The world-class Cantung and Mactung W-Cu-Au skarn deposits are hosted in limestone of the Selwyn Basin, where it is intruded by a series of Cretaceous-aged granitoids known as the Tungsten plutonic suite. Tungsten and the associated metals (e.g. Au, Cu, Mo, Sn) in W skarn settings are thought to be provided and transported by exsolved magmatic-hydrothermal fluids from the granitoids. Several mechanisms for W enrichment in fertile magmas have been proposed, all of which focus on the incompatible element (IE) behavior of W during protracted fractional crystallization due to i) mid-crustal depths of magma staging and emplacement, and/or ii) high dissolved volatile content, and/or iii) reduced oxidation conditions that prevented fractionation of W in phases such as magnetite.
This study aims to determine the major and trace element concentrations of apatite-hosted silicate melt inclusions (SMI) from the Mine Stock and associated aplite dykes at the Cantung deposit, as well as the Cirque Lake and Rockslide Mountain Stocks and leucocratic dykes at the Mactung deposit. The major element composition of homogenized SMI will be gathered via electron probe microanalysis (EPMA) to classify the tectonic environment of the magmas. The trace element abundances of homogenized and non-homogenized SMI will be acquired via laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine the W and other important ore elements (e.g., Au, Mo, Sn) concentrations of the melts during different stages of fractional crystallization. The data will also be used to model the behavior of the trace elements during fractional crystallization by using the petrography results and mineral-melt partition coefficients of the minerals in the rock samples in batch crystallization calculations.
Preliminary data of the SMI show a wide range in compositions from intermediate to felsic reflecting the trapping of melts at different stages of fractional crystallization. The transition from syn- to post-collisional may also reflect the entrapment at different stages. Tungsten content in the most evolved melts is up to three orders in magnitude higher than the average crustal values as well as the whole rock data. The inclusions with the lower IE represent melts trapped during the earliest stages of fractional crystallization. Positive correlations between Zr/Hf and Nb/Ta indicate the preferred fractionation of Nb and Zr in biotite and zircon, respectively. The negative correlation between Nb/Ta and Ta indicate the incompatible behavior of Ta during biotite crystallization. When compared to the concentrations of Ta and other IE, W shows enrichment during fractional crystallization reflecting its incompatible behavior with crystallizing phases.
This research will give insights into the source of W and magma at the Cantung and Mactung deposits and supplement the scarcity of melt inclusion data on W-skarn deposits. The controls of W enrichment (magma emplacement depth and/or volatile content and or/ fractional crystallization) in magmas associated with W skarn deposits will be evaluated by comparing the data with other melt inclusion studies from other intrusion related W deposits, porphyry deposits and barren environments.