the crustal heritage of silver and gold ratios in arizona ores the ratios of produced grades of silver and gold in ore deposits of arizona representing nearly a century of mining correlate with their geologic setting the deposits occur south and west of the colorado plateaus where two geological domains are recognizable on the basis of type and age of rocks and geological history the eastern or naco domain has a basement of proterozoic pinal schist formed from a protolith of mostly clastic sedimentary rocks and widely scattered younger proterozoic granites overlain by paleozoic platform strata and mesozoic volcanic and clastic rocks the western deserts domain is characterized by an older proterozoic section of largely submarine volcanic and volcaniclastic strata together with proterozoic granites it is largely devoid of paleozoic strata but mesozoic clastic and volcanic strata are widely exposed in both domains middle and late tertiary volcanic rocks are extensive ore deposits of many genetic types of proterozoic nevadan laramide and younger tertiary ages are present deposits of laramide and younger tertiary ages occur in both domains but deposits of proterozoic age are confined mostly to the western deserts and those of nevadan age are restricted to the naco domain the ore deposits evolved at different times under different conditions of tectonic stress the produced grades of silver and gold and the silver to gold ratios reveal a bimodal distribution most deposits of all epochs and genetic type in the naco domain show ag enrichment relative to the ratio of the ag au crustal abundance clarke ratio 17 5 1 except for two laramide porphyry copper deposits and the proterozoic massive cu pb zn ores whose ratios are close to the clarke ratio most other districts of the western deserts domain are enriched in gold with respect to the ratio the regional zoning transgresses ore deposit type time tectonic style and reported igneous rock types and metal associations it does not appear to be related to the hypothetical results of subduction processes wherein metals and magmas are directly derived from partial melting of a descending slab of oceanic rocks rather the precious metal characteristics may represent inheritance from specific kinds of crust

re os isotope systematics of sulfides from felsic igneous rocks application to base metal porphyry mineralization in chile dating sulfides and determining the source of ore forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits here we analyze sulfides from base metal porphyry deposits from chile to demonstrate the potential of the re os system to determine both the age and the source of metals for hydrothermal mineralization cogenetic chalcopyrite bornite pyrite and sphalerite from el teniente ca 5 ma have 187 re 188 os ratios from 0 3 to 21 8 and initial 187 os 188 os ratios from 0 17 to 0 22 a paragenetically late pyrite that has an initial 187 os 188 os of 0 88 indicates that the ore deposit is not isotopically homogeneous in os throughout the formation of the deposit pyrite samples from andacollo ca 100 ma have 187 re 188 os ratios from 15 to 3600 and different isochrons yield ages between 87 and 103 ma consistent with different sericite k ar ages initial 187 os 188 os ratios of 0 2 to 1 1 are similar to those of el teniente the relatively homogeneous 187 os 188 os ratios 0 19 for cogenetic sulfides from el teniente suggest that most of the os in the ore was from the causal intrusive the latestage pyrite probably incorporated os leached from the country rock by meteoric fluids which mixed with magmatic fluids at the periphery of the hydrothermal system we demonstrate that the re os isotope system can be a powerful geochronological tool in hydrothermal ore deposits the large range in re os ratios of sulfides permits the age of mineralization to be well constrained despite isotopic heterogeneities of the hydrothermal fluids

different crustal sources for au rich and au poor ores of the grasberg cu au porphyry deposit abstract the grasberg is a porphyry copper deposit that is crosscut by a second stage mineralization which is greatly enriched in gold sulfides from the porphyry type event yield a 2 9 0 3 ma re os isochron that agrees with published geochronology the initial 187os 188os ratio of the isochron is 0 56 0 02 and implies a significant crustal component for the source of os and by inference the other base metals the samples from the crosscutting secondary event do not form an isochron but form a mixing line with the older porphyry style mineralization as one of the end members and a very radiogenic end member possibly shale the initial 187os 188os ratios from samples of the second event range from 0 81 to 1 26 and correlate with gold content the most radiogenic samples have the highest gold the re os isotope data indicate different crustal sources for the ore forming elements at the grasberg cu au deposit and support a model in which gold is derived from sedimentary protoliths that may have been pre enriched by sedimentary processes the gold derived from these sources is concentrated by magmatic hydrothermal systems the requirement of a sedimentary source for gold to produce hydrothermal gold deposits has been debated for decades and this study provides strong support for the model

correspondence of ores of silver and gold with basement terranes in the american southwest the ratios of silver to gold produced from epigenetic ore districts of the american southwest reveal a consistency of value ranges differing by an order of magnitude that may be identified with either one or the other of two geologic terranes in which the ores occur a discriminating value of the ratio is about 17 5 1 the ratio of crustal abundance given by ahrens 1965 no further significance is attributed to this value at this time beyond the fact that it appears to establish a reasonable separation of values on the basis of geographic occurrence ores relatively enriched in ag occur in terranes floored by thick proterozoic clastic and paleozoic marine successions and ores relatively enriched in au lie above or within a proterozoic basement dominated by maficfelsic volcanic arc successions proterozoic granites occur in each region the values of the ratio are broadly consistent within each terrane irrespective of the age of ore formation the ore deposit style associated igneous rocks structural control differing interpreted styles of subduction and weathering histories these characteristics and associations support a hypothesis that metallogenic signatures of ore districts in this region are fundamentally related to the crust in which the ores occur

geology re os age s and o isotopic composition of the lar porphyry cu mo deposit southeast iran abstract the lar porphyry cu mo deposit is located 20 km northeast of zahedan in the sistan suture zone ssz in the southeastern part of iran lar is a sub economic porphyry cu mo deposit with average grades of 0 16 cu 0 01 mo and 0 66 ppb au the cu mo mineralization developed in post collisional shoshonitic syenitic to monzonitic intrusions in an oligocene igneous complex known as the lar igneous complex lic the host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic potassic propylitic phyllic and argillic alteration the cu mo mineralization is mainly controlled by nnw striking faults and fractures and occurs as quartz veins and veinlets disseminated sulfides and stockworks the mineralization can be divided into two types hypogene and supergene hypogene mineralization is characterized by chalcopyrite pyrite bornite molybdenite and magnetite whereas supergene mineralization is characterized by enargite covellite chalcocite digenite malachite azurite and iron hydroxides sulfur isotope values for hypogene sulfides range from 0 28 to 1 76 and suggest an igneous source or mantle derived origin for sulfur the 18o values for quartz in mineralized quartz veins range from 9 6 to 11 5 these values would be in equilibrium with waters having calculated 18oh2o values of 9 3 11 2 suggesting magmatic water re os dating of two molybdenite samples from two different drill holes yields dates of 29 72 0 11 and 31 95 0 11 suggesting that the system was active for at least 2 0 m y

cross shelf passage of coastal water transport at the south atlantic bight observed with modis ocean color sst 1 images of cross shelf fronts of ocean color parameters and sea surface temperature sst of the south atlantic bight sab area are presented and analyzed the ocean color parameters k 490 and suspended solid concentration and sst clearly indicate a penetrating front originating from the coastal water within the inner shelf the front is triangular shaped with its broad base on the coast and its sharp end penetrating into the outer shelf and merging into the gulf stream depicting a passage of coastal water transport across the shelf this passage changes in position and orientation from southwest northeast to west east the horizontal ekman number was estimated to be outside the limit of baroclinic instability suggesting that the process is deterministic thus the water transport passage may play a significant role in a non diffusive transport process of heat and land derived biogeochemical materials

age and provenance constraints on seismically determined crustal layers beneath the paleozoic southern central asian orogen inner mongolia china abstract we present 110 ages and 51 in situ 18o values for zircon xenocrysts from a post 99 ma intraplate basaltic rock suite hosted in a subduction accretion complex of the southern central asian orogenic belt in order to constrain a seismic profile across the paleozoic southern orogen of inner mongolia and the northern margin of the north china craton two zircon populations are recognized namely a phanerozoic group of 70 zircons comprising granitoid derived ca 431 99 ma n 31 peak at 256 ma meta granitoid derived ca 449 113 ma n 24 peak at 251 ma and gabbro derived 436 242 ma n 15 peaks at 264 and 244 ma grains each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation the precambrian population 2605 741 ma n 40 exhibits a prominent age peak at 2520 ma granulite facies metamorphism and four small peaks at ca 1900 1600 and 800 ma our new data together with literature zircon ages significantly constrain models of three seismically determined deep crustal layers beneath the fossil subduction zone forearc along the active northern margin of the north china craton namely 1 an upper arc crust of early to mid paleozoic age intruded by a major permian triassic composite granitoid gabbroic pluton 8 20 km depth 2 a middle crust predominantly consisting of mid meso to neoproterozoic felsic and mafic gneisses and 3 a lower crust composed predominantly of late archean granulite facies rocks we conclude that the paleozoic orogenic crust is limited to the upper crustal level and the middle to lower crust has a north china craton affinity furthermore integrating our data with surface geological petrological and geochronological constraints we present a new conceptual model of orogenic uplift lithospheric delamination and crustal underthrusting for this key ocean continent convergent margin

the fate of river discharge on the continental shelf 2 transport of coastal low salinity waters under realistic wind and tidal forcing a three dimensional numerical simulation of shelf circulation is presented we employ realistic forcing for the southeast u s continental shelf during the spring season we show that the strongest offshore transport of river induced coastal low salinity waters and associated materials occurs near the surface the preferred mean pathway is in the northeastward direction and it takes about 2 months to cross the entire shelf owing to the mean direction of surface transport and the topography of the south atlantic bight shelf the preferred location for springtime removal is off charleston south carolina and presumably in the vicinity of the charleston bump the transport and fate of the river induced coastal low salinity waters during the spring season are determined by 1 the stratification of nearshore waters which is due to the high river runoff and causes the decoupling between near surface and near bottom layers 2 the prevailing northeastward winds which cause significant offshore transport within the shallow near surface ekman layer and 3 the tidally induced bottom stirring m2 tides comparison of model and data time series of currents shows very good agreement standard deviations of the model and data computed empirical orthogonal functions are almost identical while the respective variance conserving spectra agree both in amplitude and phase

evolution of fractures and alteration in the sierrita esperanza hydrothermal system pima county arizona the sierrita esperanza porphyry copper complex is central to a large fossil hydrothermal system located in the southeastern part of the sierrita mountains arizona abundant closely spaced fractures exert a profound control of sulfide minerals and alteration in the mineralized center of the system the extent and limits of the hydrothermal system in peripheral areas have been defined in this study by determination of the distribution and abundance of the altered and mineralized fractures mapping of these elements to their limit of occurrence has resulted in recognition of an area of about 60 km 2 where fractures related to porphyry copper mineralization occur and a contiguous area of about 40 km 2 where slightly older altered fractures related to the first stages of laramide igneous activity in the region are present within the orebody distinctive fracture types as recognized by fracture style and alteration assemblages occur in a consistent paragenetic succession this same succession has been recognized in the areas peripheral to the pits but with variations for each alteration type as influenced by wall rock mineralogies the establishment of a sequence of vein alteration commencing with quartz orthoclase alteration passing through precipitation of quartz and sulfides and ending with formation of epidote orthoclase chlorite has allowed separation of times of fracture formation in the orebody periphery fracture density fracture length sample area maps have been prepared from data collected at 1 310 stations in the study area the data were processed and transformed into maps of moving average contours of fracture abundance for different times of fracture formation as interpreted from the alteration paragenesis the distinctive differences in alteration types in crosscutting veins reveal the episodic nature of fracturing and the fracture density maps show shrinking of fractured rock volumes toward the central porphyry mass with successive fracture events evaluation of the fracture area in a 1 km thick slab of the 40 km 2 area at the time of precipitation of quartz and sulfide minerals in veins indicates that 4 x 10 5 km 2 of crack area existed a slightly smaller value of 6 x 10 4 km 2 per km thick slab existed at the time of the earliest quartz orthoclase alteration

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

two layer model of summer circulation on the southeast u s continental shelf abstract the summer circulation in the south atlantic bight is investigated using a two layer finite element model simulations using a steady state mean summer wind field lead to the following conclusions i the adjustment time of the shelf circulation to sudden changes in the wind varies between 12 to 24 hours ii the main dynamical balances were geostrophic in the cross shelf direction and with the coriolis pressure gradient wind and bottom stresses all being significant in the alongshore momentum balance both findings agree with data analysis of lee and pletrafesa this experiment produced an upwelling cell that matched well with the summer upwelling observed north of cape canaveral results of a second experiment using a 9 day real wind event compared reasonably well against observed coastal sea level midshelf currents and an upwelling downwelling event momentum balance results associated with the short adjustment time compared to the time scale of the wind forcing 4 to 12 days indicate t

relationship between copper tonnage of chilean base metal porphyry deposits and os isotope ratios re os isotopes in pyrite and chalcopyrite from early high temperature hypogene alteration assemblages in chilean porphyry copper deposits identify the source of os and by inference cu in these ore systems typical concentrations for os in both pyrite and chalcopyrite are between 7 and 30 ppt 10 12 g g and for re between 0 200 and 10 ppb 10 9 g g re os isochrons yield ages and initial 187 os 188 os ratios for the sulfides the isotopic data reflect the relative contributions of copper from the mantle and crust in chilean porphyry copper deposits seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in chile were studied the initial osmium ratios of the first stage of mineralization at each of the deposits range from 0 15 to 5 these values are more radiogenic than the present chondritic mantle 0 13 and indicate significant crustal contributions of os to the magmatic and or hydrothermal systems there is a strong correspondence between the total copper content and initial os isotopic ratios in base metal porphyry deposits the larger deposits have lower initial os ratios than the smaller less significant deposits this relationship implies that larger deposits acquire a greater proportion of os from the mantle the initial os ratio of samples in the central segment of porphyry copper deposits of northern chile also decreases with decreasing age of the deposit a plausible interpretation of the re os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

the subduction of the farallon plate beneath north america as derived from relative plate motions high rates of subduction and near normal convergence of the farallon plate correlate with the duration of the laramide orogeny in the western cordillera the motion of the farallon plate relative to north america is reconstructed back to the late mesozoic using relative plate motion data this reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates two models for relative motions are considered the preferred model predicts an increase in the subduction rate of the farallon plate exceeding 150 mm yr with a direction almost due east and perpendicular to the continental margin the duration of this nonoblique normal high rate subduction correlates well with the duration of the laramide orogeny this correlation must be regarded as only suggestive the probable errors when combined and propagated through the relative motion circuit may be of the order of the effect observed however it is encouraging that these reconstructed farallon north american relative motions are similar to the published predictions of others using the alternative more direct approach of hot spot reconstructions

the study of molybdenites through the187re 187os chronometer we determined the re and os concentrations in 11 molybdenites of various ages 30 2700 m y and from various areas no common osmium has been detected and os was found to be purely radiogenic therefore by comparing the concentrations we were able to determine 187 re 187 os ages for most samples these ages are in good agreement with those measured in surrounding rocks and probably correspond to the age of mineralization however two or three re os ages seem unreliable being probably related to some rhenium leaching through later metamorphic and or hydrothermal events this shows that the petrographic characteristics of ores and the type of crystallization of mos 2 are important in choosing samples suitable for dating re and os determinations combined with careful petrographic investigations may give important clues on the genesis and evolution of molybdenum ores

the chalukou deposit in the north great xing an range of china a protracted porphyry mo ore forming system in a long lived magmatic evolution cycle abstract the chalukou deposit is located in the north great xing an range of the xing an mongolia orogen bordering and to the northeast of the north china craton the deposit is a high f type porphyry mo deposit hosted by the chalukou composite igneous body containing small intrusive bodies genetically related to mo mineralization the composite igneous body includes pre mineralization dolerite monzogranite and syenogranite syn mineralization rhyolitic porphyry granitic porphyry and fine grained monzogranite and post mineralization rhyolitic porphyry quartz porphyry dioritic porphyry and andesitic porphyry detailed laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon dating of the igneous components of the composite igneous body was carried out to determine the temporal framework for magmatism in the chalukou region the new la icp ms u pb ages constraint documented here together with the published ages indicate that there was a protracted porphyry mo ore forming event of approximately 7 million years between ca 152 when the ore related rhyolitic porphyry was emplaced and ca 145 ma when molybdenite ceased being deposited the dating reveals that the mineralization is a part of relatively long lived magmatic cycle involving the emplacement of small doleritic stocks at ca 165 ma that progressively evolved into extensive granitic intrusions at ca 164 ma and then diminished with the emplacement of mineralization related porphyries to ca 152 ma the emplacement of barren early cretaceous magmatism represented by volcanic units in the ca 136 ma guanghua formation and porphyries followed the mineralized magmatism the syn mineralization porphyry units associated with mo contain zircons assaying 15 times higher in u and th than the pre mineralization magmatic phases this indicates that there was a significant enrichment of mo u and th in the magma and directly associated with ore fluid exsolution the return to their normal levels in the three elements in the post mineralization magmatic phases indicates that they were exhausted from the magma chamber in the later phases a genetic model is proposed for the enormous introduction of ore metals and enrichment at the chalukou deposit the protracted and multiphase igneous activity during the long lived magmatism reflects a multistage enrichment of metal and may play a crucial role in the formation of a volatile enriched fertile and large volume magma chamber beneath the chalukou deposit such a chamber is envisaged to be required for the formation of porphyry mo deposits in general

proterozoic geochronologic and isotopic boundary in nw arizona u pb ages determined from zircon sphene and apatite in conjunction with pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north trending proterozoic crustal boundary in northwestern arizona two regions west of the boundary hualapai mountains and lost basin range garnet mountain are characterized by complex u pb zircon systematics evidence for inheritance of an older zircon component 1 8 2 3 ga and elevated sup 207 pb sup 204 pb from feldspars compared to the east although the discordia patterns are complex supracrustal rocks are interpreted to be ca 1 73 ga and are intruded by plutonic rocks ca 1 70 ga deformation is younger than ca 1 70 ga foliated granites and older than the 1 682 plus minus 4 ma garnet mt monzogranite the rocks in one area east of the boundary cottonwood cliffs are characterized by relatively simple u pb zircon systematics no evidence for inheritance of any older component and feldspar sup 207 sup 204 pb near model mantle values supracrustal rocks are older than 1 73 ga as they are intruded by a 1 730 plus minus 9 ma foliated granodiorite timing of deformation is constrained by the late syn kinematic valentine granite dated at 1 713 plus minus 12 ma mineral ages indicate thatmore the rocks on either side of the boundary had different cooling histories and inferentially different uplift histories west of the boundary the cooling history is inferred from minerals separated from an amphibolite metamorphic zircon is 1 687 13 minus 8 ma sphene is 1 660 plus minus 5 ma hornblende has a sup 40 ar sup 39 ar age of 1 552 plus minus 5 ma and apatite has a u pb age of 1 520 plus minus 45 ma east of the boundary an amphibolite has sphene with an age of 1 670 plus minus 11 ma and apatite with an age of 1 630 plus minus 8 ma less

rhenium osmium evidence for regional mineralization in southwestern north america more than 40 base metal porphyry ore deposits in southwestern north america are associated with the laramide orogeny about 90 million to 50 million years ago rhenium osmium dates on molybdenite a rhenium enriched sulfide common in many of the deposits reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception magnitude or duration of magmatism deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization rhenium osmium ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago most deposits that formed in the oldest interval are within the older precambrian basement of northwestern arizona whereas the younger deposits are restricted to the younger precambrian basement in southern arizona and northern mexico synchronous widespread mineralization indicates that similar crust mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes

rhenium behavior in molybdenite in hypogene and near surface environments implications for re os geochronometry abstract rhenium is concentrated mainly in molybdenite mos2 and occurs as a major cation in only a few rare minerals this affinity makes molybdenite an ideal mineral for re os geochronometry but research on the behavior of re in molybdenite is limited infrared microscope xrd back scattered electron bse and microprobe techniques have been used in this study to document re behavior in molybdenite affected by hypogene and near surface processes in the hypogene environment both 3r and 2h molybdenite can experience re loss during hydrothermal alteration which also causes increased infrared transparency ir alteration at temperatures as low as 150 c can cause re loss in the presence of advecting fluids and will affect re os dating if it occurs long after primary mineralization re loss in 3r and 2h molybdenite under supergene conditions does not increase ir transparency rhenium is not incorporated into supergene ferrimolybdite but is enriched in k al silicate intergrowths which may be illite these minute intergrowths are present in several samples and would not be detected in a simple optical examination in the supergene environment elemental os is stable whereas re is not rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187os causing erroneously old ages or it may be adsorbed into the illite intergrowths creating ages which are too young in the weathering environment proterozoic molybdenites have altered to re enriched powellite which can be detected using back scattered electron imagery combined microprobe xrd bse and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating these techniques are non destructive and should be performed before any molybdenite is dated by the re os system

mineralization at cananea sonora and the paragenesis and zoning of breccia pipes in quartzofeldspathic rock correlative early and late deposit types in the cananea district correspond to similarly classified deposit types in other porphyry copper districts potassic alteration massive silicates in the colorada orebody and prograde skarn were produced by early probably predominantly magmatic hydrothermal fluids from a limited number of porphyry stocks later stockwork disseminated deposits associated with sericitic alteration were contemporaneous with breccia pipes retrograde alteration of skarn and limestone replacement deposits fluids associated with the relatively late deposits were probably predominantly meteoric hydrothermal but geologic relations suggest that the earliest fluids in some of these deposits may have included fluid exsolved from porphyry mineralization in both early and late deposits precipitated in the sequence silicates followed by sulfides except for early molybdenite in some pipes many pipes in the district appear to have formed by collapse into open space at depth consistent paragenetic sequences structures and textures shared by these pipes suggest that collapse occurred into space formed at the same time relative to emplacement of the responsible porphyry phases and evolution of the associated hydrothermal systems a possible analogue of the space into which collapse took place is abreccia at the outer contact of a large porphyry stock small outlying structures containing sulfide mineralization and breccia or intensely fractured rock appear to be associated with the upper terminations of pipes late mineralization has high overall pyrite cu fe sulfides ratios greater than 1 wherever the sulfide sequence could be established it is 1 pyrite 2 cu fe sulfides and 3 sphalerite galena tetrahedrite except for small amounts of second stage sulfides zoning of these numbered sulfide stages in breccia pipes in quartzofeldspathic rock and also in limestone replacement deposits is inferred to be 1 1 2 1 2 3 and 1 3 from the proximal to the distal ends of the deposits pipes within disseminated deposits contain the lower portion of the pattern textures indicative of local replacement of wall rock and early gangue minerals by later phases e g sulfides are common the common occurrence of late paragenetic stages in a pipe indicates that the pipe remained open during precipitation of the main stage of economic sulfides garnet in pipes in quartzofeldspathic rock is diagnostic of limestone at depth nearby early adularia and quartz adularia alteration of wall rock fragments in the cananea duluth pipe persist to much greater distances above limestone than garnet does and might prove to be similarly diagnostic