RE: RE: RE: GTA news out
Iron formations are typically deposited as chemical sediments along with other sedimentary rocks (such as greywacke and shale), which means that they formed through chemical precipitation from seawater on the sea floor. They are commonly inter-layered with other sedimentary or volcanic rocks. They contain both iron and silica that has been derived from submarine magmatic and hydrothermal activity. Iron formations are of economic interest as they can host large gold deposits. The largest deposit hosted by iron formation is the Homestake Mine in South Dakota (125M tonnes containing 35M ounces). The Homestake deposit is over 2,400 meters deep. Examples in Canada, are Meadowbank (3.6M oz), Lupin (3.4M oz), Pickle Crow (2.7M oz) and Musslewhite (2.1M oz).
Porphyry deposits are the world's most important source of Cu and Mo, but they are also major sources of Au. In Canada, they account for more than 40% of Cu production, virtually all Mo production, and about 10% of Au production. Porphyry deposits are large, low- to medium-grade deposits and are related to felsic porphyritic intrusions. They are distinguished from other granite-related deposits by their large size and structural control, mainly stockworks, veins, vein sets, fractures, and breccias. Metal, mineral, and alteration patterns tend to be large, yielding useful clues to areas with exploration potential. Large pyritic halos, for example, may be used to delineate the extent of the deposits, and also the intensity and complexity of the hydrothermal system. The dispersed nature of sulphide distribution in porphyry deposits is particularly amenable for the application of induced polarization (IP). Porphyry deposits typically range in size from 30-200 million tonnes. Au grades typically range from 0.8 to 2 g/t Au. Examples in Canada are the Young-Davidson (3.8M oz - syenite porphyry), Eagle Gold (6.4M oz - granodiorite porphyry), Colomac (1.5M oz - quartz-feldspar porphyry) and Troilus (1.7M oz - diorite porphyry).