Second case: Study of evidence & search of evidence on abnormalities

The objective when studying evidence is analyze the interest of the discovery to decide on the possibility of moving on to more detail prospecting: tactical geochemistry, soil geophysics, trenches, ditches, and sometimes directly to drilling.

The search of evidence on abnormalities consists of searching for “in situ” mineral, starting from geochemical or geophysical abnormalities or alluvial evidence.

In this part of the search the selected area with the most substances that are being looked for is generally known.

Most the time its about, defining the extension of mineralization, starting with one ore more actual facts. The value of this mineralization is judged in relation to its recognized or believed dimensions or its pitch.

The work will mainly be done with:

• Compilation of a geological map if there are enough outcrops or an outcrop map that would be the prospecting guide.
• Carry out meticulous sampling being as through as ever.

The task will be developed following the following mentioned points:

A) Make up of net or mesh

In some case you can take advantage of existing nodes, made for geophysical or geochemical prospecting. If there aren’t already the first tasks is outline a series of regular routes starting from a base line.

For example for a 1:2.000 removal scale these can be spaced every 40m and on each one of them a reference point every 20m.

Logically the kind of mesh used will be in function besides the scale, shape & size of the evidence. For example, it can be square or rectangular with variant dimensions of 20x20 m and 80x160m. 

This work is generally done with a compass and the estimation of the incline with a clinometer.

B) Removal approach

The removal approach is no different from an evidence search: observation of boulders, observation of outcrops, relative measures of stratification, to mass, faults, determining mineralization and their percentages, etc.. In a region where outcrop is low, all water and incision courses must be followed, to the top where they begin, searching for outcrop. 

Once at a point or place that is promising enough and where formations are strongly layered, a structural study is indispensable. In this status of the search special study should be given to: particular structures, (layer hinges, fractures…), specific horizons, (carbonated, graphite, silicificated or with cherts, conglomerated,…), specific paleogeography (channels, transgressions,..), position of mineralization in relation to an intrusion, veinlets, fracture zones, hydrothermal alterations, vegetation, etc..

The way of writing out the field book will be the same in an evidence search, but there will be a lot more sections and drawings. An evidence chart will also be reported.

C) Ditches

Ditches are specially used for mineralized bodies which width is the smallest dimensions, covered by some meters of soil or alluviums. There are machines that are normally rented out from local companies & you’d have to look for the most adequate one for the task at hand. Hydraulic shovels are normally the most useful.

Ditches are normally dug perpendicularly to the probable alignment of the evidence or the elongation of the geochemical abnormalities. In each case its important to keep in mind the risk of the walls collapsing, to avoid accidents.

If data is incomplete when deciding the direction of the ditches, two ditches will begin to be dug at a right angle from each other. The extension will be based on local data. A 1m width is enough and relating to its depth it should be enough to reach the in-situ rock.  

During the digging of ditches, mineralized soil should be separated from fertile soil, piling it up on a side of the ditch.