Mt. Milligan Interpretations

Mt. Milligan
Interpretations

Geology and gold grades from drill core

This project was carried out early in the development of inversion methods. It was used because there was already extensive subsurface information about the deposit from previous work, which provided information to help validate the inversions. Such validations can be easily made if in-situ measurements of physical properties are available. Unfortunately, none existed at Mt. Milligan, and therefore, correlation with available geologic and mineralization data was done instead. Geologic logs from 600 drill holes in the analysis region were used to construct a 3D rock model that contained overburden, fault location, and five of the most common rock types including monzonite, trachytes, and latites.

A cross-section at 9600N is shown in the following figure. Also available for comparison was a 3D model of gold distribution in the MBX deposit that was composited from assayed values of drill core. Qualitative gold grades were assigned as categories 1 through 4, with 4 being the highest. Contours of these gold grades are shown on the geology figure here. The major features on this cross section are:

the monzonite stock that is truncated on the left by the Harris Fault;
the arcuate feature, called the Rainbow Dyke, extending from the stock to the surface on the right;
trachyte dykes between the Rainbow dyke and the central monzonite unit;
gold grade contours, coloured according to grade 2 in blue, grade 3 in pink, grade 4 in red.

Overlays of geophysical results with geology and gold grades

Click the buttons to see models sliced along line 9600N.

Interactive comparison of 3D models

Here are two 3D images of the Mt. Milligan ore body generated by 3D inversion of magnetic data, and full 3D inversion of chargeability.

Frame one of susceptibilty model

Frame one of chargeability model

Click either image to open new browser windows with interactive rotating animations of these models. Both can be opened at once.

Discussion of Results

Comparison of inversion results with alternate information (particularly geology and geochemistry derived from borehole sampling) is an important part of converting geophysical results into geological information that contributes to making useful decisions. Images such as those above can help enhance the understanding of geophysical results, as well as increase the reliability of geological understanding.

Here is a summary of what was learned from the 3D susceptibility model, based upon the complete paper of Li and Oldenburg, 1997:

The recovered susceptibility model shows high values associated with the monzonite stock. This is expected because of the initial magnetite content of the stock.
There is also a susceptibility low in the area enclosed by the Rainbow Dyke.
Superposition of gold onto the magnetic susceptibility indicates that the region of high gold values corresponds to a low susceptibility, or equivalently, to low magnetite concentrations. The superposition of gold concentration onto the rock model shows that high gold concentrations were centred on the boundary between the stock and the host. This is an area where intense hydrothermal activity has occurred.
A drill hole, spotted on the basis of this joint interpretation of the geophysical and geological data, would have penetrated the area of the highest gold concentration.
The anti-correlation between low susceptibility regions and zones of high gold values helped geologists modify their mineralogical model to include a secondary hydrothermal alteration event in which a second stage low temperature event deposited gold in a reaction that consumed magnetite. Details are provided in the primary reference, Li and Oldenburg, 1997.

preproc.