Page 112 - Introduction to Mineral Exploration

5: FROM PROSPECT TO PREFEASIBILITY 95

pit and the underground accesses. In addition detected, probably because of the use of a wider
the company investigated the potential of the sampling spacing. BGS also drilled two holes
area around the open pit and particularly its (Fig. 5.19) to check the subsurface extension of
strike extensions by taking soil samples for the western end of the Wheal Prosper system as
geochemical analysis along E–W lines (Hosking well as a copper anomaly adjacent to the old
1971). These suggested that the deposit contin- pit. Limited assays gave similar results (up to
ued to the south into the calc flintas. CGF then 0.3% Sn) to surface samples.
undertook a drilling program to test the poten- The third commercial program was a more
tial of the deposit at depth (Fig. 5.16). Drilling exhaustive attempt from 1979 to 1982 by Cen-
over the period 1964–66 confirmed that the tral Mining and Finance Ltd (CMF), a subsi-
mineralisation continued at roughly the width diary of Charter Consolidated Ltd, themselves
of the open pit but that the cassiterite was con- an associate company of the Anglo American
tained in thin erratic stringers. Drilling to the group. Charter Consolidated had substantial
north of the open pit showed a sharp cut-off interests in tin and wolfram mining, including
whereas drilling to the south showed that tin an interest in the South Crofty Mine and an
values continued from the slate host into a calc interest in Tehidy Minerals, the main mineral
flinta host where mineralisation became much rights owners in the area. CMF’s approach
more diffuse. Although the grades intersected included a re-evaluation of the Mulberry pit
were similar to those in the open pit, the size and a search for extensions, as well as a search
and grade were not considered economic, espe- for extensions of the Wheal Prosper system
cially when compared with Wheal Jane (even- and an attempt to identify any buried granite
tually a producing mine) which CGF was also cupolas with their associated mineralisation.
investigating, and the lease was terminated. Their program contained four phases. In the
The next phase of exploration was from first phase the Mulberry pit was resampled and
1971 to 1972 when the area was investigated regional soil geochemistry and gravity surveys
by Noranda–Kerr Ltd, a subsidiary of the Cana- were undertaken. The gravity survey identified
dian mining group Noranda Inc. This explora- a perturbation in the regional gravity field
tion built on the work of CGF and investigated along the Mulberry trend (Fig. 5.18). Regional
the possibility that the diffuse mineralisation shallow soil geochemistry samples were taken
within the calc silicates might be stratabound on 100 m centers and analysed for Sn, Cu,
and more extensive than previously suspected. As, Mo, and W. They showed an ill-defined
Noranda conducted another soil survey (Fig. southern continuation of the Mulberry system
5.17) that showed strong Sn and Cu anomalies in Cu data and a large Cu and As anomaly to
over the calc silicates. Four drillholes were the southwest of the Wheal Prosper system.
then sited to test these anomalies after pits Resampling of the Mulberry pit led to a re-
were dug to confirm mineralisation in bedrock. estimation of the resource to 1.1 Mt at 0.44%
The drillholes intersected the calc silicates but Sn using a 0.2% Sn cut off. This would form
the structure was unclear and not as predicted part of the resource for any mine.
from the surface. In addition although exten- The Phase 2 program in 1980 consisted of a
sive (>20 m thick) mineralisation was inter- detailed geochemical soil survey follow-up of
sected it was weak (mainly 0.2– 0.5% Sn) with anomalous areas identified in phase 1, as well
few high grade (>1% Sn) intersections. The as the planning of two holes to test the geo-
program therefore confirmed the exploration physical anomaly. The first hole (CMF1, Fig.
model but was terminated. 5.19) was a 540 m deep diamond-cored bore-
As part of an effort to encourage mineral hole that failed to find any indication of granite
exploration in the UK, the British Geological or associated contact metamorphism, although
Survey investigated the southern part of the it did cut vein mineralisation of 0.51% Sn over
area (Bennett et al. 1981). Their surveys also 3.1 m at 206.5 m depth. Because of the lack of
defined the southern geochemical anomaly to granite the planned second hole was cancelled.
the SW of Wheal Prosper, as well as an area of The results of the soil geochemistry survey
alluvial tin in the west of the area. The exten- encouraged CMF to progress to phase 3 with
sion of the Mulberry trend was not as clearly a deep overburden sampling program to the