# Presentation: 665959

## Geophysical Signatures Used to Constrain Geologic Mapping:  Narrowing the Search for VMS Deposits in Southeast Alaska

**Jeff Wynn****1****, Jon Doucette****1****, Sue Karl****1****,Anne McCafferty****1****, Bruce Smith****1****, and Peter Bittenbender****2**
**1U.S. Geological Survey**
**2Bureau of Land Management**** **

**Notes:**
Geophysical Signatures Used to Constrain Geologic Mapping:  Narrowing the Search for VMS Deposits in Southeast Alaska.

## The Prince of Wales Team

**Jeff Wynn, Geophysicist**

**Sue Karl, Geologist Team Leader**

**Peter Bittenbender, Mine Specialist**

**Jon Doucette, Geophysicist**

**Joe Wooden, Geochronologist**

**John Slack, Geologist**

***Not shown:***** **
**Anne McCafferty, and Bruce Smith, both geophysicists.**

**Notes:**
The POW Team at Khayyam. The team leader is Sue Karl.  Not shown are Anne McCafferty and Bruce Smith.  Joe Wooden will report separately on the geochronology the Wales Group, and John Slack will report later on the depositional setting, geochemistry, and isotopic composition of the known deposits on POW Island.

## Southeast Alaska Mining Districts

**Notes:**
The focus of this presentation is to describe the work that is being done to characterize VMS mineralization from Admiralty Island southwest of Juneau south to Prince of Wales Island, the latter found within the Ketchikan mining district in southeast Alaska.  

## Index Map: Southeast Alaska

- The driving forces behind all this work are the forced cutbacks in logging permits, and the search for a new Greens Creek-type deposit as a local economic alternative to logging
- Areas with potential for volcanogenic massive sulfide (VMS) deposits are the targets of the airborne geophysics.

**Index Map: Southeast Alaska**

**Notes:**
Work began in 1995 on Admiralty Island, then progressed south with an airborne Mag and EM survey (flown in 1996) to the Duncan-Zarembo area, and is now focused on following up a 1999 airborne Mag/EM survey on Prince of Wales Island.  
The driving forces behind all this work are the forced cutbacks in logging permits, and the search for a new Greens Creek – type VMS deposit as a local economic alternative to logging

## Bruce Smith, using a  ground mag...

**Bruce Smith, using a  ground mag...**

**Notes:**
This is Bruce Smith measuring the magnetic field on a profile that passes the Frenchie deposit on Woewodski Island.  The devise is a proton-precession (total field) magnetometer.  The “Jacob Staff “he holds in his left hand is designed to maintain the sensor (the white can on the end of the staff) a measured distance away from the ground and from the operator; this is done in order to minimize noise.

## Jeff Wynn using a VLF-EM resistivity profiler

**Jeff Wynn using a VLF-EM resistivity profiler**

**Notes:**
Jeff Wynn setting up for a VLF-EM resistivity measurement on the mountain southwards above the Niblack deposit on POW Island.  Note the electrode in his left hand; the receiver itself is beneath his right hand.

## Principles of EM, and How We Obtained Resistivities

**Principles of EM, and How We Obtained Resistivities**

**Notes:**
Before we continue it is appropriate to briefly explain the basic characteristics of a Helicopter Electromagnetic (HEM) survey.  A Helicopter EM survey of course requires a helicopter.  The EM transmitter and receiver are connected onto the helicopter by means of a long cable.  Note the position of the Mag Bird and EM bird.  Suspending the birds away from the helicopter helps reduce noise and gets the measuring devices closer to the ground.  The length of the cable is approximately 100 ft (30 meters).  
The EM bird contains a number of tuned coils in transmitter/receiver pairs.  These coils can be configured in a coplanar or coaxial orientation.  The coplanar configuration is most sensitive to flat lying conductors like a saline water-body, while the coaxial configuration responds best to vertically oriented conductors such as vertically-dipping massive sulfide bodies.
The EM system works by means of EM induction, that is, a strong alternating magnetic field is set up by the transmitter coil.  If there is a conductor nearby, eddy currents will be set up in it by the AC magnetic field, and the receiver coil will pick up the secondary field caused by these induced eddy currents.
By measuring both in-phase and out-of-phase (quadrature) components of the secondary signal, information can be gleaned about the geometry and conductivity of buried conductor.
Taking this information one step further we can calculate a Conductivity Depth Image (CDI) that resembles a “geologic” section.  We do this by realizing that the 56,000 Hz signal will penetrate rather shallowly vis-à-vis the 900 Hz signal, and plot the solutions closer to the surface accordingly.

## Zarembo Island: 7200 Hz Airborne EM Resistivity map

**We also utilized airborne geophysical data acquired by the Alaska Department of Natural Resources**

**Notes:**
This map shows the calculated resistivities for the mid-level-depth 7,200Hz frequency of the Helicopter-borne EM system for Zarembo Island.

## Zarembo Island: Airborne Magnetic Data

**Notes:**
This is a map of the helicopter-borne magnetic data for Zarembo Island.

## ...and we acquired mag and VLF-EM resistivity profiles wherever we had reasonable exposure that the team geologists could confidently map.  The following example profile is from southeastern Zarembo Island 

**Map of Zarembo Island**

**Notes:**
...and we acquired mag and VLF-EM resistivity profiles wherever we had reasonable exposure that the team geologists could confidently map.  The above example profile is from southeastern Zarembo Island.

## ...where we combined geologic mapping with ground geophysical profiles:

**Notes:**
This is an example, taken from Zarembo Island, of a geologic-geophysical profile.  Resistivities are shown in red, magnetic values in blue, and the inferred geologic cross-section (based on observed geology and the geophysics together) is shown below the geophysical profile.

## From the geologic-ground-geophysics profiles we developed these geophysical signature tables 

- From the geologic-ground-geophysics profiles we developed these geophysical signature tables 
- ...note how these forced us to subdivide the Cannery (PMc) unit into two different units, PMc1 and PMc2
- ...note also the resemblance in geophysical signatures between PMc2 and Trha.  This is causing us to reconsider the assignment of these map units.

**Notes:**
From careful selection of known geology we set up a number of ground geophysics profiles to adequately sample the main geologic units in the Duncan-Zarembo area that we were interested in.  When we analyzed the results we got these tables of geology-vs-resistivity, geology-vs-magnetic susceptibility, and geology vs-magnetic variability, a rough classification of magnetic wavelength observed in the given geologic unit.  These three taken together allow us to discriminate the key geologic units in the Duncan-Zarembo area, and even force us to reconsider the Cannery (PMc) unit as possibly two separate geologic units (note the similarity of PMc2 with the Trha unit).  In this latter case, the difference may be due to a different depositional environment or may be due to a different degree of metamorphism in two different areas.

## From these we developed Predictive Geophysical Models from the airborne geophysical data for specific geologic units that we felt we could separate out with confidence...

- From these we developed Predictive Geophysical Models from the airborne geophysical data for specific geologic units that we felt we could separate out with confidence...
- Here we have the PGM for Triassic argillite, (Trha), one of the units known to host VMS deposits on Admiralty Island to the north.

**Notes:**
We can develop Predictive Geophysical Models from these assumptions about magnetic susceptibility and resistivity for given geologic units.  Using this supervised classification scheme, we can develop a sort of “pseudo-geology”map that the geologists can field-check.  This map is a PGM for Triassic rocks in the Duncan-Canal-Zarembo Islands area.

## Trhv: Triassic Hyd Volcanic Breccia

**Notes:**
This is an example of Triassic volcanic breccia (photo by Sue Karl), a sub-unit of the Hyd Group in the Little Duncan Canal area of Kupreanof Island.  The Hyd Group is the rock unit that hosts the Greens Creek VMS deposit on Admiralty Island.

## We also used Euler Deconvolution on the airborne magnetic data to develop depth-to-source information.  Since Euler Deconvolution calculates these depths at magnetic gradient points, we have an additional defacto set of information on buried structures.  Note some of the dipping solutions (most dipping south or SSW).

**Euler Deconvolution, calculated on the airborne mag, superimposed on the shaded-relief digital topo of Zarembo Island**

**Notes:**
We also used Euler Deconvolution on the airborne magnetic data to develop depth-to-source information.  This is a method uses least-squared inversion of the data (in this case magnetic data) to fit simple bodies such as spheres, steps, and cylinders to the observed signal.  Since Euler Deconvolution calculates these depths at magnetic gradient points, we have an additional defacto set of information on buried structures.  Note some of the dipping solutions (most dipping south or SSW).

## The team effort worked extremely well in Duncan-Zarembo.  Geologists were learning geophysics, and geophysicists were learning geology.  By closely cooperating on the evolutionary steps in the analysis of the geophysical data, and tying it iteratively to the evolving geologic map, we were able to combine the geologic mapping with the Predictive Geophysical Models and the Euler Deconvolution structural geology information into a new geologic map of the Duncan-Zarembo mining district...  

**Notes:**
How the team-effort worked... almost purely electronically.

## The Revised Geologic Map, Duncan-Zarembo Area: the area underlain by Triassic rocks has increased by ~30% with this map.

**The Revised Geologic Map, Duncan-Zarembo ****Area****: *****the area underlain by Triassic rocks has increased by ~30% with this map.***

**Notes:**
This is the revised geologic map of the Duncan Canal-Zarembo Island area, by Karl, Haeussler, and McCafferty.  This map has far more detail than the original reconnaissance geologic map because it had heavy geophysical input.  A side-benefit of the effort was to increase the area underlain by Triassic rocks by over 30%-- these are the rocks known to host Greens Creek – type VMS deposits.

## Why is geophysics so important here?  

**Why is geophysics so important here?**
**For one thing, geologists can’t work very efficiently in “blueberry chainlink”... and most of southeastern Alaska is covered with water, mud-flats, or dense vegetation like this.**

**Notes:**
‘Why is geophysics so important?  Because 95% of the terrain in SE Alaska is covered with water, mud-flats, or dense vegetation such as this “Blueberry Chainlink”.

##        Red dots: sampling locations

-        **Red dots: sampling locations**

**Notes:**
This base map shows the location of the major VMS (?) occurrences that were visited during this past summers field work.  The small red dots show the locations of physical property measurements and/or rock samples.  Nearly 500 sites were occupied in two weeks.  Note the location of the towns of Ketchikan and Hydaburg; Craig is to the northwest off the map.

## HEM Flight Lines on Shaded Topography

**Notes:**
A helicopter electromagnetic and magnetic (HEM) survey was flown in 1999 over areas of Prince of Wales Island (POW) selected to cover significant volcanogenic massive sulfide (VMS) style occurrences.  Local villages, Ketchikan Gateway Borough, and the Bureau of Land Management (BLM) provided funding for this work.  Previously proprietary HEM surveys donated by Sealaska Native Corporation provide additional coverage. These data were compiled and released in August, 1999 by the Alaska Division of Geological and Geophysical Surveys (DGGS). 
The newer surveys are shown in black, grey, magenta, blue and yellow.  The older surveys are depicted in green, coral and cyan.
The nominal line spacing for the newer surveys was approximately 400 meters (1/4 mile).
The older surveys were flown at both 400 meter (1/4 mile) and 200 meters (1/8 mile).
Nominal, coplanar frequencies of 900, 7200 and 56,000 Hz were collected.  Coaxial frequencies of 900 and 7200 Hz were also sometimes collected.
Approximately 7,600 line km (4,700 line miles) were covered on the combined surveys.  

## Total Magnetic Intensity on Shaded Topography

**Total Magnetic Intensity on Shaded Topography**

**Notes:**
The Total Magnetic Intensity field is imaged on this slide.  The areas of high magnetic intensity are generally associated with plutons.  The subdued magnetic response (blue and green) is associated with sedimentary rocks, some volcaniclastics, and greenstone.  The characterization of these units will be discussed later.  Note however that  sample locations (the red dots) have covered wide areas of varying magnetic signature.

## 7200 Hz Apparent Resistivity on Shaded Topography

**7200 Hz Apparent Resistivity on Shaded Topography**

**Notes:**
This image shows apparent resistivity derived from the 7200 Hz HEM data.  The deep blue and red areas are electrically conductive (low resistivity) and the light green to white areas are electrically resistive (high resistivity).  Note the deep blue at the water margins: this is a typical response to the highly conductive seawater.  Note also, however, certain conductors northwest of Niblack – these are NOT caused by seawater, but by something buried that is highly conductive.  Massive sulfides?  Altered rock?

Note the conductive sea water.  Also note the broad areas in the northwest portion of the survey that are more conductive.  Part of the focus of this past summers field work was to explain some of these anomalies.  

## Older Geology  on Shaded Topography

- From Grybeck

**(SOsv)**

**KARHEEN (Dk)**

**WALES (CZwg)**

**Notes:**
Don Grybeck’s simplified Craig / Dixon-Entrance geology map.  As you can see a number of different geologic units were mapped but the mapping shows some inconsistencies.
The northern boundary of the olive green Descon rocks with the purple-gray Wales Group rocks is not well known, and this is important if we are searching for Proterozoic rocks that could host a VMS deposit.  
The felsic to mafic volcanic and sedimentary rocks of the Karheen Formation also have potential for VMS mineralization. In map units characterized by small scale lithologic variability, we can look for areas with alteration signatures or small areas with geophysically anomalous signatures. 
The area labeled SOsv is a time correlative unit to the Descon Formation to the north. 

## Proterozoic

- Cambrian

- Ordovician
- Early Silurian

- Silurian

- Early
- Devonian

- Jurassic

- Cretaceous

- Tertiary

**voluminous mafic to intermediate volcanic rocks and marble – *****arc***

**silicic volcanics and felsic pyroclastic deposits, sheeted dike complex, pillow basalt, conglomerate, and black carbonaceous phyllite, ****(VMS deposits)**** – *****rifted arc basins***

**metamorphism and deformation**

**voluminous mafic to intermediate volcanic rocks, limestone, mudstone and volcaniclastic turbidites, debris flows – *****arc***

**silicic volcanics and felsic pyroclastic deposits with carbonaceous graptolitic argillite and chert, ****(VMS deposits?)**** –*****rifted arc***

**voluminous andesitic breccia, volcaniclastic and calcareous graywacke turbidites -  *****arc***

**pillow basalt, limestone - *****arc***

**rhyolite flows and pyroclastic deposits, mafic to felsic dike swarms, fossiliferous limestone ****(VMS deposits?)**** – *****rifted arc***

**breccia, conglomerate, argillite, limestone**

**alkaline granite and syenite intrusions**

**contractional deformation, quartz diorite intrusions – *****arc***

**limestone reefs**

**mafic to felsic dikes and extrusive volcanics– *****strike-slip extension***

**Notes:**
The geology on POW is a product of at least 3 separate periods of rifting.  Known VMS mineralization occurs in the Proterozoic -- however, each one of these rifting events provides the appropriate geologic setting for VMS mineralization.  There may be VMS deposits in the Paleozoic part of the section as well.

## Location of Khayyam on Shaded Topography

**Location of Khayyam on Shaded Topography**

**Notes:**
A goal of the USGS is to provide an analysis of the airborne geophysics over the significant, known VMS deposits to provide a baseline for their response.   
So far, we conclude that a well-defined conductivity anomaly is not usually apparent over the known VMS occurrences.  Most of the occurrences are located in close proximity to the shore and the HEM response is adversely affected by the highly conductive seawater.  It is also suspected that the texture, grain characteristics and species of sulfide at known VMS deposits may not lend themselves well to direct geophysical detection – for instance the sulfides may be in blebs and not interconnected, so EM cannot set up an eddy current in the target.
This is a work in progress.

An exception to this observation is at Khayyam where an obvious connection between outcropping sulfide and a HEM response is clear.  Physical property data over this deposit also support this conclusion.

## Khayyam

**Notes:**
A beautiful day on Khayyam Mountain in late June. 

## 7200 Hz Apparent Resistivity

**Notes:**
We now focus on the Khayyam area, a prospect worked over and abandoned early in the last century.  An obvious resistivity anomaly is shown.  The black X’s show the location of physical property measurements.  The location of the “Khayyam Mine” appears offset from the location of the conductor... apparently because this is the adit opening leading under the mountain to where the sulfides are located. The adit was buried by snow and was not accessible when we visited. 
The point we wish to make here is that the HEM response is directly associated with outcropping sulfide mineralization and or alteration.
Note the less conspicuous anomaly to the east that has a similar shape and size to the Khayyam anomaly.  The Stumble On workings are located near the X just to the right of the “Khayyam Mine” label, and represent an outcrop of massive sulfides found there.
A conductivity-depth image (what we like to call a CDI for obvious reasons) is shown using the same color scale.  This section depicts electrical resistivity vs. depth along the line highlighted in red.  The Khayyam anomaly is a typical “pant leg” style response characteristic of a shallow, narrow conductive body.  This section supports the fact that the sulfide mineralization giving rise to this anomaly is very shallow and has only limited depth-extent (e.g., note that the white areas below the topography line are resistive).
Note the Khayyam anomaly is in the 10’s of ohm-m range... this is very conductive when we consider that the HEM system is averaging a large volume of rock.
We will now focus on the area outlined in red to have a look at the physical property data measured on the ground.

## Susceptibility on TMI Mag

**Notes:**
This image shows the Total Magnetic Intensity from the airborne survey.  The X’s pinpoint the location of magnetic susceptibility measurements, a measure of the ability of a rock to become magnetized.  
The photos show the instrument used to measure magnetic susceptibility (A Geo Instruments GMS-2, made in Australia).  It is effectively a very small EM survey, sampling about a fist-worth of rock.  The output appears on a LCD and the digital data can be stored.   Each measurement takes about 10 seconds.  A number of measurements are taken at each site to develop an average.  The Magnetic susceptibility data are in units of micro-CGS and are shown on the upper right side of the Xs.  These can be easily converted to the other common susceptibility standard, SI, by multiplying these micro-CGS values by 4*pi.
The susceptibility measurements are quite variable and don’t seem to correlate exactly to the airborne mag data.  This makes sense since the local geology is also quite variable at this site:  changing readily on the scale of meters.  Also, small magnetite grains in veinlets significantly impact the localized susceptibility measurements.  Keep in mind that the airborne magnetic data average a very large volume of rock, thus smoothing or averaging the response considerably. 
Despite these inconsistencies, this work quantifies the connection between the airborne geophysical data and geology, improves the rigor of geophysical models, and aids in the overall geologic interpretation.  A significant effort is underway to relate these physical property measurements with known geologic units to possibly help outline these units in areas of poor rock exposure and limited mapping.

## VLF on 7200 Hz Apparent Resistivity

**Notes:**
Resistivity was also measured on the ground using a Geonics EM-16R.  Unlike the magnetic susceptibility measurements, the resistivity measurements average a considerable volume of material (about a 100 meter hemisphere).  The results thus agree much better with the airborne data.
The EM 16-R uses submarine communication signals near 20,000 Hz (Very Low Frequencies, or VLF) that propagate along the earth/ionosphere interface and refract into the ground.  A measurement of the magnetic and electric fields at the surface allow us to measure both resitivity and phase (the phase data give us an idea of weather the ground is getting more conductive or more resistive with depth).
Like the magnetic susceptibility measurements, this work helps quantify the connection between the airborne geophysical data and the geology.
A piece of massive sulfide kicked loose from some nearby tundra is shown in the upper left photograph.  To be sure that the EM response was not caused by water, the electrical conductivity of a small pond of meltwater was measured.  The water was about 1 degree C and had a resistivity of about 1700 ohm-m, and so was not responsible for the anomaly.
The EM response at this location was thus most likely caused by massive sulfide mineralization.

## Preliminary Geophysical Assumptions,SE Prince of Wales Island

**Notes:**
Several of the most distinctive geologic units were used to do a preliminary interpretation of  the airborne geophysics.  Some initial assumptions were made concerning their physical properties for the preliminary, first-pass version of “Anne’s Map”.  

## Preliminary Geological Unit Classification from the airborne geophysical data

**Notes:**
This is Anne McCafferty’s first attempt at interpreting the airborne geophysics using the assumptions about the physical properties of selected lithologic units.  
With the newly acquired physical property data and improved geologic mapping and age dating, Anne will improve her statistical interpretation and extend known lithologic units in areas of poor rock exposure and limited mapping.  
This information is fundamental to understanding the age relationships, setting, and genesis of VMS mineralization on POW Island and all of southeast Alaska.

## Slide 32

**Notes:**
The major rock units mapped on Prince of Wales Island are subcategorized on this table.  
After a season of field work, our preliminary interpretation has allowed us to correlate rock units to the physical property data.  This data is critical for using the airborne geophysics to help extend the geologic mapping.
A significant effort is underway to relate these physical property measurements with known geologic units to possibly help outline the units in areas of poor rock exposure and limited mapping.
The graph shows the range and mean of resistivity grouped by the major rock units shown in the table.  Note that the range of resistivities are not terribly different from one rock unit to the next.  The carbonaceous unit stands out however.  Also the spatial variability of these data are lost in this figure, so a better distinction between units may be more evident in a map presentation.  
Note that there are a different number of samples in each category.
The photo at right shows a typical outcrop of the carbonaceous unit  (Photo taken by Jeff Wynn at station RT30 east of Cable Creek).

## Slide 33

**Notes:**
The magnetic susceptibility of major rock units are similarly displayed.  A larger contrast is apparent but the range of susceptibility is typically very wide.  
Statistical outliers are understandable since many local factors affect the magnetic susceptibility measurements including local weathering, metamorphism, alteration, and small magnetite veins.  Trying to correlate a small-scale phenomena with macroscopic geology is never straight-forward.  A finer categorization of rock units is underway to help resolve the association between geology and geophysics.
The photo by Sue Karl shows graywacke turbidites of the Descon Formation.

## Geologic Interpretation Using Geophysics

- The first-pass at a predictive geophysical model map: the
- distinctive geophysical units

- Older, reconnaissance geologic map (compiled by Don Grybeck)

**Notes:**
Our goal is to improve our understanding of the regional geologic picture on POW.  Using the physical property data and the airborne geophysics will help improve the geologic map.  We see here the first iteration based on our initial classification of the geophysical data by Anne McCafferty.

## What Lessons have we Learned?

- The physical property data and airborne geophysics can be used to help map areas of limited rock exposure
- CDI sections should help aid in the interpretation of buried geology along presently unverified contacts.
- The majority of the VMS deposits on POW do not have clear EM anomalies *(...but neither does Green’s Creek!)..*
- An interpretation using geology, geochemistry, geochronology and geophysics will allow us to subdivide the Wales Group and give us criteria for delineating prospective terrain much more reliably.
- VMS mineralization on POW may not be confined to the Proterozoic.

**Notes:**
These are the lessons we have learned after one field season in the Duncan Canal-Zarembo Island area, and one field season on Prince of Wales Island.

## Slide 36

**Notes:**
This presentation was originated by Jon Doucette, and a simplified version of it was presented at the Alaska Miners Association meeting in Anchorage, AK, in November 2000 by Jeff Wynn.  Sue Karl served as the the team leader.  She developed and organized the project. and coordinated multidisciplinary workers toward coherent, integrated geological and geophysical products. Sue also compiled the geologic maps, and cross sections for geophysical  profiles. 
Andrew Grosz, Herb Pierce, and Art Schultz provided technical reviews, and John Watson spent a great deal of time converting the PowerPoint presentation to HTML for the USGS web-site.
All Alaska presentations MUST have a bear in them somewhere (can you see it?).  Photo by Jeff Wynn.