THE ORIGIN OF TIN DEPOSITS IN THE MOUNT WELLS REGION, PINE CREEK GEOSYNCLINE, NORTHERN-TERRITORY

Tabular steeply dipping cassiterite-bearing lodes in the Mount Wells region are hosted by lower greenschist facies metasediment of the Pine Creek Geosyncline within the contact aureole of late orogenic granitoids. The latter are predominantly I-type, but S-type phases are developed near the sediment-granitoid contact. Quartz, cassiterite, pyrite, arsenopyrite, chalcopyrite and pyrrhotite are the main minerals. Two types of lodes are present: (i) Sn-quartz lodes containing 5-10 vol% sulphide minerals; and (ii) Sn-sulphide lodes containing similar to 70 vol% sulphide minerals. At the surface, the former appear as normal quartz veins and the latter as hematitequartz breccia resulting from the collapse of original sulphide-rich lodes as a consequence of volume reduction due to oxidation and leaching. Two stages of quartz veining are recognized in both types of lodes. Cassiterite is present in stage T while stage Lf is composed of barren quartz with minor pyrite. Late stage III carbonate veinlets are present in Sn-sulphide lodes. The lode-wallrock contact is sharp with weak alteration effects confined to the fringe of the lodes. The alteration minerals include sericite, quartz, tourmaline, chlorite, pyrite and minor K-feldspar. Four types of fluid inclusions are present in vein quartz and cassiterite: Type A (CO2 +/- H2O +/- CH4); Type B (H2O+similar to 20% vapour); Type C (H2O+ < 15% vapour) and Type D (H2O +/- < 15% vapour+ NaCl). Early 'primary' inclusions represented by Types A and B are present in stage I only and have a well-defined temperature mode at similar to 300 degrees C and a salinity range of 1-20 wt% eq NaCl. Types C and D inclusions are 'secondary' in stage I and primary in stage II and have a temperature mode at 120-160 degrees C and salinities from about I to more than 26 wt% eq NaCl. Variable H2O-CO2 ratios of Type A inclusions and homogenization in CO2 Or H2O phase at near identical temperature indicate entrapment at the H2O-CO2 solvus and define a pressure of similar to 100 MPa. The melting sequence of frozen inclusions suggests that the ore fluids were mainly H2O-CO2-CH4-Na-Ca-Cl brines. This is also confirmed by Raman Laser Spectrometry. Oxygen and sulphur isotope data are consistent with a magmatic origin of the ore fluids. The delta D values are up to 20% higher than those expected for magmatic fluids and probably resulted from interaction of the latter with the carbonaceous strata. This interpretation is supported by delta(13)C data on the fluid inclusion CO2. Fluid inclusions, stable isotope and mineralogical data are used to approximate the physicochemical parameters of the ore fluids which are as follows: T 300 degrees C, m Cl similar to 2, fO(2) similar to 10(-35), m Sigma S similar to 0.01, Sn similar to 1 ppm, Cu similar to 1 ppm and pH similar to 5.5. It is suggested that fluids of granitic parentage interacted with the enclosing sediment and picked up CO2, CH4 and possibly Ca. The granitic phases became reduced due to this interaction and developed S-type characteristics. Tin was probably partitioned into the CH4-bearing reduced fluids. At some stage the fluid overpressure exceeded the lithostatic lode enforcing failure of the carapace and the intruded rocks by hydraulic fracturing causing CH4 and CO2 loss resulting in the precipitation of the ore minerals.