Abstract:

Within an open pit mine the chemistry of direct rainfall, in-pit run-off and surface run-in, and groundwater inflows will all be influenced by contact with the in-pit rock materials, mobilising solutes released by weathering of the exposed rock surfaces and broken materials. Garvie et al (2014) presented a numerical assessment indicating that sulphide oxidation rates and associated solute release from talus accumulated on the pit benches would be the main source of solute release within a pit, greater than that released by oxidation and flushing of fractured wall rocks.

This paper presents a review of site-specific water chemistry data for solute release rates, mineralogical controls on in-pit chemistry for a copper deposit in Africa. Data from short term leach tests, field cribs and mine wall tests were used in the evaluation.

The data indicates that the fragmented talus material shows higher levels of solute release than in-situ mine wall or seepage into the pit. This is explained by higher surface area and longer residence time of water with the talus compared to the pit walls. Contact of seepage water with the pit walls is limited to fractures and the immediate exposed surface. Material in the field displayed little or no infiltration in the crystalline rocks. Consequently the main source of solute load into the base of the pit for sulphate and trace metals and major cation species is the talus.

When assessing mine pit water quality it is recommended to include testwork that characterises solute release from the talus material as well as the pit walls and incorporate this term into any numerical prediction. This should includes mineralogy, particularly supergene minerals, porosity and grain size variance compared to the pit walls. For crystalline rocks similar to the case study it is equally important to determine loading from the pit walls to be restricted to fractures and the surface layer of the pit. Utilizing this modification will improve predictions of wallrock seepage chemistry in open pit geochemical predictions and allow for more focused management for operations and closure. Furthermore, by recognising talus as a large source of solute release, this offers the opportunity to potentially reduce solute loading into pit contact waters by reducing or removing the talus mass on pit benches where practical and safe to do so.

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Ms. Amy Prestia