CEO Blog – September 4, 2020 – GeoMagnetic Survey
Q: I would very much like to hear more about the helicopter survey. How exactly does it work? Are there hard examples of how this truly directs drilling programs to specific areas that not only help companies save money by not hitting empty holes but also discovery that might otherwise be missed by geologists?
A: This is a very dynamic topic so thank you for your question. For anyone who hasn’t seen the recent Press Release that prompted this question, please see:
The Comstock is an expansive, world-class, historic mining district and we have consolidated the substantial majority of the Comstock, having stakes in almost 10,000 acres of land and nearly 6 miles of contiguous mineralized trends.
The geology to the north, in Storey County, NV, represents some of the richest, historic silver and gold grades discovered in the United States, and the world, with rugged topography and hundreds of miles of underground mine workings from the late 1800’s. The Comstock Lode was discovered through physical outcrops, and then followed underground.
The southern part of the District fans into a valley (including our Dayton Resource Area and our Spring Valley exploration targets) that often has 30-40 feet of pediment covering most of the bedrock and mineralized structures. There aren’t many convenient outcrops guiding us to the mineralization.
Geotech expects to be flying their proprietary Versatile Time-Domain Electromagnetic (“VTEM”) geophysical system by mid-September, and plans to deliver three-dimensional interpreted results by mid-October. Those results will let us “look through” the sedimentary cover to greatly increase the Company’s understanding of the Dayton resource area and Spring Valley resource expansion potential, along with the Company’s other exploration targets in Lyon County.
A: For the technically inclined, from the Geotech literature, VTEM™ uses a coincident vertical dipole transmitter-receiver configuration that provides a symmetric system response and features a high-power transmitter and low noise receiver.
VTEM™ generates currents that diffuse into the earth and, similar to water, always take the path of least resistance. Conductive material absorbs the currents and releases a secondary field that the VTEM™ system measures. A strong conductor absorbs and releases more or all of the VTEM™ signal. A weak conductor absorbs and releases less or none of the VTEM™ signal.
VTEM™’s capacity to vary the pulse width provides us with an important competitive advantage. A long pulse energizes the earth for a longer period of time, allowing strong conductors to absorb more signal and release a secondary field. This enables VTEM™ to accurately assess the true conductance of the material. However, a long pulse shortens the time available to listen to the earth’s response. A short pulse, in contrast, provides more time to listen and allows for deeper exploration.
Q: How about an explanation for the rest of us?
A: The airborne survey is a tool to better locate exploration drill targets. The VTEM system is a tool to map subsurface physical properties that have relative conductive variations. The conductive properties of buried “sulfide mineralization” would have a relative high response compared to rock masses barren of sulfides. The measurement of conductive properties has an accompanying attribute: the measurement of resistivity properties (quartz and silica alteration are examples of materials that have a resistive response). In addition, the helicopter will have a magnetometer positioned between the helicopter and the VTEM system. The magnetometer measures magnetic field variations over the survey area. These variations can result from rock type changes as all rock units have specific magnetic properties, or they may be the result of the depletion of original magnetic properties by alteration and mineralization.
The airborne survey will result in a three-dimensional model of conductivity/resistivity (EM inversions), a three-dimensional magnetic model, and structural analysis with lineaments. The interpretive products will be superimposed upon CMI drilled mineralized bodies at the Dayton resource area. The signature of the known mineralization will be used to identify additional targets having similar VTEM signatures. J
Q: Are there hard examples of how this truly directs drilling programs to specific areas that not only help companies save money by not hitting empty holes but also discovery that might otherwise be missed by geologists?
A: Hard examples are usually confidential and proprietary unless that information has been released to the public. CMI did find a reference from Geotech: The VTEM contributed to a recent major gold discovery by Barrick at the Tusker deposit in the Lake Victoria gold fields of Tanzania. A web search also found a report by Andiamo Exploration that reported they found “22 VTEM targets” at their South Haykota VMS Mew area.
Closer to home, Comstock Mining has had success with targeting successful drilling based on the results of previous, less comprehensive geophysical magnetic surveys, leading to two major discoveries.
In 2009, our Spring Valley discovery hole #SV09-05 was targeted using a handheld magnetometer indicating an anomalous relative magnetic low. The drill hole found 40 feet averaging 0.182 opt Au, 40 feet below the surface. In 2012, our Genesee discovery hole #SV12-05 targeted a relative magnetic low adjacent to a relative magnetic high. The discovery hole found 235 feet averaging 0.031 opt Au, 125 feet beneath the surface.
Geophysical surveys are often an early stage exploration tool. With this Dayton survey, we have the advantage of previous drilling that provides an interpretation for a portion of the survey area. That will let us calibrate the geophysical results to identify targets with similar geophysical properties elsewhere.
Image from http://geotech.ca/