Page 306 - Introduction to Mineral Exploration

12: CLIFFE HILL QUARRY AGGREGATE RESERVES 289

Aggregate Abrasion Value (AAV) and Polished E–W sections. In all cases the volumes were
Stone Value (PSV). converted to tonnages by multiplying the vol-
The Leicestershire quarries have historically ume by the average relative density of the rock
supplied stone to the profitable British Rail in a saturated, surface dried condition. In this
High Speed Track ballast market. In order to case the value 2.7 was used for the fresh rock.
qualify for this market the stone must achieve The final calculations were considered more
a very low and consistent value in a non-British precise and were accorded a confidence limit
Standard wet attrition test. A larger size mater- of ±10%.
ial is required for this test and consequently
only the 60-mm cores could be used. As testing
progressed, it proved possible to establish a us- 12.4.5 Quarry planning
able correlation between the water absorption As the drilling and sample testing continued
values determined on all samples and the wet with a progressive clarification of the com-
attrition values. bined overburden and weathered rock thick-
Over 120 samples of core were tested for nesses, and of the overall structure, shape, and
some, or all, of the properties mentioned above. quality of the deposit, three possible operating
The results provided the data necessary to de- schemes were devised for developing the new
termine which markets could be penetrated quarry in conjunction with recovering the size-
and what type of basic plant would be required able resources which would become available
to produce the materials to satisfy those outlets following the dismantling of the plant at the
in terms of both anticipated quantity and prod- old quarry.
uct specification. Bell and Hopkins (1988) described these
three options as follows. A straightforward op-
tion to work out Cliffe Hill Quarry completely,
12.4.4 Reserve estimation
while towards the end of its life, develop-
The confidence in the resources grew as the ing Stud Farm as its natural successor. This
amount of data increased and several refine- scheme had several drawbacks. The dimen-
ments of the estimation method evolved as sions of both quarries would impose a ceiling
follows: on potential output. The extension of Cliffe
1 After the initial phase of flight auger drilling Hill would require a road diversion and reloca-
an inferred resource estimate (USGS 1976) of tion of the existing plant and unfortunately the
some 20 Mt ± 100% was quoted. only feasible site for the latter would position
2 By the time the first planning application it closer to an adjacent village. This potential
was submitted, a generalized bedrock and de- plant site was also the only area suitable for
posit shape had been defined by widely spaced overburden disposal. Even if these problems
boreholes and some typical cross-sections had could be resolved for Cliffe Hill it would be
been drawn. A resource estimate based on the necessary to install a second new plant when
area of the cross-section, extended to the mid Stud Farm opened. This scheme was costed
point between sections likely to be recovered out on the basis of capital investment and the
using an average quarry configuration, gave return it would yield. The main stumbling
an enhanced figure with a confidence limit of block proved to be the cost of establishing
±20%. two plants, each with a relatively short life.
3 By 1989 with much of the relevant drilling These would have been economically viable
completed, two sets of cross-sections (N–S and above certain quarry outputs but neither
E–W) at 50-m intervals were drawn. With a quarry could supply the required outputs on a
provisional quarry development scheme super- long-term basis.
imposed, a more precise reserve estimate was The second option was the creation of a
produced. The extractable area on each cross- quarry at Stud Farm at an early date with a new
section was multiplied by the distance to the processing plant and quarry. Once this was
midpoint between the adjacent sections on commissioned the existing Cliffe Hill plant
each side. The N–S sections were considered would be demolished but crushing and screen-
best but the figures were also verified using the ing would continue using a mobile plant. An