforts and greatly facilitate the
targeting process.
Exploration implications of geologic model When the Sleeper district is viewed in the context of major low-sulfidation epithermal gold districts worldwide, a number of features relevant to exploration become apparent and need to be taken into consideration in the design of an exploration program:
• If the Sleeper and Wood veins are considered as separate ore shoots on a single northstriking structure, as seems likely, the Sleeper district contains only one substantial
vein. Most low-sulfidation districts contain a minimum of two major veins and many of them comprise three or more. Hence, the existence of at least one more major vein in the Sleeper district is considered probable. Bearing in mind that all mineralization is concealed beneath post-mineral cover and cannot be prioritized on the basis of its surface expression, there is every chance that an undiscovered vein could be longer, wider, and/or higher in grade than the Sleeper structure.
• The veins in most low-sulfidation gold districts tend to be either subparallel to one
another and/or follow directions within 45º of one another. Veins perpendicular to
one another are unusual. This observation implies that any additional veins in the
district are most likely to strike northerly or within 45º either east or west of north.
Hence, drill holes oriented at right angles to the Sleeper structure, that is to say eastwest,
are unlikely to miss any additional major vein that may exist.
• In districts containing several subparallel veins, the ore shoot(s) in each vein tend to
lie opposite one another to form mineralized corridors running across the districts at
high angles to the veins. It is evident from Figure 1 that this would be the case in the
Sleeper district should the proposed exploration targets prove to be ore bearing.
• The fact that the roots of a steam-heated horizon are preserved at Sleeper implies that much of the original vertical extent of the ore shoots is preserved, although the existence of small volumes of nearby detritus containing vein clasts shows that the tops of some shoots were eroded. This situation, which is likely to persist westward where downfaulting becomes progressively greater, maximizes the amount of gold ore present. However, it also makes exploration more difficult because it increases the likelihood that ore shoots may be blind and thereby concealed beneath barren or poorly mineralized bedrock. It is important to stress that the original elevations of the tops of low-sulfidation veins can vary by at least 200 m in some districts (e.g., El Peñón). Consequently, any additional major vein(s) in the Sleeper district will not necessarily have ore shoots spanning the same restricted elevation range as the Sleeper and Wood shoots. Indeed, the apparent top of the West Wood breccia lies 150m lower in elevation than the pre-mine top of the Sleeper and Wood shoots, although it remains to be determined if this is an original feature or the product of post-mineral faulting. No matter which is the case, it is clear that the practical significance of the
post-mineral fault displacement in the district is reduced.
• The existence of two gold mineralization types, the sulfidic breccias and stockworks
and low-sulfide chalcedony-adularia veins, in the Sleeper district poses a question for
the explorationist. Is it possible that the low-sulfide veins may not be surrounded
everywhere by the breccia-stockwork mineralization, but may also occur alone as
they do in most low-sulfidation epithermal gold districts? If occurring alone, the veins
would likely lack appreciable alteration and pyrite-marcasite halos. This possibility,
which needs to be kept firmly in mind, would obviously invalidate the use of
geologic, geochemical, and geophysical vectors developed specifically for the Sleeper
vein.
Priority exploration targets
During this review, a consensus was reached on the priority targets for future exploration in
the Sleeper district. The targets were developed using the currently available geologic,
geochemical, and geophysical databases, although all of them have been considered
previously. Particularly influential for target definition were the current geologic model and
inferred ore controls for the district, as summarized above, in combination with a structural
interpretation incorporating geologic mapping, magnetic, and seismic data by Charles
Tarnocai (Placer Dome Exploration) and a more recent structural interpretation based on
magnetic and gravity data by Jim Wright, the latter enhanced using GoCAD imaging by Vic
Chevillon.
The five principal exploration targets (Fig. 1) are summarized below, although one or more
of them may undergo some refinement once the various three-dimensional databases are
eventually merged (see below). All the targets are considered prospective for either one or
both types of Sleeper gold mineralization, although the existence of additional styles, such as
auriferous mantos (e.g., along the top of the metasedimentary basement), cannot be
precluded. It is difficult to prioritize these five targets, although their order of treatment is
probably a fair approximation.
• The West Graben target is centered on an inferred north-striking structure that runs
immediately west of the tailings disposal area. It is located approximately along the
break between the shallow (<150 m) and deeper post-mineral cover. The target is
defined on the basis of a north- to north-northeast-striking air-photo lineament
defined by Larry Martin, which coincides well with normal faults inferred by Charles
Tarnocai and a steep gradient apparent on the three-dimensional gravity
interpretation. The potentially mineralized fault is believed to mark the eastern
boundary of a graben, which is separated from the main Sleeper graben by an intervening horst. The West Graben target is
untested.
• The Northwest target is defined on the basis of a residual gravity feature and a magnetotelluric resistor. Recent drilling has intersected narrow, banded chalcedony veins containing up to 1 g/t Au, which are flanked by smectite alteration. The veins dip steeply eastward, suggesting that the controlling structure may be a hanging-wall splay off of a more important west-dipping fault. Vein-type mineralization on the west-dipping fault constitutes the principal target. The low-temperature character of the alteration halo may imply that any ore shoots are likely to occur either laterally or beneath the vein intersections or, alternatively, that the gold-bearing veins are late, low-temperature features lacking appreciable gold potential.
• The Southwest target appears to be an elongate, horst-like feature beneath a minimum
of only about 125 m of post-mineral cover based on the magnetic interpretation. The
inferred structure appears to be geometrically similar to the Sleeper vein in map view.
A magnetotelluric resistor, possibly defining a silicified zone, alongside a resistivity low defines the target, with these features being separated by a north-striking residual gravity linear. The inferred structure is further emphasized by well-defined mercury vapor and soil gas (CO2) anomalies. The Southwest target lies well beyond the outer limits of the current drilling.
• The Range Front target coincides with the range-front fault that juxtaposes the outcropping metasedimentary basement and largely concealed Miocene volcanic package. The target is defined on the basis of the exposed tectonic and, possibly,
hydrothermal brecciation, silicification, limonite after iron sulfides, and localized chalcedony veining. The structure is also defined by a broad, linear chargeability high |
