Abstract Los Azufres Volcanic Field hosts the second most important geothermal field of Mexico, with a production of 188 MW of electricity. Based on fieldwork and new geochronological data ( 14 C and 40 Ar/ 39 Ar) we define that activity at Los Azufres Volcanic Field started some 1.5 Ma with the emission of basaltic to rhyolitic lavas, and pyroclastic material. The late Pleistocene explosive activity in the southwest sector (Guangoche volcano area) of Los Azufres occurred in a narrow period of time between >31 and <26 ka. The pyroclastic stratigraphy of the S, SW, and W sectors is represented by diverse deposits of dacitic and rhyolitic composition, including a debris avalanche deposit related to a sector collapse of San Andrés volcano, several pyroclastic sequences associated with plateau forming lavas, and Guangoche volcano. Guangoche volcano was the focus of late Pleistocene eruptive activity with two Plin-ian and one subplinian events that deposited pumice-rich falls and pyroclastic flows and surges. These deposits are informally named the White Pumice (29 ka), which originated from a 23-km-high eruptive column and the ejection of 1.7 km 3 of tephra that covered an area of at least 223 km 2 with a mass discharge rate of 9 × 10 7 kg/s; the Ochre Pumice fall (<26 ka), deposited from a 16-km-high eruptive column involving 1.3 km 3 of tephra at a mass discharge rate of 1.9 × 10 7 kg/s; and the Multilayered fallout (<<26 ka) that resulted from an 11-km-high eruptive column with 1 km 3 of tephra at a mass discharge rate of 4.6 × 10 6 kg/s. The complete late Pleistocene stratigraphy suggests that explosive events at Los Azufres Volcanic Field have been intense. They are the subject of ongoing investigations to better understand this kind of large magnitude eruptions.

The Tacaná Volcanic Complex represents the northernmost active volcano of the Central American Volcanic Arc. The genesis of this volcanic chain is related to the subduction of the Cocos plate beneath the Caribbean plate. The Tacaná Volcanic Complex is influenced by an important tectonic structure as it lies south of the active left-lateral strike-slip Motozintla fault related to the Motagua-Polochic fault zone. The geological evolution of the Tacaná Volcanic Complex and surrounding areas is grouped into six major sequences dating from the Mesozoic to Recent. The oldest basement rocks are Mesozoic schists and gneisses of low-grade metamorphism. These rocks are intruded by Tertiary granites, granodiorites, and tonalites ranging in age from 12 to 39 Ma, apparently separated by a gap of 9 m.y. The first intrusive phase occurred during late Eocene to early Oligocene, and the second during early to middle Miocene. These rocks are overlain by deposits from the Calderas San Rafael (ca. 2 Ma), Chanjale (ca. 1 Ma), and Sibinal (unknown age), grouped under the name Chanjale–San Rafael Sequence, of late Pliocene–Pleistocene age. The activity of these calderas produced thick block-and-ash flows, ignimbrites, lavas, and debris flows. The Tacaná Volcanic Complex began its formation during the late Pleistocene, nested in the preexisting San Rafael Caldera. The Tacaná Volcanic Complex formed through the emplacement of four volcanic centers. The first, Chichuj volcano, was formed by andesitic lava flows and pyroclastic deposits, after which it was destroyed by the collapse of the edifice. The second, Tacaná volcano, formed through the emission of basaltic-andesite lava flows, as well as andesitic and dacitic domes that produced extensive block-and-ash flows ∼38,000, 28,000, and 16,000 yr B.P. The Plan de las Ardillas structure (the third volcanic center) consists of an andesitic dome with two lava flows emplaced on the high slope of the Tacaná ∼30,000 yr B.P. Finally, the San Antonio volcanic center was built through the emission of lava flows, andesitic and dacitic domes, and it was destroyed by a Peléan eruption at 1950 yr B.P. that produced a block-and-ash flow deposit. The Tacaná Volcanic Complex was emplaced along a NE-SW trend beginning with Chichuj, followed by Tacaná, Las Ardillas, and San Antonio. This direction is roughly the same as the NE-SW Tacaná graben (as proposed in this work), together with other faults and fractures exposed in the region. The rocks of the Chanjale-San Rafael Sequence and the Tacaná Volcanic Complex have a calc-alkaline signature with medium K contents, negative anomalies of Nb, Ti, and P, and enrichment in light rare earth elements, typical of subduction zones.

Abstract Plutonk-relcttcd gold deposits in interior Alaska occur in the apexes of middle Cretaceous (mostly 93-86 Ma) reduced plutons and in spatially associated sedimentary and metamorphic rocks. Hydrothermal micas from these deposits have cooling ages 0 to 2 m.y. younger than those of magmatic micas and amphiboles from hosting intrusions. These deposits have alteration features and trace element, fluid inclusion, and stable and radiogenic isotope signatures indicative of predominantly magmatic metallogenesis. Plutonic-related gold deposits of intetior Alaska share the following charactetistics: 1. The deposits are solely intrusion hosted (Fort Knox, Democrat, Joker-88, Table Mountain, Elephant Mountain), solely schist hosted (Christina, Cleay Hill), or hosted in both intrusions and schist (Ryan Lode, Liberty Bell). Gold occurs in contemporaneous W-Au skarns and replacement deposits. 2. The gold is typically present in and around the tops of intmsions. It occurs in closely spaced anastomosing or planar quartz veins with feldspar-white mica or quartz-sericite halos but is most predominant in brlttlely deformed planar quartz-sericite (± carbonate) shear zones and veins. Tourmaline is common in systems associated with smaller intrusions. Propylitic alteration occurs distally within intrusions. 3. Arsenopyrite and stibnite are the most common sulfides. Bismuthinite, bismuth telluride, and bismuthlend-antimony sulfosalt are common gold associates; a Bi-Au correlation is significant at many deposits. 4. Arsenopyrite geothermometry indicates temperatures or 400° to 480°C for feldspathic alteration and 300° to 350°c for scricitic alteration. Gold-associated fluid inclusion temperatures plot mostly at 300° to 350°c, Aplite, sphalerite, fluid inclusion geobaromehy, and intrusion textures indicate intrusion and mineralization pressures or 0.5 to 1.5 kbars. 5. Plutons related to or hosting gold mineralization may be multiphase. Intrusion compositions range from syenormmxonitc to granite. The most altered and/or mineralized phasc is porphyritic granite, granodiorite, or quartz monzonite. Ilmenite series intrusions with very low primary iron oxide contents are gold associative. Trace element signatures and Pb isotope ratios indicate that the plutons are 1 type. The intrusions are subduction related. Intrusive isotopic ratios indicate contamination by strongly radiogenic crust. 6. Lead and sulfur isotope ratios from sulfides ( 206 Pb/ 204 Pb = 19.0-19.45; 207 Pb/ 204 Pb = 15.58-15.7; δ 34 S = 0 ± 5‰) are similar to those of feldspars from related intrusions. Isotopic ratios are distinct from those measured from Yukon-Tanan terrane volcanogenic massive sulfide deposits and from nonplutonic-related mesotbermal gold deposits. Fluid calculated carbon, oxygen, and hrdrogen isotope ratios (δ 13 C = −9 to -10‰; δ 18 O = 5-10‰; δD = −47 to -100‰) indicate magmalic devolatilization. 7. Fluid inclusion microthermometry of plutons and gold veins indicates a wide range of salinities, the main control being the differentiation index of the host pluton. Most gold-associated inclusions are of moderate salitely (0-10 wt %), though some deposits have as much as 40 wt percent NaCI. Most of the gold-associated veins contnin abundant CO 2 (7-22 mole % avg) and exhibit evidence of H 2 O-CO 2 immiscibility. Geologic, geochronological, geocbemical, and empirical data support a model in which, high CO 2 , goldbearing fluids fractionate from the more differentiated and more potphyritic end members of reduced 1-type intrusions. The relatively low oxidation state associated with gold-favorable intntsions allows ror gold fractionation into magmatic hydrothermal fluids and favors efficient gold transport as a bisulfide complex in a reduced, vapor-rich fluid. Plutonic-related deposits in southwestern Alaska are younger (≈70 Ma), emplaced at lower pressures (0-0.4 kbarn>), and are not within the Yukon-Tanana terrane. CO 2 content is lower than that of interior plutonicrelated deposits, but carbonate alteration is more abundant. Similarilies in (1) tectonic setting and oxidation stale of the plutons, (2) alteration style and minerals, (3) fluid inclusions, and (4) stable isotope ratios suggest a similar origin for plutonic-related gold deposits in interior and southwestern Alaska.