This open file reports the results of an indicator mineral case study carried out around the Casino porphyry Cu-Mo-Au deposit in the unglaciated terrain of the west central Yukon. The research was part of the Geological Survey of Canada's (GSC) Targeted Geoscience Initiative 5 (TGI-5) Program. The purpose of the study was to test the use of indicator minerals as a porphyry Cu exploration tool in unglaciated terrain. At 22 sites around the deposit, a bulk stream sediment sample was collected to document the indicator mineral signature of the deposit. Steam water and stream silt samples were also collected at the same sites to compare geochemical to heavy mineral signatures. This open file presents indicator mineral abundance data for the bulk stream sediment samples. The Casino deposit has an obvious indicator mineral signature in the <2 mm heavy (>3.2 specific gravity (SG)) and mid-density (2.8-3.2 SG) fractions of stream sediments that consists of, in order of usefulness: gold>chalcopyrite>molybdenite>sphalerite>jarosite>goethite>pyrite that is detectable at least 14 km downstream. Similar indicator mineral patterns in other creeks indicate the presence of additional porphyry style mineralization nearby. Detailed characterization of the mineral chemistry of tourmaline and scheelite that is in progress will determine their suitability for porphyry Cu exploration in this unglaciated terrain. Government and exploration surveys will benefit from the addition of indicator mineral sampling to routine stream sediment sampling protocols.
Tourmaline has long been regarded as a petrogenetic indicator of its host environment, but its genesis in mineralized porphyry systems is poorly understood. Based on chemical and physical properties, tourmaline possesses essential features to be an effective indicator mineral. These properties, along with its abundant occurrence in mineralized copper-gold-molybdenum porphyry systems, make it potentially a key recorder of hydrothermal fluid composition, evolution and potential mineralization. A suite of tourmaline-bearing, barren and mineralized porphyry samples have been analysed as part of a broader study, including those from the Canadian Cordillera (Casino, Schaft Creek, High-land Valley and Woodjam) plus others, have been studied. Paragenetically, tourmaline is observed to be an early hydrothermal phase, predating both sulphide formation and any alteration. Tourmaline is observed to exhibit multiple growth zones, based on petrographic and electron microscope observations, which are also reflected in distinct trace-element variations. Three distinct textural types are recognized: breccia-style, vein-style and disseminated-style. Ma-jor element analyses based on SEM-EDS show a range between schorl (Na-Fe2+-rich) to dravite (Na-Mg-rich) with some minor povondraite (Na-Fe3+) component. Trace element analyses of porphyry related tourmaline via LAM-ICP-MS show distinct characteristics in comparison to that from non-porphyry settings, including redox sensitive elements (Mn, As and Sb) and large-ion lithophile elements (Sr, Ba). Elements not observed in significant concentrations include the light elements (Li, Be) and REEs, which commonly were below limit of detection. Current trace element analysis of tourmaline derived from surficial sediments points to tourmaline originating from the local porphyry system rather than an external source.
This open file reports geochemical data for stream and groundwater samples collected around the Casino porphyry Cu-Au-Mo deposit, one of the largest and highest-grade deposits of its kind in Canada. The calc-alkaline porphyry is hosted in a Late Cretaceous quartz monzonite and associated breccias in the unglaciated region of west central Yukon. Water chemistry around the deposit was investigated because: (i) the deposit has not yet been disturbed by mining; (ii) the deposit was known to have metal-rich waters in local streams; and (iii) the deposit has atypically preserved ore zones. Stream water samples were collected at 22 sites and groundwater samples were collected from eight sites. Surface and groundwaters around the Casino deposit are anomalous with respect to Cd (up to 5.4 µg/L), Co (up to 64 µg/L), Cu (up to 1657 µg/L), Mo (up to 25 µg/L), As (up to 17 µg/L), Re (up to 0.7 µg/L), and Zn (up to 354 µg/L) concentrations. The stable isotopes of O and H of the groundwaters are essentially identical to the surface waters and plot close to the local and global meteoric water lines, indicating that the waters represent modern recharge, consistent with the generally low salinities of all the waters (total dissolved solids range from 98 to 1320 mg/L). Sulfur and Sr isotopes are consistent with proximal waters interacting with the Casino rocks and mineralization; a sulfide-rich bedrock sample from the deposit has delta-34S = -1.2 permille and proximal groundwaters are only slightly heavier (-0.3 to 3.1 permille). These geochemical and isotopic results indicate that surface water geochemistry is a suitable medium for mineral exploration for porphyry-style mineralization in the Yukon, and similar unglaciated regions in Canada. The atypical geochemical signature (Mo, Se, Re, As, Cu) of these types of deposits are typically reflected in the water chemistry and S isotopes provide a more local vectoring tool.
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