Australia: The Land Where Time Began

A biography of the Australian continent 

Capricorn Orogen, Australia – In-situ U-Pb Geochronology of Xenotime and Monazite, Abra Polymetallic Deposit

The assembly and subsequent reworking of the West Australian Craton was recorded in the Capricorn Orogen from the Proterozoic, which is a major tectonic zone. Major crustal structures have been identified in recent seismic transects across the Capricorn Orogen, some of which have been associated with mineral deposits of hydrothermal mineral origin. The Abra deposit that is hosted in sedimentary rock, the largest base metal accumulation in the Capricorn Orogen, is localised within the Quartzite Well fault zone, which is cut by the Lyons River. To understand the geological history of this long-lived orogen and the processes that formed the mineral deposits it is essential to have robust radiometric dates for the timing of sediment deposition and hydrothermal mineralisation. A weighted mean of ²⁰⁷Pb/²⁰⁶Pb age of 1,594 ± 10 Ma (n=14, MSWD = 2.6) which is interpreted as a period of xenotime growth during the hydrothermal activity that was responsible for the mineralisation, was obtained by the use of U-Pb SHRIMP geochronology of xenotime intergrown with magnetite-haematite-galena from the Abra ore zone. A weighted mean age of 1,610 ±16 Ma (n = 5, MSWD = 1.5), which constrains the age of deposition of the lower Edmund Group sediments to between about 1,680 Ma (maximum age of the basal Mt Augustus Sandstone) and about 1,610 Ma, was obtained from an older coherent cluster within this group. ²⁰⁷Pb/²⁰⁶Pb ages of 1,375 ± 14 (n = 16, MSWD = 0.99) Ma was obtained from authigenic monazite from the ore zone, which Zi et al. interpreted as representing a hydrothermal event that postdates the main phase of mineralisation. Weighted mean ²⁰⁷Pb/²⁰⁶Pb ages 1,221 ± 14 Ma (n = 5, MSWD = 1.04) and 995 ± 18 Ma (n =6, MSWD = 1.3), were obtained from monazites in samples distal to mineralisation, which Zi et al. interpreted as recording discrete episodes of hydrothermal fluid flow. According to Zi et al. it is suggested by their results that the Lyons River-Quartzite Well Fault, one of the principal structures in the Capricorn Orogen, has a long history of reactivation which spans more than 600 Myrs, involving crustal extension and deposition of sediment, hydrothermal mineralisation and multiple episodes of fluid flow. Zi et al. say xenotime and monazite represent ideal chronometers for the investigation of complex histories of hydrothermal mineralisation and fluid flow in major crustal structures, as well as helping in the unravelling of the geological evolution of orogens that are intracratonic.

Conclusions

Important results have been obtained by in situ U-Th-Pb dating of xenotime and monazite from mineralised samples from the polymetallic Abra deposit and sedimentary units associated with it which place precise constraints on the time of mineralisation of the Abra mineralisation and multiple hydrothermal overprinting events. A weighted mean of ²⁰⁷Pb/²⁰⁶Pb age of 1,594 ± 10 Ma was obtained for hydrothermal xenotime from the Black Zone, which was intergrown with haematite, magnetite and galena, to precisely date the mineralisation for the first time. Zi et al. interpreted the spread of ages between about 1,610 Ma and 1,590 Ma as indicting a prolonged period of hydrothermal alteration, which is consistent with the complex relationships between the ore minerals in the deposit. A complex history of hydrothermal activity and reworking of the Abra deposit on a regional scale, as well as the sedimentary rocks of the Edmund Basin are recorded by multiple generations of hydrothermal monazite and xenotime dating to about 1,376 Ma, 1,220 Ma and 995 Ma. Veining with barite-sphalerite-chalcopyrite-dolomite-galena(-pyrite) and alteration in rocks above the unconformity on top of the mineralised zone. Synchronous with deformation and metamorphism in the underlying Gascoyne Province basement was episodic hydrothermal alteration and upper crustal fluid flow, an indication that reactivation of the fault and movement of mineralising hydrothermal fluids through the crust are intrinsically linked.

Zi et al. suggest new and critical age constraints for the timing of deposition of the Edmund Group are provided by the ages of detrital and hydrothermal xenotime and monazite from the deposit, as well as from associated sedimentary rocks. Zi et al. suggest the lower part, which includes the Yilgatherra Formation through the lower alluvial fan facies of the Kiangi Creek Formation, must have been deposited between about 1,680 Ma, the age of the youngest population of detrital zircons present in the underlying Mt Augustus Sandstone, and about 1,610 Ma, which is the age of the oldest hydrothermal xenotime from the mineralised Black Zone at Abra. It is suggested by the new age constraints that the formation of the Edmund Basin and the deposition of sediments into it, may represent a continuation of the extensional activity that was associated with the Mangaroon Orogeny that occurred 1,680-1,620 Ma. At Abra mineralisation was synchronous or shortly after sediment deposition into the Edmund Basin, with continued extensional activity on the Lyons River-Quartzite Well Fault system, which controlled the subsidence of the Edmund basin.

Zi et al. say this study has demonstrated the capabilities of the analysis of xenotime and monazite, by the use of small-spot, in situ techniques to unravel complex histories of hydrothermal mineralisation and fluid flow, which are virtually absent from the zircon record. The necessity of a multi-mineral approach when attempting to yield a more complete picture of the thermal and tectonic history of an orogen is highlighted by this study.

Zi, J.-W., B. Rasmussen, J. R. Muhling, I. R. Fletcher, A. M. Thorne, S. P. Johnson, H. N. Cutten, D. J. Dunkley and F. J. Korhonen (2015). "In situ U–Pb geochronology of xenotime and monazite from the Abra polymetallic deposit in the Capricorn Orogen, Australia: Dating hydrothermal mineralization and fluid flow in a long-lived crustal structure." Precambrian Research 260(0): 91-112.