Abstract

Many ore deposits that are zoned or limited in vertical range appear to be related to stocks, to have formed near surface, or to occur in carbonate rocks. These relations can be explained by evidence from thermochemistry, thermodynamics, and thermal conductivity. 1) Metallic sulfides appear to be carried by ore fluids as complex ions or in the gas state. Deposition of such sulfides takes place through exothermic reactions when temperatures are reduced. Energy losses caused by thermal conduction and by work losses in fluids rising to lower pressures reduce temperatures and counteract heats of sulfide deposition. 2) When ore fluids are much hotter than surrounding host rock and/or when the enclosing host is rapidly heated by ore fluids, heat losses through thermal conduction are high. Such losses, which are anticipated near intrusive rocks, should cause a steep temperature gradient outward from the heat source and may cause relatively rapid sulfide deposition, reflected by nonpersistence or zoning. 3) Work losses that are highest in rising fluids near to the surface may cause the observed nonpersistence and zoning of near-surface sulfide deposition. 4) Limestone replacement by sulfides appears to be controlled by changes in chemistry in the ore fluids caused by large interchanges of material between host and fluid and by limestone solubility, which increases when temperature decreases. In such replacements energy losses caused by thermal conduction, work, and endothermic wall-rock reactions are probably of minor importance. 5) Deposition is persistent in silica-silicate host rock and in an environment with low work and heat conductivity losses. 6) Wall-rock alteration, except for limestone replacement, is not a cause of sulfide deposition. Large quantities of material are not exchanged and alteration is commonly exothermic or neutral.

First Page Preview

First page PDF preview
You do not currently have access to this article.