Kindly help me to understand anomalous concentration of silver in the range of 2-5 ppm in Volcano-plutonic rocks of Rajasthan, India.

There is a emphasis on anomalous silver concentration in volcano-plutonic rocks of Siwana Ring Complex, Barmer District, Western Rajasthan, found in the range 2 to 5 ppm based on the analysis of a total of 41 surface samples from a area measuring 30 km x 25 km, there is a need to review whether this silver concentration is really anomalous for an area in Rajasthan state, India?.

As per the, Indian Minerals Yearbook 2012 (Part- I) 51st Edition (Advance Release),State Reviews (Rajasthan), Government of India, Ministry of Mines, Indian Bureau of Mines, Indira Bhavan, Civil Lines, Nagpur – 440 001, Dated December, 2013, it is stated that more than 85% of the country's potash, wollastonite, lead & zinc and silver resources are located in Rajasthan. The State is said to possess substantial share of the total resources of potash (94%), lead & zinc ore (89%), wollastonite (88%), silver (81%), gypsum (82%), fuller's earth (74%), diatomite (72%), marble (64%), asbestos (62%), copper ore (50%) and rock phosphate (30%).

The concentration of silver in U.S. deposits from which silver is economically minable as the principal product at 2001 silver prices was typically about four orders of magnitude greater than the crustal average, or about 700 g/t. The concentration of silver to be recovered as bye product associated with Zn-Pb-(Ag) deposit of Rampura in Rajasthan, India is 45 ppm Ag.

Moreover as per the definitions in the literature, the anomalous concentration of an element in any area has been defined/arrived at after careful detailed investigations. As per the website, http://www.frontiermining.kz/sections/Glossary , the word ‘Anomalous’ is defined as: value of a given element that is deemed to be above the background or normal value.

As far as the meaning of the word ‘Anomalous’ : adjective describing a sample, location or area at which either (i) the concentration of an element(s) or (ii) a geophysical measurement is significantly different from (generally higher than) the average background concentrations in an area. Though it may not constitute mineralization, an anomalous sample or area may be used as a guide to the possible location of mineralization.

As per the, U.S. Geological Survey Bulletin - Page B-12 - Google Books Result books.google.co.in/books?id=FCXwAAAAMAAJ , 1983 - ‎Geology, Page 812:

Analyses of an element were considered anomalous if they exceeded the concentration in 95% of the samples of typical host rocks. Elements having concentrations more than two orders of magnitude above the minimum anomalous concentration were classified as highly anomalous.

As per ‘A Discussion of Some Definitions in Economic Geology’, Anomaly – without a qualifier, this term is vague, yet it is at the heart of all the above definitions ( Beware the siren of “obviousness” – Bertrand Russell said that “obviousness is the enemy of correctness”). There are mineral anomalies and geochemical anomalies, defined statistically, but there are also gravity, induced polarization, and magnetic anomalies to name but a few, and these are also commonly used in mineral exploration. And what of alteration? When is alteration “anomalous”, as apposed to being just another exploration vector?

Therefore, one may define, generally, an anomaly as a local ‘deviation’ from the general surroundings, as defined geologically, geochemically, or geophysically.

Regarding geochemical anomalies, average crustal values are not very useful in defining an anomaly. One needs to establish the regional background. In a deeply weathered, or lateritic terrain, the background will be different from the background in a terrain comprising, e.g., altered volcanic rocks. A threshold relative to the proper background may be defined, which may be the mean plus two standard deviations (sd), making a positive anomaly something in the upper ~2.5%. Even this (which was codified in the classic exploration text by Hawkes and Webb, 1962), is controversial.

Rather than use the properties of normal distributions, and estimate dubious parameters like the population mean and sd from data that violate the assumptions of normalcy, (e.g., drawn from one population, not skewed, independence of samples from each other, etc.), some workers now prefer using the median rather than the mean, and the median of the absolute deviation of all analyses from the median (the” median absolute deviation, or MAD), rather than the sd. A positive anomaly is then defined as the median + 2MAD.

refer to the link: http://www.currentscience.ac.in/Volumes/106/02/0159.pdf

http://www.currentscience.ac.in/Volumes/106/02/0159.pdf

copy of my comments is attached herewith ( published in Current Science).

Article Anomalous silver concentration in volcano-plutonic rocks