Thickness Of Ore Body Research Articles

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO2 (LOI), MgO, Ba, K and Rb, but significant depletion of SiO2 and, to a lesser extent, Fe2O3. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO2- and H2S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.

Geophysical exploration for interlayer slip breccia gold deposits: example from Pengjiakuang gold deposit, Shandong Province, China

ABSTRACTInterlayer slipping breccia‐type gold deposit – a new type of gold deposit, defined recently in the northern margin of the Jiaolai Basin, Shandong Province, China – occurs in interlayer slip faults distributed along the basin margin. It has the features of large orebody thickness (ranging from 14 m to 46 m, with an average thickness of 30 m), shallow embedding (0–50 m thickness of cover), low tenor of gold ore (ranging from 3 g/t to 5 g/t), easy mining and ore dressing. This type of gold deposit has promising metallogenic forecasting and potential for economic exploitation.A ground gamma‐ray survey in the Pengjiakuang gold‐ore district indicates that the potassium/thorium ratio is closely related to the mineralization intensity, i.e. the larger the potassium/thorium ratio, the higher the mineralization. The gold mineralized alteration zone was defined by a potassium/thorium ratio of 0.35. A seismic survey confirms the location of the top and bottom boundaries and images various features within the Pengjiakuang gold mineralization belt. The gold‐bearing shovel slipped belt dips to the south at an angle of 50–55° at the surface and 15–20° at depth. The seismic profile is interpreted in terms of a structural band on the seismic section characterized by a three‐layered model. The upper layer is represented by weakly discontinuous reflections that represent the overlying conglomerates. A zone of stronger reflections representing the interlayer slip fault (gold‐bearing mineralized zone) is imaged within the middle of the section, while the strongest reflections are in the lower part of the section and represent metamorphic rocks at depth. At the same time, the seismic reflection survey confirms the existence of a granite body at depth, indicating that ore‐forming fluids may be related to the granite. A CSAMT survey showed that the gold‐bearing mineralized zone is a conductive layer and contains a low‐resistivity anomaly ranging from 2 Ωm to 200 Ωm.

Primary geochemical anomalies in the Caijiaying Pb-Zn-Ag deposits, Hebei, China

The Caijiaying Pb-Zn-Ag deposits consist of five zones of mineralization occurring in hornblende-plagioclase granulite. A lithogeochemical survey was carried out within the area of the ore deposits; very strong primary anomalies of Pb, Zn, Ag, Cd, As, Sb, Hg and B, and weak anomalies of I, Cu, Co and Bi were found. The primary dispersion halo of each element is distinctive. The horizontal dispersal distances range from 20 to 90 m for Pb, Zn, Ag and Cd, and from 100 to 300 m for B, I, As, Sb and Hg. Vertically, Pb, Ag, As, Sb, B, I, and Hg tend to concentrate in the upper part of the ore zones, whereas Zn, Cd, Cu, Bi, and Co concentrate in the lower part. The zoning sequence in the deposits is (from above ore to below ore): (I, B, Sb, Hg, As)-(Ag, Pb)-(Zn, Cd)-(Cu, Co, Bi). Based on the zoning sequence, the evaluation index of ore anomalies is calculated and used to assess the potential of the primary anomalies. Five distinct mineralization zones are delineated by Pb × Zn product anomalies. The product-ratio As × Sb × B × Pb Cu × Bi × Co × Zn is used to determine the level of denudation of deposits; over the known blind ore zone E, the values increase progressively from above the ore to below the ore, but this tendency is not clear over the known eroded ore zone C. The element associations and evaluation indices of mineralized zones B and D are similar to those of blind ore zone E. Therefore, blind orebodies have been inferred at some depth below the known mineralization at B and D. Ninety-meter thick orebodies were discovered by drilling 100 m into zone B in 1992.

Stressed state of a rock mass with protective extraction in the wings of tectonic faults

For the October deposit conditions extraction of the gently-sloping deposit of copper-nickel ores within the wings numerous faults intersecting the ore body is connected with increased danger of rock bursts and the requirement for carrying out a considerable amount of labor-consuming driving of dead ends. Mining is carried out in a continuous sequence by layers and chambers with stowing of the worked-out space with consolidating mixtures. Previously by driving and filling parallel cross drifts over the upper contact of the deposit a continuous concrete layer is built. The workings are orientated almost along the strike of tectonics fractures [1]. In seeking the possibility of reducing the volume of driving in zones of tectonic faults industrial experiments were carried out in two areas of the deposit intersected by faults. In contrast to the traditional scheme for constructing a protective slice there was simultaneous erection of sections of a protective slice within the wings of a fault before approaching the extraction front of the main reserves towards the tectonic fault zone. It was assumed that presence of an additional front of a protective slice leads to mutual shifting of the wings of a tectonic fault with loosening of the ore mass. As a result of this treatment of the reserves in the wings of the fault was accomplished without creating a protective slice beneath part of the ore bounded by the overlaying strata. The set of instrument and full-scale observations confirmed this assumption. In test areas both in stopes created beneath the protective slice and in the overworked part of the ore body no signs of increased mine pressure were noted [2]. However, industrial experiments were limited by the specific mining and geological conditions. Observations and measurements made in the test areas give information about the mechanical state of the rock mass at a limited number of points. Transfer of the technology to other conditions and depths requires additional substantiation and studies connected with considerable expenditure and organizational difficulties. In this situation it is possible most simply by means of mathematical modelling to reveal the most common features of stress field formation within the vicinity of a fault. Calculations of the stress-strain state for a rock mass were performed by the finite element method. In the mathematical describing material deformation the rock mass outside the plane of a tectonic fault was represented by elastic materials with corresponding mechanical properties. This assumption is correct in view of the fact that the rocks comprising the rock mass have high elasticity constants and the theological properties appear to an insignificant extent. The deformation of a tectonic fault was described by an elastoplastic material reflecting the main features of fault behavior under load [3, 4]. The original stressed state for the ore and surrounding rock masses is characterized by the presence of tectonic forces. Horizontal compressive stresses are 0.6-1.7"rH, and vertical stresses are 0.6-1.43,H (H is working depth). The most clearly expressed regions of disturbance of the original stressed state are confined to large amplitude faults. According to measured data in them both horizontal and vertical stresses may predominate which may lead to a marked difference in the mechanism of rock mass deformation within the vicinity of a fault on introducing mine working. In the present work the case is considered when within the vicinity of a tectonic fault horizontal stresses predominate. The original stress field in all of the calculations discussed below is adopted as follows: Cry = 3'H, a x = 1.53,H, rxy = 0. Corresponding to this at the outer boundary of the calculation region normal and tangential force components were prescribed. The stress -strained state of the rock mass was determined for the plane strain condition which is fulfilled here due to the considerable extent of a tectonic fault, and stoping and preparatory workings in direction perpendicular to the section in question. Values of mechanical properties for the ore body, the surrounding rocks, and stowing material were taken as typical for the October deposit conditions [5]. The corresponding values of Young's modulus were 5" 104, 8- 104 and 3.104 MPa, and for Poisson's ratio they were 0.25, 0.2, and 0.4. The ore body thickness was 25 m.

Qualitative modelling: fractal geometry in mineral deposit evaluation

Abstract The spacial distribution of the physical characteristics of a mineral deposit (its elevation, thickness, grade, etc.) can be estimated with measurable limits of accuracy by geostatistical techniques, but the estimates are smoothed. Local fluctuations of such variables as hanging-wall position or orebody thickness may be of great importance to the planning function, since they can influence choice of equipment or have an important and unavoidable effect on grade dilution or ore recovery. Fractal geometry can be used to generate qualitatively detailed views of these variables, with their complexity explicit. This paper describes several constructs using fractal concepts for two-dimensional image generation. The images can be derived from data sets and used for purposes such as the planning of mineral extraction systems; they are faithful to the original seed data. The constructs are analysed and compared, through their semi-variograms, with conventional models.

Optimum technology of mining operations for contact zones of the Karagai quarry

The choice of a rational technology for drilling and blasting and for extraction operations in working the zones of contact of thick ore bodies with the surrounding rocks (of the type found in the Karagai deposit) enables us to improve the quality indices of the extracted magnesite (by reducing the losses and impoverishment).

Structure of the Broken Hill Mine area and its significance for the genesis of the orebodies

A comprehensive reinvestigation of the Broken Hill mine area shows that the structural evidence is consistent with, and reinforces, interpretation of the orebodies as stratiform premetamorphic deposits.The fold structure is interpreted in terms of three deformation episodes (F 1 , F 2 , F 3 ). F 1 and F 2 were accompanied by high-grade metamorphism, whereas F 3 is associated with retrograde metamorphism and with the initiation of retrograde shear zones.The principal macroscopic folds of the mine area are shown to be of F 2 age, and the earlier concept of a Broken Hill anticline is reinstated as the Broken Hill antiform of F 2 age. Folds which were thought to disprove the existence of the Broken Hill anticline are now seen as F 3 folds superposed on the F 2 antiform. The orebodies lie in the northwest limb of the antiform, the hinge of which is largely replaced by a synmetamorphic slide, previously recognized as the belt of attenuation.Evidence from metamorphosed graded bedding confirms the existence of the antiform and also shows that the macroscopic F 2 folds are superimposed on a previously inverted stratigraphic succession. This inversion is the consequence of F 1 deformation.The three-fold episodes were roughly coaxial, with southwest plunge, over much of the area but have different axial plane orientations. In the North mine area the plunge of the orebody and of the F 2 fold axes is northeasterly, but sillimanite lineations continue to plunge southwest. A new analysis of this region is given and, contrary to recent views, it is shown that the lode horizon, which contains the ore lenses, is conformable with the stratigraphic succession.These conclusions lead to the establishment of a new stratigraphic succession, in which the orebodies lie close to the interface between a lower, partly volcanogenic sequence and a higher, nonvolcanogenic sequence. The lead lodes lie stratigraphically above the zinc lodes.It is inferred, therefore, that the ore constituents were originally deposited in a zone of uncertain but probably linear shape, within a limited stratigraphic interval toward the end of a phase of volcanism represented by the Potosi gneisses. There have, however, been substantial mechanical migrations of ore, within the lode horizon, during the deformation history and it is this deformation which controls the details of orebody thickness and shape.