Seismic velocity (Vp) vs alteration mineralogy in the Olympic Dam IOCG deposit

In support of a deep drilling program to define the lower extents of Fe-oxide alteration and potential sulfide mineralisation, BHP undertook a feasibility study to determine the viability of conducting Sparse and Full 3D hardrock seismic surveys of the Olympic Dam deposit. P-wave velocity measurements were systematically collected, via hand held instruments, on diamond drill core spanning the spectrum of alteration and brecciation styles present across the breccia complex and into the granite host rock. The purpose of this data was to provide measured seismic velocities into the feasibility modelling and subsequent processing and interpretation of the seismic surveys. Prior to data collection several questions were posed to focus the data collection. These questions included 1) what are the velocity (Vp) values of the various alteration and brecciation styles present in the deposit, 2) can Vp be used to directly detect sulfide mineralisation, and 3) are the Vp contrasts across faults great enough to detect the faults? The results of the velocity measurement study, correlated with bulk dry density, magnetic susceptibility, Fe-oxide alteration intensity and sulfide mineralisation clearly demonstrate that 1) disseminated sulfide mineralisation is likely not detectable via Vp measurements, 2) hematite breccias when in fault contact with granite and granite-breccias have enough of a Vp contrast to be detected in seismic surveys, 3) porosity varies significantly within the breccias and does influence Vp, and 4) Vp contrasts in the granites surrounding the deposit are too low to be used for vectoring to Fe-oxide alteration. Geochemical and mineralogical data combined with geological observations provides a powerful dataset when unravelling the characteristics of complex ore deposits. One of the major insights from this study, is that when petrophysical data (such as Vp, magnetic susceptibility, and bulk dry density) are also used, the multitude of possible geological interpretations can be significantly reduced.