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Gold/Mining/Energy : BET YOUR ASSAY - Mining Terms Explained -- Ignore unavailable to you. Want to Upgrade?

To: Walt who wrote (356)	3/19/1998 2:26:00 PM
From: Terry J. Crebs	Read Replies (2) \| Respond to of 463

Polarbear, Walt did just fine. Walt really doesn't need a geo-wizard (i.e., what geologists, geochemists, and prospectors call a mining geophysicist) to explain your questions. But here's my two cents anyway: 1. Non-magnetic sphalerite usually is the primary ore-mineral for "zinc-rich massive sulfides". Magnetic lows can be a good indicator for zinc mineralization; but magnetic lows are also caused by barren, altered rock and also be aware that certain vein- or ore-body geometries will give you prominent "north-side lows" in northern lattitudes. A good geophysicist should be able to diagnose (or interpret) the magnetic low correctly--a good geologist will always recommend a good "geo-wizard" to interpret the geophysics. (For example: Labrador's Voisey's Bay Ovoid orebody was missed on the first 5 of 6 boreholes because "somebody" didn't understand the importance of "north-side and south-side" magnetic lows .) 2) A 1-milligal (mGal) anomaly can be diagnostic. Here's the definition, 1 Gal (named after Galaleo) is a unit of acceleration of 1 centimetre-per-second-squared (i.e., the acceleration of the earth's gravity at sea level is about 980 cm/sec-2 or 980 Gal), so one milligal is one-thousandth of a Gal or about one-millionth the accerelation of the earth's gravity. (Voisey's Bay Ovoid is about 32 million metric tonnes of nickel-copper-cobalt ore at a depth of 20 metres and exhibits a Bouguer gravity anomaly of about 4 mGals.) Shallow massive sulfide mineralization almost always yields prominent gravity anomalies, but barren ultramafic rocks can also yield a pretty gravity anomaly too. (Again, a good geophysicist is usually required to interpret the gravity correctly.) 3) VLF can be hard to interpret, as its response can be degraded by hilly or rugged topography and it is also plagued by "geological or cultural noise" (i.e., bogus responses from shears, faults, conductive-sediments, fences, powerlines, etc.) which can mask the response from mineralization. Coincident VLF and Mag anomalies are always better. (VLF mapped the northern and southern edges of the Ovoid very well, and "confirmed" my interpretations of the Maxmin (another EM technique) and Mag.) 4) and 5) Sound like good base-metal drill targets to me. Time and the drilling will tell if their geophysicist interpreted the anomalies correctly. Good Luck, Terry J. Crebs Lakewood, Colorado P.S. Geophysicists have biases like everyone else--another geophysicist may have a different take on Golden Briars release.

To: Walt who wrote (356)	3/22/1998 2:19:00 PM
From: 1king	Respond to of 463

Walt, Here is a belated response to your post on geophysics. My two coins I guess. 1)Mag lows and mag highs are relative. An occurrence of massive sulfides (e.g. magnetic pyrrhotite Fe7S8) surrounded by a magnetite-rich host will still be a "relative" low. Everything is geophysics is relative. Kimberlite pipes are infamous for this. Your point on the association, either positive or negative is correct. 2) The gravity question is sort of loaded in that you call it a "prominent" feature. With no amplitude or frequency (size and shape) information, little can be said of this anomaly. The relative density contrast (delta rho) is indeed the critical factor. But remember even a very subtle density contrast (e.g. chemical variation within plutons) can cause significant anomalies at one scale or another. The big picture "gravity data" should actually contain the information on how or if the anomaly and the mineralized dyke are related. It is rarely a blind guess. 3)Coincident Mag/Inductive-VLF is rare and ambiguous. Most VLF anomalies are not caused by inductive coupling. That is the transmitted field (i.e. Cutler) induces a secondary electrical field in the body which is subsequently measured by our black box. Most anomalies are current, channelling along the contact between two features of differing physical characteristics. This is where the true power of VLF lies. I would have to say that VLF is the most misused and under interpreted geophysical data collected. It maps better than most geologists I have met! The rest seems straight forward from the tone of your text. All in all pretty interesting. But everyone should remember! Geophysical anomalies are like ---holes, everybody's got one! Discrimination and understanding are the keys, not simple possession. 1King