Alkaline Porphyry Research Articles

Geochemical Characteristics of Zircon and Apatite in the Alkaline Porphyry in Beiya Gold‐Polymetallic Deposit, Western Yunnan Province, South China

Geochemical Characteristics of Zircon and Apatite in the Alkaline Porphyry in Beiya Gold‐Polymetallic Deposit, Western Yunnan Province, South China

Delineation of geochemical anomalies using factor analysis and multifractal modeling based on stream sediments data in Sarajeh 1:100,000 sheet, Central Iran

The aim of this study is to delineate the Cu, Au, and Pb anomalies in Sarajeh 1:100,000 sheet located in Urumieh-Dokhtar ore belt, central Iran. The analyzed elements of stream sediment samples taken in the area can be classified into six groups (factors) by factor analysis. The concentration–area and number–size multifractal inverse distance weighted models were applied for recognition of the elemental thresholds which are similar in both used multifractal models. According to the thresholds, the elemental concentration distribution for Cu, Au, and Pb were divided to three lithological classifications, namely mainly c alkaline porphyry with Cu–Au mineralization, mafic and sedimentary rocks. The results illustrate that the major anomalies of Cu, Au, Pb and related factors are mostly located around intrusions, volcanics, and along NW–SE faults.

Subepithermal Au-Pd Mineralization Associated with an Alkalic Porphyry Cu-Au Deposit, Mount Milligan, Quesnel Terrane, British Columbia, Canada

At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu ( 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur.

Delineation and explanation of geochemical anomalies using fractal models in the Heqing area, Yunnan Province, China

The Heqing area, located in the Sanjiang ore belt, China, consists of the Beiya gold orefield related to the alkaline porphyry, the Emeishan volcanic mafic rocks and a series of sedimentary rocks. Thirty-nine elements of stream sediment samples taken in the 1:200,000 geochemical survey in the Heqing area can be classified into four groups using principal component analysis. Two fractal models, i.e., the concentration–area model and the number–size model, are applied in determination of the thresholds for the representative elements in the four groups. The thresholds obtained from the two models are similar. According to the thresholds, the element concentration distribution can be divided into 3 segments, each of them is mainly correlated to one type of rocks, including the alkaline porphyry related to gold-mineralized rocks, mafic rocks and sedimentary rocks. This paper reveals that the various geological events can be characterized by the different fractal models of element distribution.

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.

Geochemistry and stable isotope composition of fresh alkaline porphyry copper tailings: Implications on sources and mobility of elements during transport and early stages of deposition

Prevention of acid mine drainage (AMD) in sulfide-containing tailings requires the identification of the geochemical processes and element pathways in the early stages of tailing deposition. However, analyses of recently deposited tailings in active tailings impoundments are scarce because mineralogical changes occur near the detection limits of many assays. This study shows that a detailed geochemical study which includes stable isotopes of water ( δ 2H, δ 18O), dissolved sulfates ( δ 34S, δ 18O) and hydrochemical parameter (pH, Eh, DOC, major and trace elements) from tailings samples taken at different depths in rainy and dry seasons allows the understanding of weathering (oxidation, dissolution, sorption, and desorption), water and element pathways, and mixing processes in active tailings impoundments. Fresh alkaline tailings (pH 9.2–10.2) from the Cu–Mo porphyry deposit in El Teniente, Chile had low carbonate (0.8–1.1 wt.% CaCO 3 equivalent) and sulfide concentrations (0.8–1.3 wt.%, mainly as pyrite). In the alkaline tailings water, Mo and Cu (up to 3.9 mg/L Mo and 0.016 mg/L Cu) were mobile as MoO 4 2− and Cu(OH) 2 0. During the flotation, tailings water reached equilibrium with gypsum (up to 738 mg/L Ca and 1765 mg/L SO 4). The δ 34S vs. δ 18O covariations of dissolved sulfate (2.3 to 4.5‰ δ 34S and 4.1 to 6.0 ‰ δ 18O) revealed the sulfate sources: the dissolution of primary sulfates (12.0 to 13.2‰ δ 34S, 7.4 to 10.9‰ δ 18O) and oxidation of primary sulfides (− 6.7 to 1.7‰ δ 34S). Sedimented tailings in the tailings impoundment can be divided into three layers with different water sources, element pathways, and geochemical processes. The deeper sediments (> 1 m depth) were infiltrated by catchment water, which partly replaced the original tailings water, especially during the winter season. This may have resulted in the change from alkaline to near-neutral pH and towards lower concentrations of most dissolved elements. The neutral pH and high DOC (up to 99.4 mg/L C) of the catchment water mobilized Cu (up to 0.25 mg/L) due to formation of organic Cu complexes; and Zn (up to 130 mg/L) due to dissolution of Zn oxides and desorption). At 1 m depth, tailings pore water obtained during the winter season was chemically and isotopically similar to fresh tailings water (pH 9.8–10.6, 26.7–35.5 mg/L Cl, 2.3–6.0 mg/L Mo). During the summer, a vadose zone evolved locally and temporarily up to 1.2 m depth, resulting in a higher concentration of dissolved solids in the pore water due to evaporation. During periodical new deposition of fresh tailings, the geochemistry of the surface layer was geochemically similar to fresh tailings. In periods without deposition, sulfide oxidation was suggested by decreasing pH (7.7–9.5), enrichment of MoO 4 2− and SO 4 2−, and changes in the isotopic composition of dissolved sulfates. Further enrichment for Na, K, Cl, SO 4, Mg, Cu, and Mo (up to 23.8 mg/L Mo) resulted from capillary transport towards the surface followed by evaporation and the precipitation of highly soluble efflorescent salts (e.g., mirabilite, syngenite) at the tailing surface during summer.

Origin of lamprophyres by the mixing of basic and alkaline melts in magma chamber in Beiya area, western Yunnan, China

Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO 2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al 2O 3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K 2O(4.9–5.5 wt.%), TiO 2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/ 144Nd 0.512372–0.512536, middle 87Sr/ 86Sr 0.707322–0.707395, middle 206Pb/ 204Pb 18.50–18.59, 207Pb/ 204Pb 15.60–15.65 and 208Pb/ 204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO 2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/ 86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or 70Ab 28An 2) and 35% SiO 2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan.

Porphyry Cu–Au and associated polymetallic Fe–Cu–Au deposits in the Beiya Area, western Yunnan Province, south China

The Alkaline porphyries in the Beiya area are located east of the Jinshajiang suture, as part of a Cenozoic alkali-rich porphyry belt in western Yunnan. The main rock types include quartz-albite porphyry, quartz-K-feldspar porphyry and biotite–K-feldspar porphyry. These porphyries are characterised by high alkalinity [(K 2O + Na 2O)% > 10%], high silica (SiO 2% > 65%), high Sr (> 400 ppm) and 87Sr/ 86Sr (> 0.706)] ratio and were intruded at 65.5 Ma, between 25.5 to 32.5 Ma, and about 3.8 Ma, respectively. There are five main types of mineral deposits in the Beiya area: (1) porphyry Cu–Au deposits, (2) magmatic Fe–Au deposits, (3) sedimentary polymetallic deposits, (4) polymetallic skarn deposits, and (5) palaeoplacers associated with karsts. The porphyry Cu–Au and polymetallic skarn deposits are associated with quartz–albite porphyry bodies. The Fe–Au and polymetallic sedimentary deposits are part of an ore-forming system that produced considerable Au in the Beiya area, and are characterised by low concentrations of La, Ti, and Co, and high concentrations of Y, Yb, and Sc. The Cenozoic porphyries in western Yunnan display increased alkalinity away from the Triassic Jinshajiang suture. Distribution of both the porphyries and sedimentary deposits in the Beiya area are interpreted to be related to partial melting in a disjointed region between upper mantle lithosphere of the Yangtze Plate and Gondwana continent, and lie within a shear zone between buried Palaeo-Tethyan oceanic lithosphere and upper mantle lithosphere, caused by the subduction and collision of India and Asia.

ON THE ORIGIN OF THE PGE MINERALIZATION IN THE ELATSITE PORPHYRY Cu Au DEPOSIT, BULGARIA: COMPARISON WITH THE BAULA NUASAHI COMPLEX, INDIA, AND OTHER ALKALINE PGE-RICH PORPHYRIES

The Elatsite porphyry-copper deposit contains platinum-group minerals associated with base-metal sulfi des, i.e., merenskyite, moncheite, palladoarsenide and an undetermined Pd–Ag–Te–Bi mineral, all in a magnetite – bornite – chalcopyrite assemblage. Other minerals such as linnaeite, carrollite, siegenite, rammelsbergite, which are uncommon in typical porphyry-copper deposits, indicate a mantle derivation for the specifi c PGE–Co–Ni episode of mineralization. The Pt–Pd-rich facies is characterized by extremely low Os, Ir and Ru contents (respectively <3.0, <0.6 and <1.0 ppb), with up to 349 ppb Pt and 3440 ppb Pd. Mineralization results either from (1) multi-stage events involving a preconcentration of the PGE in magmatic disseminated (Ni–Cu–Co) sulfi des related to a mantle-derived magmatic event, with subsequent selective hydrothermal remobilization, or (more probably) from (2) particular conditions in the partial melting of the mantle and magmatic evolution that rendered the precious metals available in the magma, along with a specifi c evolution that prevented the early formation of the sulfi des whilst the PGE (mainly Pd) were concentrated in the hydrothermal fl uids. This PGE–Co–Ni episode is a very specifi c event in the main mineralizing process giving rise to the massive deposition of the Cu-porphyry system. The characteristics of the PGE mineralization deviate from those of a purely hydrothermal PGE mineralization in a mafi c environment, as exemplifi ed by the Baula–Nuasahi mineralization (India), in that they tend to converge toward (and present striking similarities with) the PGE mineralization in alkaline porphyry deposits, such as are found in British Columbia.

English

The Lake Creek watershed in Colorado is an excellent, unique, natural outdoor laboratory for studying naturally occurring acid rock drainage (ARD) and other hydrogeochemical processes involving metal transport and deposition. ARD originates in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream, headwater tributaries of Lake Creek that drain low-grade Cu-Mo hydrothermal systems associated with the Grizzly Peak Caldera, host of an Oligocene, calc- alkaline porphyry. The watershed receives no visible contribution of ARD from mining or other anthropogenic disturbances. The pH in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream starts in the 2.5-3.0 range, but is naturally neutralized by freshwater tributaries to Lake Creek and ultimately attains a pH of 7.7 at the confluence with the Arkansas River, 37 kilometers downstream. Dissolved concentrations of 277 mg/L Al, 498 mg/L Fe, 10 mg/L Cu, and 1,180 µg/L Zn in the acidic headwaters are naturally attenuated downstream to 0.014 mg/L, <0.05 mg/L, 0.0016 mg/L, and 1.57 µg/L, respectively, at the confluence. However, suspended Al and Fe concentrations, which can exceed their dissolved counterparts by a factor of 100 locally, tend to increase along significant stream reaches even as dissolved concentrations are decreasing, resulting in gradually increasing total metal loads over most of the watershed. Many kilometers of streambed are coated with mineral precipitates caused by the neutralization of Lake Creek from freshwater inflow along its course. In low-pH regimes high in the watershed, Al and Fe precipitates consist primarily of sulfates, but grade into oxides and hydroxides as pH increases downstream. Trace metal concentrations tend to mimic Al and Fe concentrations, probably due to adsorption onto precipitated phases of Al and Fe. Concentrations of dissolved Al, Fe, and other metals are strongly pH dependent, and the system displays the ability of Al and Fe to shift rapidly from dissolved to suspended forms depending on pH.

Time Sequence of Himalayan Endogenetic Mineralization on the East Side of the Qinghai‐Tibet Plateau

Abstract Among the endogenetic deposits in the Sanjiang area and at the west margin of the Yangtze platform, Himalayan deposits are the most important and contribute a large proportion of the resources of superlarge deposits. Among the controlled resources of this region, 84% of copper resources, 67% of Pb‐Zn, 31% of Ag, 77% of gold and 24% of tin come from Himalayan deposits on the east side of the Qinghai‐Tibet plateau. Himalayan endogenetic mineralization shows a relatively complete sequence evolution in the Sanjiang area and on the west margin of the Yangtze platform. Mineralization is manifested by gold deposits related to K‐rich lamprophyre, REE deposits related to alkalic complexes and Cu‐Au‐polymetallic deposits related to alkaline porphyry. Six sequences of mineralization evolution since 65 Ma B.P. in the Sanjiang area and on the west side of the Yangtze platform can be recognized. Himalayan endogenetic mineralization on the east side of the Qinghai‐Tibet plateau reached its peak before the Oligocene, corresponding to episodes I and II of the intracontinental orogenic cycle. Afterwards, mineralization waned obviously.

Lithogeochemistry of volcano-plutonic assemblages of the southern Hanson Lake Block and southeastern Glennie Domain, Trans-Hudson Orogen: evidence for a single island arc complex

This paper describes the geology, geochemistry, and age of two amphibolite facies volcano-plutonic assemblages in the southern Hanson Lake Block and southeastern Glennie Domain of the Paleoproterozoic Trans-Hudson Orogen of east-central Saskatchewan. The Hanson Lake assemblage comprises a mixed suite of subaqueous to subaerial dacitic to rhyolitic (ca. 1875 Ma) and intercalated minor mafic volcanic rocks, overlain by greywackes. Similarly with modern oceanic island arcs, the Hanson Lake assemblage shows evolution from primitive arc tholeiites to evolved calc-alkaline arc rocks. It is intruded by younger subvolcanic alkaline porphyries (ca. 1861 Ma), synvolcanic granitic plutons (ca. 1873 Ma), and the younger Hanson Lake Pluton (ca. 1844 Ma). Rocks of the Northern Lights assemblage are stratigraphically equivalent to the lower portion of the Hanson Lake assemblage and comprise tholeiitic arc pillowed mafic flows and felsic to intermediate volcaniclastic rocks and greywackes, which can be traced as far west as Wapawekka Lake in the south-central part of the Glennie Domain. The Hanson Lake volcanic belt, comprising the Northern Lights and Hanson Lake assemblages, shows strong lithological, geochemical, and geochronological similarities to lithotectonic assemblages of the Flin Flon Domain (Amisk Collage), suggesting that all of these areas may have been part of a more or less continuous island arc complex, extending from Snow Lake to Flin Flon, across the Sturgeon-Weir shear zone into the Hanson Lake Block and across the Tabbernor fault zone into the Glennie Domain.

Metallogenic evolution of convergent margins: Selected ore deposit models

Metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution, which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust. In general, tin, mercury, antimony and fluorine are most abundant in areas of thickest crust. Although the composition of ore deposits is strongly controlled by deep crustal and mantle features, the actual form and type of many ore deposits is controlled by upper crustal features, particularly the relative abundance of carbonate and silicate country rock. For instance, epithermal precious metal deposits are commonly associated with subaerial, silicic volcanism, which generally forms in the later stages of arc evolution. Deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc. Similarly, chimney-manto limestone replacement lead–zinc–silver deposits are common only in terranes with extensive thick limestone units, which are commonly formed in shallow shelf environments. Several important types of convergent margin deposits remain poorly understood. Acid-sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc. Alkaline porphyry copper deposits are closely associated with calc-alkaline porphyry copper deposits, commonly in the same volcanic arc, although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal. Massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution. In addition to tectonic and petrologic factors, metallogenic patterns probably reflect global forcing mechanisms, the most important of which are global seawater anoxia and plate reorganizations. Global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers. Plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs.

Central City, Colorado; the upper part of an alkaline porphyry molybdenum system

The distribution of molybdenite, fluorite, rhodochrosite, enargite, and telluride minerals, together with anomalous quantities of Cu, Pb, Zn, and Mo in rocks, and variations in stable isotope ratios define a complex, zoned hydrothermal system in the Central City mining district. The molybdenite mineralization is centered on a Laramide quartz bostonite or alkali rhyolite breccia pipe that occurs at the intersection of the Dory Hill fault and the Idaho Springs-Ralston shear zone. Stream sediment geochemistry suggests that significant amounts of W may be present in the system. Uranium, base, and precious metal mineralization of a similar age, but with a distribution different from the molybdenite mineralization, is also present and is considered to be a part of the same hydrothermal system.Fluid inclusion data for the molybdenite and precious and base metal mineralization indicate that fluid inclusions associated with early molybdenite veins are of two types: CO 2 rich and rare halite-bearing, with respective homogenization temperatures of 340 degrees to 420 degrees C and 240 degrees to 340 degrees C. Inclusions associated with precious metal-bearing pyrite and base metal veins are aqueous, two-phase types with homogenization temperatures in the range of 220 degrees to 380 degrees C. Those from late molybdenite veins are aqueous, two-phase inclusions with homogenization temperatures of 200 degrees to 280 degrees C. Salinities (as equiv. wt % NaCl) are between 34 to 42 for halite-bearing inclusions and 2 to 12 for all other types. No evidence of boiling has been found. Trapping pressures of CO 2 -rich inclusions are 0.46 to 0.75 kb, which suggest an emplacement depth of about 2 km for the molybdenite mineralization.Respective quartz delta 18 O and fluid inclusion delta D values are: (1) early molybdenite veins: +9.3 to +14.3 per mil and -87 to -66 per mil; (2) precious metal-bearing pyrite and base metal veins: +8.4 to +15.9 per mil and -90 to -47 per mil; and (3) late molybdenite veins: +10.1 to +15.4 per mil and -77 to -69 per mil with associated fluorite having fluid inclusion delta D values of -70 to -57 per mil. A late telluride vein has a quartz delta 18 O value of +11.2 per mil and fluid inclusion delta D value of -86 per mil. These data suggest that all except the telluride ore fluids contained large proportions of a D-rich (magmatic) fluid. The distribution of fluid inclusion delta D values defines two distinct anomalies, each zoned with delta D enrichment occuring inward toward centers which correspond to the areas of known molybdenite occurrences and precious and base metal mineralization. Likewise, delta D values for sericites developed during early hydrothermal activity increase inward toward the quartz bostonite pipe, various intrusive breccias, and molybdenite mineralization in the district. There is no indication at Central City of negative delta 18 O anomalies in the Precambrian country rock within or peripheral to the centers of mineralization suggesting that ore fluids were thus essentially confined to discrete fractures.It is concluded that molybdenite and other mineralization in this classic gold mining district may be related and may represent the upper part of an alkaline porphyry molybdenum system.

Allard stock, La Plata Mountains, Colorado—an alkaline rock-hosted porphyry copper – precious metal deposit

The Allard stock, which hosts a porphyry Cu–Ag–Au–Pt metals deposit, is an epizonal Laramide pluton at the southwest end of the Colorado mineral belt. The stock and associated mineralization evolved by a complex interaction of igneous and metasomatic processes as follows. Stage 1: forceful emplacement of syenite plutons. Stage 2: intense hydrothermal argillization of stage 1 syenites. Stage 3: intrusion of mafic syenite plutons, some of which are layered, along shear zones. Stage 4: violent escape of volatiles, producing intrusive breccia pipes. Stage 5: intense K-metasomatic alteration of the older stock units with production of a fenitelike alteration halo extending 1000 m beyond the stock and simultaneous intrusion of carbonate-bearing alkali feldspar dikes. Stage 6: emplacement of trachyte, syenite pegmatite, and mafic pegmatite dikes. Stage 7: deposition of metallic minerals, sanidine, quartz, calcite, and fluorite as veinlets and disseminated replacement masses from hydrothermal solutions.The stock shows enrichment of volatiles (CO2, F, and S), high concentrations of Ba and Sr, low concentrations of Rb, high concentrations of Cu, Ag, Au, Bi, Te, Pt, and Pd, with little or no enrichment in Nb, U, Y, and Zr.The Allard stock and its associated mineralization are unlike the calc-alkaline porphyry copper deposits of the western United States and Canada. Although the Allard syenites are silica saturated (quartz-normative), some aspects of Allard alteration and mineralization show affinities with copper and (or) precious metal deposits hosted by alkaline silica-undersaturated (nepheline- or leucite-normative) plutons and carbonatite complexes including the "alkaline suite" porphyry copper deposits of British Columbia, the Palabora, South Africa, copper deposit, and the Cripple Creek, Colorado, gold telluride deposit. Computer-assisted evaluation of major-element analyses of igneous rocks suggests that Allard-type Cu – precious metal deposits may occur in other North American alkaline rock provinces. Epithermal gold telluride deposits in subvolcanic alkaline rocks may grade downward into alkaline porphyry Cu – precious metal deposits.

Evolution of porphyry copper mineralization in an oceanic island arc; Panama

Petrologic, isotopic, and radiometric studies of intrusive rocks in the Panamanian oceanic island arc indicate that magmatism and associated porphyry copper mineralization were episodic and that they exhibit systematic compositional changes. The earliest stage of igneous activity took place about 60 to 70 m.y. ago and involved emplacement of quartz diorites with tholeiitic chemical characteristics, which did not form porphyry copper mineralization. Later magmatism was calc-alkaline in character and commenced with 50 m.y. quartz diorite and K-poor granodiorite. Later events at 35 m.y. and 5 m.y. emplaced granodioritic intrusions that are more potassic than their local predecessors. Porphyry copper mineralization was most intense and most widespread during the last intrusive event.Comparison of the Panamanian results with information on other island arcs indicates that porphyry copper mineralization is essentially absent from earliest stage, tholeiitic(?) intrusions and, with few but prominent exceptions, becomes more important in successively younger stages of arc evolution. Not all arcs exhibit potassium-enrichment trends like Panama, and late-stage quartz diorite-type porphyry copper deposits can develop. Shoshonitic or alkaline porphyry copper deposits, though absent from the tholeiitic phase of arc evolution, can form at any time during the calc-alkaline stage. Gold values are high in shoshonitic and possibly in quartz diorite-related calc-alkaline porphyry copper deposits, and molybdenum appears to be most abundant in granodiorite-related calc-alkaline deposits.