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THE ORIGIN OF CARLIN-TYPE GOLD DEPOSITS Greg B. Arehart, Department of Geological Sciences, University of Nevada, Reno, NV 89557
 

Summary: THE ORIGIN OF CARLIN-TYPE GOLD DEPOSITS
Greg B. Arehart, Department of Geological Sciences, University of Nevada, Reno, NV 89557
Although presently known to be common only in western North America, CTDs are a significant source
(~11%) of world gold production. These deposits are characterized by extremely fine-grained disseminated
gold, hosted primarily by arsenian pyrite. Gold may be present dissolved in the lattice, or as atomic clusters
in sizes below the limit of microscopic observation. The host rocks are dominantly silty carbonates, but ore
is also present in siliceous and silicified rocks as well as intrusive rocks. Alteration consists of
decarbonatization,silicification(jasperoidformation),andargillization,whicharearrangedbothspatially and
temporally in that order. Argillic alteration is zoned from kaolinite-dominated cores to sericite-dominated
margins. The deposits commonly exhibit significant structural (faults) and stratigraphic
(composition/permeability) controls.
Because of their fine-grained nature and the lack of macroscopic features such as veins, it has proven
quite difficult to extract geochemical data that are clearly related to their genesis. However, fluid inclusion
data indicate pressures corresponding to depths of 2-4 kilometers under lithostatic conditions. Temperatures
are constrained by fluid inclusions and phase equilibria to near 225C. Stable isotope data from alteration
minerals and fluid inclusions indicate that the ore fluids were dominated by meteoric waters, some of which
had clearly exchanged oxygen with wallrocks during their passage through the crust. In addition, some recent
data are supportive of the presence of a magmatic fluid. Sulfur isotope values reported from CTDs span a
wide range, from -30 to +45 (sulfides & sulfates), with ore-related sulfides (pyrite, realgar) falling between
0 and +20. The most likely ultimate source of sulfur was sedimentary; bedded barite is abundant in the

  

Source: Arehart, Greg B. - Department of Geological Sciences, Mackay School of Mines, University of Nevada, Reno

 

Collections: Geosciences