The Hamlin Lake area is located approximately 120 km southwest of the City of Thunder Bay, Ontario within the Shebandowan greenstone belt of the Wawa Subprovince, Superior Province. The purpose of this study was to understand the relationship between an extensive felsic volcanic rock package and the hydrothermal alteration.
Mapping of the area exposed five major lithologies consisting of mafic metavolcanic rocks, intermediate metavolcanic rocks, felsic metavolcanic rocks, felsic intrusive rocks and iron formation. The felsic metavolcanic rocks vary from rhyolites and banded ash to lapilli tuffs and debris flows, whereas the intermediate metavolcanic rocks are made up of dacites and andesites. The only mafic metavolcanic rocks are the debris flows. The felsic intrusive rocks consist of a quartz-eye porphyry and pink breccia. Primary textures, such as fiamme, can still be observed and which are indicative of a subaqueous environment.
Major and trace element geochemical analyses were undertaken on 200 samples of andesitic to rhyolitic volcanic and volcaniclastic rocks. The major elements have become mobile, whereas the trace elements have remained relatively immobile. Hydrothermal alteration affected the primary textures of the Hamlin Lake area so it was necessary to use trace elements to correctly classify the volcanic rocks. Primitive mantle-normalized (PM) and Zr versus Ti plots were used to separate major suites of rocks and also to determine the tectonic setting. The PM plots showed negative anomalies of the Nb and Ti elements, an indication of a supra-subduction zone setting.
A plot of Zr versus Y was used to establish that the rhyolites and andesites are both of a mildly calc-alkaline affinity and the dacites are strongly calc-alkaline. The VMS classifications of Lesher et al. (1986) and Hart et al. (2004) were used to classify the rhyolites and andesites as FII felsic volcanic rocks, whereas the dacites were classified as FI felsic volcanic rocks. FI felsic volcanic rocks are thought to have originated from a deeper source than the FII felsic volcanic rocks suggesting that the Hamlin Lake volcanic rocks were produced in a mature arc-setting.
Mapping and geochemistry was further supported by analysis of Sm-Nd and oxygen isotopes. Nine samples were analyzed for Sm-Nd isotopes and yielded εNd2700 values from -6.59 to +2.62 suggesting some samples had undergone contamination by older material. Several of the samples were close to the value for 2.7 Ga depleted mantle, but one particular sample with a εNd value of -6.59 showed that it had been affected by contamination that could only be explained by a contaminant that was much older than the surrounding 2.7 Ga rocks. The source of the contamination is likely the result of continentally derived sediments being subducted into the mantle wedge, because oceanic arcs are not underlain by older continental crust.
Thirty-seven samples were analyzed for O isotopes to recognize alteration patterns in the Hamlin Lake area. The area mapped at Hamlin Lake was not large enough to clearly show an alteration halo, but it does show that the samples at Hamlin Lake have elevated δ18O values, the result of interaction with low temperature hydrothermal fluids.
A copy of the thesis can be downloaded here