Tin mineralisation occurs at Renison Bell, near Zeehan in Tasmania, as a
group of cassiterite-bearing massive pyrrhotite bodies in Cambrian
sedimentary rocks. Mineralising fluids from the underlying Pine Hill
Granite have replaced carbonate rocks with massive sulphides and
cassiterite. The discrete blocks of massive pyrrhotite have high density
and high magnetic susceptibility contrasts with the comparatively barren
country rock. They are strongly remanently magnetised and are excellent
geophysical targets. A detailed geophysical model of the mine was created from a database
comprising 1:2500 scale geological mapping and geological logs of
approximately 4000 drill-cores. The model comprises 93 parallel
cross-sections, each 1.5 km long, spaced 20 m apart. Each cross-section is
comprised of up to 150 polygons which represent all known individual
rock types and orebodies. The magnetic response of the model was
computed and the residual between the model and the observed response,
measured by a low-level helicopter magnetic survey, revealed several
anomalies unaccounted for by the model. Drill testing of these anomalies
led to the discovery of several hitherto unknown massive sulphide
occurrences. Surface gravity data of the mine and surrounding district were
enhanced by applying digital terrain corrections. The corrected Bouguer
anomaly revealed a hitherto unknown positive anomaly associated with
the ore zones of the Renison mine. The gravity anomaly of the model was
also computed. Unfortunately, the limited resolution of the observed data
prevented the delineation of ore targets. The terrain-corrected Bouguer gravity data were also used to model the
shape of the underlying non-magnetic Pine Hill Granite. A computer
inversion method was implemented to build a 3D block model of the
granite. Results agreed with previous conventional 3D modelling
techniques but yielded improved resolution of the granite form. The project involved development of new computer software to build
and edit the large and complex 3D geophysical model of the mine and to
compute the model's magnetic and gravity responses. Software was also
developed to build and compute the gravity response of a 3D block model
of the underlying granite.