PERTH (miningweekly.com) – Gold junior ABM Resources has launched a review of its Old Pirate gold mine, in the Northern Territory.

The company said on Friday that the review would focus on the discrepancy being experienced between the mine’s estimated head grade and the mill reconciled grade.

The mill reconciliation currently accounts for only around 70% of the mine’s estimated gold production.

ABM reported that potential causes for this "unacceptably low" mine call factor have been examined, including the mining inventory model, actual mining and processing.

“It has become evident that production from areas mined to date has fallen short of the mining inventory model forecast by a greater amount than initially thought. Performance varies markedly between different mining areas, with some sections outperforming forecast and others significantly below,” the ASX-listed junior said.

The company added that while difficult to quantify, it was considered highly probable that the levels of mining inventory loss and dilution experienced during mining were also greater than provided for.

Key underlying causes of this were likely to be the inability to assign an appropriate representative grade to individual mining blocks and difficulty in cleanly excavating relatively narrow mineralised quartz veins from within blasted rock.

ABM further noted that gold lock-up in the processing plant was also proposed as a significant contributing factor in the lower production, and in an effort to assess the level of gold lock-up in the mill, a section of the mill lining was removed to recover material lodged behind it.

Although the analysis of this material was incomplete, ABM noted that preliminary estimates indicated that several hundred ounces of gold were potentially lodged behind the mill liners. However, this quantity of gold represented only around 10% of the apparent shortfall between the mine and the mill.

Other sections of the plant have also been closely examined, but only relatively small amounts of trapped gold have been identified and recovered. In addition, an independent review of the overall plant security has also been conducted, and did not identify any weakness in the security management or systems.

Detailed bench-by-bench analysis for each of the four pits would now continue in order to help establish the parameters needed to adjust the mining inventory model, ABM said.

The mining inventory model would then be updated and, along with operational data acquired since mining started at Old Pirate, would be used to re-optimise the openpit designs, revise the mine plan and update production forecasts.

The objective of the review was to excise underperforming areas from the mine plan, and where possible, expand mining in areas that were performing well.

The preparation of a revised mine plan was expected to take between three and four weeks, and ABM said that in the meantime, mining and processing operations would continue.

Old Pirate produced its first gold in June this year.