Release: Herald Resources: Upgrade of resource estimate at Lae Jehe
Tuesday, May 27 2008 - 02:28 AM WIB
The updated estimated Combined Resources at Lae Jehe are 10.5Mt at 10.6% zinc, 5.8% lead and 4g/t silver. The Lae Jehe prospect is located about 1.5km north of the main Anjing Hitam orebody and 800m north of the planned concentrator site and is the most advanced target to provide future mill feed following the initial 7 years mine life at Anjing Hitam.
The resources still remain open along strike in both directions and at depth and there is thus very good potential for increasing the Lae Jehe resources with the ongoing drill programme.
Drillhole SOP290D has extended the known resources at Lae Jehe by making the deepest intersection to date. An almost continuous mineralised intersection of 35.6m was made from 763.3m downhole. The lower 13.2m of predominantly massive sulphides which averaged 12.4% zinc and 6.5% lead, from 788.6m downhole, is interpreted to be the down dip continuation of the M2 horizon; whereas the upper 22.4m zone (assays awaited) is interpreted as veined, remobilised sulphides with 2 bands, each 1m wide, of massive suiphides within a shear/fault zone, from 763.3m downhole. The estimated true width of the 13.2m massive sulphide zone is approximately 9.2m.
This intersection is approximately 200m downdip from SOP282D (17.9m @14.5% Zn, 7.6% Pb) and is 75m vertically beneath the base of the previous resources boundary. The inferred fault zone previously shown on the long section as possibly interrupting the down dip continuation of the resources has been removed from the long section as it is now not known where, or even if, this sub-vertical fault zone will actually intersect the mineralised horizon.
The Lae Jehe shale hosted zinc-lead mineralisation, especially the main upper zone where more than 90% of the resources are located, probably corresponds to the Main Mineral Horizon at Anjing Hitam. This upper zone contains 3 separate horizons of mineralisation (M1, M2 and M3) which can be demonstrated to have good strike continuity with spacing between the sulphidic horizons generally being 5-10m and sometimes less than 5m.
The basal zone, which also consists of 3 separate horizons (M4, M5 and M6), contains less than 10% of the total resources at a lower overall grade than the upper zone but with silver grades that are about 3-7 times those of the upper zone.
The Lae Jehe resource estimates are based on the results of 50 DD holes for a total of 22,753m within the resource area. Nearly all drillholes have their collars accurately surveyed in UTM grid and all holes are downhole surveyed at 30m intervals with an Eastman camera.
Drillhole intersection spacings are approximately 100m x 100m on the local grid, although some of the more recent intersections are more closely spaced. Core size is either triple tube HQ or NQ. Assay samples of quarter or half core at intervals up to 1m are selected within geological boundaries. Sample preparation and assaying are carried out by Intertek Testing Services (ITS) at Medan and Jakarta. Internal and external assay quality controls are implemented and results have been found to be satisfactory. Density values are derived from numerous direct measurements by ITS on selected drill core samples.
Wireframe geological models have been constructed using local grid cross sectional interpretations based on mineralised envelopes constructed at a nominal 5% zinc equivalent cut-off grade. Samples within the wireframes are composited to even 1.0m intervals. The zinc equivalent used is 1% Pb = 1% Zn which reflects expected medium term metal prices and is the same as used in the previous estimate.
A local grid block model was created for the estimate with block sizes of 1m EW x 20m NS x 4m vertical with sub-cells of 0.5m x 10m x 2m. The 3D wireframe models were used to constrain the block model in which blocks were assigned grade and density values utilising domain dependent, inverse distance cubed interpolation methods. No top cuts were applied to assay values; whilst blocks with less than 5% zinc equivalent were excluded from resource estimates.
Resource classification was carried out on the basis of continuity of mineralisation and drillhole spacing. Blocks were classified as Indicated Resources when those blocks were within 50m of a drillhole intersection and within the wireframe models, whilst those blocks which were greater than 50m and less than I 50m from an intersection and within the wireframe models were classified as Inferred Resources.
Mineralisation that is within 50m of the surface is not included in the resources as a result of the recommendation in the Lae Jehe Geotechnical Study, completed in late 2007, which suggested that ?no excavation should be undertaken within 50m of surface apart from development of portals, adits and the decline.? The tonnage affected by this decision is approximately 0.3Mt at grades similar to the average resource grade.
The current Combined Resources of 10.5Mt at 10.6% Zn, 5.8% lead equates to 1.72Mt of combined metal. The previously announced Combined Resources were 9.1 Mt at 10.4% Zn, 5.6% Pb which equates to 1 .46Mt of combined metal.
The deeper portion of the M2 horizon, which contains 75% of the overall resources, is still a prime target for further exploration. This deeper M2 mineralisation has now yielded 1.1 million tonnes per 100m interval of strike and still has about 400m of potential strike length to the south and 600m of potential strike length to the north, towards Bongkaras, to be tested. Lae Jehe thus has the potential to substantially grow with further explorations. (end of edited release)