E-SCAN® 3D earth geo-electric mapping
ParadiseGREENFIELDS and BROWNFIELDS
GOLD non-placer
When inherently-permeable tuffaceous volcanics were flooded with mineralizing fluids, silica, silver and gold precipitated
throughout the receptive rock volume, up to the less-permeable constraining rock units. As a result, the entire resistive (silica-sealed) zone
is potentially ore grade. This uniform "sponge" model is thought to be a rather rare setting, compared to the more common fault- or fracture-hosted
settings where structural conduits channel fluids and also host the bulk of the precipitated metals, within a broader low-grade or no-grade resistive
(silicic) envelope.
At right are two vertical sections (from the 3D earth model) showing the three main mineralized zones as imaged by 3D E-SCAN at the Paradise Peak
minesite, Nye County, Nevada. The host volcanic background resistivity is 5 to 20 ohm-metres (red). Blue-grey anomalies are 1000 to 2000 ohm-m.
The model's upper cell dimensions are 100 by 100 feet, by 33 feet deep.
The blue-grey high resistivity anomalies are caused by intense silicification that effectively seals the connected open pore space, excluding moisture
and therefore inhibiting electrical conductivity. Note the identical signature of the non-mineralized siliceous body at left,- a reminder that in
epithermal settings we are measuring the bulk rock resistivity (connected open space), while any metal content is usually undetectable (except in the
case of sulfides, and especially massive sulfides).
It may be hard to imagine that the Paradise Peak orebody would not be detectable by the conventional survey methods that were tried.
Explorationists were so surprised that the airborne EM test survey failed, that they contracted a second, competing system of state-of-art multi-frequency airborne EM to verify.
This also failed to discriminate the resistive orebody from the surrounding shallow gravel signatures.
Ground geophysical methods, usually expected to be more reliable than airborne equivalents, also did not work.
3D E-SCAN produced useful results from a standard, no-extras survey, requiring less than 3 weeks in the field. At right, accurate imaging of the
southward dip, under cover, of the Ketchup Flat orebody is a typical example of the high resolution and accuracy of E-SCAN 3D imaging.
The imaging of a buried copper-gold refractory mineralization zone marks another example of 3D E-SCAN competence, found within in the same survey results.
Imaging a resistive target at surface eluded the best applied techniques at Paradise Peak... here the resistive target is buried beneath 150 feet of extremely
conductive desert cover, a conventionally "near-impossible" setting with virtually no expectations of success with conventional tools.
However, for 3D E-SCAN, this is a predictably routine result. See the plan and section views for a better view of the depth of conductive cover overlying this
zone of different and unexpected mineralization.
The complete sequences of plan and section views through the Paradise Peak property and its ore zones can be
reviewed here.
A common mineralization setting involves leakage of fluids into the host rocks that enclose a fracture zone or conduit. Most of the
precious metals may be precipitated within the feeder conduit structure, while a lesser volume of fluid moves more slowly into the available open
fractures in the surrounding lithology. Within the surrounding host rocks, the slower moving fluids cool, de-pressurize and precipitate dissolved minerals,
effectively sealing the open spaces and cutting off fluid flow flow. Eventually, all fluid conduits cool and self-seal.
Sealing = reduction of connected open spaces = elevation of resistivity signature. Since all areas are now somewhat sealed with precipitated silica (with and without metals), the overall resistive signature appears significantly wider than the economic resource zone that is usually located in the main fluid conduit structure(s). It all looks the same to a resistivity survey.
This broad envelope is helpful in detecting the occurrence of narrow structures of economic mineralization, using cost-effective wide-spacing surveys.
The altered (silicified) envelope can range from 2 to 10 times as wide as the main pay zone (Freedom Flats, Borealis Trend, Nevada) to 100 times the width (Phoenix Deposit British
Columbia - see next tab). Once a resistive zone is mapped, the question is this: Within the alteration envelope, where is the once-open
conduit that brought the mineralizing fluids into the area? Follow-up exploration therefore involves first drilling across
the envelope to intersect any core feeder structure and to confirm a hydrothermal origin for the resistive feature.
At right is a vertical section through the abandoned pit of the Freedom Flats mine, Borealis trend, Nevada. The upper cells are 100 feet wide, 33 feet thick;
grey-blue is high resistivity at greater than 2000 ohm-m, and red is about 10 ohm-m. Mine waste dumps are seen in the model, piled high to the left of the pit.
The resistive layer under the pit floor is silicic ore, deliberately (if reluctantly) left in place when the mine was shut down early due to slope instability.
Note the lobe of resistive (probably silicic, gold-bearing) material that is seen extending west at depth, under the pit wall and overlying dumps.
Such is the definitive signature of the Borealis-type deposit,- that of a "Christmas Tree" - a central vertical trunk with mineralized limbs spreading laterally
from the core.
Freedom Flats is a case example of the map-everything 3D E-SCAN strategy revealing a second mineralization configuration that is different from that of the initial discovery.
The case confirms that it is worthwhile to apply comprehensive and verifiably objective true 3D resistivity mapping, starting right over the new resource, immediately upon discovery,
to obtain a more complete picture of the resource setting for further investigation, mine planning (and financing).
Given sufficient grade in this unexpected buried lateral lobe, initial mine design might have been altered to allow widening of the pit to the west to exploit
the additional ore. Almost certainly, the waste dumps seen in the section would have
been piled in a location that the 3D imagery suggested was barren, and then was formally drill-condemned.
Timely information is everything, and often in the excitement of a high-grade discovery, especially for an explorer looking to incrementally finance the next
stages, exploration stops for development and production... and opportunities like this can be lost. Opportunities that can be automatically on the table... if
a sufficiently competent True 3D resistivity survey option has been selected at the start.
At left: Lawyers property, Toodoggone district, British Columbia. This is a draped plan view lying 100-125 metres below surface, through the 3D inversion earth model.
Host volcanic background (yellow) resistivity is 700-1000 ohm-metres, far higher than Nevada's more typical 20-40 ohm-metres. Blue anomalies are 2000-3000 ohm metres, a comparatively modest ratio over background levels. The image covers 800m x 1000m (station dots are 100m by 100m). The area is located just southwest of the operating mill facility. The Phoenix gold deposit is 2m wide by 60m strike length, with exceptionally high grade. The original Lawyers deposit and mill complex are located off the upper right corner of the image.
A cluster of resistive centers, each 100m to 200m wide, clearly map silicified envelopes surrounding fossil hydrothermal fluid conduits, mimicking the signature at the Lawyers deposit, mapped earlier by 3D E-SCAN. The cluster of hydrothermal indications probably overlies a deeper local heat source. Prior geophysical exploration in the district concentrated on trying to directly detect the meters-wide gold-mineralized structures that are typical of the area. Entire small-spacing survey grids had been shot within the confines of the blue resistive alteration envelopes, seen at left as we know them now. These
small surveys never got out of the resistive envelopes, so there was never an indication that the envelopes themselves were anomalous. It took
E-SCAN's bigger view to establish the context and identify the envelopes as the sole geophysical indicator of
possible mineralization. An aside for those readers who remain unsure of the difference between raw field data plots and the results of true 3D inversion
processing,- here is a comparison between the raw data plotted in plan, and an equivalent slice through the
inverted 3D earth model. 
A radically wide-spaced (for this narrow target) 100m grid 3D E-SCAN survey was tested over the main Lawyers deposit, a 15 meter wide chimney system of high
grade gold and silica, revealing the first glimpse of a 100m to 200m wide resistive alteration envelope enclosing the ore structure. With that model in hand, look-alike anomalies were identified as survey coverage was expanded wider. Duke's Ridge, a non-economic (at the time) gold-bearing pipe was the second known case example to be imaged, again displaying a roughly circular resistive envelope, though in this case extended somewhat along the known structural trend. Other more elongate envelopes were mapped, their strike also consistent with the known regional fault set, and sometimes with indications of cross-breaking structure (b to c).
All of the anomalous features "a" through "i" (except untested feature "g", which is deeply buried under cliff-bottom debris) showed evidence of silicic
alteration and mineralization near the center of their envelopes. The Phoenix Deposit ore zone was discovered by drilling across the strike of the envelope
to detect the center structure responsible for the silica flooding. Ore was was subsequently profitably mined
and milled on site, with high grade concentrate flown out for refining.
The unit cost of the successful 100m 3D mapping in the district was less than 10% of what was paid (per square kilometre) for the failed detailed-scale
geophysical surveys of the past. This regionally-proven mapping technique came about because the client was willing to pay to re-survey the known orebody
in the hope of seeing a useful exploration signature. It paid off doubly, first by seeing a completely unexpected,usable signature for the type ore setting,
and second, by virtue of confirming a lower-cost survey method, for exploration of the rest of the large property.
We have seen how a resistive anomaly may enclose the ore zone in a resistive envelope. Here, the upper parts of a high-grade basement fault system seem
to present an enclosing envelope of resistivity, but deeper parts are geo-electrically neutral. Above the interface with overlying volcanics,
a resistive bloom evidences the introduction of hydrothermal fluids from below. This could be considered a "Hishikari" case where the overlying unit
is more permeable, less capable of stopping the upward flow at the interface.
The Gwenivere-Clementine vein deposit (Hollister area, Nevada, Great Basin Gold) is an example of 3D resistivity imaging in a geo-electrically complex
setting. A somewhat weathered, permeable volcanic unit overlies a tight basement rock. In the basement, fault conduits have delivered thermal fluids
into the overlying volcanics, altering the rock, precipitating silica, and depositing low levels of disseminated gold. Within the basement, high grade
gold has been deposited with silica within the narrow (few meters wide) feeder structures, forming the reserves of the newly-proven mine.
In a classic hydrothermal system setting like this, three possible zones of economic interest occur: a large volume cap-rock alteration zone, the immediate
contact or unconformity area (Hishikari model), and the narrow conduit, potentially-bonanza deeper structure in the basement rocks (Ken Snyder,
Hollister model). The default expectations for this surveyed area are therefore:
(1) a precipitate-sealed local cap rock zone to present a distinctive resistive anomaly,
(2) an open or non-silicified basement fault feeder structure to show a measurable conductive anomaly (resistive if
silica-sealed), and
(2) deep unconformity alteration/mineralization likely to remain indistinguishable from surface due to its small volume.
A classic volcanic-hosted epithermal gold orebody (Name and location classified) extends from near surface to several hundred feet below, in the
location outlined by the rectangle on the topo map. The survey area itself is about 5400 feet square, in steeply dissected topography. The level of
the data plan view at right lies 700 feet below the ridge-tops, and well below the drilled-off orebody.
In the interest of establishing a type signature for extended exploration in the district, a known orebody was subjected to several geophysical surveys,
including EM and gradient resistivity. No anomaly was apparent, - that is, the orebody showed about the same level of resistivity as the enclosing
unaltered volcanics. This surprised the operators, given the glassy silicic nature of the bulk ore, extremely resistive in hand samples... it seemed
very likely that it would have presented some sort of high resistivity in-situ response. 3D E-SCAN was brought in to provide a more intensive
evaluation. A standard-configuration 3D E-SCAN survey confirmed that the prior geophysics had been right all along... there is no anomaly
associated with the orebody. The ore zone displays a resistivity level similar to that of the enclosing volcanic unit, i.e. 200 to 400 ohm-metres.
It seems that the fracture zone that hosts the orebody had been imperfectly re-sealed (with precipitated silica) to approximately the same resistivity
level as the original unaltered, moderately-fractured host volcanics, as a pure coincidence. Bad luck for explorationists, but no mystery here, once
sufficient field data were acquired to confirm an answer.
As always, extra-deep field data were collected by the 3D E-SCAN survey in order to be able to properly constrain the lower side of the ore zone in the
3D inversion's earth model. These data sampled to an effective penetration (nominal depth of investigation) of about 2000. Who would have guessed that
these "housekeeping" data would force the 3D algorithm to recognize an unexpected aspect of the ore zone,- a very conductive vertical sheet directly
underlying the axis of the ore mineralization. Those familiar with working with conventional sparse data sets know that this could simply be an overshoot,
an artefact of a resistive orebody that is unconstrained below (as would be the case with most conventional surveys). But there was no strongly resistive
anomaly to "encourage" such an overshoot, and furthermore, there were more thousands of hard measured data obtained in the levels between 1000 and 2000
feet below surface than were obtained through the actual ore levels! Such unconstrained behavior simply can not exist with such a comprehensive hard data
set constraining every aspect of the mid-depth 3D earth model. The conductor almost certainly exists.
Rock resistivity signatures depend upon some combination (infinitely variable) of rock chemistry, rock quality (fracture density, weathering), and the
varying temperature, pressure, chemistry, and pulse duration of the introduced hydrothermal fluids. We generally expect that silica sealing will result
in an elevation of resistivity, and that almost certainly is the case here with reference to what the system's resistivity was probably like (much lower)
during actual thermal fluid inundation. The chance that the imperfect re-sealing of this fractured zone would end up at about the same level as adjacent
unaltered rocks is low, but the odds are hardly worse than perhaps 1 in 10, for a rough match like this. Still, it fooled the expectations of earlier
geophysicists, to the point of doubting their field data.
To the author's knowledge, the presence of a conductive sheet below the ore zone was never investigated. Geologically, there is no difficulty envisioning
a fault plane lower in the epithermal system that remains quite conductive - perhaps alteration clays providing
the conduction.
With most of its gold deposition occurring under hypothermal conditions in shears in Archean meta-sediments, the setting for the Giant Yellowknife orebody
is quite different from that of the younger, simpler volcanic-hosted epithermal gold deposits of Nevada's Basin and Range province. Yet in geo-electric detail,
in the core box, the ores from both areas look and behave similarly. Quartz is abundant, and a resistive signature is expected.
Resistivity is a measurable quantity. An anomaly is dimensionless, a relative thing. 500 ohm-metres in Nevada's average background volcanic resistivity of 20 to
40 ohm-metres would be a 10:1 resistive anomaly. Here, metasediments and metavolcanics exhibit background signatures of 50,000 to 80,000 ohm-metres.
The 500 ohm-metre silica-healed shear zone signature for the Giant orebody is, in this Archean setting, a relative conductive anomaly,
in fact a 100:1 ratio conductive anomaly.
This final silica-gold signature amplifies the broader message: Whenever hydrothermal activity impacts any geological environment,
chances are that the affected zone will display geo-electric characteristics that are different from those of the host lithology.
If one applies a geophysical survey tool that can provide objective, subtly-resolved mapping of the entire conductivity-resistivity spectrum (regardless of
surface conditions), there is an excellent chance that a pattern of hydrothermal alteration, even a subtle pattern,
can be recognized against the equally fully-resolved imagery of the host geologic package. Selected details within that alteration pattern can then be methodically
drill-tested, in order to develop a geologic model and identify ore possibilities.
In many cases, absolute values of resistivity or conductivity don't matter.
In many cases, anomaly ratios don't matter.
In many cases, it's the simple, recognizable patterns that deliver key insights to the explorationist.
With no precedent DC IP or resistivity data available from the Comstock Trend, a comprehensive 3D mapping with IP and resistivity was undertaken over the low-grade gold mineralization of the South Dayton resource area (Comstock Mines Inc.)
3D resistivity provided strong graphical correlation and confirmation of many known structures striking in several orientations, but it failed to present any useful direct correlation with the known gold mineralization.
3D IP reacted to the sulfides that are known to accompany the gold mineralization at South Dayton, providing IP anomalies that correlate roughly with the mineralized zones. While there is not a sharp 1:1 correlation of elevated IP with gold values, the presence of elevated IP here is sufficient for use as an exploration guide elsewhere on the trend, in similar lithologies, until such time as the correlation is shown to be unreliable as a focused guide to mineralization.
In the images, the IP anomaly at lower right shares size and intensity characteristics, and lithology, with the resource-linked anomalies to the north, making it a target for follow-up. 3D resistivity will reveal a previously-unseen NE-striking structure that intersects the NW trending preparatory fault (seen on the images) close to the IP anomaly, elevating the level of interest in this particular IP anomaly.
The IP anomaly at lower left appears to be of similar size at 25-31 meters depth below surface, but at 50 meters it has expanded to a larger dimension, and will continue to swell at deeper levels, where it merges with a very large buried conductive unit of (unconfirmed) metavolcanic origin. This places the IP anomaly in a different category from those that represent the known-resource signature at South Dayton (north end). For this IP anomaly, the different shape and the merging with a deep conductive unit demands investigation as to mineral and alteration characteristics, and possibilities of occurrence of economic gold mineralization of an origin and type that is different from the South Dayton example.
There is no limit to the number of potentially economic signatures that may be found along the Comstock Trend, if only they are actively sought out and then tested. That is exactly what the full spectrum 3D IP and resistivity survey is doing,- objectively mapping every pattern and correlation (IP and resistivity) across all lithologies and local alteration/mineralization regimes. If there are new and recognizable geo-electric signatures for gold ore settings, even subtly expressed, true 3D mapping will capture all of them for cataloging and future comparative study. There may be one or more new mineralization signatures within this first 3D geo-electric data set; time will tell as the many untested possibilities are examined in the field.
Anyone studying the geology and mining history of the exploitation of the Comstock Lode knows that the geo-electric players are many, and probably extreme in nature, with intensive hydrothermal alteration imposing its overwhelming impact across all fresh-rock signature expectations. The spectrum of signatures ranges from very high resistivity for intact and minimally altered quartz accumulations, through to the strongly conductive zones where normally-resistive rocks are fractured and intensely altered by thermal fluids, the resulting conductive clay products being added to conductive fault gouge to deliver what would appear to be, in large scale, a probable strongly conductive expectation for most Comstock bonanza zones. The actual signatures will need to be confirmed by 3D geo-electric survey mapping.
See Example # 8 (below) for more details on the Comstock case experience.
Above - FMC Gold's Paradise Peak silver-gold orebody during active open-pit mining, circa 1989, about the time that Ketchup Flat was discovered.
Right - The Paradise Peak and Ketchup Flat silver-gold orebodies after mining finished.
Despite close proximity, Ketchup Flat required years of exploration drilling to discover. Using Paradise Peak as the test model, airborne EM surveys were tried,
various ground geo-electric surveys,- nothing could image this very large, effectively at-surface orebody. With geophysics coming up short, there was no
choice but to explore the property by drilling. Rigs drilled stepout exploration holes for several years before finding the Ketchup Flat orebody.
About the time that stripping commenced for the new Ketchup Flat orebody, FMC Gold ran a test 3D E-SCAN survey that covered both deposits. The purpose was to
see if the newer technology could acquire an exploration signature on either of the known orebodies.
The test was successful, though it was a couple of years before true 3D inversion imaging could be applied to the field data to provide the images shown here.
At right, a plan view slice through the 3D resistivity earth model shows clear and unambiguous imaging of the Paradise Peak deposit, obvious outlining of the
Ketchup Flat deposit, and imaging of previously unknown copper-gold mineralization lying deep beneath conductive cover.
Below - This composite resistivity section runs through the Paradise Peak and Ketchup Flat silver-gold orebodies. Host volcanic background resistivity is 5 to
20 ohm-metres, in red. Blue-grey anomalies are 1000 to 2000 ohm-m. Upper cell dimensions: 100 feet wide by 33 feet deep. The blue-grey high resistivity
anomalies are caused by silicification that effectively seals the connected open pore space, excluding moisture and therefore inhibiting electrical conductivity.
To the right, the south dip of the Ketchup Flat orebody under cover is accurately depicted in the E-SCAN 3D imaging. Note the identical signature of the
non-mineralized siliceous body at left,- a reminder that in epithermal settings, the relevant measurement is bulk rock resistivity (connected moisture pathways),
and never actual metal content.
Working through the operating Paradise Peak open pit mine was uneventful. A complete true 3D field data set was acquired through and under the deposit, with no
disruption to the mining schedule, including daily production blasting.
Paradise Peak was one of the first Nevada surveys for 3D E-SCAN. This post-discovery 3D E-SCAN test case history is extensively discussed on this site, under
various technical aspects, and in a comprehensive set of plan and section views presented as slide shows, which you can access here.
The best possible application of exploration geophysics is surely that which consists of simply looking for repeat examples of an already-documented local resource signature:
"The on-site resource looks like this,
so let's look for more of the same, nearby."
Hasbrouck Peak exhibits a one-to-one relationship between gold deposition and silicification. Virtually all gold intercepts lie within resistivity anomalies. All tested resistivity anomalies are mineralized.
On a district exploration scale, all resistive anomalies will warrant investigation.
A comprehensive discussion of historic 3D resistivity results at Hasbrouck Peak, including plan and section views,
is available here.
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The Sleeper-area strategy sheet (far right) shows how 3D E-SCAN can be deployed to effectively map nearby covered areas that may be positionally-prospective,
but which have little or no other available information on which to base specific drilling tests. This remarkable case shows the process from land
acquisition, to 3D geophysics, drilling, assay, evaluation, and the decisions to keep/drop acreage,- in one fast, reliable low-cost exploration sequence that
is specifically tailored for volcanic-hosted cover plays... like Sleeper.
Download the "B- or A4-size" PDF image of this strategy sheet.
The project data re-assessment sheet illustrates how older resistivity survey data can be reviewed for effectiveness of purpose, for correctness of
interpretation under current scientific understanding, and for an understanding of the significance of these shortfalls in terms of present-day opportunities
for re-opening effective exploration. This sheet shows that the initial concepts for resistivity mapping were insufficient to test the setting, and that the
interpretations that were presented are now generally known to be fundamentally misleading. In summary, while experts of the time correctly agreed that
resistivity survey was a desirable tool, its actual application and interpretation were deficient to the point disservice to the exploration effort.
Today, with high density, all-directional true 3D field data sets and modern 3D inversion algorithms, we know that the historical resistivity technique
applied at Sleeper never had any real chance of working,- not for a buried resistive target.
Questions and answers:
Has resistivity been tried and found unhelpful at Sleeper? Yes.
Has some form of proven-effective DC resistivity, one that addresses all three deficiencies of the 1980's
resistivity surveys, been tried at Sleeper? No.
Could a proven-effective 3D E-SCAN DC resistivity survey include the entire historical production area, flooded pits, leach pads, buildings, and the
surrounding wetlands, deep cover and hills? Yes.
Freedom Flats is another of the geophysically-invisible settings that 3D E-SCAN resistivity survey has mapped as a demonstration of its unique ability to
detect and define resistive targets in virtually any setting.
When there has been substantial leakage of fluids into the host rocks that enclose the mineralized fracture zone or conduit, the
resistive signature may be wider than the resource. This greater width/volume of elevated resistivity is helpful in detecting narrow high-grade orebodies
with economical wide-spacing surveys.
The altered envelope can be just 2 to 5 times as wide as the pay zone (Freedom Flats, Borealis Trend, Nevada) or 100 times the width (Phoenix Mine, British
Columbia). Follow-up exploration involves first drilling across the envelope to intersect core feeder structure(s) and confirm a hydrothermal system origin.
At right is a vertical section through the abandoned pit of the Freedom Flats mine. The upper cells are 100 feet wide, 33 feet thick; grey is high resistivity,
greater than 2000 ohm-m, and red is about 10 ohm-m. Mine waste dumps are piled high to the left of the pit.
The resistive layer under the pit floor is silicic ore, left in place when the mine was shut down a little early due to slope instability.
What is remarkable here is the lobe of resistive, silicic, gold-bearing material that is seen under the west pit wall.
Freedom Flats is a case example of the map-everything 3D E-SCAN system revealing a second mineralization configuration that is quite different from that of the initial discovery.
The case shows that it can be worthwhile to apply comprehensive 3D resistivity mapping, starting right over the new resource, immediately upon discovery,
so that you have a more complete picture for planning (and financing). Given sufficient grade in the buried lateral lobe, mine design might have been altered
to allow widening of the pit to the west to exploit the additional ore. Almost certainly, the waste dumps seen in this section would have been piled in a
location that the 3D imagery indicated was barren (then subsequently drill-condemned) to preserve future options regarding this deep extension area.
Timely information is everything, and often in the excitement of a high-grade discovery, especially for a junior explorer looking to incrementally finance next stages,
exploration stops for production and some opportunities can be lost.
Having established an effective means of mapping Borealis-style epithermal gold deposits, the surrounding covered area becomes prime exploration ground.
Download this PDF strategy sheet.
Update current claims status to see what areas are currently available for effective 3D resistive-target mapping.
Note that while the thickening cover depth to the southwest represents a mining costs issue, and certainly stopped exploration in the 1980's,
the area remains highly prospective if only there was a geophysical tool that was certain to map resistive bodies in these deeper conditions.
The case history results from the Freedom Flats orebody area (see tab, above) confirm that 3D E-SCAN work reliably for the near-surface Borealis trend orebodies.
For the expected deeper resistive targets (the conventionally "impossible" geophysical targets), the evidence of 3D E-SCAN's capability is illustrated in the
Paradise Peak case material found elsewhere on this website. The increased depth is irrelevant to 3D E-SCAN mapping, which has sufficient depth penetration
and deep target resolution to effectively survey anywhere on this map, including beneath the fresh lava flow in
the valley bottom. Given that the prolific Aurora gold district is just up the opposite side of the valley, the entire lower central valley area remains
positionally-prospective, and effectively unexplored.
The 3D E-SCAN survey over the Giant Mine property near Yellowknife, Northwest Territories once again delivers an unambiguous,
useful exploration signature for a previously geophysics-proof deposit. Even more remarkable is its performance in these conditions,- massive,
operational industrial installations, conductive tailings, slime ponds and swamps, and an active, two-shift underground mining operation with electric blasting hazards.
The image at right is about 550 feet below surface. The main ore zone shows as a yellow-orange, 500 ohm-meter feature. The host is an extremely
tight metamorphic complex, averaging 50,000 to 80,000 ohm-meters, the highest ever observed by 3D E-SCAN. It must be almost completely devoid of
connected open space. Except, of course, where the yellow and orange patterns show up.
In Nevada, 500 ohm-meters would be a very resistive anomaly, attracting considerable attention in a 20 ohm-meter background. Here, 500 ohm-meters
is actually a high-ratio conductor, 100 to 150 times more conductive than the host rock. It's not an EM target (that would be nice...),
however, since in absolute terms, it is too resistive to support the levels of induced current flow that would be sufficient to generate secondary
signals that would be detectable back at surface.)
Giant ore is hard, glassy silicified rock, easily explaining a 500 ohm-meters signature as a partially silica-healed fracture zone that once admitted
hydrothermal fluids from somewhere, long ago. Some open fractures must exist, perhaps having been remobilized after sealing, and being too brittle to
reseal under lithostatic pressure (probably unlike the surrounding more plastic metamorphics). One could expect that the other yellowish anomalous areas
might similarly be silica-healed fracture zones, perhaps also gold-bearing,- smaller satellite deposits. Some are.
This metamorphic package extends tens of miles into the distance, all of it as yet effectively unexplored. Explorationists now have the benefit of a
direct, proven, distinctive orebody signature for exploration use along the entire length of this specific setting.
At left: Lawyers property, Toodoggone district, British Columbia. This is a draped plan view lying 100-125 metres below surface, through the 3D inversion earth model.
Host volcanic background (yellow) resistivity is 700-1000 ohm-metres. Blue anomalies are 2000-3000 ohm metres, a modest ratio over background.
The image covers 800m x 1000m (station dots are 100m by 100m) located just southwest of the mill facility site. The Phoenix gold deposit is 2m wide by 60m strike length,
with exceptionally high grade. The Lawyers deposit and mill complex are located off the upper right corner of the image.
A cluster of resistive centers, each 100m to 200m wide, clearly map the silicified envelopes surrounding fossil hydrothermal fluid conduits. The cluster probably overlies
a deeper local heat source. Prior geophysical exploration in the district concentrated on trying to directly detect the meters-wide gold-mineralized structures that are typical of the area.
Entire small-spacing survey grids had been shot within the confines of the blue resistive alteration envelopes, seen at left as we know them now.
These small surveys never got out of the resistive envelopes, so there was never an indication that the envelopes themselves were anomalous.
It took E-SCAN's bigger view to establish the context and identify the envelopes as the sole geophysical indicator of possible mineralization.
A radically wide-spaced (for this narrow target) 100m grid 3D E-SCAN survey was tested over the main Lawyers deposit, a 15 meter wide chimney system of high grade gold and silica,
revealing the first glimpse of a 100m to 200m wide resistive alteration envelope enclosing the ore structure. With that model in hand, look-alike anomalies were identified as
survey coverage was expanded wider. Duke's Ridge, a non-economic (at the time) gold-bearing pipe was the second case example to be seen, again with a circular envelope, extended
somewhat along the known structural trend. Other more elongate envelopes were mapped, their strike also consistent with the known local fault set, with indications of cross-breaking
structure (b to c).
All of the anomalous features a through i (except feature g, deeply buried under cliff-bottom debris) showed evidence of silicic alteration and mineralization near the center of their
envelopes. The Phoenix Deposit ore zone was discovered by drilling across the strike of the envelope to detect the center structure responsible for the silica flooding. Ore was was
subsequently profitably mined and milled on site, with high grade concentrate flown out for refining.
The unit cost of the successful 100m 3D mapping in the district was less than 10% of what was paid (per square kilometre) for the failed detailed-scale geophysical surveys of the past.
This regionally-proven mapping technique came about because the client was willing to pay to re-survey the known orebody in the hope of seeing a useful exploration signature.
It paid off doubly, first by seeing a completely unexpected, usable signature for the type ore setting, and second, by virtue of confirming the effectiveness of a lower-cost wider-spaced
survey method for exploration of the rest of the large property.
E-SCAN's full spectrum of resolution is on display here. The pattern of blue-white high resistivities of silicic alteration and tight volcanic units yields in the upper left corner to
concentric rings of very conductive clay alteration surrounding a partly-resistive central hydrothermal feeder structure.
Mouse-over the image above. 
A classic volcanic-hosted epithermal gold orebody, described as typical of the district, extends from near surface to several hundred feet below, in the location outlined by
the rectangle on the topo map. The survey area itself is about 5400 feet square, in steeply dissected topography.
The data plan view at right shows the resistivity regime through the underground orebody, at 150 to 200 feet below surface. The image reveals no particular difference in signature
for the orebody as compared to the host volcanics... an area-normal 250 to 400 ohm-metres with some random zones of more conductive material (yellow) here and there.
Conventional surveys had failed to see the orebody. As it turns out, so did 3D E-SCAN. Here's why:
In the interest of establishing a known-ore signature to guide extended exploration in the district, this orebody was subjected to several geophysical surveys, including EM and
gradient resistivity. No anomaly was apparent, - that is, the orebody showed about the same level of resistivity as the enclosing unaltered volcanics. this surprised the operators,
given the glassy silicic nature of the bulk ore, which seemed likely to present some sort of high resistivity response. 3D E-SCAN was brought in to provide a more intensive evaluation,
while heads were scratched. A standard E-SCAN 3D mapping survey confirmed that the prior geophysics had been right all along... there is no anomaly associated with the orebody,
which responds at a level similar to that of the enclosing volcanic unit, i.e. 250 to 400 ohm-meters. The rationale that was agreed upon is that the silica-healed fracture zone
that hosts the orebody zone's fractures had been imperfectly re-sealed with silica to approximately the same resistivity level as the original unfractured host volcanics, as a pure coincidence.
Bad luck for explorationists, but certainly no geophysical mystery here.
That might have been the end of it. But despite the obvious focus on the orebody signature question, sometimes it seems quite impossible to stop 3D E-SCAN from mapping everything, everywhere,
including very deep beneath the zone of stated interest. You see, once the grid has been installed, we can double or triple the effective depth of the raw data set simply by waiting
a few minutes extra on each current shot. The system automatically scans electrodes to an increasingly wider radius, collecting deeper and deeper raw data in all directions, all over
the place, at a very fast rate of 6 to 10 measurements per minute. Nobody in the field has to work any harder... just wait a bit each time before starting the next input shot,
probably using the few extra minutes to leapfrog the next current shot into position. So the super-deep data get collected, even if the declared target is a "shallow" one like here.
Mouse-over the data image at right to show what we saw in these deep data, starting about 700 feet below surface. As the registration rectangle indicates, directly below and in line
with the overlying silicic gold orebody.
A north-south oriented vertical sheet of extreme conductivity (4 ohm-meters) directly underlies the ore zone, resembling reasonable expectations for the geo-electric signature
of the lower reaches of a classic epithermal system stack. The conductivity would be explained by the presence of intense clay alteration and remaining open fractures deep below the
precipitated silica zone, where the orebody is. Of course this deep plumbing regime could host bonanza gold deposits.
Good for you for suspecting an interaction with the overlying mine installation. But before you get too far, note that almost all of the data associated with imaging the deep
conductor originate at electrodes several hundreds to thousands of feet from the mine area itself. During processing, all data originating within 300 feet of mine workings were
eliminated and the 3D inversion re-run as a demonstration, to be sure. Same result. No industrial interference,- a verifiably valid 3D image.
Here's what is important here. The client originally wanted an exploration signature to use in the search for similar deposits in similar district conditions.
The next orebody may or may not have cooled down and re-sealed to the approximate same resistivity as the host rock, as happened here. However, if this tested system is "typical",
then other deposits may also have a conductive lower regime beneath them. At 700 feet to the top of the conductor, and with its very powerful conductivity-width characteristic,
a deep feature like this in a 250-400 ohm-meter host should be readily mapped by modern airborne EM systems. If true, imagine the cost-effectiveness of covering the entire district
from the air, instead of with 3D E-SCAN. Information is what 3D E-SCAN mapping is all about, and in this case, the information may indicate a faster and cheaper way to conduct
effective exploration in this district.
The details of this project area remain confidential. As for the object lessons, there are two:
(1) There is no universal signature for epithermal gold mineralization,- you need to keep your eyes open and maximize survey information to keep on top of possibilities.
(2) In the world of deep, high-resolution 3D earth resistivity mapping, you never know when you might be handed a time- or money-saving insight from out of left field.
The Hollister Mine (Great Basin Gold) area offers an example of 3D resistivity imaging in a geo-electrically complex setting. A somewhat weathered, permeable volcanic
unit overlies a tight basement rock. In the basement, fault conduits have delivered thermal fluids into the overlying volcanics, altering the overlying rock, precipitating
silica, and depositing disseminated gold in irregular patterns. Within the basement, high grade gold has been deposited with silica within the narrow (few meters wide)
feeder structures, forming the reserves of the new Hollister Mine.
In a classic hydrothermal system setting like this, three possible zones of economic interest occur: a potentially large-volume cap-rock alteration
pattern (Hollister, left), the immediate unconformity area (Hishikari model?), and the narrow conduit structure in the basement rocks (Ken Snyder, Hollister model).
The default geophysical expectations for epithermal gold are for
1. shallow a precipitate-sealed capping rock unit to present a distinctive resistive anomaly.
2. mid-depth contact/unconformity alteration and mineralization likely to show as anomalous resistivity, reflecting buildup and intensification of alteration as
fluid exiting basement rocks are contained by cap-rocks.
3. deep narrow basement fault feeder structure/pipe to remain in distinguishable below shallower anomaly manifestations.
In volcanic hosted or volcanic-capped cases, silica-precipitate alteration anomalies are expected to be resistive. Some other cap-rock lithologies could also show resistive
alteration. In certain geologic circumstances, some hydrothermal alteration features could show as conductive anomalies, depending upon the degree of absence of precipitate
healing of the initially conductive fracture/conduit alteration. Even with silicification as a primary element, an intensely silicic orebody anomaly could be relatively
conductive (see Giant Mine, above), or neutral (see underground producer, above)... it all depends on the background resistivity signature of the lithologic host
In the conditions at Hollister, we are truly relying upon the detailed, accurate 3D mapping and representation of complex signature patterns, from among which we can decipher
patterns that locate deep sources of rising mineralizing fluids. Unlike Paradise Peak, where the distinct, large volume resistive anomaly IS the orebody, or at Borealis or the
Toodoggone's Phoenix deposit where narrow ore zones are brilliantly marked by large envelopes of resistive anomalies, here we look for separate pattern-based evidence that
indicates the necessity for a deeper (or nearby) component where we hope to find ore-grade values.
The Comstock Trend is famous for extremely high grade bonanzas of silver and gold associated with the Comstock Lode, a miles-long mineralized fissure vein discovered around 1860 and developed for many decades thereafter.
In addition to bonanza ores, substantial volumes of lower grade silver-gold ore was also mined by open pit methods and from underground workings.
Modern extraction technologies have now reduced the cutoff grade for economically viable ore to the extent that large tonnages of low grade gold ore can now be mined and processed at a profit. The Lucerne deposit of Comstock Mines Inc. at Gold Hill is an example. At Comstock's Dayton Resource area, another body of economic low-grade gold ore has been identified for future production. The Dayton Resource was discovered by geologic mapping and sampling, including drilling.
In January 2015 an E-SCAN 3D IP and resistivity survey was undertaken for Comstock Mining over part of their South Dayton mineralization, and extending south along the trend for a further distance of 600 meters. The survey was undertaken in fs3D mode, to provide a full-objectivity, full-spectrum 3D analysis of the geo-electric characteristics of the known new resource, and of certain known structures and lithologies, while maintaining the opportunity to map any new and unexpected signatures that may be present.
The South Dayton mineralization, including local sulfides, is seen in the upper left quadrant in association with roughly spherical IP anomalies. No uniquely corresponding resistivity anomaly or pattern (conductive or resistive) is seen.
The 3D resistivity results correlate well with many known structures and contacts or a range of orientations. 3D resistivity mapping is particularly valuable in the imaging of previously-unmapped NE-trending structures which, when they intersect NW trending structural preparation faults, identify conditions known elsewhere on the Trend to be amenable to silver-gold deposition.
In summary, any untested discrete anomalous IP zones warrant investigation as possible markers of gold mineralization in the style of the South Dayton resource. Where those IP anomalies are spatially associated with NE structure intersecting NW preparatory faults, the exploration potential is increased. Evaluation of all IP anomalies and conductive and resistive linears and patterns mapped by the 3D survey is currently on hold for organizational reasons.
Click PDF to download this 2015 GSN Symposium presentation on 3D E-SCAN IP and resistivity mapping in the Comstock Trend.
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