CAMAC Energy Inc.  has announced that Sander Geophysics Limited (“SGL”) has completed shooting  airborne gravity and magnetic geophysical surveys on the Company’s Kenya onshore Lamu Basin Blocks L1B and L16 (“Blocks”). The data acquisition covers essentially the entire 12,129 square kilometers in Block L1B and the entire 3,613 square kilometers in Block L16 and satisfies the gravity and magnetic survey requirements for each Block under the relevant Production Sharing Agreements.

The Company expects to receive initial results of the shoot in the third quarter of 2013. Results will be used to optimize the placement of 2-D seismic lines by identifying faults, basement structures and intra-sedimentary volcanic layers and/or intrusions.

“I am pleased that we completed the acquisition of the airborne gravity and magnetic geophysical surveys in Kenya safely, on time, and under budget,” said Senior Vice President of Exploration and Production Segun Omidele. “Our geophysical team will now work with SGL to interpret the data and delineate optimal areas for 2-D seismic acquisition.”

A short course on the why, how, and what of geophysical inversion

Presented by the B.C. Geophysical Society

This is a 2-day workshop consisting of 1 day of lectures (April 24th) followed by a 1 day hands-on workshop (either April 25th or 26th). Delegates can register for just the lectures or both the lectures and workshop.

Registration available until April 15th at: www.bcgsonline.org

Date: April 24th – 26th 2013

Address: SFU Downtown Campus; Vancouver BC

Registration: Registration is now open!  Please fill out attached pdf form (link below) and click submit.  Registration will be complete once payment is made via Paypal link.  Delegates are asked to choose one workshop day among April 25th or 26th.

Day 1 – Introduction to inversion
– Introduction to inversion in exploration

Reasons for doing inversion
Range of inversion options: magnetics, gravity, EM, IP-resistivity, seismic
Fitting geology and data

– Background on inversion

Unconstrained/constrained
Joint and collaborative
The future of inversion

– Before inversion – forward modeling

forward modeling
petrophysics drives the story
complexity vs. adequacy

– Case studies

Potential fields
EM
IP-resistivity

Day 2 – Workshop with Case studies
Multi-disciplinary teams will use geological, geophysical and geochemical data from two deposits (Babine Lake porphyry Cu-Mo, BC and Minto Cu, YK) to develop exploration targets. Teams will present their evaluations of the data to stimulate group discussion.

Inversion results for the available geophysical surveys will be presented to demonstrate state-of-the-art technology and best practices.
Sponsors for the short course are currently being solicited. Please contact Victoria Sterritt at Victoria.Sterritt@teck.com for details. Proceeds go to the KEGS Foundation.
For those interested in more information about the short course, please email info@bcgsonline.org

Recent airborne geophysical surveys near Decorah, Iowa are providing an unprecedented look at a 470- million-year-old meteorite crater concealed beneath bedrock and sediments.

The aerial surveys, a collaboration of the U.S. Geological Survey with the Iowa and Minnesota Geological Surveys, were conducted in the last 60 days to map geologic structures and assess the mineral and water resources of the region.

“Capturing images of an ancient meteorite impact was a huge bonus,” said Dr. Paul Bedrosian, a USGS geophysicist in Denver who is leading the effort to model the recently acquired geophysical data. “These findings highlight the range of applications that these geophysical methods can address.”

In 2008-09, geologists from the Iowa Department of Natural Resources’ (Iowa DNR) Iowa Geological and Water Survey hypothesized what has become known as the Decorah Impact Structure. The scientists examined water well drill-cuttings and recognized a unique shale unit preserved only beneath and near the city of Decorah. The extent of the shale, which was deposited after the impact by an ancient seaway, defines a “nice circular basin” of 5.5 km width, according to Robert McKay, a geologist at the Iowa Geological Survey.

Bevan French, a scientist the Smithsonian’s National Museum of Natural History, subsequently identified shocked quartz – considered strong evidence of an extra-terrestrial impact – in samples of sub-shale breccia from within the crater.

“The recognition of this buried geological structure was possible because of the collaboration of a local geologist, water well drillers, the USGS STATEMAP program, and the support of the Iowa DNR concerning research on fundamental aspects of Iowa geology,” said McKay.

The recent geophysical surveys include an airborne electromagnetic system, which is sensitive to how well rocks conduct electricity, and airborne gravity gradiometry, which measures subtle changes in rock density. The surveys both confirm the earlier work and provide a new view of the Decorah Impact Structure. Models of the electromagnetic data show a crater filled with electrically conductive shale and the underlying breccia, which is rock composed of broken fragments of rock cemented together by a fine-grained matrix.

“The shale is an ideal target and provides the electrical contrast that allows us to clearly image the geometry and internal structure of the crater,” Bedrosian said.

More analysis of the data will provide additional detail. These data show the impact as a nearly circular region distinct from the surrounding area to a depth of several hundred meters.

“These data, when coupled with physical property measurements on drill core samples, will form the basis for modeling efforts to constrain the impact geometry and energy of the meteorite,” said Dr. Andy Kass, a USGS geophysicist working on the effort.

The Iowa and Minnesota airborne geophysical surveys are targeting an igneous intrusion, known as the Northeast Iowa Igneous Intrusive complex, that may be similar to the Duluth layered igneous complex exposed in the Lake Superior region of northern Minnesota. Known copper, nickel, and platinum group metal resources were deposited during the formation of the Duluth complex. Both of these complexes are associated with a large structural feature known as the Midcontinent Rift, which is exposed in the Lake Superior Region but is covered by younger rocks as it extends to the south through Iowa, Nebraska, Kansas, and Missouri.

This geophysical survey is part of a larger USGS effort to evaluate the concealed mineral resource potential of the greater Midcontinent Rift region that formed about 1.1 billion years ago.

U.S. Department of the Interior, U.S. Geological Survey Office of Communications and Publishing 12201 Sunrise Valley Dr, MS 119 Reston, VA 20192

Heidi  Koontz Robert McKay

Acquisition and Analysis of Terrestrial Gravity Data

Gravity surveys have a huge range of applications, indicating density variations in the subsurface and identifying man-made structures, local changes of rock type or even deep-seated structures at the crust/mantle boundary. This important one-stop book combines an introductory manual of practical procedures with a full explanation of analysis techniques, enabling students, geophysicists, geologists and engineers to understand the methodology, applications and limitations of a gravity survey. Filled with examples from a wide variety of acquisition problems, the book instructs students in avoiding common mistakes and misconceptions. It explores the increasing near-surface geophysical applications being opened up by improvements in instrumentation and provides more advance-level material as a useful introduction to potential theory. This is a key text for graduate students of geophysics and for professionals using gravity surveys, from civil engineers and archaeologists to oil and mineral prospectors and geophysicists seeking to learn more about the Earth’s deep interior.

Fundamentals of Gravity Exploration (Geophysical Monograph Series No. 17) covers a full range of gravity-exploration topics, including first principles, field instrumentation and operations, rock densities and density contrasts, data reduction, methods of interpretation, and geologic examples.  The subject matter includes inversion and an appendix on the Fourier transform.  This book will help students to efficiently gain knowledge and appreciation for the method, and it will provide experienced earth scientists with a valuable addition to their exploration libraries, both for reference and understanding of this important method.

CAMAC Energy Inc.  has announced it has signed an agreement with Sander Geophysics Limited (“SGL”) to shoot airborne gravity and magnetic geophysical surveys on its Kenya onshore Lamu Basin Blocks L1B and L16 (“Blocks”). The data acquisition will cover the entire 12,197 square kilometers in Block L1B and the entire 3,613 square kilometers in Block L16, exceeding the first exploration period’s gravity and magnetic survey requirements for each Block.

The results of the airborne gravity and magnetic survey will be used to optimize the placement of 2-D seismic lines by identifying faults, basement structures and intra-sedimentary volcanic layers and/or intrusions. Airborne gravity and magnetic data combined with 2-D seismic has been utilized to identify successful exploration targets in East African Rift Basins by regional operators Heritage, Tullow and Africa Oil. The Company expects SGL to commence data acquisition in the second quarter of 2013 and provide initial results in the third quarter of 2013.

Founded in 1956, the Ottawa-based SGL provides worldwide airborne geophysical surveys for petroleum and mineral exploration, and geological and environmental mapping. SGL has operated on every continent including Antarctica, and under diverse conditions ranging from the tropics, deserts, mountains and offshore.

“Executing this agreement with SGL is an important milestone for our Kenya exploration program,” said Senior Vice President of Exploration and Production Segun Omidele. “These gravity and magnetic surveys will satisfy the first requirement of the first exploration period, and most importantly, will allow us to delineate an optimal 2-D seismic program on the Blocks. This is the first step to unlocking the high potential value of our onshore Kenya acreage.”

The final seismic, gravity and magnetotelluric dataset from Geoscience Australia’s Onshore Energy Security Program has been processed and released.

The data were acquired in 2011 by Geoscience Australia in conjunction with the Geological Survey of Western Australia (GSWA) as part of the Western Australian Government’sRoyalties for Regions Exploration Incentive Scheme. The 484 kilometres of data were obtained over the Yilgarn Craton, Officer Basin and Musgrave Province of Western Australia. This survey will provide precompetitive data for energy and mineral exploration, and for potential water resources. These data, combined with the 2001 Northern Yilgarn Survey, and the 2010 Capricorn and Youanmi Surveys result in a complete cross section of central Western Australia.

Overall, more than 6500 kilometres of deep crustal seismic data were acquired during the Onshore Energy Security Program, providing valuable structural information for the Australian crust from the near surface to the Moho, boundary between the Earth’s crust and the mantle. As well as providing precompetitive data, the surveys have helped to expand understanding of the complex structures and geology of the Australia continent.

The processed SEGY data, images and ancillary information are now available for free download.

Interpretation of the seismic and magnetotelluric models undertaken by GSWA and Geoscience Australia geoscientists will be presented at a public workshop in Perth in June 2013.

Often not given its due in oil and gas geophysics, knowledge of basement geology can be critical to exploiting reservoirs including the unconventional.

by GRAHAM CHANDLER on JANUARY 24, 2013

There is an article in the last Earth Explorer issue:

New Approach to Basement Studies for Oil and Gas Explorers

The EEGS has published the latest issue of FastTIMES, news for the near-surface geophysical sciences. It is available as a pdf document optimized for screen viewing or printing.

Seismoelectric & Microgravity

Volume 17, Number 2, June 2012

• Click Here for PDF (6.2 MB)

Purepoint Uranium Group Inc. has released the results of this summer’s geophysical survey at its 100% owned Turnor Lake Project in Saskatchewan’s Athabasca Basin. The induced polarization survey has moved the exploration of this property further north, covering the high-priority Anvil South uranium target area.

____________________

-     The induced polarization (IP) survey was conducted over the Anvil South grid with the purpose of outlining possible low apparent resistivity chimneys (LARCs) in the sandstone. LARCs are considered to be excellent drill targets when found within a favourable geologic setting as they represent potential zones of hydrothermal alteration.

“The IP survey data is of high quality, free of noise and aligns well with historic drilling data, unconformity depth, and overburden thickness,” said Roger Watson, Purepoint’s Chief Geophysicist. “Numerous geophysical LARCs have now been interpreted; many of them correlating with an untested, well-defined electromagnetic conductor.”

Highlights:

• Low apparent resistivity chimneys (LARCs), possible zones of hydrothermal alteration, have been outlined within the Anvil South zone and correspond with untested EM conductors;
• The resistivity survey successfully identified a LARC that corresponds with altered sandstone intersected by historic hole OD-1 that also returned 468 ppm U over 3.4 m;
• The geophysical results collected by Purepoint at Anvil South, which includes resistivity, gravity, and highly detailed airborne electromagnetics and magnetics, have been compiled with all available historic data identifying five new high priority drill targets;
• A strong geochemical anomaly of uranium, nickel and cobalt was outlined at the Klaproth South target in 2011 and verified by re-sampling in 2012.

Anvil South

The Anvil South grid covers over four kilometers of electromagnetic (EM) conductors that were outlined by a VTEM airborne survey in 2006. The EM conductors show significant offsets interpreted to be caused by faulting. Purepoint conducted a gravity survey over the Anvil South grid in 2008 but has not yet drill tested geophysical targets in this area.

During August 2012, over thirty line-kilometers of IP surveying was completed at Anvil South by Peter E. Walcott & Associates Ltd. of Vancouver, BC. The data was inverted by Purepoint using IP inversion software developed by the University of British Columbia. The IP survey was successful in that well defined resistivity “chimneys” (LARCs) were identified in the sandstone. The Anvil South LARCs have now been plotted in relation to resistivity, gravity, EM conductors and magnetics and five priority drill targets have been identified.

The historic drill hole OD-1, drilled at Anvil South by the Saskatchewan Mining Development Corp. in 1985, intersected 3.4 m of 468 ppm U within a highly altered graphitic pelite immediately below the unconformity. Anomalous concentrations of arsenic, nickel and cobalt were associated with the alteration encountered in the OD-1 drill hole.

Turnor Lake

The Turnor Lake Project is 100% owned by Purepoint. This 9,705 hectare property covers graphitic electromagnetic (EM) conductors that are directly associated with high grade uranium showings on adjoining properties, namely Cameco’s La Rocque occurrence (up to 33.9% U3O8 over 5.5 m) to the west and Areva’s HLH-50 intercept (5.2% U3O8 over 0.38 m) located to the south. The project lies in close proximity to several uranium deposits, including Roughrider, Midwest Lake, McClean Lake and Eagle Point, and has a shallow depth to the unconformity of less than 200 metres in most areas.

A series of detailed geophysical surveys have been conducted on the property since November 2006 and numerous drill targets, including EM conductors and structurally complex areas having evidence of intersecting structures, remain untested. Drill targets have now been prioritized with the completion of the 3D Targeting Workflow Process by Mira Geoscience.

Liberty Silver Corp.  announces the successful completion of its 2012 geophysical program at the Trinity Project, located in Pershing County, Nevada. The Gravity and Induced Polarization (IP) surveys were successful in locating significant new drilling targets concealed by alluvial cover and in delineating the structural fabric of the district. The geophysics program was designed and interpreted by Jim Wright of J.L. Wright Geophysics. All the new data and the substantial amount of historic data relating to past operations at the Trinity Project have been input into a GIS database which will be used to further define future drilling and development of the current resource and multiple exploration targets.

Under the direction of Mr. Wright, Magee Geophysical Services of Reno, Nevada conducted the gravity survey consisting of 532 stations covering approximately 26 square kilometers. The survey delineated the shallow eastern edge of the Sage Valley basin. High-angle structures of two primary orientations offset the Tertiary volcanic-pre-Tertiary sediment package into a complex series of steps resulting in structurally defined basins. Most notable is a graben filled with volcanic rocks which hosts the Trinity silver deposit that is underlain by a chargeability anomaly. A second major chargeability anomaly south of the Trinity pit is located in the same graben.

A possible northern extension of the Trinity silver deposit is hypothesized based upon the gravity survey. Historic IP data, acquired in 1983 (the “Historic IP”), indicates a possible northern continuation over this area as well. Historic airborne magnetic results coupled with the gravity and new IP also suggest a continuation of the known mineralization to the southwest along the Trinity fault. The gravity and Historic IP data, coupled with several other data sets, indicate that several target areas exist, including two possible target areas along the strike extensions to the known silver mineralization, and several other untested chargeability anomalies similar to that of the Trinity silver deposit.

Based upon the gravity and historic geophysical results, eleven (11) IP lines (35 line kilometers) were completed by Zonge Geoscience to enhance the level of detail of the chargeability anomalies detected in the Historic IP survey. All eleven lines detected chargeability responses consistent with that survey. Most notable is a strong chargeability zone located approximately 500 meters (“m”) south of the known Trinity silver deposit and is defined by four IP lines from the new IP survey. The strike length of this anomaly is greater than 600m and is still open to the north and south and ranges from 500m to 700m in width in an east west direction. Furthermore, the graben interpreted from the gravity survey is further supported by the new IP results, thus increasing the thickness of the favorable host rock package. U.S. Borax (“USB”), the former operator at Trinity, drilled three holes into this zone in the mid-1980s and all three holes intersected considerable silver mineralization but due to low silver prices, further delineation of the silver mineralization was not carried out. The USB drill holes also stopped short of testing the chargeability anomaly found by the new survey. The three holes (S-21, S-70, and S-132) encountered silver mineralization ranging from 2.0 grams/tonne in an interval of 1.5m to 43 grams/tonne in an interval of 1.5m but were not drilled to conform to National Instrument (NI) 43-101 standards and, while significant, are considered as non-compliant historic data only.

Bill Tafuri, President and COO, said, “The results of these surveys are very encouraging and they demonstrate the potential for extending the known resource under the covered areas. They also have defined specific targets to drill under the covered areas making our exploration program much more focused and cost effective”.

Liberty Silver is currently completing its initial drilling program on the Trinity Property. Assayed results from the program are expected to be released in the near term.

El Niño Ventures Inc. provides an update of the exploration work performed during the period July 1 to December 31, 2011 for its Bathurst Mining Camp (BMC) Base Metals Tri-Party agreement between El Nino, Votorantim Canada (VMC) and Xstrata Zinc Joint venture project in Bathurst, New Brunswick.

VMC’s work on the Xstrata-El Nino-Votorantim lands during this period included diamond drilling and ground EM surveys. A Time Domain EM survey was completed by Eastern Geophysics Ltd. on two third party optioned properties; and a combined borehole and surface TDEM survey was performed southeast of the BM&S No.6. The following table summarizes the work completed during the period July 1 to December 31, 2011.

Table-1 Exploration Activities Performed from July to December 31, 2011

Diamond Drilling:

As of December 31, 2011 a total of 16 holes, totaling 5,011 metres, have been completed on the Bathurst Mining Camp project (Figure 1). This Phase of exploration drilling campaign focused on geophysical and geochemical targets in the Brunswick Belt, two third party optioned properties in the Brunswick Belt and two optioned properties in the southwestern part of the BMC. The only hole drilled in the Camel Back project intersected widespread alteration with common traces of sphalerite.

Ground Geophysics- Large Loop Pulse EM:

A large grid was cut over the two third party optioned properties in Brunswick Belt and adjoining parts of Xstrata’s claims. Four loops were laid out and readings were measured in-loop and out-of-loop. Only a few weak areas of low resistivity were detected. Deep drilling, which subsequently tested the at depth extent of a narrow zone of subeconomic base metal mineralization, did not intersect any significant sulphide or base-metal mineralization.

Borehole Geophysics:

Borehole and surface Pulse EM surveys were performed southeast of Nepisiguit Dam in and around two drill holes completed in 2010 that intersected stockwork style copper-iron. Only a small off-hole response was found from the Pulse EM survey at Nepisiguit Dam and no further work is planned.

Figure1 –Drill Holes Location Map

Airborne Geophysics — Airborne Gravity Gradiometry (AGG):

Fugro Airborne Surveys completed the Phase 1 AGG survey over two large areas in the Bathurst Mining Camp in May 2011. This was the first ever commercial survey using the ‘Falcon’ Gravity Gradiometer in a helicopter platform. Figure 2 shows the outlines of the surveyed areas. A decision to proceed with Phase 2 and 3 will be contingent upon results obtained in Phase 1. Final results have now been received and are being interpreted in conjunction with Votorantim’s HeliTEM magnetic and EM data and other historical data, with the objective of identifying targets for drill testing in 2012.

Figure 2 –Airborne Gravity Gradiometry Survey

2012-2013 Exploration Plans

2012-Q1: Three drill holes were completed on third party options, in the Brunswick Belt, for a total of 1975 metres to test for base metal mineralization at depth. No significant mineralization was intersected. These three drill holes as well as three holes completed in Q4 2011 were tested by borehole pulse EM, however no anomalies were detected.

2012-Q2 to Q4 and 2013 Q1: It is planned to drill-test airborne gravity gradiometer targets throughout the remainder of 2012. Ground geophysical surveys will be performed if necessary to refine some gravity targets.

Figure 3 – Land tenure map showing the location of optioned properties in the Bathurst Mining Camp

Darnley Bay Resources announces the release, filing on SEDAR and posting on its own website of the 43-101 Technical Report prepared by Stephen Reford, P.Eng., a Qualified Person for the purposes of National Instrument 43-101 and the Company’s Chief Technical Officer. The Report details the history of North America’s strongest discrete gravity anomaly and its exploration by the Company and others over a period of 17 years.

The Anomaly has been favourably compared by the Geological Survey of Canada (the “GSC”) to other prominent gravity anomalies such as those at the prolific mining camps of Noril’sk and Sudbury Basin. The Darnley Bay Anomaly is stronger than any of these comparatives by a wide margin. The GSC discovered the Anomaly in 1969 .The Company has 100% control of its exploration and potential development subject to certain back-in and other rights of the Inuvialuit Regional Corporation on whose land it occurs.

The Report details the work undertaken by the Company to identify 41 gravity, magnetic and electromagnetic targets widely distributed over the 100 km by 80 km extent of the Anomaly, in addition to larger zones of exploration significance. It recommends an exploration and drilling strategy to fully test the base and precious metal potential of the property. In view of the size and potential significance of the Anomaly, the Company has now decided to undertake a thorough search for joint venture partners to assist in the further exploration of this unique but gigantic occurrence.

http://www.darnleybay.com/DBR_Corporate_Presentation_Feb.2012.pdf

MacDonald Mines Exploration Ltd. announces results of a ground gravity survey over the Company’s VMS targets on the Butler Property in the Ring of Fire.

The presence of density anomalies associated with and adjacent to the feeder/stockwork sulphides intersected to date on Butler 3 suggests either:

• An  increased presence of sulphides or
• A different rock type of greater density or
• a combination of both

The measurement of gravity is an effective technique for defining geometry, structure, and a proven tool in mapping intrusions in sedimentary and volcanic terrains. The use of ground gravity surveys, have historical success in the search for volcanogenic massive sulphide (VMS) deposits.  The technique has been attributed to the discovery of the Sunridge Gold Embra Derho deposit in early 2007 (Approx 62.5 m/tonnes) in Eritea.

The gravity data collected over the Butler property by the Ontario Geological Survey (“OGS”) and Geological Survey of Canada (“GSC”) survey confirmed the Company’s interpretation of a large mafic – ultramafic package that exists along the eastern portion of the property (Butler 5).  This insight prompted MacDonald to utilize the technique over the highly prospective Butler 3 zone.

Ground Gravity Survey

At Butler 3, three distinct density anomalies coincident with anomalies from other geophysical techniques have been identified by Abitibi Geophysics.    These density anomalies add credence to the interpretation of a large VMS system in the Zone identified as Butler 3.  This system is associated with the stringer materials in diamond drill hole BP11-Cu06 which intersected 167 metres of 0.39% Cu and 1.13% Zn. Results for Butler 4 and Butler 5 are pending and data collection continues as exploration is ongoing.

Butler 3 – Characteristics of a Volcanic Hosted Massive Sulphide Deposit (VMS)

The Butler 3 targets are quantified by the following geological characteristics:

1. The volume of alteration is exceptional, indicating that a very large volume of hydrothermal fluid discharged in this region.
2. Butler 3 is contained in a felsic-dominated sequence.
3. The alteration zone is Cu-enriched, and the Cu content increases stratigraphically upwards towards the mineralized horizon.  Cu tenor is high (greater than 3%).
4. Zinc mineralization is less than expected – indicating a higher temperature system (Cu-enrichment).

Geophysically the targets have physical properties commonly identified with massive sulphides.  These are:

1. The Butler 3 targets are Electrically Conductive – indicative of metallic content.
2. Elevated magnetic susceptibility – signifying structure and potential sulphides.
3. Dense – suggesting the presence of sulphides.

In summary Butler 3 exhibits a multitude of exploration vectors and proximity indicators of a VMS system.  These include but are not limited to:

• Geophysical properties – conductive, dense, magnetic.

• Mineral zonation – stringer mineralization with elevated Cu values have been intersected over significant lengths.
• Alteration Indicies – the drilling has identified a large alteration system with sodium depletion.  This is an indication of lateral distance from the core of the discharge zone.
• Mineral chemistry vectors – these are coincident with the geophysical anomalies (conductance and density)
• Structural controls – the stringer zone is capped by a key marker horizon comprised of a siliceous relatively impermeable, black felsic unit

Deep Penetration and High Resolution IP Resistivity Survey

Insight Geophysics Inc. has been retained to apply their technology over Butler 3, Butler 4, Butler 5 and the Sanderson properties in the Ring of Fire.  This array is currently being utilized in the Ring of Fire by Noront Resources who have commented on the success of the technique to identify zones of nickel sulphide in their recent press release.

Preliminary results of this survey over Butler 3 have confirmed chargeability anomalies coincident with the gravity results.  Chargeability measures the ability of the sub-surface to temporarily maintain an electrical charge – sulphides are typically chargeable.  The survey is expected to be completed in early March, 2012.

MacDonald Mines Exploration Ltd. announces the mobilization of a geophysical crew to begin a ground gravity survey over the Company’s VMS, Ni and Cr targets on the Butler Property in the Ring of Fire.

The recently released Ontario Geological Survey (“OGS”) and Geological Survey of Canada (“GSC”) gravity gradiometer data over the Ring of Fire has been the catalyst for the Company’s renewed interest in the gravity technique as a proven exploration tool for identifying density variations associated with mineralization such as VMS, Ni and Cr.  The conclusion of the OGS/GSC gravity gradiometer survey was that it was probable that gravity highs were defining the package of mafic and ultramafic intrusive and volcanic rocks which host the chromite mineralization.

The measurement of gravity is an effective technique for defining geometry, structure, and a proven tool in mapping intrusions in sedimentary and volcanic terrains. Alteration can also be mapped where geological units of different density are offset and/or altered. The use of ground gravity surveys, have historical success in the  search for volcanogenic massive sulphide (VMS) deposits and has been attributed to the discovery of the Sunridge Gold Embra Derho deposit in early 2007 (Approx 62.5 m/tonnes) as well as satellite discoveries at Nevsun’s Bisha  (Approx 40 m/tonnes) both in Eritea.

The Company initiated a detailed interpretation of the available public data over the known economic discoveries in the Ring a Fire (Cliffs Natural Resources, Noront Resources, etc.).  Part of this process was the development of models for chrome and nickel. The models consist of geochemical, lithological and geophysical signatures. The attributes of the known discoveries in the area were compared to targets both on both our Butler and Sanderson properties with strong correlations.

The gravity data collected over the Butler property by the OGS – GSC gravity survey confirmed the Company’s interpretation of a large mafic – ultramafic package exists along the eastern portion of the property.  The Company has drilled through and into some of the magmatic features to corroborate this interpretation.   In 2011 drill hole V05 drilled through a mafic sill consisting of an upper portion made up of magnetite gabbros, magnetiites,  and troctolites  passing downward into Fe-rich peridotites with high background chrome concentrations. The hole was terminated in peridotites so the proportion of ultramafic materials in these sills is at least equal to or greater than the mafic material. Dr. Hulbert concludes that “The observed footwall ultramafic association to the overlying gabbroic rocks and the property wide distribution of these gabbroic bodies suggests property wide favorability for ultramafic bodies which host Ni-Cu-PGE and chrome deposits.“

3DINVER.M: A MATLAB program to invert the gravity anomaly over a 3-D horizontal density interface by Parker-Oldenburg’s algorithm.

David Gómez Ortiz and Bhrigu N P Agarwal

A MATLAB source code 3DINVER.M is described to compute 3D geometry of a horizontal density interface from gridded gravity anomaly by Parker-Oldenburg iterative method. This procedure is based on a relationship between the Fourier transform of the gravity anomaly and the sum of the Fourier transform of the interface topography.  Given the mean depth of the density interface and the density contrast between the two media, the three-dimensional geometry of the interface is iteratively calculated. The iterative process is terminated when either the RMS error between two successive approximations is lower than a pre-assigned value- used as convergence criterion, or until a pre-assigned maximum number of iterations is reached. A high-cut filter in the frequency domain has been incorporated to enhance the convergence in the iterative process. The algorithm is capable of handling large data sets requiring direct and inverse Fourier transforms effectively. The inversion of a gravity anomaly over Brittany (France) is presented to compute the Moho depth as a practical example.

Download MATLAB code and data file examples

MacDonald Mines Exploration Ltd. provides an exploration update in the mineral-rich Ring of Fire region of the James Bay Lowlands.

MacDonald began exploring in the Ring of Fire starting in 2003, shortly after the first volcanogenic massive sulphide (VMS) discovery at McFaulds Lake by Spider Resources and KWG Resources. The McFaulds VMS discovery is characterized by very high-grade intersections of VMS mineralization (McF-04-57 intersected 18.8 meters of 8.02% Copper (Cu), Mc-03-18 averaged 4.83% Zinc (Zn) over 25.75 meters) and at 250 meter-plus depths.

Exploration in the James Bay Lowlands is challenging due to the area being almost entirely a wetland. As a result, there is minimal outcrop, helicopter support is required and exploration time lines are longer. Over $18 million dollars has been spent to date on the Butler property and we have discovered six zones of multi-element mineralization with copper-zinc VMS systems, vandiferous titano-magnetite and magmatic nickel sulphide. This was accomplished through comprehensive exploration programs including geophysics, diamond drill holes and geochemistry.

Magnetic map (TF) of the Butler Property showing the location of identified multi-element, mineralized zones.

The lack of outcrop requires that a significant amount of attention and detail has to be paid to the geophysical techniques used in the area. Historically the programs focused on shallow targets; however down-hole surveys, ground magnetic and gravity surveys have indicated that the potential deposit(s) of interest lies deeper in the structural sequence.

A comprehensive suite of geophysical tools has applied at Butler:

Airborne EM, Magnetics & Gravity

• 2004 VTEM – 2138.9 line kilometers of data were collected
• 2008 VTEM – 1325.6 line kilometers of data were collected
• 2010 AeroTEM IV – 261 line kilometers of data were collected
• 2011 HeliGeoTEM – 261 line kilometers of data were collected
• OGS/GSC Airborne Gravity Gradiometer

Surface EM

• Butler 1 – 13 line kilometers of data were collected
• Butler 3 – 39.3 line kilometers of data were collected
• Butler 5 – 12.5 line kilometers of data were collected
• Butler 3, 5, 6, and 7 – 30.17 line kilometers of data were collected

Surface Magnetics

• 7 Ground Magnetic surveys completed in 2011 covering 81.1 line kilometers

Down hole EM

• 64 diamond drill holes probed and modeled

The Company is currently enhancing the geophysical model to include the geochemistry data results. Collectively, all of the above geophysical results have consistently shown that an electrically conductive unit with magnetic properties and high density lies below those prospective zones.  These are the characteristics expected from a sulphide rich zone.

Magnetic map (TF) of the Butler 3 area showing the location of the EM target and magnetic susceptibility anomaly

MacDonald’s upcoming drill program will focus on the Butler property: Prior to drilling, the targets will first be refined using a deep penetrating geophysical method, similar to that used to detect the HudBay – Lalor deposit at Snow Lake, Manitoba.

“I have been thinking for a while about writing on visualization of geophysical data. I finally got to it, and I am now pleased  to show you a technique I use often.  This tutorial has shaped up into 2 independent posts: in the first post I will show how to implement the technique with Surfer, in the second one with Matlab (you will need access to a license of Surfer 8.08 or later, and Matlab 2007a or later to replicate the work done in the tutorial).

I will illustrate the technique using gravity data since it is the data I developed it for. In an upcoming series of gravity exploration tutorials I will discuss in depth the acquisition, processing, enhancement, and interpretation of gravity data (see [1] and [4]). For now, suffice it to say that gravity prospecting is useful in areas where rocks with different density are laterally in contact, either stratigraphic or tectonic, producing a measurable local variation of the gravitational field. This was the case for the study area (in the Monti Romani of Southern Tuscany) from my thesis in Geology at the University of Rome [2].

In this part of the Apennine belt, a Paleozoic metamorphic basement (density ~2.7 g/cm³) is overlain by a thick sequence of clastic near-shore units of the Triassic-Oligocene Tuscany Nappe (density ~2.3 g/cm³). The Tuscan Nappe is in turn covered by the Cretaceous-Eocene flish units of the Liguride Complex (density ~2.1 g/cm³).

During the deformation of the Apennines, NE verging compressive thrusts caused doubling of the basement. The tectonic setting was later complicated by tensional block faulting with formation of horst-graben structures generally extend along NW-SE and N-S trends which were further disrupted by later and still active NE-SW normal faulting (see [2], and reference therein, for example [3]).”

Read more…

1] If you would like to learn more about gravity prospecting please check these excellent course notes.

[2] Niccoli, M., 2000:  Gravity, magnetic, and geologic exploration in the Monti Romani of Southern Tuscany, unpublished field and research thesis, Department of Earth Science, University of Rome La Sapienza.

[3] Moretti A., Meletti C., Ottria G. (1990) – Studio stratigrafico e strutturale dei Monti Romani (GR-VT) – 1: dal Paleozoico all’Orogenesi Alpidica. Boll. Soc. Geol. It., 109, 557-581. In Italian.

[4] Typically reduction of the raw data is necessary before any interpretation can be attempted. The result of this process of reduction is a Bouguer anomaly map, which is conceptually equivalent to what we would measure if we stripped away everything above sea level, therefore observing the distribution of rock densities below a regular surface. It is standard practice to also detrend the Bouguer anomaly to separate the influence of basin or crustal scale effects, from local effects, as either one or the other is often the target of the survey. The result of this procedure is typically called Residuals anomaly and often shows subtler details that were not apparent due to the regional gradients. Reduction to rsiduals makes it easier to qualitatively separate mass excesses from mass deficits. For a more detailed review of gravity exploration method check agai nthe notes in [1] and refer to this article on the CSEG Recorder and reference therein.