Nomenclature, concepts and classification of oreshoots in vein deposits

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Oreshoots are discrete hypogene masses usually hosted within a planar channel, surface, lode or conduit which may be either a shear zone, fissure, fault zone, or lithologic bed or unit such as a contact. Oreshoots are characterized by breadth, strike (>1000 m) and dip, and plunge (100–500 m) lengths and have higher metal contents than the adjacent parts of the host conduit. The mass of most oreshoots ranges between 1 × 10 6 and 2 × 10 4 tonnes. There is a tendency for oreshoots to be thicker and richer in the center, rather than to have uniform grade distributions. The thickness of the oreshoots may be between 0.25 and 1.75 m in shear-zone-hosted deposits, to up to 60 m in replacement deposits. Several conduits may connect to form vein systems. Vein systems have common fluid sources which result in general homogeneity of alteration, mineralization types and oreshoot control, and, therefore, commonly share the same plumbing system. The internal constituents usually reflect unique episodes relating to ore formation. The main intern constituents in oreshoots are mineralization, gangue and alteration. These constituents usually mix with each other in complex patterns, the relationships between which may be used to interpret the processes of oreshoot formation. The term “ground preparation” represents the effect of various events in the geologic history of an ore district or oreshoot area that have assisted in enhancing the rocks so that oreshoots can preferentially form in certain areas or geometries. Several types of ground preparation can be recognized: (1) sequential deformation that produces a grain in the rock, (2) severe faulting and jointing which augments permeability and areas where ore minerals can precipitate, and (3) interplay between ore fluid and deformation to produce an oreshoot. Controls of oreshoot location and shape are usually due to dilatant zones caused by changes in attitude, splays, lithologic contacts and intersections. In addition, conceptual parameters such as district fabric, magic distances and stacking are also used to describe the geometry of oreshoots. Controls in vein systems and the location and geometry of oreshoots within vein systems can be predicted by a number of qualitative concepts such as internal and external plunges, district plunge, district stacking, conduit classification, gradients and warps. These concepts have a practical and empirical application in most districts where they are useful in the exploration for ore, but are of such broad and general application that they can rarely be explained definitively.