GENERAL38607 - Laserfiche WebLink

Home Browse Search

GEOLOGY OF THE SAN LUIS GOLD DEPOSIT. COLORADO composition is 25 to 35% feldspar, 20 to 25% quartz, and 5 to 10% biotite. The remaining percentage consiata of variable amounts of hornblende, chlorite, sericite, magnetite, and specularite. Chlorite and minor magnetite are more abundant closer to and within cataclastically deformed gneiss. Gneissic foliations vary from fi- W, 15° to 25°S in the fiast Ore Zone, to N-S, 15 to 25°W in the northwestern part of the West Ore Zone. Foliations in the biotite gneiss parallel the orientation of the low-angle fault up to 80m below the clay zone. Gneissic granite ie a gray to reddish-gray 1700 tfa (Tweto, 1979) quartz-orthoclase granite. Gneissic granite occurs in the hangingwall of the fault zone and crops out in the northwestern part of the West Ore Zone (Figs. 3, 9). Gneissic granite does not host gold mineralization. A typical modal composition is 30 to 35% orthoclase and 25% quartz. The remainder of the rock consiata of muscovite, biotite, amphibole, chlorite, clay, eeroite and iron oxides. Locally extensive pegmatites of potassium feldspar and 9uartz crosscut and are locally concordant with foliation. Books of chlorite 55 cm are common along pegmatite contacts. Foliations are irregular. Diabase dikes and irregular messes occur within the gneissic granite. These equigranular and magnetite-rich intrusions are inferred to be Precambrian in age. 'Pertiwrv ianeouw ro kw Two Tertiary igneous rock types of volumetric aigniflcance are present in the San Luis deposit area: felaite dikes and andeaite dikes and flows. Feleite ie an aphanitic to weakly porPhyritic pale-green to gray intrusive rhyolite which occurs ae <1-lOm thick dikes. Intrusive contacts are commonly floe laminated and strongly spherulitic. Feleite is distinguished from altered biotite gneiss breccia by the presence of S2mm quartz and euhedral feldspar phenocryeta. Dikes trend north to north-northwest through the mine area. pikes are typically pervasively aericitized, weakly to strongly silicified, and may contain trace fluorite and up to 1% disseminated pyrite. Age dates on sericite concentrates from two feleitee are 24.0 t 1.0 Ha and 24.1 t 1.0 Ha (K/Ar; Krueger Geochron Laboratories). Although no correlation between feleitee and gold grades is evident, their altered nature end proximity to the ore zones indicates a possible genetic relation to gold mineralization. Andeeite hsa been observed in flows and dikes. Andeaite dikes contain pyroxene, plagioclase, biotite, hornblende, and magnetite (DeRuyter, 1988). Dikes typically trend north-south and apparently crosscut all rock types except Santa Fe Formation and younger alluvium. Andesite is believed to be younger than 29 !la and is not directly associated with gold mineralization. Andeaite flows, interbedded with Santa Fe sediments, occur Sn the southern part of the mine eras and are believed to b'e derived from the same source as the dikes. ' '~'ertiarv-Qnaterna ai ~+..y ro k Tertiary Santa Fe Formation at San Luie consiata of a repetitive sequence of silts. sands, gravels and 67 cobble lenses. Sentn Fe Formation directly overlies gneissic granite in the Weat Ore Zone anc overlies biotite gneiss in the fiaet Ore Zone. Cobbles of gneissic granite are more abundant in the loner beds of the Santa Fe Formation. Huaternary alluvium consists of channel-fill gravels and sands. A thin layer of alluvium ie present over much of the deposit area, and is upwards of 20 ft. thick near the Rito Seco. Structural lithologies The low-angle fault zone which hoots gold mineralization at San Luis ie divided into three atrt~ctural lithologic units: (1) fault clay, (2) biotite gneiss breccia, and (3) biotite gneiss cataclaeite. cataclaeite is here used to describe a rock in which slippade along multiple fractures has caused little rotation or dieaggregation of adjacent rock fragments. Greater than 80 volume percent of ore occurs in biotite gneiss breccia an lees than 20 percent occurs in biotite gneiss cataclaeite. The fault: clay contains no mineralization. A fault clay zone separates hangingwall gneisei~ granite from underlying biotite gneiss. The clay averages 2-3m in thickness and probably formed through extreme grain size reduction of the gneissic granite during displacement alone the low- angle fault. Clay color ranges from gray to gray- green to pale reddish brown. Fault clay Se locally silicified. Biotite gneiss breccia ie the principal host to gold mineralization. The breccia consiata of aubangular to aubrounded claete ranging in size from <1 mm to 10"e mm set in a fine-grained rock flour matrix. Clasta tvpically consist of quartz. with leeaec amounts of feldspar. In thin section, strain textures are visible in quartz fragments. Polymineralic, breccia, and quartz vein cleats are also present, but leas <:ommon. Clast size and abundance apparently de<:reesea from the breccia- cataclasite contact up-section towards the fault clay zone. This is attributed to a general increase in the intensity of both structural deformation end alteration in this direction. Typically the breccia matrix ie hydrothermally altered to an assemblage of quartz t sericite t pyrite. Biotite gneiss cntaclaslte consiata of subangular to subrounded rock fragments 1n n weakly foliated fine-grained rock flour matrix. The fine grained matrix generally constitutes lees than five volume percent of the rock. Fragments show little or no rotation or dieaggreentlon. Foliations in the cataclaeite are more irregular end lees evident than foliations in deeper, leas deformed biotite gneiss. Faults The principal structural feature of the San Lute Deposit Se the low-angle fault that separates gneissic granite from underlying biotite pneisa. In the southeast part of the West Ore Zone the fault