High-grade Gold Research Articles

Gold and native copper in supergene sulphides from the Mid-Atlantic Ridge

In 1985 black smokers and massive sulphide deposits were discovered in the TAG hydrothermal field in the rift valley of the Mid-Atlantic Ridge near latitude 26°08′ N (ref. 1). Sulphide samples from chimneys and mounds were recovered by dredging at depths between 3,620 and 3,670 m. Mineralogical and chemical analyses of the samples reveal (1) primary, unaltered Zn-Fe-Cu-sulphides containing 0.8–4.0 p.p.m. Au (0.02–0.12 oz per ton) and (2) secondary Cu-rich sulphides with native copper, formed by supergene alteration of primary Cu-Fe-sulphides, containing up to 16.4 p.p.m. Au (0.48 oz per ton) as free native gold. This is the first reported occurrence of secondary enrichment of gold and copper in recent submarine sulphides. The high gold grades and native copper associated with secondary Cu-sulphides resemble occurrences in some supergene gossans overlying ancient massive sulphide deposits on land2.

Geology of skarn and high-grade gold in the Carr Fork Mine, Utah

High-grade gold mineralization occurs as an overprint to copper-gold skarns in the Carr Fork mine. Geological and geochemical studies and geostatistical comparison of the distribution of gold in the copper-gold skarns and in the high-grade gold mineralization show that gold was deposited in two separate and distinguishable events: deposition with copper in skarn ores, under multiple controls, and high-grade gold mineralization, which formed a shoot in skarns hosted by the Parnell limestone. The gold content of material produced at this stage is much higher than that of the skarn ores.High-grade gold accompanies distinct alteration characterized by clay pyrite and quartz-pyrite(-tennantite), anomalous Au, As, Sb, Hg, Si, S, and Tl, and depletion of Pd, Ca, and Se. The high-grade gold mineralization comprises at least three episodes determined from petrographic and microprobe studies; it preceded the quartz molybdenum veins.The statistical characteristics of the high-grade gold are separable from the copper-gold mineralization in histograms, scattergrams, and variograms. The mean of the high-grade gold composites is 0.3 troy oz/short ton. Directional variograms of skarn ore show that a pervasive N 20 degrees E fault set and the distance to the Bingham stock contact controlled the copper and gold grades. Elevation seems to have been an additional control on gold grades in skarn.The shoot of high-grade gold mineralization in the Parnell limestone comprises a diluted drilled resource of 1.2 million tons. Evidence suggests that it continues some distance beyond present drilling.

The Johnson River Prospect, Alaska; gold-rich sea-floor mineralization from the Jurassic

The Johnson River prospect on the Alaskan peninsula is an unusually well preserved Jurassic example of gold-rich sea-floor mineralization accompanied by extensive anhydrite. The prospect is a gold-zinc-copper-lead deposit located within the dominantly dacitic middle portion of the lower Jurassic Talkeetna Formation, a complex succession of unmetamorphosed volcanic and volcaniclastic rocks. The mineralization at the Johnson prospect forms a discordant quartz-sulfide stockwork body in a series of subaqueous tuffs and debris flows.The mineralization and alteration can be divided into an early, pervasive nodular anhydrite-Mg chlorite-clay-sericite stage essentially barren of base metals and gold and a later quartz-sulfide gold stage associated with barite, Fe chlorite, sericite, and vein-type anhydrite. The abundance of sulfate far exceeds that of total sulfide at the Johnson River prospect. The anhydrite-bearing assemblages form a stratigraphically restricted zone in which anhydrite may constitute up to 80 percent of the rock. Textures indicating both replacement of fragments by anhydrite, and more commonly, open-space growth of anhydrite can probably be explained by anhydrite precipitation in highly porous unconsolidated tuffs. Early sulfide and oxide mineralization consists of jasper veins, sinuous pale sphalerite veins and chambers, and quartz-barite-sphalerite nodules accompanied by large volumes of chalcedonic quartz. Late sulfide mineralization includes chalcopyrite-rich veins and hydrothermal breccias accompanied by dark sphalerite, galena, pyrite, and native gold. High-grade gold is concentrated at the highest stratigraphic occurrences of significant copper mineralization.Whole-rock chemical analyses of anhydrite-chlorite rock shows that it is depleted in SiO 2 and enriched in MgO relative to stratigraphically equivalent unaltered tuff. The retrograde solubility of anhydrite, the prograde solubility of quartz, and the large Mg content of the chlorites suggest that this assemblage formed by heating of large volumes of cold seawater in a vigorous convection cell within the unconsolidated tuffs of the Talkeetna Formation. It is suggested that the convection cell was generated by the incursion of heated, metal-bearing fluids from below which eventually came to dominate the hydrothermal system with time as mineral precipitation gradually insulated the stockwork body from cold seawater.

Fluorapatite fenitization and gold enrichment in sheeted trondhjemites within the Destor–Porcupine fault zone, Taylor Township, Ontario

Pronounced fluorapatite mineralization is present in sheeted trondhjemites near Timmins. The trondhjemites intrude a deformed ultramafic to Fe-tholeiite volcanic sequence disposed in the structural hanging wall of the Destor–Porcupine Fault, and they bound a domain of high-grade gold ore. Spinifex-textured ultramafic flows record variable intensities of secondary alteration to a carbonate–Cr-muscovite assemblage. They are characterized by chondritic Al2O3/TiO2 ratios (19 ± 1.7), enhanced Cr (1570–2090 ppm) and Ni (470–1070 ppm), together with low levels of the incompatible elements Ta, Nb, and Th.Fresh quartz–oligoclase trondhjemitic intrusions are compositionally coherent with respect to SiO2 (average 63 wt.%), Al2O3 (16.3 wt. %), and Na2O (6.7 wt. %), as well as to the alteration-insensitive elements V, Sc, Zr, and Hf. Whole-rock δ18O values of 10.0–10.6‰, combined with "magmatic" quartz–feldspar (0.8–0.9‰), signify that these are primary high-18O magmas, likely derived by partial melting of 18O-enriched mafic granulites. Albitic trondhjemites are characterized by enhanced Na2O (10.8 wt. %) and are systematically enriched by ~1‰ relative to fresh counterparts, due to a preferential positive 18O shift of albite, resulting in negative quartz–feldspar. Sodium enrichment coupled with isotopic disturbance is attributed to low-temperature spilitization in the presence of marine water.A domain of high-grade gold ore is focussed in an intensely silicified unit. The ore is composed of quartz, feldspar, muscovite, carbonate, graphite, pyrite, molybdenite, and gold, with corresponding pronounced enrichments of Si, K, Rb, Ba, Cs, S, C, As, Sb, Pb, and Mo. The lithology is compositionally diverse but uniform with respect to Al2O3/TiO2 (17–27) and Ti/Zr (60–73). These features, combined with variably elevated Cr (50–955 ppm) and Ni (60–570), low contents of the incompatible elements, and a flat REE distribution, collectively indicate Fe-tholeiite and basaltic komatiite precursors. The silicified unit is bounded by trondhjemites overprinted by a secondary muscovite–Fe-dolomite alteration assemblage, with associated anomalies of CO2, S, As, Sb, Mo, and Pb. Quartz is isotopically uniform (δ18O = 14.1 ± 0.2‰ (1σ), n = 15) throughout the diverse lithologies overprinted by alteration linked to gold mineralization. Quartz is shifted ~+3‰ and albite ~−1‰ relative to fresh trondhjemites, such that Δquartz–feldspar = 1.8 to 2.6‰ in ore. Fluids implicated in gold mineralization were characterized by ambient temperatures of 330 ± 20 °C and by [Formula: see text]. Carbon-isotope compositions of hydrothermal ferroan dolomite are constrained at δ13C = −2.5 ± 0.8‰, but they are systematically heavier in graphitic rocks. δCgraphite averages −20.8‰.Fluorapatite occurs as megacrysts superimposed on trondhjemites and as microveins in a narrow domain cospatial with the Destor–Porcupine Fault. Its modal abundance correlates directly with P2O5 and F, which attain levels of 6.8 wt.% and 6800 ppm, respectively. Fluorapatite is close to end-member composition and possesses extremely enhanced REE abundances: 87Sr/86Sr = 0.7012, a value similar to that for contemporaneous mantle. Rocks have been extensively stripped of Si, K, Na, and Al in domains of fluorapatite, signifying a process allied to fenitization.

4606797 Method for recovery of high grade gold alloy from karat gold-clad base metal substrates : Arthur D Taylor, Roger W Ramsey assigned to Engelhard Corporation

4606797 Method for recovery of high grade gold alloy from karat gold-clad base metal substrates : Arthur D Taylor, Roger W Ramsey assigned to Engelhard Corporation

A Comparison of Gold Mineralization at Jerritt Canyon, Nevada, with Other Disseminated Gold Deposits of Basin and Range Province: ABSTRACT

The discovery of the gold deposits in the Jerritt Canyon area, containing in excesses of 2.25 million oz (70 million g) of recoverable gold, is one of the most significant precious metal developments in the last decade. The Bell Mine at Jerritt Canyon began production on July 4, 1981, and is currently producing in excess of 193,000 oz (6 million g) of gold per year from ore with an average grade of 0.24 oz per MT (7.4 g per MT). Disseminated sub-micron gold is hosted within oxidized and unoxidized portions of Upper Ordovician and Lower Silurian limestones, dolomites, and calcarenites that have been locally extensively silicified and subjected to low pH hydrous alteration. Hydrocarbons have been locally remobilized and oxidized and may have played an important role in gold precipitation. Structural control of mineralization is manifested by the location of higher grade gold along northeasterly trending normal faults at the intersection of favorable host lithologies. Jerritt Canyon is the most recently developed of a group of similar deposits within the Basin and Range province that collectively contain recoverable reserves in excesses of 16 million oz (500 million g) of gold. Similarities and differences between Jerritt Canyon and the other deposits are reviewed in terms of geologic setting, geochemistry, host-rock characteristics, structural controls, and ore reserves. End_of_Article - Last_Page 984------------

Evidence for a modified placer origin for auriferous conglomerates, Canavieiras Mine, Jacobina, Brazil

Gold is a trace constituent (average 0.015 ounces per ton) in the matrix of virtually all the 28 conglomerate beds exposed at the mine, and economic concentrations occur in 10 beds that are interpreted as giant foreset beds. The greatest concentrations occur close to the line where the foresets grade into topsets, supporting a placer origin for the gold. During subsequent folding, metamorphism, and intrusion, gold was redistributed to form concentrations at the top of the conglomerate beds and near dike contacts. Some gold is seen in fractures cutting conglomerate boulders. High-grade gold is associated with stringers of pitchblende. There is no evidence that gold is necessarily cogenetic with pyrite or uranium minerals.