A bauxite deposit located in the state of Jharkhand in India has been studied for geostatistical modelling in the presence of trend. The deposit is a blanket type formed by residual enrichment of aluminium oxides and selective leaching of oxides of iron, titanium, silicon, sodium and potassium among others. Population modelling of six major variables (Al 2 O 3 , loss on ignition, Fe 2 O 3 , SiO 2 , TiO 2 and thickness) revealed the presence of a log-normal distribution in all cases. Experimental semi-variograms when computed exhibited presence of trend in the sample values. An attempt to fit a trend surface reflected the presence of a third degree trend. Sample values were then made free from the trend by subtracting the trend component from the sample values thereby yielding residual values (i.e. deviations from the trend). Semi-variograms computed for the residuals exhibited regionalised phenomena. Estimates of residuals have been obtained employing block kriging. The trend component has then been added to each of the block kriged residual estimates in order to yield estimates of block grade values. A comparison of block kriged estimates and values of the nearest drill composite samples made for reconciliation of results exhibited a high degree of match as evidenced by the correlation coefficient value (r = 0·83). Suresh Pandey (E-mail: s_pandey58@yahoo.com) and Bhabesh C. Sarkar (E-mail: bhabesh_sarkar@hotmail.com) are in the Department of Applied Geology, Indian School of Mines, Dhanbad-826004, India. Kalyan Saikia (for correspondence – E-mail: saikiak@yahoo.com) is in the Central Mining Research Institute, GMD Division, Room 14, Main Building, Barwa Road, Dhanbad–826001, Jharkhand, India © 2005 Institute of Materials, Minerals and Mining and Australasian Institute of Mining and Metallurgy. Published by Maney on behalf of the Institutes. Manuscript accepted in final form 22 November 2004. Keywords: Bauxite deposit, geostatistics, trend analysis, semi- variogram, kriging Applied Earth Science (Trans. Inst. Min. Metall. B) March 2005 Vol. 114 B1 DOI 10.1179/037174505X45441 Geostatistical modelling in presence of trend – a test case of a bauxite deposit in Jharkhand, India Suresh Pandey, Kalyan Saikia and Bhabesh C. Sarkar INTRODUCTION Exploration optimisation, accurate estimation of reserves and cost effectiveness in terms of quality and productivity for a mineral deposit have a marked effect on the life of a mine and ultimately on the growth and survival of a particular mineral industry in the user market. To build a spatial model representing adequately each characteristic of a deposit in terms of quality and quantity depends essentially on the available geological information of exploration data, the unbiased nature of information and, finally, the procedure employed for evaluation of the deposit. When evaluating a deposit, keeping in view the above mentioned facts and adopting a method for unbiased characterisation of the deposit, geostatistics provide a key role. As ore bodies with complexities are difficult to model accurately using conventional estimation methods, the present paper focuses on the adaptability of geostatistical techniques for resource evaluation of a bauxite deposit in the Jharkhand state of India. The approach provides a test case for the suitability of integrated geostatistical modelling to other bauxite deposits occurring in similar geological settings for characterisation of mineralisation parameters and developing an appropriate mineral inventory. GEOLOGY OF THE DEPOSIT Indian bauxite occurrences have been studied by various workers from time to time in respect of their geological setting, mineralogy, source rock and origin. The occurrences are broadly grouped as East-coast bauxites, Central-Indian bauxites, West-coast bauxites and the bauxites of Jharkhand and Orissa. 3,5,7 The bauxite deposit in the present study is located in the state of Jharkhand, India (Fig. 1) and is included under the broad group of bauxites of Jharkhand and Orissa. It is a flat-topped plateau deposit with adequate drainage conditions and an average elevation of 1057 m above mean sea level. The deposit has a steep scarp face on the south-east margin while in the south-west margin it is highly undulating with deep valleys. The average annual rainfall (150 cm), temperature variation (5°C in winter to 40°C in summer) and the geographical location (subtropical zone) of the area favour the formation of an extensive ‘blanket-type’ bauxite deposit. The thickness of the bauxite zone in the deposit varies between 2·28 m and 19·92 m with an average of 8·8 m with aluminous laterite capping at the top. The variation in thickness may be related to the formation process of the bauxite ore body. Chemical weathering and leaching are the main processes that are responsible for the formation of the