Combined Zircon, Molybdenite, and Cassiterite Geochronology and Cassiterite Geochemistry of the Kuntabin Tin-Tungsten Deposit in Myanmar

The Kuntabin Sn-W deposit, located in southern Myanmar, is characterized by abundant greisen-type and quartz vein-type cassiterite and wolframite mineralization. We have conducted multiple geochronological methods and isotope and trace element analyses to reveal the age and evolution of the Kuntabin magmatichydrothermal system. Zircon U-Pb dating of the two-mica granite yielded a weighted mean 206Pb/238U age of 90.1 ± 0.7 Ma. Cassiterite U-Pb dating provided a lower intercept age of 88.1 ± 1.9 Ma in the Tera-Wasserburg U-Pb concordia diagram. Molybdenite Re-Os dating returned a weighted mean model age of 87.7 ± 0.5 Ma and an isochron age of 88.7 ± 2.7 Ma. These ages indicate a genetic relationship between granite and Sn-W mineralization in the Kuntabin deposit and record the earliest magmatism and Sn-W mineralization in the Sibumasu and Tengchong terranes related to subduction of the Neo-Tethys oceanic slab. Three generations of cassiterite have been identified with distinctive cathodoluminescence textures and trace element patterns, indicating the episodic input of ore-forming fluids and distinctive changes in the physical-chemical conditions of the Kuntabin magmatichydrothermal system. Sudden changes of fluid pressure, temperature, pH, etc., may have facilitated the deposition of Sn and W. Rhenium contents of molybdenite from the Kuntabin deposit and many other Sn-W deposits in Myanmar are characteristically low compared to porphyry Cu-Mo-(Au) deposits worldwide. In combination with zircon Hf isotope signatures, we infer that granites associated with Sn-W deposits in Myanmar were predominantly derived by melting of ancient continental crust and contain minimal mantle contribution. Subduction of the Neo-Tethys oceanic slab from west of the West Burma terrane reached beneath the Sibumasu terrane and led to magmatism and Sn-W mineralization at ~90 Ma when the Kuntabin deposit was formed. The Paleoproterozoic Sibumasu crust was activated during the subduction-related magmatism to form predominantly crust derived melts. After a high degree of fractional crystallization and fluid exsolution, physical-chemical changes of the hydrothermal fluid resulted in Sn and W precipitation to form the Kuntabin Sn-W deposit.

Petrogenesis and Geochronology of the late-Archean Na-rich A Type granite from the Bundelkhand Craton, India: Implication for tectonic and crustal evolution

<p>Tonalite-trondhjemite-granodiorite gneisses (TTG) are the oldest litho-units of the Bundelkhand craton. The supracrustal rocks include variable deformed mafic volcanics and Banded Iron Formation. Magmatic zircons from the TTG’s yield an upper intercept of ~ 3590 Ma. The TTG’s gradually grades to a Na-feldspar rich A type porphyric granite towards the south. In this abstract, we report mineralogical, geochemical, and geochronological information of high silica- low Ca - high Na A-type granite from Bundelkhand craton.</p><p>In the TAS diagram, the studied samples plot in the field of granite and have a metaluminous affinity with high Ga/Al and Ce + Y + Nb + Zr values typical of A-type granites. In a primitive normalized multi-element spider diagram, the studied samples exhibit negative Nb, Ti, and P anomalies characteristics of a subduction zone setting. The chondrite normalized REE’s exhibit a strong fractionated pattern with negative Eu anomaly; the LREE are enriched and the HREE depleted with moderate to high (La/Yb)<sub>CN </sub>ratios ranging from 11.12 to 26.24 ppm. The studied samples have plagioclase compositions that vary from X<sub>Ab </sub>= 0.980-0.997 and chlorite compositions varying from X<sub>Mg </sub>= 0.309-0.469.</p><p>Phase equilibria modeling yield an emplacement temperature of 700-750<sup>O</sup>C, at 1.0 GPa. Most of the zircon grains are prismatic with visible cores and rims in optical examinations. In a U-Pb concordia diagram, the grains yield an upper intercept of 2536.6 ± 1.8 Ma. The geochemical and geochronological data taken together, indicate the Na-rich A-type granite generated by the high temperature and high-pressure partial melting of Archaean supracrustal rocks.</p>

U-Th-Pb discordia regression

The actinide elements U and Th undergo radioactive decay to three isotopes of Pb, forming the basis of three coupled geochronometers. The 206Pb / 238U and 207Pb / 235U decay systems are routinely combined to improve accuracy. Joint consideration with the 208Pb / 232Th decay system is less common. This paper aims to change this. Adding 208Pb / 232Th to the mix is particularly useful for discordant samples containing variable amounts of non-radiogenic (common) Pb. The paper presents a maximum likelihood algorithm for joint isochron regression of the 206Pb / 238Pb, 207Pb / 235Pb, and 208Pb / 232Th chronometers. Given a set of cogenetic samples, the algorithm estimates the common Pb composition and concordia intercept age. U-Th-Pb data can be visualised on a conventional Wetherill or Tera-Wasserburg concordia diagram, or on a 208Pb / 232Th vs. 206Pb / 238U concordia diagram. Alternatively, the results of the new discordia regression algorithm can also be visualised as a 208Pbc / 206Pb vs. 238U / 206Pb or 208Pbc / 207Pb vs. 238U / 207Pb isochron, where 208Pbc represents the common 208Pb component. For detrital minerals, it is generally not possible to assume a shared common Pb composition and concordia intercept age. In this case the U-Th-Pb discordia regression method must be modified by tying it to a mantle evolution model. Thus also detrital common Pb correction can be formulated in a maximum likelihood sense. The new method was applied to a published monazite dataset with a Th / U-ratio of ∼ 10, resulting in a significant radiogenic 208Pb component. Therefore the case study represents a worst case scenario for the new algorithm. Nevertheless, it manages to fit the data very well. The method should work even better in low-Th phases such as carbonates. The degree to which the dispersion of the data around the isochron line matches the analytical uncertainties can be assessed using the mean square of the weighted deviates (MSWD) statistic. A modified four parameter version of the regression algorithm quantifies this overdispersion, providing potentially valuable geological insight into the processes that control isotopic closure. All the parameters in the discordia regression method (including the age and the overdispersion parameter) are strictly positive quantities that exhibit skewed error distributions near zero. This skewness can be accounted for using the profile log-likelihood method, or by recasting the regression algorithm in terms of logarithmic quantities. Both approaches yield realistic asymmetric confidence intervals for the model parameters. The new algorithm is flexible enough that it can accommodate disequilibrium corrections and inter-sample error correlations when these are provided by the user. All the methods presented in this paper have been added to the IsoplotR software package. This will hopefully encourage geochronologists to take full advantage of the entire U-Th-Pb decay system.

Rb–Sr and U–Pb dating of bentonites

A 6 in. (15 cm) bentonite in the Z coal (Cretaceous–Tertiary boundary) in eastern Montana was sampled at four different places, and biotite, sanidine, and zircon were separated from the clay. U–Pb analyses of purified zircons yielded small systematic variations from concordant U–Pb dates. Plotting the data on a concordia diagram, a short linear discordia line intersects the concordia at [Formula: see text] with an MSWD of 1.16. The systematic variation of the four sets of zircon U–Pb data on the concordia plot may be an artifact of the sampling and purification procedure, or could result from natural sample variation from minor contamination. Biotite fractions of varying specific gravity were obtained for each of the four Z coal bentonite samples and (together with the matching purified sanidine fraction) were analysed for Rb–Sr dating. Excluding those lighter biotite fractions found to have lost 30% or more of their original Rb, an isochron was obtained giving an age of 63.7 ± 0.3 Ma with an initial 87Sr/86Sr ratio of 0.7061 ± 1 and an MSWD of 1.07.To investigate further the Rb–Sr variations in altered bentonite biotite, a large biotite sample from the Ordovician Kinnekulle A1 bentonite of southwestern Sweden was separated into 11 fractions of decreasing specific gravity. Rb–Sr analysis of these fractions also showed a departure from a linear isochron when more than about 30% of the original Rb had been lost. Chemical analysis and X-ray diffraction revealed the biotite-alteration process to be vermiculitization, but gave no definite reason why biotites that retain more than 70% of their original Rb give usable Rb–Sr data. Though some of the alteration may have taken place when the bentonite was deposited as an ash, most of the alteration probably occurred in recent times. The Kinnekulle A1 bentonite Rb–Sr isochron for biotite and sanidine gives an age of 447 ± 1 Ma with an initial 87Sr/86Sr ratio of 0.7094 ± 0.0003 and an MSWD of 3.7.

U–Pb geochronology of the Bokan Mountain peralkaline granite, southeastern Alaska

The Bokan Mountain arfvedsonite–aegirine granite is the only peralkaline acid intrusion actually known on the margin of the Canadian Cordillera. It is the end product of a peralkaline magmatic evolution and is characterized by genetically associated concentrations of U and Th. The U–Pb method on zircons has been used to date two samples: one from a barren peralkaline granite and one from a low-grade U–Th mineralized albitite. A typological study has revealed two genetically different populations of zircons in the barren granite. The upper intercept age of 171 ± 5 Ma obtained on a concordia diagram dates the emplacement of the peralkaline granite. Acid wash experiments on zircons have allowed us to remove important quantities of common lead; this point is discussed as well as the abnormally low Pb content of the feldspar extracted from the barren granite.A Jurassic period of magmatic activity must be integrated into the geological history of Prince of Wales Island. Peralkaline magmatism may have occurred when the Alexander Terrane rifted away from its more southerly area of origin.