Evaluating shallow flow-system response to climate change through analysis of spring deposits in southwestern Wisconsin,USA

Tufa-depositing springs in the southern Driftless Area of Wisconsin, USA, are used to inform the response of shallow and local groundwater flow systems to changes in climate over the last 3,000 years. The springs emanate from a shallow, unconfined, sedimentary bedrock aquifer and at stratigraphic positions similar to a shallow, perched aquifer that was identified in the eastern Driftless Area. The perched aquifer was shown to be stable under current climate conditions and over decadal time scales. This study provides further evidence of the significance of the stratigraphic interval in controlling shallow groundwater flow patterns in the region and in the stability of shallow and local groundwater flow systems over thousands of years. The tufa carbonates in three cores collected from the mounds adjacent to the springs show variations in stable isotope (δ¹³C, δ¹⁸O) and elemental (Mg/Ca) values that agree with well-established paleoclimate records for the region, suggesting that the springs were active and depositing tufa in the past, during climate conditions that were similar to the present and during conditions that were drier than the present.     

Overprinting porphyry‐type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect,southwestern Sto. Tomas II (Philex), Baguio District,Philippines

The Southwest prospect is located at the southwestern periphery of the Sto. Tomas II porphyry copper–gold deposit in the Baguio District, northwestern Luzon, Philippines. The Southwest prospect hosts a copper‐gold mineralization related to a complex of porphyry intrusions, breccia facies, and overlapping porphyry‐type veinlets emplaced within the basement Pugo metavolcanics rocks and conglomerates of the Zigzag Formation. The occurrences of porphyry‐type veinlets and potassic alteration hosted in the complex are thought to be indications of the presence of blind porphyry deposits within the Sto. Tomas II vicinity. The complex is composed of at least four broadly mineralogically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. These intrusions were accompanied with at least five breccia facies that were formed by the explosive brecciation, induced by the magmatic–hydrothermal processes and phreatomagmatic activities during the emplacement of the various intrusions. Hydrothermal alteration assemblages consisting of potassic, chlorite–magnetite, propylitic and sericite–chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Elevated copper and gold grades correspond to (a) chalcopyrite–bornite assemblage in the potassic alteration in the syn‐mineralization early‐mineralization diorite (EMD) and contemporaneous veinlets and (b) chalcopyrite‐rich mineralization associated with the chalcopyrite–magnetite–chlorite–actinolite±sericite veinlets contemporaneous with the chlorite–magnetite alteration. Erratic remarkable concentrations of gold were also present in the late‐mineralization Late Diorite (LD). High X_Mg of calcic amphiboles (>0.60) in the intrusive rocks indicate that the magmas have been oxidizing since the early stages of crystallization, while a gap in the composition of Al between the rim and the cores of the calcic amphiboles in the EMD and LD indicate decompression at some point during the crystallization of these intrusive rocks. Fluid inclusion microthermometry suggests the trapping of immiscible fluids that formed the potassic alteration, associated ore mineralization, and sheeted quartz veinlets. The corresponding formation conditions of the shallower and deeper quartz veinlets were estimated at pressures of 50 and 30 MPa and temperatures of 554 and 436°C at depths of 1.9 and 1.1 km. Temperature data from the chlorite indicate that the chalcopyrite‐rich mineralization associated with the chlorite–magnetite alteration was formed at a much lower temperature (ca. 290°C) than the potassic alteration. Evidence from the vein offsetting matrix suggests multiple intrusions within the EMD, despite the K‐Ar ages of the potassic alteration in EMD and hornblende in the LD of about the same age at 3.5 ± 0.3 Ma. The K‐Ar age of the potassic alteration was likely to be thermally reset as a result of the overprinting hydrothermal alteration. The constrained K‐Ar ages also indicate earlier formed intrusive rocks in the Southwest prospect, possibly coeval to the earliest “dark diorite” intrusion in the Sto. Tomas II deposit. In addition, the range of δ³⁴S of sulfide minerals from +1.8‰ to +5.1‰ in the Southwest prospect closely overlaps with the rest of the porphyry copper and epithermal deposits in the Sto. Tomas II deposit and its vicinity. This indicates that the sulfides may have formed from a homogeneous source of the porphyry copper deposits and epithermal deposits in the Sto. Tomas II orebody and its vicinity. The evidence presented in this work proves that the porphyry copper‐type veinlets and the adjacent potassic alteration in the Southwest prospect are formed earlier and at a shallower level in contrast with the other porphyry deposits in the Baguio District.     

Paleoenvironmental dynamics in central-eastern Brazil during the last 23 000 years: tropical peatland record in the Cerrado biome

The Cerrado biome is the second largest in Brazil, but the evolution of the Cerrado during the late Quaternary is not yet fully known. This study identifies paleoenvironmental changes during the last 23 000 years, based on a tropical mountain peatland record, in the Serra do Espinhaço Meridional in central-eastern Brazil. A multi-proxy approach was used that involved palynological analysis, stable isotopes (δ¹³C, δ¹⁵N), geochemistry, radiocarbon dating and multivariate statistics derived from a peatland core from Rio Preto (Minas Gerais state). The study reveals a very humid and cold climate during the late Pleistocene, with an increase in temperature and decrease in humidity at the Pleistocene–Holocene transition. During this period there was strong instability in the landscape (episodes of erosion). At the beginning of the Holocene there was a reduction in humidity with greater landscape stability. The current sub-humid climatic conditions seem to have been established in the mid-/late Holocene, with periods of landscape instability. Our findings agree with other Cerrado records that contradict previously established hypotheses, such as the Amazonian Refuge and the Pleistocene Arc.     

Geochemical and mineralogical distinctions between Bonnin and Morris (Philadelphia, 1770–1772) porcelain and some contemporary British phosphatic wares

The major element compositions of 15 ceramic sherds from the Bonnin and Morris factory site were determined by electron microprobe. Thirteen samples are phosphatic; the others consist of (a) “soapstone” (magnesian/plombian) and (b) true porcelain, and are interpreted as exotic artifacts, as is one compositionally distinct (relatively SiO₂‐poor, P₂O₅+CaO‐rich) phosphatic sample. Although long considered to be virtually indistinguishable from Bow porcelain (London: ca. 1747–1776), the phosphatic Philadelphia wares have a relatively low mean CaO/P₂O₅ ratio (3.3 versus 3.8; molecular proportions) and high alumina content (6.6 versus 5.4 wt % Al₂O₃). Furthermore, unlike Bow, the Bonnin and Morris samples contain calcic plagioclase (bytownite), and in some instances, an orthoclase‐rich ternary feldspar. The preservation of calcic plagioclase indicates that Philadelphia porcelain was fired at (rather than above) the thermal minimum in the An‐SiO₂‐C₃P system, although the presence of Na (and other fluxes) in these wares precludes the exact determination of the maximum firing temperature from this phase diagram. These wares are also distinctive insofar as the phosphate and melt phases can contain small amounts of lead; they have bulk lead contents of approximately 0.1–1.2 wt % PbO. This component has not been detected in the body of Bow or other contemporary British phosphatic porcelains. Their principal similarity lies in the fact that both wares contain sulfate. In addition, the glazes on Bonnin and Morris porcelain (e.g., PbO ∼ 35–50 wt %; SnO₂ ∼ 1–2%) compositionally resemble those used at Bow. If feldspar is formed at all, then Al‐poor phosphatic porcelain (or those with low CaO/P₂O₅ ratios) will have comparatively low modal calcic plagioclase contents, thereby allowing the rapid depletion of this mineral via resorption by the melt phase during vitrification. Such appears to have been the case for analyzed Bow porcelain, which is therefore interpreted to have been overfired (sensu lato) relative to its Philadelphia counterpart. Conceivably, calcic plagioclase could be preserved in low‐Al wares that were fired only briefly at vitrification temperatures. Given the role of firing history in governing the mineralogy of porcelain, compositional criteria are more reliable for distinguishing these wares. © 2001 John Wiley & Sons, Inc.