Abstract The Palinpinon geothermal field (Negros Island, Philippines) is a high-temperature, liquid-dominated geothermal system. Hydrothermal alteration assemblages in the Nasuji-Sogongon region are associated with the Nasuji pluton and include K silicate (biotite, magnetite), calc-silicate (garnet, clinopyroxene), hypogene advanced argillic (andalusite, zunyite), propylitic (tremolite-actinolite, epidote), and distal illite (smectite, illite) and steam-heated advanced argillic (amorphous silica, kaolinite, alunite) assemblages. Biotite alteration and associated veins formed from magmatic-hydrothermal fluids that had temperatures from 267° to >600°C, salinities of 26 to 56 wt percent NaCl equiv and up to 0.2 wt percent Cu. Hydrothermal biotite ( 40 Ar/ 39 Ar = 0.7–0.6 Ma) and alunite (K-Ar = 0.9–0.8 Ma) formed contemporaneous with the Nasuji pluton ( 40 Ar/ 39 Ar = 0.7–0.3 Ma), implying a genetic link between intrusion emplacement and hydrothermal alteration assemblages. The emplacement of a blind intrusion in the Puhagan area at depths greater than 2.5 km has provided the heat source for present-day geothermal activity. Calc-silicate, biotite, and propylitic alteration zones developed above this intrusion at depths greater than 2 km. Parts of the biotite and propylitic alteration zones are in thermal equilibrium with the present-day geothermal system. The lack of hypogene advanced argillic alteration at Puhagan is interpreted to indicate that magma degassing has been hindered or prevented, possibly due to high lithostatic confining pressures. At <2 km, illite and steam-heated advanced argillic alteration assemblages have overprinted the biotite and hypogene advanced argillic alteration types associated with the Nasuji pluton and are in thermal equilibrium with the present geothermal system. The intimate spatial and temporal relationships between the intrusion emplacement and the styles of alteration at Palinpinon are characteristic of mineral deposits such as, porphyry, skarn, and high- and low-sulfidation epithermal. At Palinpinon, a coupled porphyry high-sulfidation epithermal alteration system formed at 0.9 to 0.8 Ma with a coupled porphyry low-sulfidation epithermal system forming today, demonstrating that these alteration systems can form simultaneously in a single mineral district. However, assays (<0.02 wt % Cu, <0.03 wt % Pb, <0.01 wt % Zn, <0.01 wt % Mo, <8 g/t Ag and <0.05 g/t Au) show that the alteration zones at Palinpinon are barren. This could be due to insufficient fracture permeability, resulting in a lack of adequate focus for high volume fluid flux. Alternatively, it could relate to an insufficient supply of metals in the magmatic-hydrothermal fluids.

Siliciclastic rocks of the upper Byrd Group in the Transantarctic Mountains record rapid denudation and molasse deposition during Ross orogenesis along the early Paleozoic convergent margin of Gondwana. These rocks, which stratigraphically overlie Lower Cambrian Byrd carbonate deposits, are dominated by fresh detritus from proximal igneous and metamorphic sources within the Ross Orogen. Biostratigraphic evidence indicates that deposition of the siliciclastic succession is late Botomian or younger (<515 Ma). The largest modes of U-Pb and 40 Ar/ 39 Ar ages from detrital zircons and muscovites respectively in the siliciclastic molasse are Early to Middle Cambrian, but based on ages from crosscutting igneous bodies and neoblastic metamorphic phases, deposition of individual molasse units continued until ∼490–485 Ma (earliest Ordovician). The entire episode of interrelated tectonic, denudational, sedimentary, deformational, and magmatic events is restricted to a time interval of 7–25 m.y. in the late Early Cambrian to earliest Ordovician, within the resolution of these stratigraphic and geochronologic data. Stratigraphic relationships suggest that the detrital zircon and muscovite in the sediments came from the same source terrain, consistent with large volumes of molasse having been shed into forearc and/or marginal basins at this time, primarily due to erosion of igneous rocks and metamorphic basement of the early Ross magmatic arc. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-clastic stratigraphic transition observed in the upper Byrd Group, reflecting an outpouring of alluvial fan and fluvial-marine clastic detritus. The short time lag between tectonism and sedimentary response is similar to that determined for the corresponding section of the Ross-Delamerian orogen in South Australia and other continental-margin arc systems, such as in the Mesozoic Peninsular Ranges of California. Mineral cooling ages from metamorphic basement adjacent to the orogen yield a syn- to late-orogenic cooling rate of ∼10 °C/m.y., which, combined with a known metamorphic geotherm, indicates a denudation rate of ∼0.5 mm/yr. Such denudation rates are comparable to those in recent convergent or collision orogens and suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced by near-synchronous erosional exhumation.