Gold-rich Porphyry Copper Deposit Research Articles

How Deep Learning Networks could be Designed to Locate Mineral Deposits

Whether using a shallow neural network with one hidden layer, or a deep network with many hidden layers, the training data must represent subgroups of the deposit type being explored to be useful. Published examples of neural networks have mostly been limited to one individual mineral deposit for training. Variation of geologic features among deposits within a type are so large that a single deposit cannot provide proper information to train a neural net to generalize and guide exploration for other deposits. Models trained with only one deposit tend to be academic successes but are not of practical value in exploration for other deposits. This is why it takes much experience examining many deposits to properly train an economic geologist—a neural network is not any different. Two examples of shallow neural networks are used to demonstrate the power of neural networks to possibly locate undiscovered deposits and to provide some suggestions of how to deal with missing data. The training data needs to include information spatially related to known deposits and hopefully information from many different deposits of the type. Lessons learned from these and other examples point to a proposed sampling plan for data that could lead to a generalized neural network for exploration. In this plan, 10 or more well-explored gold-rich porphyry copper deposits from around the world with 100 or more sample sites near and some distance from each deposit would probably capture important variability among such deposits and provide proper data to train and test a shallow neural network to predict locations of undiscovered deposits.

Petrology and U–Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong–Nujiang Ocean

We present new zircon U–Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang–Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U–Pb ages of 122 and 120Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN=13–14), high Ti/V ratios (45–64), high Zr (190–270ppm) and Nb (22–41ppm) concentrations, and Zr/Y ratios (7–9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (−6.3 to +6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong–Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc system that comprises the Bangong–Nujiang Ocean, an island arc, and a back-arc basin, respectively. The fact that this volcanic arc system was still active during the Early Cretaceous indicates that closure of the Bangong–Nujiang Ocean occurred later than the previously suggested period of between the Late Jurassic and Early Cretaceous.

Early Cretaceous arc magmatism and high-sulphidation epithermal porphyry Cu–Au mineralization in Yanbian area, Northeast China: the Duhuangling example

Yanbian area (Northeast China) is part of the Western Pacific porphyry–epithermal gold–copper metallogenic belt. Here, we present the results of a detailed study of Early Cretaceous mineralization-associated magmatic events in this region and, based on the results, identify the geological setting and mineralizing processes involved in mineral deposit formation. We focus on the timing and geodynamic mechanisms of hydrothermal alteration and metallogenesis of the Duhuangling high-sulphidation epithermal gold deposit, located ~15 km NW of the large Xiaoxinancha gold-rich porphyry copper deposit. New data are presented for zircon U–Pb, fluid inclusion Ar–Ar, whole-rock geochemical, and in situ zircon Hf isotopes for igneous rocks of the Duhuangling deposit, and the data – integrated with results of previous research – reveal that Yanbian area epithermal and porphyry Cu–Au deposits are associated with two stages of Early Cretaceous intermediate-felsic magmatism (116–118 and 112–109 Ma), with the later stage of magmatism more closely associated with mineral deposit formation. Our new data constrain the timing of formation of high-sulphidation epithermal gold deposits to 108–106 Ma and the timing of formation of gold-rich porphyry copper deposits to 111–109 Ma. The two stages of magmatism are associated with magmas derived from different sources, with the first-stage magmas potentially derived from partial melting of a depleted mantle wedge that had been metasomatized by subducted slab-derived fluids or melts; these first magmas are also mixed with material derived from the underplated lower crust. Second-stage magmas were probably generated by partial melting of subducting oceanic slab and some oceanic sediments and the interaction of these magmas with melts derived from the overlying lower crust. Most mineralization in the study area is associated with Cu- and Au-rich post-magmatic hydrothermal fluids that were generated during fractionation of hydrous, sulphur-rich, and high oxygen fugacity adakite-like/adakitic mixed magmas. The formation of both igneous rocks and mineral deposits in the study area occurred in a tectonic setting dominated by Late–Early Cretaceous subduction of the Izanagi or Pacific Plate beneath eastern Asia, indicating that the formation of epithermal and porphyry Cu–Au deposits in the Yanbian area involved subduction-derived fluids, melt modification, partial melting, magma mixing, and crystal fractionation.

Geology and ages of porphyry and medium- to high-sulphidation epithermal gold deposits of the continental margin of Northeast China

The continental margin of Northeast China, an important part of the continental margin-related West Pacific metallogenic belt, hosts numerous types of gold-dominated mineral deposits. Based on ore deposit geology and isotopic dating, we have classified hydrothermal gold–copper ore deposits in this region into four distinct types: (1) gold-rich porphyry copper deposits, (2) gold-rich porphyry-like copper deposits, (3) medium-sulphidation epithermal copper–gold deposits, and (4) high-sulphidation epithermal gold deposits. These ore deposits formed during four distinct metallogenic stages or periods, at 123.6 ± 2.5 Ma, 110–104 Ma, 104–102 Ma, and 95.0 ± 2 Ma, corresponding to periods of Cretaceous intermediate–acid volcanism and late-stage emplacement of hypabyssal magmas along the northern margin of the North China platform. The earliest stage of mineralization (123.6 ± 2.5 Ma) corresponds to the formation of medium-sulphidation epithermal copper – gold deposits and was associated with a continental margin magmatic arc system linked to subduction of the Pacific Plate beneath the Eurasia. This metallogenesis is closely related to high-K calc-alkaline intermediate–acid granite and pyroxene – diorite porphyry magmatism. The second and third stages of mineralization in the study area (110–104 Ma and 104–102 Ma, respectively) correspond to the formation of gold-rich porphyry copper, porphyry-like copper, and high-sulphidation gold deposits, with metallogenesis closely related to sodic or adakitic magmatism. These magmas formed in a continental margin magmatic arc system related to oblique subduction of the Pacific Plate beneath the Eurasia, as well as mixing of crust-derived remelted granitic and mantle-derived adakitic magmas. During the final stage of mineralization (95.0 ± 2 Ma), metallogenesis was closely related to sodic or adakitic magmatism, with diagenesis and metallogenesis related to the disintegration or destruction of the Pacific Plate, which was subducted beneath the Eurasian Plate during the Mesozoic.

High-temperature magmatic fluid exsolved from magma at the Duobuza porphyry copper-gold deposit, Northern Tibet

The Duobuza porphyry copper–gold deposit (proven Cu resources of 2.7 Mt, 0.94% Cu and 13 t gold, 0.21 g t−1 Au) is located at the northern margin of the Bangong-Nujiang suture zone separating the Qiangtang and Lhasa Terranes. The major ore-bearing porphyry consists of granodiorite. The alteration zone extends from silicification and potassic alteration close to the porphyry stock to moderate argillic alteration and propylitization further out. Phyllic alteration is not well developed. Sericite-quartz veins only occur locally. High-temperature, high-salinity fluid inclusions were observed in quartz phenocrysts and various quartz veins. These fluid inclusions are characterized by sylvite dissolution between 180 and 360°C and halite dissolution between 240 and 540°C, followed by homogenization through vapor disappearance between 620 and 960°C. Daughter minerals were identified by SEM as chalcopyrite, halite, sylvite, rutile, K–feldspar, and Fe–Mn-chloride. They indicate that the fluid is rich in ore-forming elements and of high oxidation state. The fluid belongs to a complex hydrothermal system containing H2O – NaCl – KCl ± FeCl2 ± CaCl2 ± MnCl2. With decreasing homogenization temperature, the fluid salinity tends to increase from 34 to 82 wt% NaCl equiv., possibly suggesting a pressure or Cl/H2O increase in the original magma. No coexisting vapor-rich fluid inclusions with similar homogenization temperatures were found, so the brines are interpreted to have formed by direct exsolution from magma rather than trough boiling off of a low-salinity vapor. Estimated minimum pressure of 160 MPa imply approximately 7-km depth. This indicates that the deposit represents an orthomagmatic end member of the porphyry copper deposit continuum. Two key factors are proposed for the fluid evolution responsible for the large size of the gold-rich porphyry copper deposit of Duobuza: (i) ore-forming fluids separated early from the magma, and (ii) the hydrothermal fluid system was of magmatic origin and highly oxidized.

Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-rich porphyry copper deposit in the Bangongco metallogenic belt, Tibet: Evidence from U-Pb and 40Ar/ 39Ar geochronology

The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect (inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast, the 40Ar/ 39Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a 40Ar/ 39Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. III. Zoned Copper-Gold Ore Deposition by Magmatic Vapor Expansion

Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor.

Paleozoic–early Mesozoic gold deposits of the Xinjiang Autonomous Region, northwestern China

The late Paleozoic–early Mesozoic tectonic evolution of Xinjiang Autonomous Region, northwestern China provided a favorable geological setting for the formation of lode gold deposits along the sutures between a number of the major Eastern Asia cratonic blocks. These sutures are now represented by the Altay Shan, Tian Shan, and Kunlun Shan ranges, with the former two separated by the Junggar basin and the latter two by the immense Tarim basin. In northernmost Xinjiang, final growth of the Altaid orogen, southward from the Angara craton, is now recorded in the remote mid- to late Paleozoic Altay Shan. Accreted Early to Middle Devonian oceanic rock sequences contain typically small, precious-metal bearing Fe–Cu–Zn VMS deposits (e.g. Ashele). Orogenic gold deposits are widespread along the major Irtysh (e.g. Duyolanasayi, Saidi, Taerde, Kabenbulake, Akexike, Shaerbulake) and Tuergen–Hongshanzui (e.g. Hongshanzui) fault systems, as well as in structurally displaced terrane slivers of the western Junggar (e.g. Hatu) and eastern Junggar areas. Geological and geochronological constraints indicate a generally Late Carboniferous to Early Permian episode of gold deposition, which was coeval with the final stages of Altaid magmatism and large-scale, right-lateral translation along older terrane-bounding faults. The Tian Shan, an exceptionally gold-rich mountain range to the west in the Central Asian republics, is only beginning to be recognized for its gold potential in Xinjiang. In this easternmost part to the range, northerly- and southerly-directed subduction/accretion of early to mid-Paleozoic and mid- to late Paleozoic oceanic terranes, respectively, to the Precambrian Yili block (central Tian Shan) was associated with 400 to 250 Ma arc magmatism and Carboniferous through Early Permian gold-forming hydrothermal events. The more significant resulting deposits in the terranes of the southern Tian Shan include the Sawayaerdun orogenic deposit along the Kyrgyzstan border and the epithermal and replacement deposits of the Kanggurtag belt to the east in the Chol Tagh range. Gold deposits of approximately the same age in the Yili block include the Axi hot springs/epithermal deposit near the Kazakhstan border and a series of small orogenic gold deposits south of Urumqi (e.g. Wangfeng). Gold-rich porphyry copper deposits (e.g. Tuwu) define important new exploration targets in the northern Tian Shan of Xinjiang. The northern foothills of the Kunlun Shan of southern Xinjiang host scattered, small placer gold deposits. Sources for the gold have not been identified, but are hypothesized to be orogenic gold veins beneath the icefields to the south. They are predicted to have formed in the Tianshuihai terrane during its early Mesozoic accretion to the amalgamated Tarim–Qaidam–Kunlun cratonic block.

Late Tertiary Gold-Bearing Volcanic Belt in the Sierras Pampeanas of San Luis, Argentina

In the Sierras Pampeanas of San Luis, Argentina, Late Tertiary volcanic rocks extend along a 80-km NW-SE-trending belt, between La Carolina and Sierra del Morro. Several gold deposits, among which those in the western end of the belt are better known, are genetically related to the volcanic rocks, formed during a volcanic episode that occurred between 9.5 Ma and 1.9 Ma. Located 600 km from the Peru-Chile trench, the volcanic belt represents the easternmost and youngest mineralized magmatic manifestation associated with the shallowing of the Nazca plate in the flat-slab Andean segment extending from 28° to 33° S Lat. The volcanic complex includes lavas and volcaniclastic rocks. Small-volume lavas were emplaced as domes, flows, and dikes. Pyroclastic deposits are associated with them in certain areas, such as at La Carolina, Cerro Tiporco, and Sierra del Morro. At La Carolina, phreatomagmatic breccias and base-surge deposits define a maar-diatreme volcanic setting. At Cerro Tiporco and Sierra del Morro, the volcaniclastic units are related to the formation of calderas. Mesosilicic magmas (SiO2 = 59% to 68%) belong to normal to high-K calc-alkaline and shoshonitic magma types. At both local and regional scales, K enrichment accompanies progressively decreasing age. Although the volcanic rocks differ from the typical Andean series, some geochemical features, such as Ta and Ti depletion, high large-ion-lithophile-element (LILE) contents, and arc-like Ba/La and La/Ta ratios, indicate an arc signature. In the La Carolina zone, the most important mineralization is the La Carolina volcanic-hosted, low-sulfidation, epithermal gold deposit. Here, several gold and base-metal-bearing epithermal veins cut basement rocks. In the Canada Honda district, the most important mineral deposits are the Diente Verde gold-rich porphyry copper deposit and low-sulfidation epithermal gold and base-metal veins hosted by both basement and coeval volcanics. There is no strong evidence of gold-bearing mineral deposits on the eastern side of the volcanic belt. However, there are hydrothermal alteration zones at Cerros del Rosario and El Morro as well as traces of gold at the Santa Isabel calcareous onyx deposit and inside the Sierra del Morro caldera. In addition, favorable volcanic structures, such as the calderas at Tiporco, Cerro Lomita, and El Morro, make the eastern side of the belt an interesting target for mineral exploration.

The porphyry gold deposit at Marte, northern Chile

A porphyry gold deposit has been documented for the first time at Marte in the Maricunga belt of the Andean Cordillera, northern Chile. Exploration conducted from 1981 through 1987 resulted in definition of 66 metric tons of contained gold. The deposit entered production in late 1989 as an open-pit, heap leach operation.The gold deposit is part of a linear, calc-alkaline volcanoplutonic arc constructed during the mid- to late Miocene as eastward-directed subduction was shallowing. The consequent compression of continental basement underlying the deposit and its environs created a series of high-angle, reverse faults. Stock emplacement and gold mineralization took place within a coeval andesitic stratovolcano at 13 to 14 Ma. The hornblende-biotite diorite stock is subdivided into three phases: two mineralized porphyries and a weakly porphyritic, late mineralization microdiorite. Much of the microdiorite constitutes the matrix of an intrusion breccia, which is transected locally by a closely related hydrothermal breccia.Gold mineralization is coincident with a stockwork of quartz veinlets surrounded by chloritesericite-clay alteration of intermediate argillic type. Pyrite and iron oxides are both present in dominantly disseminated form, with each making up as much as 10 vol percent of the rock. Hematite exceeds magnetite in abundance and was generated in large part by hypogene martitization. Minor chalcopyrite and even lesser quantities of molybdenite, bornite, tennantite, and enargite also occur. Gypsum after anhydrite commonly exceeds 5 vol percent and calcite and tourmaline exist as traces. Isolated remnants of hydrothermal biotite and alkali feldspar attest to the former presence of K silicate alteration, which is believed responsible for introduction of much of the veinlet quartz, magnetite, and gold. Chlorite-dominated alteration affected the late-stage microdiorite and its associated breccias. An erosional remnant of the andesitic volcanic roof to the stock is largely replaced by a chalcedony- and alunite-rich advanced argillic assemblage, which is part of a formerly more extensive tabular alteration zone. Abundant pyrite, gypsum, native sulfur, and barite and trace amounts of enargite and subeconomic gold are present in this advanced argillic cap.The relative impermeability of the cap caused development of an inverted supergene profile in which sulfide-bearing ore is underlain by jarositic leached ore. The steep, elongate orebody is made up of both ore types, which do not differ in gold content. However, the hypogene copper content, about 550 ppm, was more than halved by supergene oxidation. The gold ore is poor in silver (Ag/Au = 0.44), but highly anomalous in molybdenum as well as copper and weakly anomalous with respect to lead, arsenic, and mercury. The advanced argillic cap is marked by pronounced Au, Bi, Hg, Tl, Pb, Mo, and As anomalies, which contrast with a broad zinc halo to the orebody.Preliminary fluid inclusion studies suggest that the auriferous stockwork is a product of boiling fluids which ranged in temperature from 155 degrees to 375 degrees C and in salinity from 2 to 20 wt percent NaCl equiv. A reconstruction of the volcanic edifice combined with pressure estimates based on the fluid inclusion data implies that mineralization took place about 600 to 700 m below the paleosurface. The geologic, alteration, and mineralization characteristics of the Marte gold deposit are closely similar to those of gold-rich porphyry copper deposits, especially those in the Philippines. The only significant mineralogic difference is a deficiency of copper at Marte. The fluid inclusion population at Marte is also more reminiscent of porphyry rather than epithermal environments, although high-salinity inclusions common in porphyry copper deposits were not encountered in the few samples studied.

Some thoughts on gold-rich porphyry copper deposits

Porphyry copper deposits carrying more than 0.4 gm/ton gold are briefly described from northwest Argentina, northwest Pakistan, the Intermontane belt of British Columbia, the Philippines, Sulawesi, Papua New Guinea, Sabah and Puerto Rico. Gold is normally present in potassium silicate alteration, which commonly carries an unusually high magnetite content. High gold content is not directly related to geotectonic setting, composition of the host intrusive, nature of the wall rocks, age of mineralization, erosion level, size of the ore body, or the presence or absence of sericitic alteration. Conditions conducive to a high gold content are likely to have developed in porphyry copper systems at or near to their present sites in the crust rather than at deeper levels during magma generation and ascent.