Using information from a single bore hole

Constraints
Density values measured from drill-hole SAL-31 (see drilling details) were used to constrain the deposit scale density contrast model. In particular, the cells intersected by the drill-hole, from 150m to 475m depth, were constrained so that recovered values for those cells were within the bounds definded earlier. Background density was chosen to be 2.5 g/cm³. In terms of geology, the bounded cells correspond mostly with massive sulphides, and about 25m of interlayered, quartz rhyolite and mafic volcanics.

Results
The inversion result, displayed in Figure 2, shows that changes over the original inversion can be observed vertically above and below, and laterally adjacent to the bounded cells. Notable features are

• The anomaly that corresponds with the sulphide ore has a higher density contrast throughout most of the deposit, not just around the bounded cells (the density contrast increases from 0.3 g/cm 3 to 0.4 g/cm 3 75m away from bounded cells).
• Consequently the anomaly is less spread out, yet values are still consistent with expected values and priori information.
• The model is more constrained along the fault that marks the lower extent of the deposit.
• There is much less mass contained in the model below the deposit than previously modeled. This is in contrast to the previous gravity inversion result that had a more significant continuation of high density contrast values at depth.
• Furthermore, the small density contrast anomaly that is thought to correspond with the keel is still present.
• Away from drill-hole, the model has not been significantly changed.

A subsequent section addresses the results of incorporating all the available drill-hole information in an attempt to further improve the model, and determine the density structure below, and to the side of where drill-holes have sampled the earth.