Economic Geology 

About: Economic Geology is an academic journal published by Economic Geology. The journal publishes majorly in the area(s): Pyrite & Mineralization (geology). It has an ISSN identifier of 0013-0109. Over the lifetime, 8326 publications have been published receiving 286916 citations.

Principles of geostatistics

Abstract: Knowledge of ore grades and ore reserves as well as error estimation of these values, is fundamental for mining engineers and mining geologists. Until now no appropriate scientific approach to those estimation problems has existed: geostatistics, the principles of which are summarized in this paper, constitutes a new science leading to such an approach. The author criticizes classical statistical methods still in use, and shows some of the main results given by geostatistics. Any ore deposit evaluation as well as proper decision of starting mining operations should be preceded by a geostatistical investigation which may avoid economic failures.

Porphyry Copper Systems

Abstract: Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Systematics of sulfur and carbon isotopes in hydrothermal ore deposits

Abstract: The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors.The isotopic composition of both sulfur and carbon in hydrothermal minerals is strongly controlled by the f (sub O 2 ) and pH values of hydrothermal fluids as well as by the temperature and the isotopic composition of sulfur and carbon in the fluids (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values). For example, at 250 degrees C and within geologically important f (sub O 2 ) -pH regions, an increase in f (sub O 2 ) value by 1 log unit or in pH by 1 unit can cause a decrease in delta S 34 values of sulfur-bearing minerals by as much as 20 per mil. An increase in f (sub O 2 ) by 1 log unit or in pH by 2 units can cause a decrease of about 30 per mil in delta C 13 values of carbon-bearing minerals. Large variation in the delta S 34 values or in the delta C 13 values of hydrothermal minerals, which often have been interpreted as an indication of biogenic sulfur or carbon, could also be caused by slight variation in the f (sub O 2 ) and/or pH of ore-forming fluids during ore deposition.The concentrations in an ore solution of sulfur (or f (sub S 2 ) ) and of carbon (or f (sub CO 2 ) ) place limits on possible delta S 34 and delta C 13 values for hydrothermal minerals. Sulfur-bearing minerals and carbon-bearing minerals precipitating from sulfur- and carbon-rich solutions can have wider ranges of delta S 34 and delta C 13 values than those minerals precipitating from sulfur- and carbon-poor solutions.Sulfide minerals which precipitated in equilibrium with magnetite, hematite, or sulfate minerals, and carbonate minerals which precipitated in equilibrium with graphite, could exhibit isotopic compositions markedly different from those of the depositing fluids. Therefore, sulfides with delta S 34 values near zero per mil or carbonates with delta C 13 values near -6 per mil do not necessarily indicate a magmatic origin for the sulfur or the carbon.The mode of variation on the delta S 34 values of sulfide minerals and in the delta C 13 values of carbonate minerals in a given deposit may indicate the relative oxidation states of ore-forming fluids: variable delta S 34 + uniform delta C 13 values may suggest that the minerals were precipitated under relatively high f (sub O 2 ) conditions; uniform delta S 34 + uniform delta C 13 values, under intermediate f (sub O 2 ) conditions; and uniform delta S 34 + variable delta C 31 values suggesting deposition under relatively low f (sub O 2 ) conditions.Sulfur and carbon isotopic data, combined with geological and mineralogical data of ore deposits, may define the physico-chemical parameters (T, f (sub O 2 ) , f (sub S 2 ) , f (sub CO 2 ) , m (sub Sigma S) , m (sub Sigma C) ) and the origin (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values) of ore-forming fluids as well as the mechanisms of ore deposition.

The isocon diagram; a simple solution to Gresens' equation for metasomatic alteration

Abstract: Gresens' (1967) method of analysis of changes in volume and concentrations during metasomatism have been applied in many studies of hydrothermal alteration. This paper provides a simple method of solution of Gresens' equations, for both volume (or mass) change and concentration changes, one which requires no significant manipulation of analytical data and is readily accomplished both graphically and on a computer spreadsheet. Gresens' equation is rearranged into a linear relationship between the concentration of a component in the altered rock and that in the original. Simultaneous solution of such equations for all components that show no relative gain or loss of mass defines an "isocon." On a graph of the concentrations in the altered rock against those in the original, an isocon is a straight line through the origin. The slope of the isocon defines the mass change in the alteration, and the deviation of a data point from the isocon defines the concentration change for the corresponding component. As is shown, this can be applied to several stages of alteration simultaneously, and to other kinds of mass transfer such as migmatization.