By Dan Oancea - Twitter
Silver is an intriguing chemical element with lots of unique characteristics, properties and uses (including the folklore use as an antidote to vampires and a prime material for silver bullets). There are too many to be listed here, so I’ll just stick to the subject.
The Silver Deposit Conference hosted in Vancouver on December 14, 2006 continued with a short presentation of an applied exploration geochemist’s point of view (i.e. Carl Kuehn, IMDEX Inc.).
Sources and sinks for silver are primary and secondary minerals: native metals and alloys, sulfides, antimonides, arsenides, telurides, selenides, sulfosalts, basic sulfates and halides.
Silver could also be identified as a trace component in basic oxides, carbonates, Fe-oxides, Mn-oxides and organic matter (limonite, psilomelan, and lateritic soils could be successfully sampled for anomalous Ag concentrations). The ‘horn silver’ (chlorargyrite) occurs as a secondary mineral and it is stable in deserts (oxidizing environments). In sulphates, witnessing a supergene alteration, silver appears as a trace element.
An interesting feature is represented by the association of manganese dendrites with primary sulphides in carbonate terrains (in distal parts the only things that could be noticed are fine cracks filled with manganese oxides). This is an important district scale alteration feature very useful in silver deposit exploration (CRD type deposits) and it is called AMOM, an abbreviation which stands for Anomalous Manganese Oxide Mineralization Hypogene Effect on district scale. Remember, the little black cracks should be chip sampled and not the whole rock. Anomalous silver values would indicate a genetic association with a distal deep buried hydrothermal silver deposit.
Speaking of alteration (surface conditions) we have to note that silver shows a relative high mobility in acidic (pH<4) and oxidizing environments, where it is accompanied by Cu, Hg, As, Se (Pb, Bi, Sb, Ge, Tl). In low sulphidation deposits, characterized by small amounts of sulphides (e.g. pyrite) that could leach and generate an acidic environment, silver displays a limited mobility.
Gold-silver alloys would always display rims with a higher gold content as a result of the loss of the unstable silver. This results in pretty pure secondary gold. Actually, the process continues in placer deposits where little ‘gold’ flakes (electrum) show the same pattern of gold enriched rims. One could also factor in the fineness of the placer gold in estimating deposit’s maturity - the distance of transport.
In neutral to alkaline environments silver would display a medium (moderate) mobility. It is accompanied by another suite of chemical elements like: As, Se (Pb, Bi, Sb).
Mo and As are also good elements to keep in mind when looking for silver.
In a reducing environment silver is not mobile.
A downdip metal zonation in a primary silver deposit would look like: chlorargirite at surface, followed by native silver (secondary enrichment), then bisulphides (including silver sulphides). Silver grades to base metals at depth.
Real world strategies:
- Always go multi-element; use a larger suite As, Mo, Sb, Co, Ni, Cu Zn; look for chemical elements that are mobile and accompany silver in specific pH, Eh environments; know your target system chemistry;
- Don’t sample water or lake sediments as silver is not mobile in water;
- Collect rock chips; don’t forget to sample carbonate terrains affected by AMOM; anything starting with 2-3 ppm could be considered anomalous;
- Silver has a limited lateral dispersion in soils, which is good news; in lateritic soils silver is associated with organic material; go multi-element; any values over 1-1.5 ppm should be looked at;
- Stream sediments: Look for hydromorphic dispersion patterns over 0.5 ppm; you should also go multi-element;
- Best techniques:
MMI: Silver is well reproduced;
Biogeochemistry: Very useful; ash samples of mesquite having 1 to 10 ppm, or over, should be considered anomalous; again, go multi-element.
Questions have been raised by the audience as to why that much silver occur along the western coast of Americas. The usual subduction theory came handy, but at least we have found out that nobody knows for sure. That’s why an undisclosed amount of beer has been offered as reward for the first to come with the right answer. Nobody won, so next break pretty disappointed geologists could be seen sipping nothing else than coffee and soda.