Fluid inclusions (FI) represent the best tool available for determining temperatures and pressures of ore-forming processes and provide the only direct method for determining compositions of ore-forming fluids. More than 58 000 individual fluid inclusion measurements from approximately 680 hydrothermal ore deposits have been compiled to describe the fluid environments of ore formation in Mississippi Valley-type (MVT) Pb–Zn deposits; orogenic gold deposits; epithermal precious and base metal deposits; porphyry copper, molybdenum, and tin–tungsten deposits; volcanogenic massive sulfide (VMS) deposits; skarn deposits; and Carlin-type gold deposits. For each deposit type for which appropriate data are available, homogenization temperatures, fluid compositions, fluid sources inferred from chemical and isotopic compositions of fluids extracted from fluid inclusions, and pressures (depths) of mineralization estimated from fluid inclusion data are summarized.

13.5 – Fluid Inclusions in Hydrothermal Ore Deposits

A study of carbon and oxygen isotope ratios in fine-grained sedimentary dolomite in the Barney Creek Formation of the McArthur basin demonstrates the presence of an extensive isotope halo surrounding the giant stratiform HYC Zn-Pb-Ag deposit. Dolomite within the halo shows an 18O-enriched and 13C-depleted isotope signature ( δ 18O = 23 to 26‰ SMOW, δ 13C = –2 to –3.5‰ PDB), relative to normal Proterozoic sedimentary dolomite beyond the halo ( δ 18O = 20–23‰ and δ 13C = 0 to –2‰). The C-O isotope halo within the dolomitic siltstones extends at least 15 km southwest of the HYC deposit and approximately coincides with a previously defined lithogeochemical halo of elevated Fe, Mn, Zn, Pb, and Tl. Dolomitic siltstone lamellae within the stratiform Zn-Pb-Ag ores at HYC exhibit an isotopic range similar to that of the halo dolomites, suggesting that the ore and halo equilibrated with the same hydrothermal fluid.

Modeling of isotopic exchange accompanying fluid-rock interaction suggests that the halo dolomites equilibrated with low-temperature fluids (50°–120°C), which were enriched in 18O ( δ 18O = 5 ± 5‰) but with an average crustal carbon isotope signature ( δ 13C = –6 ± 1‰). Our preferred interpretation is that the oxygen and carbon isotope halo at HYC is related to the development of an extensive brine pool. This pool was deepest in the vicinity of the HYC deposit adjacent to the Emu fault, and it became shallower to the southwest away from the fault. Using the carbon isotope fractionation equation between dolomite and HC\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(O_{3}^{{\mbox{--}}}\) \end{document}, it is possible to estimate the temperature variation at the base of the brine pool during the accumulation of the Barney Creek Formation. Brine pool temperatures were highest in and adjacent to the HYC deposit (40°–70°C) and decreased to values of 17° to 30°C remote from the deposit. These temperatures are similar to those recorded in the Red Sea brine pool associated with the Atlantis II metalliferous sediment deposit.

Based on our work at HYC and Lady Loretta, strata-bound 18O-enriched carbonate lithogeochemical halos may be a characteristic of the Proterozoic stratiform Zn-Pb-Ag deposits of northern Australia. These halos are more extensive than the narrow 18O-depletion halos recorded in dolomites surrounding skarn, Mississippi Valley-type and Irish-style Zn-Pb deposits. This fundamental difference in the isotopic halo characteristics of these groups of zinc deposits is probably related to the low-temperature, synsedimentary brine pool origin of the North Australian SEDEX deposits in contrast to the various replacement and open space fill origins of skarn, Mississippi Valley-type, and Irish-style deposits.

Carbon and Oxygen Isotope Halo in Carbonates Related to the McArthur River (HYC) Zn-Pb-Ag Deposit, North Australia:Implications for Sedimentation, Ore Genesis, and Mineral Exploration

The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns.

Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor.

Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively.

Analyses of phase equilibria and related thermodynamic calculations indicate that the log f O2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH.

We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower f O2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone.

The Genesis of Distal Zinc Skarns: Evidence from the Mochito Deposit, Honduras

Evolution and paragenetic context of low δD hydrothermal fluids from the Panasqueira W-Sn deposit, Portugal: new evidence from microthermometric, stable isotope, noble gas and halogen analyses of primary fluid inclusions

The compositions of flinclusions trapped in garnet, pyroxene, and sphalerite, measured with laser ablation‐inductively coupled plasma‐mass spectrometry, from the distal, dominantly limestone-hosted, El Mochito Zn-Pb-Ag skarn, Honduras, are very similar to those of proximal magmatic fl uids in granitoid-related mineral deposits. However, the El Mochito mineralizing fl uids are chemically distinct from basinal brines, demonstrating that in distal skarns, magmatic fl uids dominate, and that most element concentrations and ratios are unaffected by water-rock interaction. High fl uid temperatures (~325‐425 °C) throughout the paragenesis are also consistent with this hypothesis. A variety of elements and element ratios (e.g., Rb, Mn, K/Na, Mn/Na, Rb/Na, Zn/Na) clearly discriminates between magmatic fl uids and basinal brines. Elevated Ca most likely resulted from carbonate dissolution along the fl ow path. Zinc and Pb contents indicate that the ore metals were precipitated in the deposit in proportions that approached their ratios in the fl uid.

Source of fluids forming distal Zn-Pb-Ag skarns: Evidence from laser ablation–inductively coupled plasma–mass spectrometry analysis of fluid inclusions from El Mochito, Honduras

Hydrogen and oxygen isotope evidence for origin of MVT-forming brines, southern Appalachians

El Mochito, the largest operating mine in Central America, is a typical skarn sulfide-oxide, limestone-replacement lead-zinc-silver deposit. Fluid inclusion and stable isotope geochemistry of the skarn and sulfide mineralization at El Mochito are compatible with a high-temperature (350 degrees -500 degrees ) origin from hydrothermal fluids with delta D values of -40 to -90 per mil and delta 18 O values between 10 and -1 per mil which were probably of mixed magmatic-meteoric origin. A detailed study of stable isotope compositions of the host limestone around the El Mochito deposit indicate a gradual depletion in 13 C and 18 O with approach to the ore body, indicating that the changes in isotopic composition were imposed on the limestone during the hydrothermal activity related to the mineralization. Oxygen isotope exchange between the hydrothermal fluid and limestone produced a strong depletion in 18 O of up to 18 per mil relative to background limestone compositions. 18 O depletion is easily detected for distances of up to 4 kilometers from the orebody. The depletion of 13 C is much more restricted, with changes of up to 4 per mil in delta 13 C values relative to the inferred original limestone, extending only 30 m of the ore deposit. The original stable isotope composition of the limestone, as inferred from distal, unaltered samples (delta 13 C PDB [asymp] 5.0ppm, delta 18 O SMOW [asymp] 23.5ppm), is similar to that of other Cretaceous limestones. Small-scale stable isotope analyses of the matrix, relict fossils, and late-stage veins within the limestones, and comparison to the isotopic composition of the bulk-rock samples suggests that the extent of stable isotope alteration was a function of the initial grain size, the permeability of the rocks, the fluid/rock ratio, and the temperature of the hydrothermal system, as well as the isotopic composition and dominant C-bearing species within the hydrothermal fluid. Both extensive recrystallization of lime-mud matrices and the presence of faults and fractures may have enhanced the secondary porosity and permeability of the limestones, allowing for effective fluid infiltration and increasingly pervasive fluid flow with approach to the ore deposit. The mechanisms responsible for C and O isotope alteration of the limestone were calcite recrystallization and late calcite veining. Large stable isotope alteration halos within limestones surrounding ore deposits of this type provide useful guides to exploration.

Rodrigo Vazquez

Papers

Hydrogen and oxygen isotope evidence for origin of MVT-forming brines, southern Appalachians

Carbon and oxygen isotope halos in the host limestone, El Mochito Zn-Pb-(Ag) skarn massive sulfide-oxide deposit, Honduras