One Hundredth Anniversary Volume
From the first issue in 1905 onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed?
Let us first address the question, Who were the founders? They were the 12 men who collectivelydecided a new publication was needed, who then planned the financial structure to support the venture, and who served as the original editorial group. All were employed by, or associated with, the U.S. Geological Survey. Josiah Edward Spurr suggested the need for a journal sometime in November or December 1904. After informal discussions, nine of the founders met in the office of Waldemar Lindgren in the headquarters of the U.S. Geological Survey in Washington, D.C., on May 16, 1905, and founded the Economic Geology Publishing Company. The sole purpose of the company was the publication of a journal ‘...devoted primarily to the broad application of geologicprinciples to mineral deposits of economic value, and to the scientific description of such deposits, and particularly to the chemical, physical, and structural problems bearing on their genesis.’ Initial financing for the new company was raised by the sale of 80 shares at a cost of $25 per share.
Eight of the men at the founding meeting formed the first board of directors; Spurr was president, Frederick L. Ransome, secretary, and George O. Smith, treasurer. Other members were Arthur H. Brooks, Marius R. Campbell, Walter H. Weed, Waldemar Lindgren, and a young academic from Lehigh University in Pennsylvania, John D. Irving. Theninth man at the meeting was H. Foster Bain. Irving was appointed editor. Lindgren, Ransome, and Campbell from the U.S. Geological Survey, together with three academics, James F. Kemp of Columbia University, Heinrich Ries ofCornell University, and Charles K. Leith of the University of Wisconsin, were appointed associate editors. The initial board members, the editor, and associate editors are the people we now recognize as the founders of Economic Geology. Two others, Frank D. Adams, of McGill University in Canada, and John. W. Gregory, of Glasgow University in Scotland, were subsequently added as associate editors, and a third person, W. S. Bayley of the University of Illinois, was appointed as business editor, but
Hydrothermal Replacement Model for Witwatersrand Gold
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Published:January 01, 2005
Abstract
Hydrothermal and placer origins for Witwatersrand gold have been debated ever since the discovery in 1886 of the Central Rand goldfield in South Africa. The hydrothermal model is supported by recent findings of a complex postdepositional history for the Witwatersrand Supergroup, including extensive deformation, greenschist facies metamorphism with widespread alteration and associated gold mobility. Critical to the debate is the association of half the Witwatersrand gold with migrated hydrocarbons that were not in their present position at the time of sedimentation.
Gold is mined from planar reefs that are centimeters to meters thick and of several hundred square kilometers in area. The mineralogy of the reefs is unusual in having negligible iron oxides, hydrocarbons, and abundant round pyrite. The host rocks to gold include conglomerates and sandstones, with no single depositional environment or inferred depositional process that correlates with high gold-grade areas across the basin. In contrast, the chemical association of pyrite and/or migrated hydrocarbons is ubiquitous in the orebodies.
Regional metamorphism in all goldfields generated assemblages including pyrophyllite, chloritoid, chlorite, muscovite, and pyrite, with more restricted kyanite, biotite, kaolinite, and pyrrhotite. Peak temperatures of 300° to 400°C have been inferred with isograds semiparallel to stratigraphy. A period of hydrothermal alteration near the peak of metamorphism has overprinted much of the Central Rand Group in the goldfields, extending 300 km around the basin margin. This alteration has involved loss of Si, Fe, Mg, and Ca, with addition of K and Rb. Geometrically, this hydrothermal alteration coincides vertically and laterally with the distribution of economic gold and with areas of widespread sulfide distribution in all sedimentary rock types. The hydrothermal alteration is distinguishable from weathering by its geometry and the addition of K, Rb, and sulfur.
The hydrothermal model invokes uranium introduction in meteoric waters along the uplifted basin margin. During burial diagenesis, thermal maturation of organic material in Witwatersrand shales generated hydrocarbons that were carried by the migrating fluids and precipitated near unconformities, commonly in association with preexisting uranium minerals. Gold-bearing H2O-CO2-H2S fluids at 300° to 400°C were introduced to the Central Rand Group along major basin-bounding thrust faults and were channeled between the overlying Klipriviersberg lavas and the underlying marine shales of the West Rand Group. Fluid flow was controlled by bedding subparallel fracture networks and the sedimentary architecture of the basin that favored flow along lithologically complex reef packages on unconformity surfaces. The hydrothermal model predicts the distribution of gold in the Witwatersrand reefs through sulfidation of detrital iron-rich heavy minerals and precipitation with the migrated hydrocarbons. Hydrothermal gold mineralization is inferred to predate Platberg extensional faulting that displaces the orebodies.
The hydrothermal replacement model implies significant potential for exploration in younger sedimentary basins with similar tectonic and thermal histories. Basin architecture, structure, alteration, and suitable chemical traps are important exploration criteria.