The application of computer software to orebody modelling and evaluation at South Crofty tin mine, Cornwall
Authors: Gribble P. D. 
Published: April 2008

The orebodies at South Crofty take the form of a series of steeply dipping composite fissure-veins/lodes in the classic style of this mining district. Extraction is underground. The tin bearing lodes belong to the ENE-striking series. Cassiterite mineralization is in two main stages characterized by tourmaline and chlorite gangue respectively. The paragenesis and distribution of this mineralization appears related to the structural evolution of a lode, or group of lodes.

There are two main groups based on lode composition. The first contains relatively simple veins whose dominant gangue is blue/black tourmaline, and which have limited wall rock alteration. Cassiterite mineralization is associated with a later quartz brecciation of these veins. The second group has a more obviously complex paragenesis with the development of a series of quartz and quartz-haematite phases. Cassiterite mineralization in this group is associated not only with tourmalinite veining, but also with a chlorite-fluorite event. Wall rock alteration, often haematite dominated, is often extensive.

In structural terms, both groups of lodes appear to have evolved in a similar manner, initially, with the lodes dividing in an en-echelon style. Division is limited to recognizable zones. Intralode shearing and open space mineralization, however, characterizes only the second group, where the tourmaline element of the lode zone may be lost as a result.

The resulting distribution patterns of cassiterite mineralization for these two groups are broadly elliptical when viewed in longitudinal projection.

A differing style of orebody is also present. These are zones comprising a series of relatively small flat lying veins together with replacement mineralization, making up steep ‘dipping’ sub-cylindrical bodies. Cassiterite belonging to both tourmaline and chloritic phases is present.

These distributions can be successfully modelled by computer. This is essential for grade prediction where information between mined levels is absent. In this example, SURPAC was used, enabling a powerful combination of string modelling techniques, together with a geostatistically produced block model. This last is computed using inverse-distance weighting with ellipsodial search envelopes.

Within the lodes, variation in the elliptical pattern is accounted for by modelling of differing trends, followed by extraction from each model using string intersection. The results are then combined into one model in longitudinal projection. Block grades are simply calculated by overlaying the block pattern, as string shapes, with resulting grades. Geological or mining reserves can be calculated using the same method.

Further modelling is required for the mineralized zones. This is achieved by using string structures and 3D visualization for sections through the orebody. These are defined from a combination of diamond drilling and interpretation. The use of strings ensures the original data co-ordinates are honoured. A three-dimensional triangulated surface is formed over these cross sections, with interpolation between sections controlled by the geologist. Zones of differing grade within the orebody can be similarly modelled. Volumes are simply calculated. The 3D model may contain proposed or existing underground workings, in addition to the orebody model. The complete model is then sectioned in a chosen orientation. These sections are used to enclose grade data held in a geostatistically calculated block model to complete grade calculations.