|McDonald's Oparure Lime Quarry extracts limestone from the latest Oligocene to earliest Miocene aged Otorohanga Limestone formation of the Te Kuiti Group. The quarry produces a range of limestone grades that are utilised by various industries, including the iron and steel, agricultural, and roading construction industries. One of the most important aspects of production is quality control which is driven by quality restrictions on the chemical composition of the limestones imposed by the industrial end users. In steel manufacture, silica is highly undesirable as it has detrimental effects on downstream processes such as the ability of calcium oxide to remove impurities from the steel and its abrasiveness inside pipes transporting lime into the steel making vessel. A guideline of lt;1.7% silica content in the lime is required.
McDonald's Lime Ltd and major shareholders Holcim New Zealand Ltd are aware that silica levels vary across the Oparure limestone resource, and so hold some concerns about their ability to maintain quality guidelines on their limestone products. Consequently, it is relevant that knowledge be gained about the nature, distribution, and origin of silica-rich zones within the limestone resource so as to allow for greater certainty in quarry planning and operations.
The main aims of this study were to determine the chemical stratigraphy of the limestone units in the quarry so as to assess primary sources of silica in the intact limestone (host limestone); to use field methods to measure the nature and distribution of a variety of discontinuity features which separate the intact rock blocks within the limestone rock mass; to identify sources of secondary silica-rich materials and their characteristics; and to determine the overall distribution of silica within the limestone resource. Additionally, a ground penetrating radar (GPR) trial was undertaken to establish whether this geophysical technique is successful in detecting subsurface caves that may trap silica-rich materials.
The host limestone or intact rock mass of the quarry units contains variable amounts of calcium carbonate, as follows: Caprock (av. 89.5%), Upper Steel (av. 98.2%), Aglime (av. 94.5%), High Grade (av. 97.2%), Lower Steel (av. 97.3%), and Sub-economic (av. 95%). The remaining non-carbonate component largely resides in silica-bearing minerals such as clay minerals, quartz, feldspar, and glauconite, but also in authigenic minerals such as pyrite and gypsum.
In McDonald's Oparure Lime Quarry six main discontinuity types are identified, all of which are associated with silica-rich materials. The six discontinuity types are: (1) discrete seams; (2) diffuse seams; (3) subhorizontal stylolites; (4) subvertical stylolites; (5) joints; and (6) caves and other karst features. Types (1), (2), and (3) have formed as a result of burial of the carbonate-rich sediments during which pressure dissolution has preferentially dissolved skeletal fragments, leaving behind and concentrating insoluble residue (siliceous material). These are primary sources of silica. Types (4) and (5) are produced as a result of tectonic activity, including deformation and uplift. Subvertical stylolites formed by pressure dissolution associated with tectonic stresses during uplift of the limestones, and their contained silica also derives from a primary source inside the limestone strata. Type (6) discontinuities are produced by dissolution by fresh water of the subaerially exposed limestone. Joints, caves, and other karst features trap silica-rich materials sourced mainly from overlying overburden lithologies, including the Quaternary Kauroa Ash and the early Miocene Mahoenui Group mudstone; these are secondary sources of silica. A distinctive leathery clay mineral called palygorskite occurs commonly as a chemical precipitate in both joints and caves as an infill. A final source of silica identified that is not a discontinuity material, but is operation induced, involves the accumulation of blast and road dust coating rock surfaces, here referred to as surface accumulations.
The overburden units and discontinuity materials show a range of chemical compositions and some, such as cave infills, comprise greater than 60% SiO2. The main siliceous minerals within overburden units and discontinuity materials are clays (smectite, palygorskite, gibbsite, vermiculite, halloysite, illite), quartz (and minor cristobalite), feldspar (oligoclase), and glauconite. Carbonate minerals including mainly calcite and minor dolomite.
GPR successfully detected cave structures to an effective penetration of about 10 m, but cave infills cannot be quantified using this method.
The knowledge gained in this study about the distribution, location, quantities, and nature of the silica materials in the limestone units, closely related to the different discontinuity types identified, has the potential to assist in determining appropriate processing techniques to minimise the silica issue at the Oparure Quarry.