Focussed hydrothermal alteration in upper crustal oceanic faults on Macquarie Island
Creators: name: Lewis, SJ
Publisher: name: University Of Tasmania
Year: 2023

description: Macquarie Island is an uplifted exposure of oceanic lithosphere lying deep within the Southern Ocean, less than ten kilometres east of the dextral transpressional boundary which separates the Indo-Australian and Pacific plates (54¬¨‚àû 30' S, 158¬¨‚àû 55' E). The island is a geological oddity; essentially an intra-oceanic ophiolite residing in its primary marine basin. Macquarie Island hosts a diverse spectrum of igneous rock types from all stratigraphic levels of the ocean crust, with pillow basalts and sheeted dolerite dykes most abundant. These upper crustal rocks were formed in the Late Miocene during slow crustal accretion and rifting (spreading rate ~ 20 mm per year) at the relict Proto-Macquarie Spreading Ridge (PMSR). Typical of many slow-spreading (magma-poor) mid-ocean ridges, tectonic activity played a dominant but episodic role in the geological evolution of Macquarie Island. Faults, fractures, and brittle shear zones are widespread across the island, and discrete igneous rock domains commonly have faulted margins. Many discontinuities were originally formed during arnagmatic extension at the PMSR, although more recent transpressive tectonism (&lt; 10 Ma) has further modified or reactivated most of these structures. An extensive array of neotectonic faults have also formed during ongoing uplift and deformation (post-spreading). The Major Lake, Caroline Cove, and Sellick Bay Faults are prominent structural zones which cut sharply across upper crustal rocks in central and southern Macquarie Island. These steeply dipping fault systems are oriented subparallel (NW to NNE-strike) and are regularly spaced across the island (~ 10 km apart). Each fault forms a major geological boundary separating distinct lithologic domains which host disparate rock types (pillow basalts, sheeted dykes, and their transitional volcanic‚ÄövÑvÆintrusive packages) and regional alteration assemblages (recharge-related). Variably influenced by multiple episodes of extensional and strike-slip tectonic activity, all of these significant crustal discontinuities initially developed during seafloor-spreading at the PMSR. However, their orientations are largely oblique to the ENE trend of the palaeo-rift axis, suggesting that they may have formed structural accommodation zones during the waning stages of slow extension, or that they were related to active tectonism at a proximal ridge‚ÄövÑvÆtransform intersection. Structurally focussed zones of intensely altered volcanic rocks and sheeted dolerite dykes are intimately associated (spatially and genetically) with the Major Lake, Caroline Cove, and Sellick Bay Faults. Consistent geological and hydrothermal relationships attest to their critical role as crustal pathways and fracture conduits for hydrothermal systems. These major oceanic structures host highly distinctive hydrothermal assemblages which mainly consist of semi- to pervasively altered basaltic wall rocks, abundant hydrothermal veins and breccias, and patchy domains of massive sulfide mineralisation. The alteration zones comprise (in total) six focussed hydrothermal facies, which are here defined as: (1) the vein and breccia, quartz-chlorite (VQC) facies; (2) the massive and veined, chlorite-quartz-pyrite (CQP) fades; (3) the vein-dominated, prehnite-zeolite (VPZ) fades; (4) the foliated, massive chlorite (FMC) fades; (5) the pervasive, Fe-oxyhydroxide overprint (PFO) facies; and (6) the narrow, focussed quartz vein (NQV) fades. The formation of a single diagnostic facies dominated peak hydrothermal conditions in each major structure; the VQC fades in the Major Lake Fault, the CQP facies in the Caroline Cove Fault, and the VPZ fades in the Sellick Bay Fault. In addition, both the VQC and CQP fades are partially overprinted by smaller-scale alteration assemblages that post-dated peak hydrothermal activity, i.e., the FMC and PFO fades. The dominant fault-hosted alteration facies are highly anomalous, multi-component hydrothermal assemblages. They each have significantly different alteration minerals, hydrothermal textures, and physical attributes compared to regional igneous rock domains, i.e., non-fault zone crustal blocks. Multiple episodes of superimposed fluid‚ÄövÑvÆrock interaction produced these distinctive fault-hosted assemblages, with focussed hydrothermal activity mostly post-dating the main period of axial magmatism at the PMSR (crustal accretion). However, high-temperature fluid circulation was strongly influenced (initiated and sustained) by small-scale dyke pulses, mostly injected at deeper crustal levels than the present fault exposures. Rapid fluid discharge from the basement reservoir was probably also initiated (in part) by discrete tectonic events, such as oblique extension localised along specific fault segments. The physical manifestations of these relict fluid flow events are now well exposed at six key sites on Macquarie Island, where they afford an unparalleled opportunity to study the processes and products of structurally focussed hydrothermal activity in the ocean crust. The Major Lake Fault Zone (MLFZ) is one of the best preserved and most extensive oceanic discontinuities on Macquarie Island. Mainly oriented NNW along its ~ 4 km-long strike, the MLFZ separates upper greenschist and lower amphibolite facies rocks of the Sandell Bay and Lusitania Bay Dyke Swarms (in the southern footwall) from zeolite facies pillow basalts in the northern hangingwall. The fault is the main structural host of quartz + chlorite-dominated alteration (VQC facies) and is also associated with late-stage foliated, massive 'chlorite' (FMC) fades. Furthermore, five planar alteration zones representing the narrow (&lt; 1-2 m-wide), focussed quartz vein (NQV) facies are situated in the regional footwall package ~ 1-1.5 km south of the MLFZ. There are four discrete VQC facies sites at ~ 750 m-spaced intervals along the length of the Major Lake Fault, forming 1-15 m-wide and 80-150 m-long alteration zones. The distinctive spatial segmentation of VQC fades zones likely reflects small-scale hydrothermal upflow systems, locally controlled by basement structures such as obliquely intersecting faults or slight variations in the orientation of the MLFZ, e.g., structural step-overs, kinked zones or offsets. In contrast, the FMC fades is only well exposed at a single 10 m-wide and 55 m-long outcrop segment in the central deformation corridor of the MLFZ (although its true strike extent and physical dimensions are uncertain due to the effects of surface cover and erosion). The VQC fades comprises a multi-stage paragenetic association of hydrothermally derived alteration minerals. Quartz, chlorite and albite are dominant components, whereas epidote and pyrite occur sporadically in altered basalts and dolerites. The main VQC phases are reflected by moderate to strong enrichments in whole-rock Si and Fe concentrations, consistently depleted levels of Ca (due to igneous plagioclase alteration, e.g., the conversion of labradorite to albite), and highly elevated loss-on-ignition values (enhanced volatile contents). The earliest alteration stage is characterised by selectively pervasive chlorite alteration, which is widespread throughout the primary igneous groundmass. Chlorite also forms narrow veinlets and inEtlls many vuggy cavities. Importantly, chlorites in the VQC fades are compositionally diverse and extend from Mg-rich to Fe-rich varieties (Fe # range = 0.29 to 0.64), with many also containing &gt; 1 wt. % Mn (strongly suggestive of a fluid mixing origin). Hydrothermal fluids mostly ranged from 220¬¨‚àû-260¬¨‚àû C (mean = 237¬¨‚àû C) during the precipitation of chlorite (paragenetic Stage I). In contrast, fluids were up to ~ 50¬¨‚àû C hotter during the later formation of discrete quartz veins, intricate quartz stockwork arrays, and irregularly shaped patches of massive quartz alteration (Stage II). However, this diagnostic quartz-bearing stage precipitated across a much broader thermal range (176¬¨‚àû-309¬¨‚àû C), with fluid temperature variations of up to ~ 80¬¨‚àû C occurring between discrete sites and different quartz sub-stages along the MLFZ. The distribution and abundance of pyrite trace elements in the VQC facies also varies at different outcrop sites, and are moderately well correlated with quartz precipitation temperatures. The most significant trace element enrichments are for Co, Ni, Cu, Zn, and Pb. Highly elevated levels of these elements mainly occur in pyrites associated with the lower temperature range of VQC quartz (‚Äöv¢¬ß220¬¨‚àû C), whereas only Se concentrations are consistently enriched at higher temperature sites on the MLFZ. In contrast, sulfur isotope compositions of VQC facies pyrite are not site-specific. Their 834S values encompass the known range of seafloor-hosted sulfides (-1 ‚ÄövÑ‚àû to +11.9 ‚ÄövÑ‚àû) although most are within +1 to +4 ‚ÄövÑ‚àû, indicating that sulfur was mainly derived from leached magmatic crust. The Caroline Cove Fault Zone (CCFZ) is variably exposed for ~ lkm along-strike at the south-western extremity of Macquarie Island. This NNW to NNE-oriented structure is dominated by abundant small-scale normal- and sinistral-oblique faults. Clay-bearing gouge and breccia zones (neotectonic) occur commonly in the central deformation corridor, clearly post-dating the main period of hydrothermal alteration. The CCFZ cuts across regional pillow basalts with zeolite and lower greenschist facies assemblages, and also partly forms the north-eastern boundary of the massive and veined, chlorite-quartz-pyrite (CQP) facies. The diagnostic CQP facies outcrops exclusively at Caroline Cove (northern end of the CCFZ), where it forms a fault-bound wedge of intensely altered volcanic rocks ~ 200 m-long and up to 80 m-wide (strike extent limited by neotectonic faulting and surface cover). Sporadic occurrences of the foliated, massive outcrop segment in the central deformation corridor of the MLFZ (although its true stri...