Lode Deposits

Lode deposits are a very important source of precious metals, although they also can contain base metals. They are the dominant gold deposits of the Precambrian shields of Canada, Australia, South America and Africa – veins and shear zones, holding native gold, pyrite, quartz, and carbonate minerals. They can also form stockworks or dissemination zones, occcupy “saddles” in the hinges of folded strata.

Deposits like these are found in greenstone belts, areas of metamorphosed volcanic or sedimentary rocks. Greenstone belts are most numerous in Precambrian shields, but occur throughout geological time. The deposits themselves can have almost any host rock, but mafic volcanic rocks, felsic intrusive rocks and some sedimentary rocks are the most common ones. More often than not, they occupy the contact between two different rock types.

One characteristic shared by all the lode deposits is their occurrence in tectonically deformed zones. The deposits cluster around large regional fault zones, and the mineralized bodies themselves are in zones of intense structural deformation, with fracturing, fault brecciation and shear zones.     

Intense deformation makes for a wide variety of shapes and forms for the deposits. Shear-hosted gold deposits can be comparatively straight; shear zones have been very accurately described as “zones of straightening” in which all the planar features in an area– beds, fractures, faults and dykes – are squeezed into near-parallelism. Veins can be straight or quite sinuous, disseminated mineralization can have almost any form, and stockworks and saddles often form plunging, crudely pipe-like zones. In turn, any of these shapes can be folded into more complex ones, or be cut off by a fault and reappear somewhere else.

The wallrocks of the veins often show very strong alteration. The mineralization in the veins themselves can be very irregular, marking it necessary to drill and sample these deposits very closely before making any attempt to calculate their grade and tonnage.