ENDEAVOR 26 NORTH - A PORPHYRY COPPER-GOLD DEPOSIT IN THE LATE ORDOVICIAN, SHOSHONITIC GOONUMBLA VOLCANIC COMPLEX NEW-SOUTH-WALES, AUSTRALIA

Endeavour 26 North. is the largest of 12 pipelike porphyry Cu-Au depos its found near Parkes in central-west New South Wales, within the Late Ordovician Goonumbla Volcanic Complex. The dominant volcanic rocks of the complex, the Goonumbla Volcanics, are latitic flows and volcanic breccias which are overlain by the trachytic Wombin Volcanics. The vol canic rocks are characterized by high K2O and high K2O/Na2O ratios, wh ich are typical of the shoshonite association, as well as an enrichmen t in large ion lithophile elements and a low concentration of high fie ld strength elements. The Nelungaloo Volcanics form an Early Ordovicia n, nonshoshonitic volcanic substrate to the complex which initially de veloped in a submarine and subsequently subaerial setting. The Sr-87/S r-86, epsilon(Nd), and Pb isotope data indicate the magmas were derive d from mantle sources. The shoshonitic magmas may be products of high- pressure fractionation in the subcontinental lithosphere. The porphyry deposits are associated with a gravity low which is taken to reflect a subvolcanic chamber beneath the center of the complex. All deposits are located within a pronounced circular feature in the aeromagnetic d ata that is interpreted as the margin of a collapsed caldera. Dioritic to monzonitic stocks, which in part define the caldera rim, represent a resurgent phase of magmatism. The Endeavour 31 stock which was part of this phase is ringed by six porphyry Cu-Au deposits, including End eavour 26 North, The deposits are centered on fingerlike quartz monzon ite porphyries which cut the Endeavour 31 stock at depth. Geochemical anomalies of Cu and Zn in the bedrock define the size of the alteratio n system associated with the stock. In part the distribution of the Cu -Au deposits is controlled by the Endeavour lineament. In comparison t o most other volcanic and intrusive rocks of the Goonumbla Volcanic Co mplex, the Endeavour 31 stock and the quartz monzonite porphyries show greater enrichment in SiO2 with respect to K2O, and this trend is rel ated to a buildup in volatiles in the subvolcanic chamber(s) and the f ractionation of biotite and plagioclase. Infusions associated with min eralization also show a depletion in Zr. Endeavour 26 North is a borni te-dominated quartz stockwork pipe centered on two porphyry intrusions , QMP1 and QMP2. The bulk of the mineralization is associated with QMP 1. QMP2 effectively stoped the northern section of the orebody. The fo llowing 11 stages of veins and alteration are defined, with stages 1 t hrough 5 predating the emplacement of QMP2; stage I, vein dikes (narro w quartz veins with aplitic infill) related to the Endeavour 31 stock; stage 2, albitization; stage 3, biotite-magnetite-albite-K feldspar a lteration which developed as an annulus to the deposit; stage 4, K fel dspar flooding, disseminated bornite, and fine quartz-sulfide-anhydrit e veins; stage 5, quartz stockwork with disseminated bornite, chalcopy rite, and anhydrite. Minor sericite is developed as a vein selvage in stage 5 and gold is primarily associated with bornite in this stage. S tages 6 to 8 are the equivalent of stages 1, 4, and 5, respectively, i n QMP2; stage 9 consists of zones of quartz-sericite alteration; and s tages 10 and 11 are veins of gypsum and anhydrite with minor sulfide. Sulfur isotope values for sulfides and anhydrite from the Endeavour 26 North deposit range from -8 to -2.9 per mil, whereas values for anhyd rite range from 7.3 to 16.6 per mil, respectively. The delta(34)S(flui d) is calculated to have been -1.5 per mil, and X(SO2) and X(H2S) are calculated at 0.2 and 0.8, respectively, indicating that the oxidation conditions of the hydrothermal system were close to the H2S/SO2 buffe r. All biotites are depleted in iron (100 Mg/(Mg + Fe) numbers vary be tween about 60 and 80), reflecting the oxidized environment. Hydrother mal biotite of stage 3 is estimated to have formed at temperatures gre ater than 850 degrees C. The following types of fluid inclusions were observed in vein quartz from the Endeavour 26 North deposit; type A, i nclusions with less than 60 vol percent vapor and no daughter crystals ; type B, inclusions with more than 60 vol percent vapor and no daught er crystals; type C, inclusions with less than 60 vol percent vapor wi th a halite daughter crystal; type D, inclusions with less than 60 vol percent vapor and halite and sylvite daughter crystals; type E, inclu sions with less than 60 vol percent vapor and halite and sylvite daugh ter crystals plus unknown phases. Fluid inclusion homogenization tempe ratures show a series of pronounced temperature peaks between about 35 0 degrees and 800 degrees C that are taken to reflect active periods o f the hydrothermal system and quartz sulfide deposition. Peaks at 750 degrees to 800 degrees C in stages 4 and 6 appear to be closely relate d to the emplacement of QMP1 and QMP2, respectively. Peaks around 500 degrees to 650 degrees C in stages 4 and 5 reflect the main periods of sulfide deposition and development of stockwork veining in QMP1, The apparent coexistence of type B inclusions with type C, D, and E inclus ions suggests pressure-temperature conditions consistent with the two- phase region (saline fluid plus vapor). Estimates of fluid composition s for type D inclusions for most stages center around 15, 45, and 40 w t percent H2O, NaCl, and KCl, respectively, on the ternary projection. An increase in total salinity and in the ratio of NaCl to KCl is asso ciated with stage 4.