Outer Propylitic Zone Research Articles

Genesis of the Yanghuidongzi Cu deposit, NE China: Constraints from H-O-Pb isotopic compositions and geochronological study

• The Yanghuidongzi Cu deposit classified as a porphyry type Cu deposit. • H-O isotopes indicate a mixing of magmatic water and meteoric water for ore fluids. • Pb-Re isotopes indicated a deep seated magma mixed with crustal materials for ore metals. • The Yanghuidongzi Cu deposit formed during Early Jurassic . The newly-discovered Yanghuidongzi porphyry Cu deposit is located in the southeastern part of Heilongjiang Province in NE China and represents one of the rare Jurassic porphyry Cu deposits within the easternmost segment of the Central Asian Orogenic Belt (CAOB). The Cu mineralization occurs as veins and disseminations hosted by granodiorite porphyry, associated with typical porphyry copper deposit alteration zoning, which is characterized by from an inner K-silicate zone through to a quartz–sericite zone to an outer propylitic zone, the K-silicate alteration and quartz–sericite alteration are closely related to mineralization. LA-ICP-MS zircon U-Pb data, sericite Ar-Ar and Molybdenite Re-Os data, and H-O and Pb isotopic compositions are used to constrain the ore-forming age and genesis of the Yanghuidongzi Cu deposit. LA-ICP-MS U-Pb dating of zircons from three granodiorite porphyry samples yielded weighted mean 206 Pb/ 238 U ages of 194.3 ± 1.2 Ma, 192.0 ± 2.0 Ma, and 194.6 ± 2.1 Ma respectively, whereas zircon U-Pb age of granodiorite in the study area yielded weighted mean 206 Pb/ 238 U age of 199.5 ± 1.5 Ma that are interpreted to be the emplacement age of these intrusions. 40 Ar- 39 Ar dating of sericite from a quartz-pyrite vein and a quartz-polymetallic sulfide vein yielded plateau ages of 193.9 ± 2.1 Ma and 193.6 ± 1.9 Ma, respectively, whereas five molybdenite samples yielded a Re-Os isochron age of 194.8 ± 2.1 Ma, with a weighted mean model age of 193.6 ± 1.2 Ma. Mineralization ages are consistent with the emplaced ages of the granodiorite porphyry within the analytical uncertainties, indicating that the Cu mineralization is genetically related to the granodiorite porphyry. The fluid δ 18 O fluid and δ 18 D fluid values range from −4.22‰ to 6.5‰ and from −92.8‰ to −65.7‰, respectively, indicating that the ore fluids were mixtures of magmatic and meteoric water. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb values of the pyrite and chalcopyrite are 17.547–18.327, 15.446–15.505, and 37.408–38.113, respectively, suggesting a common, magmatic source for the sulfur. Taking Re (6.75–29.53 ppm) data into consideration, the sources of ore-forming materials may be from a mixture of crustal and mantle sources. Combining our new data with the Jurassic regional tectonic setting, the Yanghuidongzi Cu deposit and the related granitoids formed in a compressional tectonic setting are associated with the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Highly differentiated juvenile crust-derived magmas linked with the Xilekuduke porphyry Mo (Cu) deposit in East Junggar, NW China

The East Junggar region links the Kazakhstan-Junggar plate with the Siberian Craton along the western part of the Central Asian Orogenic Belt. Among the major ore deposits in this region, the Xilekuduke molybdenite deposit is a unique example of Mo-dominated porphyry. Molybdenum and copper mineralization here occur as disseminated sulfides or veinlets mainly in granite porphyry and monzogranite, with a prominent alteration from an inner potassic, through sericitic to an outer propylitic zone. Zircon LA-ICP-MS U–Pb ages indicate that the granite porphyry and monzogranite were emplaced at 326.4 ± 2 Ma and 328.2 ± 1.8 Ma respectively. Both rocks show enrichment in light rare earth elements (LREE), and Rb, K, Sr, Hf, Zr, but depletion in Nb, Ta, Ti and P, suggesting magma generation in a post-collisional setting. A relatively high oxidation state is inferred from zircon δCe values and oxygen fugacity of parental magma. The ore hosted intrusions also show high Differentiation Index (DI), indicating a highly oxidized parental magma and prolonged differentiation process. The range of zircon εHf (t) values (+11.5 to +17), young zircon Hf model ages, and consistent but slightly low zircon δ18O values (+4.84 to +5.20), together with the range of Mg# (35–53), suggest derivation of the parental magma through partial melting of juvenile crust with minor contribution from mantle components and sediments. We propose that melting of juvenile oceanic components at depth with high degree of differentiation played a significant role in generating the Mo mineralization in Xilekuduke deposit within a post-collisional setting in the East Junggar domain.

Geology and geochronology of Naruo large porphyry-breccia Cu deposit in the Duolong district, Tibet

The Duolong district is located in the south Qiangtang terrane of Tibet and is the most significant ore cluster within the Bangongco-Nujiang metallogenic belt. Duolong contains one giant, three large and two medium to small-sized porphyry (±epithermal ± breccia) copper deposits and several other mineralized porphyry bodies. All deposits are closely associated with early Cretaceous (123–115 Ma) intermediate-felsic intrusions. Naruo is a poorly studied porphyry-breccia copper deposit in the north of the Duolong district. Hydrothermal alteration surrounding the ore-bearing granodiorite at Naruo is characterized by an inner potassic zone and an outer propylitic zone, overlapped locally by minor phyllic and argillic alteration assemblages. A detailed paragenetic study has identified five distinct hydrothermal veins (M, A, B, C, D) within the porphyry system. Hydrothermal B veins are strongly related to copper mineralization. Strong propylitic alteration is also observed throughout the hydrothermal breccias identified at Naruo. Sandstone breccia, diorite-bearing breccia and granodiorite-bearing breccia were identified according to the distribution and composition of clasts. U-Pb zircon dating has determined the ages of the ore-bearing granodiorite (121.6 ± 1.3 Ma) and a barren intrusion (115.5 ± 1.1 Ma) within the porphyry system, diorite clasts (122.3 ± 0.9 Ma) and later diorite matrix (120.5 ± 1.0 Ma) in the hydrothermal breccia system, suggesting that with the exception of the late barren intrusion, they all belong to the same mineralizing event at Duolong. The geological and geochemical evidence presented in this study suggest that the porphyry and breccia systems may have originated from the same magma source, but are now spatially independent.

The Chahnaly Low-Sulfidation Epithermal Gold Deposit, Western Makran Volcanic Arc, Southeast Iran

The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Stable isotope record of hydrothermal sulfate, sulfide and silicate minerals in the Darreh-Zar porphyry copper deposit in Kerman, southeastern Iran: Implications for petrogenesis and exploration

The Darreh-Zar porphyry copper deposit is found together with other porphyries such as Sar-Cheshmeh and Sungun in the Cenozoic age Urumieh–Dokhtar magmatic belt that is related to the subduction of the Arabian plate beneath Central Iranian microcontinent. The deposit is associated with Miocene granodiorite porphyry intrusions into Eocene basaltic volcanic rocks. The principal hydrothermal facies of the area are a core zone of potassic alteration enclosed by a peripheral zone of propylitic alteration. Chlorite-sericite, sericitic, advanced argillic and intermediate argillic alteration zones cut the upper part of the potassic zone and are developed near the interface between the potassic and propylitic zones. Chalcopyrite, pyrite or local bornite, and associated anhydrite are the chief hypogene sulfide minerals, and constitute <0.1 to 0.8wt.% Cu ores. A ~50-m-thick supergene zone containing 0.8 to 1.5wt.% Cu, characterized by covellite and chalcocite, underlies an oxidized and partially leached ~20-m-thick zone containing chrysocolla, malachite and azurite.The hydrogen isotopic composition of biotite from Darreh-Zar ranges from δD of −89 to −101‰, and the calculated aqueous fluids (δD of −66 to −78‰) in equilibrium with biotite are consistent with a large component of magmatic fluid. Epidote varies from δD of −56 to −53‰, and yields calculated fluids responsible for the propylitic alteration that range from −22 to −19‰. Oxygen isotopic data for the studied quartz, biotite, anhydrite and epidote vary between δ18O of 0.8 to 9.0‰. The calculated δ18O values for aqueous fluids in equilibrium with the biotite, quartz (from barren type and A veins), anhydrite (from B veins) and epidote are 7.8±0.4, 5.1±0.6, 2.6±0.5 and −1.1±0.1‰, respectively. This marked decrease likely reflects a change from magmatic-derived fluids in central potassic zone to meteoric or sedimentary-derived fluids in the outer propylitic zone. Sulfur isotope compositions are 11.8 to 14.0‰, for anhydrite (n=3), 2.4‰ for molybdenite (n=1), 1.7 to 3.9‰ for pyrite (n=17), and 1.6 to 2.9‰ for chalcopyrite (n=2). The sulfide data, alone, suggest a conventionally ‘magmatic’ value of about 1.6 to 2.9‰ for Darreh-Zar sulfur. However, the fairly oxidized granitic parental magma shows relatively heavy bulk sulfur (δ34SΣS≈+5‰) and sulfate–sulfide sulfur isotopic fractionation is consistent with an approach to isotopic equilibrium at calculated temperatures of 520±50°C for most of the coexisting anhydrite–pyrite pairs (n=4). An exploration implication is that the identification of isotopically high sulfur isotopic compositions of gypsum from near-surface samples determines a hydrothermal system that contains deeper hypogene anhydrite, a common indicator of large porphyry Cu-Mo deposits. In contrast, isotopically low sulfur isotopic values similar to and derived from weathering of pyrite and other sulfides are common in many hydrothermal environments.

Hydrothermal alteration mapping using ASTER data in Baogutu porphyry deposit, China

Remote sensing plays an important role in mineral exploration. One of its proven applications is extracting host-rock lithology and alteration zones that are related to porphyry copper deposits. An Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was used to map the Baogutu porphyry deposit alteration area. A circular alteration mineral zoning pattern was clearly observed in the classification result of potassic, phyllic, argillic, propylitic zones. The potassic is characterized by biotite and anhydrite with an absorption feature centered at 1.94 and 2.1um. The phyllic zone is characterized by illite and sericite that indicates an intense Al-OH absorption feature centered at 2.20um. The narrower argillic zone including kaolinite and alunite displays a secondary Al-OH absorption feature at 2.17 um. The mineral assemblages of the outer propylitic zone are epidote, chlorite and calcite that exhibit absorption features at 2.335um.The performance of Principal Component Analysis(PCA), Minimum Noise Fraction (MNF), band ratio(BR) and Constrained Energy Minimization(CEM) has been evaluated. These techniques identified new prospects of porphyry copper mineralization in the study areas. These results indicate that ASTER is a powerful tool in the initial steps of mineral exploration.

Geology and origin of the post-collisional Narigongma porphyry Cu–Mo deposit, southern Qinghai, Tibet

Narigongma is a poorly studied Mo-rich (~0.06wt.%) post-collisional porphyry Cu deposit located in southern Qinghai Province, Tibet, 400km northwest of the Yulong porphyry Cu–Mo–Au belt. The Narigongma deposit has a similar age (43–40Ma) to porphyry deposits in the Yulong belt, but different ore assemblages. The Narigongma deposit is associated with Eocene granodiorite and granite intrusions that were emplaced into a Permian volcanic–sedimentary rock sequence. An ~43.3Ma biotite granite stock (P1 porphyry) is the earliest Eocene intrusion, and this was itself intruded by a number of smaller, ~43.6Ma fine-grained granite porphyry stocks (P2 porphyry) and several post-ore quartz diorite porphyry dikes (~41.7Ma). The main Cu–Mo mineralization at Narigongma is associated with the P1 porphyry. Hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones, and an outer propylitic zone. Hypogene mineralization at Narigongma was characterized by early-stage precipitation of molybdenite during potassic alteration and late-stage deposition of chalcopyrite during phyllic alteration. Deposition of both the Mo and Cu mineralization stages was caused by decreasing temperature. A high degree of crystallization of the P1 porphyry occurred prior to fluid saturation that produced Mo enrichment in the residual melt due to the incompatible behavior of Mo. However, compatible Cu was sequestered by the crystallizing phases and resulted in the generation of a high-Mo/Cu magmatic–hydrothermal fluid and the final Mo±Cu mineralization assemblage. Zircon εHf(t) values of +4.1 to +7.9 are indicative of magma derivation from a depleted source. These isotopic data, coupled with other geochemical characteristics of the Narigongma porphyry, such as high SiO2 and K2O contents, low MgO contents and compatible element abundances, and highly fractionated rare earth element patterns, indicate a mixing model for the origin of the porphyry bodies. Generation of the post-collisional ore-forming porphyries occurred in two stages: (1) partial melting of metasomatized phlogopite-bearing lithospheric mantle that generated potassic to ultra-potassic mafic melts, and (2) underplating of such melts beneath thickened juvenile lower crust, which triggered partial melting of the lower crust to produce the ore-forming, high-K adakitic magma.

Occurrences of Ore Minerals and Fluid Inclusion Study on the Kingking Porphyry Copper–Gold Deposit, Eastern Mindanao, Philippines

AbstractThe Kingking deposit is a gold‐rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite‐bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu‐Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite‐bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two‐phase fluid‐rich and vapor‐rich inclusions and polyphase hypersaline inclusions from porphyry‐type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (TH) and the dissolution temperature of halite daughter crystals (TM) from the polyphase hypersaline inclusions predominantly range from 400°C up to &gt;500°C. The wide range of TH and TM may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, TH &gt; TM and in some cases, TH &lt; TM, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.

Petrology and geochemistry of propylitic alteration at Southwest Tintic, Utah

Systematic variations in mineral assemblages, fluid inclusion characteristics, mineral chemistry, and isotopic ratios document the chemical and thermal evolution of hydrothermal fluids responsible for late alteration in a small porphyry copper system near Tintic, Utah. Propylitic alteration of volcanic rock can be subdivided into actinolite, epidote, and chlorite subzones. An actinolite subzone containing actinolite, epidote, chlorite, and calcite extends 250 m away from the secondary biotite zone. An epidote subzone containing epidote, chlorite, and calcite is present up to 500 m away from the secondary biotite zone. A chlorite subzone of chlorite and calcite is present up to 2 km from the secondary biotite zone. Chlorite which occurs in each subzone varies systematically in Fe, Mg, and Mn content. The ratio Fe/(Fe + Mg) increases from 0.27 at the secondary biotite zone to 0.48 in the outer propylitic zone accompanied by a decrease in activity of clinochlore from 0.15 to 0.02. MnO increases from 0.23 to 0.44 wt percent. The Fe (super +3) content of epidote also systematically decreases from the actinolite subzone outward. The activity of clinozoisite in the epidote ranges from 0.038 at the secondary biotite zone to 0.21 700 m away from the secondary biotite zone in the outer epidote subzone. Systematic changes in chlorite and epidote compositions reflect increasing log a (sub Mg (super +2) ) /a 2 (sub H (super +) ) and log a (sub Fe (super +2) ) /a 2 (sub H (super +) ) with increasing distance from the hydrothermal biotite zone and decreasing temperature and salinity.Fluid inclusion homogenization temperatures decrease away from the secondary biotite zone with a horizontal thermal gradient of 78 degrees C/km. Temperatures range from 351 degrees to 160 degrees C in propylitic alteration. Salinities also decrease outward from 7.3 to 0.0 equiv wt percent NaCl.Hydrothermally altered rocks are systematically depleted in 18 O outward from the phyllic alteration zone into the chlorite subzone, as recorded by whole-rock samples (10.9 to -1.7ppm), calcite (10.4-1.6ppm), and quartz veins (2.8 to -0.8ppm). Calculated delta 18 O and delta D values for the actinolite subzone are +5 and -55 per mil, respectively; values as low as -15.1 and -120 per mil are recorded at the outer limits of sampling in the chlorite subzone.These latter values indicate that outer propylitic alteration is dominated by isotopically unevolved local meteoric water. However, the large variations in hydrogen and oxygen isotope compositions recorded across the zoned propylitic alteration are too large to have been produced from an isotopically homogeneous fluid, given the measured range of alteration temperatures (350 degrees -200 degrees C). Rather, the variations in isotopic compositions and salinity of the propylitic alteration fluids are consistent with the progressive mixture of unevolved ( 18 O- and D-depleted), low-salinity local meteoric water with a much more 18 O- and D-enriched, higher salinity fluid. The later phyllic alteration also formed from a similar, 18 O- and D-enriched fluid. We suggest that these isotopically enriched fluids more likely evolved from local meteoric water through extensive exchange at very low water/rock ratios with the igneous rocks at depth in the hydrothermal system at Tintic. An alternative explanation is that 18 O- and D-enriched fluids represent a late incursion of magmatic water unlike many other porphyry copper systems.Participation of such 18 O- and D-enriched fluids in the waning stages of hydrothermal alteration at Tintic contrasts with the usual domination of isotopically unevolved meteoric water at these stages (e.g., propylitic, phyllic) in many porphyry copper systems (Sheppard et al., 1971; Taylor, 1974; Gustafson, 1978). At Tintic, isotopically enriched fluids coexist with and even displace meteoric water during propylitic and phyllic alteration, respectively.

Chemical and isotopic evolution of hydrothermal solutions at Bingham, Utah

Systematic changes in mineral assemblages, mineral compositions, fluid inclusion characteristics, and hydrogen and oxygen isotope compositions in altered igneous rocks in the Bingham, Utah, porphyry copper deposit record the chemical and isotopic evolution of the hydrothermal solutions. Hydrothermal alteration in and surrounding the Bingham porphyry copper deposit consists of an outer propylitic zone of chlorite, epidote, and actinolite; a transition zone of hydrothermal biotite, actinolite, epidote, and chlorite; and an inner potassic zone of hydrothermal biotite and K-feldspar. Later sericitic and argillic alteration are superimposed on early potassic and propylitic alteration.Compositions of alteration minerals determined by electron microprobe methods are: biotite, mole fraction phlogopite (Mg/Mg + Fe) 0.58 to 0.83; actinolite, mole fraction tremolite (Mg/Mg + Fe) 0.58 to 0.79; epidote, mole fraction pistacite (Fe/Fe + Al) 0.27 to 0.30; and chlorite, mole fraction clinochlore (Mg/Mg + Fe) 0.66 to 0.76. Mineral compositions, especially MnO and MgO in chlorite, MnO in epidote, and TiO 2 in biotite, vary systematically with distance from disseminated ore. MgO in chlorite increases from 20.6 to 24.6 wt percent. MnO in chlorite and vein epidote increases from less than 0.1 to 0.59 wt percent and from 0.08 to 0.73 wt percent, respectively. The average mole proportion Ti in biotite increases from 0.1 to 0.24.Temperatures and salinities from fluid inclusions range from greater than 600 degrees C and 50 equiv wt percent NaCl for the potassic zone to approximately 370 degrees C and 33 wt percent for the propylitic zone. Coexisting liquid-rich and vapor-rich inclusions which homogenize at temperatures similar to the liquid and vapor phase, respectively, suggest boiling from the potassic core out to the potassic-propylitic transition zone. Compositions and homogenization temperatures for fluid inclusions suggest average P (sub (f)) -T conditions which vary smoothly from the potassic core to the propylitic fringe: 600 degrees C and 800 bars for the potassic zone, 450 degrees C and 500 bars for the transition zone, and 350 degrees C and 200 bars for the outer propylitic zone.Thermochemical calculations show that log f (sub H 2 O) decreases from 2.74 in the potassic core to 2.17 in the propylitic zone fringe, log f (sub S 2 ) decreases from -1 to -3.3 in the core to -8.7 in the propylitic zone, and log f (sub O 2 ) decreases from -16 to -17.5 in the core to -29 in the same interval. The log activity ratios of Ca, Mg, and K to hydrogen ion increase from the potassic to the propylitic zone. K (super +) /H (super +) increases from 3.1 to 3.4, Mg (super +2) /(H (super +) ) 2 ) increases from -2.25 to 4.25, and Ca (super +2) /(H (super +) ) 2 increases from less than 1 to 7.75.The extent of oxygen isotope exchange between the hydrothermal vein fluids and the igneous rock matrix in the Bingham stock increases with increasing temperature, abundance of alteration and quartz veins, thickness of veins, and fracture intensity. All these parameters generally increase inward from the propylitic zone into the core of the potassic zone. Within the potassic zone, there is pervasive exchange equilibrium between vein fluids and the selvage and rock matrix domains of the highly altered, veined, and fractured igneous rock. In contrast, vein fluids closely approach exchange equilibrium only with vein selvage in the propylitic zone; the igneous rock matrix has not equilibrated extensively, as might be expected in this lower temperature, less altered, and veined portion of the hydrothermal system.Calculated delta 18 O and delta D values of early fluids range from 6.8, -67 per mil in the potassic core to 5.1, -41 per mil in the outer propylitic zone, showing a systematic trend of 18 O depletion and deuterium enrichment with decreasing temperature outward from the innermost potassic zone. This trend is inconsistent with the progressive influx of local meteoric waters (delta D 18 O and delta D values for these fluids could have resulted from isotopic exchange between meteoric water and igneous rock at very low water (W)/rock (R) ratios over a range of temperature (> or =600 degrees -350 degrees C). Such water/rock exchange likely would be confined to deeper parts of the system with the exchanged fluids subsequently focused up into the present exposure of the deposit.Late-stage fluids responsible for sericitic and argillic alteration are progressively depleted in 18 O and deuterium, consistent with an increasing, and finally dominant, meteoric water component. If fluid-rock isotopic exchange has been the dominant mechanism in defining the isotopic compositions of the hydrothermal fluids at Bingham, then this later hydrothermal system was characterized by shallower circulation and higher water/rock ratios than was the system responsible for the development of the early hydrothermal fluids.

The Taysan porphyry copper deposit, southern Luzon Island, Philippines

The Taysan batholith consists of hornblende diorite to trondhjemite on a calc-alkaline trend intruded into pre-Jurassic schist on the northeast and probably Cretaceous meta-volcanics on the south and west. The youngest dikes, postmineral, were dated at 14.8 + or - 0.9 m.y., putting the intrusive sequence in the middle Miocene, the same time frame as nearly all of the Philippine disseminated deposits. The area was nearly peneplaned and in the Pleistocene the whole of southern Luzon was blanketed with volcanic ash, covering at least 95 percent of the mineralized zone.A disseminated copper deposit formed at the western edge of the batholith seemingly on the margin of a volcanic breccia pipe. It plunges about 33 degrees to the east northeast. The average copper grade is 0.43 percent with significant gold, a trace of molybdenum, and 2 to 3 percent each of magnetite and pyrite. A late pulse of mineralization superposed 0.13 percent zinc in the upper portion of the orebody.Alteration is poorly exposed but an outer propylitic zone with a strong development of epidote and some calcite, chlorite, and pyrite is present. Representing a phyllic zone, there is very strong pyritization in some portions of the fringe of the mineralized zone, much weaker in others. Soda sericite is strongly developed outside the orebody and less so within the ore zone, where silicification is prominent. Potassic alteration is represented by strong hydrothermal development of biotite with weaker and spotty orthoclase, the potassic zone coinciding with strongest mineralization.Strontium 87 Sr/ 86 Sr ratios in the Philippines are low in porphyry copper plutons ranging from 0.7035 to 0.7037, but at Taysan it is 0.7040.