John; The halo effect refers to the differential gradient in the concentration of certain elements(often traces) that have responded to the large mass of sulphide, copper,nickel, gold etc. The first type are those that are in with the sulphides, but are more mobile than the assorted sulfides, but geologically travel with them. These spread out in a halo that is highest in the deposit and decreases with distance. Chemical analyses over a large area can sometimes identify such a gradient. Think of it as a dead skunk on the highway. The skunk is the ore body and the smell is the halo. Play find the skunk, and hope your dog does nor roll in it. Wind can make the pattern teardrop shaped, like ground water flow does with the chemical halo
The other type is the depletion or absorption halo. Some elements want to live in sulfides and over time leave the area and travel to the sulfides. This is a result of a domino effect on the gradient. The traces next to the sulfide are absorbed, close traces then go down the gradient to the sulfide(where the concentration is high due to all that has been absorbed. Think of a paper towel roll placed on end
into a pool of green water, with a bit of gelatine and a bit of dye to see it with dissolved in it making it like treacle or corn syrup in viscosity 1/16" deep. The green stuff will wick up into the roll, and close to it the layer will shrink to a low value, and far away it will revert to the 1/16" deep. The circular area around the towel is a depletion halo, and the towel is full of it, and far away no effects are seen.
Trace element assays for certain transition metals and others are used for type 1 and 2 halo analysis, and can ofetn be used to guide drilling in conjunction with other geophysical methods. A halo can be tear drop shaped to reflect ground water migration patterns, and can also be folded, monged etc. Often the B horizon is sampled, as this is the point at which the O2 from the air is present as an oxidizer, thus changing the under soil area from reducing to oxidising. Certain halo metals adsorb to clay in this area and those samples can be assayed for halos. It is not easy or exactm as other factors affect the halo metals, so other calibration elements are used in conjunction to correct and level the halo measurement to find a true gradient.
The sellers of geochem assays lead you to believe that it works like a charm. few of them have become rich by finding another Kidd Creek,
so it is not a royal road.
Bill |
