Stephen L. Chryssoulis

Gold in porphyry copper deposits: its abundance and fate

Abstract Porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits. Whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust. Cu/Au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5,000,000 with a median near 40,000, which separates gold-rich and gold-poor deposits. Gold is found in porphyry copper deposits in solid solution in Cu–Fe and Cu sulfides and as small grains of native gold, usually along boundaries of bornite. SIMS (ion probe) analyses of ore minerals from the gold-rich Batu Hijau, Kingking and Skouries porphyry copper deposits show that bornite contains about 1 ppm Au, whereas chalcopyrite contains about an order of magnitude less. Chalcocite and covellite contain 10–20 ppm Au, but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits. The amount of gold presently in solid solution in Cu–Fe sulfides is not adequate to account for all the gold in porphyry copper deposits, and the remainder is present as micron-scale grains of native gold. Experiments in the Cu–Fe–S–Au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600–700°C, and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200–300°C. In contrast, Cu/Au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary (potassic) ore and alteration are thought to form. This indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in Cu–Fe sulfides when the deposit forms at high temperature, and that gold is not commonly added later from other sources, although it can be redistributed during cooling or later events. The experimental data also suggest that high-temperature (600–700°C) vapors can extract considerably more gold from porphyry copper systems than can low-temperature (300°C) alteration. Comparison to Cu/Au ratios of volcanic emissions suggests further that high-temperature processes remove copper (relative to gold) from porphyry copper systems, whereas low-temperature processes remove gold preferentially and that this can account for deposits with extremely low and high Cu/Au ratios, respectively. Deposits with Cu/Au ratios between about 20,000 and 100,000, however, probably reflect different degrees of removal of gold or copper by immiscible sulfides.

Decoupled geochemical behavior of As and Cu in hydrothermal systems

Cu-rich and As-rich growth zones in pyrite provide new insights into the composition of late-stage magmatic fluids and their host hydrothermal ore deposits. The pyrite is from the Pueblo Viejo (Dominican Republic) and Yanacocha (Peru) high-sulfidation gold-silver deposits, which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes. Electron microprobe analysis, secondary ion mass spectrometry, and elemental maps show that pyrite, the most common sulfide mineral in both deposits, contains three different types of growth zones: (1) As-rich zones that are enriched in Au, Ag, Sb, Te, and Pb, (2) Cu-rich zones with significantly lower concentrations of these elements, and (3) barren pyrite zones with no other elements. These zones are interpreted to result from mixing between the pyrite-forming fluid and vapors that invaded the main hydrothermal system episodically. Comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic-hydrothermal systems suggests that the As-rich vapor formed at high and possibly magmatic temperatures, whereas the Cu-rich vapor formed at lower temperatures, possibly during migration of the original magmatic vapor. The presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high-sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors, probably from underlying magmas.

Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/ Betze sediment-hosted disseminated gold deposit, Nevada, USA

Abstract Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sedimenthosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from δ 34 S = −29 to 23‰ and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high δ34S values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids ( δ 34 S sulfide = −12 to −29‰ ) are extremely depleted in 34S relative to main-stage ore fluids ( δ 34 S sulfide = 16–23‰ ). Although such low δ34S values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.

“Invisible” silver in chalcopyrite and bornite from the Mantos Blancos Cu deposit, northern Chile

Relatively little is known about the mineralogical occurrence and geochemical controls on the incorporation of “invisible” (refractory) silver and gold in hydrothermal sulfide minerals. Secondary ion mass spectrometry (SIMS) analysis reveals that bornite (81–649 ppm Ag) and chalcopyrite (0.61–2211 ppm Ag) are major hosts for silver in the Mantos Blancos deposit (500 Mt, @1 wt% Cu), the largest Jurassic stratabound Cu-(±Ag) deposit in the Costal Range of northern Chile. Gold concentrations are generally two orders of magnitude lower, ranging from 0.05 to 1.66 ppm Au in chalcopyrite, and 0.08 to 2.38 ppm Au in bornite. In addition to precious metals, SIMS analysis shows significant concentrations of As (~100 ppm in chalcopyrite, <10 ppm in bornite), whereas other metalloids and chalcogens, such as Sb, Se, and Te, have highly variable concentrations ranging from tens of ppb to ppm levels. These microanalytical results are consistent with a two-stage hydrothermal evolution model, as recently proposed for the Mantos Blancos deposit. Within this context, Ag, Au, As, and base metals were most likely sourced from a Late Jurassic (~155 Ma) rhyolitic dome, and partitioned into bornite and chalcopyrite in quartz-sericite veins after cooling below ~430°C. This first hypogene Cu-Ag ± Au event was followed by a second, higher-temperature alteration phase (400–600 °C) related to the emplacement of diorite and granodiorite stocks (~141–152 Ma), in which Ag and Au were partitioned into fine-grained, porous chalcopyrite in potassic alteration vein assemblages. When coupled with recent studies in the area, results presented here confirm that the high Ag endowment of Mantos Blancos is the result of multiple pulses of hypogene mineralization followed by supergene enrichment of metals.